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ECG class for MBBS pg student with ecg in it please provide pr intervals and its issue and all about qrs

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I now have comprehensive content from multiple authoritative textbooks. Let me compile the complete ECG class for MBBS/PG students.

ECG Class for MBBS/PG Students

Complete Guide to PR Interval, QRS Complex & AV Conduction

1. THE CARDIAC CONDUCTION SYSTEM

Before reading an ECG, understand the electrical highway of the heart:
Cardiac Conduction System - Goldman-Cecil Medicine
Sequence of activation:
  1. SA node (high lateral right atrium, near SVC) - dominant pacemaker
  2. Bachmann bundle - speeds depolarization to left atrium
  3. AV node - physiologic delay (only electrical bridge between atria and ventricles)
  4. Bundle of His
  5. Left bundle branch (anterior + posterior fascicles) and Right bundle branch
  6. Purkinje system - rapid conduction into ventricular muscle
The AV valve rings are electrically insulated - the AV node is the ONLY normal pathway between atria and ventricles. - Goldman-Cecil Medicine

2. NORMAL ECG INTERVALS - THE MASTER TABLE

ParameterNormal Value
Heart rate50-100 bpm
P wave duration< 0.12 sec (120 ms)
PR interval0.09-0.20 sec (90-200 ms)
QRS duration0.075-0.11 sec (75-110 ms)
QTc (male)390-450 ms
QTc (female)390-460 ms
QRS axis-30° to +90°
- Goldman-Cecil Medicine, Table 42-1
ECG paper speed: 25 mm/sec | 1 small box = 0.04 sec | 1 large box = 0.2 sec
Heart rate calculation: HR = 60,000 ÷ RR interval (in ms). Memorize: 300-150-100-75-60-50 for 1-2-3-4-5-6 large boxes between QRS.

3. THE ECG WAVES - WHAT EACH REPRESENTS

Wave/SegmentElectrical Event
P waveAtrial depolarization
PR segmentConduction through AV node + His-Purkinje (isoelectric)
PR intervalOnset of atrial depolarization → onset of ventricular depolarization
QRS complexVentricular depolarization
ST segmentPlateau of ventricular action potential (isoelectric)
T waveVentricular repolarization
U waveRepolarization of Purkinje system (seen in hypokalemia)
"The PR interval...includes the P wave and the PR segment - it is the time from initial depolarization of the atria to initial depolarization of the ventricles." - Costanzo Physiology 7e

4. THE PR INTERVAL IN DETAIL

Normal PR: 0.09-0.20 sec (90-200 ms)

What the PR interval measures:
  • Time for impulse to travel from SA node → through atrial muscle → AV node → Bundle of His → Purkinje system → onset of ventricular depolarization
  • Three contributors: atrial conduction + AV node delay + His-Purkinje conduction
  • When PR is prolonged, the delay is usually within the AV node

5. PR INTERVAL ABNORMALITIES

A. PROLONGED PR INTERVAL (> 200 ms) = HEART BLOCK

🔴 FIRST-DEGREE AV BLOCK

  • Definition: PR interval > 200 ms, every P wave is followed by a QRS
  • Mechanism: Slow conduction through AV node, delay usually in the AV node itself
  • No dropped beats - all P waves conduct
ECG (First-degree AV block):
First-degree AV block ECG - Frameworks for Internal Medicine
Note: Wide PR interval before each tall QRS - every P conducts
Clinical:
  • Usually asymptomatic, excellent prognosis
  • Common causes: vagal tone, beta-blockers, digoxin, inferior MI, rheumatic fever, Lyme disease
  • Pacemaker only if PR > 300 ms with symptoms (rare)
"First-degree AV block is defined by a PR interval >200 ms on ECG." - Frameworks for Internal Medicine

🟠 SECOND-DEGREE AV BLOCK - TWO TYPES

Definition: Some P waves conduct (followed by QRS), some don't (dropped beats)

Type 1 - Mobitz I (Wenckebach)

  • ECG pattern: PR interval progressively lengthens beat-by-beat until one P wave is completely blocked (no QRS)
  • After the dropped beat, PR resets to its shortest value
  • The RR interval shortens as PR lengthens (Wenckebach periodicity)
  • Location of block: Within the AV node itself
  • Vascular supply: Right coronary artery (inferior MI territory)
  • Memory trick: "Longer, longer, longer, drop. Then you have a Wenckebach"
ECG (Mobitz I - Wenckebach):
Mobitz I Wenckebach ECG - Frameworks for Internal Medicine
PR progressively lengthens over 4 beats, then the 5th P wave is blocked (no QRS)
Clinical: Usually benign, may be asymptomatic. Rarely needs pacing unless symptomatic.

