Question 8

Last updated 16/10/2021 - 07:10
 Topic: Cardiovascular system
Pass rate
50%
Other SAQs in this paper:
Other SAQs in this topic:

Describe the factors that affect the output of the right ventricle

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College Answer

An approach that covered the main determinants of right ventricular cardiac output
including heart rate, right ventricular preload, contractility, afterload and the
relationship with left ventricular output, ventricular interdependence, and the
respiratory system would have provided the framework for a good answer. Some
candidates used this approach but described more features of left ventricular than
right ventricular output. The observation that the right ventricle is relatively thin
walled and its output is very sensitive to changes in right ventricular preload and
afterload particularly was central to this question. The unique shape of the right
ventricle and its contraction characteristics involving ventricular interdependence
were rarely mentioned. Also details on right ventricular afterload and the importance
of factors affecting pulmonary vascular resistance were lacking in most answers.
Syllabus: C1c
Recommended sources: Review of Medical Physiology, Ganong, Chps 31 and 33,
Textbook of Medical Physiology, Guyton & Hall Chp 9 and 20

Discussion

Factors which Affect the Function of the Right Ventricle
Heart rate
  • The cardiac output of both the RV and the LV are affected by the heart rate in the same way.
Stroke volume
  • On average, though there might be beat-to-beat variations, the stroke volume both the ventricles is the same. 
  • The RV stroke volume is affected by:
    • RV afterload
    • RV preload
    • RV contractility
    • Interventricular interdependence
Afterload
  • RV afterload is affected by:
    • Thin wall  of the RV: (increases afterload)
    • Positive intrathoracic pressure (increases afterload)
    • Increased pulmonary vascular resistance increases afterload:
      • Low lung volumes
      • Low pulmonary blood flow
      • Hypoxic pulmonary vasoconstriction
      • Systemic catecholamines and an activated sympathetic nervous system
      • Increased blood viscosity (raised haematocrit)
Preload
  • RV preload is affected by:
    • Right atrial pressure
    • mean systemic filling pressure
      • Total venous blood volume
      • Venous vascular compliance
    • Pericardial compliance and pericardial contents
    • Positive intrathoracic pressure (decreases preload)
  • Ventricular wall compliance:
    • Duration of ventricular diastole
    • Wall thickness
    • Relaxation (lusitropic) properties of the muscle
    • End-systolic volume of the ventricle (i.e. afterload)
Contractility

    
For both ventricles, contractility is affected by:

  • Heart rate (Bowditch effect)
  • Afterload (Anrep effect)
  • Preload (Frank-Starling mechanism)

as well as cellular and extracellular calcium concentrations and temperature
Effect of ventricular interdependence
  • The compliance of the right ventricle is decreased in systole by the contraction of the interventricular septum

References

Franklin, Dean L., Robert L. Van Citters, and Robert F. Rushmer. "Balance between right and left ventricular output." Circulation research 10.1 (1962): 17-26.

Vincent, Jean-Louis. "Understanding cardiac output." Critical care 12.4 (2008): 174.

WIGGERS, CARL J. "Determinants of cardiac performance." Circulation 4.4 (1951): 485-495.

Theres, Heinz, et al. "Phase-related changes in right ventricular cardiac output under volume-controlled mechanical ventilation with positive end-expiratory pressure." Critical care medicine 27.5 (1999): 953-958.

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