ALVEOLAR CAPILLARY TEMPERATURE COMPARED WITH AORTIC AND BRONCHIAL WEDGE TEMPERATURES

Suprabulbar and bulbar integration of ventilation and ear vascular control during thermoregulation in the rabbit

Suprabulbar and bulbospinal integration of cardiorespiratory responses to cold and heat stress was studied in groups of normal, thalamic and pontine rabbits. The animals sat in an airconditioned environmental chamber in which ambient temperature (TA) was maintained sequentially at 22 degrees C, 12 degrees C, 22 degrees C and 35 degrees C, with an accuracy of +/- 1 degree C. Neither thalamic nor pontine rabbits could maintain core temperature in cold or heat. At TA 35 degrees C, thalamic and pontine animals did not pant, indicating that telencephalic responses were necessary for the integration of mechanisms promoting respiratory heat loss. Thalamic animals, however, could inhibit ear vascular sympathetic tone in the heat, but the response was absent in pontine animals, suggesting diencephalic responses were essential for the integration of mechanisms promoting ear skin heat loss. Thus, the neural adjustments to thermal stress depend on mechanisms of integration distributed longitudinally throughout the central nervous system, and different components of the reflex cardiorespiratory response depend on different sites in the central nervous system for their full expression.

Central nervous system 5-hydroxytryptamine and noradrenaline specificity of ear vascular and ventilation reflexes in thermoregulating rabbits

Contrasting hypotheses of mammalian thermoregulation were tested in unanesthetized rabbits in relation to the role of the central nervous (CNS) monoamines 5-hydroxytryptamine (5-HT) and noradrenaline (NA) in regulating lung function and ear skin blood flow (Doppler flowmeter). Normal rabbits and rabbits with CNS depletion of 5-HT and NA (caused by the neurotoxins 5,7-dihydroxytryptamine and 6-hydroxydopamine) were studied in an airconditioned chamber at ambient temperatures of 12 degrees, 22 degrees and 35 degrees C. The results suggest that CNS 5-HT plays an excitatory role in the heat conservation mechanism of cold-induced ear skin vasoconstriction, and that this effect is inhibited by CNS NA to cause heat dissipation during heat stress. Both CNS 5-HT and NA appear to exert a mild inhibitory restraint on ventilation even during heat stress. The data support the theory that CNS 5-HT is concerned with heat conservation and CNS NA with heat loss mechanisms in the cutaneous circulation, and that both monoamines moderate heat loss through panting. CNS monoamine-dependent thermoregulation in the rabbit thus resembles the model postulated for the cat, dog and monkey rather than, as previously proposed, for the sheep and goat.

Blood/gas equilibrium of carbon dioxide in lungs. A critical review

(1) The scope of this review is to examine the experimental evidence for the existence of negative PCO2 differences between pulmonary capillary blood and lung gas, [delta PCO2(b-G)], which have been observed both during rebreathing, when CO2 was at equilibrium, and during steady state gas exchange, particularly in hypercapnia. (2) The mechanism that have been invoked to explain negative delta PCO2(b-G) include (i) slow equilibration of the system CO2/HCO3-/H+ in blood, and (ii) effects of a negatvely charged surface of the pulmonary capillary endothelium. While the first postulated mechanism appears to be quantitatively insufficient to explain the results, the second seems to lead to serious qualitative difficulties. (3) Existence of negative delta PCO2(b-G) in CO2 equilibrium would invalidate the basis of the conventional analysis of alveolar gas exchange. (4) A critical analysis of the experimental evidence for the existence of negative delta PCO2(b-G) is presented. It includes the identification of directional experimental errors leading to spurious negative delta PCO2(b-G), and a critical review of the literature data in this regard. (5) Results of own experiments, conducted in an attempt to consider all possible sources of error, are reported, revealing (i) perfect PCO2 equality between alveolar gas and blood in rebreathing equilibrium of CO2; (ii) absence of negative delta PCO2 (b-G) during steady state gas exchange in hypercapnia. (6)Both experiments and model calculations show that negative delta PCO2 between mixed venous blood and end-expired gas observed in birds at steady state of gas exchange are explained by a particular action of the Haldane effect in avian parabronchial lungs with cross-current arrangement of gas and blood flow. (7) It is concluded that the negative delta PCO2(b-G) reported in the literature are probably artifactual and that there is no adequate evidence to invalidate the traditional view according to which blood/gas CO2 equilibration in lungs leads to equal PCO2 in both media.

Indicator-dilution measurement of cardiac output with dissolved hydrogen

Dissolved H 2 has been used as an indicator for measuring cardiac output by the techniques of constant-rate injection and sudden, single injection indicator dilution. Because of its low solubility, H 2 is eliminated essentially quantitatively in the lungs and recirculates negligibly. The constant-rate injection technique involves the gas chromatographic measurement of pulmonary (or systemic) arterial H 2 concentration during an infusion of dissolved H 2 into a vena cava (or the left atrium). Measurements in anesthetized dogs agreed well with simultaneous measurements by dye dilution, direct Fick, and direct volumetric techniques. Measurements could be repeated several times a minute, and using a linearly responding platinum electrode, intravascular H 2 concentrations could be recorded continuously. In the sudden, single injection technique, dissolved H 2 was injected into a vena cava (or the left atrium) while monitoring pulmonary (or systemic) arterial H 2 concentration with a chromatographically calibrated platinum electrode. Measurements again agreed with simultaneous measurements by dye dilution, constant-rate injection of H 2 , and the direct volumetric technique. Hydrogen curves could be repeated rapidly and integrated instantaneously. Hydrogen appears to be a useful indicator for rapidly repeated determinations of cardiac output and for measurements of output in situations in which recirculation of conventional indicators limits their usefulness.