Low End-Tidal CO2 Predicts Unsuccessful CPR
ABSTRACT & COMMENTARY
Synopsis: An end-tidal CO2 level of less than 10 mmHg after 20 minutes of advanced cardiac life support predicted failure to survive to hospital admission in 100% of patients who experienced cardiac arrest associated with electrical activity but no pulse.
Source: Levine RL, et al. N Engl J Med 1997;337: 301-306.
During cardiac arrest, end-tidal co2 levels are entirely determined by the cardiac output generated by cardiopulmonary resuscitation. Thus, end-tidal CO2 represents an indirect measure of cardiac output. Levine and colleagues, therefore, hypothesized that end-tidal CO2 level could be used to predict death in individuals who experienced a cardiac arrest.
To test this hypothesis, they prospectively followed all adults 18 years of age or older who experienced an out-of-hospital cardiac arrest associated with electrical activity but no pulse. Subjects were excluded if they had ventricular tachycardia, ventricular fibrillation, persistent asystole, and if the cardiac arrest was due to hypothermia, poisoning, trauma, tension pneumothorax, cardiac tamponade, or hypovolemia. Paramedics followed standard advanced cardiac life support protocols with on-line medical control. End-tidal CO2 analysis was performed by mainstream sampling with a lightweight sensor attached directly to the endotracheal tube. In all patients, resuscitation continued for at least 20 minutes unless spontaneous circulation was restored. After 20 minutes, resuscitation was discontinued if spontaneous circulation was restored or if there was loss of electrical activity (asystole).
During the four-year study, 150 patients were enrolled, of whom 35 (23.3%) survived to hospital admission. There was no significant difference in the mean age or initial end-tidal CO2 level between those who survived to hospital admission (survivors) and nonsurvivors. After 20 minutes of advanced cardiac life support, end-tidal CO2 levels clearly discriminated between nonsurvivors and survivors, averaging 4.4 ± 2.9 mmHg (range, 0-10) in nonsurvivors and 32.8 ± 7.4 mmHg (range, 18-58) in survivors (P < 0.001). In survivors, end-tidal CO2 rose to at least 18 mmHg before the return of clinically detectable vital signs. Patients with an end-tidal CO2 of 10 mmHg or less had a sensitivity, specificity, positive predictive value, and negative predictive value of 100%. Of the 35 survivors, 19 died in the hospital, 16 were discharged, and 14 were alive six weeks following discharge. Of those alive at six weeks, 13 were neurologically intact or had mild impairment but were able to care for themselves. End-tidal CO2 levels did not discriminate between long-term survivors and those who died in the hospital.
COMMENT BY LESLIE A. HOFFMAN, RN, PhD
A substantial body of data from both animal and human studies supports the choice of a 10 mmHg end-tidal CO2 threshold as predictive of survival following cardiopulmonary resuscitation. Nevertheless, clinicians have been reluctant to incorporate end-tidal CO2 monitoring into advanced cardiac life-support algorithms because of isolated reports of survivors of lengthy resuscitation efforts who had persistently low end-tidal CO2 levels.
To better define the predictive value of end-tidal CO2 measurements during CPR, this study was conducted in a well-defined, clinically relevant population. The entire study was performed outside the hospital, so transportation to a new environment would not be an issue. Only cardiac arrests involving electrical activity without pulse were studied, and end-tidal monitoring was initiated when resuscitation began.
There are several important observations to be made from the study findings. Among survivors to hospital admission, end-tidal CO2 level did not differentiate between long-term survivors and those who died in the hospital. However, end-tidal CO2 level did differentiate subjects who survived to hospital admission. No patient with an end-tidal CO2 of less than 10 mmHg after 20 minutes of resuscitation survived to hospital admission. If spontaneous circulation returned, end-tidal CO2 rose to at least 18 mmHg before the vital signs were clinically detectable. Also, circulation was not restored in any patient with persistent electrical activity but no pulse after 20 minutes of advanced life support.
Only a small percentage of victims survive out-of-hospital cardiac arrest. Thus, a marker that accurately identifies those individuals with no chance of survival could save an enormous amount of effort and reduce costs. In 1993, costs of futile resuscitation were estimated at over $1 billion dollars per year (JAMA 1993;270:1471). The findings of this study provide strong support for a 10 mmHg end-tidal CO2 threshold in patients with electrical activity but no pulse. An added advantage is that measurements of end-tidal CO2 are easy to accomplish. Future studies are needed to determine if these findings are reproducible by others in out-of-hospital settings and during in-hospital cardiac arrests.
In the study of end-tidal CO2 levels during CPR, non-survivors and survivors differed with respect to:
a. age.
b. prior cardiac history.
c. end-tidal CO2 after five minutes.
d. end-tidal CO2 after 10 minutes.
e. end-tidal CO2 after 20 minutes.
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