Nursing care
End-Tidal CO2 Monitoring: reading the number and acting on it
Written and reviewed by Dana Whitfield, RN, MSN · 4 min read · Updated September 2026
Short answer
End-tidal CO2, or capnography, measures the carbon dioxide exhaled with each breath, normally 35 to 45 mmHg. It confirms endotracheal tube placement faster than auscultation and tracks ventilation in real time. A sudden drop toward zero means the tube has dislodged or the heart has stopped pumping blood to the lungs.
What the test measures
Capnography measures the partial pressure of carbon dioxide at the end of exhalation, reflecting how well carbon dioxide is being carried from the tissues to the lungs and expelled. It is displayed as both a numeric value and a waveform, and the waveform shape carries as much information as the number.
Unlike pulse oximetry, which lags behind changes in ventilation by up to a minute, end-tidal CO2 changes breath by breath, giving an immediate read on ventilation adequacy. It is used continuously on intubated and sedated patients, and increasingly on patients receiving procedural sedation who are still breathing spontaneously.
Normal ranges and what moves them
Normal end-tidal CO2 is 35 to 45 mmHg, running slightly below arterial CO2 in a healthy lung due to dead space. The value rises with hypoventilation, increased metabolic production such as fever or sepsis, or rebreathing, and falls with hyperventilation, reduced metabolism such as hypothermia, or reduced pulmonary blood flow.
During cardiac arrest, end-tidal CO2 also reflects the effectiveness of chest compressions, since it depends on blood actually reaching the lungs to carry CO2 for exhalation. A value that climbs during resuscitation, sometimes to 40 mmHg or higher, can signal return of spontaneous circulation before a pulse is even palpated.
What a high result means
A rising end-tidal CO2 usually means hypoventilation, whether from oversedation, opioid effect, neuromuscular weakness, or an airway obstruction that is trapping exhaled gas. On a ventilated patient it can also indicate the ventilator rate or tidal volume is set too low for the patient's needs.
A markedly elevated value with a waveform that fails to return to baseline, sometimes called a shark-fin pattern, points to bronchospasm or airway obstruction rather than simple hypoventilation, since exhalation is being slowed rather than reduced in volume. This distinction changes the intervention: bronchodilators versus increased ventilatory support.
What a low result means
A low but present end-tidal CO2 can reflect hyperventilation, increased dead space from pulmonary embolism, or reduced cardiac output reducing pulmonary blood flow. In a patient on a ventilator, check the set rate and tidal volume first, since overventilation is a common and easily corrected cause.
A sudden drop to near zero is a different and more urgent finding. It means either the endotracheal tube has become dislodged or occluded and CO2-laden air is no longer reaching the sensor, or the heart has stopped pumping blood to the lungs, as in cardiac arrest. Either interpretation demands immediate assessment, not a wait-and-recheck approach.
Nursing actions by result
For a rising value on a spontaneously breathing sedated patient, stimulate the patient, reposition the airway, and prepare to reduce sedation or provide ventilatory support if the trend continues. Notify the provider if the value climbs despite these measures.
For a sudden drop to near zero on an intubated patient, immediately assess for tube displacement, check breath sounds bilaterally, and follow your unit's protocol for a lost airway before assuming the worst. If the patient is pulseless, this finding supports beginning or continuing CPR without delay, since it is one of the fastest available indicators that circulation has stopped.
Patient preparation and teaching
For a patient receiving procedural sedation with capnography monitoring, explain that a small nasal cannula sensor will sample their breath continuously and that this allows the team to catch slowed breathing before oxygen levels drop, since oximetry alone reacts too slowly.
For family at the bedside of an intubated patient, explain in plain terms that the number and wave pattern on the monitor confirm the breathing tube is correctly placed and working, and that the team watches it continuously as part of routine monitoring, not because of a new concern, unless they are told otherwise.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our respiratory practice questions are the closest set to what this page covers.
Common questions
How does end-tidal CO2 confirm endotracheal tube placement?
A sustained waveform with values in or near the normal range confirms the tube is in the trachea, since only the lungs produce a rhythmic CO2 signal with each breath. Absence of a waveform after intubation strongly suggests oesophageal placement and requires immediate tube removal and reintubation.
Why use capnography instead of just listening for breath sounds?
Breath sounds can be misleading, transmitted from the stomach or the opposite lung, and take time to assess. Capnography gives a continuous, objective, breath-by-breath signal that confirms placement within seconds and keeps confirming it throughout the case.
What does a capnography reading during CPR tell the team?
It reflects how well chest compressions are generating blood flow to the lungs, and a value consistently below 10 mmHg during resuscitation suggests compressions need improvement or the situation is unlikely to be reversed. A sudden sustained rise can be an early sign of return of spontaneous circulation.
Is end-tidal CO2 the same as arterial CO2?
No, though the two are normally close, with end-tidal CO2 running a few mmHg below arterial CO2 due to dead space in the airways. The gap widens with conditions like pulmonary embolism, so a normal end-tidal value does not always rule out abnormal arterial CO2.
What should be done if the capnography alarm sounds and the value is zero?
Assess the patient and the airway immediately rather than assuming equipment failure. Check tube position, breath sounds, and the patient's pulse and responsiveness, since this can indicate a dislodged tube, disconnection, or cardiac arrest.