Time required for partial pressure of arterial oxygen equilibration during mechanical ventilation after a step change in fractional inspired oxygen concentration

Abstract

Objective: To determine the time required for the partial pressure of arterial oxygen (PaO₂) to reach equilibrium after a 0.20 increment or decrement in fractional inspired oxygen concentration (FIO₂) during mechanical ventilation. Setting: A multi-disciplinary ICU in a university hospital. Patients and methods: Twenty-five adult, non-COPD patients with stable blood gas values (PaO₂/FIO₂ ≥ 180 on the day of the study) on pressure-controlled ventilation (PCV). Following a baseline PaO₂ (PaO₂b) measurement at FIO₂ =0.35, the FIO₂ was increased to 0.55 for 30 min and then decreased to 0.35 without any other change in ventilatory parameters. Sequential blood gas measurements were performed at 3, 5, 7, 9, 11, 15, 20, 25 and 30 min in both periods. The PaO₂ values measured at the 30^th min after a step change in FIO₂ (FIO₂ =0.55, PaO₂[55] and FIO₂ =0.35, PaO₂[35]) were accepted as representative of the equilibrium values for PaO₂. Each patient's rise and fall in PaO₂ over time, PaO₂(t), were fitted to the following respective exponential equations:

 PaO<SUB>2</SUB>b + (PaO<SUB>2</SUB>[55]-PaO2b)(1-e<SUP>-kt</SUP>) and PaO<SUB>2</SUB>[55] + (PaO<SUB>2</SUB>[35]-PaO<SUB>2</SUB>[55])(e<SUP>-kt</SUP>) where "t" refers to time, PaO2[55] and PaO2[35] are the final PaO₂ values obtained at a new FIO₂ of 0.55 and 0.35, after a 0.20 increment and decrement in FIO₂, respectively. Time constant "k" was determined by a non-linear fitting curve and 90% oxygenation times were defined as the time required to reach 90% of the final equilibrated PaO₂ calculated by using the non-linear fitting curves. Results: Time constant values for the rise and fall periods were 1.01±0.71 min^–1, 0.69±0.42 min^–1, respectively, and 90% oxygenation times for rises and falls in PaO₂ periods were 4.2±4.1 min^–1 and 5.5±4.8 min^–1, respectively. There was no significant difference between the rise and fall periods for the two parameters (p>0.05). Conclusion: We conclude that in stable patients ventilated with PCV, after a step change in FIO₂ of 0.20, 5–10 min will be adequate for obtaining a blood gas sample to measure a PaO₂ that will be representative of the equilibrium PaO₂ value.