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1 The Copenhagen Muscle Research Center and 3 Department of Anesthesia, Rigshospitalet, University of Copenhagen, DK-2100 Copenhagen, Denmark; and 2 Department of Medicine, Academic Medical Center, University of Amsterdam, 1012 WX Amsterdam, The Netherlands
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 (Vmean) 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 Vmean from 73 ± 4 to 62 ± 5 cm/s (P < 0.05). In both postures, the Valsalva maneuver increased central venous pressure by ~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, Vmean was lowest during the standing Valsalva maneuver (39 ± 5 vs. 47 ± 4 cm/s; P < 0.05). In healthy individuals, orthostasis induces an ~15% reduction in middle cerebral artery Vmean that is exaggerated by a Valsalva maneuver performed with 40-mmHg mouth pressure to ~50% of supine rest.
blood pressure; central venous pressure; cerebral autoregulation; cerebral blood flow; transcranial Doppler
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