The effect of body mass and gender on the accuracy of respiratory magnetometers for measurement of end expiratory lung volumes.

Respiratory magnetometers are increasingly being used in sleep studies to measure changes in end expiratory lung volume (EELV), including in obese obstructive sleep apnea (OSA) patients. Despite this, the accuracy of magnetometers has not been confirmed in obese patients, nor compared between genders. Thus, we compared spirometer-measured and magnetometer-estimated lung volume and tidal volume changes during voluntary end-expiratory lung volume changes of 1.5L, 1L, and 0.5L above, and 0.5L below, functional respiratory capacity (FRC) in supine normal weight (BMI<25kg/m²) and healthy obese (BMI>30kg/m²) men and women. Two different magnetometer calibration techniques proposed by Banzett (2) and Sackner (28) were assessed. Across all groups and target volumes, magnetometers overestimated spirometer-measured EELV by approximately 65mL (p<0.001) with no difference between techniques (p=0.07). The Banzett method overestimated the spirometer EELV change in normal weight women for all target volumes except +0.5L whereas no differences between mass or gender groups were observed for the Sackner technique. The variability of breath-to-breath measures of EELV was significantly higher for obese compared to non-obese subjects and was higher for the Sackner than Banzett technique. On the other hand, for tidal volume both calibration techniques underestimated spirometer measurements (p< 0.001), with the underestimation being more marked for the Banzett than Sackner technique (p=0.03), in obese than normal weight (p<0.001) and in men than in women (p=0.003). These results indicate that both body mass and gender affect the accuracy of respiratory magnetometers in measuring EELV and tidal volume.

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