Heart assist devices and their impact on arterial Doppler: When the waveform appears flat or serrated

Abstract

Objective

The purpose of this article was to review the principal mechanisms of three types of heart assist devices, to explore their haemodynamic impacts on spectral Doppler and to discuss the associated challenges in diagnostic ultrasound.

Conclusion

Heart assist devices are used in patients with advanced heart failure as bridge to transplantation or destination therapy. The arterial Doppler waveform features can be altered after the implantation of the devices. A continuous‐flow device converts the arterial waveform into a venous‐like waveform with a mean velocity that can be used in stenosis evaluation. When the pulsatile device produces extra pulses between the natural heartbeats, together they produce serrated arterial waveforms. When the pulsatile device serves as the only pumping source, the arterial waveforms are similar to the natural ones. Colour Doppler features are maintained despite all the spectral Doppler changes. These devices can affect both ultrasound scanning and interpretation. Therefore, it is essential to understand the principal mechanisms of these devices and their haemodynamic impacts so that an accurate ultrasound diagnosis can be made.

Introduction

Heart assist devices are used in support of failing hearts. In the long term, patients use them as the bridge to transplantation to sustain circulation and prolong life while awaiting suitable donors. Some devices have become destination therapy as an alternative to transplantation.1, 2 The devices most often used are ventricular assist devices (VAD), which are mechanical pumps used in the ventricle(s). Others include the C‐pulse^® heart assist system and total artificial heart (TAH).

The heart assist devices can alter the typical arterial Doppler waveform, depending on the type of pumps used in the devices. A pulsatile pump produces a pulsing action like the natural heart, and sometimes it produces an arterial waveform that is serrated. A non‐pulsatile pump, also known as a continuous‐flow pump, provides continuous flow during the entire cardiac cycle,3 which makes an artery appear like a vein on the spectral Doppler. This raises challenges in artery assessment as well as in venous flow detection. Such haemodynamic impacts have never been described in the literature.

With the advent of new technology, the heart assist devices have become more compact and portable. The accessories and spare battery are small enough for a shoulder bag, and patients with heart assist devices can become less hospital dependent. Instead of a long journey back to the cardiac hospital, patients with such devices may visit a local clinic for general ultrasound examinations. They may also attend the local emergency department for other medical conditions. For the benefit of these patients, it is essential that ultrasound specialists and technologists understand these devices and their impacts on spectral Doppler to achieve accurate ultrasound diagnosis.

Ventricular assist device

Ventricular assist device (VAD) is an electrically powered mechanical pump assisting the failing ventricle to sustain heart function. Such a device can be used in the left ventricle (LVAD) (Figure 1a), right ventricle (RVAD) or both together (BiVAD). LVAD is the most commonly used ventricular assist device.

Figure 1.

Illustrations of Ventricular Assist Devices (VAD). (a) A Patient with Continuous‐flow Pump LVAD and its Accessories in Carrying Cases (Image Courtesy of HeartWare International Inc). (b) Size Comparison of the Pulsatile Pump and Continuous‐flow Pump VADs (Image Courtesy of Heart and Lung Clinic, St Vincent's Hospital, Sydney, Australia).

Both types of pumps are used in ventricular assist devices. The continuous‐flow VAD is a newer generation, which is also smaller (Figure 1b) and simpler to implant.4 It has proven to be more durable than pulsatile VAD,5 and continuous‐flow LVAD has become part of the standard of care for advanced heart failure patients.5

With the insertion of a ventricular assist device, the natural ventricular movement is interfered with, as seen in Figure 2. Both carotid examinations in Figure 2 are from the same patient before and after the continuous‐flow pump LVAD implantation. Allowing for the interoperator variation, some observations can be made from the retrospective comparison.

Figure 2.

Male patient born in 1947, with Left Ventricular Failure. Carotid Ultrasound Examinations were performed by two operators with two identical machines for Pre‐operation (LVAD Insertion) Risk Assessment (a, c, e) and Pre‐transplant Workup (b, d, f) (Philips iU22 Ultrasound Machine; Philips Medical Systems, Bothell, WA, USA).

• 1Monophasic waveform, that is peak systolic velocity ≈ end‐diastolic velocity≈ mean velocity;

The post‐insertion arterial waveform shows continuously forward flow and appears like portal venous flow. The peak systolic velocity (PSV) is almost equal to the end‐diastolic velocity (EDV). In Figure 2B, D and F, only PSV values were measured, as there is only a slight difference between PSV and EDV. The term mean velocity (MV) is more appropriate to use instead of PSV or EDV.

