5 Capnography and Respiratory Monitoring

Capnography and its measurements

Capnography continuously measures carbon dioxide (CO₂) in exhaled gas. A Capnogram displays that measurement over time, while End-tidal CO₂ (EtCO₂) is the value at the end of exhalation, usually shown as a number alongside the waveform.

Capnography provides information about ventilation. Because exhaled CO₂ also depends on blood flow through the lungs and CO₂ production, it can reflect changes in perfusion and metabolism. It complements, but does not replace, patient assessment, pulse oximetry, or arterial blood gas measurement when indicated.

In a stable adult, EtCO₂ is often about $35\text{–}45\ \mathrm{mm\ Hg}$. It commonly runs a few mm Hg below arterial CO₂ (PaCO₂), but the difference can widen when ventilation–perfusion matching or pulmonary perfusion is abnormal. EtCO₂ should not be used as a direct substitute for PaCO₂ without considering the clinical context.

Reading the capnogram

A typical breath has four parts:

1. Phase I—baseline: Inspired gas and gas from the conducting airways; CO₂ is near zero.

2. Phase II—upstroke: Exhaled gas mixes with CO₂-rich alveolar gas, so CO₂ rises rapidly.

3. Phase III—alveolar plateau: Gas is predominantly alveolar. The value at the end of this phase is EtCO₂.

4. Inspiratory downstroke: Fresh inspired gas rapidly returns the trace toward baseline.

A normal trace repeats with each breath, with a near-zero baseline and a distinct plateau. Assess the shape, height, rate, and continuity of the waveform—not just the numeric value.

Interpreting changes

Interpret changes alongside respiratory rate, chest movement, oxygen saturation, circulation, and the device or airway in use.

• EtCO₂ rises: This may indicate hypoventilation, reduced minute ventilation, increased CO₂ production, or improved pulmonary blood flow. A gradual rise during sedation can warn of worsening ventilation before oxygen saturation falls, especially when supplemental oxygen is being given.

• EtCO₂ falls: This may indicate hyperventilation, reduced pulmonary perfusion, such as in shock or cardiac arrest, increased dead-space ventilation, or a sampling problem. A sudden rise during CPR may accompany return of spontaneous circulation, but confirm clinically.

• Slanted “shark-fin” plateau: This suggests expiratory airflow obstruction, as in bronchospasm or obstructive lung disease. The waveform alone does not establish the cause.

• Elevated inspiratory baseline: This suggests CO₂ rebreathing. Check valves, absorber, fresh-gas flow, and circuit setup as appropriate to the equipment.

• Notch or cleft in the plateau: This may reflect a spontaneous inspiratory effort during controlled ventilation. Assess patient–ventilator interaction and the clinical situation.

• Sudden loss of the waveform: Possible causes include apnea, disconnection, displacement, obstruction, severe loss of pulmonary blood flow, or monitor or sampling failure. Treat an abrupt change as urgent until explained.

Confirming and monitoring airway placement

After endotracheal intubation, use Continuous waveform capnography with clinical assessment to confirm and monitor tube placement. A sustained exhaled-CO₂ waveform over successive breaths strongly supports tracheal placement. Chest rise, auscultation, or tube fogging alone is not a reliable substitute.

Continue monitoring because a previously correct tube can later become displaced or obstructed. During cardiac arrest or profound low-flow states, exhaled CO₂ may be very low or difficult to detect, making an absent or weak trace less reliable as a standalone test. Immediately reassess the patient, ventilation, airway, circuit, and monitor, and use additional confirmation methods according to clinical protocol.

Do not treat a single EtCO₂ number as proof of tube position or as the sole basis for ending resuscitation.

Troubleshooting an abnormal trace

When a trace changes suddenly, use a structured check rather than focusing only on the displayed number:

1. Patient: Is the patient breathing? Have chest movement, pulse, blood pressure, or clinical status changed?

2. Airway: Could the tube or airway device be displaced, kinked, bitten, blocked by secretions, or affected by severe bronchospasm?

3. Circuit: Check for disconnection, leaks, obstruction, faulty valves, and correct connections.

4. Sampling and monitor: Check the sampling line, water trap, filter, cannula or adapter, power, and alarm settings; confirm that the sensor is functioning.

5. Reassess the trace: After correcting an identified problem, verify that a consistent waveform returns and that the patient improves.

A normal oxygen saturation does not rule out hypoventilation: supplemental oxygen can maintain saturation while CO₂ rises. Conversely, EtCO₂ can fall because of poor perfusion even when ventilation has not changed. Interpret both monitors in context and escalate promptly when the patient is deteriorating.