Type 2 - Mobitz II

  • ECG pattern: PR interval is constant, then a P wave suddenly fails to conduct (no QRS) - without any prior lengthening
  • QRS is often wide (bundle branch block pattern)
  • Location of block: Below AV node - His-Purkinje system or bundle branches
  • Vascular supply: Left anterior descending artery (anterior MI - look for ST elevation V1-V4)
  • DANGER: High risk of progression to complete (3rd degree) heart block
ECG (Mobitz II):
Mobitz II ECG - Frameworks for Internal Medicine
PR intervals are equal in conducted beats, then suddenly a P wave is blocked (after 4th P wave)
Clinical: Patients are frequently symptomatic (syncope, dyspnea). Permanent pacemaker almost always required. - Frameworks for Internal Medicine

🔴🔴 THIRD-DEGREE (COMPLETE) AV BLOCK

  • Definition: Total AV dissociation - no impulse passes from atria to ventricles
  • ECG pattern:
    • Regular P-P intervals (atrial rhythm, ~70-80 bpm)
    • Regular R-R intervals (ventricular escape rhythm, slower)
    • P waves and QRS complexes are completely independent - P waves march through QRS/T waves
    • Atrial rate > Ventricular rate
ECG (Third-degree AV block):
Third-degree (Complete) AV Block ECG - Frameworks for Internal Medicine
P waves march through at their own rate, QRS complexes (widened) fire at a much slower independent rate
QRS width tells you where the escape pacemaker is:
Escape locationQRSRate
Above Bundle of His (junctional)Narrow40-60 bpm
Below Bundle of His (ventricular)Wide20-40 bpm
Clinical: Usually very symptomatic - syncope (Stokes-Adams attacks), dyspnea. 1-year mortality with syncope can be up to 50% without treatment. Permanent pacemaker mandatory.

B. SHORT PR INTERVAL (< 120 ms)

Three causes:
  1. WPW (Wolff-Parkinson-White) syndrome - accessory pathway (Bundle of Kent) bypasses AV node, creates a delta wave (slurred upstroke of QRS) + wide QRS
  2. Junctional rhythm - impulse originates in AV junction, travels retrograde to atria
  3. Enhanced AV nodal conduction (Lown-Ganong-Levine syndrome)
"A short PR interval may reflect ventricular preexcitation (Wolff-Parkinson-White syndrome), a junctional rhythm, or enhanced AV nodal conduction." - Goldman-Cecil Medicine

Causes of PR Prolongation - Summary

CategoryExamples
DrugsDigoxin, beta-blockers, calcium channel blockers, amiodarone
IschemiaInferior MI (RCA), anterior MI (LAD)
InfectionsRheumatic fever, Lyme disease, myocarditis
Fibrosis/CalcificationAging, calcific aortic stenosis
ElectrolyteHyperkalemia
CongenitalASD, AVSD
OthersHypothyroidism, sarcoidosis

6. THE QRS COMPLEX - EVERYTHING YOU NEED

Definition

The QRS complex represents ventricular muscular depolarization - the most clinically information-rich portion of the ECG.

Nomenclature Rules (Critical for exams)

  • Capital letters (Q, R, S) = large deflections ≥ 5 mm (0.5 mV)
  • Lowercase letters (q, r, s) = small deflections < 5 mm (0.5 mV)
  • Q / q = initial negative deflection (before any R wave)
  • R / r = any positive deflection
  • S / s = negative deflection AFTER an R wave
  • QS = entirely negative complex (no R wave at all)
  • R' or r' = second positive deflection (after an S wave)
  • rSR' pattern = typical Right Bundle Branch Block (RBBB) pattern in V1

Normal QRS Duration

  • Normal: 0.075-0.11 sec (75-110 ms, ~2 small boxes)
  • Guyton notes normal as 0.06-0.08 sec
  • 0.09 sec = abnormally long (mild widening)
  • 0.12 sec = pathological block in ventricular conduction system
"If the QRS duration is prolonged, an intraventricular and/or interventricular conduction delay is present." - Goldman-Cecil Medicine

7. CAUSES OF WIDE QRS (> 120 ms)

Bundle Branch Blocks

When bundle branches are blocked, impulse conducts through slow ventricular muscle instead of fast Purkinje fibers - QRS widens to ≥ 0.14 sec.
FeatureRBBBLBBB
V1 patternrSR' ("M" shape, rabbit ears)QS or rS (broad, deep S)
V6 patternWide S waveBroad, tall R wave (no septal q)
T wave directionOpposite to terminal QRSOpposite to QRS
SignificanceOften benign; can occur with RV strain, PE, ASDMore often pathological - IHD, cardiomyopathy
Memory: In V1 - "WiLLiaM MaRRoW" (W in LBBB, M in RBBB)

