Echocardiography in Pediatric and Adult Congenital Heart Disease, 2nd Ed.

37. Echocardiographic Assessment of Mechanical Circulatory Support

INTRODUCTION

Background

Since the introduction of the first ventricular assist devices (VADs) and extracorporeal membrane oxygenation (ECMO) in the 1950s–1960s, mechanical circulatory support (MCS) has become a mainstay of treatment for patients with acute, life-threatening heart failure. Initially used in the adult population, MCS utilization continues to increase in infants and children. Consequently, echocardiographic assessment of the heart on MCS has become more common, particularly in the intensive care unit (ICU).

MCS systems have been shown to be useful for supporting hemodynamics in patients with acute, recoverable heart failure such as acute fulminant myocarditis, as well as in those with acute cardiac dysfunction following congenital heart surgery. Additionally, MCS has become an effective “bridge” to heart, lung, or heart-lung transplantation, supporting the patient with cardiopulmonary dysfunction while awaiting donor organ availability. Recently, some centers have utilized a “bridge to decision” strategy, where MCS has been used to delay decisions about transplantation until further testing or procedures can take place. Other indications for MCS include support for patients with refractory arrhythmias, for heart failure patients who need to initiate pharmaceutical regimens, and for patients with life-threatening cardiac defects that cannot undergo immediate surgical repair.

The different types of MCS currently available for children are listed in Table 37.1. MCS systems can effectively be differentiated by their expected duration of support in a particular patient. Short-term devices may offer support for days or even up to a few weeks, and include ECMO, intra-aortic balloon pumps, and centrifugal ventricular assist devices. Long-term devices may offer support for a few weeks up to years, and include most paracorporeal and implantable VADs. Selection of a particular device for any given patient is dependent on the specific patient anatomy, age and size, and expected duration of support. The specific device chosen may also be highly institution-dependent, with factors including surgeon and cardiologist preference, experience with a particular device, and the input of collaborating adult teams when available/applicable.

Importance of Echocardiography

Echocardiography plays a critical role in the evaluation of the patient with end-stage or acute heart failure, including diagnostic and functional assessments. In addition, it is the most frequently utilized imaging technique prior to, during, and after placement of the patient on MCS. Finally, echocardiography can be useful in the determination of myocardial recovery in selected patients. In the following chapter, we will discuss echocardiographic assessment of the patient before, during, and after placement on MCS, with a focus on VADs. This will be followed by a discussion of the specific roles of echocardiography in other types of MCS, including ECMO and percutaneous devices.

ECHO ASSESSMENT BEFORE MCS PLACEMENT

Diagnosis

Echocardiography plays a central role in the evaluation and management of patients for whom MCS is being considered. First and foremost, echocardiography can establish the putative diagnosis and assist in the determination of the type of device (short- versus long-term support, implantable versus paracorporeal). Echocardiography allows for detailed assessment of systolic and diastolic function of the ventricles (left, right, or single), and can help the clinician in the determination of the timing of MCS initiation. Importantly, echocardiography allows for evaluation of the structural anatomy of the heart and for the presence of any residual cardiac lesions. This assessment is critical for deciding the location of insertion of both inflow and outflow cannulae. If the patient has a history of corrective or palliative congenital heart surgery, the specific connections that have been made need to be taken into account when considering MCS choice and location. For all of these reasons, it is critical to have echocardiographers and surgeons trained in congenital heart disease making these pre-MCS assessments in children.

In cases of VAD placement in a biventricular heart, echocardiography can help the clinician decide whether a systemic VAD (most often supporting the left ventricle, LVAD) alone will be sufficient for the patient, or whether biventricular support (addition of RVAD to support the right ventricle) will be required. Primarily, this decision involves evaluation of the subpulmonary ventricle (most often the right ventricle) to determine function, size, and valvar function. A poorly functioning right ventricle may benefit from the reduction in afterload that occurs with an LVAD. However, the right ventricle may not be able to adapt to the increased preload from increased systemic output. Studies in adults have suggested that severely enlarged right ventricles (end-diastolic volume >200 mL) have a higher propensity for failing when the patient is placed on an LVAD.

