The influence of breathing on hemodynamic oscillations at presyncope

Abstract

We hypothesized that oscillations in muscle sympathetic nerve activity (MSNA), arterial pressure (AP), and middle cerebral artery blood velocity (MCAv) transition from the Mayer wave frequency (~0.1 Hz) to the prevailing breathing frequency immediately prior to presyncope during progressive central hypovolemia. Ten subjects were instrumented for ECG, end‐tidal CO2 (surrogate for respiration), AP, MCAv, and MSNA, then completed a lower body negative pressure (LBNP) test to presyncope. The 6‐min immediately prior to presyncope were divided into two discrete 3‐min periods, “sub‐max” and “max”. High frequency (HF) and low frequency (LF) powers were calculated for mean AP (MAP), mean MCAv, and MSNA at each time period. The LF/HF ratio for each metric was also calculated as an index of balance between HF oscillations (predominantly influenced by respiration) and LF oscillations (predominantly influenced by sympathetic activity). Peak respiratory frequency remained in the HF range (0.15–0.4 Hz), and did not change (P=0.90) from sub‐max to max. The LF/HF ratio decreased (P≤0.08) from sub‐max to max for MAP (8.7±2.4 to 6.1±1.4), mean MCAv (4.1±0.8 to 2.2±0.4), and MSNA (0.74±0.15 to 0.51±0.09), indicative of a shift from LF oscillations to HF oscillations. In support of our hypothesis, hemodynamic oscillations transitioned from sympathetically‐mediated to respiratory‐mediated as subjects reached presyncope.