Airway Pharmacology

Chapter 6 Airway Pharmacology





I. Introduction


II. Airway Pharmacology in Normal Airway Physiology


















III. Drugs with a Direct Effect on the Airway







IV. Drugs with an Indirect Effect on the Airway









V. Airway Pharmacology for the Treatment of Asthma






















VI. Conclusions


VII. Clinical Pearls



I Introduction


Airway pharmacology is concerned with the effect of drug action on airway function, with relevance to anesthesia and critical care. We define a drug as a medicinal substance prescribed and administered with therapeutic intent. This definition excludes unprescribed substances consumed for social or recreational purposes. Drugs should be used rationally, with knowledge of effect (efficacy, the therapeutic intent) and side effect (toxicity). A measure of the safety margin between efficacy and toxicity is the therapeutic ratio, and the expected benefit should reasonably outweigh the risk. Drug selection should be a rational choice based on three principles:



In common with all forms of medical treatment, drug therapy carries risk, which should be considered and managed. Drug therapy must be viewed in context. Airway management involves the application of knowledge, skills, and attitudes. Drug administration is only one part of this process, used as part of an overall therapeutic plan. Use of drugs must be viewed within the context of this plan and individualized for a particular patient with a particular set of challenges.


This chapter offers a clinical perspective on airway pharmacology. It is divided into two parts. The first part reviews the actions of common anesthetic drugs on normal airway physiology. The second part reviews the use of drugs in managing the most common airway disorder, asthma, and its analog, intraoperative bronchospasm.


The effects of drugs on the upper airway (nares to glottis) and on the lower airway (glottis to terminal bronchioles) are integral to the practice of safe and effective airway management. Drug administration is often deliberate, either as a treatment for airway pathology or as part of an airway management plan for anesthesia. On occasion, however, there are unwanted effects that may contribute to airway compromise.



II Airway Pharmacology in Normal Airway Physiology



A Clinical Issues


The relevance of airway pharmacology can be considered in relation to a number of clinically important issues.









B Physiology of the Upper Airway


Physiology of the upper airway is considered in detail in Chapter 5. In the awake patient, airway patency is maintained by muscle tone in the head and neck, particularly the pharynx and tongue. The upper airway muscles are influenced by afferent information from a number of sources (Fig. 6-1), including receptors in the upper airway and lung that respond to changes in airway pressure and a variety of chemical and mechanical stimuli.



A number of nonpharmacologic factors influence the patency of the upper airway. These include the following.








C Effects of Drugs on the Airway




2 Indirect Effect


Drugs that are administered intravenously (IV) as part of general anesthesia often affect the airway systemically.




III Drugs with a Direct Effect on the Airway



A Local Anesthetics


Local anesthetics produce a rapid, reversible depression of nerve conduction, particularly with regard to sensory nerves, by binding to sodium channels and interfering with their function, thereby preventing propagation of action potentials. Chemically, they usually consist of a lipophilic group connected to an ionizable group, typically a tertiary amine, by either an ester or an amide link (Fig. 6-2).



Local anesthetics are usually effective topically on airway mucosa. The clinical effect is to reduce airway reactivity. At the same time, there is a reduction in airway caliber and a reduction in the reflexes that protect against aspiration.


Topicalization of the upper airway is a common technique in the “awake” management of a difficult airway (DA), and it is also used during general anesthesia to facilitate upper airway instrumentation when NMB agents are being deliberately avoided. Topical local anesthesia is also used as an adjunct to general anesthesia in both upper airway and lower airway surgery.


Most local anesthetics are weak bases; they are formulated as chloride salts by combining them with acids to maximize their solubility. In plasma, they remain largely in the ionized or protonated form (BH+) before converting to the non-ionized (lipophilic) form to penetrate the nerve. The proportion of the drug in non-ionized form depends on its acid dissociation constant (pKa): the lower the pKa (i.e., the closer to pH 7.4), the more “free base” (B) available to enter the nerve cell, and the more rapid the action. This relationship is defined by the Henderson-Hasselbalch equation:



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1 Complications of Airway Local Anesthesia


In infected tissues, topical local anesthesia is often ineffective because of the lower pH of the tissues; this results in a relative increase in the ionized form of the drug, which will not enter the nerve. Alternative techniques may be required.


