Modulation of the Spontaneous Beat-to-beat Fluctuations in Peripheral Vascular Resistance during Activation of Muscle Metaboreflex

Abstract

Continuous measurement of leg blood flow (LBF) using Doppler ultrasound with simultaneous noninvasive arterial blood pressure measurement permits beat-to-beat estimates of leg vascular resistance (LVR) in humans. PURPOSE: We tested the hypothesis that the beat-to-beat fluctuations in LVR and the dynamic relationship between mean arterial blood pressure (MAP) and LVR are modulated during activation of the muscle metaboreflex. METHODS: Twelve healthy subjects performed a 1-min isometric handgrip exercise at 50% maximal voluntary contraction, which was followed by a period of imposed post-exercise muscle ischemia (PEMI). We then employed transfer function analysis to examine the dynamic relationships between MAP and LBF, and MAP and LVR, both at rest (control) and during PEMI. RESULTS: (a) The spectral power for LBF and LVR in low-frequency (LF: 0.03 ∼ 0.15 Hz) range significantly increased from control during PEMI without a significant change in the high-frequency (HF: 0.15 ∼0.35 Hz) power. The spectral power for MAP within both the LF and HF ranges did not differ during control and PEMI periods, (b) The transfer function gains for MAP and LBF and MAP and LVR in the LF range (0.05 ∼ 0.15 Hz) were significantly increased during PEMI (vs. control), (MAP and LBF: 10 ± 1.2 vs. 15 ± 1.9 ml min-1 mmHg-1, P < 0.05; MAP and LVR: 0.009 ± 0.002 vs. 0.015 ± 0.003 LVRu mmHg-1, P < 0.05) but were unchanged in the HF range (0.2 ∼0.3 Hz). (c) The phase relationships between MAP and LBF, and MAP and LVR did not different during the control and PEMI periods (MAP and LBF in LF: 74.5 ± 11.0 vs. 91.6 ± 6.3 degrees, HF: 9.1 ± 5.6 vs. 1.2 ± 7.8 degrees; MAP and LVR in LF: −47.5 ± 7.0 vs. −44.3 ± 6.5 degrees, HF: −38.5 ± 23.9 vs. −2.1 ± 20.1 degrees). The phase relationship between MAP and LVR revealed that changes in MAP were followed by directionally similar changes in LVR, which is consistent with the characteristics of intrinsic vascular regulatory mechanisms such as the myogenic response of the resistance arteries. CONCLUSION: We suggest that, in humans, modulation of the dynamic relationship between MAP and LVR during activation of the muscle metaboreflex reflects complex interactions between intrinsic vascular regulatory mechanisms and sympathetic vascular regulation.