Acute microcirculatory effects of medium frequency versus high frequency neuromuscular electrical stimulation in critically ill patients - a pilot study

Background: Intensive care unit-acquired weakness (ICUAW) is a common complication, associated with significant morbidity. Neuromuscular electrical stimulation (NMES) has shown promise for prevention. NMES acutely affects skeletal muscle microcirculation; such effects could mediate the favorable outcomes. However, optimal current characteristics have not been defined. This study aimed to compare the effects on muscle microcirculation of a single NMES session using medium and high frequency currents. Methods: ICU patients with systemic inflammatory response syndrome (SIRS) or sepsis of three to five days duration and patients with ICUAW were studied. A single 30-minute NMES session was applied to the lower limbs bilaterally using current of increasing intensity. Patients were randomly assigned to either the HF (75 Hz, pulse 400 mu s, cycle 5 seconds on-21 seconds off) or the MF (45 Hz, pulse 400 mu s, cycle 5 seconds on-12 seconds off) protocol. Peripheral microcirculation was monitored at the thenar eminence using near-infrared spectroscopy (NIRS) to obtain tissue O-2 saturation (StO(2)); a vascular occlusion test was applied before and after the session. Local microcirculation of the vastus lateralis was also monitored using NIRS. Results: Thirty-one patients were randomized. In the HF protocol (17 patients), peripheral microcirculatory parameters were: thenar O-2 consumption rate (%/minute) from 8.6 +/- 2.2 to 9.9 +/- 5.1 (P = 0.08), endothelial reactivity (%/second) from 2.7 +/- 1.4 to 3.2 +/- 1.9 (P = 0.04), vascular reserve (seconds) from 160 +/- 55 to 145 +/- 49 (P = 0.03). In the MF protocol: thenar O-2 consumption rate (%/minute) from 8.8 +/- 3.8 to 9.9 +/- 3.6 (P = 0.07), endothelial reactivity (%/second) from 2.5 +/- 1.4 to 3.1 +/- 1.7 (P = 0.03), vascular reserve (seconds) from 163 +/- 37 to 144 +/- 33 (P = 0.001). Both protocols showed a similar effect. In the vastus lateralis, average muscle O-2 consumption rate was 61 +/- 9%/minute during the HF protocol versus 69 +/- 23%/minute during the MF protocol (P = 0.5). The minimum amplitude in StO(2) was 5 +/- 4 units with the HF protocol versus 7 +/- 4 units with the MF protocol (P = 0.3). Post-exercise, StO(2) increased by 6 +/- 7 units with the HF protocol versus 5 +/- 4 units with the MF protocol (P = 0.6). These changes correlated well with contraction strength. Conclusions: A single NMES session affected local and systemic skeletal muscle microcirculation. Medium and high frequency currents were equally effective.