Windkessel model of hemodynamic state supported by a pulsatile ventricular assist device in premature ventricle contraction

Abstract

Background: Counter-pulsation control (CPC) by ventricular assist devices (VADs) is believed to reduce cardiac load and increase coronary perfusion. However, patients with VADs have a higher risk of arrhythmia, which may cause the CPC to fail. Consequently, CPC has not been applied by VADs in clinical practice. The phase-locked loop (PLL) algorithm for CPC is readily implemented in VADs; however, it requires a normal, consistent heartbeat for adequate performance. When an arrhythmia occurs, the algorithm maintains a constant pumping rate despite the unstable heartbeat. Therefore, to apply the PLL algorithm to CPC, the hemodynamic effects of abnormal heartbeats must be analyzed.

Objectives: This study sought to predict the hemodynamic effects in patients undergoing CPC using VADs, based on electrocardiogram (ECG) data, including a wide range of heart rate (HR) changes caused by premature ventricular contraction (PVC) or other reasons.

Methods: A four-element Windkessel hemodynamic model was used to reproduce the patient's aortic blood pressure in this study. ECG data from 15 patients with severe congestive heart failure were used to assess the effect of the CPC on the patients' hemodynamic state. The input and output flow characteristics of the pulsatile VAD (LibraHeart I, Cervika, Korea) were measured using an ultrasound blood flow meter (TS410, Transonic, USA), with the aortic pressure maintained at 80-120 mmHg. All other patient conditions were also reproduced.

Results: In patients with PVCs or normal heartbeats, CPC controlled by a VAD reduced the cardiac load by 20 and 40%, respectively. When the HR was greater for other reasons, such as sinus tachycardia, simultaneous ejection from the heart and VAD was observed; however, the cardiac load was not increased by rapid cardiac contractions resulting from decreased left ventricle volume. These data suggest that the PLL algorithm reduces the cardiac load and maintains consistent hemodynamic changes.

Keywords: Arrhythmia; Counter-pulsation control; Phase-locked loop; Pulsatile ventricular assist device; Windkessel model.

Fig. 1

Phase-locked loop (PLL) algorithm for counter-pulsation control (CPC)

Fig. 2

a 4-element Windkessel model, including the ventricular assist device (VAD), b aortic blood pressure (AoP), and c pressure–volume curves before and during VAD perfusion

Fig. 3

a Representative electrocardiogram (ECG) data showing temporary premature ventricle contraction (PVC) obtained from a congestive heart failure patient, and b the patient’s AoP, reproduced by the Windkessel model

Fig. 4

a Experimental setup for the flow measurement of the pulsatile VAD, and b the inflow and outflow waveform of the VAD

Fig. 5

Representative curves for a patient ECGs, b AoP without VAD perfusion, c AoP with VAD perfusion, d pressure–volume (PV) curves of LV without VAD, and e PV curves of LV with VAD. At first, the patient’s heart beat was normal (①, ②). However, when PVC occurred, the heart rate increased abruptly (③) and then immedicately decreased (④)

Fig. 6

Representations of a patient ECGs, b LV volumes, c LV pressures, d AoP and e PV curves during normal (①) and subsequent abnormal heart beat caused by sinus tachycardia (②, ③)

Fig. 7

a Predicted cardiac load reduction ratio according to the ratio of T_p-d to T_R-R, and b histogram of the ratios of T_p-d to T_R-R for observed patients’ heartbeats