Increased Physical Activity

Although some wilderness activities are of the sedentary variety (e.g., car-based safaris), many other activities involve physical exertion. The level of exertion may be comparable with that which the person already performs on a regular basis but may, in some cases, involve more activity than the person’s typical baseline. As a result, all wilderness travelers with underlying medical problems must consider the level of activity on their planned trip and whether they are capable of doing the work necessary for that activity. Many commercial outfitters label trips with a difficulty rating that takes into account the level of exertion, remoteness, and exposure to high altitude. Ratings generally include easy, moderate, and strenuous categories. This can be useful to the physician evaluating the patient before the trip, especially if the physician is not familiar with the environment or the activity that will be pursued, although the physician must be aware that these ratings are not based on hard criteria and what constitutes a “moderate” trip for one company may be labeled as a “strenuous” trip by another. Some companies that offer adventure trips require potential clients to complete a medical history questionnaire and may also have a consultant physician review the questionnaire and provide advice to select clients.^77,78 Erb⁴⁷ has developed a scale of trip difficulty and correlated that with objective parameters of exercise capacity required to complete that type of activity (see Table 101-2, Chapter 101). “Extreme-performance ventures” are the most physically demanding and require above-average exercise capacity for participation. This category would include activities such as mountaineering at high altitude and alpine climbing. “High-performance ventures,” such as high-altitude trekking or hunting, or jungle trekking are the next most demanding category of activities and require an average exercise capacity. The third level of wilderness trips, “recreational activities” requires just below-average exercise capacity and includes activities such as hiking on mild-moderate terrain in a variety of environments. The last category of wilderness trips is “therapeutic activities,” which may be appropriate for persons with chronic cardiovascular or pulmonary disease that limit activity. Even though Erb defines categories of wilderness trips and assigns objective exercise capacity parameters to them, formal cardiac stress testing or cardiopulmonary exercise testing is not required in most cases to determine if a patient has adequate exercise capacity to complete a planned wilderness trip. The most important predictor is successful completion by the patient of similar activities in the past. Still, formal cardiopulmonary exercise testing may be useful in patients with chronic medical conditions to define maximum exercise capacity objectively to select the appropriate level of a wilderness trip. This helps to ensure a more enjoyable and safe experience for patients and their trip partners.

Trip Preparation

Patients with chronic medical conditions have an obligation to inform the other people involved in a trip, or the trip leader in the case of a commercial trip, about medical issues that might affect their ability to complete the planned itinerary. This is important because chronic medical conditions may influence not only the patient but also other members of the party. Notification will allow the trip leaders or other participants to anticipate situations in which the underlying medical problem may worsen, facilitate monitoring of the patient’s condition, and allow preparations to alter the group’s plans or arrange an evacuation when serious problems arise. It is also important that everyone in the group accept the limitations of a chronic medical condition and make conservative decisions during the trip that prevent the patients from overexerting themselves.

Pretrip preparation includes identifying the closest medical facility and resources available should evacuation become necessary. Rescue insurance and repatriation medical insurance are available and should be strongly considered by the patient. Patients should carry an adequate supply of medications and keep medications with them at all times to ensure that they do not become lost (e.g., pack in carry-on baggage rather than checked baggage). Patients should travel with a list of their medications and a concise list of their medical issues, including a baseline electrocardiogram for patients with cardiac problems, and contact information for their regular health care providers. They must also recognize that in remote settings they may encounter facilities not equipped for or physicians lacking experience dealing with complex medical problems.

Cold

Exposure to cold causes peripheral vasoconstriction, increased systemic blood pressure, and tachycardia and, as a result, increases myocardial oxygen consumption. When core body temperature drops below normal, shivering ensues as a metabolically demanding activity that occurs as a means to generate heat and defend against further drops in temperature. Although such physiologic responses are important for protecting the individual, learned behavioral responses, such as avoiding moisture or donning dry clothing, also play an important role during cold exposure. Even with appropriate preparation, however, some chronic medical conditions still increase the risk for accidental hypothermia or frostbite. Alternatively, cold conditions may exacerbate the underlying chronic medical condition. Situations in which such problems may arise include the following:

Heat

Peripheral vasodilation is a primary response to heat exposure that leads to sweat production and evaporative heat loss. In high-humidity conditions, however, heat loss from sweating is diminished. Radiant heat loss has no usefulness when environmental temperature exceeds skin temperature. Unlike cold adaptation, heat adaptation depends more on physiologic acclimatization than on behavior. Prior heat exposure reduces the skin temperature at which vasodilation and sweating occur, thus initiating cooling earlier. Physical training improves heat tolerance through the same mechanisms and improves efficiency of the cardiovascular system. Poor physical fitness, obesity, lack of heat acclimatization, and dehydration decrease heat tolerance. Chronic medical conditions increase the risk for heatstroke, whereas increased temperature may exacerbate chronic medical conditions:

Susceptible individuals can improve heat tolerance through a program of heat acclimatization before or during their trip.⁵ Before a wilderness trip to a hot environment, exercising daily in the heat for limited periods of at least 1 hour’s duration for at least several days improves heat tolerance. If such pretrip training is not feasible, then the individual should restrict exertion to limited periods during cool parts of the day for the first week of the trip. Regardless of the acclimatization program, once a trip commences, individuals should maintain volume status through adequate intake of water or electrolyte drinks with copious clear urine output being a good indicator of adequate hydration status. Individuals engaged in prolonged bouts of exercise (lasting several hours or more) should supplement water intake with meals or salty foods to maintain electrolyte balance and prevent hyponatremia.

