Introduction Current techniques used to monitor Functional Residual Capacity (FRC) present significant clinical limitations which often restrict patients to those who are conscious, cooperative and mobile. There is a clinical need for an operator – independent device that is able to simultaneously and continuously monitor FRC in mechanically ventilated patients, the morbidly obese and infants. The InspiWave™ is one such device which utilises a sinusoidal Nitrous Oxide (N2O) tracer gas technique to non – invasively monitor various parameters of cardiopulmonary function, including end tidal alveolar volume (VA) and Bohr deadspace volume (VD). This study evaluates the performance of the InspiWave™ in estimating lung volume using two unique postural change protocols in healthy volunteers.
Method 5 subjects were asked to adopt 3 stationary postures (sitting upright, supine, 45° elevation) using a tilt table. 21 subjects adopted a different protocol, with a single transition phase between sitting and supine.
Results In the first protocol (n = 5), VA estimations were consistently smaller at supine than at sitting (31%). At 45° elevation, VD estimations were the smallest of all three postures. Comparison of absolute flow signal (∆∫Flow dt) data at the point of postural transition with VA estimations by the device (n = 21) yielded a Pearson’s correlation (r) of 0.453 ( p < 0.05).
Conclusion Estimations by the InspiWave™ demonstrated acceptable correlation between absolute flow signal change and device estimation, suggesting good accuracy. FRC changes by approximately 25% from sitting to supine, due to added pressure from abdominal contents on the diaphragm when gravity shifts with postural change.1 Data from this study closely mirrored this value (31%), demonstrating a good degree of device performance. VD estimations were smallest at 45° elevation of the three postures which may be due to airways being kept more open, consistent with respiratory benefits of the 45° “beach chair position”. Postural studies provide an alternative over conventional comparative studies with reference techniques (such as body plethysmography) in assessing device performance, when comparing variables which are not entirely equivalent.
Reference 1 Ibanez J, Raurich JM. Normal values of functional residual capacity in the sitting and supine positions. Intensive Care Med. 1982;8(4):173–177