Effect of Functional Electrical Stimulation (FES) on OH

Discussion

FES may be an important treatment adjunct to minimize cardiovascular changes during postural orthostatic stress in individuals with SCI. Several studies have suggested that FES-induced contractions of the leg muscles increases cardiac output and stroke volume, which increases venous return (Raymond et al. 2001). Subsequently, this increases ventricular filling and left ventricular end-diastolic volume (i.e., enhanced cardiac preload). According to the Frank-Starling effect, an increase in ventricular preload will lead to greater stretch of the myocytes and a concomitant increase in left ventricular stroke volume. The increased stroke volume may produce greater cardiac output and in turn, greater arterial blood pressure. In this manner, FES-induced contraction of the leg muscles may attenuate the drop in systolic BP in response to an orthostatic challenge.

FES-induced contraction of the leg muscles may also restore the body’s ability to redistribute blood from below the level of the lesion back to the heart.In fact, it is through this means that Davis et al. (1990) attributes FES’s effectiveness during an orthostatic challenge. In their study, Davis et al. found FES of leg muscles resulted in increased cardiac output and stroke volume in 6 males with paraplegia performing maximal arm-crank exercise. These results suggest that FES of leg muscles could alleviate the lower limb pooling effect during the orthostatic challenge. Chi et al. (2008) suggest that alleviation of the pooling effect could be further enhanced when FES of leg muscles is combined with passive mobilization. The clinical utility of this combination must be examined further in subjects with SCI because subjects in Chi et al. (2008) were able-bodied. A cross-sectional study by Yoshida et al. (2013) compares isometric FES of leg muscles vs. passive stepping vs. isometric FES + passive stepping. They found that both FES and passive stepping increased stroke volume and mean BP and that the highest increase in these two resulted from combined FES + stepping; however, the two interventions did not interact to synergistically increase stroke volume and mean aBP.

FES results in a dose-dependent increase in BP independent of the stimulation site that may be useful in treating OH (Sampson et al. 2000) andmay be an important treatment adjunct to minimize cardiovascular changes during postural orthostatic stress in individuals with acute SCI. Three level 2RCTs (Faghri & Yount 2002; Elokda et al. 2000; Sampson et al. 2000) and five non-randomized controlled trials (Chao & Cheing 2005; Raymond et al. 2001; Faghri et al. 2001; Faghri et al. 1992; Davis et al. 1990) with small sample sizes provide support for use of FES in individuals with SCI. FES of the lower extremity could be used by persons with SCI as an adjunct during standing to prevent OH and circulatory hypokinesis. An FES-induced leg muscle contraction is an effective adjunct treatment to delay OH caused by tilting; it allows people with tetraplegia to stand up more frequently and for longer durations (Elokda et al. 2000; Sampson et al. 2000).This effect may be more beneficial to those with tetraplegia who have a greater degree of decentralized cardiovascular autonomic control and may not be able to adjust their hemodynamics to the change in position (Faghri et al. 2001).

Current protocols predominantly evaluate BP after a single application of FES with a single change in position. The feasibility and practicality of implementing FES to influence orthostatic BP over time needs to be further explored.

Conclusion

There is level 2 evidence (from small, lower quality RCTs: Faghri & Yount 2002; Elokda et al. 2000; Sampson et al. 2000) that FES is an important treatment adjunct to minimize cardiovascular changes during postural orthostatic stress in individuals with SCI.