Effect of counter-pulsation control of a pulsatile left ventricular assist device on working load variations of the native heart

Flow assessment as a function of pump timing of tubular pulsatile pump for use as a ventricular assist device in a left heart simulator

INTRODUCTION

Although mechanical circulatory support saved many lives during the last decade, clinical observations have shown that the continuous flow pumps are associated with a much higher incidence of gastrointestinal bleeding and kidney problems, among others, compared with the earlier generation pulsatile pumps. However, the presence of several moving mechanical components made pulsatile pumps less durable, bulky, and prone to malfunction, ultimately leading to bias in favor of continuous flow designs.

OBJECTIVE

The aim of the current work is to create a prototype tubular pulsatile pump and to test the timing of the pump in a left heart simulator.

METHODS

A left heart simulator to mimic pumping from a failing heart was created. This was used to experimentally test the output of a prototype ventricular assist device relative to a failing heart in the form of flow and pressure. The effect of pulsation timing was quantified.

RESULTS

A failing heart was simulated with an average flow rate of 1.1 L/min and a systolic pressure of 47 mmHg. With the pump, the flow rate increases to 4.8 L/min and a systolic pressure of 110mmHg, in a copulsation mode, while activating for 300-400 ms. If the activation time is reduced, or increased, the pump becomes less effective. Load on the heart is reduced when the pump operates in a counterpulsation mode.

CONCLUSION

A pulsatile pump, like the one proposed, provides adequate output for mechanical circulatory support, while minimizing the number of moving parts that could otherwise lead to tribological wear.

A novel intra-ventricular assist device enhances cardiac performance in normal and acutely failing isolated porcine hearts

Background:

We recently demonstrated that a novel intra-ventricular membrane pump (IVMP) was able to increase the pump function of isolated beating porcine hearts. In follow-up, we now investigated the impact of the IVMP on myocardial oxygen consumption and total mechanical efficiency (TME) and assessed the effect of IVMP-support in acutely failing hearts.

Methods:

In 10 ex vivo beating porcine hearts, we studied hemodynamic parameters, as well as arterial and coronary venous oxygen content. We assessed cardiac power (CP), myocardial oxygen consumption (MVO₂), and TME (CP divided by MVO₂) under baseline conditions and during IVMP-support. Additionally, five isolated hearts were subjected to global hypoxia to investigate the effects of IVMP-support on CP under conditions of acute heart failure.

Results:

Under physiological conditions, baseline CP was 0.36 ± 0.10 W, which increased to 0.65 ± 0.16 W during IVMP-support (increase of 85% ± 24, p < 0.001). This was accompanied by an increase in MVO2 from 18.6 ± 6.2 ml/min at baseline, to 22.3 ± 5.0 ml/min during IVMP-support (+26 ± 31%, p = 0.005). As a result, TME (%) increased from 5.9 ± 1.2 to 8.8 ± 1.8 (50 ± 22% increase, p < 0.001). Acute hypoxia-induced cardiac pump failure reduced CP by 35 ± 6%, which was fully restored to baseline levels during IVMP-support in all hearts.

Conclusion:

IVMP-support improved mechanical efficiency under physiological conditions, as the marked increase in cardiac performance only resulted in a modest increase in oxygen consumption. Moreover, the IVMP rapidly restored cardiac performance under conditions of acute pump failure. These observations warrant further study, to evaluate the effects of IVMP-support in in vivo animal models of acute cardiac pump failure.

Effects of an intra-ventricular assist device on the stroke volume of failing ventricle: Analysis of a mock circulatory system

BACKGROUND:

A novel intra-ventricular assist device (iVAD) was established as a new pulsatile assist device to address various disadvantages, such as bulky configuration and reduced arterial pulsatility, observed in conventional ventricular assist devices.

OBJECTIVE:

Analyzed the native left ventricular stroke volume (SV) after iVAD support in vitro.

METHODS:

The SV of iVAD was examined in a home-designed mock circulatory system (MCS) at different heart rates and drive pressures and the SV of a failure ventricle was examined with iVAD at 75, 90, 120 bpm and 120–180 mmHg drive pressure after iVAD support. Data pertaining to native left ventricular SV before and after iVAD support were compared.

RESULTS:

The native ventricular SV was improved by iVAD when its drive pressure (DP) was slightly greater than that of the mock system. Conversely, the native ventricular SV was decreased when DP was much greater than that (150 mmHg) of MCS. A high DP had a significant effect on SV.

CONCLUSIONS:

The proposed device improved the dysfunctional native left ventricular SV when DP of iVAD was slightly greater than that of MCS. However, iVAD reduced the SV when the drive pressure was greater than that of MCS.

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

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.