High Altitude

The primary physiologic insult at high altitude is hypobaric hypoxia. This leads to a decrease in the partial pressure of inspired oxygen (PIO₂), which in turn leads to drops in the alveolar and arterial PO₂. For persons with normal gas exchange, significant decreases in arterial oxygen saturation will not occur on acute exposure until they are higher than approximately 3000 m (9,843 feet), because of the sigmoidal shape of the oxyhemoglobin dissociation curve. In acclimatized individuals, arterial oxygen saturation (SaO₂) decreases below 90% at elevations above approximately 3500 m (11,483 feet). Any individual with gas exchange abnormalities or right-to-left shunts at sea level may become very hypoxemic at significantly lower elevations, whereas individuals with diseases exacerbated by hypoxia (e.g., severe coronary artery disease or heart failure) may have significant problems in this environment as well.

Regardless of their underlying health status, all individuals undergo physiologic responses that facilitate acclimatization to hypobaric hypoxia. Individuals with certain chronic conditions, however, may either fail to mount these responses or may suffer adverse consequences as a result of the responses. For example, low arterial PO₂ triggers an increase in minute ventilation, referred to as the hypoxic ventilatory response. This response, which occurs within hours of ascent to altitudes greater than 2000 m (6562 feet) and varies in magnitude between individuals, helps to raise alveolar and arterial oxygen tensions and may affect susceptibility to altitude illness. Persons with chronic diseases that restrict ventilatory capacity (e.g., very severe chronic obstructive pulmonary disease [COPD], obesity hypoventilation syndrome, and neuromuscular diseases with respiratory system involvement) may be unable to raise their minute ventilation and may experience severe hypoxemia and exercise intolerance.

Low alveolar oxygen tensions also trigger hypoxic pulmonary vasoconstriction and a rise in pulmonary artery pressure. The response is seen above 2,000 m (6562 feet) in elevation and varies in magnitude between individuals. Most people tolerate the rise in pulmonary artery pressure, but individuals with underlying pulmonary hypertension or right-to-left shunts may be at risk for high-altitude pulmonary edema, worsening right heart function, or increased hypoxemia.

Important cardiovascular responses also occur that may not be tolerated well by all individuals. Increased sympathetic tone occurs acutely after ascent to high altitude and increases heart rate and blood pressure. Heart rate and blood pressure gradually decrease over several days at high altitude, but remain higher than sea level baseline values for the duration of stay at high altitude. Despite the increase in sympathetic tone, most persons with mild to moderate cardiovascular disease do well after ascent to moderate altitudes of approximately 2500 m (8202 feet),^99,137 although individuals with unstable angina, severe cardiomyopathy, or poorly controlled hypertension might not tolerate such changes.

Increased erythropoiesis is a long-term adaptation to high altitude requiring weeks to complete that increases red cell mass and oxygen-carrying capacity of the blood. Persons with severe iron deficiency or chronic marrow suppression are unable to increase erythropoiesis to complete the normal long-term hematologic acclimatization.

The following chronic diseases may worsen at high altitude or predispose to impaired tolerance of hypobaric hypoxia:

Asthma

Asthma is a disorder of reversible airflow limitation marked by the presence of cough, wheezing, chest tightness, and shortness of breath. Affected individuals can have long symptom-free periods punctuated by exacerbations and worsening symptoms that are often triggered by stimuli such as respiratory infections, exercise, or allergen exposures. Given the high prevalence of the disorder in the general population, it is likely that many wilderness travelers suffer from this disorder. Despite this fact, little data are available as to how wilderness travel affects these patients. In the only prospective study of asthma patients engaged in wilderness travel, Golan and colleagues⁶⁶ studied 203 patients with mild to moderate asthma presenting to a travel clinic before departure. Forty-three percent of these individuals reported an exacerbation during their trip, whereas 20% reported worsening asthma control and 16% reported the worst exacerbation of their life. The leading risk factors for exacerbations during the trips were frequent rescue inhaler use (>3 times per week) before the trip and participation in intensive physical activity during the trip itself. Pretrip exercise testing with pretest and post-test spirometry was not useful in predicting which patients would develop an exacerbation.

Because the potential for exacerbations exists and pretrip identification of patients likely to experience such problems is difficult, all asthma patients should strongly consider pretravel evaluation to ensure that their disease is under adequate control at the time of their trip and to establish the means to monitor for and respond to exacerbations.

Treatment of a patient with asthma planning a wilderness activity should begin with the basic elements of treatment for any patient with asthma: monitoring of symptoms, pharmacologic therapy, avoidance of triggers, and patient education. A pretrip evaluation for asthma is an opportunity to ensure that the patient has optimal baseline treatment for asthma, which will also help prevent asthma attacks during the sojourn. Although not necessary before every short day-long or weekend excursion, such evaluations should take place before any long trips, particularly those to international and/or remote destinations.

The first part of this evaluation is to review the state of the patient’s current symptoms and determine if the patient is on the appropriate pharmacologic regimen. The National Institutes of Health’s Guidelines for the Diagnosis and Management of Asthma¹¹⁸ provides definitions of categories of severity for patients with asthma and the appropriate treatment (Box 34-1 and Table 34-1). Pretrip evaluation for a patient with asthma should include a review of these guidelines in relation to the patient’s symptoms and consideration of escalating therapy before a wilderness trip. For example, a patient who usually uses inhaled bronchodilators only on an as-needed basis for mild persistent asthma may consider adding an inhaled corticosteroid for improved control, although this practice has never been formally studied for wilderness travel. Because a primary trigger for asthma on a wilderness trip may be exercise, consideration can be given to adding the leukotriene receptor blocker montelukast to the patient’s controller regimen, because this has been shown to be effective adjunctive therapy for exercise-induced asthma.⁹⁷ Patients can also use short-acting β₂ agonists before and during exercise.¹¹⁴ Individuals reporting worsening control or who are in the midst of a severe exacerbation should strongly consider postponing their trip until symptoms are under better control.

