Mesenteric artery response to head‐up tilt‐induced centralhypovolaemia and hypotension

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.