Dissection of carotid sinus hypersensitivity: the timing of vagal and vasodepressor effects and the effect of body position.

We assessed the timing of vagal and sympathetic factors that mediate hypotension during CSM (carotid sinus massage) in patients with carotid sinus hypersensitivity. We hypothesized that a fall in cardiac output would precede vasodepression, and that vasodepression would be exaggerated by head-up tilt. We performed pulse contour analyses on blood pressure recordings during CSM in syncope patients during supine rest and head-up tilt. In a subset we simultaneously recorded muscle sympathetic nerve activity supine. During supine rest, systolic blood pressure decreased from 150±7 to 107±7 mmHg (P<0.001) and heart rate from 64±2 to 39±3 beats/min (P<0.01). Cardiac output decreased with heart rate to nadir (66±6% of baseline), 3.1±0.4 s after onset of bradycardia. In contrast, total peripheral resistance reached nadir (77±3% of baseline) after 11±1 s. During head-up-tilt, systolic blood pressure fell from 149±10 to 90±11 mmHg and heart rate decreased from 73±4 to 60±7 beats/min. Compared with supine rest, cardiac output nadir was lower (60±8 compared with 83±4%, P<0.05), whereas total peripheral resistance nadir was similar (81±6 compared with 80±3%). The time to nadir from the onset of bradycardia did not differ from supine rest. At the onset of bradycardia there was an immediate withdrawal of muscle-sympathetic nerve activity while total peripheral resistance decay occurred much later (6-8 s). The haemodynamic changes following CSM have a distinct temporal pattern that is characterized by an initial fall in cardiac output (driven by heart rate), followed by a later fall in total peripheral resistance, even though sympathetic withdrawal is immediate. This pattern is independent of body position.