kindly provide how to interpreted ecg in emergency department , please provide details about that.
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I now have comprehensive content from Tintinalli's Emergency Medicine, Harrison's Principles, and Kaplan & Sadock's. Let me compile the full detailed answer.
ECG Interpretation in the Emergency Department
A systematic, stepwise approach is the standard for every ECG read in the ED. Errors in ECG interpretation are most often errors of omission - things you failed to look for, not things you misidentified. Always correlate with the patient's age, sex, and clinical presentation, and compare with any prior ECGs.
Step 1: Technical Checklist Before You Read Anything
Before interpreting wave morphology, confirm:
- Calibration: Standard is 10 mm = 1 mV amplitude, paper speed 25 mm/sec
- Lead placement: Incorrect lead placement (e.g. limb lead reversal) is a common artifact that mimics pathology
- Artifact: Tremor, movement, or poor contact can mimic arrhythmias
Step 2: Heart Rate
Normal: 60-100 bpm
Two methods exist:
Rate-by-squares method (for regular rhythms):
- Count large boxes between two consecutive R waves
- Divide 300 by that number
- 1 large box = 300 bpm, 2 = 150, 3 = 100, 4 = 75, 5 = 60, 6 = 50
QRS-count method (for irregular rhythms such as atrial fibrillation):
- Count the total number of QRS complexes on a standard 10-second ECG strip
- Multiply by 6 (since 10 sec × 6 = 1 minute)
| Finding | Threshold | Common Causes |
|---|---|---|
| Bradycardia | < 60 bpm | Medications (beta-blockers, CCBs, lithium), hypothyroidism, AV block, increased vagal tone, sinus node disease |
| Tachycardia | > 100 bpm | Pain, fever, hypovolemia, PE, anticholinergic drugs, sympathomimetics |
- Kaplan & Sadock's Comprehensive Textbook of Psychiatry, p. 6824
- Tintinalli's Emergency Medicine, Chapter 49
Step 3: Rhythm
Ask three questions:
- Is there a P wave before every QRS?
- Is there a QRS after every P wave?
- Are the P-P and R-R intervals regular?
Normal sinus rhythm: P wave precedes every QRS; P is upright in leads I, II; regular rate 60-100 bpm.
Classify the rhythm by two features:
| QRS Width | Rhythm Type |
|---|---|
| Narrow (< 100 ms / 2.5 small boxes) | Supraventricular origin - follows the normal His-Purkinje system |
| Wide (≥ 100 ms) | Ventricular origin OR conduction delay (bundle branch block, aberrant conduction) |
Narrow complex tachycardias include sinus tachycardia, atrial fibrillation/flutter, SVT (AVNRT, AVRT), MAT.
Wide complex tachycardias include ventricular tachycardia (VT), SVT with aberrancy, pre-excitation syndromes (WPW).
Key rule in the ED: If a patient is hemodynamically unstable with a wide complex tachycardia, treat as VT until proven otherwise.
Step 4: Cardiac Axis
The axis represents the net vector of ventricular depolarization across the myocardium.
Normal axis: -30° to +90°
Quick 2-lead method - check Lead I (0°) and aVF (+90°):
| Lead I | aVF | Axis |
|---|---|---|
| Positive | Positive | Normal (-30° to +90°) |
| Positive | Negative | Left axis deviation (LAD) |
| Negative | Positive | Right axis deviation (RAD) |
| Negative | Negative | Extreme / "northwest" axis (rare) |
Clinical significance:
-
LAD: Left ventricular hypertrophy, left anterior fascicular block, inferior MI, LBBB
-
RAD: Right ventricular hypertrophy, left posterior fascicular block, lateral MI, PE (acute cor pulmonale), RBBB, dextrocardia
-
Kaplan & Sadock's, p. 6825
Step 5: Intervals
The ECG paper grid: each small box = 0.04 sec, each large box = 0.20 sec.
PR Interval
- Normal: 0.12-0.20 sec (3-5 small boxes)
- Short PR (< 0.12 sec): Pre-excitation (WPW), accelerated AV nodal conduction, junctional rhythm
- Long PR (> 0.20 sec): First-degree AV block
- Progressive lengthening then dropped QRS: Second-degree AV block (Mobitz I / Wenckebach)
- Constant long PR with dropped QRS: Mobitz II (more dangerous - prone to complete block)
- No relationship between P and QRS: Third-degree (complete) AV block - requires pacing
QRS Duration
- Normal: < 0.12 sec (< 3 small boxes)
- Wide QRS (≥ 0.12 sec) = bundle branch block or ventricular origin
Differentiating BBB:
| Feature | RBBB | LBBB |
|---|---|---|
| V1 morphology | RSR' ("rabbit ears" / M-pattern) | QS or rS (broad negative) |
| V6 morphology | Wide S wave | Broad monophasic R wave |
| Clinical concern | RBBB may be normal variant; new RBBB can indicate PE or RV strain | New LBBB - evaluate for ACS, cardiomyopathy |
QT / QTc Interval
- Measured from start of QRS to end of T wave
- Correct for heart rate using Bazett's formula: QTc = QT ÷ √(R-R interval in seconds)
- Normal QTc: < 440 ms in men, < 460 ms in women
- Prolonged QTc is a risk for Torsades de Pointes (TdP) - a polymorphic VT
- Common ED causes of QT prolongation: electrolyte disturbances (hypokalemia, hypomagnesemia, hypocalcemia), antipsychotics, antiarrhythmics (amiodarone, sotalol), antibiotics (fluoroquinolones, macrolides), tricyclic antidepressants, hypothyroidism
Note: A wide QRS (bundle branch block or ventricular pacing) artificially prolongs the measured QT. In these cases, the AHA/ACC/HRS recommends correcting for QRS duration or using the JT interval instead.
