Absence of a capnogram after positive end-expiratory pressure

Abstract

Objective. The objective of this study was to evaluate the effect of positive end-expiratory pressure (PEEP) on capnography.Design. The study design was experimental and open, and it was performed in the Anesthesiology Experimental Research Laboratory.Methods. Six dogs (9.8±0.8 kg) were anesthetized and intubated. The animals' lungs were ventilated with a tidal volume of 137±34 ml and a respiratory frequency of 34±10 breaths/min to produce a Paco ₂ of 35 to 45 mm Hg. Application of 20 cm H₂O of PEEP was initiated for 1 minute, then repeated twice after 10-minute stabilization periods. Arterial pH and gas tensions were measured, and capnogram, airway gas flow, and airway pressure were recorded continuously. Airway gas flow was electronically integrated to calculate tidal volume.Results. Mean values before application of PEEP were as follows: pHa, 7.37±0.04 mm Hg; Paco ₂, 37.1±3.2 mm Hg; PaO₂, 93.4±1.6 mm Hg; andPetco ₂, 32.0±3.5 mm Hg. Compliance of the ventilator circuit was 3.3 ml/cm H₂O. Mean deflation lung-thorax compliance was 41.5±10.3 ml/cm H₂O. After application of PEEP, no capnogram was reported for 1 to 6 breaths, an average of 2.7±1.8 breaths.Conclusions. These results demonstrated that absence of gas flow immediately after the application of PEEP may transiently abolish a capnogram when the lung volume increases.