A 56 year old female with septic shock and multiple organ failure is admitted to intensive care. She is endotracheally intubated and ventilated. A central venous catheter is inserted into her right subclavian vein, a Vas Cath is inserted into her right femoral vein and a pulmonary artery catheter is inserted via her left subclavian vein.
a) Blood gas samples are simultaneously taken from all three catheters. The oxygen saturations are as follows:
50%
60%
67%
State which site each of the blood gas samples is taken from and justify your answer:
College Answer
60% = pulmonary artery catheter - pooled blood from SVC and IVC
67% = subclavian vein (superior vena cava) - in sepsis, cerebral blood flow relatively maintained initially
50% = femoral vein (inferior vena cava) - in sepsis, regional O2 consumption and extraction in gut/splanchnic circulation increases
Discussion
Central venous saturation measurements are discussed in greater detail elsewhere. These measurements are a means of assessment of the adequacy of oxygen delivery.
In brief summary, the further up you move away from the pulmonary artery, the greater your SvO2becomes; whereas if you go further down, the very hypoxic splanchnic venous blood will decrease the SvO2.
References
Chawla, Lakhmir S., et al. "Lack of equivalence between central and mixed venous oxygen saturation." CHEST Journal 126.6 (2004): 1891-1896.