Aortic pressure and heart rate patterns during the respiratory cycle in different autonomic conditions in conscious dog

The relation between respiratory cycle and aortic pressure and heart rate patterns has been studied in the spontaneously breathing conscious dog in four experimental conditions: presence or absence of both vagal and sympathetic activity, presence of activity of only one autonomic branch. Chronic treatment with guanethidine was used to block adrenergic system, while acute, reversible blockade of activity in the cervical vagus was obtained by cooling the nerve. Vagal block was performed before and after guanethidine. In absence of vagal activity, aortic pressure decreased during inspiration and increased during expiration. This pattern was reversed by vagal activity, while it was enhanced by sympathetic activity. The integrative effects of the two branches are responsible for the classic "pulsus paradoxus". The comparison of the four experimental conditions is compatible with the hypothesis that variations of left ventricular preload play the major role in the regulation of ventricular output during respiratory cycle, while the relation between left ventricular output and aortic pressure is largely dependent on autonomic activity