In addition to the classical carotid and aortic baroreceptors, experiments in animals have demonstrated the existence of baroreceptors in the coronary arteries. Coronary baroreceptors have an effect on systemic vascular resistance (SVR) similar in magnitude to carotid baroreceptors (Drinkhill et al. 2001, Wright et al. 2000; Bennetts et al. 2002). Whilst the importance of carotid baroreceptors in blood pressure control is widely recognised in man the contribution of the coronary baroreceptors is unknown. The aim of this study was to determine whether evidence of the existence of coronary baroreceptor reflex could be obtained in man during cardiac surgery. All procedures were carried out with the approval of local ethical committee.

A human model was designed using cardiopulmonary bypass and modified use of a single pass blood cardioplegia device following aortic cross-clamping and ventricular fibrillation during open heart surgery for mitral valve replacement. Excluded from the study were those patients with coronary artery disease, peripheral vascular disease, diabetes and aortic valve incompetence. The selected patients all had normal coronary anatomy as determined angiographically. Anaesthesia involved temazopam premedication, alfentanil, etomidate and pancuronium induction and propofol and isoflurane maintenance. The proximal ascending aorta and coronary arteries were perfused through the cardioplegia device. The systemic circulation perfused at constant blood flow from the heart-lung machine. Blood temperature was 32°C for the first six patients and 37°C for the next four. Changes in mean systemic blood pressure provided a measure of SVR. Coronary sinus blood samples were measured for troponin T, lactate and oxygen saturation to identify any myocardial ischaemia. Patients were randomly selected to have their coronary pressure held high (80 mmHg) for 90 seconds then reduced to low pressure (50 mmHg) for a further 90 seconds (H-L) or receive the reverse order of coronary pressure change (L-H). SVR during 30 second segments of coronary perfusion were calculated and compared with the SVR immediately before the change in coronary pressure.

Ten patients were initially recruited to the study, three were subsequently excluded due to technical problems or blood pressure instability during the experimental procedure. Increasing the coronary sinus pressure from 50 mmHg to 80 mmHg in 3 patients resulted in a decrease in vascular resistance in each patient (-10.1% mean, range -5.9 to -18.7). In four patients coronary pressure was decreased from 80 mmHg to 50mmHg. SVR increased in three patients (4.8% mean, range 1.4 to 7.9) but decreased in one -15.8 %). The values of troponin T, lactate and oxygen saturation were not different during the two states of coronary pressure. These preliminary results would suggest that coronary baroreceptors exist in man and result in reflex vascular responses similar to those previously reported in animal models.