Time to steady state after changes in FIO(2) in patients with COPD

International guidelines recommend that when changing FIO2 in patients with COPD receiving Long-Term Oxygen Therapy (LTOT), 30 minutes should be waited for steady state before measurement of arterial blood gasses. This study evaluates whether 30 minutes is really necessary, as a smaller duration might improve the logistics of care, potentially reducing the time spent by patients at the out-patient clinic.

12 patients with severe to very severe COPD according to the GOLD guidelines were included. Patients had a median FEV1% of 23% of the predicted value (range 15-64%), median FEV1/FVC 0.43 (range 0.26-0.63), and chronic respiratory failure necessitating LTOT, 1-4 liters/minute, minimum 16 hours/day. Following a FIO2 reduction (wash out), arterial blood gases were measured at 0, 1, 2, 4, 8, 12, 17, 22, 32 and 34 minutes. FIO2 was then increased to baseline levels (wash in) and blood gasses measured at 0, 1, 2, 4, 8, 12, 17, 22, 32, and 34 minutes. Data were analyzed to examine the dynamics of arterial PO2 and saturation (SO2) wash out and wash in by calculating the time constants, tau (ô), and to evaluate the time required to reach values which might be considered clinically stable, defined as PO2 within 0.5 kPa and SO2 within 1% of equilibrium values.

For arterial PO2 values of time constants were about 3 minutes and similar for both wash out and wash in. A median of 5 minutes was required to reach clinically stable values of PO2 in both wash out and wash in, with 7-8 minutes sufficient in 75% of patients, and in the worst case 14 minutes. For SO2, values of the time constant were 4.5 and 1.4 minutes for wash out and wash in, respectively. The time required to reach clinically stable values was different in the two phases. For wash out the median time was 7.4 minutes, and in the worst case 15.6 minutes. For wash in the median time was 2.6 minutes and in worst case 6.8 minutes. No significant changes in PCO2 or pH were seen during FIO2 changes.

This study shows that oxygen equilibration relevant for clinical interpretation requires only 10 minutes following an increase and 16 minutes following a decrease in FIO2. over the range studied.