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.