Reference value for expiratory time constant calculated from the maximal expiratory flow-volume curve

BACKGROUND: The expiratory time constant (RC(EXP)), which is defined as the product of airway resistance and lung compliance, enable us to assess the mechanical properties of the respiratory system in mechanically ventilated patients. Although RC(EXP) could also be applied to spontaneously breathing patients, little is known about RC(EXP) calculated from the maximal expiratory flow-volume (MEFV) curve. The aim of our study was to determine the reference value for RC(EXP), as well as to investigate the association between RC(EXP) and other respiratory function parameters, including the forced expiratory volume in 1 s (FEV(1))/ forced vital capacity (FVC) ratio, maximal mid-expiratory flow rate (MMF), maximal expiratory flow at 50 and 25% of FVC (MEF(50) and MEF(25), respectively), ratio of MEF(50) to MEF(25) (MEF(50)/MEF(25)). METHODS: Spirometric parameters were extracted from the records of patients aged 15 years or older who underwent pulmonary function testing as a routine preoperative examination before non-cardiac surgery at the University of Tokyo Hospital. RC(EXP) was calculated in each patient from the slope of the descending limb of the MEFV curve using two points corresponding to MEF(50) and MEF(25). Airway obstruction was defined as an FEV(1)/FVC and FEV(1) below the statistically lower limit of normal. RESULTS: We retrospectively analyzed 777 spirometry records, and 62 patients were deemed to have airway obstruction according to Japanese spirometric reference values. The cut-off value for RC(EXP) was 0.601 s with an area under the receiver operating characteristic curve of 0.934 (95% confidence interval = 0.898–0.970). RC(EXP) was strongly associated with FEV(1)/FVC, and was moderately associated with MMF and MEF(50). However, RC(EXP) was less associated with MEF(25) and MEF(50)/MEF(25). CONCLUSIONS: Our findings suggest that an RC(EXP) of longer than approximately 0.6 s can be linked to the presence of airway obstruction. Application of the concept of RC(EXP) to spontaneously breathing subjects was feasible, using our simple calculation method.