Capnography tips

Bhavani Shankar Kodali MD

Tips on using capnography- Abnormal values and shapes

The following guidelines can be used to assess abnormal capnograms. However, a thorough understanding of underlying principles of capnography will to help you to maximize the benefit of capnography in rectifying a clinical abnormality during anesthesia.

1. Rule out technical causes of abnormal capnograms. The guidelines listed in the previous two pages should help in minimizing abnormal waveforms as a result of technical errors. The author is privileged to have been called when abnormal capnograms were observed in the operating rooms. The most frequent abnormal capnograms which were as a result of technical problems included a loose connection between the sampling tube and the monitor, cracked connection at the monitor end of sampling tube, and sticking unidirectional valves of the closed circuit. The abnormal capnograms observed are as follows.

Sticking Inspiratory unidirectional valve	Loose connection between the sampling tube and the monitor

Details	Details

2. Look at the PETCO2 value: Determine if low, normal or high. Follow the differential diagnosis scheme as outlined in the table (Clinical aspects).

3. A low PETCO2 is commonly due to hyperventilation and progressively decreasing metabolism during anesthesia. A normal shape of a capnogram may generally rule out sampling errors. A progressive decrease in PETCO2 for a given ventilation may suggest low cardiac output and therefore, attention should be directed to the cardiovascular system. The author (after reviewing several papers on cardiac output and PETCO2) believes that a significant decrease in PETCO2 for given ventilation is most likely due to a reduction in cardiac output and uses this concept in day-to-day practice during anesthesia. A decreased blood pressure but unchanged PETCO2 generally suggests that cardiac output is probably still maintained in the light of decreased SVR, a common scenario during anesthesia. However, If PETCO2 also decreases with decreasing blood pressure, then it probably suggests reduced pulmonary blood flow consequent to a low cardiac output. Of course, one has to remember rarer causes of such a scenario, including air embolism or thromboembolism.

4. If PETCO2 is high, determine if the capnogram’s baseline is at zero or elevated. If at zero, hypoventilation is suggested. If PETCO2 increases despite progressively increasing ventilation, a hypermetabolic state should be kept in mind (rare but can happen, such as malignant hyperpyrexia or thyrotoxic storm).

5. Baseline elevation in increased, normal, or low PETCO2 presence suggests rebreathing. A rebreathing can be normal in rebreathing circuits, but definitely abnormal in a closed circuit anesthesia and suggests exhaustion of soda lime or defective unidirectional inspiratory or expiratory valves.

6. A sudden elevation of baseline and PETCO2 usually suggests contaminated monitors.

7. If capnogram is abnormal in shape, proceed to analyze starting from baseline (phase I), upstroke (phase II), alveolar plateau (phase III), and descending limb (phase 0). Commonly seen abnormal capnograms

Phase I: Elevation: Rebreathing
Phase II: Prolongation: Airway obstruction
Phase III: Increased slope: Airway obstruction; Cleft in phase III: Spontaneous effort during controlled ventilation

Phase 0: Descending limb usually makes a nearly 90 angle with an alveolar plateau. An increase in this angle (a sloping descending limb) usually suggests rebreathing.For an atlas of abnormal capnogram details: see Capno-encyclopedia.A sample list of abnormal capnograms is as follows:

Bronchospasm	Curare cleft

Ripple effect	Contamination

Sampling tube leak	Hypoventilation

Bain circuit	Apnea

Inspiratory unidirectional valve defect	Spontaneously breathing capnograms in children

Capnography tips

Tips on using capnography- Abnormal values and shapes

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