CARBON-DIOXIDE SPIROGRAM (BUT NOT CAPNOGRAM) DETECTS LEAKING INSPIRATORY VALVE IN A CIRCLE CIRCUIT

Expiratory valve incompetence in the circle circuit is diagnosed by us ing capnography (Pco(2) versus time) when significant CO2 is present t hroughout inspiration. However, inspiratory valve incompetence will al low CO2-containing expirate to reverse flow into the inspiratory limb. CO2 rebreathing occurs early during the next inspiration, generating a short extension of the alveolar plateau and decreased inspiratory do wnslope of the capnogram, which may be indistinguishable from normal. We hypothesized that CO2 spirography (Pco(2) versus volume) would corr ectly measure inspired CO2 volume (Vco(2)) during inspiratory valve le ak. Accordingly, a metabolic chamber (alcohol combustion) was connecte d to a lung simulator, which was mechanically ventilated through a sta ndard anesthesia circle circuit. By multiplying and integrating airway flow and Pco(2) overall, expired, and inspired Vco(2) (Vco(2,br) = Vc o(2,E)-Vco(2,I)) were measured. When the inspiratory valve was comprom ised (by placing a wire between the valve seat and diaphragm), Vco(2,I ) increased from 2.7 +/- 1.7 to 5.7 +/- 0.2 mL (P < 0.05), as measured by using CO2 spirography. In contrast, the capnogram demonstrated onl y an imperceptible lengthening of the alveolar plateau and did not mea sure Vco(2,I). To maintain effective alveolar ventilation and CO2 elim ination, increased Vco(2,I) requires a larger tidal volume, which coul d result in pulmonary barotrauma, decreased cardiac output, and increa sed intracranial pressure; Implications: Circle circuit inspiratory va lve leak will allow CO2-containing expirate to reverse flow into the i nspiratory limb, with subsequent rebreathing during the next inspirati on. This CO2 rebreathing causes imperceptible lengthening of the alveo lar plateau of the capnogram and is detected only by using the CO2 spi rogram (Pco(2) versus volume).