Understanding Waveform Capnography
Capnography is the continuous measurement and graphical display of exhaled CO₂ throughout the respiratory cycle. The waveform, or capnogram, represents the CO₂ concentration over time and provides real-time data about ventilation, perfusion, and metabolism. The use of wave form capnography is the gold standard in prehospital monitoring and management of intubated and ventilated patients. It is required for all Paramedic agencies performing medication facilitated intubation or rapid sequence intubation (MFI/RSI). It serves, in conjunction with visual confirmation of tube placement, chest rise, bilateral lung sounds and absent epigastric sounds as continued evidence of ventilatory adequacy. Since field ABG is not practical or generally possible today, the wave form capnogram is a critical diagnostic monitoring tool providing information to providers during ventilatory support.
Waveform Physiology
Carbon dioxide is produced in the mitochondria of all body cells as a byproduct of aerobic metabolism. It diffuses into the bloodstream, is transported primarily as bicarbonate and bound to hemoglobin, and is delivered to the lungs. In the alveoli, CO₂ diffuses from blood into the alveolar space and is expelled during exhalation.
Capnography measures the partial pressure of CO₂ in exhaled gas. Healthy adults maintain an ETCO₂ of approximately 35-45 mmHg, which closely mirrors arterial CO₂ (PaCO₂) because the lungs are normally efficient at eliminating CO₂. When ventilation, perfusion, or metabolism changes, so does the waveform. This tool, in conjunction with clinical correlation, is assistive in interpolating acid/base balance.
- ETCO₂ < 35 mmHg: Typically indicates hyperventilation or decreased perfusion. In a cardiac arrest patient, low ETCO₂ (<10 mmHg) suggests minimal perfusion and very low likelihood of return of spontaneous circulation (ROSC).
- ETCO₂ 35-45 mmHg: Normal range. Indicates adequate ventilation and perfusion.
- ETCO₂ > 45 mmHg: Typically indicates hypoventilation, hyperthermia, or increased metabolic rate (fever, seizure). Can also occur with muscle relaxant administration if ventilation is not increased proportionally.
The waveform itself—not just the number—provides additional diagnostic information that numbers alone cannot convey .These include bronchospasm, airway compromise or obstruction, rebreathing and return of spontaneous respiration.
Waveform Phases and Clinical Correlation
PHASE 1: The Baseline and Early Expiration
At the end of inspiration, the monitor reads near zero mmHg (dead space). As exhalation begins, gas from the anatomic dead space (airways) is expelled first; this contains little or no CO₂. As the waveform begins to rise, you are seeing the transition from dead space to alveolar gas.
PHASE 2: The Upstroke
The CO₂ concentration rises rapidly as alveolar gas is expelled. This phase is normally steeply angled. A gentler upstroke (less steep) suggests uneven ventilation or perfusion heterogeneity. In COPD patients or those with significant airway obstruction, you may see a more gradual upstroke.
PHASE 3: The Plateau
As pure alveolar gas is being exhaled, the waveform reaches a relatively flat plateau. The end of this plateau (the peak) is the ETCO₂ value. A sloping (rather than flat) plateau can indicate ventilation/perfusion mismatch or diseased lung units emptying at different rates.
PHASE 0: The Downstroke and Return to Baseline
Inspiration begins, and fresh air with essentially zero CO₂ rushes into the airways, rapidly dropping the measured value back to baseline. The downstroke is normally very sharp and nearly vertical. A prolonged downstroke can indicate obstruction to inspiration (upper airway pathology) or rebreathing (equipment malfunction).
Waveform Patterns
The normal capnogram consists of four distinct phases: baseline (expiratory), upstroke (early exhalation), plateau (alveolar exhalation), and downstroke (inspiration). The waveform resembles a ski slope or a square wave with rounded corners. The plateau phase is typically nearly horizontal, and the rapid downstroke to baseline represents the transition to inspiration.
A normal shaped capnogram, ETCO₂ of 60 mmHg as depicted, reflects effective alveolar emptying but inadequate CO₂ clearance or production at an abnormal rate. This may occur in cases of hypoventilation including CNS depression secondary to overdose from opioids, benzodiazepines or barbiturates. This also occurs in traumatic brain injury, stroke or cases of rising ICP. Another common scenario is ventilator mismanagement such as respiratory rate or tidal volume too low for patient’s physiologic demand. Increased CO₂ production is also found, in cases of malignant hyperthermia, sepsis, thyrotoxicosis or following an aggressive administration of sodium bicarbonate.
A shark fin waveform, as depicted above with ETCO₂ of 22mm/Hg is often present in cases of lower airway obstruction of bronchospasm. The most common clinical scenarios are exacerbation of asthma, exacerbation of COPD, anaphylaxis and mechanical airway obstruction such as a kinked ET tube or a partially blocked ventilator. The first step in an intubated patient depicting shark fin pattern is to check your ventilatory circuit.
