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Understanding Cardiac Output and Its Factors

Mar 7, 2025

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Cardiac Output

Definition

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

Heart Rate (HR)

  • Units: Beats per minute (bpm)
  • Normal Range: 60-100 bpm
    • < 60 bpm: Bradycardia

    • 100 bpm: Tachycardia

  • Set by: SA Node (Sinus Rhythm)

Influences on Heart Rate

  • Sympathetic Nervous System:
    • Releases norepinephrine, epinephrine
    • Acts on beta-1 adrenergic receptors
    • Positive chronotropic effect (increases HR)
  • Parasympathetic Nervous System:
    • Releases acetylcholine
    • Acts on muscarinic type 2 receptors
    • Negative chronotropic effect (decreases HR)
  • Thyroid Hormones (T3 and T4):
    • Increase HR and basal metabolic rate
  • Body Temperature:
    • Increased temperature raises HR
  • Ions:
    • High calcium: Increases HR
    • High potassium: Can lead to cardiac arrest
    • Low levels can cause arrhythmias
  • Chemo Receptors:
    • Respond to low O2, high CO2, low pH
    • Increase HR
  • Age and Gender:
    • Fetus: 120-140 bpm, Adults: 60-100 bpm
    • Females generally have a faster HR than males

Stroke Volume (SV)

  • Equation: End-Diastolic Volume (EDV) - End-Systolic Volume (ESV)
    • Normal SV: ~70 mL per beat

Influences on Stroke Volume

  • Preload:
    • Degree of stretch in cardiac muscle
    • Influenced by EDV; more blood = more stretch
    • Increased by venous return (muscle milking, respiratory pump, venoconstriction)
  • Contractility:
    • Strength of contraction
    • Influenced by sympathetic stimulation, hormones (T3, T4), drugs (digitalis, dopamine)
  • Afterload:
    • Resistance to blood ejection from ventricles
    • Increased by hypertension, aortic stenosis, plaque buildup
    • Inversely proportional to SV

Key Concepts

  • Frank-Starling Law: Greater stretch leads to stronger contraction
  • Inotropic Agents:
    • Positive: Increase contractility (epinephrine, calcium)
    • Negative: Decrease contractility (beta-blockers, calcium channel blockers)

Additional Points

  • Age and Heart Rate Influences
    • Fetus have very high HR, adults vary with gender
  • Clinical Relevance:
    • High afterload common in hypertension
    • Preload influenced by blood return and muscle activity

This extensive exploration of cardiac output covers its calculation, factors influencing heart rate and stroke volume, and how these play a crucial role in cardiovascular health.

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