Lecture Notes: Cardiac Output

Definition of Cardiac Output

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

Heart Rate (HR)

• Measured in beats per minute (bpm)
• Normal heart rate: 60-100 bpm
• SA node sets the sinus rhythm (60-80 bpm)
• Influences on heart rate:
  • Sympathetic Nervous System: Releases norepinephrine and epinephrine, increasing HR (positive chronotropic agent)
  • Parasympathetic Nervous System: Releases acetylcholine, decreasing HR (negative chronotropic agent)
  • Hormones: Thyroid hormones (T3 and T4) increase HR
  • Body Temperature: Increase in temperature raises HR
  • Ions: Calcium (high Ca²⁺ increases HR, low Ca²⁺ decreases HR), Potassium (high K⁺ decreases HR)
  • Chemoreceptors: Activated by low O₂, high CO₂, low pH, stimulates HR
• Age and Heart Rate:
  • Fetus/Infants: Higher heart rates (120-140 bpm)
  • Adults: Males (64-72 bpm), Females (72-80 bpm)
  • Bradycardia: HR < 60 bpm
  • Tachycardia: HR > 100 bpm

Stroke Volume (SV)

• Volume of blood pumped by ventricles per beat
• SV = End Diastolic Volume (EDV) - End Systolic Volume (ESV)
• Average SV: 70 mL/beat

Factors Affecting Stroke Volume

1. Preload

• Degree of stretch of cardiac muscle
• Increased by EDV
• Influenced by:
  • Venous return (muscular milking, respiratory pump, venomotor tone)
  • Filling time
  • Health of myocardium (no scar tissue from MIs)
• Frank-Starling Law: Greater stretch = Greater contraction

2. Contractility

• Strength of contraction at any given preload
• Increased by:
  • Sympathetic stimulation (norepinephrine/epinephrine)
  • Hormones (T3, T4, glucagon)
  • Drugs (Digitalis, Dopamine)
• Decreased by:
  • Beta-blockers
  • Calcium channel blockers
  • Ions: High Ca²⁺ increases contractility, High K⁺ decreases contractility
• Positive inotropic agents: Increase contractility
• Negative inotropic agents: Decrease contractility

3. Afterload

• Resistance ventricle must overcome to eject blood
• Increased by:
  • Hypertension
  • Plaque buildup
  • Valve stenosis
• Decrease in afterload increases SV

Additional Concepts

• Atrial Bainbridge Reflex: Increases HR in response to increased venous return

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This lecture covers the concepts of cardiac output, heart rate, stroke volume, and their influencing factors. Understanding the physiological mechanisms behind these parameters is crucial for comprehending cardiovascular function and its regulation.