measurement of arterial-venous oxygenation gradients (A-V O 2 ), mixed venous
saturations (SvO 2 ), acid base, and lactate.
A-V O 2 is the difference between arterial oxygen saturation and SvO 2 , and
is normally <30%. Changes in A-V O 2 are reflective of variations in CO when
oxygen demands are stable in the absence of anemia or hypoxia. A high A-V O 2
in combination with elevated serum lactate could indicate an inability of the
tissues to consume oxygen the cellular level.
SvO 2 is the oxygen saturation within the pulmonary artery following the
mixing of the systemic venous return (from the superior and inferior vena cavae)
and the coronary venous circulation. It measures the overall balance of oxygen
transport and consumption and thus provides the clinician with critical information
pertaining to ability of the patient’s CO to meet metabolic demands. Ideally, these
measurements are taken form a catheter in the pulmonary artery but if these are
not present, samples may be taken from central venous or right atrial catheters.
This measurement must be made using co-oximetry since this value cannot be
determined solely using arterial oxygen tension values (i.e. PaO 2 ). Normal SvO 2
values range from 70-75%. Values <65% suggest increased oxygen extraction at
the tissue level and is indicative of impaired tissue perfusion. Importantly, SvO 2 is
affected by all 4 components that affect oxygen delivery (CO, Hgb, PaO 2 , and
SaO 2 ). Furthermore, an increase in oxygen consumption without a compensatory
increase in oxygen delivery will also lead to low SvO 2 values.