EKG

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EKG/ECG - Beginner's Structured Study Guide

1. What Is an EKG?

An electrocardiogram (EKG or ECG) is a recording of the electrical activity of the heart, captured by electrodes placed on the skin. When a cardiac impulse travels through the heart, electrical currents spread into the surrounding tissues and reach the body surface. By recording the voltage differences between electrodes on opposite sides of the heart, you get the characteristic waveform tracing. - Guyton & Hall Textbook of Medical Physiology

2. The Cardiac Conduction System

Before reading an EKG, understand what generates the signal:

The normal heartbeat originates at the sinoatrial (SA) node, the heart's natural pacemaker. Depolarization then spreads in the following sequence:

SA node - fires spontaneously (60-100 bpm at rest)
Right and left atria - depolarize and contract
AV node - briefly delays conduction (allows ventricles to fill)
Bundle of His - passes through the fibrous skeleton
Right and left bundle branches - rapidly distribute the impulse
Left anterior and posterior fascicles - subdivisions of the left bundle
Purkinje fibers - deliver impulse to ventricular myocardium
Ventricular myocardium - depolarizes endocardium to epicardium

Harrison's Principles of Internal Medicine, 22E

3. The Normal EKG Waveform

Normal ECG waveform with labeled P, QRS, ST, T, U, and intervals

Wave/Segment	What It Represents
P wave	Atrial depolarization
PR interval	Time from atrial to ventricular depolarization (AV conduction)
QRS complex	Ventricular depolarization
Q wave	First negative deflection before the R wave
R wave	First positive deflection
S wave	Negative deflection after R wave
ST segment	Plateau phase of ventricular action potential (isoelectric)
J point	Junction between QRS end and ST segment start
T wave	Ventricular repolarization
U wave	Late ventricular repolarization (especially visible in hypokalemia)

Guyton & Hall; Harrison's, 22E

4. Normal Intervals

The EKG is recorded on paper at 25 mm/s. Each small box = 1 mm = 40 ms (0.04 s). Each large box = 5 mm = 200 ms (0.20 s).

Interval	Normal Range	Clinical Significance
PR interval	120-200 ms (3-5 small boxes)	AV conduction time; prolonged = heart block
QRS duration	≤110 ms (≤2.75 small boxes)	Ventricular depolarization; wide = bundle branch block
QT interval	<450 ms men, <460 ms women	Ventricular repolarization; prolonged = risk of torsades
RR interval	Determines heart rate	300 ÷ (# large boxes) = heart rate in bpm

Quick heart rate calculation: Count the number of large boxes between two R waves, then divide 300 by that number. For example, 4 large boxes between R waves = 300/4 = 75 bpm.

Harrison's Principles of Internal Medicine, 22E

5. The 12 Leads - What Each One "Sees"

A standard EKG uses 12 leads (vantage points), split into two groups:

Limb leads (frontal plane):

Lead I - Left side of the heart (lateral)
Lead II - Inferior (best for rhythm monitoring)
Lead III - Inferior
aVR - From the right arm; P wave normally negative here
aVL - High lateral
aVF - Inferior

Precordial (chest) leads - horizontal plane:

V1-V2 - Right ventricle / septum
V3-V4 - Anterior wall
V5-V6 - Lateral wall (left ventricle)

Think of the 12 leads as 12 different cameras all filming the same event from different angles. Abnormalities visible in certain lead groups tell you which region of the heart is affected.

6. The Systematic Approach - How to Read Any EKG

Always follow this sequence so you never miss something:

Step 1 - Rate

Normal: 60-100 bpm
Bradycardia: <60 bpm
Tachycardia: >100 bpm

Step 2 - Rhythm

Is it regular? (Are RR intervals equal?)
Is there a P wave before every QRS?
Is there a QRS after every P wave?
Is the QRS narrow or wide?

Step 3 - Axis (QRS axis)

Normal axis: -30° to +90°
Look at leads I and aVF:
- Both positive = normal axis
- I positive, aVF negative = left axis deviation (LAD)
- I negative, aVF positive = right axis deviation (RAD)
- Both negative = extreme/northwest axis

Step 4 - P wave morphology

Normal: upright in I and II, negative in aVR
Broad/notched (lead II) = left atrial enlargement
Tall/peaked >2.5 mm (lead II) = right atrial enlargement

Step 5 - PR interval

Short <120 ms = pre-excitation (e.g., WPW)
Prolonged 200-300 ms = 1st degree heart block
Progressively lengthening = 2nd degree (Mobitz I/Wenckebach)
No relationship P to QRS = 3rd degree (complete heart block)

Step 6 - QRS complex

Width, morphology, amplitude
Wide QRS (>120 ms) = bundle branch block or ventricular rhythm

