Mixed venous oxygen saturation (SvO2) monitoring

USES

• measurement of oxygenation saturation from mixed venous blood (SvO2) in the pulmonary artery
• requires Pulmonary Artery Catheter insertion in most clinical settings

DESCRIPTION

• measures the end result of O2 consumption and delivery

METHOD OF INSERTION AND/OR USE

O2 flux = (cardiac output x (Hemoglobin concentration x SpO2 x 1.34) + (PaO2 x 0.003)) – oxygen consumption

• SvO2 = mixed venous oxygen saturation
• measured via a sample of blood from a pulmonary artery catheter (PAC)
• measures the end result of O2 consumption and delivery
• is used in ICU as a measure of O2 extraction by the body
• normal SvO2 = 65-70%
• SvO2 > ScvO2 as it contains blood from both SVC and IVC
• if SvO2 low then either consumption elevated or demand high
• 0.5 corresponds to a theoretical critical PvO2 of 26mmHg -> level where tissue dysoxia is highly likely
• > 0.8 corresponds with high flow states: sepsis, hyperthyroidism, severe liver disease

OTHER INFORMATION

Usefulness

• it can be used as a marker of how well O2 is being delivered to the peripheral tissues by extrapolation (if SvO2 low and patient in multiorgan failure then we can add a inotrope to help increase cardiac output ie. in severe sepsis)
• its surrogate ScvO2 has used as a treatment goal in severe sepsis and has been shown to decrease mortality and morbidity (Rivers study)
• continuous measurement obtained once inputting data about patient (thus can see trends with changes in therapy – fluid, inotropes, vasodilators, dialysis)
• good information quickly

Problems

• must be measured from a PAC thus patient exposed to risks associated with pulmonary artery catheterization (arrhythmia, pulmonary infarction, embolism, bleeding, pneumothorax, line sepsis)
• blood taken from a normal central line to estimate SvO2 (referred to as ScvO2 not true result and not as accurate and may mainly be blood from SVC which has a different O2 saturation than SvO2 -> used as a treatment goal in severe sepsis and has been shown to decrease mortality and morbidity (Rivers))
• 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

COMPLICATIONS

• see complications associated with PAC use

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

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)

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

References and Links

LITFL

• CCC — SvO2 versus ScvO2
• CCC — Central venous oxygen saturation (ScvO2) monitoring
• CCC — Pulmonary Artery Catheter

Journal articles

• Robin E, Costecalde M, Lebuffe G, Vallet B. Clinical relevance of data from the pulmonary artery catheter. Crit Care. 2006;10 Suppl 3:S3. Review. PubMed PMID: 17164015; PubMed Central PMCID: PMC3226125.