Valve Function

Valve function can be assessed comprehensively using echocardiography. Significant valvar regurgitation or stenosis can have a deleterious effect on MCS capability, sometimes requiring modification of the technique.

Aortic insufficiency is very important to assess prior to VAD placement. Not uncommonly, the degree of aortic insufficiency may appear to worsen after placement of a VAD. This is likely due to underestimation of the severity of regurgitation in the setting of severe ventricular dysfunction. When severe, aortic insufficiency causes the VAD circuit to be inefficient, with much of the VAD output lost back into the ventricle. If this occurs, aortic valve replacement or oversewing the valve should be considered at the time of VAD implantation. Aortic stenosis, when present, is often not a major concern in patients with pulsatile VADs; however, it becomes an issue for any device with partial support, or those which cross the aortic valve (such as the Impella Recover, Abiomed, Danvers, MA).

Mitral valve dysfunction commonly occurs in patients with severely dilated left ventricles in the setting of dilated cardiomyopathy. This can result from annular dilatation as well as from ischemia of the papillary muscles. Most often, appropriate decompression of the left ventricle after VAD initiation will result in improvement in this functional mitral valve regurgitation. Mitral stenosis may affect inflow to an LVAD apical cannula. In this situation, the inflow cannula may need to be placed in the left atrium, or the mitral valve might require surgical intervention.

In patients with poor right ventricular function, significant tricuspid regurgitation may be an important association and it often results in worsening of cardiac output. Tricuspid regurgitation may be exacerbated by the septal shift that occurs with decompression of the left ventricle. Consideration should be given for addressing clinically significant degrees of tricuspid regurgitation at the time of VAD implantation (i.e., moderate or greater). Alternatively, the tricuspid regurgitation may be indicative of poor right ventricular function, and the clinician should be prepared for the possibility of needing biventricular support (BiVAD).

Pulmonary valve problems including insufficiency and stenosis are rare in patients undergoing VAD placement. In the setting of RVAD (or biventricular VAD), severe pulmonary insufficiency may result in inefficiency of the RVAD circuit, with much of the outflow lost back into the ventricle. Similar to patients with severe aortic insufficiency, patients with significant pulmonary insufficiency may rarely require valve repair or oversewing techniques.

Shunts

Detection of intracardiac shunts is important in the patient being placed on MCS, and is particularly critical in the VAD patient. Defects of the atrial septum allow for atrial level shunting. This is of particular importance in patients with poor biventricular function being supported by an LVAD alone. Right-to-left atrial shunting occurs due to decompression of the left atrium and ventricle after LVAD placement. Such shunting may result in hypoxia or paradoxical embolism. Thus, intracardiac shunts should be closed during VAD placement. In a study of 32 pediatric patients being placed on a Berlin Heart EXCOR device (Berlin Heart AG, Berlin, Germany), 11 were found to have intracardiac shunts prior to placement of the device. Surgical closure was performed in all.

Thrombus Formation

Assessment of the heart for thrombus formation is important, and may affect where the surgeon is able to place the inflow cannula. Common locations for thrombus formation include the systemic ventricular apex (the most common insertion site for an inflow cannula (Fig. 37.1, Video 37.1A,B)), and the atrial appendages. The ascending aorta, a common location of insertion of the outflow cannula, may harbor atheromatous disease in adults, but this is rarely a concern in children.

Figure 37.1. Magnified apical four-chamber view with focus on the left ventricle in a 2-year-old patient with dilated cardiomyopathy and poor systolic function. Note the thrombus in the left ventricular apex measuring 6 × 8 mm.

