Heart Rate During Short Periods of Apnea in Curarized Dogs

¹ From the Foundation Marie Lannelongue, Paris, France, and the Department of Physiology, University of Minnesota, Minneapolis, Minnesota

Under light Pentothal anesthesia, 11 dogs were fitted with catheters in the right atrium and femoral artery. An electrocardiogram was taken in the standard leads. Arrest of ventilation was induced by intravenous injection of d-tubocurarine and mechanical breathing installed. Photokymographic records of heart rate, right atrial pressure and femoral artery pressure were taken during mechanical ventilation and at the end of a 90-second period of ‘apneic oxygenation’ which is a period of arrest of ventilation following partial denitrogenation of the animal. Arterial blood samples were collected before, during and after the apneic period for measurement of ph and samples of end expiratory air were withdrawn for measurement of pCO₂. A significant bradycardia was noticeable after 60 seconds of ‘apneic oxygenation;’ after 90 seconds it amounted to a 7% fall of the control heart rate which averaged 148/min. This bradycardia was accompanied by a significant rise in systemic pressure averaging 5.6 mm Hg, and a significant fall in central venous pressure averaging 2.1 mm Hg. During the same time end expiratory pCO₂ increased by 16 mm Hg and blood arterial ph dropped by 0.02. Similar changes were observed after bilateral cervical vagotomy, and when apnea was associated with moderate hypoxia. These results indicate that respiratory acidosis is the main factor responsible for the bradycardia of apnea observed under the present experimental conditions.