End-Tidal-to-Arterial P CO 2 Ratio as Signifier for Physiologic Dead-Space Ratio and Oxygenation Dysfunction in Acute Respiratory Distress Syndrome

BACKGROUND:

The ratio of end-tidal CO
2
pressure to arterial partial pressure of CO
2
(




P



ETCO

2





/P




aCO

2






) was recently suggested for monitoring pulmonary gas exchange in patients with ARDS associated with COVID-19, yet no evidence was offered supporting that claim. Therefore, we evaluated whether




P



ETCO

2





/P




aCO

2






might be relevant in assessing ARDS not associated with COVID-19.



METHODS:

We evaluated the correspondence between




P



ETCO

2





/P




aCO

2






and the ratio of dead space to tidal volume (V
D
/V
T
) measured in 561 subjects with ARDS from a previous study in whom




P



ETCO

2






data were also available. Subjects also were analyzed according to 4 ranges of




P



ETCO

2





/P




aCO

2






representing increasing illness severity (≥ 0.80, 0.6–0.79, 0.50–0.59, and < 0.50). Correlation was assessed by either Pearson or Spearman tests, grouped comparisons were assessed using either ANOVA or Kruskal-Wallis tests and dichotomous variables assessed by Fisher Exact tests. Normally distributed data are presented as mean and standard deviation(SD) and non-normal data are presented as median and inter-quartile range (IQR). Overall mortality risk was assessed with multivariate logistic regression. Alpha was set at 0.05.



RESULTS:





P



ETCO

2





/P




aCO

2






correlated strongly with V
D
/V
T
(r = –0.87 [95% CI –0.89 to –0.85],
P
< .001). Decreasing




P



ETCO

2





/P




aCO

2






was associated with increased V
D
/V
T
and hospital mortality between all groups. In the univariate analysis, for every 0.01 decrease in




P



ETCO

2





/P




aCO

2






, mortality risk increased by ∼1% (odds ratio 0.009, 95% CI 0.003–0.029,
P
< .001) and maintained a strong independent association with mortality risk when adjusted for other variables (odds ratio 0.19, 95% CI 0.04–0.91,
P
= .039).




P



ETCO

2





/P




aCO

2






< 0.50 was characterized by very high mean ± SD value for V
D
/V
T
(0.82 ± 0.05,
P
< .001) and high hospital mortality (70%).



CONCLUSIONS:

Using




P



ETCO

2





/P




aCO

2






as a surrogate for V
D
/V
T
may be a useful and practical measurement for both management and ongoing research into the nature of ARDS.