HVAD Flow Waveform Morphologies: Theoretical Foundation and Implications for Clinical Practice

Abstract

Continuous-flow ventricular assist device (cfVAD) performance and patient hemodynamic conditions are intimately interrelated and dynamic, changing frequently with alterations in physiologic conditions, particularly pre- and afterloading conditions. The Heartware cfVAD (HVAD) provides a unique feature among currently approved VADs of providing an estimated instantaneous flow waveform, the characteristics of which can provide significant insights into patient and device properties. Despite being readily available, HVAD waveforms are poorly understood, underutilized, and insufficiently leveraged, even by clinicians who regularly manage HVAD patients. The purpose of this review is to provide the theoretical foundation for understanding the determinants of HVAD waveform characteristics and to provide practical examples illustrating how to interpret and integrate changes of HVAD waveforms into clinical practice. Heartware cfVAD waveforms should be considered a complimentary tool for the optimization of medical therapies and device speed in HVAD patients.

Figure 1.

Pump flow (Q) is dependent on pressure gradient between the inflow cannula and outflow graft (H) and pump RPM. Increased RPMs results in increased flow at a given pressure gradient. RPM, revolutions per minute.

Figure 2.

A: Typical AOP and LVP tracings in a continuous flow ventricular assist device patient. At a fixed set speed, the pressure gradient between AOP and LVP during systole is relatively small and flow will be at its greatest value. Conversely, during ventricular diastole, the pressure gradient is relatively large and flow will be at its lowest value. B: A typical HVAD waveform of three consecutive beats. The peak of each waveform represents maximum instantaneous flow, the trough represents the minimum instantaneous flow, and the difference between peak and trough is referred to as the pulsatility. The mean flow, which is displayed on the patient monitor, is the average waveform during a beat. The time interval between waveform peaks reflects the heart rate; because the HVAD monitor displays 10 seconds of data, heart rate can be calculated by counting the number of waveforms displayed on the monitor and multiplying it by six. AOP, aortic pressure; HVAD, Heartware continuous-flow ventricular assist device; LVP, left ventricular pressure.

Figure 3.

A: Waveform from a 62 year old man 4 weeks post HVAD implant. The patient had evidence of volume overload on examination and the waveform showed high flow (mean value 6.0 L/min) with high pulsatility (approximately 6 L/min). B: Waveform from a 56 year old man 6 weeks post HVAD implant who presented to clinic for routine follow-up. He reported feeling well but had a complaint of occasional, intermittent dizziness. His waveform showed relatively low flow (mean 4.1 L/min) and low pulsatility (~1 L/min) indicative of a relative hypovolemic state. Provocative maneuvers including positional changes induced intermittent suction with standing and a marked increase in flow and waveform pulsatility with supine positioning. As a result of the history, waveform and waveform changes, HVAD speed was reduced by 100 RPM, and diuretics were discontinued. HVAD, Heartware continuous-flow ventricular assist device; RPM, revolutions per minute.

Figure 4.

A: Clinical hypovolemia associated with a low-flow, low-pulsatility waveform with patient sitting upright and legs dangling. B: Both flow and pulsatility increased within minutes after a passive leg raise maneuver.

Figure 5.

A: A 65 year old man with an ischemic cardiomyopathy, 6 months post HVAD implant presents to clinic for a follow-up visit. A low-flow, high-pulsatility waveform with a low-flow trough was seen on the HVAD monitor suggestive of relative hypertension. Systolic pressure by Doppler was 110 mm Hg. B: A 44 year old man 2 month post HVAD implant developed bacteremia related to a driveline infection. A high-flow, low-pulsatility waveform was seen on the HVAD monitor suggestive of relative hypotension due to excessive vasodilatation. Systolic blood pressure by Doppler was 60 mm Hg. HVAD, Heartware continuous-flow ventricular assist device.

Figure 6.

