Total Respiratory Heat Loss Research Articles

Computational fluid dynamics model of avian tracheal temperature control as a model for extant and extinct animals

Respiratory evaporative cooling is an important mechanism of temperature control in bird. A computational simulation of the breathing cycle, heat and water loss in anatomical avian trachea/air sac model has not previously been conducted. We report a first attempt to simulate a breathing cycle in a three-dimensional model of avian trachea and air sacs (domestic fowl) using transient computational fluid dynamics. The airflow in the trachea of the model is evoked by changing the volume of the air sacs based on the measured tidal volume and inspiratory/expiratory times for the domestic fowl. We compare flow parameters and heat transfer results with in vivo data and with our previously reported results for a two-dimensional model. The total respiratory heat loss corresponds to about 13–19% of the starvation metabolic rate of domestic fowl. The present study can lend insight into a possible thermoregulatory function in species with long necks and/or a very long trachea, as found in swans and birds of paradise. Assuming the structure of the sauropod dinosaur respiratory system was close to avian, the simulation of the respiratory temperature control (using convective and evaporative cooling) in the extensively experimentally studied domestic fowl may also help in making simulations of respiratory heat control in these extinct animals.

Influence of passive humidification on respiratory heat loss in tracheotomized patients

The aim of this study was to evaluate changes in total respiratory heat loss during use of a heat and moisture exchanger (HME) in tracheotomized patients. Tracheal humidity and temperature were measured before the application and during use of the HME (plastic foam impregnated with CaCl2), and total respiratory heat loss was calculated. No significant difference was found between the convective heat exchange before and after use of the HME for a 10-minute period. When the HME was placed on the tracheal opening, the evaporative heat exchange and the total respiratory heat loss decreased significantly. The results indicate that passive airway humidification is effective in tracheotomized patients even after a 10-minute period. However, the positive effect on the energy balance of the tracheal mucosa after prolonged use of the HME remains to be proven.

Influence of exercise-induced bronchoconstriction on refractoriness.

This study determined if the degree of exercise-induced refractoriness is determined by the degree of exercise-induced bronchoconstriction. In 12 patients with exercise-induced asthma (mean [SEM] age 27 [3] years) we performed 2 pairs of exercise challenges 45 min apart at different work loads on 2 days. Mean (SEM) total respiratory heat loss during low and high work loads was 3.4 (0.2) and 5.1 (0.4) kcal, respectively. After the first and second exercise challenge at low work loads, mean (SEM) SRaw increased by 107 (15) and 73 (16)% (n.s.), as compared to 361 (40) and 98 (25)% at high work loads (p less than 0.005). We found a correlation between the initial airways response and refractoriness (r = 0.58, p less than 0.005) and conclude that the degree of refractoriness after exercise-induced bronchoconstriction is in part dependent on the severity of exercise-induced bronchoconstriction.

Longitudinal distribution of canine respiratory heat and water exchanges.

We assessed the longitudinal distribution of intra-airway heat and water exchanges and their effects on airway wall temperature by directly measuring respiratory fluctuations in airstream temperature and humidity, as well as airway wall temperature, at multiple sites along the airways of endotracheally intubated dogs. By comparing these axial thermal and water profiles, we have demonstrated that increasing minute ventilation of cold or warm dry air leads to 1) further penetration of unconditioned air into the lung, 2) a shift of the principal site of total respiratory heat loss from the trachea to the bronchi, and 3) alteration of the relative contributions of conductive and evaporative heat losses to local total (conductive plus evaporative) heat loss. These changes were not accurately reflected in global measurements of respiratory heat and water exchange made at the free end of the endotracheal tube. Raising the temperature of inspired dry air from frigid to near body temperature principally altered the mechanism of airway cooling but did not influence airway mucosal temperature substantially. When local heat loss was increased from both trachea and bronchi (by increasing minute ventilation), only the tracheal mucosal temperature fell appreciably (up to 4.0 degrees C), even though the rise in heat loss from the bronchi about doubled that in the trachea. Thus it appears that the bronchi are better able to resist changes in airway wall temperature than is the trachea. These data indicate that the sites, magnitudes, and mechanisms of respiratory heat loss vary appreciably with breathing pattern and inspired gas temperature and that these changes cannot be predicted from measurements made at the mouth. In addition, they demonstrate that local heat (and presumably, water) sources that replenish mucosal heat and water lost to the airstream are important in determining the degree of local airway cooling (and presumably, drying).

Nasal heat and water exchange in gray seals.

Metabolic rate (MR), expired air temperature (Tex), respiratory frequency (f), respiratory minute volume (V), and skin (Ts) and body (Tb) temperatures were recorded in three gray seals (Halichoerus grypus) at ambient air temperatures (Ta) between -40 and +20 degrees C. At Ta within the thermoneutral zone, MR averaged 3.7 W.kg-0.75, while mean V was 0.26 1.min-1.kg-0.75. At Ta below -11 degrees C [apparent lower critical temperature (Tlc)], both MR and V increased linearly with decreasing Ta. Average maximum MR (9.6 W.kg-0.75) and V (0.57 1.min-1.kg-0.75) were both recorded at Ta of -40 degrees C. Tex decreased with decreasing Ta to an average minimum value of 8 degrees C at Ta of -30 degrees C. The highest Tex recorded was 32 degrees C at Ta of +20 degrees C. At Ta of -20 degrees C, both total respiratory heat loss, with one exception, and respiratory evaporative water loss reached their lowest values. At this Ta, 66% of the heat and 80% of the water added to the inspired air were regained on expiration. We suggest that nasal heat exchange may be of considerable importance for thermal and water balance in many pinnipeds.

Absence of refractoriness in asthmatic subjects after exercise with warm, humid inspirate.