A reduction in upper airway patency has been demonstrated after topicalization of the airway.1,2 This occurs because of the loss of afferent feedback and the resultant decrease in motor tone, which may lead to upper airway obstruction, particularly in an airway that is already compromised. Hence, care is needed when considering an awake airway management technique in a patient with evidence of upper airway narrowing.


Systemic toxicity is extremely rare in patients receiving local anesthesia for instrumentation or examination of the upper or lower airway. Most reports suggest that plasma lidocaine levels very rarely approach the reported toxic plasma concentration of 5 to 6 µg/mL. However, one study demonstrated near-toxic levels after gargling of large volumes of lidocaine.3 Toxicity tends to involve the CNS with irritability or convulsions and the cardiovascular system with circulatory compromise. The safe maximum dose for airway administration for the most commonly used agent, lidocaine, is uncertain, but in one study in which patients received 9 mg/kg lidocaine, none had plasma concentrations in excess of 5 mg/mL.4 However, the death of a healthy volunteer due to presumed lidocaine toxicity after topicalization for fiberoptic bronchoscopy has been reported in the literature, so some consideration of the dose administered is advisable.5 The British Thoracic Society recommendation for the use of topical lidocaine with a flexible fiberoptic bronchoscope in adults is not to exceed 8.2 mg/kg.6


Topical lidocaine, particularly in higher doses, is an irritant to the upper airway and may stimulate unwanted reflexes. Other side effects include cardiotoxicity, allergy, and abuse potential.






IV Drugs with an Indirect Effect on the Airway



A Sedatives and Upper Airway Patency


The effect of general anesthetic agents on the upper airway is crucial to the practice of safe airway management, largely in terms of airway patency but also in terms of airway reactivity and airway protection against aspiration. There is a decrease in muscle tone associated with the loss of wakefulness, and this is compounded by specific drug-induced inhibition of upper airway neural and muscle activity and suppression of protective arousal responses. These processes tend to narrow the airway lumen, leading to airway obstruction. The mechanism of the reduction in muscle electrical activity relates, in part, to depressed hypoglossal nerve activity that occurs in excess of a similar depressant effect on other respiratory muscles, such as the diaphragm or intercostal muscles, which are more resistant to this dose-dependent loss of muscle tone.12 This reduction in hypoglossal nerve activity results in reduced tone in the muscles of the tongue and soft palate.


The differential suppression of respiratory muscles seems to vary among sedative agents, being less pronounced, for example, with ketamine than with other agents.13 Ketamine also has an indirect effect on the lower airways, causing bronchodilation by inducing the release of endogenous catecholamines. However, the loss of muscle tone caused by sedatives may not only relate to a specific effect of motor nerves. The depressed state of arousal results in a reduction of inputs from the reticular activating system to these motor nerves. This theory is borne out by changes in upper airway patency demonstrated during natural sleep, which show similarities to those induced by sedative drugs.


Originally, the upper airway narrowing associated with anesthesia was attributed to a posterior shift of the base of the tongue. Evidence now suggests that the soft palate and, to a lesser extent, the epiglottis are more relevant (Fig. 6-3). One study using lateral radiography demonstrated that occlusion of the airway under general anesthesia occurred most consistently at the level of the soft palate and sometimes at the level of the epiglottis, but in no patient did the tongue base come into contact with the posterior pharyngeal wall.14



Despite the assumption that loss of upper airway patency relates to a reduction in tone of the surrounding muscles, a consistent relationship between electromyographic activity and upper airway resistance has not been demonstrated. Thiopental causes measurable changes in muscular activity, but the changes are inconsistent and do not correlate with the onset of airway obstruction.15 Similarly, benzodiazepines increase upper airway resistance but again do not appear to have a consistent effect on genioglossus activity.16 Therefore, airway obstruction under general anesthesia may not relate to a simple diminution of muscular activity but to a disruption of the normal coordination of muscle activity that provides overall control of upper airway function.


