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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 (PCO2-a; PCO2-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 = (etPCO2-a PCO2-b)-(etPCO2-b PCO2-a)/PCO2-b-etPCO2-b-PCO2-a + etPCO2-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

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Michel, E., Peper, J. Non-invasive two-point estimation of arterial PCO2 and alveolar deadspace. Crit Care 4, P108 (2000). https://doi.org/10.1186/cc828

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Keywords

  • Adult Patient
  • Emergency Medicine
  • Treatment Response
  • Accurate Estimation
  • Full Text