Cardiac Output — Determinants

Cardiac output is the amount of blood pumped into the systemic circulation per unit time.

Normal Cardiac Output = 5l/min

Normal Cardiac Index = 3l/min/m²

Cardiac Output = SV x HR

Stroke Volume

SV = Amount of blood pumped into the circulation per contraction

Usually 70mls

Influenced by Preload, Afterload, Contractility

• Preload / Venous Return:
  • = (MSFP – RAP) / RVR
    • MSFP: Mean Systemic Filling Pressure
      • Equilibration of pressures in the systemic circulation if cardiac output was abolished
      • Describes the filling state of the circulation and tone of capacitance vessels
    • RAP: Right Atrial Pressure
    • RVR: Resistance to Venous Return
      • Calibre of transmitting venous system according to Poisuille-Hagen equation (where resistance ∝ 1/r⁴)
      • Physical obstructions to venous return
        • Mechanical obstruction
        • Valvular stenosis
  • Thoracic pump (negative intrathoracic pressure created during inspiration)
  • Skeletal muscle pump
  • One way valves
  • Pump function of the ventricle
  • Venous tone

• Afterload
  • Proportional to aortic pressure and ventricular size
  • Inversely proportional to ventricular wall thickness
  • Affected by obstruction to the LVOT
• Contractility
  • Affected by adrenergic or muscarinic stimulation
  • Availability of Ca

Heart Rate

• Affected by automatic rhythmicity of cell and sympathetic/muscarinic input

Cardiac Output regulated by venous return

• Ventricles have ability to adapt to various degrees of filling according to the Frank-Starling Principal
  • Increasing preload increases cardiac output to a degree, before cardiac output falls
  • Due to the optimal alignment of actin and myosin filaments at sarcomere length of 2.2μm
  • Also due to some stretch related release of Ca²⁺ increasing contractility

Autonomic Regulation

• Medullary vasomotor area
  • Inputs to sensory area in NTS
  • Vasoconstrictor area in anterolateral upper medulla
    • Sympathetic output
  • Vasodilator area in anterolateral lower medulla
    • Parasympathetic output
• Adrenaline and to a lesser extent noradrenaline stimulates β₂ adrenoceptors
  • G_sPCR
  • Increased cAMP due to upregulation of adenylyl cyclase
  • Activation of PKA leading to phosphorylation and activation of intracellular enzymes
    • Inotropy
      • Increased Ca²⁺ release from and SR and influx from T-tubules
      • Increased intracellular [Ca²⁺]
      • Increased Ca²⁺ – troponin C interaction
      • Increased Actin and myosin interaction due to exposure of myosin binding sites
      • Increased contractility
    • Chronotropy

• Acetylcholine
  • G_iPCR → decreased cAMP and reversing the above
  • Muscarinic receptor coupled K⁺ channels hyperpolarize the cell reducing chance for action potential
• At rest
  • Parasympathetic and Sympathetic tone
    • Parasympathetic > Sympathetic
      • HR ~60
        • + Propranolol → HR 50
        • + Atropine → HR 110
        • + Propranolol + Atropine → HR 100

Reflexes

Baroreflex

• Sensor
  • Aortic arch and carotid sinus stretch receptors
  • Transmit via vagal nerve and glossopharyngeal nerve respectively
• Control centre
  • Vasomotor area in medulla
  • Increased stretch causes inhibition of sympathetic and promotion of parasympathetic output
• Effector
  • Parasympathetic innervation of the heart via vagal nerve upregulated
  • Sympathetic innervation via thoracic segments of the sympathetic chain

Bainbridge reflex

• Input
  • Increased venous return or obstruction to atrial outflow causes atrial stretch
  • Atrial stretch receptors signal to vasomotor area
  • Increased sympathetic output to heart à tachycardia

Sakurai 2016

Answer by Sakurai