Question 6

College Answer

Answers should have included a description of the need for a pressure gradient for flow and a discussion on right atrial pressure, mean systemic filling pressure and resistance to blood flow. The discussion of each of these factors included definitions, normal values, factors affecting them and the direction of change on venous return. Diagrams were not essential, but their use assisted some candidates in explaining the effects of RAP on venous return.

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.