Other Causes of Wide QRS

CauseMechanism
Ventricular tachycardia (VT)Depolarization starts in myocardium, not Purkinje
HyperkalemiaSlows conduction in all myocardial cells
Pre-excitation (WPW)Accessory pathway bypasses His-Purkinje
Ventricular hypertrophyLonger path for impulse, QRS 0.09-0.12 sec
Ventricular pacingArtificial pacing from ventricle = wide QRS (LBBB morphology)
Antiarrhythmic drugsSodium channel blockers (quinidine, procainamide, flecainide)
"When the Purkinje fibers are blocked, the cardiac impulse must be conducted by ventricular muscle instead...QRS duration increases to 0.14 sec or longer." - Guyton & Hall Medical Physiology

8. PATHOLOGICAL Q WAVES

  • A pathological Q wave = Q wave ≥ 1 small box wide (≥ 40 ms) AND ≥ 25% depth of the R wave in the same lead
  • Indicates transmural (full-thickness) myocardial infarction (old or established)
  • Represents electrically silent dead myocardium - the electrode "sees through" the infarct to the opposite wall's negative forces
Q wave locations and territory:
Q waves in leadsInfarcted territoryArtery
V1-V4AnteriorLAD
II, III, aVFInferiorRCA (80%)
I, aVL, V5-V6LateralLCx
V1-V2 tall RPosteriorRCA/LCx

9. QRS AXIS

Normal axis: -30° to +90°
AxisDegreesSignificance
Normal-30° to +90°Normal
Left axis deviation (LAD)-30° to -90°Left anterior fascicular block, inferior MI, LVH
Right axis deviation (RAD)+90° to +180°RVH, PE, RBBB, lateral MI
Extreme axis±180°VT, severe disease
Quick bedside rule:
  • Lead I positive + aVF positive = Normal axis
  • Lead I positive + aVF negative = Left axis deviation
  • Lead I negative + aVF positive = Right axis deviation

10. QRS IN VENTRICULAR HYPERTROPHY

Left Ventricular Hypertrophy (LVH) - Sokolow-Lyon criteria:
  • S in V1 + R in V5 or V6 > 35 mm
  • R in aVL > 11 mm
  • Plus "strain pattern": ST depression + T wave inversion in lateral leads (I, aVL, V5-V6)
Right Ventricular Hypertrophy (RVH):
  • Dominant R wave in V1 (R > S in V1)
  • Right axis deviation
  • ST depression + T inversion in V1-V3/V4

11. QUICK COMPARISON TABLE - AV BLOCKS

Feature1st Degree2nd Degree Mobitz I2nd Degree Mobitz II3rd Degree
PR interval>200 ms, constantProgressively lengthens, then dropsFixed, then sudden dropCannot measure (AV dissociation)
Dropped beatsNoneYes, after PR prolongationYes, suddenAll - none conducts
QRS widthNarrowUsually narrowOften wideNarrow (junctional) or Wide (ventricular)
Location of blockAV nodeAV nodeHis-PurkinjeComplete
RiskLowLowHIGHVERY HIGH
TreatmentNone usuallyNone usuallyPermanent pacemakerPermanent pacemaker

12. HIGH-YIELD EXAM POINTS

  • PR > 200 ms = 1st degree AV block (but it's a conduction delay, not true block)
  • PR progressively lengthens then drops = Mobitz I (Wenckebach)
  • Fixed PR then sudden drop + wide QRS = Mobitz II (dangerous, needs pacemaker)
  • P waves and QRS completely independent = Complete (3rd degree) heart block
  • Short PR + delta wave + wide QRS = WPW
  • QRS > 120 ms = Bundle branch block / VT / hyperkalemia / drugs
  • Pathological Q = transmural MI in that territory
  • RBBB: rSR' in V1 ("M" shape) | LBBB: broad notched R in V5/V6, no septal q
  • Paced rhythm always produces wide QRS with LBBB morphology
  • 1-year mortality of complete heart block with syncope = up to 50% without pacemaker
Sources:
  • Goldman-Cecil Medicine International Edition, 2-Volume Set (Chapter 42)
  • Costanzo Physiology, 7th Edition (Chapter 4)
  • Frameworks for Internal Medicine (Chapter 3)
  • Guyton and Hall Textbook of Medical Physiology (Chapter 12)
  • Harrison's Principles of Internal Medicine, 22nd Edition (2025)
  • Tintinalli's Emergency Medicine, Comprehensive Study

More ecgs and in pdf format with qrs diseases

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