TABLE 34-1 Stepwise Approach to Managing Asthma in Patients 12 Years of Age and Older

As noted above, many exacerbations are triggered by identifiable factors such as allergens, respiratory infections, irritants, chemicals, exercise, and emotional stress. The pretravel assessment also provides an opportunity to review the triggers that could contribute to worsening control during the trip. Patients should realize, however, that on certain wilderness trips it may be difficult to avoid certain triggers, such as breathing cold, dry air during exercise on a mountaineering expedition. In such cases, effort can be made instead to minimize exposure by, for example, wearing and breathing through a balaclava or pretreating with asthma medications before exertion. Asthma patients should also anticipate exposure to triggers that might not be an issue at home. For example, adventure travel often requires transit through major international cites, such as Bangkok, Thailand, or Kathmandu, Nepal, where local air quality may be poor and contribute to worsening control before the actual adventure activity begins.

Another important part of the pretravel assessment is to devise a program for objectively monitoring disease status during the trip. Asthma patients commonly monitor asthma control using measurements of peak expiratory flow (PEF), an objective parameter measured after the patient inhales to total lung capacity and then forcefully exhales into a peak flow meter. Patients establish their baseline peak flows when their disease is under good control by performing the maneuver several times a day and recording the results in a diary. Typically, PEF is highest in the morning and lowest in the afternoon. The highest measured PEF becomes the baseline for the patient, and comparison of further measurements with that baseline can be used to identify disease exacerbation and therefore escalate therapy. The National Institutes of Health’s Guidelines for the Diagnosis and Management of Asthma¹¹⁸ recommends using a zone scheme for categorizing results of PEF: green is a PEF greater than 80% of personal best, yellow is a PEF 50% to 80% of personal best, and red is a PEF less than 50% of personal best. A PEF in the green zone indicates the patient should continue maintenance medications, whereas a PEF in the yellow or red zone requires adjustments in treatment according to a predetermined plan, as well as seeking evaluation by a physician.

This type of self-monitoring could easily be used on a wilderness trip. For example, a predetermined treatment plan for PEF in the yellow zone associated with symptoms of asthma might include acute treatment with an inhaled short-acting β₂-adrenergic agonist followed by reevaluation in 1 hour, and if not improved to the green zone, initiation of oral corticosteroids and an increase in the dose of inhaled corticosteroids. A treatment plan for PEF in the red zone associated with symptoms of an asthma exacerbation would specify immediate therapy with inhaled β₂ agonists, to be repeated at frequent intervals, and initiating an oral corticosteroid. Persistent symptoms associated with PEF in the red zone suggest that evacuation from the wilderness environment should be initiated.

An alternative to peak-flow monitoring is the PiKo-1, which measures both PEF and forced expiratory volume in 1 second (FEV₁). This electronic device has a 2-year battery life and is small and easy to pack. Patients should be aware that the PiKo-1 and PEF meters, particularly variable orifice peak flow meters, may generate readings significantly lower than actual PEF under conditions of cold temperatures or high altitude.^127,132 If concern exists about such problems on a trip, the individual should rely on an assessment of trends in the measured PEF rather than the absolute values.

In addition to devising a monitoring plan and means for recording the data (e.g., an asthma diary), the health care provider should observe the patient’s inhaler technique and correct any deficiencies, because inhaled medications are of less benefit when not administered properly. Finally, all patients should travel with an adequate supply of rescue inhalers (e.g., short-acting bronchodilators) and a supply of prednisone to treat an exacerbation during the trip. If traveling in a cold environment, patients should be instructed to keep their inhalers warm, because cold exposure can decrease their effectiveness.

With regard to specific types of wilderness activities, two types of activities that warrant further attention in the asthma patient are high-altitude travel and diving. The effect of high-altitude exposure on asthmatic patients has not been well studied, but available evidence suggests that patients with mild to moderate disease, well controlled at the time of their trip, can tolerate significant altitude exposure. Several studies have shown that exposure to elevations as high as 5000 m (16,404 feet) is associated with decreased bronchial hyperresponsiveness.^2,32 A small study of patients with mild to moderate disease climbing Mt Kilimanjaro found a non–statistically significant improvement in PEF between 2700 and 4700 m (8858 and 15,420 feet), no difference in the incidence of acute mountain sickness or summit success compared with nonasthmatic patients, and no evidence of exacerbations during the excursion.¹⁵⁶ Because of interindividual disease heterogeneity, however, it is difficult to draw broad conclusions that apply to all patients. In the end, how a patient fares at altitude may be a function of the particular triggers for their disease. Patients whose disease is triggered primarily by allergens may fare well at altitude,¹⁶⁶ where, for example, the number of dust mites decreases with increasing elevation, whereas patients whose disease is triggered by breathing cold, dry air may have difficulty during mountaineering or ski excursions that include significant exposure to such conditions. Epidemiologic evidence suggests that asthma incidence is increased among cross-country skiers and ski mountaineers, athletes whose activities entail high levels of minute ventilation in cold, dry environments.^42,94

The primary concern with scuba diving in patients with asthma is that active airflow obstruction could lead to air trapping and significantly increase the risk for pulmonary barotrauma with changes in barometric pressure on ascent back to the surface of the water. As discussed further in Chapter 77, asthma was previously considered an absolute contraindication to diving but is now permitted provided the patient is (1) an asymptomatic adult with a childhood history of asthma, (2) has well-controlled disease with known triggers, (3) has normal pulmonary function tests with a less than 20% change in peak expiratory flow after exercise, and (4) no evidence of cold- or exercise-induced bronchospasm.