Improving hemodynamics of cardiovascular system under a novel intraventricular assist device support via modeling and simulations

Ventricular assist devices (LVADs) are increasingly recognized for supporting blood circulation in heart failure patients who are non-transplant eligible. Because of its volume, the traditional pulsatile device is not easy to implant intracorporeally. Continuous flow LVADs (CF-LVADs) reduce arterial pulsatility and only offer continuous flow, which is different from physiological flow, and may cause long-term complications in the cardiovascular system. The aim of this study was to design a new pulsatile assist device that overcomes this disadvantage, and to test this device in the cardiovascular system. Firstly, the input and output characteristics of the new device were tested in a simple cardiovascular mock system. A detailed mathematical model was established by fitting the experimental data. Secondly, the model was tested in four pathological cases, and was simulated and coupled with a fifth-order cardiovascular system and a new device model using Matlab software. Using assistance of the new device, we demonstrated that the left ventricle pressure, aortic pressure, and aortic flow of heart failure patients improved to the levels of a healthy individual. Especially, in state IV level heart failure patients, the systolic blood pressure increased from 81.34 mmHg to 132.1 mmHg, whereas the diastolic blood pressure increased from 54.28 mmHg to 78.7 mmHg. Cardiac output increased from 3.21 L/min to 5.16 L/min. The newly-developed assist device not only provided a physiological flow that was similar to healthy individuals, but also effectively improved the ability of the pathological ventricular volume. Finally, the effects of the new device on other hemodynamic parameters are discussed.

Outflow monitoring of a pneumatic ventricular assist device using external pressure sensors

BACKGROUND

In this study, a new algorithm was developed for estimating the pump outflow of a pneumatic ventricular assist device (p-VAD). The pump outflow estimation algorithm was derived from the ideal gas equation and determined the change in blood-sac volume of a p-VAD using two external pressure sensors.

OBJECTIVES

Based on in vitro experiments, the algorithm was revised to consider the effects of structural compliance caused by volume changes in an implanted unit, an air driveline, and the pressure difference between the sensors and the implanted unit.

METHODS

In animal experiments, p-VADs were connected to the left ventricles and the descending aorta of three calves (70-100 kg). Their outflows were estimated using the new algorithm and compared to the results obtained using an ultrasonic blood flow meter (UBF) (TS-410, Transonic Systems Inc., Ithaca, NY, USA).

RESULTS

The estimated and measured values had a Pearson's correlation coefficient of 0.864. The pressure sensors were installed at the external controller and connected to the air driveline on the same side as the external actuator, which made the sensors easy to manage.

Pumping Rate Study of a Left Ventricular Assist Device in a Mock Circulatory System

The aim of this work was to investigate the hemodynamic influence of the change of pump rate on the cardiovascular system with consideration of heart rate and the resonant characteristics of the arterial system when a reliable synchronous triggering source is unavailable. Hemodynamic waveforms are recorded at baseline conditions and with the pump rate of left ventricular assist device (LVAD) at 55, 60, 66, and 70 beats per minute for four test conditions in a mock circulatory system. The total input work (TIW) and energy equivalent pressure (EEP) are calculated as metrics for evaluating the hemodynamic performance within different test conditions. Experimental results show that TIW and EEP achieve their maximum values, where the pump rate is equal to the heart rate. In addition, it demonstrates that TIW and EEP are significantly affected by changing pump rate of LVAD, especially when the pump rate is closing to the natural frequency of the arterial system. When a reliable synchronous triggering source is not available for LVAD, it is suggested that selecting a pump rate equal to the resonant frequency of the arterial system could achieve better supporting effects.

Heart monitoring using left ventricle impedance and ventricular electrocardiography in left ventricular assist device patients

Background

Patients who develop critical arrhythmia during left ventricular assist device (LVAD) perfusion have a low survival rate. For diagnosis of unexpected heart abnormalities, new heart-monitoring methods are required for patients supported by LVAD perfusion. Ventricular electrocardiography using electrodes implanted in the ventricle to detect heart contractions is unsuitable if the heart is abnormal. Left ventricular impedance (LVI) is useful for monitoring heart movement but does not show abnormal action potential in the heart muscle.

Objectives

To detect detailed abnormal heart conditions, we obtained ventricular electrocardiograms (v-ECGs) and LVI simultaneously in porcine models connected to LVADs.

Methods

In the porcine models, electrodes were set on the heart apex and ascending aorta for real-time measurements of v-ECGs and LVI. As the carrier current frequency of the LVI was adjusted to 30 kHz, it was easily derived from the original v-ECG signal by using a high-pass filter (cutoff: 10 kHz). In addition, v-ECGs with a frequency band of 0.1 – 120 Hz were easily derived using a low-pass filter. Simultaneous v-ECG and LVI data were compared to detect heart volume changes during the Q-T period when the heart contracted. A new real-time algorithm for comparison of v-ECGs and LVI determined whether the porcine heartbeats were normal or abnormal. Several abnormal heartbeats were detected using the LVADs operating in asynchronous mode, most of which were premature ventricle contractions (PVCs). To evaluate the accuracy of the new method, the results obtained were compared to normal ECG data and cardiac output measured simultaneously using commercial devices.

Results

The new method provided more accurate detection of abnormal heart movements. This method can be used for various heart diseases, even those in which the cardiac output is heavily affected by LVAD operation.