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Perfluorocarbon and accuracy of tidal volume estimation.

We read with interest the report by Dr. Devabhaktuni and colleagues (1) on the effect of nitric oxide, perfluorocarbon, and heliox on differential pressure pneumotachometer characteristics. We have experienced problems when using a hot wire anemometer (Bear NVM, Medical Systems, Riverside, CA) to measure tidal volume during partial liquid ventilation with a perfluorocarbon. Ethical approval was obtained from the Animal Care Committee to undertake a study to investigate pulmonary blood flow distribution during partial liquid ventilation. Seven male pigs (mean weight, 17.4 [SD 2.3] kg) were studied. Animals were premedicated with intramuscular ketamine and acepromazine, anesthetized with inhaled halothane, and intubated with a cuffed endotracheal tube. Anesthesia was maintained throughout the study with an infusion of pentobarbital sodium and intermittent doses of a muscle relaxant (pancuronium 0.1 mg/kg). Animals were studied in the supine position. Animals were initially ventilated using conventional gas ventilation with the following ventilator settings using a Humming V ventilator (Medtran, Japan): FIO2 1.0; frequency, 16 breaths/min; positive end-expiratory pressure (PEEP), 5 cm H2O; peak inspiratory pressure to achieve a tidal volume of 12 mL/kg; inspiratory time, 1.5 secs. Before partial liquid ventilation (PLV), aliquots of 10 mL/kg warmed, preoxygenated perfluorocarbon (RM 101, Mercantile Development) were instilled via the endotracheal tube until a meniscus remained visible within the tube when disconnected from the ventilator. During PLV, this is referred to as liquid “functional residual capacity” and in these animals required approximately 40 mL/kg of perfluorocarbon (PFC). During PLV, the inspiratory pressure was adjusted to maintain inspired gas tidal volume at 12 mL/kg. The other ventilator settings remained unchanged. In two animals, we investigated the effect of altering PEEP from 5 cm H2O through 20 cm H2O. We noted a considerable discrepancy between the measured inspired tidal volume and expired tidal volume during PLV (mean [SD]: 207.3 [18.6] mL vs. 353.7 [29.1] mL) that had not been apparent during the period of conventional gas ventilation that preceded partial liquid ventilation (206.9 [20.4] mL vs. 219.4 [20.6] mL). Interestingly, the magnitude of the difference between inspired tidal volume and expired tidal volume fell as PEEP was progressively increased during PLV in two of the animals (Fig. 1). We confirmed these findings in a benchtop experiment. Two identical 200-mL flasks were set up, one containing 50 mL of PFC and the other air (Fig. 2). Calibration of flow was done separately for both sensors using a Bear hot wire anemometer and a Fleisch pneumotachometer. Flow was set at 150 mL/sec using the air-filled bottle (air1); the lid was then transferred onto the PFCcontaining bottle and finally returned to the air-filled bottle (air2). Whereas the Fleisch pneumotachometer recorded similar flow during all three phases, the bear recorded a consistently higher flow during the PFC phase of the experiment (Fig. 3). The experiment was repeated with the positions of the Bear anemometer and Fleisch pneumotachometer reversed, and the same results were obtained. We found that perfluorocarbon interferes with flow measurement when using the Bear monitor, tending to overestimate flow. We hypothesize that this is due to the different thermal properties of PFC vapor. The overestimation of exhaled tidal volume falls with increasing PEEP, perhaps as a result of liquid PFC being held out of the larger proximal airways during exhalation (2).