ScvO2 vs functional hgb saturation?

offlabel · Oct 7, 2016

Good question...What they're doing is increasing O2 delivery by increasing the CO with dobutamine. They've probably maximized oxygenation and ventilation and hgb as much as they can for now, so the arterial O2 content is a fixed value.

At a lower CO, the tissue extracts so much oxygen that the mixed venous or central venous saturation coming back to the lungs is too low for the lungs to boost it back up to the desired arterial saturation, say 90% for arguments sake.

If they raise the CO with dobutamine, more O2 will be delivered to the tissues, so at the same rate of extraction, more will be left over in the venous blood returning to the lungs and the higher the arterial saturation will be coming out the other end.

Was that your question or did I misunderstand?

Cowboyardee · Oct 7, 2016

I'm not sure I'm understanding you, but I'll try to take a stab.

Where I've worked, we don't explicitly use the term 'functional hgb saturation' to distinguish from a fractional saturation. But my understanding is that functional hgb sat is the same value as the SaO2 you get on an ABG.

IF this is the case where you work, then I understand your question to be whether a venous hgb saturation (ScvO2) is interchangeable with an arterial hgb saturation (SaO2). And the answer is it's not. Largely for reasons that Offlabel indicated: an arterial saturation tells you primarily about a patient's ability to oxygenate their blood via pulmonary function; meanwhile a central venous saturation tells you about how much of the oxygen has been used in circulating around the body giving you clues as to the patient's perfusion status and/or the metabolic oxygen consumption rate of his or her tissues.

On the other hand, IF an arterial sample wasn't used and you're asking me whether the ScvO2 is a different value than a venous functional hgb saturation... I'm honestly not sure. I believe they are two terms for the exact same thing. But since I've never had to make a distinction between fractional and functional hgb sat where I've worked (and certainly not with respect to venous blood), I could be mistaken.

offlabel · Oct 7, 2016

I'm not sure I'm understanding you, but I'll try to take a stab.
Where I've worked, we don't explicitly use the term 'functional hgb saturation' to distinguish from a fractional saturation. But my understanding is that functional hgb sat is the same value as the SaO2 you get on an ABG.
QUOTE]
yah...I'm not familiar with "functional hgb sat" either. Maybe the question is if there is a difference in where sampling occurs, ie, peripheral vein, central line or main pulmonary artery. Those venous saturations (functional hgb sat?) will all be different.

christophermiles · Oct 8, 2016

Thanks for the reply, offlabel.

I understand the role of dobutamine as it relates to this pt's scenario. My confusion arises from the apparent fact that the MICU team was using functional hgb sat from the co-ox panel as if it were ScvO2. Their progress note basically said dobutamine was uptitrated in an effort to raise the ScvO2, but a co-ox panel doesn't give you ScvO2 (right?). (A co-ox panel gives you the following: total hgb; O2 sat - fuctional and fractional; carboxyhemoglobin; methemoglobin; and O2 content.)

I understand ScvO2 is used to gauge tissue perfusion. Does functional hgb sat basically tell you the same thing? If no, what role does this value play?

More pt info: pt admitted for Rt-side HF 2/2 pulmonary htn and failed to respond to sildenafil and Remodulin. Listed for lung tx. SBPs 90-100. HR 90-100. SpO2 mid-90's on 2L NC. RR upper teens. On vasopressin at 0.02, Lasix 24mg/hr and dobutamine at 9mcg. Vasopressin was uptitrated unsuccessfully in an effort to improve ScvO2 and reduced back to 0.02. Pt seemed to modestly improve ScvO2 with increase of dobutamine.

christophermiles · Oct 8, 2016

Thanks for the response, cowboyardee.

Arterial sample was NOT used. Sample was mixed venous drawn from a PICC.

After doing some Googling it appears functional and fractional hgb sat can be used interchangeably. For this pt, the results were separated by only 1-2 percentage points difference.

To summarize I'm essentially asking can a fract hgb sat be used interchangeably with ScvO2?

Sorry for my muddied post. I think the lack of clarity reflects my lack of understanding regarding the pathophysiology of this pt's condition.

offlabel · Oct 8, 2016

Thanks for the reply, offlabel.
I understand the role of dobutamine as it relates to this pt's scenario. My confusion arises from the apparent fact that the MICU team was using functional hgb sat from the co-ox panel as if it were ScvO2. Their progress note basically said dobutamine was uptitrated in an effort to raise the ScvO2, but a co-ox panel doesn't give you ScvO2 (right?). (A co-ox panel gives you the following: total hgb; O2 sat - fuctional and fractional; carboxyhemoglobin; methemoglobin; and O2 content.)

I don't take care of patients waiting for hearts and lungs, but I do take care of a lot of cardiothoracic surgical patients that use PA catheters with SvO2 capability. I think the last time I even saw a co-ox panel was in the late 1980's.

That said, any "saturation" of venous or arterial blood refers specifically to hgb. In this case it's venous and whether the specific test is ScvO2 or "functional" hgb saturation, I'll take an educated guess and say they are, for all practical purposes the same, perhaps with a bias toward the "functional hgb saturation" as it is a direct measurement by a different means than saturation of venous blood.

I suppose one could split hairs over one or the other, but practically speaking, at the end of the day, I think you'd probably end up on the same drip rates for either one.

Do follow up for us with your CC service and let us know their thinking. You have me curious now.