Timing of sampling for blood gases following change in oxygen therapy

Patients with chronic obstruction pulmonary disease (COPD) may need long-term oxygen therapy, monitored with blood gas analysis (pO₂(a) and sO₂(a)). It is important that when adjustments are made to this therapy (either increase or decrease in fraction inspired oxygen, FIO₂) there should be delay until a new steady state is achieved, before sampling blood for assessment of oxygenation status. 

Current guidelines recommend a delay of 20 to 30 minutes – but is such a delay really necessary? This is the question addressed by a recently published clinical study of 12 patients with severe COPD. All require long-term oxygen therapy (1 to 4 liters/min) delivered for a minimum of 16 hours per day; some require it for 24 hours/day.

Each of the 12 study subjects had arterial blood sampled, via an indwelling arterial cannula, for pO₂(a) and sO₂(a) measurement at baseline whilst receiving their prescribed oxygen dose, and at the following timings after withdrawal of oxygen: 1, 2, 4, 8, 12, 17, 22, 32 and 34 minutes. 

The prescribed oxygen therapy was then re-instated and arterial blood sampled again at the same time intervals as before. Thus two sets of pO₂(a) and sO₂(a) data were generated for each study subject: the first for oxygen ”wash out”, simulating dose reduction; and the second for oxygen ”wash in”, simulating dose increase.

Analysis of the data revealed that the median time to reach clinically stable pO₂(a) values was 5 minutes for both ”wash in” and ”wash out”; 7-8 minutes were sufficient time for 75 % of patients, and the longest time was 14 minutes. The median time to reach clinically stable sO₂(a) values was longer for ”wash out” (7.4 minutes) than for ”wash in” (2.6 minutes). 

More detailed analysis of the data allowed the authors to conclude that sampling blood for assessment of oxygenation (pO₂(a) and sO₂(a)) following reduction of oxygen dose (reduction in FIO₂) should be delayed for 16 minutes. When assessing the effect of dose increase (i.e. increased FIO₂), blood sampling need only be delayed by 10 minutes.