Introduction:
End-tidal carbon dioxide (ETCO2) levels during cardiopulmonary resuscitation (CPR) depend primarily on the blood flow generated by chest compressions, on the ventilation rate and on the metabolic activity of the patient tissues. A better characterization of the influence of each factor is warranted.
Objective:
To assess the quantitative relation between ETCO2 and ventilation rate from retrospective analysis of adult out-of-hospital resuscitations.
Methods:
Monitor recordings of 163 adult out-of-hospital cardiac arrest cases treated by Tualatin Valley Fire & Rescue (Tigard, OR) from 2012 through 2017. Recordings contained concurrent capnogram, ECG, transthoracic impedance (TI) and compression depth. Episodes were manually inspected to identify pairs of segments with different ventilation rates (
vr
1,
vr
2) and meeting the following criteria: 1) Reliable capnogram to annotate the ETCO2 value of each segment (
ET
1,
ET
2). 2) A maximum of 2 min separation between segments so that similar metabolic activity could be assumed. 3) Absence of circulation, by inspecting the ECG and the TI signal. 4) Compression depth and rate variation between segments < 4 mm and 8 compressions per minute, respectively.
We applied curve fitting to the data with a custom equation (see figure) and non-linear least squares as fit option.
Results:
Forty pairs of segments were included. The figure shows the measurements and the resulting fitting curve normalized to
vr
1 = 10 ventilations per minute (vpm). Factor
K
was 0.93, and the coefficient of determination R
2
was 0.90. Thus, for instance, the ETCO2 at 5 vpm would be 1.64 times the ETCO2 at 10 vpm. Similarly, the ETCO2 at 15 vpm would be 0.80 times the ETCO2 at 10 vpm.
Conclusions:
The obtained curve may help to eliminate the influence of ventilation rate on ETCO2 during cardiopulmonary resuscitation. This finding is a step forward to a better understanding of ETCO2 variation during the course of resuscitation.
Changes in expired end-tidal carbon dioxide during cardiopulmonary resuscitation in dogs: A prognostic guide for resuscitation efforts
