End-tidal to arterial PCO2 ratio: a bedside meter of the overall gas exchanger performance

Matteo Bonifazi,Mattia Busana,Irene Steinberg,Simone Gattarello,Leif Saager,Paola Palermo,Davide Chiumello,Luciano Gattinoni,Konrad Meissner,Maria Michela Palumbo,Michael Quintel,Federica Romitti

doi:10.1186/s40635-021-00377-9

Matteo Bonifazi, Mattia Busana + Show 10 more

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https://doi.org/10.1186/s40635-021-00377-9

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Abstract

BackgroundThe physiological dead space is a strong indicator of severity and outcome of acute respiratory distress syndrome (ARDS). The “ideal” alveolar PCO2, in equilibrium with pulmonary capillary PCO2, is a central concept in the physiological dead space measurement. As it cannot be measured, it is surrogated by arterial PCO2 which, unfortunately, may be far higher than ideal alveolar PCO2, when the right-to-left venous admixture is present. The “ideal” alveolar PCO2 equals the end-tidal PCO2 (PETCO2) only in absence of alveolar dead space. Therefore, in the perfect gas exchanger (alveolar dead space = 0, venous admixture = 0), the PETCO2/PaCO2 is 1, as PETCO2, PACO2 and PaCO2 are equal. Our aim is to investigate if and at which extent the PETCO2/PaCO2, a comprehensive meter of the “gas exchanger” performance, is related to the anatomo physiological characteristics in ARDS.ResultsWe retrospectively studied 200 patients with ARDS. The source was a database in which we collected since 2003 all the patients enrolled in different CT scan studies. The PETCO2/PaCO2, measured at 5 cmH2O airway pressure, significantly decreased from mild to mild–moderate moderate–severe and severe ARDS. The overall populations was divided into four groups (~ 50 patients each) according to the quartiles of the PETCO2/PaCO2 (lowest ratio, the worst = group 1, highest ratio, the best = group 4). The progressive increase PETCO2/PaCO2 from quartile 1 to 4 (i.e., the progressive approach to the “perfect” gas exchanger value of 1.0) was associated with a significant decrease of non-aerated tissue, inohomogeneity index and increase of well-aerated tissue. The respiratory system elastance significantly improved from quartile 1 to 4, as well as the PaO2/FiO2 and PaCO2. The improvement of PETCO2/PaCO2 was also associated with a significant decrease of physiological dead space and venous admixture. When PEEP was increased from 5 to 15 cmH2O, the greatest improvement of non-aerated tissue, PaO2 and venous admixture were observed in quartile 1 of PETCO2/PaCO2 and the worst deterioration of dead space in quartile 4.ConclusionThe ratio PETCO2/PaCO2 is highly correlated with CT scan, physiological and clinical variables. It appears as an excellent measure of the overall “gas exchanger” status.

Highlights

The physiological dead space, which includes both the anatomical and alveolar dead space, is a strong indicator of severity and outcome of acute respiratory distress syndrome (ARDS) [1, 2]
The improvement of P ETCO2/PaCO2 was associated with a significant decrease of physiological dead space and venous admixture
When PEEP was increased from 5 to 15 cmH2O, the greatest improvement of non-aerated tissue, PaO2 and venous admixture were observed in quartile 1 of PETCO2/ PaCO2 and the worst deterioration of dead space in quartile 4

Summary

Introduction

The physiological dead space, which includes both the anatomical and alveolar dead space, is a strong indicator of severity and outcome of acute respiratory distress syndrome (ARDS) [1, 2]. The computation of the physiological dead space is based on the dilution of the ideal alveolar P CO2 (PACO2) This ideal P CO2, introduced by Riley, cannot be measured directly and it is assumed to be equal to the capillary P CO2 (PcCO2) [3] which leaves the ventilated/perfused pulmonary units [4]. To compute the alveolar dead space, we may assume that the end-tidal CO2 (PETCO2) is representative of the actual alveolar gases. The “ideal” alveolar PCO2, in equilibrium with pulmonary capillary PCO2, is a central concept in the physiological dead space measurement. As it cannot be measured, it is surrogated by arterial P CO2 which, may be far higher than ideal alveolar PCO2, when the right-toleft venous admixture is present. Our aim is to investigate if and at which extent the PETCO2/ PaCO2, a comprehensive meter of the “gas exchanger” performance, is related to the anatomo physiological characteristics in ARDS

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