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.