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Effect of pattern and severity of respiratory muscle weakness on carbon monoxide gas transfer and lung volumes

¹ Respiratory Muscle Laboratory, and ² Lung Function Unit, Royal Brompton Hospital, and ³ Dept of Respiratory Medicine & Allergy, Guy's, King's and St Thomas' School of Medicine, King's College Hospital, London, UK

CORRESPONDENCE: N. Hart, Respiratory Muscle Laboratory, Royal Brompton Hospital, Fulham Road, London, SW3 6NP, UK. Fax: 44 2073518939. E-mail: drnhart@aol.com

Keywords: carbon monoxide transfer, lung volumes, respiratory muscle weakness

Received: October 16, 2001
Accepted January 14, 2002

N. Hart was funded by the Dorothy Osbourne Legacy/Royal Brompton Hospital Trust Award and A.H. Nickol was funded by the British Lung Foundation.

In clinical practice, an elevated carbon monoxide (CO) transfer coefficient (K_CO) and restrictive ventilatory defect are taken as features of respiratory muscle weakness (RMW). However, the authors hypothesised that both pattern and severity of RMW effect gas transfer and lung volumes.

Measurements of CO transfer and lung volumes were performed in patients with isolated diaphragm weakness (n=10), inspiratory muscle weakness (n=12), combined inspiratory and expiratory muscle weakness (n=5) and healthy controls (n=6).

Patients with diaphragm weakness and inspiratory muscle weakness had reduced total lung capacity (TLC) (83.6% predicted and 68.9% pred, respectively), functional residual capacity (FRC) (83.9% pred and 83.6% pred) and transfer factor of the lung for CO (T_L,CO) (86.2% pred and 66.2% pred) with increased K_CO (114.1% pred and 130.2% pred). Patients with combined inspiratory and expiratory muscle weakness had reduced TLC (80.9% pred) but increased FRC (109.9% pred) and RV (157.4% pred) with decreased T_L,CO (58.0% pred) and K_CO (85.5% pred).

In patients with diaphragm weakness, the increase in carbon monoxide transfer coefficient was similar to that of normal subjects when alveolar volume was reduced. However, the increase in carbon monoxide transfer coefficient in inspiratory muscle weakness was often less than expected, while in combined inspiratory and expiratory muscle weakness, the carbon monoxide transfer coefficient was normal/reduced despite further reductions in alveolar volume, which may indicate subtle abnormalities of the lung parenchyma or pulmonary vasculature. Thus, this study demonstrates the limitations of using carbon monoxide transfer coefficient in the diagnosis of respiratory muscle weakness, particularly if no account is taken of the alveolar volume at which the carbon monoxide transfer coefficient is made.