## Abstract

We simulated the use of simultaneous sinusoidal changes of inspired O_{2} and N_{2}O (Williams et al., J Appl Physiol, 1994; 76: 2130-9) at fractional concentrations up to 0.3 and 0.7, respectively, to estimate FRC and pulmonary blood flow (PBF) during anaesthesia, using O_{2} as an insoluble indicator. Hahn's approximate equations, which neglect the effect of pulmonary uptake and excretion on expiratory flow, estimate dead space and alveolar volume (V(A)) with systematic errors less than 10%, but yield systematic errors in PBF which are approximately proportional to F1(N)_{2}(O) in magnitude. A correction factor (I - P̄)^{-1} for Hahn's equations for PBF (where P̄ is the mean partial pressure of the soluble indicator) reduces the dependence of PBF estimates on F1(N)_{2}(O), and the solution of equations describing the simultaneous mass balance of both indicators yields accurate results for a wide range of mean F1(N)_{2}(O). However, PBF estimates are sensitive to measurement errors and a third gas must be present to ensure that the indicator gases behave independently.

Original language | English |
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Pages (from-to) | 371-378 |

Number of pages | 8 |

Journal | British Journal of Anaesthesia |

Volume | 85 |

Issue number | 3 |

Publication status | Published - 2000 |

Externally published | Yes |

## Keywords

- Computers
- Lung, blood flow
- Lung, dead space
- Lung, volume
- Measurement techniques, pulmonary arterial