Acute Hypovolemia Does Not Affect Dynamic Cerebral Autoregulation in Humans

Abstract

While it is well known that cerebral perfusion pressure is the primary determinant of cerebral blood flow, it has been suggested that cardiac output and stroke volume may also play an important role. To test the role of stroke volume and cardiac output on dynamic cerebral autoregulation we performed sit to stand tests on a group of volunteers prior to and following administration of furosemide to reduce blood volume. These subjects were participating in a larger artificial gravity study. As expected furosemide resulted in a significant reduction in stroke volume while seated (63±5 vs. 51±4 mL, P=0.000). In contrast cardiac output was unchanged due to increases in heart rate (72±3 vs 94±3 bpm, P=0.000). Upon standing subjects demonstrated similar drops in SV, cardiac output and increases in HR from pre to post infusion. Blood pressure was unchanged by infusion while sitting and had similar drops upon standing (−20±4 vs −21±12 mmHg). Similarly cerebral flow velocity decreases were unchanged (−15±7 vs −17±8 mmHg). Examination of the Autoregulatory Index found no change from pre to post infusion (4.7±0.6 vs 4.8±0). These data demonstrate that hypovolemia and reduced stroke volume do not affect dynamic cerebral autoregulation or the cerebrovascular response to standing. Further work is needed to determine the role of cardiac output. Supported by NASA and VA.