SvO2 & ScvO2 monitoring

My small presentation on practical aspect of Mixed venous oxygen and tissue utility of oxygen

1. SvO2 and ScvO2 Monitoring Dr. Ahsan Ahmed

2. Measurement of oxygenation saturation from mixed venous blood (SvO2) in the pulmonary artery Requires Pulmonary Artery Catheter insertion SvO2

3. The pulmonary artery receives a mixture of blood from the superior vena cava, inferior vena cava, and coronary sinus. It serves as a sample of whole body oxygen utilization. Mixed venous oxygen saturation is measured from blood in the pulmonary artery to sample deoxygenated blood entering the pulmonary artery before passing through the lungs

4. Why measure it? If SvO2 decreases, it indicates that the tissues are extracting a higher percentage of oxygen from the blood than normal.  A rise in SvO2 demonstrates a decrease in oxygen extraction, and usually indicates that the cardiac output is meeting the tissue oxygen need.  A rise in SvO2 in the presence of a rising lactate is inappropriate - the patient who has resorted to anaerobic metabolism

5. Unfortunately, the cardiac output measurement only gives us a value, it does not indicate whether the measured cardiac output is meeting the patient's needs An SvO2  in the normal range, along with a normal lactate, suggests that the cardiac output is adequate.

6. Measuring SvO2 before and after a change can assist in determining whether the therapy made the patient better or worse By measuring the SvO2 before and after a change in PEEP, the optimal level of PEEP can be determined.  The "best" PEEP is the level that improves the SaO2 without causing the SvO2 to fall.

7. Physiology Tissue oxygen need is met when the amount of oxygen being delivered to the tissues is sufficient to meet the amount of oxygen being consumed (VO2).  When the oxygen delivery falls below oxygen consumption needs, lactic acidosis develops. VO2 (Oxygen Consumption) = Cardiac Output X Hb   X (SaO2 - SvO2) Oxygen Delivery (DO2) = Cardiac Output X Oxygen Content ( Hb X SaO2)

8. Physiology There are 4 fundamental causes for a drop in SvO2: The cardiac output is not high enough to meet tissue oxygen needs The Hb is too low The SaO2 is too low Oxygen consumption has increased without an increase in oxgyen delivery 

9. Physiology ATP (energy) is needed for all cell function and survival. Tissues require oxygen in order to make ATP (energy).  If the amount of oxygen being received by the tissues falls below the amount of oxygen required (because of an increased need, or decreased supply), the body attempts to compensate as follows: First Compensation: Cardiac Output increases Second Compensation: Tissue oxygen extraction increases. Third Compensation: Anaerobic Metabolism increases  

10. Techniques Intermittent sampling Spectrophotometry Co-oxymetry Indwelling catheter Incorporating optical fibre in PA and CVC catheter Continuous spectrophotometry

11. Utility Correct clinical interpretation of Sv O2 , or its properly measured Scv O2  surrogate, can be used to- Estimate cardiac output using the Fick equation Better understand whether a patient's oxygen delivery is adequate to meet their oxygen demands Help guide clinical practice, particularly when resuscitating patients using validated early goal directed therapy treatment protocols Understand and treat arterial hypoxemia, and Rapidly estimate shunt fraction (venous admixture).

12. Utility Scv O2  and  SvO2  measurements should never be interpreted in isolation. Rather, clinical context must always be considered. For example,  SvO2 over 70% is generally a good indicator. Yet in the setting of extreme vasodilatory shock or following mitochondrial poisoning where organ function is poor and lactate is rising, an  SvO2 of 90% provides is not good at all

13. Disadvantage Must be measured from a PAC thus patient exposed to risks associated with pulmonary artery catheterization (arrhythmia, pulmonary infarction, embolism, bleeding, pneumothorax, line sepsis) Can be high in a number of situations (sepsis, liver failure, wedged PAC, administration of high FiO2) Can be low in a number of situation (multiple organ failure, cardiac arrest) Requires calibration for changing haematocrit Gattinoni RCT showed no benefit from SvO2 monitoring

14. INTERPRETATION High SvO2 increased O2 delivery (increased FiO2, hyperoxia , hyperbaric oxygen) decreased O2 demand (hypothermia, anaesthesia , neuromuscular blockade) high flow states: sepsis, hyperthyroidism, severe liver disease

15. INTERPRETATION Low SvO2 decreased O2 delivery: 1. decreased Hb ( anaemia , haemorrhage , dilution) 2. decreased SaO2 ( hypoxaemia ) 3. decreased Q (any form of shock, arrhythmia) increased O2 demand (hyperthermia, shivering, pain, seizures)

16. INTERPRETATION Causes of High SvO2 despite evidence of End-organ Hypoxia microvascular shunting (e.g. sepsis) histotoxic hypoxia (e.g. cyanide poisoning) abnormalities in distribution of blood flow

17. Problems R ecent data showing that lactate clearance in sepsis is non-inferior to continuous ScVO2 monitoring (Jones, JAMA, 2010) T itration of end of resuscitation to ScvO2 may not be required (ICU Monitor, 2010 – summary of Jones, JAMA, 2010) ScvO2 does not reflect myocardial perfusion (upstream from the opening of the coronary venous sinus)

18. COMPLICATIONS Same as those associated with central line insertion & PAC Equipment failure CeVox can block a CVC lumen completely and is prone to drift Potential misinterpretation of the measured values if devices are incorrectly calibrated or malpositioned

19. Clinical application

20. Clinical application

21. Thank You Taking the earth as the individual and its satellite the moon as the relative, moonlight can be seen as what can still shed light during the night of an unconscious state

22. Shunt fraction=Q˙S/Q˙T=(CcO2−CaO2)/(CcO2−CvO2),Shunt fraction=Q˙S/Q˙T=(CcO2−CaO2)/(CcO2−CvO2), (Eq.7) where Cco 2  is the oxygen concentration in maximally saturated pulmonary end-capillary blood (i.e., Sco 2  = 1). Ignoring dissolved oxygen, this can be simplified to:Shunt fraction=(1 − SaO2)/(1 − SvO2).Shunt fraction=(1 − SaO2)/(1 − SvO2). For example, when pulse oximeter oxygen saturation is 90% and Sv−v− O2  is 60%, the shunt fraction is (1 − 0.9)/(1 − 0.6) = 25%, or when pulse oximeter oxygen saturation is 85% and  Sv¯O2  Sv¯O2  is 70%, the shunt fraction is (1 − 0.85)/(1 − 0.7) = 50%. With this simple equation in mind, bedside estimates of the clinical effect of diuresis , PEEP, and other treatment strategies on shunt fraction becomes very straightforward.