Abstract
BackgroundThe expiratory time constant (RCEXP), 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 RCEXP could also be applied to spontaneously breathing patients, little is known about RCEXP calculated from the maximal expiratory flow-volume (MEFV) curve. The aim of our study was to determine the reference value for RCEXP, as well as to investigate the association between RCEXP and other respiratory function parameters, including the forced expiratory volume in 1 s (FEV1)/ forced vital capacity (FVC) ratio, maximal mid-expiratory flow rate (MMF), maximal expiratory flow at 50 and 25% of FVC (MEF50 and MEF25, respectively), ratio of MEF50 to MEF25 (MEF50/MEF25).MethodsSpirometric 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. RCEXP was calculated in each patient from the slope of the descending limb of the MEFV curve using two points corresponding to MEF50 and MEF25. Airway obstruction was defined as an FEV1/FVC and FEV1 below the statistically lower limit of normal.ResultsWe 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 RCEXP was 0.601 s with an area under the receiver operating characteristic curve of 0.934 (95% confidence interval = 0.898–0.970). RCEXP was strongly associated with FEV1/FVC, and was moderately associated with MMF and MEF50. However, RCEXP was less associated with MEF25 and MEF50/MEF25.ConclusionsOur findings suggest that an RCEXP of longer than approximately 0.6 s can be linked to the presence of airway obstruction. Application of the concept of RCEXP to spontaneously breathing subjects was feasible, using our simple calculation method.
Highlights
The expiratory time constant (RCEXP), 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
Given that RCEXP is defined as the product of airway resistance and lung compliance [2], the variable serves as a dynamic measurement that can reflect the mechanical properties of the respiratory system [3, 4]
Airway obstruction was defined as an forced expiratory volume in 1 s (FEV1)/
Summary
The expiratory time constant (RCEXP), 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. Given that RCEXP is defined as the product of airway resistance and lung compliance [2], the variable serves as a dynamic measurement that can reflect the mechanical properties of the respiratory system [3, 4]. RCEXP is obtained when maximal expiratory flows at low lung volumes are accurately ascertained, as the slope of the effort-independent portion is known to be expressed as the reciprocal of the time constant of the respiratory system [1, 7]. Unlike flow-based parameters, the value of RCEXP would contain qualitatively different information closely related to respiratory mechanics, given its ability to describe the features of the lung unit with its unique elasticity and capacity to resist airflow
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