Twelve asthmatic adults each completed two six minute treadmill runs separated by an interval of 20 minutes. Running speed was constant for each subject, and inspired air temperature averaged 5.5 degrees C (SD 1.5 degree) for both tests. Total minute ventilation and total respiratory heat loss showed no significant difference between the two runs. Forced expiratory volume in one second (FEV1) was measured before exercise and at five minute intervals throughout the recovery periods, during which subjects breathed room air at an average temperature of 17.8 degrees C (1.8 degree). Reduction in FEV1 from pre-exercise readings averaged 39.3% (13.3%) for the first run and 11.5% (7.3%) for the second. On another day the subjects underwent an identical procedure except that the first exercise period was performed with the saturated inspirate at 37.3 degrees C (1.7 degree). This run produced a mean FEV1 reduction of only 3.1% (7.3%). The ensuing run, during which the inspiratory temperature averaged 6.0 degrees C (2.0 degrees), led to a mean fall in FEV1 of 37.3% (17.3%). This was not significantly different from the value recorded for the first of the paired runs with cool air. We therefore have been unable to confirm that exercise with warm humid inspirate may induce refractoriness to exercise induced asthma. Our data are compatible with the theory that refractoriness may be due to depletion of mediators during an initial exercise induced asthma attack.

Aspects of nasal heat exchange in resting reindeer.

Total respiratory heat loss (t.r.h.l.) in resting reindeer has been calculated from exhaled air temperature (Te) and respiratory minute volume (V) obtained during heat and cold stress in both summer and winter. Exhaled air temperature decreased with decreasing ambient temperature (Ta) in both summer and winter. The lowest Te recorded was +6 degrees C at a Ta of -45 degrees C in winter and the highest +38 degrees C at a Ta of + 29 degrees C in summer. At any Ta, Te was higher in winter than in summer. V was constant with increasing Ta up to +25 degrees C and -10 degrees C in summer and winter, respectively. Further increase in Ta resulted in thermal tachypnea. During summer t.r.h.l. did not change significantly in the ambient temperature range from -25 to +25 degrees C. In winter, however, t.r.h.l. increased linearly with increasing ambient temperature from -30 to +15 degrees C. Minimum t.r.h.l. was found to coincide with the animals' lower critical temperature. We suggest that the heat exchange in the upper respiratory tract of resting reindeer is actively regulated to attain fine tuning of thermal balance in this high arctic mammal undergoing great seasonal changes in fur insulation.

Oxygen consumption, ventilation and respiratory heat loss in a parrot,Bolborhynchus lineola, in relation to ambient temperature

1. Bolborhynchus lineola, a parrot of the humid tropics, has an unusually narrow thermoneutral zone (TNZ) (28–30 °C) and a total (wet) conductance 40% greater than predicted based upon its mass. 2. Within thermal neutrality mean tidal volume was 1.22 ml/breath and respiratory frequency was 28.6 breaths/min. These values are, respectively, 2.11 and 0.68 times allometrically predicted values (Lasiewski 1972). The mean rate of oxygen consumption in the TNZ during simultaneous measurement of other respiratory parameters was 1.87 ml O2/min and mean inspiratory minute volume equaled 34.3 ml/min (1.12 times the predicted value). 3. Mean oxygen extraction efficiency in the TNZ was 27.2%, 1.25 times the predicted value for a nonpasserine bird of 55.7 g and 1.66 times the predicted value for a similarly sized mammal. Maximum extraction efficiencies were about 53% at 27 °C and the highest mean value, 40.0%, was measured at that temperature. Mean oxygen extraction efficiency at allTa's belowTlc were higher than within TNZ or above it. 4. Total respiratory heat loss accounted for 9% of total heat production atTlc and only 3% of total heat production at 5 °C. Elevated oxygen extraction efficiencies belowTlc help to reduce respiratory heat loss. 5. Normal respiratory responses of resting birds to changes in ambient conditions can best be measured in totally unrestrained animals because respiratory frequency and oxygen consumption, and therefore oxygen extraction efficiency, are very labile and any disturbance masks normal responses.

Net energetic and thermoregulatory efficiency during panting in the sheep

1. 1. The total heat production and that proportion attributable to panting, together with the total body and respiratory heat loss of shorn Merino sheep, were studied during exposure to various combinations of air temperature and humidity. 2. 2. While exposure to moderately hot environments resulted in panting with mean respiratory frequencies of up to 270 breaths min −1, the total heat production was not significantly higher than that in a thermoneutral environment and therefore the heat produced by panting appeared to be zero. 3. 3. The mean maximum heat loss through panting was approximately 60 per cent of the total heat loss and 56 per cent of the total heat production; this was due entirely to the evaporation of moisture. 4. 4. The results indicate that the sheep relies heavily on panting for heat dissipation and that in moderately hot environments the thermoregulatory efficiency of panting is high. 5. 5. It is concluded that, in the light of more recent observations, the method used here and in previous studies cannot be used to provide an estimate of the energy expenditure due to increased respiratory activity; rather, an estimate of the net change in metabolic rate is obtained.

Respiratory water and heat loss of the black duck during flight at different ambient temperatures.

Pulmonary ventilation and temperature of expired air and of the respiratory passages has been measured by telemetry during flight in the black duck (Anas rubripes) and the respiratory water and heat loss has been calculated.During flight, temperature of expired air was higher than at rest and decreased with decreasing ambient temperatures. Accordingly, respiratory water loss as well as evaporative heat loss decreased at low ambient temperatures, whereas heat loss by warming of the inspired air increased. The data indicated respiratory water loss exceeded metabolic water production except at very low ambient temperatures. In the range between −16 °C to +19 °C, the total respiratory heat loss was fairly constant and amounted to 19% of the heat production. Evidence for the independence of total heat loss and production from changes in ambient temperature during flight is discussed.