![]() The error arising from unmeasured alveolar–arterial differences for the two volatile agents is likely to be small and will effectively only scale the uptake calculations but will not substantially alter the results. Because the arterial partial pressure might have been lower than indicated by the end-expired partial pressure, the data presented in figure 5 may slightly overestimate uptake and elimination. Second, we did not measure blood–gas partition coefficients but retrieved them from the literature 4 and assumed they would match this in the animals used in this study. We believe this to be the case because the pigs’ lungs were healthy with minimal V A/Q abnormality, as evidenced by the absence of a significant alveolar–arterial partial pressure difference for carbon dioxide before methacholine administration. The partial pressure at this point in the oxygen cascade can be determined by using the alveolar gas equation. First, the calculation of the uptake of inhaled agent across the alveolocapillary membrane was based on a calibration factor that assumed that the end-tidal anesthetic partial pressure approximated the arterial anesthetic partial pressure. When the gas reaches the alveoli the partial pressure of oxygen will again decrease as some oxygen is absorbed and CO2 is excreted. Two assumptions were made that allow us to derive uptake from the partial pressures and cardiac output measurements.
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