- Kaplan & Sadock's, p. 6825-6826
Step 6: P Waves
- Normal P wave: Upright in I, II; inverted in aVR; biphasic in V1 (initial positive, terminal negative deflection)
- Peaked, tall P in II (> 2.5 mm): Right atrial enlargement ("P pulmonale") - seen in COPD, PE, pulmonary hypertension
- Broad, notched P (> 0.12 sec in II) or prominent negative component in V1: Left atrial enlargement ("P mitrale") - seen in mitral stenosis, LV failure
- No P waves: Atrial fibrillation (irregularly irregular), sinus arrest
- Sawtooth P waves at ~300 bpm: Atrial flutter (typically 2:1, 3:1, or 4:1 block)
- Retrograde P after QRS: Junctional rhythm
Step 7: QRS Voltage and Morphology
Voltage criteria for Left Ventricular Hypertrophy (LVH):
- Sokolow-Lyon: S in V1 + R in V5 or V6 > 35 mm
- Cornell: R in aVL + S in V3 > 28 mm (men) or > 20 mm (women)
Low voltage:
- QRS amplitude < 5 mm in all limb leads or < 10 mm in all precordial leads
- Causes: pericardial effusion, cardiac tamponade, hypothyroidism, COPD/emphysema, obesity
Pathological Q waves (> 0.04 sec wide or > 1/4 height of the R wave in same lead):
- Indicate prior transmural myocardial infarction
- Location tells you the territory affected (see STEMI table below)
Precordial R-wave progression:
- R wave should progressively increase from V1 to V5-V6
- Loss of R-wave progression (poor R-wave progression) may indicate anterior MI or LVH
Step 8: ST Segments
The ST segment represents the isoelectric phase between ventricular depolarization and repolarization. Measured at the J point (junction of QRS and ST).
ST Elevation - STEMI Criteria
Per Tintinalli's Emergency Medicine (Table 49-4):
| Territory | Leads with ST Elevation | Likely Vessel |
|---|---|---|
| Anteroseptal | V1, V2, (V3) | Proximal LAD |
| Anterior | V1-V4 | LAD |
| Anterolateral | V1-V6, I, aVL | Proximal LAD or LCx |
| Lateral | I, aVL | LCx or diagonal branch |
| Inferior | II, III, aVF | RCA (80%) or LCx (20%) |
| Inferolateral | II, III, aVF + V5, V6 | RCA or LCx |
| True posterior | Tall R waves in V1-V2, R/S ≥ 1 | RCA or LCx |
| Right ventricular | II, III, aVF + ST elevation in V3R-V6R | Proximal RCA |
Reciprocal changes (ST depression in leads opposite to the injury zone) indicate a larger infarct, greater severity of CAD, and increased mortality risk.
Inferior MI: Always obtain a right-sided ECG (V4R) to rule out right ventricular infarction (important because nitrates and diuretics are relatively contraindicated if RV infarct is present).
Posterior MI: Check for tall R waves and ST depression in V1-V2 (mirror image); confirm with posterior leads V7-V9 showing ST elevation.
