title: Hypoxia and hyperoxia in man: immediate effects on ventilation and heart rate
Creators: name: Jennett, Sheila
Publisher: name: University of Glasgow
Year: 1969

description: Three problems related to ventilatory and cardiovascular control in resting man have been studied; emphasis was placed on the timing of response to changes in concentration of inspired gases, and thus on the assessment of the part played by arterial chemo-reflexes. Observations were made on the immediate effects, by continuous recording of ventilatory parameters, and in many instances of heart rate also. The response of ventilation to increase in inspired CO2 concentration during hyperoxia. This was studied in 9 normal subjects. Ventilation was recorded during a series of 2-breath tests in which 7% CO2 was substituted briefly for the background of air, or of oxygen respectively. In a few subjects only, there was evidence of an arterial chemoreflex response in hyperoxia. 2. The changes in ventilation and in heart rate during the first few minutes of breathing 100% oxygen. Ventilatory responses were studied in 11 normal subjects, and heart rate also in 3. Initial transient depression of ventilation was confirmed in many instances: more particularly timing of the secondary stimulation was studied; and it was found to develop most often in the second minute. Bradycardia, when it occurred developed more gradually from the first half minute. 3. The tachycardia induced by hypoxia, and its relationship to ventilatory changes. This was the major problem studied, and the largest section of the thesis is devoted to it. The mechanism of the tachycardia of hypoxia in man is not known: earlier evidence has only partly supported the most likely explanation from animal studies, namely that central hypoxic stimulation leads to increased sympathetic activity and thence to neural pacemaker stimulation. Subjects were exposed to moderate brief hypoxia (10% oxygen for 3 - 10 min); this was abruptly relieved by 100% oxygen. The changes at reoxygenation were observed particularly closely. The following were studied: i) 6 normal men, repeatedly, in three different series of experiments. ii) 8 patients with complete transection of the cervical spinal cord. iii) A patient following bilateral removal of carotid body tumours. In all these subjects, heart rate increased during hypoxia, and decreased to normal abruptly after a delay of 10-18 sec from the beginning of the first breath of oxygen. The changes were not linked to changes in ventilation; his was observed on many individual occasions, and was conclusively established by a series of experiments in which normal subjects were trained to maintain their breathing constant during the period of the change from 10% to 100% oxygen: the heart rate nevertheless decreased and after a similarly short delay. The normality of the heart rate changes in the patients with cervical cord transection indicates that medullary stimulation and increased sympathetic efferent activity is not the mechanism, (or not the only mechanism), responsible for hypoxic tachycardia in man. The timing of the response is consistent with an arterial chemoreflex: the normal response in the patient with absent carotid body function leaves open the possibility of the aortic bodies as the receptors. This and other possibilities, such as a link with pulmonary vascular changes, are discussed, and further human and animal studies suggested, to elucidate the cause of the tachycardia.