Chronic Obstructive Pulmonary Disease

COPD is a syndrome of progressive airflow limitation caused by chronic inflammation of the airways and lung parenchyma with a prevalence of approximately 4 to 7 per 1000 persons in developed countries.⁶ The extent to which COPD limits an individual’s planned wilderness activities is a function of disease severity, assessment of which can be made using criteria specified by the Global Initiative for Chronic Obstructive Lung Disease (GOLD).¹³³ According to these guidelines, disease severity is graded based on the decrement in the patient’s postbronchodilator FEV₁ of a forced vital capacity (FVC) maneuver (Table 34-2). Patients who fall in the mild disease category (FEV₁ ≥80% predicted) will probably do well on a wilderness activity, provided the planned activity does not far exceed their usual exercise tolerance. Once patients meet the criteria for moderate disease (FEV₁ 50% to 80% predicted), careful evaluation is warranted to determine their suitability for the planned activity. Cardiopulmonary exercise testing⁴ should be considered to determine exercise capacity, which can then be compared with the expected level of exertion on the planned trip. Of note, patients whose disease may not appear too severe based on their pulmonary function test results may have significant air trapping during exercise that impairs pulmonary mechanics and leads to significant exercise limitation.¹²⁴Patients in the severe (FEV₁ 30% to 50% predicted) or very severe (FEV₁ ≤30% predicted) categories, or those with carbon dioxide retention or right heart failure, should be advised against any wilderness activity in which exertion above their baseline level of tolerance is expected, or if the planned trip is to a higher altitude than their current residence. Such patients may, however, tolerate car or horse-led activities such as safaris or fishing that do not require much in the way of physical exertion. In considering suitability for travel, it is important to remember that many patients with COPD have comorbid conditions, such as coronary artery disease, that may also affect their tolerance for a planned activity and that will require attention in the pretravel assessment.

TABLE 34-2 Global Initiative for Chronic Obstructive Lung Disease (GOLD) Classification of Severity of COPD and Therapy

Once a decision is made that a patient can undertake a given activity, pretrip pulmonary evaluation is warranted to evaluate several important aspects of disease management around the time of the trip. The first element of this evaluation is to review the patient’s pharmacologic regimen. Inhaled bronchodilators (selective β₂-agonists or anticholinergics) are the foundation of pharmacotherapy for COPD because of their capacity to alleviate symptoms, decrease frequency of disease exacerbations, and improve exercise tolerance by decreasing hyperinflation and airflow limitation.^25,157 These medications have clear benefits, even in patients who may not meet official pulmonary function testing criteria for bronchodilator responsiveness (increase in FEV₁ or FVC by 200 mL and 12% compared with prebronchodilator testing). As indicated in Table 34-2, patients with mild disease are typically on a regimen of short-acting bronchodilators used on an as-needed basis. The selective β₂-agonist albuterol and the anticholinergic ipratropium bromide are equally effective for this purpose.¹⁵⁷ In addition to as-needed short-acting bronchodilators, there is a need for patients with moderate disease severity to be started on scheduled long-acting bronchodilator therapy with either a long-acting β-agonist (salmeterol or formoterol) or the long-acting anticholinergic tiotropium. Patients are typically started on a single agent, but combination therapy with a β-agonist and anticholinergic can be used in those who fail to respond to monotherapy. Inhaled corticosteroids are considered for patients with poor symptom control on such therapy or frequent exacerbations, or whose disease falls in the severe or very severe categories.⁵¹

Patients whose symptoms are not under good control before their planned wilderness trip may require escalation of therapy. Any such changes should be made several weeks before the planned departure, because the benefits of tiotropium may not occur for a week after starting therapy, whereas those of corticosteroids may require up to several weeks. Although theophylline is sometimes used as an adjunctive therapy in patients whose disease is not well controlled with the other agents, it has a narrow therapeutic window and increased risk for toxicity compared with other therapies and should not be added to a patient’s regimen before a wilderness trip. Patients already taking theophylline on a regular basis should have their levels checked before departure and should be counseled regarding how to recognize symptoms and signs of toxicity.

As with asthma patients, COPD patients are at risk for disease exacerbations. Often triggered by viral or bacterial infections, these exacerbations are characterized by increased dyspnea and a change in the frequency or character of sputum production and can lead to worsening hypoxemia and overt respiratory failure. The literature does not contain information on the incidence of COPD exacerbations during wilderness travel and, as a result, all COPD patients traveling into the wilderness should prepare for the possibility of this complication. They should arrange an evacuation plan in the event of severe symptoms and carry a supply of rescue medications, including short-acting bronchodilators, prednisone, and oral antibiotics (azithromycin, levofloxacin, doxycycline, or trimethoprim-sulfamethoxazole). In hospital settings, short-acting bronchodilators are often given in the form of nebulized solutions. Such treatments are infeasible in the field, but with proper technique, metered-dose inhaler therapy can be of equal efficacy. Similarly, prednisone has good oral bioavailability and can be substituted for the intravenous formulations that are commonly used in the inpatient setting. Patients on formoterol should be aware that its onset of action is fast enough that it can be used as a rescue inhaler if albuterol is not available, although its long half-life precludes use for more than every 12 hours. Salmeterol’s onset time is too slow to be of use in rescue situations.

COPD patients with a baseline resting arterial partial pressure of oxygen (PaO₂) less than 55 mm Hg or SpO₂ less than 88% should be treated with continuous supplemental oxygen, because this has been shown to have a mortality benefit in this patient population. These patients should continue supplemental oxygen on any planned wilderness trip, whereas patients not regularly on oxygen, but whose exacerbations are associated with worsening hypoxemia, might consider supplemental oxygen for the purpose of their trip. The traditional supplemental medical oxygen delivery system is a continuous flow of 100% oxygen from a compressed gas cylinder delivered by nasal cannula. The disadvantage to this system is its inefficient use of oxygen, because only a small percentage of the oxygen delivered to the nose actually reaches the lungs. In a wilderness setting, the weight and space required to carry medical oxygen cylinders create a significant burden and limit the trip duration. A more efficient alternative is for the patient to use a pneumatic nonelectronic demand valve that delivers flow of oxygen only on inspiration.¹⁶³ Portable liquid oxygen units with demand valves are an alternative to oxygen cylinders and offer the advantage of lighter weight for a comparable amount of oxygen but are more expensive. Any patient using a demand valve system for a wilderness trip should be evaluated during rest and during exercise to ensure that adequate oxygenation is maintained.¹⁶² Finally, portable oxygen concentrators are available that obviate the need for liquid or compressed gas cylinders and increase patient mobility. However, their usefulness is limited by short battery life.¹⁰¹ Depending on the length of the planned trip, access to power sources, or ability to carry spare batteries, they may not represent a suitable alternative.

