COUGH: AN INDICATOR OF AIRWAY FUNCTION

Abstract

The flow-volume loop obtained during coughing resembles that during a forced vital capacity manoeuvre, with the maximal flow rate during forced expiration being exceeded by the peak flow of each cough effort. The extent by which maximum expiratory flow could be exceeded by coughing was investigated as a non-dimensional means of quantifying the degree of airflow limitation.

Fifty subjects over the age range 3 to 56 years were investigated. Subjects breathed through a pneumotachograph and signals of flow and volume were recorded and plotted using a Jaeger Bodytest. Each subject was asked to inspire to total lung capacity and cough out as forcefully as possible to residual volume. In addition each subject performed a forced vital capacity manoeuvre, and plethysmographic measurements of thoracic gas volume and airways resistance were made. The slope of expiratory peak flow during coughing showed a linear relationship with airway conductance measured plethysmographically in the 10 normal subjects below the age of 21 years (r = 0.854). The cough flow rates therefore provide information about airway function independently of plethysmographic measurements.

The ratio of maximal expired airflow (conventionally thought to be related to dynamic compression of airways) to peak airflow induced by coughing (i.e. release of static compression) at the same lung volume may represent a non-dimensional means of quantifying airflow limitation in individual subjects. This cough ratio was found to decrease during adult life, possibly reflecting the change in elastic properties of the airways with age.

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