Table 3.

Cerebral hemodynamic responses during progressive LBNP to presyncope in males and females

	LBNP Stage					P Values
	0	15	30	45	60	LBNP	Sex	Interaction	PS-1	P Value
Mean MCAv, cm/s
Male	60.4 ± 2.2	57.8 ± 2.1*†	55.7 ± 2.4*	53.4 ± 2.3*	51.4 ± 1.9*†	<0.0001	0.02	0.77	42.1 ± 1.5	P = 0.007
Female	68.5 ± 3.2	66.2 ± 3.5	62.7 ± 3.3*	60.2 ± 3.3*	55.1 ± 4.2*				51.1 ± 3.0
Mean PCAv, cm/s
Male	43.6 ± 2.1	42.1 ± 2.0	40.8 ± 2.4*	40.8 ± 2.4*	37.4 ± 1.6*	<0.0001	0.38	0.99	32.1 ± 1.6	P = 0.23
Female	46.7 ± 2.4	44.3 ± 2.9	42.5 ± 2.6*	41.1 ± 3.1*	40.6 ± 3.1*				35.8 ± 2.6
Mean MCAv CVR,  mmHg/(cm/s)
Male	1.6 ± 0.1	1.7 ± 0.1	1.7 ± 0.1	1.7 ± 0.1*	1.7 ± 0.1*	<0.0001	0.04	0.79	1.9 ± 0.1	P = 0.01
Female	1.4 ± 0.1	1.5 ± 0.1	1.5 ± 0.1	1.5 ± 0.1	1.6 ± 0.1				1.6 ± 0.1
Mean MCAv CVR, %Δ
Male	–	3.8 ± 1.6	5.3 ± 2.7*	8.3 ± 3.0*	9.1 ± 3.1*	<0.0001	0.29	0.65	18.1 ± 4.2	P = 0.14
Female	–	0.8 ± 1.2	3.8 ± 2.0	4.4 ± 2.3	5.9 ± 3.2				9.5 ± 3.6
Mean PCAv CVR,  mmHg/(cm/s)
Male	2.3 ± 0.1	2.3 ± 0.1	2.3 ± 0.1	2.2 ± 0.1	2.3 ± 0.1	0.003	0.63	0.30	2.4 ± 0.1	P = 0.44
Female	2.1 ± 0.1	2.2 ± 0.1	2.2 ± 0.1	2.3 ± 0.2*	2.2 ± 0.2*				2.3 ± 0.2
Mean PCAv CVR, %Δ
Male	–	1.4 ± 1.2	1.0 ± 1.2	1.0 ± 1.3	3.3 ± 2.5	0.006	0.23	0.39	9.8 ± 4.0	P = 0.81
Female	–	2.3 ± 1.5	3.6 ± 2.5	5.9 ± 3.2*	5.7 ± 3.6*				8.3 ± 4.5
ScO2, %
Male	69.5 ± 1.3†	68.5 ± 1.3*†	67.6 ± 1.3*†	66.5 ± 1.4*†	66.1 ± 1.5*†	<0.0001	0.01	0.08	64.2 ± 1.4	P = 0.06
Female	63.3 ± 1.9	62.4 ± 1.9*	61.7 ± 2.0*	60.6 ± 2.4*	59.3 ± 3.6*				59.6 ± 2.1
HbO2, µM
Male	27.4 ± 1.5*†	26.6 ± 1.7*†	26.6 ± 1.6*†	26.0 ± 1.9*†	26.2 ± 2.6*†	<0.0001	0.0004	0.01	25.4 ± 1.6	P = 0.002
Female	38.4 ± 2.2*	37.2 ± 2.1*	36.1 ± 2.0*	35.4 ± 2.2*	34.1 ± 2.6*				34.0 ± 1.9
dHb, µM
Male	15.7 ± 1.0	15.8 ± 1.0	16.3 ± 1.0*	16.7 ± 1.3*	17.8 ± 1.9*	<0.0001	0.46	0.01	17.0 ± 1.1	P = 0.30
Female	16.5 ± 1.0	16.8 ± 1.1	17.1 ± 1.1	17.6 ± 1.2*	17.4 ± 1.6*				18.7 ± 1.1
Respiratory Rate, breaths/min
Male	14.6 ± 0.8	13.7 ± 0.7	13.1 ± 0.8*	13.1 ± 0.9*	13.6 ± 1.1	0.0002	0.84	0.46	15.1 ± 1.1	P = 0.53
Female	14.9 ± 1.0	14.0 ± 0.8	13.5 ± 0.8	12.9 ± 0.8*	12.1 ± 1.0*				14.0 ± 1.3
etCO2, mmHg
Male	40.3 ± 0.7	38.9 ± 0.8	37.2 ± 1.0*	35.3 ± 1.3*	34.6 ± 1.4*	<0.0001	0.90	0.63	27.8 ± 1.1	P = 0.08
Female	39.4 ± 0.9	38.7 ± 0.7	37.4 ± 0.8*	35.3 ± 0.9*	33.0 ± 1.6*				30.9 ± 1.3

Data are presented as absolute and relative means ± SE. dHb, deoxygenated hemoglobin concentration; etCO₂, end tidal carbon dioxide; HbO₂, oxygenated hemoglobin concentration; LBNP, lower body negative pressure; MCAv, middle cerebral artery velocity; PCAv, posterior cerebral artery velocity; ScO₂, cerebral oxygen saturation. PS-1 time point refers to the 1 min prior to presyncope. A two-factor linear mixed model analysis with repeated measures from baseline to −60 mmHg LBNP was performed, followed by Holm-corrected post hoc tests for multiple comparisons (run on the least squared means generated by the linear mixed model analysis).

*P ≤ 0.07 compared to baseline within a group. †P ≤ 0.09 between male and female groups. Sex differences at presyncope were assessed using unpaired t tests. Exact P values are reported for all comparisons.