Venous Return

This section describes determinants of cardiac output from the perspective of venous return. The relationship to and importance of myocardial function is covered elsewhere.

Define the components and determinants of cardiac output

The venous system has two key cardiovascular functions:

• Blood reservoir
  Contains 65% of blood volume.
• Conduit for return of blood to the heart

Venous return is the rate at which blood is returned to the heart (in L.min^-1). At steady state, venous return is equal to cardiac output, and can be expressed as:
$VR = {MSFP - RAP \over RVR}$, where:

• $VR$ is venous return
• $MSFP$ is the mean systemic filling pressure
  This is the mean pressure of the circulation when there is no flow. It is an indicator of circulatory filling, and is a function of circulating volume and vascular compliance.
  • Normal mean systemic filling pressure is ~7mmHg
• $RAP$ is the right atrial pressure
  An elevated RAP reduces venous return.
• $RVR$ is the resistance to venous return

This relationship can be expressed graphically:

• When venous return is 0, the measured right atrial pressure is an indication of mean systemic filling pressure

• Alterations to circulating volume and compliance affect both venous return and mean systemic filling pressure

• Alterations to the resistance to venous return affect venous return but mean systemic filling pressure is unchanged

Factors Affecting Venous Return

Venous return will be altered by any of the variables in the above equation:

• MSFP
  • Volume
    e.g. Haemorrhage, resuscitation.
  • Compliance
• RAP
  • Respiratory pump
    Negative intrathoracic pressure reduces RAP, improving venous return.
  • Positive pressure ventilation
  • Pericardial compliance
    • Constriction
    • Tamponade
• Resistance to Venous Return
  • Posture
  • Vascular compression
    • Obesity
    • Pregnancy
    • Laparoscopy

• Other factors affecting venous return
  • Skeletal muscle pump
    Contraction of leg muscles in combination with an intact venous system propels blood back towards the heart.

Interaction between Venous Return and Cardiac Function Curves

Guyton's curve can be superimposed on Starling's curve to examine the interaction between venous and cardiac function over a range of conditions:

References

1. Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.
2. Brandis K. The Physiology Viva: Questions & Answers. 2003.
3. Henderson WR, Griesdale DE, Walley KR, Sheel AW. Clinical review: Guyton - the role of mean circulatory filling pressure and right atrial pressure in controlling cardiac output. Critical Care. 2010;14(6):243. doi:10.1186/cc9247.