Mixed Venous Saturation - Importance in Septic Shock

Mixed Venous Saturation - Importance in Septic Shock

Concept of Mixed Venous Saturation in Septic Shock : Mixed venous saturation (Svo2) is an important concept while managing patients with Septic Shock. This is one of the sepsis management guidelines - the guideline is that every attempt should be made to keep mixed venous sat > 70% . This mixed venous sat is obtained by collecting sample from central venous catheter. So, a central line must be placed in any patient with septic shock not only to start fluids and pressors but also it helps you to get frequent samples to test Sv02. a) Now, what is this mixed venous sat? - Remember, saturation on arterial side in a well-oxygenated patient is around 99% to 100%. So, when you have arterial sat at 100, you should expect mixed venous sat to be at least > 70%. If it is less than 70% ( A-v difference more than 30) ; it means tissues are sucking out oxygen from arterial blood so, by the time blood gets to venous side the oxygen content is so low. So, what if tissues suck out oxygen ..is not this a good thing? Yes, but the question should arise why are they sucking out so much 02 more than usual ...... the reason usually, is because the 02 delivery to tissues is impaired so, they want to get as much as they can out of what they are supplied with. So, that means if mixed venous sat is low ..it indirectly means there is low 02 delivery to the tissues. b) Now, apply the formula of 02 delivery to understand this concept better. 02 delivery = Oxygen content x Cardiac Output . Oxygen content = (Hgb x 1.36 x SaO2) . So, O2 delivery = (Hgbx1.36x SaO2) x Cardiac Output . c) So, if someone has mixed venos sat < 70% that means low o2 delivery ..how can we improve that ? The only modifiable parameters in the formula above are HG and Cardiac output if Sa02 is already at 99%. So, blood transfusion to maintain hg > 10gm% is first step. If that goal is met, second step is to increase cardiac output by adding dopamine + dobutamine in a septic shock patient. d) So, if mixed venous sat < 70% this is how you should work 1) Check HGB. if less than 10gm%, transfuse 2) If HGB > 10gm%, add dobutamine to a pressor to improve cardiac output. Other interventions aimed at improving cardiac output is also acceptable. Try to maintain mixed venous sat > 70% by doing the above two steps!