Discussion
Uhl, pelvic fractures, MPE, venous stasis, DVT, VTE, MP
BLOOD CLOTS, EVANS, VENOUS STASIS, DVT
The main finding of this study was that the PVV in both the popliteal and the femoral veins exhibited a dose–response relationship to the current intensity administered to the calf, for both the TTE and the MPE setups. The TTE setup required higher current intensity than the MPE setup to induce a plantar flexion (ML I) and to reach ML II, but also demonstrated superior increases of PVV in the popliteal vein at both ML I and ML II. However, PVV in the femoral vein was equally increased with both setups. Moreover, at ML I, the MPE setup caused less discomfort than the TTE setup. None of the participant characteristics caused more discomfort or significantly altered the increase of PVV.The finding that the PVV in both the popliteal and the femoral vein exhibited significant median increases of 50–100% from baseline to ML I conform with results from earlier studies on NMES (Williams et al. The finding of an intensity-dependent relationship between the current intensity of applied NMES (frequency: 36 Hz) and increased venous return is supported by the literature (Corley et al. The presumable explanation to the observed dependence between current intensity and increased PVV lies in the earlier knowledge that there is a relationship between the current intensity and muscle force production (Flodin et al. An intensity-dependent increase in venous blood flow velocity using calf-NMES is important for the prevention of DVT and VTE, since it may reduce one of their underlying causes, i.e., venous stasis in the lower limbs (Hajibandeh et al. The second most important finding of this study was the identification that the TTE setup, produced an equal or even better hemodynamic response in the popliteal vein compared to the MPE setup. The most plausible explanation to the enhanced hemodynamic response seen would be that the TTE setup required a higher current to reach ML I. This may theoretically have resulted in a broader stimulation over the muscle bulk, activating more muscle fibers as compared to the more focused stimulation resulting from the MPE setup. The TTE setup was configured with electrodes transversely, approximately at the thickest portions of both the lateral and medial gastrocnemius muscle heads, which may be a good electrode placement to cause a broader muscle fiber activation for better venous return (Uhl et al. However, since the determination of when a plantar flexion was performed did not differ between the two electrode setups, another explanation to the observed increased hemodynamic response with TTE may be that the higher current intensity also induced indirect, local effects on blood vessels via the nervous system (Maffiuletti et al. The observation in this study of a more pronounced response of the calf-NMES treatment in the popliteal vein as compared to the femoral vein could most reasonably be explained by two factors. The distance from the calf to the popliteal vein is shorter than to the femoral vein and the diameter of the popliteal vein is smaller as compared to the femoral vein. Thus, changes in peak venous velocity after calf-NMES treatment will be more easily detected in the popliteal as compared to the femoral vein. Another possible influencing factor may be the body position. However, in this study patients were analyzed in the semirecumbent position with the legs horizontal, mimicking a clinical situation in a hospital bed. Thus, the observed increases in venous velocity in the femoral vein may be of importance for patient with eg. Hip- and pelvic fractures, in which blood clots may develop in more proximal veins. However, this conclusion will necessitate further studies.Another aspect to consider when deciding the optimal placement of electrodes is the discomfort of the patients. The observation that the MPE setup exhibited less discomfort at ML I as compared to the TTE setup suggests a better compliance with treatment when electrodes are placed on MP, which is in line with earlier studies (Gobbo 2011). The finding that there was a difference in hemodynamic effect, but no difference in discomfort between the MPE and TTE setup at ML II, on the other hand, suggests that a compromise of several aspects must be taken into account to optimize patient outcome. For example, that compliance to treatment is also affected by the usability of the NMES electrodes. A standardized sock does not require the motor point scan and should therefore increase the usability. However, the issue with discomfort needs to be solved.The great differences in PVV observed, both at baseline and with calf-NMES, are suggestive of vast individual variations, which are supported by earlier studies (Evans et al. A possible limitation of the study is the relatively low number of participants, which hinders conclusions especially on associations between subject characteristics and outcome. Venous velocity, which was assessed with Doppler ultrasound, can be influenced by a number of factors such as venous pressure and vein diameter that were not assessed during stimulation. However, the vein diameter was assessed at baseline and since the stimulation time was so short it is less likely that it affected the blood pressure or the release of cortisol to change the vein diameter. Another limitation is the potentially differing impedances of the TTE and MPE setups, as well as of the skin of the different participants, which is likely to have influenced results. However, we believe that such measurements would have been unreliable considering that manipulation of the calf in the form of ultrasound measurements and NMES, among other factors (Euler et al. 