An abundance of new unproven technologies have emerged to replace the disfavored Swan-Ganz catheter for obtaining physiologic measurements — mainly cardiac output. Can we more accurately use a simpler tool we’re already familiar with — a central line with ScvO2 measurements? wonders Keith Walley in this Blue Journal review.
He elegantly proposes a derivation of “critical oxygen extraction ratio” (VO2/DO2, must be 0.7 or less to prevent ischemia) from cardiac output. The result of his proof is that mixed venous O2 saturation is close to (1 – ERO2) in most states of indeterminate cardiac output (i.e., not very high or very low). With important caveats (catheter placement, etc.) ScvO2 is an accurate surrogate when properly obtained, and can therefore be used to infer ERO2.
Despite his impressive mastery of the subject, IMHO the essential problem remains: how to interpret a low (or normal, or even high) ScvO2 in a septic patient when cardiac function is also indeterminate:
Example: ScvO2 is 58% in a man with septic shock, HR 130. Does the slightly hypocontractile LV on TTE represent true failure and concomitant cardiogenic shock, or is it actually a compensated high output state, resulting from the tachycardia? I don’t see how ScvO2 can ever answer questions like this reliably. That’s why the new noninvasive technologies purporting to measure cardiac output are so appealing.
Helpfully, Walley advises that intermittent ScvO2 measurements are sufficient except when minute-by-minute assessments are required (e.g., in the O.R.), when continuous would be preferred.
Walley KR. Use of Central Venous Oxygen Saturation to Guide Therapy. Am J Resp Crit Care Med 2011;184:514-520.