The phase of the respiratory cycle is also relevant to the patency of the upper airway during spontaneous ventilation under general anesthesia. In a study by Nandi and colleagues, the movement of the soft palate during expiration was similar to that during apnea. However, during attempted inspiration, there was a significant reduction in the airway lumen at all levels, with the tongue now reaching the posterior pharyngeal wall in some cases and thus contributing further to the airway obstruction.14 This has relevance when one considers upper airway patency during anesthesia with or without muscle paralysis. The dynamic collapse of the upper airway that occurs during spontaneous inspiration anesthesia may be insignificant when pharmacologic paralysis is achieved. Similarly, the application of continuous positive airway pressure (CPAP) may also reduce this effect and help to maintain gas flow.


In summary, the increased collapsibility associated with anesthesia and sedation seems to be universal, regardless of whether the route of administration is inhalational or parenteral. Although this collapsibility lessens with decreasing anesthetic depth, the two do not necessarily have a linear relationship.17,18 The extent of the upper airway narrowing may vary subtly among agents, but the only clinically relevant difference appears to be the described lesser effect with ketamine.


At a laryngeal level, the effect of general anesthesia is probably different. One study demonstrated that the width of the laryngeal vestibule increased after administration of thiopental and succinylcholine.19






E Opioid Analgesics and the Airway


When fentanyl is combined with propofol, the depressant effect on upper airway reflexes is considerably more profound than with propofol alone, with the cough reflex being the most suppressed reflex and laryngospasm being the most resistant.21 The central mechanisms for this depression of reflexes are unclear and are probably complex.


Muscle rigidity associated with opioid use has been described25; although rare, it may be the direct cause of a significant increase in lower airway resistance that can make ventilation extremely difficult. Others suggest that this rare phenomenon in fact has more to do with vocal cord closure than with decreased pulmonary compliance secondary to chest wall rigidity.26 If the increase in airway resistance is truly linked to muscle rigidity, then its clinical manifestation is unpredictable and relates to many factors, including the opioid drug, its dose, its speed of administration, the effects of concomitant sedatives and muscle relaxants, and the age of the patient.


Short-acting narcotics are used at times in place of NMB agents, in combination with sedatives, to facilitate endotracheal intubation. However, upper airway conditions for airway instrumentation may be inferior, and the risks of arterial hypotension and laryngeal trauma may be higher.27


Remifentanil merits specific discussion because it is a relatively new drug in anesthesia and its use in facilitating safe and effective airway management is increasing. It is a potent, ultrashort-acting synthetic opioid that is administered typically by continuous infusion because it is immediately metabolized in the plasma. It causes profound respiratory depression and therefore is used clinically to successfully prevent spontaneous respiratory effort when assisted ventilation is the chosen mode of gas exchange. It also reduces muscle tone considerably and can be used to facilitate endotracheal intubation in the absence of NMB drugs.28 Use of remifentanil to facilitate smooth extubation is well described, and for some anesthesiologists,29 this technique has replaced deep general anesthesia extubation. Continuing remifentanil while discontinuing sedation results in a predominant effect of suppression of upper airway reflexes, in excess of any sedative effect. This allows extubation with good endotracheal tube (ETT) tolerance and a minimum of coughing. However, care is required when using the drug because of its potency. Apnea is likely and can even occur in a patient who is receiving remifentanil alone for conscious sedation.30 As with other synthetic opioids, muscle rigidity is described but not common.31


The main effect of opioids on the lower airway is similar to that described earlier, namely reduced airway reactivity and attenuated reflex bronchoconstriction. Mechanisms of action are again a combination of a neural effect and a direct dose-dependent effect in relaxing airway smooth muscle. For example, there is evidence that morphine attenuates vagally mediated bronchoconstriction in asthmatics.32 Opioid administration has been shown to cause histamine release, and there is also evidence that they cause an increase in tracheal smooth muscle tone, but neither effect has been associated with clinically demonstrable bronchoconstriction. A condition termed wooden chest syndrome has been described after administration of IV opioids, particularly if given at high dose; in this syndrome, thoracic and abdominal muscle rigidity apparently results in difficulty with assisted ventilation. The clinical significance of this finding is uncertain, but it has not restricted the use of these drugs worldwide.