INTRAOPERATIVE ASSESSMENT OF MCS

Cannula Position

Transesophageal echocardiography (TEE) can be very effective in guiding placement of the cannulae during VAD implantation. The inflow cannula in the apex of the heart can be guided away from the ventricular walls, avoiding potential sources of obstruction (Fig. 37.2). In a typical four-chamber imaging plane, the inflow cannula should align with the mitral inflow stream. Color Doppler should demonstrate unidirectional, laminar flow into the inflow cannula (Fig. 37.3, Video 37.2A,B). Appropriate decompression of the left ventricle can be assessed (Fig. 37.4, Videos 37.3 and 37.4A-C). Similarly, outflow cannula placement can be guided in the aorta or pulmonary artery (Fig. 37.5, Videos 37.5 and 37.6). The outflow cannula in an LVAD is best visualized in the long-axis view of the aorta, typically from the mid-esophagus (~120 degrees). Doppler velocities up to 2 m/s are often seen on interrogation of the outflow color Doppler signal.

When an RVAD is placed, similar views can be obtained of the inflow and outflow cannulae. The inflow cannula in the right atrium can often be best visualized in a bicaval view (~120 degrees looking towards the atrial septum). The outflow cannula in the pulmonary artery is quite anterior and is therefore more difficult to visualize using TEE. However, with sufficient image quality, a short-axis view at the base of the great vessels may allow for visualization of the outflow cannula.

De-airing

In every VAD placement, a thorough evaluation for intracardiac air is mandatory prior to unclamping the aorta and taking the patient off of cardiopulmonary bypass. Intracardiac air can be detected as small bubbles noted on TEE imaging. Common entrapment sites for air bubbles include the anastomotic sites of the great vessels, as well as the anterior portions of the left and right ventricles, the left atrium, and the left atrial appendage.

Figure 37.2. A: Apical four-chamber view in a 21-month-old patient status post placement of a Berlin EXCOR device in the left ventricular apex. Note the inflow cannula visualized in the apex. B: Transesophageal transgastric image with the left ventricle in a short-axis plane. Note the Berlin EXCOR device inflow cannula seen in cross section in the lumen of the left ventricle.

Right Ventricular Function

Meticulous monitoring of right ventricular and tricuspid valve function is critical in the immediate postoperative period following LVAD placement in children. If significant right ventricular dysfunction develops after LVAD placement, medical therapies to lower pulmonary vascular resistance such as nitric oxide may be considered. Increasing severity of tricuspid regurgitation may occur in concert with right ventricular dysfunction; however, it may also occur in the setting of a decompressed left ventricle with a leftward septal shift causing tricuspid valve malcoaptation. In this case, the degree of tricuspid regurgitation may improve with management of the VAD settings.

Figure 37.3. Transesophageal four-chamber view with color Doppler. The inflow cannula of a Berlin EXCOR device is seen in the left ventricular apex with normal Doppler flow.

ECHO ASSESSMENT OF MCS AFTER PLACEMENT

Inflow/Outflow Obstruction

In an appropriately functioning LVAD, the ventricular septum position should be relatively neutral, without favoring either ventricle. If the ventricular septum shifts into the unsupported ventricle, VAD circuit obstruction needs to be ruled out. Common causes of obstruction include kinking of the inflow or outflow cannulae, thrombus formation adjacent to a cannula, or abnormal inflow cannula position resulting in entrapment within ventricular trabeculations or valvar tissue. Cannula obstruction may occur in “underfilled” states including dehydration, sepsis, or pericardial effusion. Finally, valve destruction within pulsatile VADs can cause inflow or outflow obstruction.

On transthoracic echocardiography of VADs, the inflow cannula is best visualized in the standard apical four-chamber imaging plane (see Fig. 37.2), while the outflow cannula is often best seen in the right parasternal long-axis view. In some children, appropriate cannula flow can be assessed from subcostal or apical windows (see Fig. 37.5, Video 37.5). Adult studies have demonstrated specific cutoff values for diagnosis of inflow and outflow obstruction; these include spectral Doppler velocities >2.3 m/s (inflow) and >2.1 m/s (outflow) for pulsatile pumps, and >2.0 m/s for axial flow pumps. These Doppler velocities can be very difficult to obtain, due to poor imaging windows and the inability to orient the probe parallel to the cannula flow for accurate measurement.

Figure 37.4. Transesophageal four-chamber views before (top) and after (bottom) placement of a Berlin EXCOR device. Note the difference in left ventricular size between the images, as well as the “neutral position” of the ventricular septum in the lower image, indicative of appropriate LV decompression.

