Abstract
Objective
In a previous study in anesthetized animals, the slope of percent decreases in exhaled CO2 versus percent decreases in cardiac output (\( \dot{\rm Q}{\sc{T,}} \) inflation of vena cava balloons) was 0.73. To examine the mechanisms underlying this exhaled CO2-\( {\dot{\hbox{Q}}{\sc T}} \) relationship, an iterative numerical analysis computer model of non-steady state CO2 kinetics was developed.
Methods
The model consisted of a large peripheral tissue compartment connected by venous return and \( {\dot{\hbox{Q}}{\sc T}} \) to a small central pulmonary compartment. Equations were developed to describe the movement of CO2 in this system. Decreases in \( {\dot{\hbox{Q}}{\sc T}} \) were accompanied by experimentally measured increases in alveolar dead space fraction (Vd alv/Vt alv), generated by decreased pulmonary vascular pressure during the \( {\dot{\hbox{Q}}{\sc T}} \) decrease.
Results
When the model was perturbed by a 40% decrease in \( {\dot{\hbox{Q}}{\sc T}} \) and an increase in Vd alv/Vt alv from 5 to 20.6%, average alveolar expired \( {\hbox{P}}_{{{\rm{CO}}_{ 2} }} \) (\( {\hbox{P}}{{\overline{\sc{AE}}_{{{\rm{CO}}_{ 2} }} }}\)) decreased from 37.5 to 29.4 mm Hg, similar to the animal experiments. Due to the high peripheral tissue compliance for CO2, the computer model demonstrated that, after a decrease in \( {\dot{\hbox{Q}}{\sc T}} \), at least 1 h was required for compartment CO2 stores to approach a new equilibrium state.
Conclusions
The numerical analysis computer model helps to delineate the mechanisms underlying how decreased \( {\dot{\hbox{Q}}{\sc T}} \) resulted in decreased exhaled CO2. The model permitted deconvolution of the effects of simultaneous variables and the interrogation of parameters that would be difficult to measure in actual experiments.
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Breen PH. How do changes in exhaled CO2 measure changes in cardiac output? A numerical analysis model.
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Breen, P.H. How do changes in exhaled CO2 measure changes in cardiac output? A numerical analysis model. J Clin Monit Comput 24, 413–419 (2010). https://doi.org/10.1007/s10877-010-9263-z
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DOI: https://doi.org/10.1007/s10877-010-9263-z