Q1. List the techniques / measurements that are available to assess the circulation status of a patient in the intensive care unit.
Q2. a) How do you calculate the oxygen extraction ratio (O2ER)?
Q2. b) In a patient with septic shock, how would you interpret the following values for the oxygen extraction ratio (O2ER):
- (i) O2ER = 0.5
- (ii) O2ER = 0.2
Answer and interpretation
- Physical examination [warm hands, urine output, mentation]
- Vital signs – heart rate, blood pressure, oxygenation
- Urine output
- Blood pressure response to passive leg raise or fluid challenge
- Invasive arterial monitoring [Vigileo/LiDCO (cardiac output, stroke volume variation, stroke volume)]
- Central venous pressure measurement, central venous oxygen saturation
Invasive cardiac monitoring
- PiCCO measurements [Intra thoracic blood volume, global end diastolic volume, cardiac output, stroke volume variation]
- Pulmonary Artery Flotation Catheter [pulmonary artery occlusion pressure, cardiac output, mixed venous oxygenation]
Non-invasive cardiac monitoring
- Echocardiogram [cardiac output, left ventricular ejection fraction, IVC collapsibility]
- Transcutaneous Doppler [cardiac output/stroke volume variation]
- Techniques for measuring microvascular perfusion eg contrast US, SDF
- Techniques for measuring tissue oxygenation eg, gastric tonometry [delta pCO2], sublingual tonometry, microdialysis
- Impedance cardiography [cardiac output, stroke volume variation, stroke volume]
2a) How do you calculate the oxygen extraction ratio (O2ER)?
- O2ER = VO2 / DO2
2b) In a patient with septic shock, how would you interpret the following values for the oxygen extraction ratio (O2ER):
- (i) The normal value is around 0.2 – 0.3 and if the value is higher this suggests that the tissues are extracting excessive amounts because oxygen delivery is inadequate due to inadequate cardiac output from either inadequate contractility or inadequate preload and may respond to inotropes and/or fluid resuscitation.
- (ii) A low normal OER in this patient suggests failure of the microcirculation with inadequate oxygen uptake due to shunting and microvascular occlusion and resultant tissue ischaemia. This would be confirmed by rising lactate levels.
Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also a Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.
After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.
He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE. He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.
His one great achievement is being the father of three amazing children.
On Twitter, he is @precordialthump.