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Use of the Gas Exchange Threshold to Noninvasively Determine the Lactate Threshold in Patients With Cystic Fibrosis^*

^* From the Departments of Physiology (Drs. Thin and McLoughlin) and Medicine and Therapeutics (Drs. Linnane, McKone, and FitzGerald), Conway Institute of Biomedical and Biomolecular Research, University College, Dublin; and Education and Research Center (Dr. Freaney) and Department of Respiratory Medicine and National Adult Cystic Fibrosis Unit (Dr. Gallagher), St Vincent’s University Hospital, Dublin, Ireland.

Correspondence to: Paul McLoughlin, MB, PhD, Department of Physiology, Conway Institute of Biomedical and Biomolecular Research, University College Dublin, Earlsfort Terrace, Dublin 2, Ireland; e-mail: paul.mcloughlin{at}ucd.ie

Objective: The anaerobic threshold (AT) is a submaximal index related to endurance exercise performance, which is usually determined by the measurement of blood lactate concentration during an incremental exercise test (lactate threshold [LT]). The LT, and thus the AT, can also be detected noninvasively in normal subjects by means of the gas exchange threshold (GET). This study was undertaken to validate the use of GET in patients with cystic fibrosis (CF) with a wide range of disease severity, and to assess the reproducibility of this index.

Methods: In patients with CF (FEV₁ range, 23 to 118% of predicted) and control subjects, gas exchange was measured breath by breath during the incremental exercise tests to allow determination of the GET. Arterialized-venous blood was sampled for determination of the LT. The GET and LT were determined in a blinded manner.

Results: The mean differences (GET - LT) for control subjects (n = 18) and patients with CF (n = 23) were - 40 mL/min and + 10 mL/min, respectively, neither being significantly different from zero. The limits of agreement were ± 550 mL/min and ± 410 mL/min, respectively. The mean test-retest differences in GET for control subjects (n = 14) and patients with CF (n = 12) were - 50 mL/min and 0 mL/min, respectively, neither being significantly different from zero; the respective limits of reproducibility were ± 450 mL/min and ± 350 mL/min.

Conclusions: This study demonstrates that in patients with CF, the GET can be used to obtain an unbiased estimate of the LT, and that the GET is reproducible.

Key Words: cystic fibrosis • gas exchange threshold • lactate threshold • noninvasive • submaximal

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