Step 7 - ST segment

ST elevation = acute injury/STEMI (key territory by lead group)
ST depression = ischemia or posterior MI
Compare to isoelectric PR baseline

Step 8 - T waves

Normal: upright in most leads, inverted in aVR
Tall peaked T waves = hyperkalemia or early ischemia
T wave inversions = ischemia, strain, bundle branch block

Step 9 - QT interval

Correct for rate using QTc formula
Prolonged QTc = risk of torsades de pointes

7. Key Patterns to Recognize

Bundle Branch Blocks

Right BBB (RBBB): Wide QRS (≥120 ms) + rSR' ("rabbit ears") in V1 + wide S wave in V6. The terminal QRS vector points right and anterior. - Harrison's, 22E
Left BBB (LBBB): Wide QRS + broad notched R (or QS) in V1 + tall broad R in V6. Septal activation reverses (right to left). Often a marker of coronary disease, hypertensive heart disease, cardiomyopathy, or aortic valve disease. - Harrison's, 22E

Acute MI (STEMI territory by leads)

Territory	Leads with Changes	Artery
Inferior	II, III, aVF	RCA
Lateral	I, aVL, V5-V6	LCx
Anterior	V1-V4	LAD
Posterior	ST depression V1-V2 (reciprocal)	LCx/RCA

Common Arrhythmias

Arrhythmia	Key Features
Sinus tachycardia	Rate >100, normal P-QRS
Sinus bradycardia	Rate <60, normal P-QRS
Atrial fibrillation	No P waves, irregularly irregular, narrow QRS
Atrial flutter	Saw-tooth P waves ~300 bpm, typically 2:1 or 4:1 block
SVT	Narrow QRS tachycardia, P often hidden in T
Ventricular tachycardia	Wide QRS tachycardia, rate >100, AV dissociation
1° AV block	PR >200 ms, every P conducts
2° AV block (Mobitz I)	Progressive PR lengthening, then dropped QRS
2° AV block (Mobitz II)	Constant PR, sudden dropped QRS
3° AV block	Complete dissociation P from QRS

8. Electrolyte Effects on EKG

Electrolyte	Change	EKG Finding
Hyperkalemia	High K+	Peaked T waves → wide QRS → loss of P wave → sine wave
Hypokalemia	Low K+	Flat/inverted T waves, prominent U waves, long QT
Hypercalcemia	High Ca²+	Short QT interval
Hypocalcemia	Low Ca²+	Long QT interval

9. Memory Aids

Rate trick: 300 - 150 - 100 - 75 - 60 - 50 (count large boxes between R waves: 1, 2, 3, 4, 5, 6)
LBBB mnemonic: WiLLiaM MaRRoW (W pattern in V1 = LBBB; M pattern in V1 = RBBB)
Normal P wave: "P is positive in I and II, negative in aVR" - if not, not sinus rhythm
ST elevation rule: ≥1 mm in limb leads, ≥2 mm in precordial leads = significant

Sources

Guyton and Hall Textbook of Medical Physiology - Fundamentals of Electrocardiography
Harrison's Principles of Internal Medicine, 22E (2025) - Chapter 247: Basic ECG Waveforms and Intervals
The Harriet Lane Handbook, 23rd ed. - Basic Electrocardiography Principles

Generate a comparison table of normal vs abnormal EKG findings

Based on the detailed content already retrieved in this session from Harrison's 22E and Guyton & Hall, here is the comparison table:

Normal vs. Abnormal EKG Findings

Rate & Rhythm

Parameter	Normal	Abnormal	What It Suggests
Heart rate	60-100 bpm	<60 bpm	Sinus bradycardia, heart block, hypothyroidism
Heart rate	60-100 bpm	>100 bpm	Sinus tachycardia, SVT, AF, VT
Rhythm regularity	Regular (equal RR intervals)	Irregular	Atrial fibrillation, PACs, PVCs, Wenckebach
P before every QRS	Yes, 1:1	Missing P waves	AF, junctional rhythm, ventricular rhythm
QRS after every P	Yes, 1:1	Dropped QRS	2nd or 3rd degree heart block

P Wave

Parameter	Normal	Abnormal	What It Suggests
Morphology in lead II	Upright, smooth, rounded	Absent / saw-tooth / chaotic	AF (absent), flutter (saw-tooth), AF (chaotic)
Morphology in aVR	Negative (inverted)	Positive (upright)	Ectopic atrial rhythm, retrograde conduction
Duration	<120 ms	Broad/notched ("P mitrale")	Left atrial enlargement
Amplitude in lead II	≤2.5 mm	Tall peaked >2.5 mm ("P pulmonale")	Right atrial enlargement
Axis	Positive in I and II	Negative in II	Non-sinus (ectopic or retrograde) pacemaker

