Table 4.

Frequency domain responses during progressive LBNP to presyncope in males and females

	LBNP Stage					P Values
	0	15	30	45	60	LBNP	Sex	Interaction	PS 3-Min	P Value
RRI HF, ms2
Male	856 ± 165	567 ± 174	319 ± 48*	267 ± 56*	79 ± 20*	<0.0001	0.36	0.73	92 ± 33	P = 0.47
Female	1070 ± 224	830 ± 219	451 ± 104*	187 ± 34*	132 ± 24*				135 ± 52
MAP LF, mmHg2
Male	5.9 ± 0.6	6.5 ± 0.9	11.3 ± 1.6*	12.7 ± 1.6*	14.6 ± 2.4*	<0.0001	0.58	0.05	15.1 ± 2.3	P = 0.49
Female	6.0 ± 1.2	5.7 ± 0.8	6.8 ± 1.1	11.1 ± 2.1*	17.1 ± 4.7*				12.9 ± 2.0
Mean MCAv LF, (cm/s)2
Male	3.7 ± 0.5	4.0 ± 0.8	5.5 ± 1.1	4.9 ± 0.9	5.5 ± 1.0	0.17	0.49	0.57	5.0 ± 0.8	P = 0.67
Female	4.6 ± 0.8	4.6 ± 1.7	4.3 ± 0.6	5.1 ± 0.8	4.9 ± 0.7				5.5 ± 0.8
Mean PCAv LF, (cm/s)2
Male	2.2 ± 0.4	1.7 ± 0.3	2.3 ± 0.5	2.6 ± 0.4	2.1 ± 0.4†	0.10	0.05	0.13	2.2 ± 0.5	P = 0.66
Female	2.0 ± 0.3	2.2 ± 0.6	2.1 ± 0.4	2.6 ± 0.4	3.1 ± 0.5*				2.5 ± 0.5
MAP-mean MCAv LF Coherence
Male	0.65 ± 0.02	0.64 ± 0.03	0.69 ± 0.02	0.69 ± 0.02	0.69 ± 0.03	0.14	0.74	0.38	0.68 ± 0.03	P = 0.63
Female	0.65 ± 0.04	0.64 ± 0.03	0.67 ± 0.03	0.61 ± 0.04	0.71 ± 0.04				0.70 ± 0.03
MAP-mean MCAv LF Gain, cm/s·mmHg−1
Male	0.72 ± 0.04†	0.67 ± 0.04†	0.65 ± 0.05†	0.59 ± 0.03*†	0.56 ± 0.03	<0.0001	<0.0001	0.37	0.56 ± 0.03	P = 0.09
Female	0.84 ± 0.09	0.80 ± 0.08	0.82 ± 0.08	0.58 ± 0.04*	0.59 ± 0.07*				0.70 ± 0.08
Normalized MAP-mean MCAv LF Gain, %·mmHg−1
Male	1.19 ± 0.05	1.17 ± 0.06	1.14 ± 0.06	1.12 ± 0.06	1.15 ± 0.06	0.21	0.33	0.18	1.24 ± 0.05	P = 0.99
Female	1.21 ± 0.09	1.18 ± 0.08	1.28 ± 0.08	1.02 ± 0.07	1.05 ± 0.09				1.24 ± 0.08
MAP-mean PCAv LF Coherence
Male	0.65 ± 0.03	0.64 ± 0.03	0.67 ± 0.02	0.65 ± 0.03	0.63 ± 0.04	0.85	0.84	0.81	0.65 ± 0.03	P = 0.62
Female	0.65 ± 0.04	0.63 ± 0.04	0.65 ± 0.03	0.62 ± 0.04	0.69 ± 0.04				0.67 ± 0.04
MAP-mean PCAv LF Gain, cm/s·mmHg−1
Male	0.54 ± 0.05†	0.50 ± 0.04†	0.46 ± 0.04†	0.44 ± 0.04†	0.42 ± 0.04†	0.02	<0.0001	0.97	0.42 ± 0.03	P = 0.65
Female	0.60 ± 0.08	0.58 ± 0.10	0.54 ± 0.06	0.45 ± 0.04	0.49 ± 0.05				0.44 ± 0.05
Normalized MAP-mean PCAv LF Gain, %·mmHg−1
Male	1.25 ± 0.10	1.17 ± 0.07	1.12 ± 0.08	1.07 ± 0.10	1.11 ± 0.08	0.49	0.07	0.94	1.23 ± 0.07	P = 0.79
Female	1.22 ± 0.09	1.24 ± 0.13	1.24 ± 0.08	1.12 ± 0.10	1.22 ± 0.10				1.20 ± 0.09
SAP-RRI LF Coherence
Male	0.60 ± 0.03	0.59 ± 0.03	0.60 ± 0.03	0.68 ± 0.03	0.62 ± 0.04	0.10	0.23	0.82	0.58 ± 0.03	P = 0.05
Female	0.65 ± 0.03	0.61 ± 0.04	0.66 ± 0.04	0.67 ± 0.03	0.65 ± 0.05				0.66 ± 0.03
SAP-RRI LF Gain, ms/mmHg
Male	14.45 ± 1.32	13.26 ± 1.03	8.83 ± 0.55*	7.41 ± 0.64*	5.38 ± 0.68*	<0.0001	0.18	0.91	3.28 ± 0.51	P = 0.07
Female	15.80 ± 2.09	13.43 ± 1.79	10.73 ± 1.07*	7.47 ± 0.84*	6.35 ± 1.00*				5.60 ± 1.11

Data are presented as means ± SE. HF, high frequency; LBNP, lower body negative pressure; LF, low frequency; MAP, mean arterial pressure; MCAv, middle cerebral artery velocity; PCAv, posterior cerebral artery velocity; RRI, R-to-R interval; SAP, systolic arterial pressure. PS 3-min refers to the 3 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 ≤ 0.05 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.