F Neuromuscular Blockade and the Airway


NMB agents have several properties that may affect airway function. They are used in clinical practice to facilitate endotracheal intubation by abolishing airway reflexes and to facilitate mechanical ventilation by eliminating spontaneous respiratory effort. Similar to the effects of sedative agents on the respiratory muscles, these drugs also demonstrate a differential effect, with the diaphragm and the adductor muscles of the larynx being more resistant to paralysis than some of the muscles affecting upper airway patency.33 This relative sparing of the diaphragm permits the maintenance of respiratory effort even during complete paralysis of peripheral muscles. However, it also means that upper airway patency may be compromised at levels of paralysis that otherwise permit maintenance of normal spontaneous ventilation. Clinically, this is most relevant at extubation. A patient with residual NMB drug may maintain adequate ventilation with an artificial airway, such as an ETT or a supralaryngeal airway (SLA), in situ. After removal of the device, airway obstruction leading to hypoxemia may develop.


Removal of protective airway reflexes can have negative consequences if there is a risk of aspiration. Similarly, elimination of spontaneous respiratory effort may be deleterious if assisted ventilation cannot be delivered. Hence, care and experience are required for the safe use of NMB agents, and in certain clinical scenarios, it may be prudent to withhold them. However, the major clinical debate regarding their use seems to relate to their effect on upper airway patency. There is evidence that paralysis of the upper airway musculature improves upper airway patency. A recent study published data indicating that NMB using rocuronium facilitated bag-mask ventilation in anesthetized patients (Fig. 6-4).34



Another study on the physiologic comparison of spontaneous ventilation and positive-pressure ventilation (PPV) in laryngotracheal stenosis concluded that spontaneous ventilation creates a negative inspiratory extrathoracic intratracheal pressure that causes indrawing of the mobile tracheal segments, further narrowing the airway.35 During expiration, there is positive intratracheal pressure, and a similar reduction in airflow does not occur. During PPV with muscle paralysis, however, a positive intraluminal pressure is created during inspiration that improves ventilation. Whether the conclusions of this paper can be extrapolated to the management of the upper airway under general anesthesia per se is uncertain, but several leading airway experts feel strongly that they should. The argument states that muscle relaxation makes upper airway management easier (and therefore safer) in terms of ventilation through a facemask or SLA and in terms of airway instrumentation, including intubation.36,37


Historically, there has been a fear that the prolonged effect of muscle paralysis during failing airway management under anesthesia contributes to severe hypoxic damage, but there does not appear to be any convincing evidence to support this supposition. The fear appears to relate to the loss of reversibility of the anesthetic technique, although it can never be certain whether the awakening of a patient during failing airway management is feasible before catastrophic desaturation occurs. The evidence suggests that, when faced with impossible bag-mask ventilation, very few anesthesiologists choose to discontinue the general anesthesia or avoid NMB.38 Most choose to provide oxygenation by intubating the trachea or by ventilating through an SLA device or performing an emergency cricothyrotomy. If ventilation is possible but intubation has been unsuccessful, “wake up” is strongly advocated by discontinuing any sedative drugs while reestablishing muscular tone and respiratory drive and maintaining oxygenation.


