Shock Syndromes I

iii.

Elevated lactate concentrations may be the result of increased production, decreased clearance,

or both.

supply).

of impaired lactate clearance or medication-related causes (e.g., metformin, epinephrine,

linezolid, or toxic alcohols).

iv.

Arterial and venous lactate concentrations are slightly different in value but may be used

interchangeably. When trending lactate values over time, consistent methods of collection

should be used.

Venous oximetry (Scvo2 and Svo2)

Central venous oxygen saturation (Scvo2) and mixed venous oxygen saturation (Svo2) are the

oxyhemoglobin saturations of venous blood obtained from a central vein and the pulmonary

artery, respectively, and are expressed as a percentage.

ii.

Scvo2 is obtained from a central venous catheter (i.e., subclavian or internal jugular access)

where the catheter tip terminates in the superior vena cava. As such, Scvo2 is more reflective

of oxygen extraction in the brain and upper body than of systemic oxygen extraction. Of

importance, the oxyhemoglobin saturation of blood obtained from a central venous catheter

terminating in the IVC (i.e., femoral access) cannot be used interchangeably with the Scvo2

obtained from a central venous catheter terminating in the superior vena cava and should not

be used as a marker of perfusion.

iii.

Svo2 better represents systemic oxygen extraction because it represents the mixing of venous

blood from the superior vena cava, inferior vena cava (IVC), and coronary sinus.

iv.

In normal physiology, Scvo2 is about 2%–3% lower than Svo2 because the upper body and the

brain extract more oxygen than the lower body.

In the setting of shock, Scvo2 exceeds Svo2 by about 5%–8% because of increased mesenteric

and renal oxygen extraction with a similar cerebral extraction ratio.

vi.

The difference between Scvo2 and Svo2 decreases in low CO states.

vii.

Although Scvo2 and Svo2 are not equivalent, they have a good (though not perfect) correlation,

and Scvo2 may be a reasonable approximation of Svo2, given its requirement for a PAC for

accurate measurement.

viii.

A decreased Scvo2 or Svo2 is a sign that tissue oxygen demands are not completely met by Do2

(more discussion on this topic later in this chapter).

ix.

Rearrangement of the Fick equation shows that a decrease in Svo2 indicates a decrease in CO,

whereas an increase in Svo2 indicates an increase in CO.

In general, Svo2 values above 70% are considered adequate, whereas Svo2 values less than

40% are considered critically low and approach the critical O2ER where anaerobic metabolism

will occur and lactate concentrations will increase. Svo2 values of 50%–70% by themselves

do not lead to firm conclusions about the O2ER and must be interpreted in the context of other

markers of tissue perfusion (e.g., lactate concentrations).

xi.

Svo2 values above 80% likely indicate poor tissue oxygen extraction capacity.

flow (i.e., microcirculatory dysfunction), peripheral shunting of oxygen past the tissues, or

impaired mitochondrial oxygen use.

6 hours of their treatment had a higher mortality than those with normoxia (Scvo2 71%–

89%).