AB

Week 2: Cardiac Cycle

Cardiac Conduction System

  • \text{SA node} (right atrium) = primary pacemaker; fires 70!\text{–}!80\,\text{bpm}
  • Conduction pathway: 1) SA node → 2) atrial myocardium → 3) AV node → 4) \text{AV bundle (Bundle of His)} → 5) left & right bundle branches → 6) Purkinje fibers → ventricular myocardium
  • Damage to SA node → ectopic focus (usually AV node, 40!\text{–}!50\,\text{bpm}); further failure → 20!\text{–}!40\,\text{bpm} (requires pacemaker)

Cardiac Rhythm & Arrhythmias

  • Normal rhythm = sinus rhythm (60!\text{–}!100\,\text{bpm})
  • Tachycardia: persistent >100\,\text{bpm}
  • Bradycardia: persistent <60\,\text{bpm}
  • Key arrhythmias:
    • Ventricular fibrillation: chaotic, irregular ECG; medical emergency
    • Atrial fibrillation: no distinct P waves; irregular QRS
    • Premature atrial/ventricular contractions: early P (PAC) or wide QRS w/o P (PVC)

Electrocardiogram (ECG) Essentials

  • \text{P wave}: atrial depolarization
  • \text{QRS complex}: ventricular depolarization
  • \text{T wave}: ventricular repolarization
  • Intervals:
    • \text{PR interval} – atria → ventricles conduction time
    • \text{QT interval} – total ventricular depol. + repol.

Cardiac Cycle Phases (occurring in both ventricles)

  1. Ventricular Filling (diastole)
    • AV valves open, semilunar closed; EDV ≈ 130\,\text{mL}
  2. Isovolumetric Contraction
    • All valves closed; S1 ("lub"); pressure ↑, volume constant
  3. Ventricular Ejection
    • Semilunar valves open; SV ≈ 70\,\text{mL}; left ventricle peaks \approx120\,\text{mmHg}
    • Ejection fraction = \dfrac{SV}{EDV} (normal 50!\text{–}!70\%)
    • ESV = EDV - SV \approx60\,\text{mL}
  4. Isovolumetric Relaxation
    • All valves closed; S2 ("dub"); pressure ↓, volume constant

Pressure–Flow Principles & Heart Sounds

  • Blood flows high → low pressure; valve motion is passive
  • S1: AV valve closure (start systole)
  • S2: semilunar valve closure (start diastole)

Cardiac Output (CO)

  • Formula: CO = HR \times SV
  • Resting example: HR = 75\,\text{bpm},\; SV = 70\,\text{mL} \Rightarrow CO \approx5.25\,\text{L\,min}^{-1}
  • Cardiac reserve = \text{CO}{\max} - \text{CO}{\text{rest}}

Regulation of Heart Rate (Chronotropic)

  • Autonomic centers (medulla):
    • Cardioacceleratory → sympathetic → ↑ SA firing, ↑ contractility
    • Cardioinhibitory → vagus (parasymp.) → hyperpolarize SA → ↓ HR
  • Positive chronotropes: epinephrine, dopamine, thyroid hormones
  • Negative chronotropes: acetylcholine, beta-blockers, Ca^{2+} channel blockers

Determinants of Stroke Volume (Inotropic)

  • Preload: venous return/EDV; governed by Frank–Starling law (stretch ↑ → SV ↑)
  • Contractility: myocardial force for given preload; ↑ by positive inotropes (catecholamines, insulin, thyroid hormones)
  • Afterload: arterial resistance; ↑ afterload (e.g., hypertension, aortic stenosis) → SV ↓

Key Equations & Values

  • SV = EDV - ESV
  • Typical volumes: EDV \approx130\,\text{mL},\; SV \approx70\,\text{mL},\; ESV \approx60\,\text{mL}
  • Equal ventricular output mandatory; mismatch → pulmonary or systemic congestion

Valve Status Summary

  • Ventricular Filling: AV open, semilunar closed
  • Isovolumetric Contraction & Relaxation: all valves closed
  • Ventricular Ejection: AV closed, semilunar open