Cardioinhibitory-vasodepressor response to head-up tilt without hypoxaemia or myocardial ischaemia

Summary. We evaluated whether hypoxaemia and/or myocardial ischaemia are of importance for development of the bradycardic hypotensive phase (cardioinhibitory-vasodepressor syncope) of central hypovolaemia. Arterial blood gas variables and a twelve-lead electrocardiogram (ECG) were followed during head-up tilt in seven men. During tilt, before presyncopal symptoms appeared, mean arterial pressure (MAP) increased (from 67±7 to 78±6 mmHg) (meanae) as did heart rate (HR) (61 ±4 to 99±8 beats min-1) and total peripheral resistance (TPR) (ll±lto17±l mmHg min l-1) (P < 0.01), while cardiac output (5.9±0.5 to 4.6±0.6 1 min-1) and central venous pressure (CVP) (4.2±0.4 to l.3±0.7 mmHg) decreased (P < 0.01). After 40±7 min of head-up tilt presyncopal symptoms appeared together with a decrease in MAP to 48 ±7 mmHg, HR to 71±11 beats min-1 and TPR to 9±2 mmHg min l-1 (P < 0.01). Arterial oxygen tension was not changed and there was no ST-segment depression of the ECG. The results indicate that during central hypovolaemia decreases in HR and TPR are elicited during normoxaemia and without electrocardiographic signs of myocardial ischaemia.