Objective. The present study aimed at determining the relative influences of tidal volume and thoraco-abdominal separation (relative thoracic and abdominal contribution to the tidal volume) on the respiratory induced intensity variations (RIIV) of the photoplethysmographic (PPG) signal. The effects were studied in two body positions.
Methods. Respiratory inductive plethysmography (RIP) was used for quantifying thoracoabdominal separation and for assessing tidal volumes. 10 subjects were trained to perform widely varying degrees of thoraco-abdominal separations at different tidal volumes. The relationship between the RIIV signal peak-to-peak value (measured at the forearm), and the tidal volume and separation was investigated in two body positions with the use of multiple linear regression.
Results. Larger tidal volume and more thoracic contribution to respiration were found to increase the RIIV peak-to-peak value (p<0.0005). In the supine position, tidal volume had a stronger influence than separation, and in the sitting position, the opposite was seen.
Conclusions. The effects on the RIIV signal from changes in thoraco abdominal separation and tidal volume are of similar magnitude. In the supine position, the influence of separation is less than in the sitting position, but the regression model fit is reduced. PPG is a promising technique for monitoring tidal volumes. However, in situations where the relative thoracic and abdominal contributions are likely to vary, the tidal volume information is less reliable.