Cerebral Blood Flow Velocity During Combined Lower Body Negative Pressure and Cognitive Stress.

BACKGROUND: Lower body negative pressure (LBNP) decreases middle cerebral artery blood velocity (MCAv) and can induce hypotension. Mental stress increases MCAv, but the MCAv response to combined LBNP and mental stress (COMBO) is unknown. We hypothesized that performing a stressful cognitive challenge (i.e., mental stress) concurrently with LBNP would prevent LBNP-induced reductions of MCAv.

METHODS: There were 18 subjects (9 men, 9 women; ages 20.1±0.3 yr) who completed 3 randomized 3-min trials: 1) LBNP (-40 mmHg); 2) mental stress (serial subtraction); and 3) COMBO (LBNP+mental stress). All reported values are mean±SE. Mean arterial pressure (MAP), heart rate (HR), forearm blood flow (FBF), and MCAv were measured continuously. Subjects also reported perceived stress following the mental stress and COMBO trials.

RESULTS: LBNP decreased MAP (Δ-1.4±0.5 mmHg), MCAv (Δ-2.6±1.1 cm s(-1)) and FBF (Δ-0.8±0.1 units), and increased HR (Δ2.7±1.2 bpm). Mental stress increased MAP (Δ10.1±1.3 mmHg), HR (Δ17.4±2.2 bpm), and FBF (Δ2.4±0.4 units), while MCAv (Δ2.8±1.3 cm s(-1)) tended to increase. COMBO increased MAP (Δ5.3±2.3 mmHg) and HR (Δ21.3±2.6 bpm), and tended to increase FBF (Δ0.5±0.3 units). However, MCAv (Δ-4.6±2.0 cm s(-1)) decreased during COMBO. Decreases in MCAv during COMBO were not statistically different from LBNP-induced decreases (Δ-4.6±2.0 vs. Δ-2.6±1.1 cm s(-1)). Subjective ratings of perceived stress (standard 0 to 4 scale) tended to be higher during COMBO than mental stress (2.9±0.1 vs. 2.5±0.1 units).

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