Impaired neuromuscular transmission secondary to partial pharmacologic blockade, even to a degree insufficient to evoke respiratory symptoms, markedly impairs upper airway dimensions and function. This may be explained by an impairment of the balance between upper airway dilating forces and the negative intraluminal pressure generated during inspiration. The same effect does not appear to occur during expiration.39 Furthermore, reflex glottic activity may be preserved, with a tendency toward coughing and laryngospasm, and it may also have a negative effect on gas flow. Complete muscular paralysis necessitating assisted PPV appears to have the reverse effect and increases upper airway patency.


The lower airway behaves differently under muscle paralysis, with a potential loss of airway patency when NMB agents are administered. This may relate to the loss of the subatmospheric intrapleural pressure that normally supports the intrathoracic bronchial tree. Therefore, whereas muscle paralysis may benefit upper airway patency, it may hinder lower airway patency. This can be clinically relevant during management of the scenarios of acute upper and lower airway obstruction.


Histamine release, caused by many of the muscle relaxants, tends to increase lower airway resistance, although clinically this is rarely significant. Some agents, such as atracurium and suxamethonium, are more likely to cause histamine release, although a relationship between these drugs and bronchoconstriction is not clear. Severe bronchoconstriction secondary to anaphylactoid or anaphylactic reactions is described for most muscle relaxants.


Monitoring of the degree of NMB during anesthesia involving muscle relaxants is indicated, but there are limitations with the available techniques. The degree of blockade is frequently monitored by the response of the adductor pollicis to stimulation of the ulnar nerve, although this muscle is more sensitive than the diaphragm to paralysis.33 At present, monitoring methods that would reliably demonstrate the return of power required to maintain and protect the airway with an effective cough are lacking, and anesthesiologists tend to use a combination of monitoring and clinical assessment.


Reversal of nondepolarizing NMB before extubation and discontinuation of assisted ventilation is standard anesthetic practice. Use of a cholinesterase inhibitor, such as neostigmine, to flood the neuromuscular junction with acetylcholine and thereby restore neuromuscular function has traditionally been the only technique available for reversing the paralysis. These drugs could theoretically cause bronchospasm, but they appear to have no significant effect on airway resistance when combined with an anticholinergic drug such as atropine or glycopyrrolate. More importantly, inadequate reversal of the block, which is probably much more common than most anesthesiologists appreciate, can result in respiratory compromise, morbidity, and mortality.40 A meta-analysis of current reports of NMB in anesthesia revealed a 41.3% incidence of residual paralysis.41


The recent introduction of sugammadex has provided an option for immediate and complete reversal of profound nondepolarizing NMB—when aminosteroid agents such as rocuronium or vecuronium have been used for muscle relaxation—to promote a rapid return of full muscle power and spontaneous respiratory effort. Sugammadex is a geometric donut with a lipophilic core and a hydrophilic periphery; it works in the plasma rather than the neuromuscular junction. The drug encapsulates rocuronium, binding it to its lipophilic core.




V Airway Pharmacology for the Treatment of Asthma


Asthma (from the Greek, asthma, “panting”), as defined by the Global Initiative for Asthma (GINA),44 is a “chronic inflammatory disorder of the airways in which many cellular elements play a role. The chronic inflammation is associated with airway hyperresponsiveness that leads to recurrent episodes of wheezing, breathlessness and coughing, particularly at night or in the early morning. These episodes are usually associated with widespread, but variable, airflow obstruction within the lung that is often reversible either spontaneously or with treatment.”



A Overview


The complex, recursive interplay of inflammation, hyperresponsiveness, and airflow limitation determines the disease course, which over time produces structural airway changes, termed remodeling. There is presently greater recognition of the essentially inflammatory nature to asthma, with treatment directed at reducing this component over the long term as well as providing treatment to alleviate symptoms.45 An idea of the interplay of these factors is shown in Figure 6-5.




