Equipment review: Continuous assessment of arterial blood gases

Abstract

par-tial pressure and pH is traditionally performed by inter-mittent arterial blood sampling. However, this methodpresents a number of disadvantages such as the need foriterative and uncomfortable arterial punctures, and canbe associated with substantial blood loss [1,2]. Despitethese major impediments to serial measurements, arter-ial blood gas values are the most frequently orderedlaboratory examinations in the intensive care unit (ICU)and the operating room [3]. In the ICU, except foronce-daily analysis which represents a‘spot check’ ofthe physiological state of the patient, indications forsampling are essentially the result of a deleterious event[4]. In these situations, sampling depends greatly uponthe judgement of the physician, nurse or other healthcare provider to determine whether a blood gas mea-surement is needed. The delay between the event itselfand blood sampling, plus the delay in obtaining theresults, means that this sort of analysis may be mislead-ing. For example, clinically important changes in apatient’s blood gas status may go undetected or mayoccur after a sample has been drawn and while it isbeing analysed [5]. Moreover, considerable spontaneousvariation in blood gases frequently occurs, even in stableICU patients [6].Because clinical decisions need to be made on thebasisoftrendsinbloodgasesaswellaswiththerapiddetection of an acute event [6,7], continuous non-inva-sive monitoring techniques, such as pulse-oximetry andcontinuous capnography, have been developed. Unfortu-nately, these devices are not always accurate or reliablein acute situations such as shock, hypothermia, or dur-ing the use of vasopressors [8,9]. Moreover, pulse-oxi-metry does not measure oxygen tension, and majordrawbacks also exist for continuous capnography.Continuous arterial blood gas monitoringsystems?Since the 1980’s several attempts has been made todevelop equipment which is able to overcome the disad-vantages of intermittent arterial blood sampling andthose of non-invasive monitoring. The goal of theresearch has been to develop a real-time continuousblood gas monitoring system.This led to the development of‘blood gas monitors’which were defined as ‘patient-dedicated apparatus usedto measure arterial pH, PaCO