Non-invasive two-point estimation of arterial PCO₂ and alveolar deadspace

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Introduction

Often end-tidal PCO2 (etPCO₂) gives a good estimate of arterial PCO₂ (PaCO₂). However, there are conditions, namely diseases with pulmonary ventilation/perfusion mismatch, where PaCO₂ is severely underestimated.

Objective

We aimed to develop a method for the accurate non-invasive estimation of PaCO₂ from etPCO₂.

Method

The method is based on the model assumption that etPCO₂ is a mixture of PCO₂ from both the ventilated and perfused alveoli, and the ventilated but non-perfused ones ('alveolar deadspace'). Knowing the etPCO₂ under conditions of inspiratory gases with different CO₂ content (PCO_2-a; PCO_2-b), it is possible to calculate the PCO₂ of the ventilated and perfused alveoli (PCO₂alv). Knowledge of PCO₂alv allows to calculate the fraction of alveolar deadspace ventilation. As long as para-alveolar (i.e. cardiac) shunting is less than 20%, PCO₂alv accurately reflects PaCO₂.

PCO₂alv = (etPCO_2-a PCO_2-b)-(etPCO_2-b PCO_2-a)/PCO_2-b-etPCO_2-b-PCO_2-a + etPCO_2-a

The model was applied on ventilated adult patients while inpiring different levels of PCO₂. Estimates of PaCO₂ and alveolar deadspace were compared to the results of invasive determination.

Results

Preliminary results indicate a good correlation between invasively and non-invasively determined PaCO₂and alveolar deadspace (Table 1).

Conclusions

Using gas void of CO₂ as one of the two inspiratory gases further simplifies the formula. Our preliminary findings show that a difference in the levels of inspired PCO₂ of at least 3 kPa gives an estimate of PaCO₂ with an accuracy of 10% or better. We provide a non-invasive method for the accurate estimation of PaCO₂ and alveolar deadspace ventilation. We suggest its implementation in ventilators for the close monitoring of pulmonary treatment response.

Table 1