How do changes in exhaled CO₂ measure changes in cardiac output? A numerical analysis model

Area of Science:

• Physiology
• Computational Modeling
• Respiratory System

Background:

• A prior study established a correlation between decreased cardiac output (Q(T)) and reduced exhaled CO₂ in animals.
• Understanding the underlying physiological mechanisms of this relationship is crucial for interpreting respiratory gas exchange.

Purpose of the Study:

• To develop and utilize a numerical analysis computer model simulating non-steady state CO₂ kinetics.
• To elucidate the mechanisms linking decreased cardiac output to reduced exhaled CO₂.

Main Methods:

• A computational model was created, featuring a peripheral tissue compartment linked to a central pulmonary compartment.
• Equations described CO₂ movement, incorporating experimentally measured increases in alveolar dead space fraction (VD/VT) with decreased Q(T).

Main Results:

• A 40% Q(T) decrease and increased VD/VT simulated a drop in average expired PCO₂ (PAE(CO₂)) from 37.5 to 29.4 mm Hg.
• The model indicated that high peripheral tissue CO₂ compliance necessitates approximately 1 hour for CO₂ stores to reach equilibrium after Q(T) reduction.

Conclusions:

• The developed computer model successfully delineated the mechanisms by which decreased Q(T) reduces exhaled CO₂.
• This modeling approach allowed for the separation of effects from simultaneous variables and the investigation of parameters challenging to measure directly.