A: A 58 year old woman postoperative day 7 post Heartware continuous-flow ventricular assist device implant. Inotropes were stopped and several hours later, she developed a low-flow alarm with this low-flow, low-pulsatility waveform suggestive of a low left ventricular filling state; the proximity in time to discontinuation of inotropes suggested the possibility of right heart failure. Intermittent suction events were also occurring (not shown), which resolved upon lowering the speed from 2700 to 2400 RPM. B: After reinitiating inotropes for right heart failure, flow and pulsatility increased. RPM, revolutions per minute.

Figure 7.

This high-flow, low-pulsatility waveform derived from a cardiovascular simulation^, is representative of what is seen in cases of severe aortic regurgitation.

Figure 8.

A 62 year old man 2 days post HVAD implant showed an abrupt drop in VAD flows and flow pulsatility, followed by a gradual drop in blood pressure and urine output. Waveforms had relatively long intervals of constant flow between peaks. Echocardiography confirmed a large pericardial effusion with biventricular collapse. A pericardial window was performed with immediate improvement in hemodynamics and restoration of a normal HVAD waveform. HVAD, Heartware continuous-flow ventricular assist device.

Figure 9.

A: Changes in waveforms with changes in LV contractility at a constant RPM. A decrease in contractility results in a decrease in waveform peak and mean values, an increase in trough, and a decrease in pulsatility. The opposite occurs with an increase in contractility. B: Changes in waveform with changes in RPMs from a baseline value of 2800 RPMs with constant LV contractility. With RPMs decreased to 2400, peak, trough, and mean flows decreased and pulsatility increased. The opposite occurs when RPMs are increased to 3200. Waveforms derived from a cardiovascular simulation (6,7). LV, left ventricle; RPM, revolutions per minute.

Figure 10.

A 64 year old woman ~1 year post HVAD implant found during a routine office visit to have irregularly timed HVAD waveforms with varying peak flows (with high flows occurring after longer intervals) suggestive of atrial fibrillation, which was confirmed by electrocardiography. HVAD, Heartware continuous-flow ventricular assist device.

Figure 11.

A: A premature ventricular contraction occurred (ECG not shown) resulting in a premature contraction with waveform of lower peak flow and reduced pulsatility (red arrow) followed by a compensatory pause and postextrasystolic potentiation reflected in the waveform with higher peak flow and greater pulsatility (green arrow). The remainder of the waveform is otherwise normal. B: A waveform showing a “bigeminy” pattern (alternating red and green arrows) consistent with the patient’s underlying sinus rhythm with ventricular bigeminy noted on ECG (C). ECG, electrocardiogram.

Figure 12.

A 27 year old man post HVAD implant presented for a routine office visit. The HVAD waveform (A) showed normal flow, low pulsatility, and a heart rate of ~170 bpm (number of waveforms on the monitor × 6). He denied any symptoms. An ECG (B) confirmed sustained ventricular tachycardia at 170 bpm. HVAD, Heartware continuous-flow ventricular assist device.

Figure 13.

HVAD waveforms and corresponding arterial pressure tracings in an IABP-supported patient immediately after an HVAD implant during standby (A), 1:3 (B), 1:2 (C), and 1:1 IABP (D) trigger modes, respectively. Red bars reflect flow during “unsupported” IABP beats and green bars reflect flow during “supported” beats. HVAD, Heartware continuous-flow ventricular assist device; IABP, intra-aortic balloon pump.

Figure 14.

A low-flow, low-pulsatility waveform in an Heartware continuous-flow ventricular assist device-supported patient in a supine position with suspected hypovolemia (A). Changing position to sitting (B) induced intermittent suction followed by continuous suction when the patient stood up (C).

Figure 15.

These Heartware continuous-flow ventricular assist device log file tracings from Jorde et al. show examples of how power consumption changes over time with different time courses of pump thrombus formation (A, B, and C) and with an occlusion of the inflow cannula or outflow graft. See reference for details. Reproduced with permission from Elsevier.