There has been a steady increase in the number of continuous-flow left ventricular assist devices (LVADs) implanted annually because the HeartMate II LVAD was approved for bridge to transplant in 2008 and destination therapy in 2010.1 Certain subsets of patients who undergo destination therapy LVAD may have outcomes competitive with transplantation at 2 years.2 Given improvements in LVAD technology, patients are increasingly likely to undergo LVAD placement and remain on LVAD therapy for prolonged periods of time. Left ventricular assist device–related complications are thus assuming increasing importance. In a recent Interagency Registry for Mechanically Assisted Circulatory Support analysis,3 >20% of deaths in LVAD patients were attributed to cardiovascular causes, including arrhythmias, right ventricular failure, and device failure. Accurate assessment of the physiology of the LVAD-heart complex is important in minimizing these adverse outcomes.
Imaging modalities that can assess LVAD function include transthoracic and transesophageal echocardiography, computed tomography, and LVAD angiography. Transthoracic echocardiography (TTE) can evaluate the LVAD inflow cannula position, aortic valve, right ventricle, and the interventricular septum.4–6 Transesophageal echocardiography is widely used intraoperatively to assess cannula position, adequacy of deairing, patent foramen ovale, and valvular abnormalities.7,8 Limitations of echocardiographic techniques include suboptimal views, artifacts, and operator dependence. Catheter-based techniques have been used to diagnose cannula obstruction, regurgitation, thrombus, and valvular regurgitation.,9–11 but the reported experience is limited and primarily with pulsatile pumps. Computed tomographic angiography (CTA) is another useful imaging modality in the management of patients with LVADs. Three-dimensional reconstruction, a wide field of view, and reproducibility of measurements are advantages of CTA over echocardiographic techniques, and the noninvasive evaluation afforded by CT is an advantage over catheter-based approaches in anticoagulated LVAD patients.
We describe the use of retrospectively gated CTA to diagnose partial inflow cannula obstruction in systole in two patients who had recurrent heart failure symptoms after LVAD implantation and discuss the role and limitations of gated CTA in LVAD assessment.
Materials and Methods
Retrospectively gated contrast-enhanced cardiac CTA was performed using a 64 detector Philips scanner (Philips Healthcare, Andover, MA) with the following parameters: collimation, 64 × 0.625 mm; rotation time, 400 msec; pitch, 0.2; tube voltage, 120 kV; tube current, 600–900 mAs (depending on patient’s body habitus); and slice thickness, 1.4 mm at 0.7 mm increment. Bolus tracking method was used to trigger the scan after reaching a predetermined threshold following intravenous contrast injection. The region of interest was placed in the proximal descending thoracic aorta away from the cannula insertion site to avoid false early triggering of scanning from high-attenuating inflow cannula in the left ventricle. Approximately 80–100 ml of Omnipaque 350 contrast followed by 40 ml of saline chaser was administered at 4.5 ml/second injection rate, and scans were acquired in craniocaudal direction from thoracic inlet to below the LVAD cannulae. Electrocardiogram (ECG) dose modulation was used during systolic phase of the cardiac cycle to reduce radiation exposure. All 10 phases of CTA data set were reconstructed at 10% interval (0–90%), and images were further analyzed using Brilliance 2.1 Philips as well as TeraRecon (TeraRecon Inc, Foster City, CA) thin client workstations.
A 47-year-old man with ischemic cardiomyopathy, prior coronary bypass grafting, uncontrolled DM, and hepatitis C progressed to New York Heart Association (NYHA) IV symptoms, despite optimal medical therapy and cardiac resynchronization. Transthoracic echocardiography showed left ventricular (LV) ejection fraction 25% and severe mitral regurgitation (MR). Coronary angiograms showed native 3 vessel disease and one patent graft, with poor targets for repeat revascularization. He underwent HeartMate II LVAD placement as destination therapy. Intraoperative transesophageal echocardiogram (TEE) was reported as unremarkable. Early postoperative course was uncomplicated, but 1 week after surgery, he started having high LVAD powers, up to 12 W, associated with drops in pulsatility index to 3. Transthoracic echocardiography showed poor LV function, persistent severe MR, and LVAD cannula in the apex without apparent obstruction. Right heart catheterization at a LVAD speed of 9,800 RPM showed a cardiac index (CI) of 2.2 L/min/m2, pulmonary capillary wedge pressure (PCWP) 25 mm Hg, and right atrial pressure (RAP) of 11 mm Hg.
Further evaluation included gated CTA. On the diastolic phase images, the inflow cannula position was appropriate (Figure 1). However, during systolic phase of the cardiac cycle, it was evident that the inflow cannula was abutting the posteromedial papillary muscle (Figure 2), with partial inflow cannula obstruction that was clearly seen on reconstructed 4D cine of the cardiac CT data (Movie 1, Supplemental Digital Content 1, http://links.lww.com/ASAIO/A30).
The coronary sinus lead on the defibrillator was turned off, and he was placed on β-blockers to help improve LV filling. He was initially asymptomatic with these episodes, so he was discharged home with close follow-up. Over the next few weeks, however, he started having frequent high-power episodes and associated dizziness and had two episodes of transient LVAD pump stoppage. Symptoms persisted despite controller exchange. Repeat right heart catheterization showed CI 2.18 L/min/m2 and PCWP 22 mm Hg. He underwent reoperation and repositioning of the inflow cannula 2 months after the initial operation. He did well after reoperation, with no further alarms or symptoms.
