Cardiac Output

Master answer

LV Systolic Function

LV systolic function is a function of its contractility.

Contractility = the change in force generated independent of preload

Factors affecting Contractility

Contractility is primarily dependent on intracellular Ca²⁺. Determinants include:

• Metabolic:
  • substrate supply – glucose, fat, protein
  • metabolic/electrolyte homeostasis
  • hormones – thyroid, insulin/glucagon
• Physiological:
  • integrity of myofilaments
  • co-ordinated depolarisation
  • functional muscle mass
  • autonomic tone – sympathetic and parasympathetic
• Drugs: Digoxin, b agonists, PDE3 inhibitors
• Minor increases in contractility occur as an intrinsic response to:
  • increased afterload
    • Anrep Effect: Autoregulation method in which myocardial contractility increases with afterload.
  • increased heart rate
    • Bowditch Effect: Autoregulation method by which myocardial tension increases with an increase in heart rate. Also known as the Treppe phenomenon, Treppe effect or staircase effect .
    • (Contractility improves at faster heart rates. This is because the myocardium does not have time to remove calcium, so it accumulates intracellularly.)
• Disease
  • Ischaemia / Coronary Blood flow
    Reduced ATP production secondary to hypoxia, which impairs sarcoplasmic reticulum Ca²⁺ function. Further exacerbated by intracellular acidosis from anaerobic metabolism.
  • Heart Failure
    Impaired contractility reserve, i.e. minimal increase in contractility with sympathetic stimulation.
    • Reduced peak Ca²⁺and sarcoplasmic reticulum uptake of Ca²⁺

LV Diastolic Function

LV diastolic function is determined by its compliance. LV compliance is primarily determined by  myocardial characteristics and load.

Factors affecting LV diastolic function:

• Normal HR and rhythm
• LV systolic function
• Wall thickness
• Chamber geometry
• Duration, rate and extent of myocyte relaxation
• LV untwisting and elastic recoil
• Magnitude of diastolic suction
• LA-LV pressure gradient
• Passive elastic properties of LV myocardium
• Viscoelastic effects (rapid LV filling and atrial systole)
• LA structure and function
• Mitral valve structure and function
• Pulmonary venous blood flow
• Pericardial restraint
• RV loading conditions and function
• Ventricular interdependence
• Coronary blood flow and vascular engorgement
• Compression by mediastinal masses

JC 2019

Answer by JC