A new instrument for the measurement of rib cage and abdomen circumference variation in respiration at rest and during exercise.

A simple and inexpensive new extensometer for measuring changes in chest wall circumference during human respiratory movements is presented. The instrument detects the delay between ultrasound emission and reception at opposite ends of two rubber tubes encircling the rib cage and abdomen. Assuming a two degree of freedom model of the chest wall and employing an isovolume procedure for determination of volume-motion coefficients, extensometer estimation of tidal volume (VT) from changes of rib cage and abdomen circumference was compared with spirometer measurements at rest and during exercise on a cycle ergometer (55-155 W) in six subjects and, in four of them, on a treadmill (4-12 km.h-1). In three subjects hypercapnic hyperpnoea at rest was also studied. The slopes of the linear relationship between extensometer and spirometer VT (litres) averaged 0.9967 (SD 0.0117) (r2 = 0.995-0.998; n = 90-143) for cycle ergometer exercise, 1.0072 (SD 0.0078) (r2 = 0.991-0.998; n = 75-93) for treadmill exercise and 0.9942 (SD 0.0188) (r2 = 0.997-0.998; n = 18-25) for hypercapnic hyperpnoea. In all instances the slope of the regression line was consistent with the model of the identity line (slope = 1). The changes in end-expiratory lung volume between respiration at rest and during exercise were determined by the extensometers, and were nearly identical (98.4% on average) to those measured with the spirometer (r2 = 0.945; n = 24). It is concluded that determination of chest wall circumference with this new instrument is suitable for quantitative measurement of ventilation and lung volume variations in humans under most physiological conditions.