ANSWER TO QUIZ OF THE WEEK FOR MAY 11, 2007

PvO2 is the partial pressure of oxygen from a venous blood gas sample. Normal PvO2 is 40 mmHg.

ScvO2 is the measurement of venous oxygen saturation obtained from a CENTRAL VENOUS catheter (obtained from a catheter close to the superior or inferior vena cava). The usual site for measuring ScvO2 is a jugular or subclavian line. Normal ScvO2 drawn from either of these two sites is >70% (representing the amount of oxygen still bound to hemoglobin after the HEAD and UPPER EXTREMITIES have been oxygenated).

SvO2 is the measurement of a MIXED venous oxygen saturation. SvO2 is obtained from the pulmonary artery port of a pulmonary artery catheter and is a mixture or average sampling of venous blood returning from the head and upper extremities (from the superior vena cava), from the gut and lower extermities (from the inferior vena cava) AND the heart muscle (from the coronary sinus). The normal range for SvO2 is 60-80%. It differs from an ScvO2 because it includes coronary and lower extremity extraction and reflects the "leftover" oxygen from the entire body.

Femoral lines are not the first choice for venous oxygen saturation. This is because femoral lines are often quite proximal to the inferior vena cava, and may be more reflective of local venous blood flow. Most of the published experience has also been with jugular or subclavian samples. Inferior vena cava blood reflects the "leftover" oxygen after the legs and gut have extracted oxygen. At rest, the venous saturation from the inferior vena cava may be normally as high as 80%. Because the gut is often the first organ to become ischemic during shock, IVC oxygen saturation may be much lower than normal.

Currently, most of the literature for continuous monitoring is based on samples taken close to the superior vena cava or from the pulmonary artery. In the absence of a subclavian, jugular venous, or pulmonary artery catheter, a femoral line may be used to follow trends in oxygen saturation.

Brenda Morgan, Clinical Nurse Specialist, CCTC

 

References:

Chawla, L. et al. (2004). Lack of equivalence between central venous and mixed venous oxygen saturation. Chest: 126, 1891-1896.

Kopterides, P. et al. (2005). Central or mixed venous oxygen saturation? Chest: 128, 1073-1075.

 

Brenda Morgan
Clinical Nurse Specialist, CCTC
Last Update: April 6 , 2007

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