Pulsed or continuous flow in long-term assist devices: a debated topic

How well do we understand pulsatility in the context of modern ventricular assist devices?

BACKGROUND

Modern ventricular assist devices (VADs) use a continuous flow design. It has been suggested that a lack of pulsatility contributes to a range of adverse outcomes including pump thrombus, gastrointestinal bleeding and stroke. To better assess the role of pulsatility in these adverse events, we first require a clear definition of 'pulsatility' in the setting of a severely impaired ventricle and a modern continuous flow VAD.

METHODS

A literature review was conducted to elucidate the understanding of pulsatility in modern VAD literature. Search engines used included PUBMED, EMBASE and the Cochrane library. Articles were appraised on three aspects: Whether they mentioned pulsatility; whether they mentioned which pulsatility measure was used and finally which methodology was used to obtain the value.

RESULTS

Of 354 articles reviewed, only 13 met our broad inclusion criteria. Of these articles, the most cited measure was pulsatility index (PI) - used by 11 of the publications. The methodology used to obtain the value was not uniform and five articles did not clearly state it. Other measures included pulse pressure and surplus haemodynamic energy. The majority of articles did not directly discuss pulsatility in the setting of patient-pump interaction.

CONCLUSION

Most publications did not provide a definition for pulsatility. In those that did, the most common measure was PI. Measuring PI was not standardised. Few papers addressed the impact of intrinsic ventricular function and arterial compliance on pulsatility. We suggest that future publications adopt a uniform definition which encompasses both patient and pump characteristics.

Computational fluid dynamics-based study of possibility of generating pulsatile blood flow via a continuous-flow VAD

Until recent years, it was almost beyond remedy to save the life of end-stage heart failure patients without considering a heart transplant. This is while the need for healthy organs has always far exceeded donations. However, the evolution of VAD technology has certainly changed the management of these patients. Today, blood pumps are designed either pulsatile flow or continuous flow, each of which has its own concerns and limitations. For instance, pulsatile pumps are mostly voluminous and hardly can be used for children. On the other hand, the flow generated by continuous-flow pumps is in contrast with pulsatile flow of the natural heart. In this project, having used computational fluid dynamics, we studied the possibility of generating pulsatile blood flow via a continuous-flow blood pump by adjusting the rotational speed of the pump with two distinct patterns (sinusoidal and trapezoidal), both of which have been proposed and set based on physiological needs and blood flow waveform of the natural heart. An important feature of this study is setting the outlet pressure of the pump similar to the physiological conditions of a patient with heart failure, and since these axial pumps are sensitive to outlet pressures, more secure and reliable results of their performance are achieved. Our results show a slight superiority of a sinusoidal pattern compared to a trapezoidal one with the potential to achieve an adequate pulsatile flow by precisely controlling the rotational speed.

A monitoring and physiological control system for determining aortic valve closing with a ventricular assist device

OBJECTIVES

Real-time monitoring of the aortic valve function and the loading state of the left ventricle (LV) during mechanical circulatory support is essential. Therefore, we developed a system that determines accurately the aortic valve closing moment based on integrals derived from the pump inlet pressure and the pump power [pressure-power area (PPA)].

METHODS

A Deltastream diagonal pump was implanted in 10 healthy Rhoen sheep. Changes in ventricular volume and pressure in different assist levels were measured by a conductance catheter placed in the LV and were correlated with intrinsic pump signals, motor power, voltage and current. Measurements were obtained in the state of normal as well as decreased left ventricular contractility induced by β-blockers.

RESULTS

Complete datasets were obtained in seven animals. The PPA-feedback signal reached its maximum at the speed of aortic valve closing. This was validated by pressure-volume (PV)-catheter measurements both at the baseline and in the state of decreased contractility. In both cases, zero-crossing occurred at the point of aortic valve closing speed.

CONCLUSIONS

With this trial, we deliver the experimental basis for the development of an automatic feedback controller that would allow periodic speed changes in accordance with the loading state of the native ventricle and the opening state of the aortic valve. This would deliver real-time data to treating physicians and enable the establishment of a standard weaning protocol.

Minimally invasive implantation of the myopore sutureless myocardial pacing lead

Cardiac resynchronization therapy improves symptoms and survival of patients with congestive heart failure. Usually, the transvenous placement of the left ventricular lead is feasible, but in case of anatomic abnormalities of the coronary sinus, an unintended left phrenic nerve stimulation, a dislodgement of the percutaneous electrode, or a loss of capture of the electrode, surgical treatment should be considered. From January 2010 to September 2011, 15 patients underwent surgical implantation of the left ventricular lead after failure of transvenous placement. The MyoPore sutureless myocardial pacing lead (MSMPL) was implanted through a left minithoracotomy (~5 cm) under selective right lung ventilation. Time of surgery was 38.5 ± 3.0 minutes, and no surgical complications or early deaths are reported so far. After 10.7 ± 8.3 months of follow-up, no cases of late mortality, dislodgement, or loss of capture of the electrode are described. The use of the MSMPL is not novel, although the association with a minimally invasive approach may represent an alternative for a high-risk population. The screw-in of the lead ensures low impedance and threshold of stimulation (1.1 ± 0.6 V at 0.5 milliseconds) both in early and medium terms. In conclusion, in case of failure of the transvenous approach, the MSMPL may be easily implanted through a left minithoracotomy, and the results are noteworthy.