Understanding Cardiac Output Mechanics

Feb 13, 2025

Cardiac Output

Definition

  • Cardiac Output (CO): Heart Rate (HR) x Stroke Volume (SV)
  • Units: Milliliters per minute (mL/min)

Components

  • Heart Rate (HR): Beats per minute (bpm)
  • Stroke Volume (SV): Volume of blood ejected per beat (mL/beat)

Normal Values

  • Average Heart Rate: 70 bpm
  • Average Stroke Volume: 70 mL
  • Average Cardiac Output: 5,000 mL/min or 5 L/min

Factors Influencing Cardiac Output

Heart Rate (HR)

  • Intrinsic Regulation: Set by SA Node (60-100 bpm)
  • Sympathetic Nervous System: Releases norepinephrine and epinephrine, increasing HR (positive chronotropic effect)
  • Parasympathetic Nervous System: Releases acetylcholine, decreasing HR (negative chronotropic effect)
  • Hormonal Influence: Thyroid hormones (T3, T4) can increase HR
  • Temperature Influence: Increased body temperature increases HR
  • Ionic Influence:
    • High calcium: Increases HR
    • High potassium: Decreases HR, risk of cardiac arrest
  • Chemoreceptors: Low O2, high CO2, low pH activate chemoreceptors to increase HR
  • Age and Gender:
    • Higher in infants and children
    • Females have slightly higher HR than males

Stroke Volume (SV)

  • Equation: SV = End Diastolic Volume (EDV) - End Systolic Volume (ESV)
  • Average Values:
    • EDV: ~120 mL
    • ESV: ~50 mL
    • SV: ~70 mL

Influencing Factors

  • Preload:
    • Degree of myocardial stretch
    • Increased by high venous return, muscular milking, respiratory pump, venoconstriction
    • Frank-Starling Law: Greater stretch increases contraction strength
  • Contractility:
    • Increased by sympathetic nervous system, hormones, and certain drugs (positive inotropic agents)
    • Decreased by high potassium or sodium, low calcium, acidity (negative inotropic agents)
  • Afterload:
    • Resistance to ventricular ejection
    • Increased by high systemic vascular resistance, valve stenosis, or plaques
    • Increased afterload reduces SV

Conditions Affecting HR and SV

  • Bradycardia: HR < 60 bpm
  • Tachycardia: HR > 100 bpm
  • Exercise: Increases CO due to increased HR and SV
  • Endurance Athletes: High SV allows lower resting HR

Key Concepts

  • Chronotropic Effects: Affect HR
  • Inotropic Effects: Affect force of contraction
  • Atrial Bainbridge Reflex: Increased venous return increases HR through stretch receptors

These notes summarize the detailed discussion on cardiac output and its influencing factors, providing a comprehensive overview of the key concepts and physiological mechanisms involved.

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