Effects of lumbar puncture position on arterial blood gases.

We observed the changes in partial pressure of arterial oxygen (PaO ) and carbon dioxide (PaCO ) before and during assumption of the lateral position prior to lumbar puncture in 81 patients to investigate whether lung volume decreased and ventilation was suppressed. PaO significantly decreased while the patients were in the lateral position, while PaCO remained unchanged. There was a negative correlation between the change in PaO and age [change in PaO (mmHg)=-0.13×age (years)+4.28,P<0.01]. The fact that closing volume increases with age implies that the decrease in functional residual capacity in the lateral position could have caused the decrease in PaO . It is therefore advisable to continuously monitor arterial oxygenation using a noninvasive monitor, such as a pulse oximeter, while performing spinal or epidural block, especially in elderly patients.