Abstract The benefit of a continuous intra-arterial blood gas monitor is apparent to any physician who has ever waited for the return of blood gas values at the bedside of a critically ill patient. The potential for rapid detection and prompt treatment of blood gas changes, coupled with immediate evaluation of the effectiveness of a therapeutic intervention, appears great. Whereas these systems may offer a tremendous advance in the physician's ability to track and treat alterations in cardiopulmonary physiology, no large-scale in vivo human trials have yet been reported. The only data available on accuracy, bias, precision, drift, and complication rate are from relatively small trials. Hopefully, when large-scale trials are completed, these systems will live up to their tremendous potential.