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1 Respiratory Muscle Laboratory, Royal Brompton Hospital and 2 Dept of Respiratory Medicine and Allergy, Guy's, King's and St Thomas' School of Medicine, Hospital, King's College Hospital, London, UK, 3 Dept of Pulmonary Medicine, Sahlgrenska University Hospital, Gothenborg, Sweden
CORRESPONDENCE: N. Hart, Respiratory Muscle Laboratory, Royal Brompton Hospital, London, SW3 6NP, UK. Fax: 44 2073518939. E-mail: E-mail: drnhart@aol.com
Keywords: load and capacity, negative pressure threshold loading, respiratory muscle endurance
Received: May 28, 2001
Accepted July 13, 2001
N. Hart was supported by the Dorothy Osbourne Legacy.
Impaired respiratory muscle endurance (RME) could reduce exercise tolerance and contribute to ventilatory failure. The aim of the present study was to develop a clinically-feasible method to measure RME using negative-pressure inspiratory-threshold loading. It was hypothesized that endurance time (tlim) could be predicted by normalizing oesophageal pressure-time product (PTP) per total breath cycle (PTPoes) for maximum oesophageal pressure (Poes,max); the load/capacity ratio. The corresponding mouth pressures, PTPmouth and Pmouth,max were also measured.
The RME test was performed on 30 healthy subjects exposed to the same target pressure (70% of Poes,max). Eight patients with systemic sclerosis/interstitial lung disease were studied to assess the validity and acceptability of the technique.
Normal subjects showed a wide intersubject variation in tlim (coefficient of variation, 69%), with a linear relationship demonstrated between log tlim and PTPoes/Poes,max (r=0.88). All patients with systemic sclerosis/interstitial lung disease had normal respiratory muscle strength, but six out of eight had a reduction in RME.
In conclusion, endurance time can be predicted from the load/capacity ratio, over a range of breathing strategies; this relationship allows abnormal respiratory muscle endurance to be detected in patients. Oesophageal and mouth pressure showed a close correlation, thus suggesting that the test could be applied noninvasively.
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