The cerebrovascular response to lower body negative pressure versus head-up tilt

Lower body negative pressure (LBNP) has been proposed as a MRI-compatible surrogate for orthostatic stress. Although the effects of LBNP on cerebral hemodynamic behavior have been considered to reflect those of orthostatic stress, a direct comparison with actual orthostasis is lacking. We assessed the effects of LBNP (-50 mmHg) versus head-up tilt (HUT; at 70°) in 10 healthy subjects (5 female) on transcranial Doppler determined cerebral blood flow velocity (CBFv) in the middle cerebral artery and cerebral perfusion pressure (CPP) as estimated from the blood pressure signal (finger plethysmography). CPP was maintained during LBNP but decreased after 2 min in response to HUT leading to a ~15% difference in CPP between LBNP and HUT (p≤0.020). Mean CBFv initially decreased similarly in response to LBNP and for HUT but from minute 3 on, the decline became ~50% smaller (p≤0.029) during LBNP. The reduction in end-tidal PCO₂ was comparable but with an earlier return towards baseline values in response to LBNP but not during HUT (p=0.008). We consider the larger decrease in CBFv during HUT versus LBNP attributable to the pronounced reduction in PetCO₂ and to gravitational influences on CPP and this should be taken into account when applying LBNP as an MRI-compatible orthostatic stress modality.

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