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Utility of the Breathing Reserve Index at the Anaerobic Threshold in Determining Ventilatory-Limited Exercise in Adult Cystic Fibrosis Patients^*

^* From the Division of Pulmonary and Critical Care Medicine, Drexel University College of Medicine, Philadelphia, PA.

Correspondence to: William P. Sexauer, MD, FCCP, Division of Pulmonary/Critical Care Medicine, Drexel University College of Medicine, 3300 Henry Ave, Philadelphia, PA 19129; e-mail: William.Sexauer{at}drexel.edu

Objectives: Cardiopulmonary exercise testing in cystic fibrosis (CF) patients is useful to assess functional status and prognosis. Using the current interpretation guidelines, the utility of this testing will be limited in those patients who cannot exercise to a near-maximal level. This study investigates the utility of the breathing reserve index at the anaerobic threshold (BRIAT), which is defined as minute ventilation at the anaerobic threshold (AT)/maximum voluntary ventilation (MVV), to distinguish ventilatory-limited (VL) CF patients from nonventilatory-limited (NVL) CF patients.

Design: Exercise studies on 53 adult CF patients at baseline clinical status performed from 1993 to 1999 were reviewed, of which 40 met the inclusion criteria. The studies were performed via ramp protocol to the symptom-limited maximum on a cycle ergometer with breath-by-breath expired gas analysis. AT was determined noninvasively via the V-Slope method. The patients were classified as VL if they had abnormal spirometry findings, reduced exercise capacity, and a breathing reserve index at maximum exercise (BRImax) of 0.7. NVL patients had a normal BRImax and met the criteria for a maximal study.

Results: VL patients (21 patients) had significantly lower FVC, FEV₁, MVV, and body mass index than NVL patients (19 patients). The BRIAT for the VL group was significantly higher than that for the NVL group (p < 0.001). Logistic regression analysis revealed that BRIAT discriminated VL patients from NVL patients better than a variety of nonexercise variables tested. The BRIAT correlated extremely well with BRImax (r = 0.89; p < 0.01), FVC (r = -0.67; p < 0.001), FEV₁ (r = -0.76; p < 0.001), and FEV₁/FVC ratio (r = -0.683; p < 0.001). A BRIAT value of 0.29 distinguished VL CF patients from NVL CF patients with 95.2% sensitivity and 84.2% specificity.

Conclusions: The BRIAT assessed noninvasively correlates well with commonly used measurements of pulmonary function and accurately distinguishes CF patients with and without a ventilatory limitation to exercise. The BRIAT may have utility in the interpretation of exercise studies in CF patients who are unable to exercise to a maximal level.

Key Words: anaerobic threshold • cystic fibrosis • exercise testing • ventilation