Conditions Mimicking STEMI (False Positives)
The following can produce ST elevation without acute MI. Always correlate clinically:
| Condition | Key Differentiator |
|---|---|
| Early repolarization | Concave upward ST elevation; notch at J point; young healthy patient |
| Pericarditis | Diffuse saddle-shaped ST elevation; PR depression (especially in II); pleuritic chest pain |
| LVH | ST elevation in V1-V3 with reciprocal changes; voltage criteria met |
| Ventricular aneurysm | Persistent ST elevation in prior MI territory; Q waves present |
| LBBB (new or old) | ST elevation in V1-V3 by nature of block; use Sgarbossa criteria |
| Takotsubo cardiomyopathy | Diffuse ST elevation; post-stress; predominantly female; apical ballooning on echo |
| Hyperkalemia | Peaked T waves + ST changes; broad QRS; sine wave pattern |
| Hypothermia | Osborn (J) wave at QRS-ST junction |
| Myocarditis | Diffuse ST elevation; may be young patient with viral prodrome |
- Tintinalli's Emergency Medicine, Table 49-6
ST Depression
Indicates subendocardial ischemia or reciprocal change. Common causes:
- Active ischemia/NSTEMI
- Reciprocal to STEMI in opposite wall
- Digoxin effect (downsloping, "reverse tick" pattern in V5-V6)
- Hypokalemia
- Cor pulmonale / right heart strain
Step 9: T Waves
- Normal: Upright in I, II, V3-V6; inverted in aVR; variable in III, aVL, V1
- Repolarization is an active (ATP-dependent) process, so T waves are especially sensitive to ischemia and metabolic disturbance
T-wave changes of clinical importance:
| Finding | Differential Diagnosis |
|---|---|
| Hyperacute T waves (tall, peaked, asymmetric) | First ECG sign of STEMI (precedes ST elevation); also hyperkalemia |
| T-wave inversion | Ischemia/NSTEMI, Wellens' syndrome, PE (V1-V4), intracranial hemorrhage, myocarditis, pericarditis, RBBB, post-tachycardia pattern, mitral valve prolapse |
| Wellens' syndrome | Deep symmetric T-wave inversion OR biphasic T in V2-V3 in a pain-free patient after angina - indicates critical LAD stenosis, high risk of anterior STEMI |
| Peaked T waves (symmetrical) | Hyperkalemia (early sign) |
| Flattened T waves | Hypokalemia, hyper/hypocalcemia, normal variant, drug effect |
Wellens' warning: 15% of unstable angina patients show Wellens' sign. These patients must receive urgent interventional management because of the high risk of imminent anterior MI.
- Tintinalli's Emergency Medicine, p. 3328
Step 10: U Waves
- A small positive deflection after the T wave, best seen in V2-V3
- Prominent U waves: Hypokalemia (classic finding), bradycardia, certain drugs (quinidine)
- Negative U waves: Suggest LVH or ischemia (pathological)
Step 11: Common ED-Specific Patterns to Recognize
Life-Threatening Arrhythmias
| Pattern | Key ECG Feature | Immediate Action |
|---|---|---|
| VF | No organized QRS; chaotic undulation | Defibrillate immediately |
| Pulseless VT | Wide complex tachycardia, regular | Cardioversion |
| Complete (3rd degree) AV block | P waves and QRS with no relationship; slow escape rate | Atranscutaneous pacing |
| Torsades de Pointes | Twisting of QRS axis around isoelectric line; long QTc | Magnesium IV; stop offending drugs |
| WPW + AF | Irregularly irregular wide complex tachycardia (delta waves) | Procainamide; avoid AV nodal blockers (can cause VF) |
Other Important Patterns
| Pattern | Key ECG Features |
|---|---|
| Pulmonary embolism | Sinus tachycardia (most common); S1Q3T3 (S in I, Q and T inversion in III); new RBBB; T inversions V1-V4 |
| Cardiac tamponade | Sinus tachycardia + low voltage + electrical alternans (QRS amplitude changes beat to beat) |
| Hyperkalemia | Peaked T waves → widened QRS → PR prolongation → sine wave → VF/PEA |
| Brugada syndrome | Coved ST elevation in V1-V2 with pseudo-RBBB morphology; risk of sudden cardiac death |
| Digoxin toxicity | "Reverse tick" ST depression + slowed AV conduction + bidirectional VT |
Key Pearls for the ED
- Always compare with old ECGs - a "normal" QRS width is not normal if the patient previously had a narrow QRS.
- New LBBB is not automatically a STEMI equivalent - < 10% of patients with new LBBB have AMI. Use the Sgarbossa criteria (concordant ST elevation ≥ 1 mm in leads with positive QRS, or concordant ST depression ≥ 1 mm in V1-V3, or discordant ST elevation ≥ 5 mm) to identify STEMI within LBBB.
- Repeat ECGs - a normal initial ECG does not exclude ACS. In high-risk patients with persistent symptoms and non-diagnostic first ECG, serial ECGs can detect evolving changes.
- Right-sided and posterior leads should be obtained in every inferior STEMI to detect RV infarction and posterior wall extension.
- Reciprocal changes confirm STEMI - their presence increases the likelihood of true transmural infarction and indicates a larger injury area.
- Never rely on a computer interpretation alone - automated ECG systems have significant limitations in arrhythmia diagnosis and require mandatory clinician review.
- Harrison's Principles of Internal Medicine 22E, p. 1919
- Tintinalli's Emergency Medicine, Chapter 49
- Kaplan & Sadock's Comprehensive Textbook of Psychiatry, p. 6824-6826
Summary: The 14-Parameter Systematic Checklist (Harrison's)
- Calibration and technical quality
- Rhythm
- Heart rate
- PR interval / AV conduction
- QRS duration
- QT / QTc interval
- Mean QRS electrical axis
- P wave morphology
- QRS voltages
- Precordial R-wave progression
- Abnormal Q waves
- ST segments
- T waves
- U waves
Following this order on every ECG ensures no critical finding is missed in the time-pressured ED environment.
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