Monitoring Temperature

Care should be taken when using the tympanic thermometer, because poor technique can render the measurement inaccurate. To ensure that the temperature measurements are accurate, the tympanic thermometer probe should be positioned to fit snugly in the ear canal. This prevents ambient air at the opening of the ear canal from entering it, resulting in a false low temperature measurement (Bickley and Szilagyi 2009). Ear canal size, wax, operator technique and the patient’s position can affect the accuracy of the measurements (Knies 2003).

Chemical Dot Thermometers

Chemical dot thermometers are flexible polystyrene strips with a temperature sensor at one end, designed for single oral/axilla use. These thermometers are unsuitable in patients with hypothermia because their temperature range is restricted to 35.5–40.4°C (Mains et al. 2008).

Best Practice – Tympanic Temperature Monitoring

Use the same ear for consecutive measurements

Install a new disposable probe cover for each measurement

Ensure that thermometer probe is positioned snugly in the external auditory meatus

Aim the thermometer towards the tympanic membrane

Measure the patient’s temperature following the manufacturer’s instructions

Consider the temperature reading alongside other systemic observations and overall condition of the patient

Store the thermometer following manufacturer’s instructions

Jevon and Jevon (2001)

If the oral route is used, the strip should be placed in the sublingual pocket of tissue at the base of the tongue, which is close to the thermoreceptors, which respond rapidly to changes in core temperature (Mains et al. 2008). It is important to ensure that the strip is placed in the right or left sublingual pocket (Mains et al. 2008) and not in the area under the front of the tongue because there may be a difference in temperature of up to 1.7°C between the two areas (Dougherty and Lister 2011).

Oral temperature measurements can be affected by the temperature of ingested foods and fluids and by the muscular activity of chewing (Dougherty and Lister 2011). In addition a respiratory rate of >18 breaths/min will reduce core temperature values (Hussein 2009).

The axilla is an alternative route for temperature monitoring if the oral route is unsuitable, e.g. in a convulsive patient. However, it can be difficult to obtain an accurate and reliable measurement because the site is not close to a major blood vessel and the surface temperature of the skin can be affected by the environment (Mains et al. 2008). If the axilla is used, the strip should be placed in the centre of the armpit with the patient’s arm positioned firmly against the side of the chest. As the temperature can vary between arms, the same site should be used for serial measurements.

Regardless of what site is used for temperature measurements, the same one should be consistently used, because switching between different sites can produce measurements that are both misleading and difficult to interpret (Dougherty and Lister 2011).

Oesophageal/Nasopharyngeal Probes

The oesophageal probe should be accurately positioned in the lower quarter of the oesophagus. The nasopharyngeal temperature can be affected by air leaking around the tracheal tube. Both probes can be interfaced with the patient monitoring system, thus offering an accurate and continuous reading.

Bladder Probe

Bladder and pulmonary artery temperature correlate well and, as most critically ill patients require a urinary catheter, this method of measuring body temperature avoids additional invasive equipment. A thermocouple attached to the distal end of the catheter can interface with the patient monitoring system and offer continuous temperature measurements. This method of temperature measurement is considered to be very reliable (Lefrant et al. 2003).

Pulmonary Artery Catheter

Although the pulmonary artery catheter is the gold standard for temperature measurement (Sturgess and Morgan 2009), it is highly invasive and its sole use for temperature measurements cannot be justified. However, if one is inserted, the thermistor sited in the distal end can interface with the patient’s monitoring system and provide continuous temperature measurements.

PHYSIOLOGICAL EFFECTS OF HYPOTHERMIA

Hypothermias defined as body temperature <35°C and may be mild (34–35°C), moderate (30–34°C, severe (<30°C) or profund (<20°C) (Farley and McLafferty 2008) can occur when the body loses too much heat or cannot maintain its normothermic state. There are several risk factors (Table 12.1). Table 12.2 shows the various signs and symptoms of hypothermia at different levels of temperature.

Table 12.1 Predisposing factors for hypothermia

Reproduced by kind permission of Butterworth Heinemann Elsevier from Hussein (2009).

Age	Extremes of age
Environmental	Exposure to cold Immersion Poor living conditions
Drugs	Anaesthetic agents Phenothiazines Barbiturates Alcohol
Central nervous system disorders	Cerebrovascular accident Trauma Spinal cord transections Brain tumours Wernicke’s encephalopathy Alzheimer’s and Parkinson’s diseases Mental illness
Endocrine dysfunction	Hypoglycaemia Diabetic ketoacidosis Hyperosmolar coma Panhypopituitarism Hypoadrenalism Hypothyroidism
Trauma	Major trauma
Debility	Severe cardiac, renal or hepatic impairment Malnutrition, sepsis
Skin disorders	Burns Exfoliative dermatitis

Table 12.2 Signs and symptoms of hypothermia at different levels of temperature

Reproduced by kind permission of Cambridge University Press from Skinner et al. (1997).