Thrombus Formation

Thrombus formation continues to be a long-term concern after MCS placement. In initial reports, up to 30% of patients on a Berlin Heart have had neurologic injury. Meticulous monitoring of coagulation parameters is essential, though it does not completely alleviate the risk. Echocardiography can be useful to detect thrombi which typically occur in the same locations as they occur prior to MCS initiation (Fig. 37.6). Transthoracic imaging is often sufficient in children who have good acoustic windows; however, TEE should be considered if transthoracic image quality is poor. The most common site of thrombus formation after MCS initiation is around the cannulae themselves. As such, care needs to be taken during device manipulation.

Figure 37.5. Transthoracic apical view demonstrating outflow cannula flow from a Berlin EXCOR device in a patient with hypoplastic left heart syndrome and a systemic right ventricle.

Valvar Regurgitation

In adults on MCS, the primary concern is the development of significant aortic insufficiency, which may result in inefficiency of the VAD circuit. Aortic valve regurgitation may worsen over time on a VAD. It has been theorized that this decompensation may be due to alterations in ascending aorta flow dynamics, endocarditis, or aortic dissection. These findings have not been observed in the pediatric population. Of note, mitral regurgitation may improve over time in children while on VAD support, likely secondary to the decompression of the ventricle.

Recovery of Function

In patients with reversible causes of cardiopulmonary failure such as myocarditis, recovery of function may occur. Signs of recovering ventricular function include reduced VAD flow rates, increased forward flow across the aortic valve (often demonstrated as improved aortic valve opening), and improvement in spontaneous function as assessed by echocardiography. In adult studies, predictors of successful recovery include a left ventricular end-diastolic dimension z-score that becomes <2 and an EF that becomes >45%.

Depending on the type of device used, different protocols for weaning off devices in the setting of improved intrinsic function and myocardial recovery have been established. Many VAD companies have devised specific guidelines for this purpose. The weaning protocols are typically methodical and slow by design, meant to have the patient prove their readiness to be off the VAD in order to avoid the complications of premature weaning.

Figure 37.6. Transesophageal imaging in a patient with a Berlin EXCOR device, with thrombus noted in the right atrium.

VADS IN THE OUTPATIENT SETTING

VADs are rarely used in the outpatient setting in the pediatric population. However, children on VADs outside the hospital may become more common in the future as more implantable devices become available for clinical use. As part of the standard routine echocardiographic assessment of adult outpatients on VADs, the following elements should be included: left and right ventricular size and function, ascending aorta size and presence/absence of dissection, intracardiac mass or thrombus, valvar abnormalities, ventricular septal position, and cannula position.

SPECIFIC MCS CONSIDERATIONS

ECMO

Imaging

Echocardiographic assessment of patients on or being considered for ECMO is essential to any cardiac ICU service. The potential indications for use of ECMO are vast (Table 37.2), and may include acute or chronic respiratory failure, acute or chronic cardiac failure, or combined cardiopulmonary failure. Many of the evaluations discussed above for pre-VAD assessment are also important in the assessment prior to placement on ECMO.

The specific imaging requirements for ECMO depend on the manner of support. The two primary types of support are venovenous (VV) ECMO and venoarterial (VA) ECMO. VV-ECMO is used in instances of respiratory failure when gas exchange is the primary goal; it does not provide circulatory support and thus requires reasonable cardiac function. In VV-ECMO, the blood is drained from the systemic veins (SVC, IVC, or both) and returned to the right atrium having been oxygenated through the circuit. VA-ECMO is similar to cardiopulmonary bypass and is primarily used in patients who need cardiac support or cardiac and respiratory support. It requires access to a primary artery with a large-bore catheter. In VA-ECMO, the blood is drained from the right atrium, and is returned to the systemic circulation either centrally by way of the ascending aorta or peripherally in a large artery (usually the carotid artery).

