Study notes on Cardiac Out By Mr. Critical

Cardiac Output (CO)

Definition

• Cardiac Output (CO) = Heart Rate (HR) × Stroke Volume (SV)

• Units: milliliters per minute (mL/min)

• Normal value: ~ 5 L/min (70 bpm × 70 mL/beat)

Heart Rate (HR)

• Normal: 60–100 bpm

• Intrinsic pacemaker: SA node → sets sinus rhythm (60–80 bpm at rest)

Regulators of Heart Rate

1. Autonomic Nervous System

   • Sympathetic (NE, Epi on β1 receptors) → ↑ HR (positive chronotropy)

   • Parasympathetic (ACh on M2 receptors) → ↓ HR (negative chronotropy)

2. Hormones

   • Thyroid hormones (T3, T4) → ↑ HR, ↑ basal metabolic rate

   • Other stimulators: epinephrine, norepinephrine

3. Body Temperature

   • ↑ Temp → ↑ HR

   • ↓ Temp → ↓ HR

4. Electrolytes

   • Calcium (Ca²⁺)

     • ↑ Ca²⁺ → ↑ HR (positive chronotropy)

     • ↓ Ca²⁺ → ↓ HR

   • Potassium (K⁺)

     • ↑ K⁺ (hyperkalemia) → dangerous ↓ HR, may cause arrest

     • ↓ K⁺ (hypokalemia) → arrhythmias

5. Chemoreceptors

   • Located in carotid and aortic bodies

   • Stimulated by:

     • ↓ O₂ (hypoxia)

     • ↑ CO₂ (hypercapnia)

     • ↓ pH (acidosis)

   • Effect: ↑ HR (secondary to ↑ sympathetic drive; stronger effect on respiration than HR)

6. Age & Sex

   • Fetus/infant: very high HR (~120–140 bpm)

   • Adults: 60–100 bpm

     • Males: ~64–72 bpm

     • Females: ~72–80 bpm

   • Elderly: HR may increase again

7. Clinical Conditions

   • Bradycardia (<60 bpm): causes → ↑ parasympathetic tone, drugs, endurance athletes (↑ SV compensates for ↓ HR)

   • Tachycardia (>100 bpm): causes → sympathetic activation, thyroid hormone excess, anxiety, drugs

Stroke Volume (SV)

• Formula: SV = End-Diastolic Volume (EDV) – End-Systolic Volume (ESV)

• Normal: 120 mL – 50 mL = ~70 mL/beat

Determinants of Stroke Volume

1. Preload

   • Definition: degree of stretch of myocardium before contraction (≈ EDV)

   • Factors increasing preload:

     • ↑ Venous return via:

       • Muscular pump (skeletal muscle contraction)

       • Respiratory pump (↓ thoracic pressure, ↑ abdominal pressure)

       • Venoconstriction (↑ venous tone via sympathetic activity)

     • Adequate filling time (↓ HR allows more filling)

   • Frank-Starling Law: greater stretch (EDV) → stronger contraction (↑ SV)

   • Limiting factor: infarcted/fibrotic myocardium (↓ compliance)

2. Contractility

   • Definition: strength of contraction at a given preload

   • Increased by:

     • Sympathetic stimulation (NE/Epi on β1 → ↑ Ca²⁺)

     • Thyroid hormone (↑ β1 receptor expression)

     • Glucagon

     • Drugs: digitalis, dopamine, dobutamine, isoproterenol, atropine

   • Decreased by:

     • β-blockers (e.g., propranolol, metoprolol)

     • Ca²⁺ channel blockers (e.g., verapamil, diltiazem)

   • Ions:

     • ↑ Ca²⁺ → ↑ contractility

     • Severe ↑ K⁺ → ↓ contractility

3. Afterload

   • Definition: resistance the ventricles must overcome to eject blood

   • Determined by: arterial pressure & vascular resistance

   • ↑ Afterload → ↓ SV (e.g., hypertension, aortic stenosis)

Summary

• CO = HR × SV (~5 L/min at rest)

• HR regulators: ANS, hormones, temperature, electrolytes, chemoreceptors, age, clinical states

• SV regulators: preload (EDV & venous return), contractility (Ca²⁺, sympathetic drive, hormones, drugs), afterload (vascular resistance)

• Exercise → ↑ CO (by ↑ HR and ↑ SV)

• Endurance training → ↑ SV, resting bradycardia.