| Abstract: | Superior mesenteric artery (SMA) blood flow and impedance were evaluated byduplex ultrasound during head-up tilt (HUT)-induced central hypovolaemia and hypotension ineight healthy volunteers. HUT induced a reduction in cardiac stroke volume from88·8±6·3 to 64·7±6·3 ml(mean±SEM; P<0·01) and an increase in thoracic electricimpedance from 38·6±2·1 to 42·6±2·1Ω (P<0·01) reflecting a reduced central blood volume. Maintainedtilt provoked a 30% reduction in mean arterial pressure (from 87·1±3·3to 63·4±3·6 mmHg; P<0·01) and the appearanceof presyncopal symptoms. During both the normotensive and the hypotensive phase of HUT, theSMA diameter (5·7±0·03 mm) and blood flow (514±75 ml min?1) did not change significantly, although the end-diastolic velocity increasedfrom 9·7±4·8 to 39·7±4·0 cm s?1 (P<0·01). The increase in diastolic velocity, despite amaintained or reduced arterial pressure, supports a reduction in the SMA impedance as it wasreproduced during a meal test when a moderate reduction in mean arterial pressure (87±4to 80±4 mmHg; P=0·04) was accompanied by a ninefoldincrease in the end-diastolic velocity (P<0·01). The results indicate areduction in the mesenteric vascular impedance to the extent that superior mesenteric artery bloodflow is maintained during HUT-induced central hypovolaemia and hypotension. |
