D. Journois et D. Safran, CONTINUOUS MONITORING OF MIXED VENOUS OXY GEN-SATURATION, Annales francaises d'anesthesie et de reanimation, 12(4), 1993, pp. 393-408
Mixed venous oxygen saturation (SVO2BAR), measured on pulmonary artery
blood, is a convenient indicator of matching between O2 transport (Ta
O2) 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 depend
s on five parameters : SVO2BAR = SaO2 - (VO2/CI x Hb x PO) where SaO2,
CI and PO respectively represent arterial O2 saturation, cardiac inde
x and O2 affinity. SVO2BAR does not always reflect tissue O2 tension :
when considering a given value of SVO2BAR, PVO2BAR will depend upon t
he 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% at
tests that O2 tissue delivery is appropriate. Under certain conditions
a continuous monitoring of SVO2BAR allows to assess another index suc
h as ventilation-perfusion index or the O2 tissue extraction index.Usu
ally SVO2BAR variations are more informative than the absolute SVO2BAR
value. However, their interpretation should be cautious. First and fo
remost, the ability of each of the four main SVO2BAR determinants to i
nfluence the SVO2BAR is unequal as the numerical ranges of variation o
f 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 s
een in clinical practice. Finally as the mathematical relationship bet
ween SVO2BAR and its determinants is linear (SaO2 and VO2), or hyperbo
lic (CI and Hb), the weight of SaO2 or VO2 is independent of their abs
olute 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 ju
st provides informations on the aerobic part of the metabolism. Moreov
er, SVO2BAR is just a global indicator for tissue O2 oxygenation statu
s which does not give any indication about regional flow distribution.
Therefore, SVO2BAR enables systemic imbalance supervision only. Final
ly, the existence of a right-to-left shunt will modify the SVO2BAR val
ues through various mechanisms. However the SVO2BAR measured, in the p
ulmonary artery, remains reliable, whereas the presence of a left-to-r
ight shunt will highly alter SVO2BAR basal value, only its time course
remaining significant. SVO2BAR monitoring, element of diagnosis and m
onitoring, as well as a warning signal, has a priori specific indicati
ons poorly assessed, so far. The cost/efficiency ratio determination c
omes up against a problem of definition and evaluation of the expected
benefit. In septic ICU patients, SVO2BAR monitoring enables the asses
sment of systemic circulation, so that TaO2 will not become the limiti
ng factor to tissue O2 supply but does not allow, if normal or even hi
gh, to rule out hypoxia in some organs.