Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital, Dallas, Texas and Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, Texas;
Am J Physiol Regul Integr Comp Physiol. 2013 Sep 15;305(6):R604-9. doi: 10.1152/ajpregu.00052.2013. Epub 2013 Jul 17.
Heat-related decreases in cerebral perfusion are partly the result of ventilatory-related reductions in arterial CO2 tension. Cerebral perfusion likely contributes to an individual's tolerance to a challenge like lower body negative pressure (LBNP). Thus increasing cerebral perfusion may prolong LBNP tolerance. This study tested the hypothesis that a hypercapnia-induced increase in cerebral perfusion improves LBNP tolerance in hyperthermic individuals. Eleven individuals (31 ± 7 yr; 75 ± 12 kg) underwent passive heat stress (increased intestinal temperature ∼1.3°C) followed by a progressive LBNP challenge to tolerance on two separate days (randomized). From 30 mmHg LBNP, subjects inhaled either (blinded) a hypercapnic gas mixture (5% CO2, 21% oxygen, balanced nitrogen) or room air (SHAM). LBNP tolerance was quantified via the cumulative stress index (CSI). Mean middle cerebral artery blood velocity (MCAvmean,) and end-tidal CO2 (PetCO2) were also measured. CO2 inhalation of 5% increased PetCO2 at ∼40 mmHg LBNP (by 16 ± 4 mmHg) and at LBNP tolerance (by 18 ± 5 mmHg) compared with SHAM (P < 0.01). Subsequently, MCAvmean was higher in the 5% CO2 trial during ∼40 mmHg LBNP (by 21 ± 12 cm/s, ∼31%) and at LBNP tolerance (by 18 ± 10 cm/s, ∼25%) relative to the SHAM (P < 0.01). However, hypercapnia-induced increases in MCAvmean did not alter LBNP tolerance (5% CO2 CSI: 339 ± 155 mmHg × min; SHAM CSI: 273 ± 158 mmHg × min; P = 0.26). These data indicate that inhaling a hypercapnic gas mixture increases cerebral perfusion during LBNP but does not improve LBNP tolerance when hyperthermic.
热相关的脑灌注减少部分是由于通气相关的动脉二氧化碳张力降低所致。脑灌注可能有助于个体对下体负压（LBNP）等挑战的耐受。因此，增加脑灌注可能会延长 LBNP 的耐受时间。本研究旨在检验以下假设：高碳酸血症引起的脑灌注增加可改善高温个体对 LBNP 的耐受。11 名参与者（31 ± 7 岁；75 ± 12kg）在两天内分别进行被动热应激（肠道温度升高约 1.3°C）和逐步 LBNP 耐受挑战（随机）。从 30mmHg 的 LBNP 开始，受试者分别（盲法）吸入高碳酸血症混合气体（5%CO2、21%氧气、平衡氮气）或空气（SHAM）。通过累积应激指数（CSI）来量化 LBNP 耐受。还测量了大脑中动脉平均血流速度（MCAvmean）和呼气末二氧化碳（PetCO2）。5%的 CO2 吸入使 PetCO2 在约 40mmHg 的 LBNP 时（增加 16 ± 4mmHg）和在 LBNP 耐受时（增加 18 ± 5mmHg）与 SHAM 相比有所增加（P < 0.01）。随后，在 5%CO2 试验中，MCAvmean 在约 40mmHg 的 LBNP 时（增加 21 ± 12cm/s，约 31%）和在 LBNP 耐受时（增加 18 ± 10cm/s，约 25%）均高于 SHAM（P < 0.01）。然而，高碳酸血症引起的 MCAvmean 增加并未改变 LBNP 的耐受（5%CO2 CSI：339 ± 155mmHg×min；SHAM CSI：273 ± 158mmHg×min；P = 0.26）。这些数据表明，在 LBNP 期间吸入高碳酸血症混合气体可增加脑灌注，但在高温时并不能改善 LBNP 的耐受。
