The temperature within the circle of Willis versus tympanic temperature in resting normothermic humans

A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 2: physiological measurements

In this, the second of four historical reviews on human thermoregulation during exercise, we examine the research techniques developed by our forebears. We emphasise calorimetry and thermometry, and measurements of vasomotor and sudomotor function. Since its first human use (1899), direct calorimetry has provided the foundation for modern respirometric methods for quantifying metabolic rate, and remains the most precise index of whole-body heat exchange and storage. Its alternative, biophysical modelling, relies upon many, often dubious assumptions. Thermometry, used for >300 y to assess deep-body temperatures, provides only an instantaneous snapshot of the thermal status of tissues in contact with any thermometer. Seemingly unbeknownst to some, thermal time delays at some surrogate sites preclude valid measurements during non-steady state conditions. To assess cutaneous blood flow, immersion plethysmography was introduced (1875), followed by strain-gauge plethysmography (1949) and then laser-Doppler velocimetry (1964). Those techniques allow only local flow measurements, which may not reflect whole-body blood flows. Sudomotor function has been estimated from body-mass losses since the 1600s, but using mass losses to assess evaporation rates requires precise measures of non-evaporated sweat, which are rarely obtained. Hygrometric methods provide data for local sweat rates, but not local evaporation rates, and most local sweat rates cannot be extrapolated to reflect whole-body sweating. The objective of these methodological overviews and critiques is to provide a deeper understanding of how modern measurement techniques were developed, their underlying assumptions, and the strengths and weaknesses of the measurements used for humans exercising and working in thermally challenging conditions.

The parietal foramen anatomy: studies using dry skulls, cadaver and in vivo MRI

OBJECTIVE

The aim of this study was to describe the anatomical features encountered in the parietal foramen in a series of 178 human bones and 123 head MRI examinations. A cadaveric specimen was also dissected to demonstrate the trajectory of a superficial scalp vein through the parietal foramen as far as the dura mater. A literature review was performed regarding prevalence of parietal foramen in different populations.

METHODS

Totally, 178 paired adult bones were used to investigate the presence, shape and number of the parietal foramina. In addition, 123 brain MRI examinations were also studied.

RESULTS

The parietal foramina were encountered in 75/89 (84.3%) skulls [32/38 (84.2%) in women vs. 43/51 (84.3%) in men, p > 0.05]. The parietal foramen was present bilaterally in 44.73% of females and 54.9% of males. Regarding unilaterality of the parietal foramen, a right or left laterality was observed in female 21% right versus 18% left; and 16% versus 14% (left) in males (p > 0.05). The accessory parietal foramen was present in the right parietal in 2.6% and in 7.9% on the left side of the females, while 5.9% and 3.9% of the males on the right or left sides, respectively. The parietal foramina located in the proximity of the sagittal suture (male 7.1 ± 2.5 mm vs. female, 7.4 ± 2.7 mm). There was a positive correlation between the right and left parietal foramina regarding the distance from the median line. The distance from a foramen to the contralateral one was 16 ± 4 mm in men and 18 ± 5 mm in women, respectively (p > 0.05).

CONCLUSION

No major differences were encountered between sexes regarding the anatomical features of parietal foramen.

Panting as a human heat loss thermoeffector

The human autonomic nervous system participates in the control of thermoregulatory responses that are employed to regulate core temperature following deviations of skin temperature and/or core temperature from their respective resting values. This permits a regulation of the core temperature (T_C) at 37.0 ± 1°C with superimposed circadian variations in both sexes and menstrual cycle-associated variations in premenopausal women. When rendered hyperthermic, passively by heat exposure while at rest or actively during exercise, humans engage heat loss or thermolytic responses, including eccrine sweating and cutaneous vasodilatation. A third, less studied, human thermolytic response is thermal panting, and this response is the focus of this review. Human thermal panting was first described over a century ago. It has since been shown to be a reproducible response showing some similar patterns of breathing in species that employ panting as their sole thermolytic heat loss response. The contribution of human panting as a thermolytic response, however, remains controversial. This review highlights both past and recent evidence supporting that hyperthermic humans have a panting pattern of breathing that plays an important role in human thermoregulation.

