Effects of acute hypoxia on the volume of blood in the thorax

Two different methods were used to study the effect of acute hypoxia on the volume of blood in the thorax. The first method entailed estimating the central blood volume by the Stewart-Hamilton dye-dilution technic; the second involved the use of a teeter-board. The 2 methods gave comparable results. Both indicated that the volume of blood in the thorax is not appreciably altered by hypoxia.

Effect of mild hypoxia on ventilation during exercise

Hypoxia of mild degree (PaOO2 above 60 mm Hg) produces little or no ventilatory response in resting man during the steady state. To evaluate the possibility that the effectiveness of a hypoxic chemoreceptor drive might be enhanced by exercise, the ventilatory response to mild hypoxia was measured in two human subjects during rest and exercise. Though no significant increase in ventilation occurred at rest above a PaOO2 of 60 mm Hg, a decrease in PaOO2from 100 to 94 mm Hg produced a statistically significant increase in steady state ventilation during moderate exercise. In addition, temporary block of the sympathetic innervation to the carotid and aortic bodies in one subject resulted in a diminution of work hyperpnea. This suggests that increased sympathetic tone during exercise, by reducing blood flow through the chemoreceptors, might result in increased neural discharge and hence increased ventilation even though arterial POO2 is the same as at rest. Thus, activity of the chemoreceptors as modified by sympathetic control of their blood supply may be an important determinant of the ventilatory response to exercise. Since work hyperpnea is enhanced by even mild hypoxia, this ventilatory response may be sufficient to initiate respiratory acclimatization to altitudes so low that resting ventilation on acute exposure is unaffected.

Submitted on July 31, 1961

Adrenal cortical response to chronic hypoxia

Although acute oxygen lack causes increased adrenal cortical activity, there is evidence that continued exposure to hypoxia is accompanied by a return of adrenal cortical function to its sea level status. To evaluate the adrenal cortical response in men living for 14-21 days above 21,000 ft, urinary output of 17-hydroxycorticoids was measured in ten members of a Himalayan mountaineering expedition and compared to values obtained subsequently at sea level. No significant difference in 17-hydroxycorticoid output was observed between the two altitudes. The response of four subjects to the administration of Su-4885 (Methopyrapone) showed that the pituitary-adrenal cortical system was still capable of response to additional stimulation.

Submitted on July 31, 1961

Insufficiency of O2 supply to pregnant uterus

Pregnant goats under anesthesia were studied while subjected to various degrees of hypoxemia. Lactate and pyruvate concentrations were determined in maternal arterial and uterine venous bloods and in umbilical arterial and venous bloods. The same highly heterogeneous values were found as reported for nonhypoxemic animals, and no clear systematic relationships could be discerned. However, when the values were viewed as being determined by exchanges of excess lactate on each side of the placenta, several patterns of metabolic activity were discernible. 1) No net anaerobic metabolism occurred in the pregnant uterus in the well-oxygenated mother. 2) When maternal blood oxygen was extremely low, anaerobic metabolism occurred in the fetus and in the uterus as a whole. 3) However, in the whole range of mild and moderate hypoxemia, the pattern consisted of production of excess lactate by the fetus and its reoxidation by the placenta; consequently, there was no net anaerobic metabolism by the pregnant uterus as a whole. Such fetuses, even when umbilical blood oxygen was virtually absent, survived and showed no signs of deleterious effect.

Effect of short periods of arterial occlusion on blood flow and oxygen uptake

In 17 experiments, performed on the forearm of normal subjects, the effect of 2½, 5, and 10 min of arterial occlusion was studied. Blood flow was obtained with the venous occlusion plethysmograph, and oxygen uptake was calculated using the Fick principle. Arterial occlusion resulted in the production of an oxygen debt which was subsequently repaid. With progressively longer periods of anoxia there was a proportionate increase in the magnitude of the debt. Similar conclusions could not be drawn from blood flow studies alone, since the vascular change represented only one means of repayment of the oxygen debt during reactive hyperemia, the other being a greater extraction of oxygen from each unit of blood early in the postocclusion period. The constant overswing on either side of the control base line, observed in the records of oxygen uptake, suggested the absence of delicately balanced and efficient checks on the mechanisms responsible for repayment of the oxygen debt incurred in the period of tissue anoxia.

