PETCO2 (partial pressure of end-tidal carbon dioxide) is the amount of CO2 measured in a patient’s exhaled breath at the very end of exhalation. In ACLS, it serves as a real-time indicator of how well chest compressions are generating blood flow during cardiac arrest. It’s measured in millimeters of mercury (mmHg) using a device called a capnograph, and current ACLS guidelines recommend monitoring it with continuous waveform capnography throughout resuscitation.
Why CO2 Tells You About Blood Flow
During cardiac arrest, the body’s cells continue producing carbon dioxide as a metabolic waste product. That CO2 travels through the bloodstream to the lungs, where it gets exhaled. The key insight is that CO2 can only reach the lungs if blood is actually flowing there. So the amount of CO2 detected in exhaled breath directly reflects how much blood the heart (or chest compressions) is pushing through the pulmonary circulation.
When cardiac output drops critically, blood flow to the lungs slows down. CO2 accumulates in the veins but never reaches the lungs to be exhaled. The result: PETCO2 falls. When compressions improve or the heart restarts, blood flow increases, more CO2 reaches the lungs, and PETCO2 rises. This tight correlation between PETCO2 and cardiac output is the foundation for everything capnography does in ACLS.
The Three Uses in ACLS
Monitoring CPR Quality
PETCO2 gives rescuers a continuous, objective measure of whether chest compressions are effective. The target is a PETCO2 of at least 20 mmHg during CPR. If values fall below that, it signals that compressions aren’t generating enough blood flow and the team needs to adjust: pushing harder, faster, or swapping in a less fatigued compressor. This is more reliable than visual assessment alone, which tends to overestimate compression quality.
Detecting Return of Spontaneous Circulation
A sudden, sharp jump in PETCO2 is one of the earliest signs that the heart has restarted. When spontaneous circulation returns, cardiac output surges, flushing a backlog of CO2 through the lungs. This spike often appears on the capnograph before a pulse is palpable. Rescuers watching for this change can identify return of spontaneous circulation (ROSC) quickly without pausing compressions to check for a pulse.
Confirming Airway Placement
After a breathing tube is placed, capnography confirms it’s in the trachea and not the esophagus. A consistent CO2 waveform with each breath means the tube is in the right location. No CO2 waveform suggests the tube is in the esophagus, where no gas exchange occurs. ACLS guidelines recommend this as the most reliable method of confirming tube placement.
The 10 mmHg Threshold at 20 Minutes
One of the most studied applications of PETCO2 in ACLS is its role as a prognostic tool. A landmark study published in the New England Journal of Medicine found that a PETCO2 of 10 mmHg or less after 20 minutes of resuscitation predicted death with 100% accuracy in patients with pulseless electrical activity. In that study, nonsurvivors averaged just 4.4 mmHg at the 20-minute mark, while survivors averaged 32.8 mmHg.
This finding has practical implications for resuscitation decisions. A persistently low PETCO2 after 20 minutes of high-quality ACLS reflects an extremely low cardiac output that has proven incompatible with survival. While no single measurement should be used in isolation to stop resuscitation, this threshold is one of the strongest objective data points available to help guide that difficult decision.
What Can Throw Off the Reading
PETCO2 is useful precisely because it tracks cardiac output so closely, but several situations can produce misleading values. Sodium bicarbonate, commonly given during prolonged resuscitation, breaks down into CO2 in the bloodstream. This can cause a temporary spike in PETCO2 that mimics the surge seen with ROSC, even when the heart hasn’t restarted. Rescuers need to account for timing if bicarb was recently administered.
Massive pulmonary embolism creates a different problem. A large clot blocks blood flow to a section of the lungs, so even if the heart is generating reasonable output, CO2 can’t reach the blocked alveoli for exhalation. This produces falsely low PETCO2 readings that don’t accurately reflect compression quality. Excessive ventilation rates can also artificially lower PETCO2 by washing out CO2 faster than it arrives in the lungs, which is another reason ACLS emphasizes controlled ventilation rates during arrest.
The cause of arrest matters too. Patients who arrest from asphyxia (suffocation, drowning) tend to have higher initial PETCO2 values because CO2 has been building up during the period of inadequate breathing before the arrest. This is different from the pattern seen in cardiac arrests caused by abnormal heart rhythms, where initial PETCO2 values start lower. Understanding these patterns helps rescuers interpret the numbers in context rather than applying the same benchmarks rigidly to every situation.
How It Appears on the Monitor
Waveform capnography displays PETCO2 as a continuous graph, with each breath producing a wave that rises as CO2 is exhaled and falls as fresh air is inhaled. During CPR, the height of each wave corresponds to how much CO2 is being delivered to the lungs by chest compressions. A flat line means no CO2 is being detected, either because the airway tube is misplaced or because there is essentially no blood flow reaching the lungs.
The numeric PETCO2 value displayed alongside the waveform is the peak of each wave, measured in mmHg. During effective CPR, you’d expect to see values at or above 20 mmHg. A sudden rise to 35 or 40 mmHg during compressions strongly suggests the heart has regained its own rhythm. The combination of the waveform shape and the numeric value gives the resuscitation team continuous feedback without interrupting chest compressions, which is why capnography has become a standard monitoring tool in modern ACLS.