Abstract Body: Background: End-tidal carbon dioxide (ETCO2) has been proposed as a surrogate for cardiac output during cardiopulmonary resuscitation (CPR). Chest compressions alter the capnogram waveform, complicating the calculation of ETCO2 values. To explore the utility of ETCO2 as a surrogate marker, we have conducted a retrospective analysis of the relationship between local maxima in the capnogram and hemodynamic metrics in preclinical resuscitation studies.
Methods: In eight domestic swine (~30 kg), the IVC blood flow, aortic pressure (AOP), right atrial pressure (RAP), and capnogram were measured during resuscitation. Coronary perfusion pressure (CPP) is the difference between the AOP and the RAP during relaxation. ETCO2 is calculated using three methods: averaging the maximum CO2 level reached during each chest compression, selecting the global CO2 maximum measured during each breath, and identifying the maximum CO2 level from the last compression for each breath. The data are modeled using multivariate linear regression as follows:
ETCO2= IVC + CPP + CC Depth + CC rate + Epoch # + Animal #

Results: Table 1 shows R-squared values relating intra-compression CO2 maxima to hemodynamic measures and compression characteristics. The maximum CO2 value in a breath shows stronger correlations with these parameters than the other CO2 measurements. Table 2 presents regression coefficients for the model using this measure. CC Depth and CPP have coefficients of 3.67 and 2.05 respectively, indicating significant positive influences, while IVC has a coefficient of 0.84, indicating a weaker positive impact compared to CC Depth and CPP.

Table 1. Method for Calculating the ETCO2
Calculating ETCO2
R-Squared

Global CO2 Maximum per breath
0.67

Average of CO2 Maxima
0.64

CO2 Maximum of Last Compression
0.36


Table 2. Variable Coefficients
Outcomes
Regression Coefficients

CPP
2.05

IVC
0.84

CC Depth
3.67

CC Rate
-0.02

Epoch Number
-0.57


Conclusions: The experimental results show that the global CO2 maximum per breath has the closest relationship to the hemodynamics in this experiment. The analysis suggests that increasing CC Depth, CPP, and IVC flow also increases the calculated ETCO2.