8.3 Continuous Waveform Capnography

Fundamentals of Quantitative Waveform Capnography vs Capnometry

Continuous quantitative waveform capnography is the gold-standard non-invasive monitoring technology required by the American Heart Association (AHA) for all patients undergoing advanced airway management during cardiac arrest and post-resuscitation care. Capnography provides a real-time graphic display of partial pressure of carbon dioxide (PCO2) plotted against time throughout the respiratory cycle, accompanied by a precise numerical End-Tidal CO2 (ETCO2) measurement.

Distinguishing Capnographic Technologies

It is vital to distinguish quantitative waveform capnography from less sophisticated monitoring devices:

• Qualitative Colorimetric CO2 Detectors: These disposable paper-indicator devices attach to the airway and change color (e.g., from purple to yellow) in the presence of exhaled CO2. While useful for initial verification in resource-limited settings, they do not provide continuous numerical data, are susceptible to false readings from gastric acid or epinephrine contamination, and cannot display waveform morphology.
• Quantitative Capnometry: Provides a continuous numerical value for ETCO2 without displaying a graphical waveform. While superior to colorimetric indicators, capnometry lacks the visual waveform necessary to identify technical artifact, airway obstruction, or breathing effort.
• Continuous Quantitative Waveform Capnography: Combines real-time graphical waveform analysis with exact numerical ETCO2 tracking. It is mandatory because it continuously confirms tube placement, monitors CPR quality, detects ROSC instantly, and alerts providers to airway dislodgement.

The Three Pillars of ETCO2 Physiology

End-Tidal CO2 is not merely a respiratory parameter; it represents the dynamic interaction of three fundamental physiological systems:

1. Cellular Metabolism: Tissues generate CO2 as a byproduct of aerobic metabolism.
2. Cardiovascular Perfusion: Venous blood transports CO2 from peripheral tissues to the pulmonary capillary bed. In cardiac arrest, pulmonary blood flow is determined entirely by the cardiac output generated by chest compressions.
3. Alveolar Ventilation: Exhaled gas removes CO2 from the pulmonary alveoli through the respiratory tract.

Detailed Morphological Analysis of the Four Capnogram Phases

A normal capnogram exhibits a rectangular or box-like shape, reflecting the sequential movement of gas during exhalation and inspiration. Understanding the four distinct phases of the capnogram and its key angles allows providers to rapidly interpret respiratory and circulatory dynamics.

Morphological Phases of the Capnogram

• Phase I (Anatomical Dead Space Baseline): Phase I occurs at the beginning of exhalation. The gas initially passing the sensor originates from the anatomical dead space (nasopharynx, trachea, and major bronchi), where no gas exchange occurs. Because dead space gas contains no carbon dioxide, the capnogram remains flat at the zero PCO2 baseline.
• Phase II (Expiratory Upstroke): As exhalation continues, CO2-rich gas from the pulmonary alveoli begins mixing with dead space gas and clearing the airway. This produces a steep, almost vertical upward deflection on the display, representing the rapid transition in PCO2.
• Phase III (Alveolar Plateau): Phase III represents the exhalation of pure alveolar gas from the pulmonary acini. The waveform plateaus, sloping gently upward due to slight variations in emptying rates across different lung segments. The peak value at the very end of Phase III immediately prior to inspiration is recorded as the End-Tidal CO2 (ETCO2) value (normally 35 to 45 mmHg in a healthy, spontaneously breathing patient).
• Phase IV (Inspiratory Downstroke): Phase IV marks the start of the next inspiration. Fresh, oxygenated gas containing zero CO2 sweeps past the sensor, causing the PCO2 curve to drop precipitously back to the Phase I zero baseline.

Critical Capnographic Angles

• Alpha Angle: The transition angle between Phase II and Phase III (normally 100° to 110°). An increased alpha angle (slanted upstroke) indicates expiratory airflow obstruction, such as asthma or bronchospasm.
• Beta Angle: The transition angle between Phase III and Phase IV (normally approximately 90°). An elevated beta angle occurs during rebreathing of CO2.

Clinical Applications: Airway Verification, CPR Quality, and ROSC Detection

Continuous quantitative waveform capnography serves three indispensable clinical functions during ACLS resuscitations.

