Cardiac Output

Jul 15, 2024

Lecture Notes: Cardiac Output

Overview

  • Definition: Cardiac output (CO) = Heart Rate (HR) x Stroke Volume (SV)
  • Units
    • Heart Rate (HR): Beats per minute (bpm)
    • Stroke Volume (SV): Milliliters per beat (mL/beat)
  • CO Measurement: mL/min or L/min

Normal Values

  • Average Heart Rate: 60-80 bpm (Use 70 bpm as a standard reference)
  • Average Stroke Volume: 70 mL/beat
  • Average Cardiac Output: 5000 mL/min or 5 L/min
  • Adjustments
    • Increases: Exercise, sympathetic activation
    • Decreases: Parasympathetic activation, heart diseases

Heart Rate (HR) Influences

Normal Range

  • Normal HR: 60-100 bpm
  • SA Node: Sets sinus rhythm, usually 60-80 bpm
  • Tachycardia: >100 bpm
  • Bradycardia: <60 bpm

Influences

  • Sympathetic Nervous System
    • Neurotransmitters: Norepinephrine, Epinephrine
    • Effect: Positive chronotropic, increases HR
  • Parasympathetic Nervous System
    • Neurotransmitter: Acetylcholine
    • Effect: Negative chronotropic, decreases HR
  • Hormones
    • Thyroid Hormones (T3 and T4): Increase HR, basal metabolic rate, and body temperature
  • Body Temperature: Higher temps increase HR
  • Ions
    • Calcium: High levels increase HR, low levels decrease HR
    • Potassium: High levels decrease HR (can cause cardiac arrest), low levels can cause arrhythmias
  • Chemoreceptors
    • Location: Carotid bodies, aortic arch
    • Stimuli: Low O2, high CO2, low pH
    • Effect: Minor increase in HR

Age Influences

  • Fetus/Infants: High HR, 120-140 bpm
  • Adults: 60-100 bpm, generally lower with females having slightly higher values
  • Older Adults: HR can increase again

Conditions

  • Tachycardia: Sympathetic activity, T3/T4, drugs, anxiety
  • Bradycardia: Parasympathetic activity, drugs, endurance training (increased stroke volume)

Stroke Volume (SV) Influences

Definition

  • Formula: SV = End Diastolic Volume (EDV) - End Systolic Volume (ESV)
  • EDV: Blood in ventricles before contraction (120-140 mL)
  • ESV: Blood remaining post-contraction (50-70 mL)
  • Average SV: 70 mL

Factors Affecting SV

  1. Preload: Degree of stretch of cardiac muscle
    • Increases With: High EDV, venous return, muscular milking, respiratory pump, venomotor tone, adequate filling time
    • Starling's Law: Greater stretch leads to greater contraction
  2. Contractility: Force of muscle contraction
    • Increases With: Sympathetic activation, calcium levels, T3 and T4, glucagon, certain drugs (digitalis, dopamine, epinephrine)
    • Decreases With: Beta blockers, calcium channel blockers, acidosis, hyperkalemia, hypernatremia, hypocalcemia
  3. Afterload: Resistance overcoming to eject blood
    • Increased By: Hypertension, valve stenosis, systemic vascular resistance, plaque buildup
    • Effect: Increased afterload decreases SV

Practical Implications

  • Endurance Training: Increases SV, can cause bradycardia
  • Diseases: Hypertension, valve issues, coronary artery disease can increase afterload leading to decreased SV

Other Factors

  • Atrial Bainbridge Reflex: Increased venous return stimulates HR through cardiac accelerator center

Summary: Understanding the balance and influences on heart rate and stroke volume is essential for comprehending how cardiac output adjusts to physiological demands and pathological conditions.