Anatomical dead space, ventilatory pattern, and exercise capacity in chronic heart failure.

BACKGROUND Patients with chronic heart failure have an excessive ventilatory response to exercise, characterised by an increase in the slope of the relation between ventilation and carbon dioxide production (VE/VCO2 slope). Patients have an altered respiratory pattern with an increased respiratory rate (f) at a given tidal volume (VT), which may result in increased anatomical dead space ventilation. METHODS The ventilatory responses in 88 patients with chronic heart failure and 43 age matched controls during maximal incremental treadmill exercise were analysed. Peak oxygen consumption (VO2), VE/VCO2 slope, and the slope of the relation between f and VT were derived. Anatomical dead space was estimated from a standard formula and anatomical dead space ventilation calculated. RESULTS Peak VO2 was greater (mean (SD)) (33.2 (8.5) v 19.4 (6.7) ml/min/kg; P < 0.001) and the VE/VCO2 slope lower in the controls (25.96 (4.16) v 35.14 (9.80); P < 0.001). During matched submaximal exercise VT was higher (1.97 (0.92) v 1.68 (0.62) 1; P < 0.05) and flower in the controls (18.23 (6.48) v 24.28 (7.58); P < 0.001). At peak exercise there was no difference in f, but VT was higher in the controls (2.66 (0.97) v 1.90 (0.61) 1; P < 0.001). The VT/f slope was the same (0.04 (0.04)) in both groups. The intercept of the relation was greater for the control group (1.31 (1.28) v 0.59 (0.83); P < 0.001). Anatomical dead space ventilation was lower in the controls at submaximal work load (4.17 (1.56) v 5.58 (1.93) l/min; P < 0.001). At peak exercise anatomical dead space ventilation was the same in both groups, but was lower expressed as a percentage of total VE in the control group (9.8 (3.3) v 13.5 (4.0); P < 0.001). There were weak relations within the heart failure group alone between VT/f slope and peak VO2 and VE/VCO2 slope. CONCLUSIONS The relation between anatomical dead space ventilation and VE/VCO2 slope is expected: as f increases, so do VE/VCO2 slope and anatomical dead space ventilation. The VT/f slope was the same in patients with chronic heart failure and controls, so change in respiratory pattern cannot explain the increase in VE/VCO2 slope. The stimulus causing the increased f has yet to be identified.