Assessment of submaximal exercise capacity in patients with left ventricular assist devices

Long-term implantable left ventricular assist devices (LVADs) are increasingly used as a bridge to cardiac transplantation and may be used as long-term therapy for end-stage heart failure. After insertion of an LVAD, patients frequently become ambulatory. Sensors contained within these devices can measure the hemodynamic demands of submaximal exercise. In this study, we performed serial assessment of submaximal exercise capacity in patients with LVADs early (< 3 months), mid (3 to 6 months), and late (> 6 months) after device implantation to determine whether submaximal exercise capacity increased over time and whether this was related to improvements in the cardiac output response. Moreover, we compared the sub-maximal exercise capacities of these patients with those of normal subjects and patients with mild to severe heart failure.

An encouraged 6-minute walk test with metabolic measurements was used to assess submaximal exercise capacity in 14 patients with LVADs, 20 patients with mild to moderate congestive heart failure (CHF), 14 patients with severe heart failure dependent on dobutamine (DB), and 6 normal subjects. Cardiac output measurements at rest and during exercise were obtained in the patients with devices. Distance walked was significantly greater for LVAD patients than DB patients and was similar to that for patients with mild CHF (LVAD, 1562 +/- 404; DB, 948 +/- 241; and CHF, 1358 +/- 278 ft; P < .01). Vo2 was also greater in the LVAD than DB or CHF patients (LVAD, 16.3 +/- 6.5; DB, 9.8 +/- 4.8; and CHF, 11.2 +/- 2.0 mL.kg-1.min-1; P < .05). Vo2 (23.4 +/- 7.4 mL.kg-1.min-1) and distance walked (2142 +/- 408 ft) in normal subjects was significantly greater than for all patient groups (all P < .01). Serial assessment of submaximal exercise capacity in LVAD patients demonstrated continued sustained improvement over time (early, 1261 +/- 341; mid, 1538 +/- 345; and late, 1867 +/- 265 ft; P < .05). However, peak cardiac output response was unchanged (early, 8 +/- 1.3; mid, 8.6 +/- 1.4; and late, 8.6 +/- 1.4 L/min; P = NS).

The submaximal exercise capacity of LVAD patients is comparable to that of patients with mild CHF but significantly better than that of dobutamine-dependent patients. Hemodynamic and metabolic assessment during 6-minute walk tests demonstrates that in patients with heart failure and LVAD, cardiovascular demands are generally > 85% of maximum. A significant sustained improvement in submaximal exercise capacity occurs with chronic LVAD therapy. Lack of alteration of the cardiac output response to exercise suggests that this functional improvement results from peripheral mechanisms.