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Our results suggest that mental stress does not effectively preserve MCAv when combined with central hypovolemia (i.e., LBNP).

Lower body negative pressure (LBNP) decreases mean middle cerebral blood velocity (MCAv) and mental stress (MS) typically increases MCAv, but the MCAv response to these combined autonomic stressors is unknown. We hypothesized that performing MS during LBNP would result in an additive MCAv interaction. Eighteen participants (9 men, 9 women) completed three randomized trials: 1) 3 min LBNP (−40 mmHg), 2) 3 min MS (serial subtraction), and 3) combined LBNP and MS. Mean arterial pressure (MAP), heart rate (HR) and MCAv were continuously measured. Baseline MAP (81±2 vs. 80±1 vs. 80±1 mmHg) and HR (65±2 vs. 64±2 vs. 65±2 beats/min) were similar preceding each trial. LBNP significantly (p<0.05) decreased MCAv (Δ−3±1 cm/s), but did not alter MAP or HR. MS significantly (p<0.05) increased MAP (Δ10±1 mmHg), HR (Δ17±2 beats/min) and MCAv (Δ3±1 cm/s; p=0.05). Combined LBNP and MS significantly (P<0.05) increased MAP (Δ7±3 mmHg) and HR (Δ18±4 beats/min) and significantly decreased MCAv (Δ−5±2 cm/s). Increases in MAP and HR were similar during the combination trial when compared to the calculated sum of the individual trials. However, the MCAv (Δ−5±2 vs. Δ0±2 cm/s) response was blunted during the combination trial vs. the sum of individual trials. In conclusion, our findings are contrary to our hypothesis and suggest there is not an additive MCAv interaction during combined cognitive and orthostatic stress.Supported by NASA.

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