CONTINUOUS MONITORING OF MIXED VENOUS OXYGEN-SATURATION

Mixed venous oxygen saturation (SVO2BAR), measured on pulmonary artery blood, is a convenient indicator of matching between O2 transport (TaO2) and O2 body consumption (VO2). The measurement technique is based on the haemoglobin reflection spectrophotometry principle using two or three wave lengths. The Fick principle points out that SVO2BAR depends on five parameters : SVO2BAR = SaO2 - (VO2/CI x Hb x PO) where SaO2, CI and PO respectively represent arterial O2 saturation, cardiac index and O2 affinity. SVO2BAR does not always reflect tissue O2 tension : when considering a given value of SVO2BAR, PVO2BAR will depend upon the position of the oxyhaemoglobin dissociation curve. It is impossible to establish in the absolute a << normal >> value of SVO2BAR. However, in most clinical circumstances, an SVO2BAR ranging from 60 to 80% attests that O2 tissue delivery is appropriate. Under certain conditions a continuous monitoring of SVO2BAR allows to assess another index such as ventilation-perfusion index or the O2 tissue extraction index.Usually SVO2BAR variations are more informative than the absolute SVO2BAR value. However, their interpretation should be cautious. First and foremost, the ability of each of the four main SVO2BAR determinants to influence the SVO2BAR is unequal as the numerical ranges of variation of these determinants are very different. Moreover, the attribution of a variation of SVO2BAR to one of its determinants implies that each of them is independent from the others, a feature which is very rarely seen in clinical practice. Finally as the mathematical relationship between SVO2BAR and its determinants is linear (SaO2 and VO2), or hyperbolic (CI and Hb), the weight of SaO2 or VO2 is independent of their absolute value, whereas CI or Hb weights will depend on their value. The limits of SVO2BAR monitoring are linked first to the occurrence of an anaerobic metabolism state when TaO2 becomes too low ; SVO2BAr then just provides informations on the aerobic part of the metabolism. Moreover, SVO2BAR is just a global indicator for tissue O2 oxygenation status which does not give any indication about regional flow distribution. Therefore, SVO2BAR enables systemic imbalance supervision only. Finally, the existence of a right-to-left shunt will modify the SVO2BAR values through various mechanisms. However the SVO2BAR measured, in the pulmonary artery, remains reliable, whereas the presence of a left-to-right shunt will highly alter SVO2BAR basal value, only its time course remaining significant. SVO2BAR monitoring, element of diagnosis and monitoring, as well as a warning signal, has a priori specific indications poorly assessed, so far. The cost/efficiency ratio determination comes up against a problem of definition and evaluation of the expected benefit. In septic ICU patients, SVO2BAR monitoring enables the assessment of systemic circulation, so that TaO2 will not become the limiting factor to tissue O2 supply but does not allow, if normal or even high, to rule out hypoxia in some organs.