Patients seeking to travel with supplemental oxygen need to be aware of important logistic issues that may affect their plans. For those traveling by car to their destination, there should be few problems bringing oxygen to their destination, but persons traveling by airplane may encounter significant problems. As a general rule, patients are not allowed to bring liquid or compressed gas oxygen cylinders on board aircraft as either carry-on or checked baggage. The Federal Aviation Administration permits the use of small portable oxygen concentrators on aircraft in the United States, but use may not be permitted on all airlines worldwide. Unfortunately, standard practices for supplemental oxygen vary across the airline industry, with the availability of service, feasibility of using personal systems, and the fees varying between countries, airlines, and domestic and international flights.^104,172 Patients planning to obtain oxygen sources on arrival at their destination will need to confirm whether such sources are available, because access will vary based on whether the person is traveling in the developed or developing world. Even in the developed world, access to supplies might be limited in more remote settings.

Similar to asthma patients, two specific wilderness activities that deserve further attention in patients with COPD are high-altitude travel and diving. The biggest concern with regard to high-altitude travel is the potential for worsening hypoxemia. Few data are available about COPD patients in actual mountain environments, but data from the literature on COPD patients and commercial airline flights clearly indicate that patients with FEV₁ values of 1 to 1.5 L experience significant hypoxemia when exposed to altitudes equivalent to 2440 m (8005 feet), with further drops in their PaO₂ with minimal exertion, such as walking on flat ground or cycling at very modest work rates (20 to 30 W).^101,103 Patients already using supplemental oxygen at home should increase their flow rates at high altitude and can consider portable pulse oximetry as a means to decide on the appropriate adjustment. Depending on their baseline disease severity, patients not already on supplemental oxygen should undergo pretravel assessment to determine the need for oxygen at high altitude. This can be done using either high altitude simulation testing⁴⁰ or a variety of prediction rules that take into account various factors such as the PaO₂ on room air at sea level, the FEV₁, or the target altitude.^31,67,116 Patients who develop symptomatic hypoxemia (PaO₂ <50 to 55 mm Hg) during the high altitude simulation testing or who are predicted to have a PaO₂ less than 50 to 55 mm Hg using one of the other prediction tools should be strongly encouraged to use supplemental oxygen at high altitude. Decisions to use oxygen should not be based on the PaO₂ alone but should reflect whether or not the patient develops associated symptoms (dyspnea, light-headedness, dizziness, altered mental status, exercise intolerance). Patients who become hypoxemic (PaO₂ <50 to 55 mm Hg) but remain asymptomatic with preserved exertion tolerance can travel without supplemental oxygen. They should monitor symptoms and oxygen saturation on arrival at high altitude using portable pulse oximetry or through periodic clinic visits and should carry a prescription for supplemental oxygen that can be filled at their destination if they develop problems following arrival. Patients whose PaO₂ remains above these thresholds can travel without supplemental oxygen but may also consider monitoring symptoms and SpO₂ during their sojourn.¹⁰¹

There are no data on the frequency of exacerbations at high altitude, whereas data on measures of airflow obstruction are limited and conflicting. As a result, any COPD patient traveling to high altitude must be prepared for the possibility of exacerbations as described above. There is no evidence to suggest that patients with severe bullous disease are at increased risk for pneumothorax at high altitude, despite the fall in barometric pressure.¹⁰³

Diving carries the same concerns for COPD patients as it does for asthma patients. Airflow obstruction could lead to significant air trapping, which would increase the risk for barotrauma when trapped air expands in response to barometric pressure changes on ascent. Any blebs or bullae that do not adequately communicate with the environment could also expand and rupture with ascent to the surface. For these reasons, COPD is considered a contraindication to diving.

Sleep Apnea

Sleep-disordered breathing refers to respiratory disturbances that occur during sleep and includes entities such as obstructive sleep apnea (OSA), central sleep apnea (CSA), and sleep-related hypoventilation. OSA is the most common form of sleep-disordered breathing and is marked by the presence of repeated reductions (hypopneas) or cessation (apnea) of airflow that occur as a result of either partial or complete occlusion of the upper airway during sleep. Present in up to 28% of the general population, the disease is more common in men and among older individuals and may occur in people who lack other underlying medical problems.¹⁷⁸ CSA is also marked by recurrent apneas or hypopneas, but, unlike in OSA, alterations in airflow occur because of changes in respiratory signaling and effort rather than upper airway occlusion. Although idiopathic forms of the disease occur, it is most frequently seen among individuals with severe cardiomyopathy. As indicated in Chapter 1, CSA is also common among otherwise healthy people at high altitude. Sleep-related hypoventilation refers to abnormally high arterial carbon dioxide tension levels during sleep and is usually seen in the context of severe obesity, obstructive lung diseases, and various neuromuscular disorders, such as muscular dystrophy or amyotrophic lateral sclerosis. The various forms of sleep-disordered breathing are significant not only because of their ability to disrupt sleep quality but also due to their adverse effects on daytime function, including, for example, excessive daytime somnolence and impaired concentration. In addition to treatments directed at the underlying disease (e.g., heart failure), the standard treatment approach for each of these disorders is nocturnal use of intermittent noninvasive positive pressure ventilation (NIPPV) or continuous positive airway pressure (CPAP). Individuals with these disorders who seek to travel into wilderness environments must consider several key issues before their trip, including (1) what will happen to the underlying disorder in that environment, (2) whether it is necessary to continue treatment while engaged in the wilderness activity, and (3) how to facilitate continued treatment if such treatment is deemed necessary. With regard to the first question, little is known about what happens to the various patterns of sleep-disordered breathing in the wilderness. There is little theoretic basis to expect changes in the incidence and severity of these problems when sleep is conducted at or near the same elevation at which the patient normally resides. Although changes in the severity of preexisting CSA following ascent to high altitude have not been studied, limited data suggest that the severity of OSA decreases considerably with ascent to high altitude. In one study of normal individuals, the OSA index fell from 5.5 + 6.9 events/hr to 0.1 + 0.3 events/hr at 5050 m (16,568 feet),²² whereas in a study of adults with moderate OSA at baseline, the obstructive respiratory disturbance index fell from 25.5 + 14.4 events/hr to 0.5 + 0.5 events/hr at 2750 m (9022 feet).²¹ Of note, however, was the fact that in both studies, the decrease in obstructive events was offset by marked increases in the frequency of central apneas. The reason for the observed changes was not elucidated in these studies, but they may be due to alterations in air density, increased respiratory drives, and increased upper airway tone.²¹ Because the various forms of sleep-disordered breathing are generally associated with intermittent nocturnal hypoxemia, it can be expected that high-altitude travel will lead to greater degrees of nocturnal desaturation, with the magnitude of hypoxemia likely being a function of the altitude attained, as well as the duration of apneas and hypopneas.