1. Immediate and Continuous Airway Verification

Following endotracheal intubation or SGA placement, capnography is the primary objective method for confirming airway placement. If the tube is correctly positioned in the trachea, a clear, multi-phased waveform with consistent CO2 detection will appear within 2 to 3 breaths.

• Esophageal Placement Rule: If the ETT is inadvertently inserted into the esophagus, the stomach contains negligible CO2. Consequently, the capnograph will display a flat line with an ETCO2 reading of 0 mmHg. Clinical Rule: A flat capnogram after intubation indicates esophageal placement until proven otherwise. The tube must be immediately removed and bag-mask ventilation resumed.
• False Initial CO2: If the patient recently consumed carbonated beverages, a small amount of CO2 may briefly register during the first 1–2 ventilations. However, this false CO2 rapidly decays to 0 mmHg within 3 to 4 breaths, confirming esophageal misplacement.

2. Real-Time CPR Quality and Perfusion Monitoring

During cardiac arrest, ventilation is held constant (10 breaths/min), meaning changes in ETCO2 directly reflect changes in pulmonary blood flow (cardiac output) generated by chest compressions. ETCO2 correlates directly with Coronary Perfusion Pressure (CPP):

• ETCO2 < 10 to 20 mmHg: Signals inadequate chest compressions or severe low-flow state. If ETCO2 is <10 mmHg, the team leader must immediately instruct the compressor to increase depth and rate, ensure complete recoil, or rotate compressors. Persistence of ETCO2 <10 mmHg after 20 minutes of high-quality ACLS indicates extremely low cardiac output and predicts unsuccessful resuscitation.
• ETCO2 > 20 mmHg: Indicates high-quality chest compressions generating effective cardiac output.

3. Detection of Return of Spontaneous Circulation (ROSC)

When the heart spontaneously restarts, cardiac output increases instantly from the low levels generated by CPR (15-25% of normal) to native levels. This sudden surge in blood flow washes out accumulated cellular CO2 from ischemic peripheral tissues directly to the lungs. Capnography detects ROSC instantly as a sudden, sustained rise in ETCO2 to ≥35 to 40 mmHg (often doubling or tripling from baseline during CPR). This spike frequently occurs before a central pulse becomes palpable, allowing the team leader to pause compressions for a rhythm check without wasting valuable time.

Diagnostic Interpretation of Waveform Abnormalities & Troubleshooting

Recognizing specific abnormal capnogram patterns allows ACLS providers to rapidly diagnose underlying pulmonary pathologies, mechanical equipment failures, and physiological changes.

Common Abnormal Capnogram Patterns

• Shark-Fin Waveform (Obstructive Pattern): Characterized by a sloped Phase II upstroke, loss of the distinct alpha angle, and a steep Phase III plateau. This pattern signifies expiratory airflow obstruction, commonly caused by bronchospasm (asthma, COPD), mucous plugging, or a kinked endotracheal tube. Treatment includes nebulized bronchodilators, airway suctioning, or un-kinking the ETT.
• Curare Cleft: A distinct downward notch or dip occurring during Phase III of the alveolar plateau. It represents a brief inspiratory effort by the patient against mechanical ventilation, indicating that muscle relaxants/paralytics are wearing off or that the patient is regaining spontaneous breathing effort.
• Progressively Decreasing ETCO2 Trend: A step-wise drop in ETCO2 over several minutes during CPR indicates declining cardiac output due to rescuer fatigue, acute hypovolemia, massive pulmonary embolism, or worsening cardiac tamponade.
• Elevated Baseline (Rebreathing CO2): When Phase I fails to return to the zero baseline (remaining elevated above 0 mmHg), the patient is rebreathing exhaled CO2. Causes include exhausted CO2 absorber medium in anesthesia circuits, a malfunctioning expiratory valve, or insufficient fresh gas flow in bag-mask circuits.
• Loss of Waveform (Sudden Zero): A sudden, total disappearance of the capnogram indicates total airway occlusion, complete ETT extubation, circuit disconnection, or sudden cardiac arrest in a previously beating heart.