19A08: Exam Report

Compare and contrast the measurement (40% of marks) and interpretation (60% of marks) of both central venous and mixed venous oxygen saturations.

8% of candidates passed this question.

Many candidates did not appreciate that ScvO2 refers to SVC / RA junction venous oximetry and not femoral or peripheral venous oximetry. Methods of measurement such as co-oximetry and reflectance spectrophotometry needed to be explained. Marks were awarded for the normal values. Discussion of the relationship between ScvO2 and SmvO2 and changes during shock
attracted marks. Better answers quoted the modified Fick equation and related this to cardiac output and factors affecting oxygen consumption versus delivery.

G6iii / 19A08: Compare and contrast the measurement (40% of marks) and interpretation (60% of marks) of both central venous and mixed venous oxygen saturations

Definitions

Central venous O2 (ScvO2) refers to oxygen saturations of venous blood at the cavo-atrial junction, measured with an IJ/SCV CVC.

Mixed venous O2 (SvO2) refers to oxygen saturations of venous blood after mixing in the RV, measured with a pulmonary artery catheter.

Background

ScvO2 is occasionally used as a surrogate for SvO2, which is a marker of oxygen flux

Fick equation: CO = VO2/CaO2 – CvO2

  • DO2 (oxygen delivery) = CO x CaO2
    • Determined by cardiac output + arterial O2 content ([Hb] x 1.34 x SaO2 + PaO2 x 0.003)
    • .: increased O2 delivery from increased cardiac output, arterial O2 content will INCREASE SvO2
  • VO2 (oxygen consumption) = CO x (CaO2 – CvO2)
    • Determined by cardiac output + arteriovenous O2 content difference
    • .: increased O2 consumption (e.g. stress, pain, shivering) will increase arteriovenous O2 content difference, and DECREASE SvO2
  • Oxygen flux = DO2 – VO2
    • = CO x CaO2 – CO x CaO2 + CO x CvO2
    • = CO (CvO2)
    • = CO [(Hb x 1.34 x SvO2) + (pVO2 x 0.003)]
  • SvO2 = SaO2 x (1-ER)
    • Mixed venous sats approx. equal to arterial sats x (1- extraction ration of O2)
    • Generally, lower SvO2 reflects increased total body oxygen consumption

Measurement Differences

ScvO2

Mixed Venous O2

Sampling Site

ScvO2

SVC/RA junction

Mixed Venous O2

Pulmonary artery

Sampling Method

ScvO2

Internal jugular/subclavian central line

Mixed Venous O2

Pulmonary artery catheter – more invasive

Advantages

ScvO2

Accessible – most ICU patients will have a CVC

Mixed Venous O2

Disadvantages

ScvO2

Complications of CVC insertion including bleeding, infection

Mixed Venous O2

PAC can’t be used with severe TR, intra-cardiac shunt, MS

All complications inferred by CVC insertion + Arrhythmias/catheter knotting/myocardial perforation/death

Measurement Method

Same methods of measurement:
Continuous monitoring via reflection spectrophotometry

  • LED emits specific near-IR wavelength, reflected wavelength measured
  • Differences in reflected wavelength proportional to haemoglobin saturation
  • Concentrations of deoxy/oxyhaemoglobin calculated from log ratio of signal strength

Intermittent monitoring (w ABG machine) via

  1. Absorption spectrophotometry – SvO2 derived from pO2/pH/pCO2 using OxyHb dissociation curve
  2. Co-oximetry – absorption of near IR light by Hb species measured, Beer-Lambert law used to calculate oxyHb/deoxyHb concentrations and .: saturations

Mixed venous O2 can only be measured intermittently via either method when PA catheter floated

Interpretation

ScvO2

Mixed Venous O2

Value Reflects

ScvO2

Upper body oxygen consumption

Mixed Venous O2

Balance of whole body oxygen consumption/delivery

Normal Values

ScvO2

65-80%

Lower normal value due to increased upper body O2 consumption & extraction (brain, heart have high O2 consumption) relative to lower body (kidneys have higher blood flow, low O2 consumption)

Mixed Venous O2

70-75%

Consists of blood from SVC, IVC, coronary sinus mixed in RA and RV

Shock State

ScvO2

<50% with tissue hypoxia

Mixed Venous O2

<65%

High Flow State

ScvO2

>75%

Mixed Venous O2

>80% with shunting, hyperoxic/cytotoxic states

Interpretation

Elevated reading with:

  • Anaesthesia (increased CBF, decreased CMRO2)
  • Septic shock (decreased flow to splanchnic circulation = increased lower body O2 extraction, much lower IVC sats)
  • Cyanide toxicity
  • High output cardiac failure
  • Hypothermia

Low reading with:

  • Cardiogenic, septic shock
  • Malignant hyperthermia

Derived Data

ScvO2

CVP

Mixed Venous O2

CO/CI
PCWP, Pulm A pressure, RA pressure, RVSP, RVDP
Core body temperature

Author: Audrey Guo