A SIMPLE AND RELIABLE METHOD TO CALIBRATE RESPIRATORY MAGNETOMETERS AND RESPITRACE

We present a simple and reliable method to calibrate respiratory magne tometers and Respitrace to infer respiratory volume changes. As in ear lier methods, we assume two degrees of freedom in the chest wall and t hat volume displacement depends linearly on surface motion at the rib cage and abdomen. Because the area of the rib cage is larger, a given motion of its surface produces a greater lung volume change; therefore , the rib cage motion signal is given a larger gain before the two sig nals are added to estimate volume. In contrast to earlier methods, we use a ''standard ratio'' to weight relative gains of the rib cage and abdominal signals for all subjects rather than determining a gain rati o for each individual subject. Our procedure does not require subjects to perform the sometimes difficult isovolume maneuvers used in the ca libration method of Konno and Mead (J. Appl. Physiol. 22: 407-422, 196 7), does not require statistical computation used in the multiple-brea th linear regression method, and does not produce the occasional subst antial errors in gain ratio that may occur with the other methods. Whe n magnetometers are used, the standard ratio is 4:1 (rib cage-to-abdom en); when Respitrace is used, the standard ratio is 2:1. In 11 subject s, calibration with standard ratios was as accurate as the isovolume a nd linear regression techniques. Accuracy during normal breathing was nearly always within 10% (median 2%), but occasional large errors occu rred with both instruments.