PR Interval

Parameter	Normal	Abnormal	What It Suggests
Duration	120-200 ms (3-5 small boxes)	<120 ms	Pre-excitation (WPW), accelerated AV conduction
Duration	120-200 ms	>200 ms (fixed)	1st degree AV block
Duration	120-200 ms	Progressively lengthening → dropped beat	2nd degree AV block, Mobitz I (Wenckebach)
Duration	120-200 ms	Fixed, then sudden dropped QRS	2nd degree AV block, Mobitz II
P-QRS relationship	1:1, consistent PR	No consistent relationship	3rd degree (complete) AV block

QRS Complex

Parameter	Normal	Abnormal	What It Suggests
Duration	≤110 ms (<2.75 small boxes)	110-120 ms	Incomplete bundle branch block
Duration	≤110 ms	≥120 ms (3+ small boxes)	Complete BBB, ventricular rhythm, hyperkalemia
Morphology V1	Small r, deep S (rS)	rSR' ("rabbit ears" / M pattern)	Right bundle branch block (RBBB)
Morphology V1	Small r, deep S	Broad QS or W pattern	Left bundle branch block (LBBB)
Morphology V6	qR (small q, tall R)	Wide S wave with rSR' in V1	RBBB
Morphology V6	qR	Tall broad R, no septal q	LBBB
Q waves	Absent or septal q <40 ms, <25% R height	Pathologic Q: ≥40 ms wide OR ≥25% of R amplitude	Prior MI (necrosis)
R wave progression V1→V6	Gradually increasing R amplitude	Poor progression or R wave loss	Anterior MI, LBBB, RV hypertrophy
Amplitude	Normal voltage	SV1 + RV5/V6 >35 mm	Left ventricular hypertrophy (LVH)
Amplitude	Normal voltage	R in V1 tall, deep S in V5/V6	Right ventricular hypertrophy (RVH)
Axis	-30° to +90°	Left axis deviation (<-30°)	LBBB, LVH, left anterior fascicular block, inferior MI
Axis	-30° to +90°	Right axis deviation (>+90°)	RBBB, RVH, left posterior fascicular block, lateral MI

ST Segment

Parameter	Normal	Abnormal	What It Suggests
Position	Isoelectric (flat, at baseline)	Elevation ≥1 mm limb leads / ≥2 mm precordial	STEMI, Prinzmetal angina, pericarditis, early repolarization
Position	Isoelectric	Depression ≥0.5-1 mm	NSTEMI/UA, posterior MI (V1-V2), digoxin effect
Shape	Flat/gently upsloping	Saddle-shaped elevation, diffuse	Acute pericarditis
Shape	Flat/gently upsloping	Concave upward elevation in V1-V3	Brugada pattern
ST-T discordance	Concordant with QRS polarity	Discordant T wave opposite to QRS	Expected in BBB; primary change = ischemia

T Wave

Parameter	Normal	Abnormal	What It Suggests
Polarity	Upright in I, II, V3-V6; inverted in aVR	Inverted in I, aVL, V4-V6	Lateral ischemia, LVH strain, LBBB
Polarity	Upright	Inverted in V1-V4	Anterior ischemia, RVH, RBBB, PE
Amplitude	Proportionate, asymmetric	Tall, peaked, symmetric ("hyperacute")	Early STEMI, hyperkalemia
Morphology	Asymmetric (slow upstroke, fast downstroke)	Deeply inverted, symmetric (Wellens' T waves)	Critical LAD stenosis (Wellens' syndrome)

QT Interval

Parameter	Normal	Abnormal	What It Suggests
QTc (Bazett)	≤450 ms men / ≤460 ms women	Prolonged QTc	Drugs (amiodarone, antipsychotics), hypocalcemia, hypokalemia, long QT syndrome - risk of torsades de pointes
QTc	≤450 ms men	Short QTc (<350 ms)	Hypercalcemia, digoxin effect, short QT syndrome

U Wave

Parameter	Normal	Abnormal	What It Suggests
Presence	Small, may be present (especially V2-V3)	Prominent U waves (>T amplitude)	Hypokalemia, bradycardia
Polarity	Positive when present	Negative U wave	Myocardial ischemia, LVH

Quick Reference Summary

Component	Key Normal Values
Rate	60-100 bpm
PR interval	120-200 ms
QRS duration	≤110 ms
QTc	≤450 ms (M) / ≤460 ms (F)
P wave	Upright I & II, inverted aVR
ST segment	Isoelectric
T wave	Upright I, II, V3-V6
Axis	-30° to +90°

Sources: Harrison's Principles of Internal Medicine 22E (2025); Guyton and Hall Textbook of Medical Physiology

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EKG Lead Placement

A standard 12-lead EKG uses 10 electrodes (4 limb + 6 chest) that generate 12 different views of the heart's electrical activity.

