Manifestations of circulatory assist devices in noninvasive arterial duplex evaluation: A pictorial review

Abstract

Spectral Doppler and plethysmographic waveforms vary significantly in the presence of circulatory assist devices. Understanding the effect these devices have on the waveforms and flow characteristics is necessary for the appropriate interpretation of duplex ultrasound and other noninvasive vascular studies. We review the different arterial waveforms that can be seen with circulatory assist devices.

Introduction

Circulatory assist devices such as left ventricular assist devices (LVADs), intra-aortic balloon pump (IABP), and extracorporeal membrane oxygenation (ECMO) are used in cardiac or pulmonary failure refractory to medical management.

Duplex ultrasound is frequently used in critically ill patients treated with these devices to evaluate for vascular access site complications, vascular thrombosis, and distal limb malperfusion. The effect of the assist devices on hemodynamics produces sonographic flow patterns, which are important to recognize for accurate interpretation of the vascular studies.

In this pictorial review, we discuss normal Doppler and pulse volume recording waveforms, and changes observed in the presence of circulatory assist devices.

Normal peripheral arterial waveforms

In a resting upper or lower extremity, spectral Doppler waveforms are typically multiphasic in nature (Figure 1). The first phase is characterized by a brisk systolic upstroke, which is followed by the second phase – reversal of flow in early diastole. The final phase is a low, or absent, end-diastolic flow in an upward deflection, reflecting forward flow due to elastic recoil of the vessel wall. ¹

Figure 1.

Normal peripheral arterial spectral Doppler waveform.

Normal carotid artery spectral Doppler waveforms

The spectral Doppler waveforms of the carotid arteries are reflective of the vascular bed being supplied. ¹ Common carotid artery (CCA) spectral Doppler waveform has characteristics of the waveforms seen in the internal carotid (ICA) and external carotid arteries (Figure 2A). Owing to a predominance of CCA flow through the ICA, the waveform in CCA is mostly that of a low-resistive pattern, which is characterized by forward flow throughout diastole.

Figure 2.

(A) Normal CCA spectral Doppler waveform. (B) Normal ICA spectral Doppler waveform.

ICA spectral Doppler waveform has a monophasic low-resistive flow pattern. It is characterized by a sharp upstroke during systole followed by a continuous antegrade flow throughout diastole (Figure 2B).

Normal peripheral arterial physiologic study

A normal plethysmographic waveform has a sharp systolic rise, a narrow peak, and a dicrotic notch during downslope. ² An ankle–brachial index (ABI) ⩾ 1.0 is considered normal. An ABI of ⩽ 0.9 is diagnostic of peripheral artery disease (PAD), whereas an ABI of 0.91–0.99 is considered borderline. In cases of noncompressible vessels and an ABI of > 1.40, the use of a toe–brachial index is recommended. A value of < 0.7 is abnormal and diagnostic of PAD. Figure 3 shows an example of a normal pulse volume recording with no segmental pressure gradients and bilaterally normal ankle–brachial indices.

Figure 3.

Normal PVR and ankle–brachial indices.

Intra-aortic balloon pump

IABP is a device used to increase cardiac output in cases of cardiogenic shock, and/or as a support device in patients at risk for hemodynamic compensation during high-risk coronary interventions. ³ The device consists of a helium-filled balloon that is inserted into the aorta, and it operates by deflating during systole and inflating during diastole.

Typical spectral Doppler waveform in the setting of an IABP at 1:1 setting demonstrates two separate systolic peaks. The first peak corresponds to the unassisted systole and the second peak corresponds to the inflation of the balloon during diastole. The second peak is followed by the reversal of flow due to the deflation of the balloon (Figures 4A and 4B). Plethysmographic waveform in a patient undergoing treatment with IABP retains the characteristic two peaks in 1:1 setting (Figure 4C).

Figure 4.

(A) Spectral Doppler waveform in the ICA in the presence of the intra-aortic balloon pump at 1:1 setting. (B) Spectral Doppler waveform in the SFA in the presence of the intra-aortic balloon pump at 1:1 setting. (C) Plethysmographic waveform in a patient undergoing treatment with the intra-aortic balloon pump at 1:1 setting. (D) Spectral Doppler waveform in the SFA in a patient undergoing treatment with the intra-aortic balloon pump at 1:2 setting.

The level of assistance provided by an IABP can be adjusted to assist every cardiac cycle (1:1 setting) as in Figures 4A and 4B, or less frequently (1:2 setting) as in Figure 4D. Changing the level of assistance will change the frequency of the augmented systolic peak in relation to the unassisted beat. Velocities cannot be measured accurately unless the IABP is turned off. If this is not possible, the peak systolic velocity is measured from the unassisted peak as in Figures 4A and 4B.

Left ventricular assist device

A LVAD is a device used in cases of end-stage heart failure. The device consists of a pump that has its inlet in the apex of the left ventricle and its output in the ascending or descending aorta. ⁴ The device works by continuously pumping blood from its inlet to the outlet, augmenting cardiac outflow.

