ETCO2 Waveforms Explained: A Practical Guide for EMS

The ETCO2 number catches your eye. The waveform helps you understand how that number was produced. For EMTs and paramedics, learning to read both adds useful information to an assessment of breathing, airway function, and circulation.

This guide explains normal ETCO2 values, common capnography waveforms, and questions to ask when the trace changes. Open CE Solutions' interactive Capno Lab alongside the article to explore the patterns as you read.

What Is ETCO2, and What Is a Normal Value?

End-tidal carbon dioxide, or ETCO2, is the partial pressure of carbon dioxide measured at the end of exhalation. It is commonly displayed in millimeters of mercury, or mmHg. A time-based capnogram shows how measured CO2 changes throughout successive breaths.

A common adult reference range is 35–45 mmHg. Treat that range as a reference within the patient's clinical context. Ventilation, pulmonary blood flow, metabolism, and sampling quality all influence the reading. A value within the reference range does not establish that the patient is stable.

Pulse oximetry and capnography provide different information. SpO2 estimates oxygen saturation; capnography monitors exhaled CO2. A patient receiving supplemental oxygen can maintain a reassuring SpO2 while ventilation deteriorates.

How to Read a Normal Capnography Waveform

A typical waveform has a roughly rectangular outline with a slightly rising top. Follow one breath through four features:

  1. Baseline: CO2 is near zero during inspiration and the beginning of expiration, when gas from the conducting airways reaches the sensor.
  2. Expiratory upstroke: CO2 rises as exhaled gas contains an increasing proportion of alveolar gas.
  3. Alveolar plateau: Mostly alveolar gas passes the sensor. The end of this portion provides the ETCO2 measurement.
  4. Inspiratory downstroke: Fresh inspired gas produces a rapid return toward baseline.
Normal, obstructive, and elevated-baseline capnography patterns Three original schematic traces. A normal trace rises steeply to a nearly flat plateau and returns to zero. An obstructive trace has a sloping shark-fin appearance. An elevated-baseline trace does not return to zero between breaths. Time moves left to right; height represents carbon dioxide. CO2 waveform shapes Normal Steep rise; plateau Obstructive pattern Sloping “shark fin” Elevated baseline CO2 remains above zero 0 0 0 Time →
Original schematic illustrations. Time increases from left to right; vertical height represents CO2. These are teaching examples, not patient recordings or fixed diagnostic thresholds.

Common ETCO2 Waveforms and What They Can Mean

Use the shape and trend to guide further assessment. Several conditions can produce a similar pattern.

Capnography patterns to recognize in EMS
Pattern What you see What to consider
Rising ETCO2 Increasing waveform height over successive breaths. Reduced alveolar ventilation, increased CO2 production, or a change in circulation. Interpret with breathing depth and rate.
Falling ETCO2 Decreasing waveform height. Increased alveolar ventilation, reduced pulmonary blood flow, or a sampling problem.
Shark-fin waveform A slower expiratory rise and upward-sloping plateau. Expiratory airflow obstruction, including bronchospasm or an obstructed artificial airway. The shape does not identify the cause by itself.
Elevated baseline The trace remains above zero during inspiration. CO2 rebreathing or a technical problem. Assess the breathing system and monitor.
Curare cleft A notch within the plateau during mechanical ventilation. Possible spontaneous inspiratory effort. Assess the patient and ventilator interaction.
Sudden loss of waveform A previously visible trace disappears. Immediately assess the patient, airway, ventilation, and equipment. Causes include apnea, disconnection, airway displacement or obstruction, and severely reduced perfusion.

Why a Low ETCO2 Does Not Always Mean Hyperventilation

CO2 must reach the lungs through the circulation before it can be exhaled. A decrease in pulmonary blood flow can lower ETCO2 even when the ventilation rate has not changed. Look at perfusion, blood pressure, breathing, and the reliability of the sample together.

