Pott F, van Lieshout J J, Ide K, Madsen P, Secher N H
The Copenhagen Muscle Research Center, Rigshospitalet, University of Copenhagen, DK-2100 Copenhagen, Denmark.
J Appl Physiol (1985). 2000 May;88(5):1545-50. doi: 10.1152/jappl.2000.88.5.1545.
Occasionally, lifting of a heavy weight leads to dizziness and even to fainting, suggesting that, especially in the standing position, expiratory straining compromises cerebral perfusion. In 10 subjects, the middle cerebral artery mean blood velocity (V(mean)) was evaluated during a Valsalva maneuver (mouth pressure 40 mmHg for 15 s) both in the supine and in the standing position. During standing, cardiac output decreased by 16 +/- 4 (SE) % (P < 0.05), and at the level of the brain mean arterial pressure (MAP) decreased from 89 +/- 2 to 78 +/- 3 mmHg (P < 0.05), as did V(mean) from 73 +/- 4 to 62 +/- 5 cm/s (P < 0.05). In both postures, the Valsalva maneuver increased central venous pressure by approximately 40 mmHg with a nadir in MAP and cardiac output that was most pronounced during standing (MAP: 65 +/- 6 vs. 87 +/- 3 mmHg; cardiac output: 37 +/- 3 vs. 57 +/- 4% of the resting value; P < 0.05). Also, V(mean) was lowest during the standing Valsalva maneuver (39 +/- 5 vs. 47 +/- 4 cm/s; P < 0.05). In healthy individuals, orthostasis induces an approximately 15% reduction in middle cerebral artery V(mean) that is exaggerated by a Valsalva maneuver performed with 40-mmHg mouth pressure to approximately 50% of supine rest.
偶尔，举重物会导致头晕甚至昏厥，这表明，尤其是在站立位时，呼气用力会损害脑灌注。在10名受试者中，分别在仰卧位和站立位进行瓦尔萨尔瓦动作（口腔压力40 mmHg，持续15秒）时评估大脑中动脉平均血流速度（V(mean)）。站立时，心输出量下降了16±4（SE）%（P<0.05），在脑水平，平均动脉压（MAP）从89±2 mmHg降至78±3 mmHg（P<0.05），V(mean)也从73±4 cm/s降至62±5 cm/s（P<0.05）。在两种体位下，瓦尔萨尔瓦动作使中心静脉压升高约40 mmHg，MAP和心输出量出现最低点，在站立时最为明显（MAP：65±6 mmHg对87±3 mmHg；心输出量：为静息值的37±3%对57±4%；P<0.05）。此外，站立位瓦尔萨尔瓦动作时V(mean)最低（39±5 cm/s对47±4 cm/s；P<0.05）。在健康个体中，直立位会使大脑中动脉V(mean)降低约15%，而在口腔压力为40 mmHg的瓦尔萨尔瓦动作时，这种降低会加剧，降至仰卧位静息时的约50%。
