Simultaneous Quantification of Chest Wall Distortion by Multiple Methods in Preterm Infants

Abstract

Three methods for volume partitioning were simultaneously used to study 8 preterm infants: (1) miniature magnetometers, which measured the anteroposterior diameter of the chest wall and abdomen, (2) mercury-in-rubber strain gauges, which measured circumference changes, and (3) an inductance plethysmograph (Respitrace), which reportedly measures volume changes of a coil of fine wire placed around the chest and abdomen. Volume changes of the chest wall and abdomen were calibrated against lung volume changes measured with a pneumotachograph using a multiple linear regression (MLR) method, and the partitioning was checked by airway occlusion. The abdominal signals of the 3 systems were similar. The chest wall signals from the strain gauges and inductance plethysmograph were also similar but differed from those of the magnetometers (in 2 of the infants) primarily due to differences in the phase relationship of the signals. The 95% confidence limits for estimates of changes in lung volume by the MLR method in most infants was .+-. 5% using all 3 systems. The airway occlusion check, however, could not corroborate the accuracy of volume partitioning by the magnetometers in half the infants. The minute volume displacement of the diaphragm was the same for the strain gauges and the inductance plethysmograph, ranging from 48 to 127% of the pulmonary minute ventilation, and was significantly underestimated by the magnetometers (p < 0.001). These results suggest that distortion of the sternal portion of the chest wall, as detected by the magnetometers, represents less volumetric loss than the volume lost for the entire chest wall due to paradoxical breathing. This part of the chest wall may be supported by the area of apposition of the diaphragm to the chest wall, unlike the more cephalad portion which is distorted more by the negative intrapleural pressure.