Cardiac Function & ECG Study Guide Notes

Component Roles in Cardiac Function

• SA Node:
  • Primary pacemaker, initiates the heartbeat.
• AV Node:
  • Delays the impulse, allowing the atria to contract first.
• Conduction System Overall:
  • Relies on both SA and AV nodes to coordinate contractions.

ECG Interpretation

• Atrial Fibrillation (AFib):
  • Irregularly irregular rhythm.
  • No identifiable P waves.
• Ventricular Fibrillation:
  • Chaotic, wave-like ECG pattern
  • No effective contraction (medical emergency).
• Asystole:
  • Flatline, no electrical activity or contractions.
• Junctional Beats:
  • Originate in or near the AV node.
  • May have absent or inverted P waves.
• Atrial Bigeminy:
  • Every other beat is a premature atrial contraction (PAC).
• T Wave:
  • Typically the last major wave on the ECG (ventricular repolarization).

Key Visual Identification on ECGs

• T Wave:
  • Appears after the QRS complex; represents ventricular repolarization.
• Asystole:
  • Flatline - requires immediate resuscitation.
• Ventricular Fibrillation:
  • No recognizable waveforms, coarse or fine undulating pattern.
• Atrial Fibrillation:
  • Rapid, irregular baseline with variable R-R intervals.

Quick Review Questions

• What is preload?
  • Volume of blood entering ventricles before contraction.
• Which valves close to produce S2?
  • Aortic and pulmonic valves.
• Which ECG pattern is most dangerous and requires immediate defibrillation?
  • Ventricular fibrillation.
• What structure keeps the AV valves from inverting?
  • Chordae tendineae.

Cardiac Cycle & Hemodynamics

• Preload:
  • Definition: Blood volume filling the ventricles before contraction (end-diastolic volume).
• Afterload:
  • Definition: Resistance the ventricles must overcome to eject blood (e.g., aortic pressure).
• Stroke Volume:
  • Definition: Amount of blood ejected by a ventricle during a single contraction.
• Systolic Pressure:
  • Definition: Peak pressure in the arteries during ventricular systole.

Heart Sounds

• S1 ("lub")
  • Cause: Closure of mitral & tricuspid valves (start of systole).
• S2 ("dub")
  • Cause: Closure of aortic & pulmonic valves (end of systole).

Heart Anatomy

• Chordae Tendineae:
  • Function/Description: "Heartstrings" that prevent valve prolapse into atria during ventricular contraction.
• Papillary Muscles:
  • Function/Description: Contract to hold the chordae tendineae taut.
• Pericardium:
  • Function/Description: Double-layered sac enclosing the heart.
• Interventricular Septum:
  • Function/Description: Muscular wall dividing the right and left ventricles.
• Venae Cavae:
  • Function/Description: Superior & inferior venae cavae return deoxygenated blood to the right atrium.
• Aortic Stenosis:
  • Function/Description: Narrowing of the aortic valve that impedes blood flow from the left ventricle.
• Tachycardia
  • Resting adult heart rate >100 bpm.