A normal capnogram (waveform) suggests correct tracheal intubation, whereas an absence of an expected capnogram will indicate the possibility of oesophageal intubation, even though all proper tests to verify tube placement are valid.

Normal capnogram:
(1) Starting with a low flat trace, as CO2 free dead space gas is exhaled, (2) expiratory upstroke: a rapid rise as a mixture of alveolar and dead space gas leaves the major airways, (3) the alveolar plateau of mostly alveolar gas, (4) a rapid decline as the next breath is taken.

Deficient plateau:
Could be due to obstructions which limit expiration, such as a kinked tube, herniated cuff, or bronchospasm.

Curare cleft:
A diaphragmatic twitch pulling some fresh gas past the sampling tube, usually seen with high CO2 levels.

Cardiogenic oscillations:
Caused by the beating of the heart against the lungs. More common in paediatrics, because the heart takes up relatively more space in the chest.

Possibility due to unequal emptying of the lungs, lateral position, or the tube touching the carina.

Note: A "shark fin" waveform suggests the need for a bronchodilator in, for example, patients with COPD. A down sloping alveolar plateau suggest emphysema.


Slow decrease in CO2 level:
Reasons could be: (1) Hyperventilation, (2) Fall in body temperature, (3) Falling lung or body perfusion.

Sudden drop in end tidal CO2 to nil:
In spontaneously breathing and ventilated patients: (1) Kinked ET tube, (2) Kinked or disconnected sampling tube, (3) Patient extubated, (4) Anaesthetic circuit disconnected. In a ventilated patient: the ventilator has failed.

A sudden drop in end tidal CO2 - but not to nil:
Reasons include: (1) Obstruction or leak (eg deflated cuff) in the circuit, (2) Airway obstruction eg, acute bronchospasm, (3) Leak in the sampling tube, which draws in room air.

A sudden rise in the baseline:
Reasons include: (1) Stuck valve in the circle absorber system, (2) Exhausted CO2 absorber, (3) Monitor calibration error, (4) Water in the system.

An exponential decrease in end tidal CO2:
Reasons include: (1) Cardiac arrest, (2) Hypovolaemia, (3) Pulmonary embolism, (4) Severe hyperventilation, (5) Sudden hypoventilation/hypovolaemia.

Gradual increase in end tidal CO2:
Reasons include: (1) Hypoventilation, (2) Absorption of CO2 from peritoneal cavity, (3) Rapidly rising body temperature, (4) Partial airway obstruction, (5) Reactive airway disease.

Sudden increase in end tidal CO2:
Reasons include: (1) Injection of sodium bicarbonate, (2) Release of tourniquet, (3) Sudden increase in blood pressure, (4) Malignant hyperthermia, (5) Ventilation of previously unventilated lung.