Association between Prehospital CPR Quality and End-Tidal Carbon Dioxide Levels in Out-of-Hospital Cardiac Arrest

<p><b>Introduction:</b> International Guidelines recommend measurement of end-tidal carbon dioxide (EtCO<sub>2</sub>) to enhance cardiopulmonary resuscitation (CPR) quality and optimize blood flow during CPR. Numerous factors impact EtCO<sub>2</sub> (e.g., ventilation, metabolism, cardiac output), yet few clinical studies have correlated CPR quality and EtCO<sub>2</sub> during actual out-of-hospital cardiac arrest (OHCA) resuscitations. The purpose of this study was to describe the association between EtCO<sub>2</sub> and CPR quality variables during OHCA. <b>Methods:</b> This is an observational study of prospectively collected CPR quality and capnography data from two EMS agencies participating in a statewide resuscitation quality improvement program. CPR quality and capnography data from adult (≥18 years) cardiac resuscitation attempts (10/2008–06/2013) were collected and analyzed on a minute-by-minute basis using RescueNet™ Code Review. Linear mixed effect models were used to evaluate the association between (log-transformed) EtCO<sub>2</sub> level and CPR variables: chest compression (CC) depth, CC rate, CC release velocity (CCRV), ventilation rate. <b>Results:</b> Among the 1217 adult OHCA cases of presumed cardiac etiology, 925 (76.0%) had a monitor-defibrillator file with CPR quality data, of which 296 (32.0%) cases had >1 minute of capnography data during CPR. After capnography quality review, 66 of these cases (22.3%) were excluded due to uninterpretable capnography, resulting in a final study sample of 230 subjects (mean age 68 years; 69.1% male), with a total of 1581 minutes of data. After adjustment for other CPR variables, a 10 mm increase in CC depth was associated with a 4.0% increase in EtCO<sub>2</sub> (<i>p</i> < 0.0001), a 10 compression/minute increase in CC rate with a 1.7% increase in EtCO<sub>2</sub> (<i>p</i> = 0.02), a 10 mm/second increase in CCRV with a 2.8% increase in EtCO<sub>2</sub> (<i>p</i> = 0.03), and a 10 breath/minute increase in ventilation rate with a 17.4% decrease in EtCO<sub>2</sub> (<i>p</i> < 0.0001). <b>Conclusion:</b> When controlling for known CPR quality variables, increases in CC depth, CC rate and CCRV were each associated with a statistically significant but clinically modest increase in EtCO<sub>2</sub>. Given the small effect sizes, the clinical utility of using EtCO<sub>2</sub> to guide CPR performance is unclear. Further research is needed to determine the practicality and impact of using real-time EtCO<sub>2</sub> to guide CPR delivery in the prehospital environment.</p>