Temperature (°C )	Signs and symptoms
37.6	‘Normal’ rectal temperature
37	‘Normal’ oral temperature
36	Increased metabolic rate to attempt to balance heat loss
35	Shivering maximum at this temperature; hyper-reflexia, dysarthria, delayed cerebration
34	Patients usually responsive and with normal blood pressure; lower limit compatible with continued exercise
33–31	Retrograde amnesia, consciousness clouded, blood pressure difficult to obtain, pupils dilated, most shivering ceases
30–28	Progressive loss of consciousness, increased muscular rigidity, slow pulse and respiration, cardiac arrhythmias develop if heart irritated
27	Voluntary motion lost along with pupillary light reflex, deep tendon and skin reflexes; appears dead
26	Victims seldom conscious
25	Ventricular fibrillation may appear spontaneously
24–21	Pulmonary oedema develops (100% mortality rate in shipwreck victims in World War II)
20	Heart standstill
18	Lowest adult accidental hypothermic patient with recovery
17	Isoelectric EEG
15.2	Lowest infant accidental hypothermic patient with recovery
9	Lowest artificially cooled hypothermic patient with recovery
4	Monkeys revived successfully
1–7	Rats and hamsters revived successfully

Sometimes hypothermia is intentionally induced, e.g. during some cardiac surgery or after a cardiac arrest, either by heat exchange through a heart/lung machine or by surface cooling using ice. Induced hypothermia after a cardiac arrest has been shown to decrease (Morley 2009):

cerebral oxygen consumption
free radical formation
excitatory amino acids
neuron-specific enolase
cerebral lactate
cerebral oedema
intracranial pressure
cell destructive enzymes
neutrophil migration to ischaemic tissue
ongoing inflammatory response.

When monitoring a patient with hypothermia it is important to understand the physiological effects that it has on the various systems in the body. The main effects of hypothermia on the bodily systems are detailed below.

Cardiovascular System

Initially there is sympathetic stimulation, which increases heart rate, blood pressure and cardiac output. However, with increasing hypothermia, there is progressive cardiovascular depression leading to a reduction in tissue perfusion and oxygenation (Hussein 2009), and cardiac arrhythmias, e.g. bradycardia, atrial fibrillation and ventricular fibrillation, can become a problem (Resuscitation Council UK 2010). Rough movement and activity should be avoided because this can precipitate a cardiopulmonary arrest. Sometimes the patient’s pulse may be difficult to detect.

Respiratory System

Following an initial reflex stimulation of respiration, there is a progressive decrease in respiratory rate, tidal volume and minute volume (Hussein 2009) leading to hypoxaemia and hypoxia. Sometimes the patient’s respirations may be difficult to detect (Resuscitation Council UK 2010).

Neurological System

Cerebral blood flow reduces at a rate of 7% for each 1°C drop in temperature (Hudak et al. 1998) resulting in confusion, decreased reflexes, cranial nerve deficits and lack of voluntary motion. Increased blood viscosity, decreased oxygen availability, lack of shivering and muscle rigidity also develop (Kelly et al. 2001).

Renal System

In mild hypothermia (34–35°C) sympathetic activity leads to an increase in cardiac output, resulting in a ‘cold’ diuresis. However, with progressive hypothermia, renal blood flow and glomerular filtrate fall. Sodium and water losses may be evident due to metabolic failure of the renal tubules (Morley 2009).

Gastrointestinal System

If the temperature is <34°C, gut motility decreases which can lead to vomiting and malabsorption.

Metabolic System

Metabolic acidosis occurs due to accumulation of lactate and failure to secrete hydrogen ions. Hyperkalaemia resulting from the failure of membrane sodium/potassium pumps and hypoxic liver damage may also occur (Morley 2009).

Endocrine System

Below 30°C pituitary, pancreatic functions and catecholamine secretion are diminished. Increased serum glucose occurs as a result of increased glycogenolysis and insulin resistance (Hussein 2009).

Haematology

Hypothermia increases blood viscosity by 2% and potential complications include thrombocytopenia, leukopenia and coagulopathy (Hussein 2009). In severe hypothermia platelet dysfunction and disseminated intravascular coagulation (DIC) are common (Hussein 2009).

MONITORING PRIORITIES OF A PATIENT WITH HYPOTHERMIA

The monitoring priorities of a patient with hypothermia include the following:

Regular assessment of vital signs: airway, respirations, blood pressure, pulse and core temperature
Arterial blood gas analysis
ECG monitoring to detect cardiac arrhythmias
Urine output measurements
Blood sugar measurements to detect hypoglycaemia
Neurological function observations.

Methods of re-warming should be appropriate to the degree of hypothermia. Methods include warmed fluids, warm blankets. Monitoring during re-warming is also important. A patient warmer (Fig. 12.2) is commonly used for active re-warming, although other methods are available (Farley and McLafferty 2008). Re-warming should not exceed increases of 1–2°C per h (Farley and McLafferty 2008); in cases of mild hypothermia aim for 0.3–1.2°C per h, but rapid re-warming of >3°C per h may be necessary if there are severe hypothermia and cardiovascular instability (Carson 1999).

Fig. 12.2 Patient warmer.