Middle cerebral artery blood velocity during a Valsalva maneuver in the standing position

Occasionally, lifting of a heavy weight leads to dizziness and even to fain ting, suggesting that, especially in the standing position, expiratory stra ining compromises cerebral perfusion. In 10 subjects, the middle cerebral a rtery 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 p osition. 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 similar to 40 mmHg with a nadir in MAP and card iac 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 indu ces an similar to 15% reduction in middle cerebral artery V-mean that is ex aggerated by a Valsalva maneuver performed with 40-mmHg mouth pressure to s imilar to 50% of supine rest.