Model-based optimization of adaptive external counterpulsation therapy

Abstract

External counterpulsation therapy (ECP) is a non-invasive method to assist the circulatory system. The main principle of ECP is to initiate a diastolic pulse wave in the arterial system by squeezing the inner leg vessels. Superficial and low veins are compressed due to muscle contractions, which are triggered by functional electrical stimulation (FES). In this work, a new trigger method for the stimulation is proposed. Blood flow is determined by measuring the pulsatile change of conductivity in the observed segment by impedance plethysmography. The proposed “adaptive stimulation” allows the automatic adjustment of the start and duration of the stimulation to improve transport of venous blood. For this purpose, a mathematical circulation model was developed for optimization of the adaptive functional electrical stimulation. The model takes into account the effects of gravity, muscle pump with or without FES, and venous regurgitation. The model was shown to have a behavior similar to that of clinical measurements. The simulation showed a flow enhancement of up to 14% and, furthermore, that the start of the stimulation is less important than the duration of each stimulation phase. Therefore, an adaptation of the heart rate seems necessary, as the expulsion and refill times must remain relatively constant for an optimal pumping result. For a practical approach, it is reasonable to adapt the duration of stimulation to 56% of the cardiac cycle in order to reach a maximum flow.