ECMO and the DO2/VO2 ratio
(Oxygen delivery/oxygen consumption ratio)
By Trina Augustin, MD
Peer reviewed by Eric Leiendecker, MD & Lewis McLean, MD
What is this?
The ratio of oxygen delivery (DO2) to metabolizing tissues compared to the tissue’s oxygen consumption (VO2).
Why do I need to know this?
Because an ECMO consult is basically a consult for inadequate DO2.
The goal of ECMO is adequate DO2 delivery for oxygen- dependent (aerobic) metabolism thus preventing anaerobic metabolism and its resultant metabolic and cardiovascular collapse with end organ dysfunction and death.
Normal DO2 is approximately 600 ml/min/m2 and normal VO2 approximately 120ml/min/m2 with a DO2/VO2 ratio of 5:1. This redundancy in delivery is imperative for our ability to exercise and perform activities.
VO2 is dynamic, increasing due to catecholamines, fever, sepsis, agitation, shivering, endocrine derangements, and exercise. DO2 is regulated by homeostatic mechanisms to be maintained 5 times VO2 under normal physiological conditions. If DO2 is impaired (decreased CO, hypoxemia, anemia) more oxygen is extracted leading to a lower venous oxygen content. This is tolerated till the DO2:VO2 ratio nears 2, at this time anaerobic metabolism will occur with resultant lactic acidosis, multi-organ failure, and cardiovascular collapse.
ALERT: Physiological Precipice
The beauty of ECMO is it allows safe DO2/VO2 ratio to be restored while avoiding harmful ventilator settings and vasoactive drugs. ECMO itself does NOT FIX the underlying etiology of heart/lung failure, but instead supports the patient by providing adequate DO2 while enabling time for diagnosis, interventions, and/or recovery.
Ok, so how do I know if my ECMO patient’s DO2/VO2 ratio is adequate (≥3)?…
…WITHOUT COMPLEX MATH
Reminder: pre-oxygenator gas represents blood drained from multistage drainage cannula typically draining IVC +/- right atrium (RA) in VV ECMO and IVC + SVC + RA in VA ECMO (assuming bicaval drainage) thus frequently thought of as a ScvO2 whereas SvO2 measured in the pulmonary artery is reflective of SVC + IVC + coronary sinus blood (which may not be captured by drainage cannula)
Source: Bartlett R. H. (2017). Physiology of Gas Exchange During ECMO for Respiratory Failure. Journal of intensive care medicine, 32(4), 243–248. https://doi.org/10.1177/0885066616641383
Example: Individual with normal lung physiology breathing room air with 100% SaO2 and ScvO2 80%, extracted 20% with a ratio of DO2/VO2 5:1
Basically its…
Caveats:
- On VV ECMO it is possible to have recirculation with high pre-oxygenator venous saturation despite poor oxygen delivery to the body; hence, it is important to ascertain that the gas represents global oxygen delivery.
- On VA ECMO with differential hypoxia (upper body poorly oxygenated from native flow while lower body well oxygenated from ECMO) it is possible pre-oxygenator may not be fully reflective of global oxygen delivery especially if not adequately draining SVC +RA
So, what if my DO2/VO2 is ≤ 3:1?
Then it is time to increase DO2 or reduce VO2!
Increasing DO2 (oxygen delivery)
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Increase ECMO flows
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- Increase RPMs/ add additional drainage cannula
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- VV ECMO–> increasing flow–> increases DO2 by increasing % of total CO (less native poorly oxygenated flow shunting past ECMO circuit) comprised of oxygenated blood thus increased O2 content
- VA ECMO–>increasing flows increases CO mainly, while to a smaller extent also increasing O2 content
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- Typical O2 metabolism is 3ml/kg/min for adult (which after performing MORE math allows you to calculate their approximated VO2 and then estimate their DO2 needs. Then divide the DO2 by their calculated 02 content and arrive at their needed cardiac output (estimated ECMO flow requirements)
- OR more realistically…
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Transfuse to a higher hemoglobin
- Note O2 content with same Sa02/PaO2 and Hgb of 15 versus 7.5 has twice the 02 content
Source: Bartlett R. H. (2017). Physiology of Gas Exchange During ECMO for Respiratory Failure. Journal of intensive care medicine, 32(4), 243–248. https://doi.org/10.1177/0885066616641383
Source: Katrina Augustin. Note significant drop in 02 content (CtO2) due to a decline in Hgb (stable SaO2/PaO2) over a 6-day ECMO run.
