Our results complement those of studies of first-generation pulsatile LVADs which demonstrated improvements in peak oxygen consumption of 3–4 ml/kg/min, 3–6 months after implantation.13–15 More recently, a few small studies demonstrated similar results with continuous-flow LVADs achieving peak oxygen consumption of 15–16 ml/kg/min, though none of these cohorts had preimplantation data for comparison.16–18 Two small studies with paired pre- and postimplant VO2 showed similarly small increases in 2 max">VO2 max after 3–6 months of support.19,20 Recently, Nahumi et al.21 measured 2 max">VO2 max and 6 min walk distance in 26 patients supported with a continuous flow LVAD. When compared with a matched cohort of chronic heart failure patients, those supported with an LVAD were able to walk a longer distance in 6 min (80 m), but failed to reach a similar peak oxygen consumption (2 max">VO2 max 12.4 vs. 15.0 ml/kg/min). This suggests that measurable differences in exercise capacity exist between heart failure and LVAD-supported patients with the latter demonstrating a greater submaximal capacity for any given level of VO2.
Patients supported with an LVAD are similar to heart transplant recipients in that they, too, fail to normalize VO2 to predicted values. Within 6–12 months after transplantation, 2 max">VO2 max improves significantly to approximately 20–21 ml/kg/min (60% of predicted values), but fails to normalize despite the fact that patients anecdotally report excellent quality of life and experience greater functional capacity.22,23 This apparent disconnect between modest improvements in peak oxygen consumption and marked improvements in self-reported functional class suggest that improvements in the latter are less dependent upon improvements in cardiac output. Nonetheless, the inability to increase peak VO2 to predicted levels may be a result of persistent right ventricular dysfunction, pulmonary hypertension, changes in skeletal muscle and deconditioning, as well as an inability to sufficiently augment cardiac output during maximal exercise.
In our cohort, no preimplantation hemodynamic parameters, including right atrial pressure, pulmonary artery pressure, or right ventricular stroke work index correlated with postimplantation VO2. This suggests that peripheral mechanisms such as skeletal muscle atrophy or diaphragmatic weakness may have a more significant effect limiting peak oxygen consumption independent of cardiac output.
In advanced heart failure patients requiring mechanical circulatory support, the restoration of cardiac output and adequate peripheral blood flow is necessary but insufficient for full recovery. The failure to objectively normalize VO2 suggests there are ongoing physiologic derangements which LVAD therapy is insufficiently able to reverse. This may be less of a concern in short-term support, but more important when trying to achieve durable, long-term results in destination therapy patients. Ongoing physical reconditioning and training regimens improve exercise capacity in patients with LVADs. Efforts should be directed toward treating, medically optimizing, and, if possible, reversing, other significant comorbidities in these patients.
Our study was limited by its retrospective nature and the possibility of selection bias and confounding. The number of patients studied was small when viewed as a percentage (35%) of all patients receiving LVADs at our center. Only 10 patients had matched studies before and after LVAD placement, and these studies did not include VE/VCO2 slope data for comparison. Because many of our patients who completed exercise testing had other noncardiac comorbidities which could limit exercise, these conditions may have contributed to submaximal tests. Specifically regarding the low RER, our use of the Innocor device may explain the occasionally observed low RER as similar findings have been corroborated in verbal and written communication with other centers using this inert gas rebreathing system. Despite the lower than expected RER, 35 out of 37 patients studied reached anaerobic threshold, and the two who did not were excluded from the analysis. RVEF could not be quantified as this was not systematically recorded in our echocardiography studies.
Peak oxygen consumption improves significantly with continuous-flow LVAD support but fails to normalize to predicted age-, gender-, and BMI-matched values, despite improvement in NYHA functional class. There is no difference in postimplantation 2 max">VO2 max or VE/VCO2 between heart failure etiologies or when stratifying by Intermacs levels. Further investigation is required to elucidate the mechanisms responsible for the profound symptomatic improvement despite an inability to normalize peak oxygen consumption.
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