Contemporary LVADs are continuous flow (CF) devices with either axial (e.g., HeartMate II, Abbott, Abbott Park, IL, USA; Impella, ABIOMED, Danvers, MA, USA) or centrifugal pump (e.g., HeartMate III, Abbott) designs. In CF-LVADs, blood flow is inversely related to the pressure differential between the left ventricle and aorta. ⁵ Centrifugal pumps are more sensitive to changes in preload and afterload; consequently, for the same changes in pressure, centrifugal pumps generate greater changes in flow and this results in greater pulsatility to waveforms. Therefore, LVADs with centrifugal pump designs produce a more pulsatile waveform relative to LVADs with axial pump designs.^5,6 In addition to pump design, the systolic pulsatility of the Doppler waveforms of LVAD patients depends on the device settings, and the patient’s myocardial reserve. ⁶ Patients with poor myocardial reserve will have a decreased pulsatility as compared to patients with a preserved myocardial reserve. Furthermore, the HeartMate III can be programmed using pump speed modulation to generate more pulsatility from the LVAD and to intermittently increase flow from the left ventricle during systole and diastole. ⁵

Doppler waveform in the patients with a poor myocardial reserve treated with a CF-LVAD with an axial pump design demonstrates a monophasic and low-resistive waveform with a prolonged systolic upstroke and minimal pulsatility, as in Figure 5A. Monophasic and low-resistive spectral Doppler waveforms with preserved pulsatility is seen in patients with a CF device with a centrifugal pump design, as in Figure 5B.

Figure 5.

(A) ICA spectral Doppler waveform in a patient with a poor myocardial reserve treated with the percutaneous continuous flow LVAD with an axial pump design. (B) ICA spectral Doppler waveform in the presence of the continuous flow LVAD with a centrifugal pump design. (C) Peripheral arterial spectral Doppler waveform in the presence of the continuous flow LVAD with a centrifugal pump design. (D) Plethysmographic waveforms in a patient treated with the LVAD.

Peripheral arterial spectral Doppler waveform demonstrates a monophasic, low-resistive pattern in the presence of an LVAD (Figure 5C). LVAD-associated qualitative and quantitative hemodynamic changes limit accuracy of the duplex ultrasound for the estimation of stenosis.^7,8 Further investigation on the use of additional parameters such as velocity ratios and plaque characterization is needed in such cases.

Plethysmographic waveforms in a patient with LVAD are dampened throughout with loss of dicrotic notch bilaterally (Figure 5D). Morphology of the plethysmographic waveforms may be variable, dependent on the LVAD pump settings. The localization and severity of the disease cannot be accurately determined based on the waveforms in the presence of the LVAD.

Extracorporeal membrane oxygenation

ECMO is a method of oxygenating the blood outside the body. There are two types of ECMO – veno-venous (VV) and veno-arterial (VA). VA ECMO runs parallel to circulation, where blood is obtained from the right atrium or vena cava and reintroduced through peripheral cannulations or central cannulation to the ascending aorta. ⁹

Doppler waveform in the presence of ECMO demonstrates a high-flow, low-resistive pattern with variable systolic pulsatility, depending on the myocardial reserve of a patient and degree of ECMO support. ⁶ Monophasic low-resistive spectral Doppler waveform with diminished systolic pulsatility suggests poor myocardial reserve but may also be seen with increased VA ECMO support (Figure 6A). High-flow, monophasic, low-resistive spectral Doppler waveform with systolic pulsatility suggests preserved myocardial reserve (Figure 6B).

Figure 6.

(A) CCA spectral Doppler waveforms in a patient treated with ECMO. (B) ICA spectral Doppler waveforms in a patient treated with ECMO. (C) Peripheral arterial spectral Doppler waveforms in a patient treated with ECMO. (D) Plethysmographic waveforms in a patient treated with ECMO (with the arterial cannula in the left common femoral artery).

Peripheral arterial spectral Doppler waveforms obtained in the presence of ECMO show monophasic, low-resistive pattern and delayed systolic upstroke (Figure 6C). Change in the waveform morphology and limited visualization due to presence of cannulas present challenges in evaluation of stenosis in patients treated with ECMO. There are no established criteria to accurately assess stenosis in the presence of LVAD or ECMO devices. Further research into the role of additional hemodynamic parameters is necessary.

Vascular ultrasound has a role in the diagnosis of complications caused by circulatory devices. Severely dampened left ankle plethysmographic waveforms and nonaudible pedal Doppler signals were demonstrated in a patient with VA ECMO with an arterial cannula in the left common femoral artery, which indicate severely impaired limb perfusion (Figure 6D).

Conclusion

Presence of the cardiac assist devices has a distinct effect on the morphology of the spectral Doppler and plethysmographic waveforms, flow velocities, and pulsatility. Recognition of these changes and limitations of the diagnostic criteria in the presence of mechanical assist devices are essential for the accurate interpretation of the arterial studies.