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- ELSO guidelines evolving on exact Hgb target and variable based on patient condition (bleeding vs not bleeding) 2021 ELSO guidelines
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Optimize SaO2-NORMALLY NOT EFFECTIVE
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- Add additional oxygenator/membrane lung (not effective because oxygenator flow limited)
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- Optimize lung function- often limited to maintain lung protective ventilation settings; can diurese
- Use caution with attempting beta blockade on VV ECMO to decrease shunt fraction (high fraction of native CO not captured by VV ECMO) because while it may increase SaO2 it is overall decreasing total cardiac output and thus DECREASING DO2- may be select times for use (see below)
Decrease VO2 (oxygen consumption)
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Decrease metabolic demand
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- Treat fever with antipyretics
- Treat agitation
- Treat shivering
- Cool patient
- Paralyze
- ? consider with caution B-blockade if tachycardic and hypertensive (increased sympathetic response) on VV ECMO to decrease O2 demand/metabolic rate in patients with hyperdynamic EF
Take Home Points
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An ECMO consult is a consult for failure in oxygen delivery (DO2)
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The goal of ECMO is to provide adequate DO2 to prevent metabolic/cardiovascular collapse, end organ dysfunction, and death
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ECMO allows safe a safe DO2/VO2 (oxygen consumption) ratio to be restored while avoiding harmful ventilator settings/vasoactive drugs and enables time for diagnosis, interventions, and recovery
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Goal DO2/VO2 ratio on ECMO is ≥3:1 (ideally 3:1 to 4:1) targeting a ScvO2 of 66-75%
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DO2 can be optimized by increasing ECMO flows and transfusing to higher hemoglobin levels
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VO2 can be minimized by interventions to decrease the metabolic rate
Sources/Additional Resources
- Bartlett R. H. (2017). Physiology of Gas Exchange During ECMO for Respiratory Failure. Journal of intensive care medicine, 32(4), 243–248. https://doi.org/10.1177/0885066616641383
- THE PHYSIOLOGY OF EXTRACORPOREAL LIFE SUPPORT By Bartlett R. H
- 2021 ELSO Adult and Pediatric Anticoagulation guidelines
Read this PulmCrit Post to See this all Related to Non-ECMO Patients
Now on to the Podcast
- CV-EMCrit Wee – MCS Minute: ECMO and the DO2/VO2 ratio with Trina Augustin - January 31, 2024
- CV-EMCrit 327 – Acute Valve Disasters Part 2 – Management of Critical Aortic Stenosis - July 1, 2022
- EMCrit 321 – CV-EMCrit – Acute Valve Disasters – Critical Aortic & Mitral Regurgitation and Bonus: VSDs with Trina Augustin - April 7, 2022
This was excellent thank you – a great summary of physiology that is often hard to summarize. Would be very interested in future discussions on how to optimize wakefulness on ECMO. You mentioned this is your (Trina’s) typical practice, and it is a goal for me too. In my hospital’s relatively young program, we basically have never been able to wake anyone up successfully, until just about ready to decannulate. We started with COVID and are largely doing sepsis/ARDS, not pre-transplant patients, so this may be a significant factor. I would also love a discussion/your thoughts on how effective a… Read more »
Thermodilution derived CO isn’t accurate (aspiration of indicator into extracorporeal circuit leads to overestimation of CO probably proportional to ECBF). Pressures should be accurate incl PAPi. Mixed venous sat will no longer necessarily be an accurate reflection of DO2:VO2 (in V-V) as it will be a mixture of ‘true venous’ blood which hasn’t been extracorporeal and post-oxy blood. The same will apply to gas tensions
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9067097/
Great Discussion. Agree with Alex and the article sited regarding inaccuracy of thermodilution using standard PAC (CCO swan may actually have potential to be more accurate though to my knowledge no great data) to measure CO and mixed venous gas on VV ECMO. I do not use Thermodilution on a PAC to determine CO on my VV ECMO patients as not only does the rate ECBF effect if but also the recirculation fraction. Also agree with the statements on mixed venous gas. In my opinion, Pressures may be somewhat nuanced based on VV ECMO configuration. With V-PA (venous-pulmonary artery as… Read more »
I really enjoyed reading this post. I need more such topic