Whether individuals with sleep-disordered breathing should continue treatment while engaged in wilderness activity will be a function of the severity of their disease and associated symptoms and characteristics of their planned activities. For example, individuals with OSA but minimal or no daytime symptoms may be able to forego CPAP for several days while away on a trip, although excessive snoring may be a nuisance to other people on their excursion. On the other hand, individuals with severe daytime symptoms or who expect to perform activities that require high levels of concentration may find it necessary to continue therapy.

For individuals who plan to continue NIPPV or CPAP, the most important issue will be ensuring reliable access to power supplies. When such access is available (e.g., hotel, generators), travel should be relatively straightforward, because most machines are small and light enough to facilitate travel and can be brought on airplanes as carry-on luggage. Significant challenges arise, however, when power access is not available. Some manufacturers now make devices with rechargeable lithium batteries, but battery life is short, and use for more than 1 to 2 days would require access to recharging facilities or ample numbers of back-up batteries. An alternative is to obtain a special power cord from the device manufacturer that allows one to draw power off a 12-volt battery (deep cycle marine batteries offer the best battery life), but this option is limited by the size and weight of these batteries, expense, and logistic issues associated with obtaining a battery at the destination or traveling with one in hand.¹⁴⁶ Individuals should contact the device manufacturer for information about the power consumption of their device to guide anticipated battery needs and should be aware that use of heated humidity systems will increase power needs. Finally, although most commercially available machines can run off the voltage levels used with outlets in the United States (110 to 120 V) and Europe (220 to 240 V), individuals should confirm the range for their machine and ensure that they are carrying the appropriate plug adapters.

Diabetes

Approximately 17.9 million people with a diagnosis of diabetes live in the United States (6% of the U.S. population), so it is likely that diabetes will be encountered in persons pursuing wilderness activities.¹¹⁹ Diabetes encompasses the disorders of type 1 diabetes, previously known as insulin-dependent or juvenile-onset diabetes, and type 2 diabetes, previously known as non–insulin-dependent or adult-onset diabetes. Approximately 5% to 10% of diabetic patients have type 1 disease, and 80% have type 2, with the remainder of cases due to other causes. The pathophysiology of type 1 diabetes results from inadequate insulin production, whereas the pathophysiology of type 2 diabetes is due to peripheral resistance to insulin action. Patients with type 1 diabetes must be treated with insulin, whereas type 2 diabetes may be treated with diet and exercise, oral and injectable hypoglycemic agents, or insulin.

A number of issues need consideration for diabetics pursuing wilderness activities. Diabetics may be remote from medical help and need to ensure that adequate medication is available. Carrying two or three times as much medication and devices (syringes, glucometer, glucose and ketone test strips) as anticipated and splitting up medication and medical device supplies among group members will mitigate against theft, loss, or unanticipated delays on longer trips (Table 34-3). Anticipate erratic meals, time changes, and increased levels of physical activity, and factor how they may change medication regimens. Engaging in nonroutine activities also creates certain safety issues for the patient with diabetes, so advising the patient on how to manage diabetes appropriately during exercise in an outdoor environment is essential. High-risk wilderness activities, such as mountaineering or rock climbing, where loss of focus and concentration may result in death, would not be appropriate for a diabetic patient who is susceptible to hypoglycemia, unless the person is constantly monitored by a climbing partner, does not climb in the lead, and is always secured by a rope. Similarly, solo wilderness activities may not be appropriate for diabetic patients who may become hypoglycemic.

TABLE 34-3 Diabetes-Specific Supplies

Patients with both type 1 and type 2 diabetes require evaluation for several issues before a wilderness trip. The extent of increased exercise that will be undertaken on the trip needs to be determined in relation to the patient’s current level of physical activity and his or her glucose control. Increased physical activity is beneficial for diabetic patients, particularly those with type 2 disease, and wilderness activities may be encouraged as part of lifestyle modification. In addition to exercise, the specific activity and associated travel are likely to effect a change in diet. Screening for conditions associated with diabetes, such as cardiovascular disease, peripheral and autonomic neuropathies, proliferative retinopathy, and nephropathy, should be done. These conditions may significantly influence the ability of diabetic patients to pursue wilderness activities safely and may require further evaluation and testing before the trip. Last, the effect of the change in environment on the diabetic patient is considered, including potential problems with exposure to heat or cold.