1 Diagnosis


Asthma is a clinical diagnosis, based on the symptoms and signs. Symptoms are variable, with one or more of the following being present: wheeze, breathlessness, cough, and chest tightness.46,47 None of these symptoms is specific to asthma. For example, wheeze is a function of any airway narrowing as a consequence of reduction of FRC. Signs of asthma include clinical verification by auscultation, spirometry, and lung function testing, which may include gas transfer measurements.48


There are no standard definitions for types of symptoms, nor for frequency or severity.47



2 Prevalence


Worldwide, asthma is estimated to affect more than 300 million people, 7% to 10% of the global population.49 About 15 million daily adjusted life-years (DALYs) are lost each year, which places asthma 20th in the current disability ranking, similar to schizophrenia and diabetes. There are about 225,000 asthma deaths each year, amounting to 1 in 250 deaths from all causes.50 In the United States, 7.3% of the population is diagnosed with asthma, and asthma accounted for 17 million physician consultations in 2006.51 The worldwide prevalence of asthma is predicted to increase by another 100 million by 2025.46



3 Distribution


There are marked differences in national and geographic distribution. Although methodologic factors may confound accurate measurement, there is widespread agreement that asthma is a disease of industrialized nations.46 Indoor aeroallergens, especially house dust mite52; tobacco smoking53; and obesity are some implicated factors.54,55


Genetic factors may contribute. Inheritance of an “asthmatic genotype” is complex with variable transmission.56 Atopy, the development of immunoglobulin E (IgE) antibodies to specific allergens, is the strongest identifiable risk for developing asthma.46,57,58 A family history of asthma, particularly on the maternal side, is a strong risk factor.59 Other inheritable traits are the forced expiratory volume in 1 second (FEV1) and response to drug therapies.56


Additionally, there are gender differences, with a higher incidence of asthma in prepubertal boys compared to girls.60 In middle age, however, there is a higher incidence among women.



4 Pathology


The pathology of asthma is incompletely understood and is an area of active research.61,62





c Inflammation


One crucial aspect of asthma is considered to be the relative dominance of pro-inflammatory over anti-inflammatory factors driven by a variety of cell types in complex, dynamic, and variable ways. CD4+ lymphocytes, particularly the T helper type 2 (Th2) subtype, are presently thought to have a key role, with a predominance of this population over the T helper 1 (Th1) group. Relative overactivity of the Th2 group, especially at crucial developmental (childhood) stages, with expression of inflammatory cytokines such as interleukin-4 (IL-4), IL-5, IL-9, and IL-13, is considered to play a pivotal role.70


The inflammatory cytokines recruit and activate mast cells and eosinophils, leading to cellular infiltration, the density of which is often linked to disease severity.71 Some interleukins (IL-4 and IL-13) promote B-lymphocyte production of IgE in response to inhalation of allergens (aeroallergens). IgE binds to mast cell surface receptors, conferring immunologic memory and triggering release of mast cell contents (degranulation) when exposed to the specific aeroallergen.


A variety of substances such as histamine, proteases (including mast cell tryptase, a time-sensitive marker of degranulation), and cysteinyl-leukotrienes are released.72 At least 100 mediators have been described.73 Other stimuli, such as mechanical changes around these cells, may account for mast cell degranulation in response to cold-induced and exercise-induced asthma.74


Other cells, such as natural killer cells,75 Th17 cells,76 regulatory T cells,77 dendritic cells,78,79 macrophages,80 mast cells,81 and neutrophils, may be involved.82 Neutrophil activity is associated with severe disease and sudden deterioration.83



d Remodeling


Poorly controlled asthma may be complicated by anatomic changes in airway structure, which are often correlated to worsening of the disease84 but may start before diagnosis.85 A variety of cells are implicated in causing smooth muscle hypertrophy and hyperplasia, subepithelial fibrosis, angiogenesis, and mucus gland hyperplasia—collectively called the epithelial-mesenchymal trophic unit.


Recent attention has focused on the role of pulmonary blood flow in asthmatics, which is higher at rest compared with control subjects and increases during episodes of asthma enough to interfere with airflow.86

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Apr 12, 2017 | Posted by in ANESTHESIA | Comments Off on Airway Pharmacology

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