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Noramal ecg

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normal ECG electrocardiogram waves intervals

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Here is a comprehensive overview of the Normal ECG (Electrocardiogram):

Normal ECG — Overview

The ECG measures tiny potential differences on the body surface that reflect the electrical activity of the heart. Electrodes detect the sequence and timing of depolarization and repolarization across the myocardium.

ECG Diagram — Waves, Segments & Intervals

Normal ECG waveform diagram showing P, Q, R, S, T waves with PR, ST, and QT intervals labeled

Normal ECG on grid paper with all intervals, J point, U wave, and calibration markings labeled

Waves and What They Represent

Wave / Feature	Electrical Event	Normal Value
P wave	Atrial depolarization (SA node → atria)	Duration < 120 ms; upright in leads I, II, aVF
PR interval	Atrial depol. → start of ventricular depol. (includes AV node delay)	120–200 ms (normal ~160 ms)
QRS complex	Ventricular depolarization	< 120 ms (≈ 80–100 ms)
ST segment	Between end of ventricular depol. and start of repolarization	Isoelectric (flat); no elevation or depression
T wave	Ventricular repolarization	Upright in most leads; concordant with QRS
QT interval	Ventricular depol. + repolarization (QRS onset → T wave end)	QTc: 350–440 ms (men), up to 460 ms (women)
J point	Junction of QRS end and ST segment start	At baseline (isoelectric)
U wave	Possibly Purkinje fiber repolarization (not always visible)	Small, positive deflection after T wave

Key distinction: Intervals include waves; segments do not.

Normal Sinus Rhythm Criteria

Rate: 60–100 beats/min (measured by R-R interval: HR = 1/cycle length)
Rhythm: Regular — consistent R-R intervals
P wave before every QRS — upright in leads I, II; negative in aVR
1:1 P:QRS relationship
PR interval: 120–200 ms
QRS duration: < 120 ms (narrow)
ST segment: Isoelectric — no elevation or depression
T waves: Upright and concordant with QRS in lateral leads

ECG Paper & Calibration

Measurement	Value
Paper speed	25 mm/sec
Small box (1 mm)	0.04 sec (40 ms) horizontally; 0.1 mV vertically
Large box (5 mm)	0.2 sec (200 ms) horizontally; 0.5 mV vertically
Standard gain	10 mm = 1 mV

12-Lead ECG — A Normal Example

Normal sinus rhythm 12-lead ECG at 90–100 bpm with narrow QRS complexes and no ST changes

This 12-lead tracing shows:

Regular P waves preceding every QRS in all leads
Narrow QRS complexes with normal morphology
No ST elevation or depression
Upright T waves in V2–V6
No pathological Q waves

Why the QRS Isn't Wider Than the P Wave

Though the ventricles are much larger than the atria, the His-Purkinje system conducts at very high velocity, allowing the entire ventricular myocardium to depolarize almost as quickly as the atria. This keeps QRS duration similar to P wave duration.

Sources: Costanzo Physiology 7th Ed., p. 149 | Goldman-Cecil Medicine, p. 421

Heart attack ecg

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myocardial infarction heart attack ECG ST elevation STEMI changes

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ECG evolution myocardial infarction sequence hyperacute T wave ST elevation Q wave changes over time

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Here is a comprehensive guide to the Heart Attack (Myocardial Infarction) ECG:

Heart Attack ECG — Overview

A myocardial infarction (MI) occurs when a coronary artery is occluded, cutting off blood supply to heart muscle. The ECG is the first-line diagnostic tool — in STEMI, it is diagnostic on its own. ECG changes evolve over time in a predictable sequence.

The 4 Classic ECG Changes of MI (in order of appearance)

Stage	ECG Finding	Timing	Meaning
1. Hyperacute	Tall, broad-based, symmetric T waves	Minutes	Very early ischemia (often missed)
2. Acute	ST-segment elevation (convex/tombstone)	Minutes–hours	Active transmural injury — call for reperfusion NOW
3. Evolving	Q wave formation + T wave inversion	Hours–days	Myocardial necrosis developing
4. Old/Chronic	Persistent pathological Q waves	Days–weeks onward	Permanent scar

Stage 1 — Hyperacute T Waves (Earliest Sign)

ECG showing hyperacute tall broad-based T waves in V2-V4, earliest sign of anterior MI

Disproportionately tall, broad, symmetric T waves in precordial leads (V2–V4 here)
Mild ST elevation beginning
Seen within minutes of coronary occlusion — often missed clinically

Stage 2 — Acute STEMI (Anterolateral)

12-lead ECG showing acute anterolateral STEMI with convex ST elevation in V1-V6, I, aVL and reciprocal depression in inferior leads

This is a classic anterolateral STEMI (LAD occlusion):

Convex ("tombstone") ST elevation in V1–V6, I, aVL
Reciprocal ST depression in inferior leads (II, III, aVF) — this confirms true elevation
Peaked hyperacute T waves in V2–V4

Stage 2 — Acute Inferior STEMI

12-lead ECG showing acute inferior STEMI with ST elevation in II, III, aVF and reciprocal ST depression in I, aVL

Classic inferior STEMI (RCA or circumflex occlusion):

ST elevation in II, III, aVF
Reciprocal ST depression + T wave inversion in I, aVL
Usually due to right coronary artery (RCA) occlusion

Stages 2→4 Evolution — Inferior MI Before & After

Comparison of inferior STEMI at admission (Panel A: ST elevation in II, III, aVF with reciprocal depression in I, aVL) vs 1 week later (Panel B: ST resolved, pathological Q waves in inferior leads)

Panel A (Acute): Red arrows = ST elevation in II, III, aVF; Blue arrows = reciprocal depression in I, aVL
Panel B (1 week later): ST normalized; Black arrows = pathological Q waves now permanent in inferior leads → confirms old infarction

STEMI Diagnostic Criteria (Fourth Universal Definition)

Lead Group	Cut-off for STEMI
Most leads (≥2 contiguous)	ST elevation ≥ 1 mm
V2–V3, females	≥ 1.5 mm
V2–V3, males ≥ 40 yrs	≥ 2 mm
V2–V3, males < 40 yrs	≥ 2.5 mm

MI Localization by Lead

MI Territory	Leads with Changes	Artery
Anterior	V1–V4	LAD
Anterolateral	V1–V6, I, aVL	Proximal LAD
Lateral	I, aVL, V5–V6	LCx or diagonal
Inferior	II, III, aVF	RCA (or LCx)
Posterior	Reciprocal depression V1–V3 (mirror image)	RCA or LCx
Septal	V1–V2	Septal LAD branches

Pathological Q Wave Criteria

A Q wave is pathological (indicating necrosis) when:

Width ≥ 0.04 sec (1 small box)
Depth ≥ 25% of the R wave amplitude
Present in ≥ 2 contiguous leads

Q waves usually appear 8–12 hours after infarction onset, but can appear within the first hour. They may persist permanently as markers of old MI.

NSTEMI vs STEMI

Feature	STEMI	NSTEMI
ECG	ST elevation	ST depression / T inversion / normal
Artery	Complete occlusion	Partial occlusion
Q waves	Usually develop	Usually absent
Treatment	Emergency PCI / fibrinolysis	Anticoagulation, urgent PCI
Biomarkers	Elevated (troponin, CK-MB)	Elevated

Sources: Rosen's Emergency Medicine | Harrison's Principles of Internal Medicine 22E | Goldman-Cecil Medicine

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