Time required for PaO₂ equilibration during mechanical ventilation with changes in FiO₂

Introduction

Although it's a general practice to obtain blood gas sample after 30 min when FiO₂ was changed during mechanical ventilation, exact timing for blood gas analysis has not been very well documented. In this study we aimed to investigate the time required for PaO₂ equilibration during pressure controlled ventilation (PCV) with changes in FiO₂.

Patients and methods

25 adult patients with stable blood gas values (5 female, 20 male) on PCV (Servo 900C Siemens, Elema, Sweden) with FiO₂:0.35, frequency: 12 breaths/min, tidal volume: 10 ml/kg and PEEP: 5 cmH₂O were studied.

Following a control blood gas measurement at FiO₂:0.35 and determination of PaO₂ (PaO₂35), FiO₂ was increased to 0.55 for 60 min and again returned to 0.35 without any change in ventilatory parameters. Blood samples were obtained and immediate blood gas measurements were performed on 3, 5, 7, 9, 11, 15, 20, 25, 30 min in both periods and at the 60th minute in FiO₂:0.55 period. The PaO₂ value measured at the 60th minute of FiO₂:0.55 (PaO₂55) was accepted as representative of equilibration.

The timing of PaO₂ values reaching either the PaO₂55 and PaO₂35 levels were recorded for each patient and average (Mean ± SD) of these values were calculated.

Results

The average (Mean ± SD) time period necessary to reach PaO₂55 and PaO₂35 were 9 ± 7.33 min and 7.08 ± 6.02 min respectively. The numbers and the percentages of the patients reaching their own PaO₂55 and PaO₂35 values for each sampling time were listed in Table 1.

Conclusion

We concluded that after FiO₂ changes during PCV, 10 min will be adequate for obtaining the blood gas sample representative for equilibration of PaO₂.

Table 1 The numbers and the percentages of the patients reaching their own PaO₂55 and PaO₂35 values for each sampling time