Am. J. Respir. Crit. Care Med., Volume 161, Number 3, March 2000, 880-885

Respiratory Muscle Recruitment and Exercise Performance in Eucapnic and Hypercapnic Severe Chronic Obstructive Pulmonary Disease

MARIA MONTES de OCA and BARTOLOME R. CELLI

Division of Pulmonary and Critical Care Medicine, St. Elizabeth's Medical Center, Tufts University, Boston, Massachusetts; and Pulmonary Division, Hospital Universitario de Caracas, Universidad Central de Venezuela, Caracas, Venezuela

If chronic hypercapnia in patients with severe COPD occurs as a consequence of respiratory muscle (RM) weakness or fatigue, we would expect that ventilatory muscle recruitment (VMR) and exercise performance in stable hypercapnic patients would differ from those in eucapnic patients. We evaluated exercise performance and RM function at rest and during exercise in 19 eucapnic (PCO₂ 40 ± 3 mm Hg), and 13 hypercapnic (PCO₂ 52 ± 10 mm Hg) patients with severe COPD. A metabolic cart was used to determine E, O₂, CO₂, and HR. Gastric (Pg) and esophageal (Ppl) balloons were used to measure Pg, Ppl, and Pdi. Ventilatory muscle recruitment pattern (VMR) was partitioned using end-inspiratory and end-expiratory Pg and Ppl. Hypercapnic patients had lower FEV₁ (0.60 ± 0.24 versus 0.95 ± 0.31 L, p < 0.001), MVV (28 ± 11 versus 41 ± 13 L, p < 0.001), resting PO₂ (61 ± 11 versus 70 ± 11 mm Hg, p < 0.001), peak PO₂ (60 ± 20 versus 75 ± 22 mm Hg, p < 0.005), and E_max (24 ± 10 versus 32 ± 12 L/min, p < 0.001). Patients in both groups had similar FRC (5.7 ± 1.6 versus 5.0 ± 1.5 L), O₂max (0.58 ± 0.30 versus 0.76 ± 0.32 L/min), Watts (45 ± 48 versus 71 ± 59), E/MVV (88 ± 33 versus 79 ± 14), and HRmax (117 ± 17 versus 128 ± 18 beats/min). PI_max (67 ± 28 versus 65 ± 32 cm H₂O) and PE_max (98 ± 34 versus 96 ± 40 cm H₂O) were also similar in both groups. VMR (Pg/Ppl) at rest (0.28 ± 0.51 versus 0 ± 0.35) and during exercise (0.4 ± 0.2 versus 0.39 ± 0.15) was equally affected in both groups. We conclude that exercise capacity and ventilatory muscle recruitment are similarly impaired in eucapnic and hypercapnic patients with severe COPD. These findings make inability of the lung to increase ventilation and not respiratory muscle dysfunction a more attractive explanation for CO₂ retention in stable hypercapnic patients. Montes de Oca M, Celli BR. Respiratory muscle recruitment and exercise performance in eucapnic and hypercapnic severe chronic obstructive pulmonary disease.

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