Confirming adequate sampling

Capnography may be the only guide to detect the adequacy of gas sampling

Normal	Sampling leaks/dilution of expired gases

Capnography may be the only guide to detect the adequacy of gas sampling from the airways, the presence of leaks in the sampling system, and the malfunction of CO2 measuring equipment. The dilution of expired CO2 by the atmospheric air during nasal sampling, and by the fresh gas flow (FGF) during endotracheal tube (ETT) sampling, can result in false low PETCO2 values. This can be easily detected by monitoring the shape of CO2 waveform where an adequate sampling of CO2 results in a good and consistent capnogram, whereas dilution of expired CO2 by the atmospheric air/FGF results in an abnormal capnogram (see clips above).

Characteristic abnormal waveforms can help in the diagnosis of underlying clinical or technical abnormalities

Normal	Bronchospasm

Apnea / Circuit disconnection / Accidental tracheal extubation / ventilator failure

 

It is more advantageous to have a continuous recording of the capnograms rather than a digital display of PETCO2, or occasional recordings of capnograms. An analysis of the capnograms gives more information and better insight into the clinical situation and offers an early clue to the diagnosis of potentially life threatening conditions such as partial airway obstruction, accidental extubation and circuit disconnections, hypermetabolic states, etc. This permits early institution of the corrective measures before an irreversible damage is done to the patient. It also gives information regarding the type of breathing and V/Q abnormalities in the lung (see clips above). There is only one normal capnogram and all variations must be recognized and corrected where possible. Abnormalities should be found by analyzing the various phases of capnogram for individual breaths as well as observing trends over a period of time. Five characteristics should be inspected, viz., height, frequency, rhythm, base line and shape. Abnormalities in each phase of the capnogram are associated with either patient- or equipment-related problems as shown below.

Therapeutic aid

The efficacy of remedial measures undertaken can be assessed from the continuous evaluation of capnograms

Before treatment with bronchodilators	After treatment with bronchodilators

Once a diagnosis of a clinical condition is established based on the shape of capnogram, the efficacy of remedial measures undertaken can be assessed from continuous evaluation of capnograms. An example is shown in the fig as above.