ETCO2 also differs from PaCO2, the carbon dioxide partial pressure measured in arterial blood. The AHA's post–cardiac arrest guidance notes that ETCO2 may underestimate PaCO2 after cardiac arrest. An apparently acceptable ETCO2 cannot establish an adequate arterial CO2 level in that setting; blood gas assessment helps guide ventilation when available.

Waveform Capnography During Cardiac Arrest

The 2025 AHA adult advanced life support guidelines recommend continuous waveform capnography together with clinical assessment to confirm and monitor endotracheal tube placement in adults with cardiac arrest.

ETCO2 trends can also help clinicians assess CPR effectiveness. An abrupt increase may signal return of spontaneous circulation, or ROSC. Ventilation changes, medications, and other factors can affect the reading, so confirm the finding through the resuscitation assessment process while minimizing interruptions in compressions.

ETCO2 alone should not determine when resuscitation ends. Follow your system's applicable protocols and medical direction.

A Consistent Approach to Capnography Interpretation

Work through the same questions each time you reassess:

  1. Patient: What are the mental status, breathing effort, chest movement, and signs of perfusion?
  2. Signal: Is there a reliable waveform, and is the sampling setup intact?
  3. Shape: What do the baseline, upstroke, plateau, and downstroke look like?
  4. Rate and value: How do the measured breaths and ETCO2 fit the examination?
  5. Trend: What changed after treatment, movement, or a change in the patient's condition?

With nasal or oral sampling, shallow breaths and sampling limitations can make the number less representative of arterial CO2. Interpret the trace alongside a direct assessment of ventilation.

Three Capnography Practice Cases

These fictional cases are designed for discussion. Consider your interpretation before opening each explanation.

Case 1: A Reassuring Oxygen Saturation

An adult receiving supplemental oxygen becomes increasingly drowsy after an opioid. Respirations slow to six per minute, SpO2 is 97%, and a consistent capnogram shows ETCO2 rising to 58 mmHg. What deserves attention?

View explanation

The slowing breathing, drowsiness, and rising ETCO2 raise concern for ventilatory impairment. Supplemental oxygen can delay desaturation. Promptly assess airway patency and breathing adequacy, support ventilation as indicated, and follow the relevant protocol.

Case 2: A Sloping Expiratory Waveform

A patient with wheezing and increased breathing effort has a shark-fin capnogram. Does the waveform establish a diagnosis of asthma?

View explanation

The pattern supports concern for airflow obstruction. Clinical history, examination, and airway assessment help identify the cause. Bronchospasm is one possibility; a mechanical obstruction can also distort the waveform. Reassess the patient and trace after intervention.

Case 3: A Sudden Rise During CPR

During an adult resuscitation, ETCO2 abruptly rises from 14 to 38 mmHg with no apparent ventilation change. What could explain the rise?

View explanation

ROSC is a possibility because improved circulation delivers more CO2 to the lungs. The numerical change does not confirm ROSC by itself. Correlate it with the clinical assessment and resuscitation protocol, keeping compression interruptions to a minimum.

Practice ETCO2 Waveforms in Capno Lab

Explore CE Solutions' Capno Lab waveform capnography simulator. It includes 11 cases, adjustable physiology controls, and a blind mode for identifying patterns.

Start with a normal case. Change one variable at a time, such as respiratory rate, airway obstruction, or pulmonary perfusion. Predict the effect, observe the trace, and explain why it changed. Then try blind mode.

The simulator uses simplified, illustrative physiology for learning. Its displayed values are not clinical predictions or treatment guidance.

Continue Learning With CE Solutions

CE Solutions has provided EMS continuing education since 1997. Explore our online EMS continuing education for EMTs, AEMTs, and paramedics, and use Capno Lab as a practice resource alongside your department's training.

This article and its practice cases are educational resources. Apply your scope of practice, local protocols, device instructions, and medical direction to patient care.

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