During a cardiac arrest with CPR the blood flow is entirely dependent upon the quality of compressions. However, even with high quality CPR the output is just 25%-30% of normal. ETCO₂ in this situation remains persistently low, often plateauing as depicted between 10-22 mm/Hg. An abrupt increase indicates that the myocardium has resumed spontaneous pumping
The above capnogram represents a curare cleft, which is a negative deflection during the alveolar plateau phase of the capnogram. This indicates spontaneous respiratory effort by the patient during the expiratory phase of mechanical ventilation. The primary clinical scenario is the waning or wearing off of the neuromuscular blockade, patient-ventilator asynchrony which is found in cases of inadequate sedation, and diaphragmatic spasms. The name curare cleft originates from the historical use of the paralytic drug curare. The isolation of curares active ingredient d-tubocurarine, revolutionized modern surgery and was introduced into clinical anesthesiology in 1942. This has been replaced in modern medicine by safer synthetic neuromuscular blocking agents such as rocuronium and vecuronium. For more on airway management see this clinical case.
Paramedic Assessment: Real Time Approach
When assessing a capnography waveform, ask yourself three questions:
First ask, is there a waveform at all? A flatline indicates no CO₂ is being detected. This could mean (a) the tube is not in the trachea (esophageal intubation), (b) the tube has become dislodged, or (c) there is no pulmonary blood flow (cardiac arrest, profound shock). Always verify tube position clinically if waveform is absent and confirm the integrity of the ventilatory circuit. This is troubleshooting step one.
Secondly ask, what is the ETCO₂ value, and does it match your clinical assessment? Integrate the waveform value with your patient exam (e,g. clinically correlate). A patient with a normal ETCO₂ but absent breath sounds on the left and diminished on the right is concerning for pneumothorax, not cardiopulmonary collapse. Trust neither the number nor the physical exam in isolation—use them to corroborate each other.
Lastly ask is the waveform shape normal, and is it trending the right direction? An abnormal waveform shape or an acute change in ETCO₂ demands investigation. A sudden drop in an intubated patient might indicate tube dislodgement, pulmonary embolism, decompensation, or just a problem with the monitor/sensor. Never ignore an abrupt change.
Evidentiary Matter
The use of waveform capnography in EMS is recognized broadly as the standard of care. Capnography (mainstream or sidestream) has sensitivity and specificity >99% for confirming endotracheal tube position in cardiac arrest and other settings where perfusion is present. A study by Grmec and Klemen (2000) in prehospital cardiac arrest found that capnography detected all 50 esophageal intubations in their cohort, preventing iatrogenic hypoxemia and improving outcomes.
As noted above, multiple studies have demonstrated that ETCO₂ values during CPR correlate strongly with resuscitation outcomes. Levine et al. (2006) found that patients with ETCO₂ >20 mmHg during resuscitation had significantly higher rates of ROSC and hospital discharge than those with ETCO₂ <20 mmHg. ETCO₂ <10 mmHg throughout resuscitation is associated with very low likelihood of meaningful recovery.
Among the most helpful uses of capnography is the objective measurement of ventilation adequacy. The use of portable transport ventilators and ventilator circuits in the field introduce risk variables. Hyperventilation—common in stressed paramedics—can be immediately corrected when you see the ETCO₂ dropping below target range. This has led to improved outcomes in traumatic brain injury and cardiac arrest, where excessive ventilation is harmful.
References
- American Heart Association. (2020). Highlights of the 2020 American Heart Association guidelines for CPR and ECC. Retrieved from https://cpr.heart.org
- Grmec, Š., & Klemen, P. (2000). Does the end-tidal carbon dioxide (ETCO₂) concentration have prognostic value during out-of-hospital cardiac arrest? European Journal of Emergency Medicine, 7(2), 144–160.
- Levine, R. L., Wayne, M. A., & Miller, C. C. (2006). End-tidal carbon dioxide and outcome of out-of-hospital cardiac arrest. New England Journal of Medicine, 337(5), 301–306.
- Mikkelsen, S., Ringdal, K. G., Moons, P., Soreide, K., Hirlekar, G., Kramer-Johansen, J., ... & Søreide, E. (2016). Capnography for detection of inadvertent esophageal intubation: A systematic review. Resuscitation, 120, 36–43.
- Paiva, E. F., Paxton, J. H., & O'Neill, B. J. (2018). The use of capnography during cardiopulmonary resuscitation in emergency medicine. Annals of Emergency Medicine, 47(4), 344–350.
- Skipper, J., & Ramirez, A. G. (2003). Prehospital capnography and cardiac arrest outcomes. Prehospital Emergency Care, 7(2), 225–231.
- Soni, S., Kunte, M., Agrawal, V., & Pandey, S. (2011). Capnography: Continuing monitoring for carbon dioxide. Indian Journal of Emergency Medicine, 15(2), 34–40.
- Tobias, J. D. (2008). Capnography in non-intubated patients. Current Opinion in Anesthesiology, 20(6), 540–543.