In patients being placed on ECMO outside of the operating room setting, “peripheral” cannulation is typically performed (Figs. 37.7 and 37.8, Video 37.7). In these cases, transthoracic echocardiographic guidance of cannula placement can be very helpful. In neonates, the venous cannula can be seen well from the subcostal coronal plane, entering the right atrium from the systemic veins. Companies producing ECMO equipment often have guidelines for echocardiographic guidance of cannula placement. In the setting of VA-ECMO initiation after failure to wean from cardiopulmonary bypass, the ascending aorta and right atrium are typically cannulated (“central cannulation”). This allows for larger-bore access and avoids damage to the neck vessels: this cannulation can often be well visualized using TEE in the operating room.

After placement on VA-ECMO, the LV dysfunction may be so severe that decompression is required. Decompression is typically performed in the interventional cardiac catheterization laboratory, and involves balloon atrial septostomy or blade septostomy to facilitate “off-loading” of the hypertensive left atrium. Transthoracic, TEE, or intracardiac echocardiography can be used to help guide these procedures.

Figure 37.7. High parasternal transthoracic image in an infant on ECMO with peripheral cannulation after congenital diaphragmatic hernia repair. Note the venous cannula (asterisk) in the superior vena cava directed to the right atrium, running lateral to the ascending aorta (Ao).

Finally, complications of ECMO can be evaluated by echocardiography even after removal of the patient from the circuit. Narrowing of the superior vena cava and intracardiac thrombi are not uncommon following this type of support. Moreover, fibrinous casts may remain within the atrium after removal of the venous access cannula. For these reasons, follow-up echocardiography is often ordered in patients who have been successfully weaned from ECMO.

Monitoring during Wean

In ECMO patients with reversible causes of cardiopulmonary failure, signs of cardiopulmonary recovery are important to recognize early, so that support can be discontinued as soon as possible. Signs of recovering ventricular function may include increased pulse pressure, improved aortic valve opening, maintenance of systemic venous oxygenation with weaning, and improvement in spontaneous function as assessed by echocardiography.

Unlike implantable VAD weaning protocols that may take several days to weeks, ECMO teams can do intermittent trials of weaning flow (“turn-downs”) to assess the hemodynamic response. Echocardiography, either by transthoracic or TEE, can be useful in these situations. During weaning, assessment of left and right ventricular function, atrioventricular valve function, aortic valve opening, and identification of potential thrombus is typically performed. In situations where ECMO is used for the critically ill neonate with severe pulmonary hypertension, echocardiography can be used during ECMO “turn-downs” to assess right-sided heart size and function, and to predict pulmonary artery pressures using the tricuspid regurgitant or pulmonary regurgitant velocities. In adult studies of patients on VA-ECMO for cardiogenic shock, ECMO can typically be successfully discontinued once the left ventricular ejection fraction is greater than 35%, and the aortic valve time-velocity integral is greater than 10 cm. Data are not available in children to predict the success of weaning trials. Weaning patients from VV-ECMO is more dependent on oxygenation and pulmonary compliance than circuit blood flow. Therefore, echocardiography is not typically used to monitor patients after placement. However, echocardiography can be useful to monitor the complications of ECMO, including thrombus formation and vessel obstruction.

Figure 37.8. Suprasternal transthoracic image in an infant on ECMO with peripheral cannulation after congenital diaphragmatic hernia repair. Note the color Doppler flow in the aortic arch representing outflow from the arterial cannula.

Impella Device

For patients receiving an Impella Recover device (Abiomed, Danvers, MA), TEE in the cardiac catheterization laboratory can help guide placement. Ideally, the inflow of the Impella should be 3–4 cm below the aortic valve, well into the ventricle, with the outflow located 1–2 cm above the aortic sinuses.

SUGGESTED READING

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Questions

1.Which of the following are the most common sites of thrombosis that need assessment prior to VAD placement in a pediatric patient?

A.Systemic ventricular apex and atrial appendage

B.Atrial septum and ventricular septum

C.Aortic arch and left pulmonary artery

D.Coronary sinus and right atrium

E.Septal and anterior leaflets of the tricuspid valve

2.Which of the following is NOT a common entrapment site for air bubbles during VAD placement?

