Effect of Acute Hypoxia on Cerebral Blood Velocity during Lower Body Negative Pressure

Abstract

BackgroundThe ability to maintain adequate cerebral blood flow and oxygenation determines tolerance to central hypovolemia. It is unclear whether exposure to acute hypoxia changes cerebral blood flow during central hypovolemia. We hypothesized that cerebral blood velocity during lower body negative pressure would be lower during hypoxic versus normoxic conditions.MethodsTen healthy subjects (35 ± 3 y, BMI 25 ± 1 kg/m2) were exposed to stepwise lower body negative pressure (LBNP, 5 minutes at 0, −15, −30, and −45 mmHg) during both normoxia and hypoxia (FiO2 = 0.12–0.15 O2). Normoxia and hypoxia protocols were randomized and blinded. Mean arterial blood pressure (MAP, brachial arterial catheter), middle cerebral artery blood velocity (MCAv, transcranial doppler) and end tidal CO2 (ETCO2, mass spectrometer) were continuously recorded. We calculated cardiac output (CO) and total peripheral resistance (TPR) from the arterial pressure waveform. Cerebral vascular conductance (CVC) was calculated as MCAv/MAP. The cumulative stress index (CSI) was calculated as the cumulative product of LBNP pressure and time at each LBNP stage.ResultsDuring the hypoxia protocol, 7 participants developed presyncopal symptoms and the protocol was terminated early. Three of these 7 did not complete the normoxia protocol due to presyncopal symptoms as well. Subsequently, CSI was lower during hypoxia (278 ± 63 vs. 513 ± 47 mmHg·min−1, p < 0.01). MAP (p < 0.01), TPR (p < 0.01), MCAv (p = 0.06) and ETCO2 (p = 0.01) were lower during hypoxia than normoxia – Table 1. CO was higher (p = 0.03) during hypoxia than normoxia, while CVC (p = 0.35) was similar.ConclusionThe attenuated rise in TPR during the hypoxia protocol was associated with a reduced MAP observed during hypoxia. These findings indicate that lower MCAv and LBNP tolerance during hypoxia are due to reductions in cerebral perfusion pressure (MAP). The similar CVC during both conditions renders it unlikely that significant changes in cerebrovascular resistance contribute to the differences between normoxia and hypoxia in LBNP tolerance and MCAv.Support or Funding InformationFunding: W81XWH‐13‐2‐0038 Physiological responses to LBNP with hypoxia and normoxia Baseline LBNP−15mmHg LBNP−30mmHg LBNP−45mmHg MAP, mmHg Normoxia 103 ± 3 104 ± 3 101 ± 3 98 ± 41 * Hypoxia 100 ± 3 94 ± 4 93 ± 4 83 ± 4 CO, 1/min Normoxia 4.6 ± 0.3 4.5 ± 0.3 4.1 ± 0.3 3.7 ± 0.31 * Hypoxia 5.4 ± 0.3 4.8 ± 0.3 4.1 ± 0.3 3.6 ± 0.3 TPR, mmHg·min·mL−1 Normoxia 22.6 ± 1.3 23.7 ± 1.3 25.0 ± 1.3 26.6 ± 1.41 * Hypoxia 18.5 ± 1.3 19.8 ± 1.3 22.7 ± 1.4 23.7 ± 1.6 MCAv, cm·s−1 Normoxia 65 ± 4 64 ± 4 61 ± 4 55 ± 5 * Hypoxia 58 ± 4 59 ± 4 57 ± 5 54 ± 6 CVC, cm·s−1·mmHg−1 Normoxia 0.67 ± 0.05 0.65 ± 0.05 0.63 ± 0.05 0.58 ± 0.05 Hypoxia 0.61 ± 0.05 0.64 ± 0.05 0.63 ± 0.05 0.64 ± 0.06 ETCO2, mmHg Normoxia 38 ± 1 38 ± 1 38 ± 1 36 ± 1 * Hypoxia 37 ± 1 36 ± 1 35 ± 1 34 ± 2 Different vs. normoxia p<0.06, } indicates a main effect.