• 2Post‐insertion MV≈ Pre‐insertion EDV;

Post‐insertion MV values are very close to the pre‐insertion EDV values (Figure 2a–d). This is probably due to the uninterrupted arterial wall compliance and distal vascular resistance.

• 3Same colour Doppler information;

Colour Doppler provides the same information about the flow direction (Figure 2a–f).

• 4Other unaffected features;

1. The temporal tapping effect remains evident on the external carotid artery (ECA) waveform (Figure 2f), a method often used in differentiating ICA and ECA.6

2. The diastolic spectral window is still present in the low‐resistant common carotid artery (CCA) (Figure 2b) and internal carotid artery (ICA) (Figure 2d), indicating the majority of blood cells are moving at the same speed.6

C‐Pulse^® heart assist system

The C‐Pulse^® heart assist system, which uses an extra‐aortic cuff/balloon, is an external aortic counter‐pulsation supporting system. The C‐Pulse^® is designed to assist the patient with moderate to severe heart failure by increasing coronary blood supply and decreasing cardiac workload. The extra‐aortic cuff is placed outside the ascending aorta and above the aortic valve level. A balloon within the cuff inflates and deflates during the cardiac cycle. ECG sensing leads are placed near the left ventricle to monitor the natural heartbeats, so that the cuff pumps between them (Figure 3a). The cuff deflates just before the heart pumps blood, to open up the aorta and minimise the impact on the heart's workload (Figure 3b). After the heartbeat and the closing of the aortic valve, the cuff inflates to give a second pulse. By pushing the blood through the aorta, it reduces the total workload of the heart and increases the blood supply to the heart muscle through the coronary arteries (Figure 3c).7

Figure 3.

Illustrations of C‐Pulse^® Heart Assist System (Image Courtesy of Sunshine Heart Inc). (a) Patient with C‐Pulse^® Heart Assist System and its Accessories in a Carrying Case. (b) The Cuff deflates before the heart beat. (c) The Cuff inflates after the heart beat.

The C‐Pulse^® is a non‐blood‐contacting device. Hence, anticoagulation therapy is not required after the implantation. The device may also be disconnected for short periods when required. The C‐Pulse^® system is currently undergoing an FDA‐approved clinical trial to evaluate its safety and efficacy.8

Because of the extra‐cardiac location of C‐Pulse^® device, the Doppler arterial waveform does not reflect the native cardiac movement (Figure 4). Allowing for interoperator variation, the following features can be seen in the post‐implant images (Figure 4b and d):

Figure 4.

Male patient born in 1949, with End‐stage Heart Failure, was a participant of C‐Pulse^® Heart Assist System Clinical Trial. Carotid Ultrasound performed by two operators with two identical machines for Pre‐operation (C‐Pulse^® System Implantation) Risk Assessment (a, c) and Pre‐transplant Workup (b, d) (Philips iU22 Ultrasound Machine; Philips Medical Systems, Bothell, WA, USA).

• 1Two pulses in one cycle

There are two pulses in each cardiac cycle. The first pulse is from the genuine ventricular contraction. The second pulse is produced by C‐Pulse^® and appears at the beginning of the diastolic phase (Figure 4b and d). The dicrotic notch is still visible in the transition from systole to diastole, which coincides with the aortic valve closure (Figure 4b and d).6

• 2Pulsatile diastolic flow with no true EDV values;

High resistance and pulsatile flow is seen in the diastolic phase, with reversed component at the end‐diastolic phase. This replaced the expected forward and low resistance flow (Figure 4b and d). The measured EDV values during the diastolic phase (in Figure 4b and d) no longer reflect the true arterial wall compliance and distal vascular resistance.

• 3Post‐implant PSV ≈ Pre‐implant PSV;

PSV value in the post‐implant scan is very close to the pre‐implant PSV value (Figure 4a–d), probably because the native ventricular contraction was preserved.

• 4Same colour Doppler information;

Colour Doppler provides the same information about the flow direction (Figure 4a–d).

Total artificial heart

Total artificial heart (TAH) is a fully implantable artificial heart. The implantation of TAH includes the following:

• Removing both native heart ventricles and four native valves;

• Sewing the quick connects of the TAH into the native atria, aorta and pulmonary artery;

• Attaching the TAH through the quick connects.9

TAH replaces the entire failing heart in a similar way to a transplanted heart (Figure 5). By implanting TAH, native heart complications such as arrhythmias, failing ventricles and malfunctioning heart valves can be eliminated.

Figure 5.

Illustrations of Total Artificial Heart and Human Heart (Image courtesy of syncardia.com). (a) Patient with Total Artificial Heart and its Supporting System. (b) Implanted Total Artificial Heart. (c) Human Heart.

