Waveform Capnography, Pulse Oximetry, and Continuous Monitoring

Why Continuous Monitoring Matters in Airway Management

Once an advanced airway is placed, your job is not finished. Tube dislodgement, bronchospasm, and patient deterioration can occur at any moment during transport. Waveform capnography and pulse oximetry together give you a continuous, real-time picture of oxygenation and ventilation. Used correctly, they are among the most powerful tools in the paramedic's kit. Used without understanding their limitations, they can create false reassurance.

Waveform Capnography: The Basics

Capnography measures the concentration of carbon dioxide (CO₂) in exhaled air and displays it as a waveform over time. The numeric value displayed is the end-tidal CO₂ (EtCO₂), expressed in millimeters of mercury (mmHg). Normal EtCO₂ in a healthy adult at rest is roughly 35–45 mmHg, closely approximating arterial CO₂ levels.

The waveform itself has a recognizable rectangular shape with four phases:

• Phase I: Baseline — exhaled dead-space air with near-zero CO₂
• Phase II: Rapid rise — alveolar gas mixes with dead-space gas
• Phase III: Alveolar plateau — CO₂-rich alveolar gas exhaled; the EtCO₂ reading is taken at the peak of this phase
• Phase IV (descending): Inspiration — CO₂ falls sharply to baseline

A normal waveform has a flat alveolar plateau and returns cleanly to baseline with each breath.

Recognizing Abnormal Waveform Patterns

Absent waveform (flat line): No CO₂ detected. In an intubated patient this strongly suggests esophageal intubation or complete tube obstruction. Treat as misplacement until proven otherwise — immediately confirm tube position clinically and with direct laryngoscopy.

Hyperventilation: Normal waveform shape but EtCO₂ below 35 mmHg. Ventilation rate or tidal volume is too high. Slow the rate and check tidal volume.

Hypoventilation: Normal waveform shape but EtCO₂ above 45 mmHg. Ventilation is inadequate. Increase rate or volume as guided by protocol.

Shark-fin pattern: The alveolar plateau slopes upward rather than remaining flat, creating a pointed peak. This indicates obstructive airflow, most commonly bronchospasm or reactive airway disease. Administer bronchodilator therapy per protocol.

Cardiac arrest waveform: Very low EtCO₂ (often below 10 mmHg) with small, irregular waveforms. EtCO₂ reflects pulmonary perfusion; low values in arrest reflect poor cardiac output. A sudden sustained rise in EtCO₂ during CPR may signal return of spontaneous circulation (ROSC).

Gradually declining EtCO₂: Consider tube displacement (partial), decreasing perfusion, or a leak in the ventilation circuit. Reassess immediately.

Using Capnography to Confirm Tube Placement

Continuous waveform capnography is the gold standard for confirming and monitoring endotracheal tube placement in the prehospital environment. A consistent, rectangular waveform with appropriate EtCO₂ values across multiple breaths confirms the tube is in the trachea. Colorimetric CO₂ detectors change color but do not provide the ongoing monitoring that a waveform device does. Any time the waveform disappears or becomes erratic, treat it as a tube problem until you rule one out.

Pulse Oximetry: Value and Limitations

Pulse oximetry (SpO₂) measures the percentage of hemoglobin saturated with oxygen using light absorption. A reading of 95% or higher is generally considered acceptable in most patients, though clinical context always applies.

SpO₂ is fast and non-invasive, but it has important limitations every paramedic must know:

• Carbon monoxide poisoning: CO binds hemoglobin similarly to oxygen. Pulse oximeters cannot distinguish carboxyhemoglobin from oxyhemoglobin, so the SpO₂ will read falsely high in CO-poisoned patients. These patients may appear well-saturated while being critically hypoxic.
• Peripheral vasoconstriction: Cold extremities, hypoperfusion, or vasoconstrictors reduce the signal. Readings may be unreliable or unobtainable.
• Nail polish and artificial nails: Dark or opaque nail polish can interfere with light transmission. Move the probe to another digit or the earlobe.
• Motion artifact: Patient movement, shivering, or transport vibration can cause falsely low or erratic readings.
• Anemia: Severely anemic patients can have normal SpO₂ readings while carrying inadequate oxygen for tissue needs, because the hemoglobin that is present is fully saturated.

Because of these limitations, SpO₂ must always be interpreted alongside capnography, clinical assessment, and patient history. Never withhold oxygen from a symptomatic patient because SpO₂ appears normal.

Integrating Both Monitors: A Safety Framework

Think of capnography and pulse oximetry as two different windows into the same patient. Capnography tells you about ventilation (CO₂ elimination) and tube position in near-real time — changes appear within a breath or two. Pulse oximetry tells you about oxygenation, but it lags behind ventilation problems by seconds to minutes because of oxygen reserves already in the blood. A patient who loses their airway will show a capnography change before the SpO₂ drops. This lag makes capnography the earlier warning system.

Document EtCO₂ and SpO₂ values at regular intervals on your patient care report. Trending matters: a gradual rise in EtCO₂ or a slow fall in SpO₂ may be more significant than a single abnormal reading.

Safety Priorities

• An absent capnography waveform after intubation is an emergency. Do not assume equipment failure without ruling out esophageal intubation.
• Never adjust ventilation rate or volume based solely on SpO₂.
• In suspected CO poisoning, treat with high-flow oxygen regardless of SpO₂ reading.
• Any unexplained change in waveform or values requires immediate reassessment of the patient and airway.