The effect of increased exercise is important for both type 1 and type 2 diabetic patients because it may precipitate hypoglycemia or hyperglycemia, depending on the timing of the last dose of insulin and blood glucose level at the onset of exercise.^70,173 The normal response to exercise in nondiabetic persons is a decrease in insulin secretion as serum glucose levels fall because of uptake of glucose by exercising muscle, and an increase in hepatic glucose release in response to catecholamines, glucagon, and growth hormone to maintain blood glucose levels. Patients with type 1 diabetes who are hypoinsulinemic when they exercise (as may occur if they excessively decrease their insulin in an effort to accommodate the increased physical activity) may become hyperglycemic because of increased hepatic release of glucose into the blood in addition to insufficient insulin to allow glucose to enter the cells. Thus hyperglycemia results in decreased exercise capacity, because exercising muscle is depleted of glucose as a result of insufficient insulin to enable glucose to enter the cells. Under these conditions, the substrate for fuel becomes free fatty acids released from adipocytes, with generation of ketone bodies by the liver. Hypovolemia may occur due to glycosuria; if the process persists, diabetic ketoacidosis may ensue.

Patients with type 1 diabetes who have exogenous insulin in excess may become hypoglycemic when they exercise. Exercise results in an increase in insulin secretion, rather than the expected physiologic decrease in insulin with exercise. The increased insulin levels enhance glucose uptake by the muscles and decreases release of glucose from the liver. Both result in a fall in blood glucose to a greater degree than in nondiabetic persons during exercise. Type 1 diabetic patients also have defective glucagon secretion that will augment defective counter-regulatory responses to hypoglycemia. Delayed hypoglycemia may occur hours after exercise as the resting muscle takes up glucose from the blood to replenish glycogen stores. Because insulin is metabolized by the kidneys, dehydration can result in decreased renal insulin clearance and prolong insulin action. Therefore monitoring glucose levels, nutrition and hydration, and adjusting the dose of insulin are also important after exercise.

Another factor contributing to hypoglycemia in the exercising patient with type 1 diabetes is increased exogenous insulin mobilization from subcutaneous tissue because of increased blood flow.^60,85 Inadvertent intramuscular injection would exaggerate this phenomenon. It is important for patients with type 1 diabetes to administer their dose of subcutaneous insulin before exercise in a location away from exercising muscle. They should avoid injections into the arms and legs, instead using the abdomen or back of the neck. Insulin absorption is fastest and most consistent when it is injected into the abdomen. Absorption is generally slower from the arms and slowest from the thighs or buttocks, but the rate may be more inconsistent depending on the type of exercise. If the planned exercise uses primarily arm muscles, such as sport climbing, then it may be beneficial to give the insulin injection in the abdomen to avoid increased absorption from the exercising arm muscles. Likewise, if the planned activity is hiking or skiing, then it would be beneficial to use the arm or abdomen to avoid increased absorption from the exercising leg muscles. Absorption of subcutaneous insulin during exercise may also depend on the type of insulin used. A study of the long-acting insulin analog glargine, injected subcutaneously into the thigh on the evening before an intense 30-minute exercise session in patients with type 1 diabetes, did not show an increased rate of absorption, but plasma glucose fell during exercise.¹³⁰

Another measure to prevent exercise-associated hypoglycemia is to reduce the dose of insulin that will be in effect during exercise.¹⁰⁹ The best strategy for a patient with type 1 diabetes is to monitor blood glucose before, during, and after exercise to predict changes, and adjust insulin doses accordingly. This means that before a wilderness trip, the type 1 diabetic patient should exercise daily at a level of physical activity similar to that anticipated on the wilderness trip, so that adjustments in insulin dosing can be made. Differences in nutritional intake, in addition to increased exercise, during the wilderness trip should also be anticipated when planning insulin regimens. Patients with type 1 diabetes may engage in strenuous physical activity without experiencing problems, but it is important for the patient to focus on the timing of exercise in relation to meals and insulin dosing.^86,159 It is also important to anticipate the nature of the exercise that will be undertaken. Long endurance activities have different implications for type 1 diabetes management (more risk for hypoglycemia) than do short bursts of high-intensity exercise (more risk for hyperglycemia).⁷⁰

Another consideration is whether regular insulin or a rapidly acting insulin analog (lispro or aspart) is used for prandial dosing. Rapidly acting insulin alters the timing of exercise-related hypoglycemia. Patients who exercise early in the postprandial period (1 to 3 hours after a meal) require a decrease in the dose of rapidly acting insulin, whereas those who exercise later (3 to 5 hours) require a smaller or no change.⁷⁹ A predictable exercise schedule on a wilderness trip would be useful for the patient with type 1 diabetes, although this may be difficult depending on the type of trip and the environment.

Other members of the group need to be aware of signs and symptoms of hypoglycemia in the diabetic patient and how to render appropriate treatment. This is especially important for patients with type 1 diabetes, but also is significant for patients with type 2 diabetes who are on oral hypoglycemic drugs or insulin, because this group of diabetic patients also experiences a decrease in serum glucose during exercise.^131,173 Preventive measures include ingesting extra food in the form of 15 to 30 g (0.5 to 1 oz) of quickly absorbed carbohydrate (e.g., glucose tablets, whole milk, hard candies, or juice), which should be taken 15 to 30 minutes before exercise and approximately every 30 minutes during exercise.^69,70 Patients are also at risk for late hypoglycemia (i.e., 4 to 8 hours after the termination of exercise) because of replenishment of depleted glycogen stores. This can usually be avoided by ingesting slowly absorbed carbohydrates (dried fruit, granola bars, or trail mix) immediately after exercise.¹⁵² Fluid intake should also be increased to ensure adequate hydration, because dehydration prolongs insulin action by decreasing insulin clearance from the blood.

The acute symptoms of hypoglycemia are mediated by the adrenergic autonomic nervous system and are manifested by shakiness, tremors, palpitations, cold sweats, hunger, confusion, and nervousness/anxiety. If these are ignored, not sensed (owing to hypoglycemic unawareness), or cannot be treated immediately, the person may experience neuroglycopenic symptoms because of inadequate amounts of glucose reaching the brain. Neuroglycopenic symptoms consist of headache, loss of concentration, irritability, and, if allowed to proceed long enough, even seizures and loss of consciousness. Hypothermia is a common sign of hypoglycemia. All persons with diabetes should be questioned about their history of hypoglycemic reactions (frequency and severity) as well as their ability to sense the reactions (history of hypoglycemic unawareness). Travel into the wilderness should be discouraged in persons with a history of severe or frequent reactions or a history of hypoglycemic unawareness. Companions should be taught to recognize the signs and be knowledgeable in the treatment of hypoglycemia.

