Background: The multiple inert gas elimination technique was developed to measure shunt and the ratio of alveolar ventilation to simultaneous alveolar capillary blood flow in any part of the lung (V(A)'/Q') distributions. Micropore membrane inlet mass spectrometry (MMIMS), instead of gas chromatography, has been introduced for inert gas measurement and shunt determination in a rabbit lung model. However, agreement with a frequently used and accepted method for quantifying deficits in arterial oxygenation has not been established. We compared MMIMS-derived shunt (M-S) as a fraction of total cardiac output (CO) with Riley shunt (R-S) derived from the R-S formula in a porcine lung injury model.
Methods: To allow a broad variance of atelectasis and therefore shunt fraction, 8 sham animals did not receive lavage, and 8 animals were treated by lung lavages with 30 mL/kg warmed lactated Ringer's solution as follows: 2 animals were lavaged once, 5 animals twice, and 1 animal 3 times. Variables were recorded at baseline and twice after induction of lung injury (T1 and T2). Retention data of sulfur hexafluoride, krypton, desflurane, enflurane, diethyl ether, and acetone were analyzed by MMIMS, and M-S was derived using a known algorithm for the multiple inert gas elimination technique. Standard formulas were used for the calculation of R-S.
Results: Forty-four pairs of M-S and R-S were recorded. M-S ranged from 0.1% to 35.4% and R-S from 3.7% to 62.1%. M-S showed a correlation with R-S described by linear regression: M-S = -4.26 + 0.59 x R-S (r(2) = 0.83). M-S was on average lower than R-S (mean = -15.0% CO, sd = 6.5% CO, and median = -15.1), with lower and upper limits of agreement of -28.0% and -2.0%, respectively. The lower and upper limits of the 95% confidence intervals were -17.0 and -13.1 (P < 0.001, Student's t-test).
Conclusions: Shunt derived from MMIMS inert gas retention data correlated well with R-S during breathing of oxygen. Shunt as derived by MMIMS was generally less than R-S.