Cardiac Output and Its Regulation

Cardiac Output: The Basics - Heart's Vital Stats

• Cardiac Output (CO): Blood pumped/ventricle/min; ~5-6 L/min.
  • Formula: $CO = HR \times SV$.
• Stroke Volume (SV): Blood pumped/beat; ~70-80 mL.
  • Formula: $SV = EDV - ESV$.
• Cardiac Index (CI): CO/Body Surface Area (BSA); ~2.5-4.2 L/min/m².
  • Formula: $CI = CO / BSA$.
• Ejection Fraction (EF): Percentage of End Diastolic Volume (EDV) ejected per beat; normally 55-70%.
  • Formula: $EF = (SV / EDV) \times 100%$.

  ⭐ EF is a crucial measure of left ventricular function; values < 40% suggest systolic dysfunction/heart failure.

Heart Rate Regulation - The Cardiac Conductor

• Pacemaker: SA node (intrinsic rate 60-100 bpm). Highest discharge frequency.
• Autonomic Control (ANS):
  • Sympathetic (SNS): Norepinephrine (NE) on β1 receptors → ↑HR, ↑conduction (+ve chronotropy & dromotropy).
  • Parasympathetic (PNS - Vagus): Acetylcholine (ACh) on M2 receptors → ↓HR, ↓conduction (-ve chronotropy & dromotropy).

    ⭐ Vagal tone predominates at rest, keeping resting HR ~70 bpm (vs. SA intrinsic ~100 bpm).

• Hormonal:
  • Adrenaline (β1): ↑HR.
  • Thyroid hormones (T3/T4): ↑HR (↑β1 receptor sensitivity).
• Other Factors:
  • Temperature: ↑Temp → ↑HR.
  • Ions: K+ (HyperK+ ↓HR); Ca2+ (HyperCa2+ ↑HR).
  • Bainbridge Reflex: ↑Venous return → ↑Atrial stretch → ↑HR.

Stroke Volume Regulation - The Ventricular Trio

• Preload: Ventricular stretch at end-diastole (EDV).
  • Frank-Starling Mechanism: ↑Preload → ↑SV (optimal myofilament overlap).
  • Factors: Venous return (volume status, venoconstriction, muscle/respiratory pumps).
• Afterload: Force opposing ventricular ejection.
  • LV: Aortic pressure, Systemic Vascular Resistance (SVR).
  • RV: Pulmonary artery pressure, Pulmonary Vascular Resistance (PVR).
  • ↑Afterload → ↓SV.
• Contractility (Inotropy): Intrinsic strength of myocardial contraction.
  • ↑Sympathetic tone (e.g., catecholamines), ↑Ca²⁺ influx → ↑Contractility → ↑SV.
  • Independent of loading conditions. 📌 Mnemonic: PAC (Preload, Afterload, Contractility) drives Stroke Volume.

⭐ The Frank-Starling mechanism ensures that the heart automatically adjusts its output to match venous return, preventing blood accumulation.

Factors Modulating CO - The Output Shifters

• CO ↑ (Increased Output):
  • Physiological:
    • Exercise (up to 5-7x)
    • Anxiety, excitement (sympathetic ↑)
    • Pregnancy (↑ blood volume, ↑ metabolism)
    • Eating, high environmental temperature
  • Pathological:
    • Hyperthyroidism (↑ BMR)
    • Anemia (↓ O2 capacity, ↓ viscosity)
    • Arteriovenous (AV) shunts (↓ TPR)
    • Beriberi (Thiamine deficiency → vasodilation)
• CO ↓ (Decreased Output):
  • Physiological:
    • Standing (postural change, transient ↓ VR)
  • Pathological:
    • Heart failure (↓ contractility)
    • Myocardial Infarction (MI)
    • Hemorrhage, hypovolemia (↓ VR)
    • Shock (e.g., cardiogenic, hypovolemic)
    • Severe valvular disease (stenosis/regurgitation)
    • Cardiac tamponade (↓ filling)
    • Hypothyroidism (↓ BMR)

⭐ Anemia significantly ↑ CO (can double) due to ↓ blood viscosity & tissue hypoxia leading to peripheral vasodilation.

Cardiac Output Measurement - Quantifying the Flow

• Fick Principle: Uses O₂ consumption. $CO = \frac{VO_2}{C_aO_2 - C_vO_2}$. Invasive; original gold standard.
• Indicator Dilution:
  • Thermodilution: Cold saline via pulmonary artery catheter (Swan-Ganz). Stewart-Hamilton equation. Common.
  • Dye dilution (Indocyanine green): Historically used, now rare.
• Echocardiography: Widely used, non-invasive. Doppler (VTI (aortic)) or 2D for Stroke Volume (SV). $CO = SV \times HR$.
• Pulse Contour Analysis: Minimally invasive (arterial line); SV from arterial waveform, allows continuous monitoring.

⭐ Thermodilution via pulmonary artery catheter (PAC) is a key invasive reference method in critical care.

High‑Yield Points - ⚡ Biggest Takeaways

• CO = HR × SV; normal adult value is ~5 L/min.
• Frank-Starling Law: ↑Preload (venous return, EDV) directly ↑SV.
• Afterload: Resistance to ventricular ejection (e.g., systemic vascular resistance, aortic pressure).
• Contractility: Intrinsic myocardial pump effectiveness, altered by inotropes and ANS.
• ANS: Sympathetic ↑CO (↑HR, ↑SV); Parasympathetic (vagal) ↓HR, thus ↓CO.
• Fick Principle: CO = O₂ consumption / (Arterial O₂ content - Venous O₂ content).