A 50-year-old man with a history of dilated cardiomyopathy presented with NYHA IV symptoms and multiple hospitalizations for heart failure requiring inotropes. He underwent HeartMate II LVAD placement. The intraoperative TEE reported optimal cannula position. He underwent sternal closure the next day. Postoperatively, he continued to have dyspnea, swelling, and intermittent episodes of high power to 13 W. Transthoracic echocardiography showed an enlarged right ventricle with moderate dysfunction and a small left ventricle with the LVAD inflow cannula adjacent to the septum. There was no Doppler evidence of cannula obstruction. Right heart catheterization showed PCWP 18 mm Hg, RAP 22 mm Hg, and CI 2.3 L/min/m2. Retrospectively gated CTA showed that the inflow cannula was trapped between the apical ventricular septum and an unusually hypertrophied lateral papillary muscle, resulting in cavity obliteration and cannula obstruction in systole (Figures 3 and 4; Movie 2, Supplemental Digital Content 2, http://links.lww.com/ASAIO/A31).
It was believed that surgical revision would entail significant LV reconstruction, and medical management was first attempted. Inotropes were weaned, and he was treated for right ventricular (RV) dysfunction with nitric oxide and sildenafil. Repeat TTE 2 weeks later showed a smaller right ventricle with improved function, and the cannula was not abutting the septum. Patient improved clinically, with good exertional capacity, no dyspnea, and no further alarms. He was discharged home in good condition. He presented 2 months later with elevated powers, hemolysis, and clinical evidence of heart failure. A repeat-gated CTA showed worsening of the inflow cannula obstruction with systolic collapse of the interventricular septum over the cannula opening causing significantly more systolic obstruction (Figures 5 and 6; Movies 3 and 4, Supplemental Digital Content 3 and 4, http://links.lww.com/ASAIO/A32 and http://links.lww.com/ASAIO/A33). He was listed for cardiac transplantation.
Perioperative management, patient selection, and device characteristics are broadly known to affect outcomes after LVAD placement.12 Persistent left heart failure, worsening right heart failure, arrhythmias, hypotension, and hemolysis can be indicators of suboptimal LVAD function or related complications, such as tamponade or aortic regurgitation. Accurate early recognition and management of these symptoms are important. Clinical evaluation, LVAD parameters and logs, ramp studies, laboratory studies for anemia and hemolysis, right heart catheterization, and imaging studies all have specific limitations and advantages. In an important subset of challenging patients, multiple modalities of assessment may be necessary to guide appropriate management. We describe two cases where laboratory and echocardiographic techniques were not diagnostic, but the findings were suggestive of inadequate unloading of the left ventricle, and CTA was used to arrive at the correct diagnosis.
Computed tomography has been used successfully to diagnose several common LVAD complications. Computed tomographic angiography can delineate inflow and outflow cannula position, abnormal angulation, cannula thrombi, pericannula fluid collection, pericardial effusion, and driveline position.13–16
Current continuous-flow LVADs have less motion than pulsatile pumps, which may minimize motion artifacts. In addition, gated and dynamic CTA can provide functional information, including right ventricular function17 and cardiac output.18 Limitations of CTA include the need to use contrast, radiation exposure, inability to visualize thrombus within the LVAD pump itself, and the limitations in transporting critically ill patients to the radiology suite.
Gating of the tomographic images to the ECG is commonly performed in cardiac CT. Gating minimizes cardiac motion artifacts and improves visualization of smaller structures. Both prospective and retrospective ECG gating are available. In prospective gating, imaging is done in axial scan mode at a predetermined interval from the preceding R wave, and usually image acquisition occurs in late diastole when the cardiac motion is minimal. Images are obtained every other heart beat with table moves in between; therefore, such an acquisition is also known as step and shoot mode. Because radiation exposure occurs only for a short period in diastole, this reduces the radiation exposure to the patient. Prospective gating can only be done in patients with low and regular heart rates, and because no systolic information is gathered, cardiac functional analysis cannot be performed. In retrospective gating, imaging is done in helical mode and continues throughout the cardiac cycle. Therefore, the radiation exposure to the patient is significantly higher. Because both diastolic and systolic data are captured, functional analysis of the CT data can be performed to determine ejection fraction and ventricular volumes. In both the patients described above, the partial inflow cannula obstruction was only visible in the systolic phase of the imaging with retrospective gating and led to important changes in clinical management. This information would have not been detected if only diastolic phase imaging had been done using prospective gating.
Since the early days of mechanical circulatory support, evaluation of human thoracic anatomy by CT has facilitated LVAD design. The apex position, LV axis orientation, rib orientation, chest wall thickness, aortic position, and chest wall shape were determined using CT and used in the design of the inflow cannula shape, pump position and curvature, and the outflow cannula position and alignment of a Nimbus LVAD.19,20 It was also recognized at that time that “accurate anatomical dimensions may be the key to successful clinical implantation” and that CT could be used clinically as a preoperative pump fit diagnostic test. More recent reports have focused on the use of CT and particularly CTA to diagnose problems that occur after LVAD implantation, but there is a major underused potential for using CT preoperatively to plan optimal incision site, cannula position and angulation, and pump housing pocket site. As multiple devices with varying designs get approved by regulatory agencies, some devices may be more anatomically appropriate for some patients, and this could be delineated by CT. Finally, patients’ RV function, LV geometry, and LV function may change over time on LVAD support, and this may change the inflow cannula position with respect to the ventricle. Aggressive use of heart failure medications and LVAD unloading may lead to reverse remodeling and a smaller LV chamber size. As more patients are living for years on LVAD support, the unique anatomic and physiologic changes in these patients may require periodic re-evaluation, particularly if there are clinical changes. Along with clinical and laboratory parameters, imaging studies such as CTA have an important role in the ongoing optimization of LVAD function. These present exciting areas for further research.
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Keywords:Copyright © 2013 by the American Society for Artificial Internal Organs
heart failure; left ventricular assist device; computed tomography