Unlike the transplant heart, the TAH is immediately available on clinical request. There is no necessity for antirejection therapy, which makes TAH superior to the transplant heart in renal impairment patients. Current TAH to date has supported a patient's life up to 1,374 days (3 years and 9 months) prior to a successful heart transplant.10 It is considered that these machines may have a greater role than just biding time, given the shortage of donor organs and complications of heart transplant surgery.2

The TAH works as combined mechanical valves and BiVAD, without the need of pacemaker or defibrillator. The pulsatile pump in the TAH produces a pulsing movement like a biological heart. Thus, the arterial flow waveform (Figure 6a) is also very similar to the one by the biological heart (Figure 6b).

Figure 6.

Male patient born in 1955, with End‐stage Heart Failure, Ischaemic Cardiomyopathy, who was awaiting heart transplant. He had a TAH Implanted in March 2011 because of increasing shortness of breath and tachycardia, followed by a heart transplant in May 2012. The figures show two renal ultrasound examinations performed because of decreased renal function and by two operators. The Resistant Index (RI) was measured as part of the renal assessment. (a) With Total Artificial Heart Implanted. (b) With Transplanted Heart (Philips iU22 Ultrasound Machine; Philips Medical Systems, Bothell, WA, USA).

Discussion

Colour Doppler features are consistent despite the type of devices in use, which helps in confirming the flow direction and detecting turbulent flow. Colour Doppler plays an important role when the arterial waveform becomes monophasic. On the basis of the vascular anatomy and the flow direction, the operators can differentiate arteries from veins during the scan. This differentiation prevents the operator from performing the arterial assessment in the adjacent veins. The assist in detecting true venous flow is also critical for patients with suspected ovarian or testicular torsion in emergency settings.

Spectral Doppler is widely used to assess the blood flow velocity. In addition to the pulsatile flow patterns, the commonly used values are PSV, EDV, resistant index (RI) and velocity ratios. PSV is favoured in most of the velocity ratio calculations, such as ICA/ECA PSV ratio for carotids and renal/aortic PSV ratio for renal arteries.

In the case of the continuous‐flow LVAD in Figure 2, the evaluation of the carotid artery stenosis relies on the plaque morphology, lumen diameter change (with NASCET method)6 and colour Doppler information on direction change or turbulence. The presence of spectral broadening confirms a turbulent flow. Besides, the mean velocity (MV) measures almost the same as the pre‐insertion EDV. This may add diagnostic information for ICA stenosis grading. Even though rarely used, EDV and EDV ratio also serve as markers for ICA stenosis grading. When EDV is over 52 cm/s, the stenosis is greater than 50%. In a 75% stenosis, EDV reaches 120 cm/s. The less favoured EDV ratio (ICA‐EDV/CCA‐EDV) is also useful in this situation. A stenosis greater than 50% will have an EDV ratio more than 2.4. When the EDV ratio is beyond 7.5, the ICA stenosis will be greater than 75%.11

The pulsatile pumps cause less confusion in interpretation compared to the continuous‐flow pumps, because the arterial waveforms stay pulsatile. The challenge lies in the lack of true EDV values, when the device produces extra pulses between the natural heartbeats. The true EDV value becomes unmeasurable, as the expected monophasic diastolic waveform is replaced by the additional pulsatile waveform. Nevertheless, the PSV and PSV ratio are available for the diagnostic criteria. Care should be taken in measuring the PSV as both the heart and the device produce pulses. The inconsistency shown in Figure 4b (first peak by the genuine ventricular contraction was measured) and Figure 4d (second peak by C‐Pulse^® was measured) may decrease the accuracy of the clinical diagnosis. When the pulsatile device is the only pumping source, such as in the total artificial heart, the arterial waveforms are like the natural waveforms.

Some arterial spectral Doppler features are preserved, such as the dicrotic notch, spectral window and temporal tapping effect, because of the unchanged nature of the arteries. These features can provide additional diagnostic information.

Heart assist devices are predominantly used as a bridge to transplantation clinically. Such application limits the case variation in this retrospective comparison review. Most of the patients were already heart transplant candidates before the implantation of heart assist devices. This makes it difficult to find those patients with other significant illness. It is also common that patients with high‐grade carotid stenosis undergo prophylactic endarterectomy to reduce the future risk of perioperative stroke for heart transplant. Therefore, the cases selected in this review are based on physiological changes. With the increasing use of heart assist devices as ‘destination therapy’ in the near future, the candidate selection will be less restricted. It will be feasible to observe more physiological and pathological changes in these patients.