Tidal Volume and Respiratory Timing Derived from a Portable Ventilation Monitor

Study Objectives: To determine the accuracy of a portable magnetometer designed to measure tidal volume (VT), inspiratory time (TI), and expiratory time (TE).

Participants: Fourteen healthy subjects.

Design: Subjects breathed over a sixfold range of VTs while at rest (sitting and standing) and during treadmill exercise. We then compared VT, TI, and TE measured by magnetometry (VTmag, TImag, and TEmag) with VT, TI, and TE measured by spirometry (VTspiro, TIspiro, and TEspiro, respectively).

Setting: Pulmonary function and exercise physiology laboratories.

Measurements And Results: The sternal-umbilical distance and the anteroposterior displacements of the rib cage and abdomen were measured with two pairs of magnetometer coils. VT was calculated from the sum of these three signals, and was simultaneously measured using a spirometer or flow meter. A total of 1,111 breaths were analyzed for the resting condition, and 1,163 breaths were analyzed for the exercise condition. We found that VTmag was highly correlated with VTspiro at rest (R2 = 0.90) and during exercise (R2 = 0.79) for pooled data. The slope of this relationship approached the line of identity. The mean percentage differences between VTmag and VTspiro were 10.1 +/- 6.6% at rest and 13.5 +/- 8.6% with exercise. By Bland-Altman analysis, the mean differences between VTmag and VTspiro were 38 mL at rest with changes in posture, and 182 mL during exercise. TImag and TIspiro values and TEmag and TEspiro values also were highly correlated (R2 = 0.97 and R2 = 0.95, respectively) for pooled data.

Conclusion: A portable magnetometer system can give useful measures of VT, TI, and TE over a wide range of VTs in sitting, standing, and exercising subjects.