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ECG waveform interpretation P QRS T wave

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I now have all the content I need for a comprehensive guide. Here is a complete, systematic approach to ECG interpretation.

How to Read an ECG: A Systematic Approach

A standard 12-lead ECG records 10 seconds of cardiac electrical activity at a paper speed of 25 mm/sec. Each small box = 0.04 sec (40 ms), each large box = 0.2 sec (200 ms). Voltage is 10 mm = 1 mV at standard calibration. Always use a stepwise approach — most errors are errors of omission.

The 12 Leads at a Glance

Normal ECG — HR ~78 bpm, axis +60°, PR 140 ms, QRS 90 ms, QTc 360 ms. Goldman-Cecil Medicine

Lead Group	Leads	Anatomic View
Inferior	II, III, aVF	Inferior wall (RCA territory)
Lateral	I, aVL, V5, V6	Lateral wall (LCx territory)
Anterior/Septal	V1–V4	Anterior wall (LAD territory)
Right-facing	aVR	Right side (always inverted in normal)

Step 1 — Rate

Formula: HR = 60,000 ÷ RR interval (in ms)

Quick method (regular rhythm): Count large boxes between two R waves → divide 300 by that number.

1 box = 300 bpm, 2 = 150, 3 = 100, 4 = 75, 5 = 60, 6 = 50

Normal resting rate: 50–100 bpm (physiologic range 50–90 bpm)

Step 2 — Rhythm

Is it regular or irregular? (Compare R–R intervals)
Are P waves present? Is every P followed by a QRS?
What is the P wave axis? (Upright in II, inverted in aVR = sinus origin)
Are there premature beats or pauses?

Normal sinus rhythm = regular rhythm, upright P in II, P before every QRS, rate 60–100 bpm.

Step 3 — Intervals

Normal ECG Intervals (Goldman-Cecil Medicine, Table 42-1)

Parameter	Normal Range
Heart rate	50–100 bpm
P wave duration	< 120 ms (< 3 small boxes)
PR interval	90–200 ms (2.25–5 small boxes)
QRS duration	75–110 ms (< 3 small boxes)
QTc (males)	390–450 ms
QTc (females)	390–460 ms
QRS axis	−30° to +90°

PR interval: onset of P → onset of QRS. Prolonged (>200 ms) = 1st degree AV block. Short with delta wave = WPW preexcitation.

QRS duration: Prolonged >110 ms = intraventricular conduction delay; ≥120 ms = bundle branch block.

QT/QTc: Measure Q onset → T wave end. Correct using Bazett's formula: QTc = QT ÷ √RR. Prolonged QTc raises risk of torsades de pointes.

Step 4 — Axis

Frontal plane axis zones. Goldman-Cecil Medicine

Axis	Degrees	Causes
Normal (NL)	−30° to +90°	—
Left axis deviation (LAD)	−30° to −90°	LBBB, LAFB, inferior MI
Right axis deviation (RAD)	+90° to +180°	RVH, RBBB, lateral MI, normal variant in young
Extreme RAD (ERAD)	−90° to ±180°	Severe pathology

Quick axis check: If QRS is positive in both Lead I and Lead II → normal axis. If positive I, negative II → LAD. If negative I, positive II → RAD.

Step 5 — P Waves

Normal: Upright in I, II, aVF, V4–V6; inverted in aVR; biphasic in V1
Duration < 120 ms; amplitude < 2.5 mm in II
Broad, notched P (P mitrale) → left atrial enlargement
Tall, peaked P (P pulmonale, >2.5 mm in II) → right atrial enlargement
Absent P waves → AF, junctional rhythm, hyperkalemia

Step 6 — QRS Morphology

Naming convention:

Capital letters (Q, R, S) = amplitude ≥ 5 mm
Lowercase (q, r, s) = amplitude < 5 mm
QS = entirely negative deflection

R-wave progression (precordial leads V1→V6):

V1: small r, deep S (rS pattern)
V3–V4: transition zone — R amplitude begins to exceed S
V6: large R, small or no S
Poor R-wave progression (transition delayed to V5/V6) suggests anterior MI or RVH

Pathological Q waves:

Width > 40 ms (1 small box) OR depth > 25% of the R wave in the same lead
Present in a regional distribution → prior transmural infarction
Septal q waves in V5–V6 and small q in inferior leads = normal variant

Step 7 — ST Segments

Measured from the J point (where QRS meets ST segment) to the onset of the T wave.

Finding	Significance
ST elevation ≥ 1 mm (limb leads) or ≥ 2 mm (precordial) in ≥ 2 contiguous leads	STEMI, pericarditis (diffuse saddle-shaped elevation), early repolarization
ST depression ≥ 1 mm	NSTEMI/UA, posterior MI (V1–V3), digoxin effect (scooped), strain
Diffuse saddle-shaped ST elevation + PR depression	Pericarditis

Step 8 — T Waves

Normal: Upright in I, II, V3–V6; inverted in aVR; variable in III and V1
Peaked, symmetric T waves → hyperkalemia, hyperacute STEMI
Deep, symmetric T wave inversions → ischemia, Wellens syndrome (V2–V3)
Asymmetric inversions → right/left ventricular strain, normal variant in lateral leads

Step 9 — U Waves

Small deflection after the T wave, best seen in V2–V3
Prominent U waves → hypokalemia, bradycardia, drug effects (quinidine, amiodarone)
Inverted U waves → may indicate ischemia or LV volume overload

The Harrison's 14-Point Checklist (complete systematic read)

Standardization and technical features (calibration, lead placement, artifacts)
Rhythm
Heart rate
PR interval / AV conduction
QRS interval
QT / QTc
Mean QRS electrical axis
P waves
QRS voltages
Precordial R-wave progression
Abnormal Q waves
ST segments
T waves
U waves

Always compare with prior ECGs — this is invaluable. — Harrison's Principles of Internal Medicine, 22E

Common Patterns to Recognize

Pattern	Key Features
STEMI	ST elevation in ≥2 contiguous leads, reciprocal ST depression
LBBB	QRS ≥120 ms, broad notched R in I/V6, QS in V1
RBBB	QRS ≥120 ms, rSR' (bunny ear) in V1, wide S in I/V6
1st° AV block	PR >200 ms, 1:1 P:QRS
2nd° AV block (Mobitz I)	Progressive PR lengthening → dropped QRS
2nd° AV block (Mobitz II)	Fixed PR, sudden dropped QRS
3rd° AV block	P waves & QRS completely dissociated
Atrial fibrillation	Irregularly irregular, absent P waves, fibrillatory baseline
Atrial flutter	Sawtooth flutter waves ~300 bpm, regular ventricular response
WPW	Short PR, delta wave, wide QRS
Hyperkalemia	Peaked T → widened QRS → sine wave
Hypokalemia	Flattened T, prominent U waves

Sources: Goldman-Cecil Medicine (International Ed.), Table 42-1/42-2; Harrison's Principles of Internal Medicine 22E, Chapter 247

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