Acute unloading of the work of breathing extends exercise duration in patients with heart failure

OBJECTIVES: This study investigated whether maximal exercise performance can be improved by acutely decreasing the work of breathing in these patients. BACKGROUND: Exertional dyspnea is a frequent limiting symptom in patients with heart failure. It may result from increased work of breathing. METHODS: Fifteen patients with heart failure and nine age-matched normal subjects underwent two maximal exercise tests. Subjects exercised twice in randomized, single-blind manner using room air (RA) and a 79% helium/21% oxygen mixture (He). Respiratory gas analysis, Borg scale recordings of perceived dyspnea and near infrared spectroscopy of an accessory respiratory muscle were obtained during exercise. RESULTS: In normal subjects there was no significant difference in peak oxygen uptake (Vo2) ([mean ± SD] RA 38 ± 8 vs. He 35 ± 7 ml/kg per min), exercise duration (RA 724 ± 163 vs. He 762 ± 123 s) or peak minute ventilation (RA 97 ± 27 vs. He 97 ± 28 liters/min, all p = NS). Only three of nine control subjects thought that exercise with the He mixture was subjectively easier. In contrast, patients with heart failure exercised an average of 146 s longer with the He mixture (RA 868 ± 293 vs. He 1,014 ± 338, p < 0.01). Peak Vo2 (RA 19 ± 4 vs. He 18 ± 5 ml/kg per min) and peak minute ventilation (RA 53 ± 12 vs. He 53 ± 15 liters/min) were unchanged (both p = NS). The respiratory quotient at peak exercise was lower with the He mixture (RA 1.05 ± 0.08 vs. He 0.98 ± 0.06, p < 0.05). Thirteen of the 15 patients thought that exercise with the He mixture was subjectively easier (p < 0.02 vs. control group). CONCLUSIONS: In patients with heart failure, pulmonary factors, including respiratory muscle work and airflow turbulence, contribute to limiting exercise performance. Therapeutic interventions aimed at attenuating work of breathing may be beneficial.