Carbon dioxide retention and capnography

Published on 07/02/2015 by admin

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Last modified 07/02/2015

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Carbon dioxide retention and capnography

Michael G. Ivancic, MD

The monitoring of CO₂, the most abundant gas produced by the human body during anesthesia, has become a standard of practice strongly encouraged by the American Society of Anesthesiologists. CO₂ is a byproduct of cellular metabolism, transported to the lungs by the systemic venous system and eliminated from the alveoli during ventilation (see Chapter 22).

CO₂ retention

Rebreathing of CO₂ is undesirable, although, during mechanical ventilation, allowing the patient to rebreathe CO₂ is infrequently used to achieve normocarbia in those patients who are being hyperventilated (i.e., when large tidal volumes may be desirable for other reasons). A leak or obstruction in the anesthesia machine circuit, common gas outlet, or fresh gas supply line may also cause an increase in CO₂ concentration.

When Mapleson systems are used, inadequate fresh gas flow is the primary cause of an increase in CO₂ because these systems do not contain unidirectional valves or absorbent canisters. Specifically, systems with inner tubes, such as the Bain system, can cause rebreathing if there is any dysfunction (kink) in that tube. The Mapelson D (Bain circuit) is the most efficient for controlled ventilation with regard to a relatively low flow of fresh gas, whereas the Mapleson A is most suitable for patients who are spontaneously breathing (see Chapter 193). Specific minimum fresh gas flow rates for the various Mapleson apparatuses are recommended for spontaneous ventilation as well as controlled ventilation (Box 9-1).

Increased dead space, whether mechanical or physiologic, can increase rebreathing and CO₂ retention if the dead space is particularly large, especially in smaller patients. A heat and moisture exchanger in the breathing circuit may also be a source of dead space, with the larger the volume of the heat and moisture exchanger, the larger the dead space.

In the closed-circle systems used in modern anesthesia machines, minimal rebreathing of CO₂, if any, should occur; however, malfunction of either of the unidirectional valves may lead to CO₂ rebreathing. If an inspiratory valve is stuck open, rebreathing can occur because, during expiration, alveolar gas can backfill the inspiratory limb of the circle. A malfunctioning expiratory valve can lead to CO₂ rebreathing in a spontaneously breathing patient because, during inspiration, the negative pressure generated by the patient can entrain alveolar gas from the expiratory limb of the circuit.

Other causes of inadvertent CO₂ rebreathing usually involve the CO₂ absorber. If the absorbent color indicator malfunctions—and, therefore, is not reflecting the true level of CO₂ in the system—rebreathing can occur without the anesthesia provider being aware of the problem. In older anesthesia machines, the CO₂ absorber could be bypassed. Older absorbent canisters had a rebreathing valve on them that, if engaged, would lead to CO₂ rebreathing. Channeling of gas through the canister without contacting any active absorbent can also lead to CO₂ rebreathing. Independent of the cause, absorbent malfunction is best corrected immediately by increasing the fresh gas flow and then troubleshooting the underlying cause.

Capnography

During induction of and emergence from anesthesia, rebreathing of CO₂ will lengthen each process because of alterations in alveolar tensions associated with rebreathing of exhaled alveolar anesthetic gases. Capnography can help troubleshoot malfunctioning equipment, such as the problems noted above. The classic rebreathing pattern on the capnogram will show an elevation of the waveform baseline that does not return to 0, as well as a higher end-tidal CO₂ (PETCO₂) reading, although the PaCO₂ value may be normal, depending on the degree of alveolar ventilation.

Terminology

Capnometry is the measurement and numeric display of the partial pressure or gas concentration of CO₂. A capnometer is the device that measures and displays the concentration of CO₂. Capnography is the graphic record of CO₂ concentration, and the capnograph is the device that generates the waveform, with the capnogram being the actual waveform.

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