A.Anastomotic sites of the great vessels

B.Left atrium

C.Left atrial appendage

D.Coronary sinus

E.Right ventricular outflow tract

3.Which of the following is true regarding inflow/outflow obstruction in pediatric VADs?

A.“Underfilled” states such as dehydration or sepsis are unlikely to cause cannula obstruction.

B.In an appropriately functioning LVAD, the ventricular septal position should be shifted into the unsupported ventricle.

C.If the ventricular septum shifts into the unsupported ventricle, VAD obstruction needs to be ruled out.

D.The inflow cannula is usually best visualized in the suprasternal notch view.

E.The outflow cannula is usually best visualized in the sub-costal sagittal plane.

4.Which of the following is true regarding ECMO subtypes?

A.VV-ECMO is used commonly in the setting of circulatory collapse.

B.VA-ECMO is required when respiratory failure occurs but cardiac function is normal.

C.VA-ECMO can be performed through a small peripheral artery

D.In VA-ECMO, the blood is typically drained from the systemic veins and returned to the right atrium.

E.In VV-ECMO, the blood is typically drained from the systemic veins and returned to the right atrium.

5.Which of the following is an indication for VV-ECMO?

A.Graft failure after heart transplant

B.Acute respiratory distress syndrome (ARDS)

C.Cardiogenic shock

D.Toxic myocardial depression

E.Incessant cardiac arrhythmias

Answers

1.Answer: A. Assessment of the heart for thrombus formation is important, and may affect where the surgeon is able to place the inflow cannula. Common locations for thrombus formation include the systemic ventricular apex (the most common insertion site for an inflow cannula), and the atrial appendages.

2.Answer: D. In every VAD placement, a thorough evaluation for intracardiac air is mandatory prior to unclamping the aorta and taking the patient off of cardiopulmonary bypass. Intracardiac air can be detected as small bubbles noted on TEE imaging. Common entrapment sites for air bubbles include the anastomotic sites of the great vessels, as well as the anterior portions of the left and right ventricles, the left atrium, and the left atrial appendage. The coronary sinus is not a common entrapment site for air.

3.Answer: C. In an appropriately functioning LVAD, the ventricular septal position should be relatively neutral, without favoring either ventricle. If the ventricular septum shifts into the unsupported ventricle, VAD circuit obstruction needs to be ruled out. Common causes of obstruction include kinking of the inflow or outflow cannulae, thrombus formation adjacent to a cannula, or abnormal inflow cannula position resulting in entrapment within ventricular trabeculations or valvar tissue. Cannula obstruction may occur in “underfilled” states including dehydration, sepsis, or pericardial effusion. Finally, valve destruction within pulsatile VADs can cause inflow or outflow obstruction.

On transthoracic echocardiography of VADs, the inflow cannula is best visualized in the standard apical four-chamber viewing plane, while the outflow cannula is often best seen in the right parasternal long-axis view. In some children, appropriate cannula flow can be assessed from subcostal or apical windows.

4.Answer: E. The two primary types of support are venovenous (VV) ECMO and venoarterial (VA) ECMO. VV-ECMO is primarily used in instances of respiratory failure when gas exchange is the primary goal; it does not provide circulatory support and thus requires reasonable cardiac function. In VV-ECMO, the blood is drained from the systemic veins (SVC, IVC, or both) and returned to the right atrium after being oxygenated through the circuit. VA-ECMO is similar to cardiopulmonary bypass and is primarily used in patients who need cardiac support or cardiac and respiratory support. It requires access to a primary artery with a large-bore catheter. In VA-ECMO, the blood is drained from the right atrium, and is returned to the systemic circulation either centrally by way of the ascending aorta or peripherally in a large artery (usually the carotid artery).

5.Answer: B. Acute respiratory distress syndrome is an indication where VV-ECMO may be useful, providing full oxygenation capabilities while not needing to provide circulatory support. If circulatory support is needed, the circuit can be converted to VA-ECMO. The other indications listed are all for VA-ECMO.