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Capnography: reading end-tidal CO2
The fastest confirmation that a tube is in the trachea, and the earliest warning that a circulation is failing.
The normal waveform
A capnogram has four parts. A flat baseline at zero during inspiration and the first part of exhalation, when dead-space gas with no CO2 in it is leaving. A steep upstroke as alveolar gas arrives. A near-flat alveolar plateau with a slight upward slope, whose highest point is the end-tidal value. Then a vertical drop back to zero as the next breath begins.
A trace with all four parts and a plateau that is genuinely flat is a trace you can trust the number from.
The rule that never breaks
End-tidal CO2 is at or below the arterial CO2. It is never above it.
So whatever the capnograph reads, the arterial CO2 is at least that much. A reading of 55 does not mean the PaCO2 is 55; it means the PaCO2 is 55 or worse.
An end-tidal value above the arterial one is a sampling or calibration fault, with one rare physiological exception in late pregnancy.
The gradient is the signal
The space between the two numbers measures dead space: alveoli that are ventilated but not perfused, diluting the exhaled gas with CO2-free air.
| Pattern | Reading |
|---|---|
| Both stable, gradient 3 to 5 | Healthy gas exchange |
| Both rising | Hypoventilation, or more CO2 being produced: fever, sepsis, malignant hyperthermia, bicarbonate given, a tourniquet released |
| End-tidal falling, arterial steady or rising | Dead space increasing: pulmonary embolism, falling cardiac output, shock, overdistension from too much PEEP |
| Both falling | Hyperventilation, hypothermia, a slowing metabolism |
A widening gradient with a falling end-tidal value and an unchanged ventilator is a circulatory problem until proven otherwise.
Shapes worth knowing
| Shape | Cause |
|---|---|
| Sloped plateau, the shark fin | Bronchospasm or COPD: alveoli emptying at different rates |
| A notch cut into the plateau | A spontaneous diaphragmatic effort through incomplete paralysis |
| Baseline that never reaches zero | Rebreathing: exhausted absorbent, an incompetent valve, inadequate fresh gas flow |
| Sudden fall to zero | Disconnection, complete obstruction, an esophageal tube, or cardiac arrest |
| Sudden fall, but not to zero | A partial disconnect, or an abrupt drop in cardiac output |
When the trace disappears
Loss of the waveform is the single best indicator that a tube is not in the trachea. Work the differential in order and do not pause to adjust settings first.
- Displacement: esophageal, supraglottic, or down a mainstem bronchus.
- Obstruction: a kinked tube, a mucus plug, bronchospasm, a patient biting.
- Pneumothorax, particularly a tension pneumothorax.
- Equipment: a contaminated sensor, water in the sampling line, a disconnect beyond the sensor, a ventilator fault.
- Stacked breaths: hyperinflation cutting venous return and so cutting the output that delivers CO2 to the lung.
Disconnect from the ventilator and bag with 100 percent oxygen. If the patient improves immediately, the problem was the ventilator or the circuit.
During CPR
With no spontaneous circulation, end-tidal CO2 tracks the blood flow that chest compressions are generating, which makes it the only continuous feedback on compression quality available at the bedside.
A value persistently below 10 mmHg after twenty minutes of resuscitation is associated with very poor survival. A sudden rise, often above 35 to 40, is one of the earliest signs of return of circulation and usually appears before a pulse can be felt.
Nobody ever mastered a ventilator by reading about one.
Drop the cardiac output on a simulated patient and watch the end-tidal value fall while the ventilator settings do not change. The Ventilator Workbook puts a live ventilator in front of you with a simulated patient behind it, so you can break things that cannot be harmed. Three modules and time on the simulator are free, and no card is needed.
Keep reading
- How to read an arterial blood gas
- Patient-ventilator dyssynchrony: the patterns
- Ventilator glossary, for the terms in this article
- Every topic in the library