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

• Published in: BMC pulmonary medicine Vol. 19; no. 1; pp. 208 - 9
• Main Authors: Ikeda, Takamitsu, Yamauchi, Yasuhiro, Uchida, Kanji, Oba, Koji, Nagase, Takahide, Yamada, Yoshitsugu
• Format: Journal Article
• Language: English
• Published: London BioMed Central 11.11.2019; BioMed Central Ltd; BMC
• Subjects: Adolescent; Airway obstruction; Airway Obstruction - diagnosis; Airway Obstruction - physiopathology; Critical Care Medicine; Exhalation - physiology; Expiratory time constant; Feasibility Studies; Female; Humans; Intensive; Internal Medicine; Lung - physiopathology; Lung volume measurement; Male; Maximal expiratory flow-volume curve; Maximal Expiratory Flow-Volume Curves - physiology; Mechanical properties; Medicine; Medicine & Public Health; Pneumology/Respiratory System; Predictive Value of Tests; Preoperative Period; Pulmonary function test; Pulmonary function tests; Reference Values; Research Article; Respiratory critical care; Respiratory physiology; Retrospective Studies; ROC Curve; Spirometry; Surgery; Surgical Procedures, Operative; Japan; Spirometry; Pulmonary function test; Maximal expiratory flow-volume curve; Respiratory physiology; Expiratory time constant
• ISSN: 1471-2466; 1471-2466
• DOI: 10.1186/s12890-019-0976-6

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.