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.