BP104T • Unit 3Topic 2 of 3⏱ 5 Min Read

2. Cardiac Anatomy, Conduction System, Cardiac Cycle & ECG Waveforms

💡 Core Concept & Clinical Context:

The heart is a four-chambered muscular pump enclosed within the pericardial sac. Myocardial auto-rhythmic pacemaker cells spontaneously generate periodic action potentials without neural input. The cardiac conduction hierarchy originates at the Sinoatrial (SA) Node (the primary pacemaker, firing at 60-100 bpm), spreads across atria via internodal tracts to the Atrioventricular (AV) Node (where a physiological 0.1-second delay occurs allowing ventricular filling), travels down the Bundle of His, branches into Left and Right Bundle Branches, and terminates in Purkinje Fibers, triggering synchronous ventricular systole.

Clinical Scenarios & Pharmacopoeial Examples:

1Correlation of Cardiac Cycle Phases with Standard 12-Lead ECG

Electrophysiological events and their surface ECG representation.

Step-by-Step Breakdown:

•1. P Wave: Atrial Depolarization (Spread of action potential from SA node across atria; leads to atrial contraction).

•2. PR Interval (0.12 - 0.20 sec): Time taken for impulse to travel from SA node through AV node to ventricles (AV nodal conduction delay).

•3. QRS Complex (0.08 - 0.10 sec): Rapid Ventricular Depolarization (Atrial repolarization is masked inside QRS).

•4. ST Segment: Plateau phase of ventricular action potential (Isoelectric; elevation indicates acute transmural myocardial infarction).

•5. T Wave: Ventricular Repolarization (Ventricular relaxation / diastole).

•6. QT Interval: Total duration of ventricular electrical activation and recovery.

•Heart Sounds:

•• S1 ('LUB'): Closure of Atrioventricular valves (Mitral & Tricuspid) at start of ventricular systole.

•• S2 ('DUB'): Closure of Semilunar valves (Aortic & Pulmonary) at start of ventricular diastole.

📋Key Takeaways & Exam Highlights

Cardiac Output ($CO$): $CO = \text{Heart Rate (HR)} \times \text{Stroke Volume (SV)}$ (Normal resting $CO \approx 70 \text{ bpm} \times 70 \text{ mL} = 4.9 \text{ to } 5.0 \text{ L/min}$).
Frank-Starling Law of the Heart: The force of cardiac contraction is directly proportional to the initial resting length of the myocardial fibers (End-Diastolic Volume / Preload).
Ejection Fraction (EF): $EF = \frac{SV}{EDV} \times 100$ (Normal healthy EF is 55% to 70%).

🇮🇳 हिन्दी व्याख्या (2. Cardiac Anatomy, Conduction System, Cardiac Cycle & ECG Waveforms)

SA नोड हृदय का प्राकृतिक पेसमेकर है। ECG में P तरंग आलिंद (Atria) के सिकुड़ने को, QRS कॉम्प्लेक्स निलय (Ventricles) के सिकुड़ने को, और T तरंग विश्राम (Repolarization) को दर्शाती है। S1 आवाज (LUB) और S2 आवाज (DUB) वॉल्व बंद होने से आती है।