Submitted on March 27, 1961

Conditions affecting rate of development of anoxic block in C fibers

The isolated splenic nerve of sheep conducted impulses for an average of 10 minutes in the complete absence of oxygen at 37.5°C. If, immediately following conduction failure, the nerve was allowed to recuperate for 2 hours in oxygen it became able to conduct impulses as long during a repeated period of anoxia as it had initially. If the initial period of anoxia was prolonged beyond 28 minutes the nerve sustained irreversible damage and its ability to conduct impulses during subsequent periods of anoxia was shortened proportionally. For recuperation periods shorter than 2 hours the conduction block during the course of the second period of anoxia appeared earlier. Temperatures below 37.5°C or CO2 concentrations above 10% prolonged the time to conduction block during anoxia; temperatures above 37.5°C or frequencies of stimulation greater than 1/sec. shortened the time to conduction block during anoxia.

Transient effect of sudden mild hypoxia on respiration

Recent studies of carotid body chemoreceptor activity in cats show that the activity of these organs increases markedly over a range of alveolar pO2 considerably higher than the alveolar pO2 of 50–60 mm Hg known to produce an increase in ventilation in the steady state. In an attempt to explain this discrepancy between chemoreceptor activity and ventilation, the transient ventilatory response to two breaths of a low O2 mixture was observed and correlated with the alveolar pO2 during this brief hypoxic stimulus. A transient increase in ventilation could be detected at an alveolar pO2 of 93 mm Hg, that is, considerably higher than the highest pO2 known to increase ventilation during the steady state. These findings are interpreted as evidence of an increased chemoreceptor drive when alveolar O2 tension is lowered only slightly below that existing at sea level. Possible reasons for the difference between the steady state and transient responses are discussed.

Submitted on June 6, 1960

Effect of sleep on CO2 stimulation of breathing in acute and chronic hypoxia

The effect of sleep on the respiratory response to graded increases in CO2 was studied in four adult male, sea-level residents during hypoxia at sea level, during 23 days at 14,250 ft. and after return to sea level. One subject was also studied at sea level breathing CO2 mixtures with alternating high and low O2. Hypoxia had no apparent effect on the change in respiratory response to CO2 associated with sleep under any of the conditions studied, but it shifted the curve relating respiratory minute volume to alveolar CO2 tension about 1 mm Hg to the left awake and during sleep. It was concluded that sleep and hypoxia act independently rather than interacting in their effect on the respiratory response to CO2.

Submitted on June 6, 1960

Effect of sleep on hypoxic stimulation of breathing at sea level and altitude

The effect of sleep on the level of hypoxia in four adult male, sea-level residents was studied at sea level, during 23 days at 14,250 ft. and after return to sea level. The subjects, awake and asleep, breathed graded O2-N2 mixtures, each followed by a high O2 gas. Sleep did not significantly affect the hypoxic drive of breathing as evaluated from the steady-state levels of ventilation or from the immediate respiratory depression produced by interrupting hypoxia. At the ambient altitude O2 tension, arterial O2 saturation indicated by ear oximeter fell 4–8% with sleep. This fall in arterial oxygenation is believed to be sufficient to help explain the increased severity of mountain sickness which is commonly associated with sleep at altitude.

Submitted on June 6, 1960

Effects of Acute Hypoxia and Exercise on the Pulmonary Circulation

The effects of acute hypoxia, of graded exercise, or both, on the pulmonary circulation were studied in 17 normal subjects. In 3 of these subjects, and in 3 patients with restricted vascular beds, the effects of these stimuli were compared during successive test periods.

In 7 normal subjects, moderate exercise, associated with an increase in cardiac output of approximately 25 per cent, resulted in an average rise in pulmonary arterial mean pressure of 3 mm.Hg. More strenuous exercise in the same subjects, associated with a further increase in cardiac output of approximately 25 per cent, did not elicit further increment in pulmonary arterial mean pressure.

In contrast, 10 subjects with normal pulmonary circulations, who responded to acute hypoxia with an average reduction of arterial oxygen saturation of 17 per cent, manifested an average increase in cardiac output of 5 per cent and an average rise in mean pulmonary arterial pressure of 4 mm.Hg. The critical level of arterial blood oxygen saturation for a significant rise in pulmonary arterial pressure was 85 per cent: in 6 subjects, with arterial blood O 2 saturation reduced below this level, the average pulmonary artery pressure rise was 7 mm. Hg.

In the 3 subjects with normal pulmonary circulations who performed exercise and were exposed to acute hypoxia, the contrasting effects of these stimuli on the pulmonary circulation were striking. In the 3 patients with restricted vascular beds exposed successively to both stimuli, the rise in pulmonary arterial mean pressure appeared to be linearly related to the increase in pulmonary blood flow.

In 1 subject, the surgical resection of the preganglionic fibers and ganglia which supply sympathetic efferent nerves to the pulmonary vascular tree was without effect on the responses of the pulmonary circulation to acute hypoxia and to exercise.

The present study failed to identify the mechanism and the site of action of acute hypoxia on the pulmonary circulation.