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 when 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‐time data but the accuracy usually does not match that of in vitro measurements. In vivo monitoring therefore finds its application in the detection of trends of change, and it is needed only for quantities that change rapidly and unpredictably and where a suitable therapeutic action is available. In critically ill patients, this applies to the arterial p O 2 , p CO 2 , and pH, and the mixed venous p O 2 . Ideal in vivo monitoring techniques are not available for all these quantities. In the newborn, the arterial p O 2 may be monitored with a transcutaneous p O 2 electrode. In the adult, the arterial p O 2 may be monitored indirectly by monitoring the arterial oxygen saturation with a pulse oximeter and the mixed venous p O 2 by monitoring the mixed venous oxygen saturation with a catheter tip sensor. The arterial p CO 2 may be monitored with a transcutaneous p CO 2 electrode or by capnography, i. e., by monitoring the end‐expiratory p CO 2 . Other in vivo monitoring techniques such as gastric tonometry for the gastric mucosal pH and thoracic impedance measurement have found some routine application, whereas near‐infrared spectrometry for oxy‐ and deoxyhaemoglobin in the brain, and magnetic resonance spectroscopy for tissue ATP are at the stage of research and development.