If the person with diabetes is alert and cooperative, he or she may be treated with 15 g (0.5 oz) of a rapidly absorbed carbohydrate (e.g., glucose tablets, juice, or hard candies). The blood glucose should be checked in 15 minutes to ensure that the glucose level has increased to a safe level (>100 mg/dL) before continuing with the physical activity. The person should be closely watched for evidence of recurrent symptoms. If the person becomes uncooperative or loses consciousness, a glucagon injection kit should be available. Glucagon releases glucose stored in the liver and helps raise the glucose level to the point at which the person becomes alert and treatment may be continued with an oral, rapid-acting carbohydrate. The dose of glucagon is 1 mg (1 ampule) given either subcutaneously or intramuscularly. Another route for delivering glucose is to deposit glucose formulations, such as table sugar, under the tongue.

Activity limitations for diabetic patients also depend on associated complications. Patients with proliferative retinopathy should avoid activities that can rapidly or explosively elevate intraocular pressure because vitreous hemorrhage may occur. Bleeding from damaged retinal capillaries may also occur during a Valsalva maneuver, while lifting heavy objects, or during a collision. An expert should do an examination for early signs of retinopathy at least once a year.^41,169

Patients with peripheral neuropathy are at risk for pressure ulceration to the feet and should avoid traumatic weight-bearing exercise (long-distance running or prolonged downhill skiing). Well-fitting protective footgear helps prevent this complication. Diabetic patients with peripheral neuropathy are also more susceptible to cold injury of the feet in a cold environment, and patients, as well as group members, need to monitor their feet meticulously. In a cold-weather environment, it is especially important to have well-insulated warm footgear with dry socks. Diabetic patients should check all surfaces of their feet at least once a day. A mirror may be required to visualize the plantar surface. Any sign of inflammation, including blistering or abrasion, should be promptly addressed. The importance of well broken in footwear and the need to wear clean and dry socks at all times should be emphasized. Diabetic patients should be advised to never walk barefoot.

Autonomic neuropathy also poses problems with increased activity in a wilderness environment. Cardiac responsiveness may be delayed during rapidly changing exercise levels. Gastroparesis may make absorption of carbohydrates unpredictable and could result in a mismatch in timing between insulin and glucose peak.

Certain drugs that may be used at high altitude also need special consideration in diabetics. Acetazolamide is commonly used for prevention of acute mountain sickness through a mechanism of inhibition of renal carbonic anhydrase, causing bicarbonate diuresis and non–anion gap metabolic acidosis that result in compensatory hyperventilation. Acetazolamide could worsen metabolic acidosis and promote volume depletion in diabetic patients. Dexamethasone is commonly used to treat severe acute mountain sickness or high-altitude cerebral edema and, as a glucocorticoid, increases insulin resistance and predisposes to hyperglycemia. Lastly, metformin is commonly used to treat type 2 diabetes. It is contraindicated in clinical conditions that may cause hypoxemia. This may be a consideration in travel to high altitude.

Ensuring that insulin does not freeze and glucose testing equipment works properly are also important for the insulin-dependent diabetic patient in the wilderness. Strategies to ensure that insulin does not freeze include carrying the medication inside a jacket next to the body or storing the insulin in an insulated container and putting the container in the sleeping bag at night. Glucose testing equipment should be reliable, rapid, and used frequently to help identify impending hypoglycemia and increase carbohydrate ingestion before incapacitation occurs. The accuracy of glucose testing equipment can be affected by high altitude and cold. Both overestimation and underestimation of glycemia and of standard glucose control solutions have been reported for all types of glucose meters.¹⁹ Glucose meters using the oxygen-insensitive enzyme glucose dehydrogenase may perform better at high altitude than those using the enzyme glucose oxidase, but both types perform poorly at low temperatures.¹²³ High glucose levels seem to be misreported to a greater extent at altitude than are low to normal glucose levels.¹⁹ The use of multiple meters with control glucose solutions can lend some confidence. Carrying glucose monitoring equipment next to the skin may prevent the problems associated with battery malfunction at cold temperatures.

Provided that the insulin-treated patient with diabetes has reviewed all the factors associated with a wilderness trip that may be complicated by diabetes and has chosen to undertake the preparation required and accept the risks, a treatment plan needs to be generated. The most challenging wilderness trips for maintaining glucose control in diabetic patients are those where the level of exercise is consistently increased above usual. Intensive diabetes management can be achieved with multiple daily injections of insulin or an insulin pump and numerous checks of blood glucose throughout the day (Tables 34-4 and 34-5). A standard strategy to offset the increased efficiency of insulin due to exercise is to ingest 15 to 30 g (0.5 to 1 oz) of carbohydrate for every half hour of moderate aerobic exercise. Monitoring blood glucose during periods of exercise of several hours’ duration is important.^70,173 Adjusting the dose of insulin downward may be required if carbohydrate supplementation does not prevent hypoglycemia. It may be necessary to decrease the dose of insulin by 30% or more.^41,109 Blood glucose should be checked more often early during a wilderness trip to assess how to balance insulin, carbohydrate intake, and exercise. It is also critical to prevent delayed hypoglycemia associated with exercise that often occurs after muscle and liver glycogen stores have been depleted and not replenished. The use of intermediate-acting insulin may need to be shifted from the afternoon or early evening to bedtime, or a long-acting insulin preparation that does not peak may be used. Adequate hydration is essential to ensure that the duration of action of insulin is not increased.

TABLE 34-4 Matching Insulin Treatment Schedules With Exercise Schedules

TABLE 34-5 Types, Names, and Uses of Insulin

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