Noninvasive evaluation of electrical stimulation impacts on muscle hemodynamics via integrating diffuse optical spectroscopies with muscle stimulator

Abstract

Technologies currently available for the monitoring of electrical stimulation (ES) in promoting blood circulation and tissue oxygenation are limited. This study integrated a muscle stimulator with a diffuse correlation spectroscopy (DCS) flow-oximeter to noninvasively quantify muscle blood flow and oxygenation responses during ES. Ten healthy subjects were tested using the integrated system. The muscle stimulator delivered biphasic electrical current to right leg quadriceps muscle, and a custom-made DCS flow-oximeter was used for simultaneous measurements of muscle blood flow and oxygenation in both legs. To minimize motion artifact of muscle fibers during ES, a novel gating algorithm was developed for data acquisition at the time when the muscle was relaxed. ES at 2, 10, and 50 Hz were applied for 20 min on each subject in three days sequentially. Results demonstrate that the 20-min ES at all frequencies promoted muscle blood flow significantly. However, only the ES at 10 Hz resulted in significant and persistent increases in oxy-hemoglobin concentration during and post ES. This pilot study supports the application of the integrated system to quantify tissue hemodynamic improvements for the optimization of ES treatment in patients suffering from diseases caused by poor blood circulation and low tissue oxygenation (e.g., pressure ulcer).

Figures

Fig. 1

A schematic illustration of the integrated system, including a custom-made diffuse correlation spectroscopy (DCS) flow-oximeter (a), a laptop controller (b), a rectifying circuit (c), and a four-channel muscle stimulator (d). A pair of electrodes was placed on the right leg and two fiber-optic probes were taped on the two legs, respectively (e).

Fig. 2

A gating algorithm for optical data acquisition during electrical stimulation (ES) at 2 Hz. The outputs from channel #1 (CH1) (a) and channel #2 (CH2) (c) of muscle stimulator were synchronized; thus the muscle status (contraction or relaxation) controlled by CH2 could be read from the CH1 output (“2 s on” or “2 s off”). A digital counter in the A/D board was used to detect the ES pulses generated from CH1. The pulse counts increased continuously when ES was on and remained unchanged when ES was off (b). Optical data were recorded within 1.5 s when the ES was off (d).

Fig. 3

Relative blood flow (rBF) responses during 3-min ES at 2 Hz, measured by the DCS flow-oximeter without (a) or with (b) the gating algorithm. The time period between the first and second marks (vertical lines) indicates the transition to adjust the ES current from zero to the maximum tolerant level (i.e., ES setup). The optical data collected during the period of ES setup were excluded due to the large noises induced by the adjustment of ES current. The ES started from the second mark and ended at the third mark. The gating algorithm (b) significantly reduced noise due to motion artifact.

Fig. 4

The average time course physiological responses [rBF, Δ[HbO2], Δ[Hb], arterial blood pressure (ABP) and heart rate] to ES over 10 subjects measured from the stimulated muscles in the right legs with ES at 2 Hz (a), 10 Hz (b), and 50 Hz (c). Data obtained from 10 subjects were aligned based on the marks (vertical lines) made at the beginning and end of ES.

Fig. 5

The average time course hemodynamics responses to ES over 10 subjects measured from the control muscles in the left legs without ES. The hemodynamic responses (rBF, Δ[HbO2], Δ[Hb]) to ES at low (2 Hz), medium (10 Hz), and high (50 Hz) frequencies are displayed in (a), (b), and (c), respectively. The two vertical lines (marks) indicate the beginning and end of ES.

Fig. 6

Average physiological changes (mean±SE) over 10 subjects in rBF (a), Δ[HbO2] (b), Δ[Hb] (c), ABP (d), and heart rate (e) during ES at 2, 10, and 50 Hz. The changes for each subject were calculated by averaging the data over the last 3 min during ES.

Fig. 7

Average physiological changes (mean±SE) over 10 subjects in rBF (a), Δ[HbO2] (b), Δ[Hb] (c), ABP (d), and heart rate (e) after ES at 2, 10, and 50 Hz. Individual changes were calculated by averaging the physiological data over the last 3 min of the recovery period.