Patient Setup

Patient lies supine on the exam table, arms at sides, legs flat and not touching each other
Expose the chest and distal extremities; keep the rest covered (prevents shivering = artifact)
Clean electrode sites with an alcohol swab; shave if needed; gentle skin abrasion improves contact
Paper speed: 25 mm/s | Amplitude: 1 mV = 10 mm

Pfenninger and Fowler's Procedures for Primary Care

Part 1 - Limb Electrodes (4 electrodes)

These go on the wrists and ankles (or upper arms/thighs to reduce motion artifact). They generate the 6 frontal plane leads.

Electrode	Label	Color (US/AHA)	Placement
Right Arm	RA	White	Right wrist or upper right arm
Left Arm	LA	Black	Left wrist or upper left arm
Right Leg	RL	Green	Right ankle or lower right leg (ground electrode)
Left Leg	LL	Red	Left ankle or lower left leg

Memory trick: "White on right, smoke (black) over fire (red)" - White = RA, Black = LA, Red = LL, Green = RL

Note: The right leg (green) is a ground electrode only - its exact position does not affect any displayed lead.

The 4 limb electrodes generate 6 frontal leads:

Bipolar: I, II, III
Augmented unipolar: aVR, aVL, aVF

Part 2 - Precordial (Chest) Electrodes (6 electrodes)

These generate the 6 horizontal plane leads (V1-V6). Placement is entirely based on bony landmarks - do not use nipples as landmarks in women.

Precordial lead placement V1-V6 on chest diagram

How to find the landmarks:

First, locate the sternal angle (Angle of Louis) - the ridge where the manubrium meets the body of the sternum. The 2nd rib attaches here. Count down from there to find each intercostal space.

Lead	Color (US)	Anatomical Position
V1	Red	4th intercostal space (ICS), right sternal border
V2	Yellow	4th intercostal space (ICS), left sternal border
V3	Green	Halfway between V2 and V4 (diagonal, not necessarily on a rib)
V4	Blue	5th ICS, mid-clavicular line
V5	Orange/Tan	Anterior axillary line, same horizontal level as V4
V6	Purple/Violet	Mid-axillary line, same horizontal level as V4 and V5

Key rule: V4, V5, and V6 must all be on the same horizontal plane (same transverse level), even if they fall in slightly different intercostal spaces.

Step-by-Step Landmark Technique

1. Find the sternal angle (Angle of Louis) - the bony ridge on the sternum
2. The 2nd rib attaches here → count down to the 4th ICS
3. Place V1 at 4th ICS, RIGHT of sternum
4. Place V2 at 4th ICS, LEFT of sternum
5. Drop down one space → 5th ICS, mid-clavicular line = V4
6. Place V3 midway between V2 and V4
7. Draw an imaginary horizontal line through V4
8. V5 = anterior axillary line on that line
9. V6 = mid-axillary line on that line

What Each Lead "Sees"

Lead(s)	Region of Heart	Coronary Territory
I, aVL, V5, V6	Lateral wall (LV)	Left circumflex (LCx)
II, III, aVF	Inferior wall	Right coronary artery (RCA)
V1, V2	Septum / RV	LAD (septal branches)
V3, V4	Anterior wall	LAD
aVR	Cavity / right atrium	Left main (when diffusely elevated)

Common Placement Errors and How to Spot Them

Error	How to Recognize
RA/LA reversal (most common)	Lead I is inverted; P wave negative in I but normal in aVF; computer flags new right axis deviation
V1/V3 reversal	R wave does not progressively increase V1→V3; T wave inverted in V3
V leads placed too high	Tall R waves disappear; poor R-wave progression
Precordial leads not on same horizontal plane	Sudden jump in R-wave amplitude between V4-V6
Electrodes on wrong limb	Bizarre axis change vs. prior tracing

Pfenninger and Fowler's Procedures for Primary Care

Monitoring vs. Standard 12-Lead Placement

In the ICU or OR, limb electrodes are often placed on the torso rather than the extremities (Mason-Likar modification):

Right and left arm leads: just below the clavicles
Leg leads: above the hips

This reduces motion artifact but slightly alters QRS amplitude and axis compared to a standard 12-lead. ST-segment measurements remain generally reliable. - Miller's Anesthesia, 10e

Best Monitoring Lead for Specific Purposes

Goal	Best Lead
Rhythm monitoring	Lead II (clearest P waves)
Ischemia detection	Lead V5 (most sensitive - lateral wall)
If 2 channels available	II + V5
If 3 channels available	aVF + V2 + V5
RV ischemia	Right-sided leads V3R-V6R
Posterior MI	Posterior leads V7-V9

Pfenninger and Fowler's Procedures for Primary Care; Miller's Anesthesia, 10e

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