Mechanisms of action, physiological effects, and complications of hypothermia

BACKGROUND

Mild to moderate hypothermia (32-35 degrees C) is the first treatment with proven efficacy for postischemic neurological injury. In recent years important insights have been gained into the mechanisms underlying hypothermia's protective effects; in addition, physiological and pathophysiological changes associated with cooling have become better understood.

OBJECTIVE

To discuss hypothermia's mechanisms of action, to review (patho)physiological changes associated with cooling, and to discuss potential side effects.

DESIGN

Review article.

INTERVENTIONS

None.

MAIN RESULTS

A myriad of destructive processes unfold in injured tissue following ischemia-reperfusion. These include excitotoxicty, neuroinflammation, apoptosis, free radical production, seizure activity, blood-brain barrier disruption, blood vessel leakage, cerebral thermopooling, and numerous others. The severity of this destructive cascade determines whether injured cells will survive or die. Hypothermia can inhibit or mitigate all of these mechanisms, while stimulating protective systems such as early gene activation. Hypothermia is also effective in mitigating intracranial hypertension and reducing brain edema. Side effects include immunosuppression with increased infection risk, cold diuresis and hypovolemia, electrolyte disorders, insulin resistance, impaired drug clearance, and mild coagulopathy. Targeted interventions are required to effectively manage these side effects. Hypothermia does not decrease myocardial contractility or induce hypotension if hypovolemia is corrected, and preliminary evidence suggests that it can be safely used in patients with cardiac shock. Cardiac output will decrease due to hypothermia-induced bradycardia, but given that metabolic rate also decreases the balance between supply and demand, is usually maintained or improved. In contrast to deep hypothermia (<or=30 degrees C), moderate hypothermia does not induce arrhythmias; indeed, the evidence suggests that arrhythmias can be prevented and/or more easily treated under hypothermic conditions.

CONCLUSIONS

Therapeutic hypothermia is a highly promising treatment, but the potential side effects need to be properly managed particularly if prolonged treatment periods are required. Understanding the underlying mechanisms, awareness of physiological changes associated with cooling, and prevention of potential side effects are all key factors for its effective clinical usage.

Components and mechanisms of thermal hyperpnea

The pattern of breathing during a hyperthermia-induced hyperventilation varies across different species. Thermal tachypnea is a first phase panting response adopted during hyperthermia when tidal volume is minimized and the frequency of breathing is maximized. Blood-gas tensions and pH are maintained during this hyperventilation, and the associated heat loss helps the animal regulate its body temperature. A second pattern of breathing adopted in hyperthermia is thermal hyperpnea; this response is the focus of this review. This form of hyperventilation is evident after an increase in core temperature and it is apparent in humans. Increases of tidal volume as well as frequency of breathing are evident during this response that results in a respiratory alkalosis. The cause of thermal hyperpnea is not resolved; evidence of the potential mechanisms underlying this response support that modulators of the response act in either a multiplicative or additive manner with body temperatures. The details of the designs and methodologies of the studies supporting or refuting these two views are discussed. A physiological rationale for thermal hyperpnea is presented in which it is suggested this response serves a heat-loss role and contributes to selective brain cooling in hyperthermic humans. Ongoing research in this area is focused on resolving the mechanisms underlying thermal hyperpnea and its contribution to cranial thermoregulation. The direct application of this research is for the care of febrile and hyperthermic patients.

Mechanisms for the control of respiratory evaporative heat loss in panting animals

Panting is a controlled increase in respiratory frequency accompanied by a decrease in tidal volume, the purpose of which is to increase ventilation of the upper respiratory tract, preserve alveolar ventilation, and thereby elevate evaporative heat loss. The increased energy cost of panting is offset by reducing the metabolism of nonrespiratory muscles. The panting mechanism tends to be important in smaller mammalian species and in larger species is supplemented by sweating. At elevated respiratory frequencies and body temperatures alveolar hyperventilation begins to develop but is accompanied by a decline in the control of carbon dioxide partial pressure in arterial blood, probably through central chemoreceptors. Most heat exchange takes place at the nasal epithelial lining, and venous drainage can be directed to a special network of arteries at the base of the brain whereby countercurrent heat transfer can occur, which results in selective brain cooling. Such a phenomenon has also been suggested in nonpanting species, including humans, and although originally thought to be a mechanism for protecting the thermally vulnerable brain is now considered to be one of the thermoregulatory reflexes whereby respiratory evaporation can be closely controlled in the interests of thermal homeostasis.

