Question 19

College Answer

The factors that influence VR are captured in 2 formulae; VR = CO, and VR = (MSFP-RAP) / Venous Resistance. Candidates that used these as the backbone structure of their answer scored well. Quite a few candidates failed to consider factors that affect left heart CO also effect VR. Recognising that CO does = VR appeared to elude some candidates.

Discussion

• Venous return is the rate of blood flow into the heart from the veins.
• At a steady state, venous return and cardiac output are equal.
• Venous return can be expressed as VR = (MSFP - RAP) / VR = HR × SV
  where MSFP is mean systemic filling pressure, RAP is right atrial pressure and VR is the venous resistance 
• Factors which influence venous return are therefore:
  • Right atrial pressure (RAP):
    • The mean pressure in the right atrium, which corresponds to the  mean central venous pressure (CVP) 
    • As RAP increases, venous return decreases
    • Factors which affect right atrial pressure
      • Intrathoracic pressure (spontaneous vs. positive pressure ventilation)
      • Pericardial compliance (eg. tamponade, open chest)
      • Right atrial compliance (eg. infarct, dilatation)
      • Right atrial contractility (i.e. AF vs sinus rhythm)
      • Tricuspid valvular competence and resistance
  • Mean systemic filling pressure, MSFP:
    • The pressure exterted by the tone of the vascular smooth muscle on the systemic blood volume, in the absence of pulsatile flow
    • As MSFP increases, the venous return increases
    • Factors which affect mean systemic filling pressure
      • Total venous blood volume
      • Venous smooth muscle tone (which affects the size of the  "stressed volume"
  • Venous resistance, VR:
    • The resistance to flow in the venous circulation
    • As VR increases, venous return decreases
    • Factors which affect venous resistance
      • Mechanical factors
        • Posture
        • Intraabdominal pressure
        • Skeletal muscle pump
        • Obstruction to venous flow (eg. pregnancy,  SVC obstruction)
        • Hyperviscosity (polycythemia, hyperproteinaemia)
      • Neuroendocrine factors
        • Autonomic tone
        • Vasoactive drugs (eg. noradrenaline, GTN)
  • Cardiac output, CO:
    • The amount of blood pumped by the heart in L per minute
    • Cardiac output and venous return are essentially the same thing; as cardiac output decreases, the venous return decreases.
    • Factors which affect cardiac output
      • Preload
      • Afterload
      • Contractility

For a diagram, the exam candidate would probably have to reproduce something like this:

Beard, Daniel A., and Eric O. Feigl. "Understanding Guyton's venous return curves." American Journal of Physiology-Heart and Circulatory Physiology 301.3 (2011): H629-H633.

Guyton, Arthur C. "Regulation of cardiac output." Anesthesiology: The Journal of the American Society of Anesthesiologists 29.2 (1968): 314-326.

Guyton, A. "Circulatory Physiological: Cardiac Output and Its Regulation." Philadelphia and London: WB Saunders Company (1963).

Guyton, Arthur C. "Determination of cardiac output by equating venous return curves with cardiac response curves." Physiological reviews 35.1 (1955): 123-129.

Levy, MATTHEW N. "The cardiac and vascular factors that determine systemic blood flow." Circulation research 44.6 (1979): 739-747.

Guyton, Arthur C., Arthur W. Lindsey, and Berwind N. Kaufmann. "Effect of mean circulatory filling pressure and other peripheral circulatory factors on cardiac output." American Journal of Physiology-Legacy Content 180.3 (1955): 463-468.

Guyton, Arthur C., James H. Satterfield, and Jeff W. Harris. "Dynamics of central venous resistance with observations on static blood pressure." American Journal of Physiology-Legacy Content 169.3 (1952): 691-699.

Guyton, Arthur C., Dimitri Polizo, and George G. Armstrong. "Mean circulatory filling pressure measured immediately after cessation of heart pumping." American Journal of Physiology-Legacy Content 179.2 (1954): 261-267.

Guyton, Arthur C., et al. "Venous return at various right atrial pressures and the normal venous return curve." American Journal of Physiology-Legacy Content 189.3 (1957): 609-615.

Bendjelid, Karim. "Right atrial pressure: determinant or result of change in venous return?." Chest 128.5 (2005): 3639-3640.

Bendjelid, Karim. "Right atrial pressure: determinant or result of change in venous return?." Chest 128.5 (2005): 3639-3640.

Bressack, Michael A., and Thomas A. Raffin. "Importance of venous return, venous resistance, and mean circulatory pressure in the physiology and management of shock." Chest 92.5 (1987): 906-912.