What is the significance of a persistent capnography waveform with an end-tidal CO2 (ETCO2) of 8 mmHg in an intubated patient during cardiac arrest?

Significance of ETCO2 of 8 mmHg During Cardiac Arrest

An ETCO2 of 8 mmHg during cardiac arrest indicates inadequate cardiac output from chest compressions and predicts extremely poor likelihood of achieving return of spontaneous circulation (ROSC)—you must immediately optimize CPR quality by improving chest compression depth, rate, and minimizing interruptions. 1

Critical Interpretation

The presence of a persistent waveform confirms correct endotracheal tube placement in the trachea, which is essential information during resuscitation. 1 The waveform itself demonstrates that:

• The tube is properly positioned (not esophageal)
• There is some pulmonary blood flow, albeit severely inadequate
• Ventilation is occurring through the endotracheal tube

However, the ETCO2 value of 8 mmHg falls below the critical 10 mmHg threshold and indicates that cardiac output generated by chest compressions is insufficient to achieve ROSC. 1, 2

Physiologic Basis

During cardiac arrest with ongoing CPR, ETCO2 directly reflects cardiac output because: 1, 2

• CO2 continues to be produced by cellular metabolism
• Ventilation is relatively constant with mechanical ventilation
• Pulmonary blood flow becomes the primary determinant of CO2 delivery to the lungs
• The measured ETCO2 therefore correlates with the cardiac output generated by chest compressions

Normal ETCO2 is 35-40 mmHg; during effective CPR, values typically range from 10-20 mmHg. 1, 2 Your patient's value of 8 mmHg indicates critically low pulmonary blood flow.

Immediate Management Algorithm

Step 1: Optimize Chest Compressions Immediately

If PETCO2 is <10 mmHg, it is reasonable to immediately attempt to improve CPR quality by optimizing chest compression parameters (Class IIb, LOE C). 1, 2 Specifically:

• Ensure compression depth of at least 2 inches (5 cm) but not exceeding 2.4 inches (6 cm)
• Maintain compression rate of 100-120 per minute
• Allow complete chest recoil between compressions
• Minimize interruptions to <10 seconds
• Rotate compressors every 2 minutes to prevent fatigue 1, 2

Step 2: Verify Technical Factors

Confirm there are no technical issues causing falsely low readings: 2, 3

• Check for ETT kinking or obstruction
• Verify no circuit disconnections or leaks
• Ensure proper ETT positioning (not too deep causing bronchial intubation)
• Rule out mucous plugging

Step 3: Monitor for ROSC

An abrupt sustained increase in PETCO2 to 35-40 mmHg is a sensitive indicator of ROSC and often precedes detection of pulse or blood pressure (Class IIa, LOE B). 1, 2 Continue monitoring the waveform continuously during compressions.

Step 4: Consider Reversible Causes

While optimizing compressions, systematically address H's and T's, as persistently low ETCO2 may reflect: 1

• Severe hypovolemia (reducing venous return)
• Massive pulmonary embolism (reducing pulmonary blood flow)
• Tension pneumothorax (impairing venous return)

Prognostic Implications

Multiple studies demonstrate that ETCO2 values <10 mmHg after 20 minutes of advanced cardiac life support predict death with high accuracy. 4, 5 Specifically:

• One landmark study showed 100% sensitivity and specificity for predicting death when ETCO2 remained ≤10 mmHg at 20 minutes 4
• Another prehospital study confirmed 97.3% sensitivity and 100% specificity for this threshold 5

However, ETCO2 should guide CPR optimization efforts but not serve as the sole criterion for terminating resuscitation. 2 The American Heart Association does not recommend using ETCO2 alone to determine when to cease resuscitation efforts, though persistently low values indicate poor prognosis. 1, 2

Common Pitfalls to Avoid

Do not misinterpret transient changes in ETCO2 after medication administration: 1

• Sodium bicarbonate causes transient ETCO2 increase (converted to CO2 and water)—this does NOT indicate improved CPR quality or ROSC
• High-dose vasopressors (>1 mg epinephrine) may cause small ETCO2 decrease due to increased afterload reducing cardiac output—this does NOT indicate worsening CPR quality

Do not assume the tube is misplaced based solely on low ETCO2. 1 The persistent waveform confirms tracheal placement; the low value reflects inadequate pulmonary blood flow, not tube malposition.