Direct cooling of the human brain by heat loss from the upper respiratory tract

This study is the first report on human intracranial temperature in conscious patients during and after an upper respiratory bypass. Temperatures were measured in four subjects subdurally between the frontal lobes and cribriform plate (T(cr)) and on the vault of the skull (T(sd)). Further measurements were taken in the esophagus (T(es)) and on the tympanic membrane. Reinstitution of airflow in the upper respiratory tract under conditions of mild hyperthermia gave a rapid drop in T(cr) of 0.4-0.8 degrees C. In three patients the intracranial temperature at the basal aspect of the frontal lobes fell below T(es). Thus local selective cooling of the brain surface below that of the trunk temperature was shown to occur. Intensive breathing by the patients after extubation for a 3-min period produced a cooling at the site of T(cr) measurement at a rate of up to 0.1 degrees C/min, and this response could be evoked on demand. The results support the view that cooling of the upper airway can directly influence human brain temperature.

Control of brain temperature during experimental global ischemia in rats

Temperature control during experimental ischemia continues to be of major interest. However, if exposure of brain tissue is necessary during the experiment, regional heat loss may occur even when the core temperature is maintained. Furthermore, valid non-invasive brain temperature monitoring is difficult in small rodents. This paper describes a method for both monitoring and maintenance of brain temperature during small animal preparations in a stereotaxic frame. The device used includes an ear-bar thermocouple probe and a small near-infrared radiator. The new equipment permitted to maintain peri-ischemic brain temperature at a desired level while carrying out non-invasive continuous recordings of cerebral blood flow (laser Doppler-flowmetry) and of electrical brain function (EEG). In contrast, without extracranial heat application, superficial and basal brain temperatures decreased during global cerebral ischemia by 4.1 +/- 0.1 and 4.6 +/- 0.4 degrees C (mean +/- SEM), respectively, returning to baseline values at 15-30 min of reperfusion while rectal (core) temperature remained stable at baseline values. The ear-bar thermocouple probe (tympanic membrane) reliably reflected basal brain temperature, and temperature in superficial brain areas correlated well with that in the temporal muscle. Our data show that the new system allows to exclude unwanted hypothermic neuroprotection, and does not interfere with optical and electrical measurement techniques.

Oesophageal, rectal, axillary, tympanic and pulmonary artery temperatures during cardiac surgery

PURPOSE

The gradient between temperatures measured at different body sites is not constant; one factor which will change this gradient is rapid changes in body temperature. Measurement of this gradient was done in patients undergoing rapid changes in body temperature to establish the best site to measure temperature and to compare two brands of commercial tympanic thermometers.

METHOD

A total of 228 sets of temperatures were measured from probes in the oesophagus, rectum, and axilla and from two brands of tympanic thermometer and compared with pulmonary artery (PA) temperature in 18 adults during cardiac surgery.

RESULTS

Measurements from the oesophageal site was closest to PA readings (mean difference 0.0 +/- 0.5 degree C) compared with IVAC tympanic thermometer (mean difference -0.3 +/- 0.5 degree C), Genius tympanic thermometer (mean difference -0.4 +/- 0.5 degree C), axillary (mean difference 0.2 +/- 1.0 degrees C) and rectal (mean difference -0.4 +/- 1.0 degree C) readings. When data during cooling were analysed separately, all sites had similar gradients from PA except for rectal, which was larger. On rewarming, oesophageal readings were closest to PA readings; tympanic readings were closer to PA than were rectal or axillary readings. Readings from the two brands of tympanic thermometer were equivalent.

CONCLUSION

Oesophageal temperature is more accurate and will reflect rapid changes in body temperature better than tympanic, axillary, or rectal temperature. When oesophageal temperature cannot be measured, tympanic temperature done by a trained operator should become the reading of choice.