Clinical refinement of the Automatic Lung Parameter Estimator (ALPE)
Supplementary material
The mathematical model included in the ALPE method is shown in figure 1. The model is a three compartment model with two ventilated and perfused compartments representing gas exchange in the lungs, and the third representing pulmonary shunt. In the right hand side of figure 1, the previously published [1] model equations describing transport of oxygen from inspired gasses into the tissues are given with explanations in the figure legend. The model includes three parameters: pulmonary shunt, the fraction of blood flow not contributing to gas exchange; fA2, the fraction of alveolar ventilation distributed to one of the two ventilated compartment; and f2, the fraction of non-shunted blood flow distributed to one of the two ventilated compartments.
Figure 1: The mathematical model of oxygen transport (from 1 with permission). The model has three compartments, where two are ventilated and perfused representing gas exchange in the lungs, and the third representing pulmonary shunt. The equations describe the transport of oxygen at steady state from the a ventilator or air into the tissues: 1-4) oxygen flow into the alveoli and blood (VO2) in total and addition from each compartment; 5) total expired oxygen fraction (FEO2); 6-7) Oxygen partial pressure (PcO2(1), PcO2(2)) in the compartments; 8) drop in O2 partial pressure from expired gas to capillary blood; 9) mixed concentration of arterial blood (CaO2); 10-14) relationship between oxygen partial pressure (PO2), saturation (SO2) and concentration (CO2) in the capillary compartments calculated from the oxygen dissociation curve (ODC) and blood variables; 15-16) concentration of oxygen in the lung capillary compartments (CcO2(1), CcO2(2)) combining venous concentration (CvO2) and the increase in oxygen concentration resulting from alveolar equilibration; 17) venous oxygen concentration (CvO2) combining arterial oxygen concentration (CaO2) and the drop in oxygen concentration as a result of consumption in the tissues.
References
1. Kjaergaard S, Rees S, Malczynski J, Nielsen JA, Thorgaard P, Toft E, Andreassen S: Non-invasive estimation of shunt and ventilation-perfusion mismatch. Intensive Care Med 2003, 29(5):727-734.