The Cardiovascular Effects of Coronary Sinus Distension in the Anaesthetised Dog

Abstract

Experiments were carried out to determine whether the systemic hypotension caused by distension of the coronary sinus is reflex in origin. In anaesthetized open-chest dogs, indwelling Morawitz balloon cannulae were used to distend the sinus. Eighty-three distensions in fourteen experiments with intact vagi caused systemic hypotension and tachyeardia, but there was no significant change in this response in a further twenty-two tests after the cervical vagi had been cut, or cooled (to less than 5°C). Recordings were made of sympathetic efferent nerve activity in few-fibre preparations from the ventral ansae subelaviae. Activity was reduced during the hypotension reflexly caused by intrapericardial nicotine but was increased after coronary sinus distension. The increase in activity occurred both with intact and sectioned vagi. It was concluded that the excitation of cardiac mechanoreceptors in the coronary sinus wall by distension does not produce measurable reflex depression of the circulation, and that the hypotension observed after sinus distension is mechanically caused, rather than reflex in origin as earlier studies have implied.For many years there has been histological evidence of a rich afferent innervation of the heart, and there have been numerous reports of cardiovascular reflex effects from the heart [Heymans and Neil, 1958;Sleight, 1971].Szentivanyi and Juhasz-Nagy [1960, 1962] reported that distension without obstruction of the coronary sinus of the anaesthetised dog by an indwelling Morawitz balloon cannula [Morawitz and Zahn, 1914] produced systemic arterial hypotension. This hypotension was abolished by bilateral cervical vagotomy, selective cardiac denervation, or the application of cocaine to the outside of the sinus; heart rate was not measured. The average fall in blood pressure reported was about 20 mm Hg with a range of 10-70 mm Hg from resting mean blood pressure levels of about 55-140 mm Hg. The pressure in the distending balloon was between 10-40 mm Hg and the hypotension produced persisted as long as the balloon remained inflated, with only a small, slow rise towards the previous level. Hypotension was maintained for as long as 2 hr in one experiment. This pattern is very different from that caused by maintained distension of an isolated carotid sinus with intact vagi, when the arterial blood pressure returns to a near normal value within a few minutes [Scher and Young, 1963;Gillam, 1968].Szentivanyi and Juhasz-Nagy cited three pieces of evidence to support the conclusion that the response was reflex. First, the response could not be obtained after bilateral vagotomy, but was present after atropine: second, it was abolished by applying cocaine to the 'environment' of the sinus; and third,