FROM IN-VITRO TO IN-VIVO MONITORING

In vitro monitoring is inherently invasive with discrete measurements on blood samples and the results are often delayed an hour or more whe n the analyses are performed in the central laboratory. The delay may be greatly reduced if the analyses are performed near the patient. In vivo monitoring may be non-invasive and may provide continuous real-ti me data but the accuracy usually does not match that of in vitro measu rements. In viva monitoring therefore finds its application in the det ection of trends of change, and it is needed only for quantities that change rapidly and unpredictably and where a suitable therapeutic acti on is available. In critically ill patients, this applies to the arter ial pO(2), pCO(2), and pH, and the mixed venous pO(2). Ideal in vivo m onitoring techniques are not available for all these quantities. In th e newborn, the arterial pO(2) may be monitored with a transcutaneous p O(2) electrode. In the adult, the arterial pO(2) may be monitored indi rectly by monitoring the arterial oxygen saturation with a pulse oxime ter and the mixed venous pO(2) by monitoring the mixed venous oxygen s aturation with a catheter tip sensor. The arterial pCO(2) may be monit ored with a transcutaneous pCO(2) electrode or by capnography, i.e., b y monitoring the end-expiratory pCO(2). Other in vivo monitoring techn iques such as gastric tonometry for the gastric mucosal pH and thoraci c impedance measurement have found some routine application, whereas n ear-infrared spectrometry for oxy- and deoxyhaemoglobin in the brain, and magnetic resonance spectroscopy for tissue ATP are at the stage of research and development.