Estimation of respiratory volumes from the photoplethysmographic signal. Part 2: a model study

A Windkessel model has been constructed with the aim of investigating the r espiratory-volume dependence of the photoplethysmographic (PPG) signal. Exp erimental studies show a correlation between respiratory volume and the pea k-to-peak value of the respiratory-induced intensity variations (RIIV) in t he PPG signal. The model compartments are organised in two closed chambers, representing the thorax and the abdomen, and in a peripheral part not dire ctly influenced by respiration. Cardiac pulse and respiration are created b y continuous adjustment of the pressures in the affected compartments. Toge ther with the criteria for heart and venous valves, the model is based on a set of 17 differential equations. These equations are salved for varying t horacic and abdominal pressures corresponding to different respiratory volu mes. Furthermore, a sensitivity analysis is performed to evaluate the prope rties of the model. The PPG signals are created as a combination of periphe ral blood flow and pressure. From these signals, the respiratory synchronou s parts are extracted and analysed. To study some important limitations of the model, respiratory type and rate are varied. From the simulations, it i s possible to verify our earlier experimental results concerning the relati onship between respiratory volume and the peak-to-peak value of the RIIV si gnal. An expected decrease in the amplitude of the respiratory signal with increased respiratory rate is also found, which is due to the lowpass chara cteristics of the vessel system. Variations in the relationship between tho racic and abdominal respiration also affect the RIIV signal. The simulation s explain and verify what has been found previously in experimental studies .