ECG interpretation guide
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| ECG Made Easy: How to Diagnose Electrolyte Imbalances & Heart Blocks |
Struggling with ECG interpretation? Master the criteria for WPW syndrome, complete third-degree heart block, and distinct T wave abnormalities with our quick-review guide.
P wave Abnormalities
A normal P wave is less than 3 small squares in width and less than 2.5 mm in height.
P wave is < 3 small squares (< 0.12 sec)
Vertical Ht --> < 2.5mm limb lead / greater than 2.5 mm chest lead.
-->P-Pulmonale: vertical height
Absent P Waves
P waves may be absent in:
Atrial fibrillation
Sick sinus syndrome
Hyperkalemia
In atrial fibrillation, atrial activity is chaotic and no distinct P waves are visible. The R-R interval is typically irregular.
Wide and Notched P Waves (P Mitrale)
As Left atrial is dilated, current will take more time to go across this dilated Left atria.
Wide and notched P waves indicate left atrial enlargement, commonly seen in mitral stenosis. Because the left atrium is enlarged, depolarization takes longer, resulting in a broad, bifid P wave.
Tall and Peaked P Waves (P Pulmonale)
Tall, peaked P waves indicate right atrial enlargement, often due to right atrial overload.
Note: Pseudo p-pulmonale (P > 2.5 mm in absence of PAH) seen in hypokalemia.
Causes include:
Pulmonary hypertension
Chronic lung disease
Pulmonary stenosis
Hypoxemia
P pulmonale is characterized by P waves greater than 2.5 mm in amplitude.
Short PR Interval
A PR interval shorter than 0.12 seconds occurs when the normal AV nodal delay is bypassed. In this situation, the ventricles are activated prematurely through an accessory conduction pathway.
The classic example is Wolff–Parkinson–White (WPW) syndrome, in which an accessory pathway known as the Bundle of Kent allows impulses to reach the ventricles without passing through the AV node. ECG findings include:
Short PR interval (<0.12 s)
Delta wave
Premature ventricular activation
QT Interval Abnormalities
The QT interval reflects the total time for ventricular depolarization and repolarization and is influenced by blood calcium levels.
Short QT Interval
A short QT interval is commonly associated with:
Hypercalcemia
Certain Class IB antiarrhythmic drugs (e.g., lidocaine)
Prolonged QT Interval
A prolonged QT interval is associated with:
Hypocalcemia
Hypokalemia
Class IA antiarrhythmic drugs (e.g., quinidine, procainamide)
Electrolyte disturbances, particularly abnormalities of calcium, potassium, and magnesium, can significantly affect the QT interval.
T Wave Abnormalities
The T wave represents ventricular repolarization. Its height should generally be less than 5 mm in limb leads and less than 10 mm in chest leads.
Tall T Waves
Tall, peaked T waves are classically seen in hyperkalemia. These T waves are narrow-based and sharply peaked.
Inverted T Waves
T-wave inversion may occur in:
Hypokalemia
Myocardial ischemia
Ventricular strain patterns
Mild ischemia at the apex of the ventricles commonly produces inverted T waves.
Biphasic T Waves
A biphasic T wave, in which the wave has both positive and negative components, may be seen with digitalis toxicity and certain ischemic conditions.
These ECG abnormalities provide important clues to electrolyte imbalances, atrial enlargement, conduction disturbances, and myocardial ischemia, making ECG interpretation an essential clinical skill.
PR Interval and AV Blocks
The PR interval is the time from the beginning of the P wave to the beginning of the QRS complex. A prolonged PR interval is commonly seen in bradycardia and heart block.
Mobitz Type I (Wenckebach) Second-Degree AV Block
Mobitz Type I is an intermittent AV nodal conduction problem. The AV node progressively delays conduction, causing the PR interval to become longer with each beat until a ventricular beat is not conducted and a QRS complex is missed. After the dropped beat, the cycle repeats. This phenomenon is known as the Wenckebach phenomenon. The PR interval progressively increases until a QRS complex is not conducted.
Mobitz Type II Second-Degree AV Block
Mobitz Type II is usually caused by a conduction problem below the AV node, commonly in the Bundle of His.The PR interval of conducted beats is normal but some P waves are not conducted.
Unlike Mobitz Type I, the PR interval remains constant, but occasional QRS complexes are suddenly dropped. Because the conduction system is more severely affected, this type often requires temporary pacing.
Third-Degree (Complete) Heart Block
In third-degree heart block, there is complete failure of conduction between the atria and ventricles. The AV node no longer conducts impulses from the SA node.So lack of co-ordination, normally atria & ventricle, As a result, the atria and ventricles beat independently of each other. The atrial rate is determined by the SA node, while the ventricular rate is controlled by an escape pacemaker located in the AV junction or ventricles.
Normally, the atria and ventricles contract in a coordinated manner, but in complete heart block this coordination is lost, producing AV dissociation. The P-P intervals remain regular and the R-R intervals remain regular, but there is no relationship between the P waves and QRS complexes. Because of the slow ventricular rate, cardiac output decreases, causing bradycardia, postural hypotension, dizziness, and syncope.
When the ventricular rate falls to around 40 beats/minute or less, cerebral blood flow decreases significantly, leading to transient loss of consciousness (syncope). This condition is known as Stokes–Adams syndrome. In complete heart block, more P waves than QRS complexes are present because many atrial impulses fail to reach the ventricles.
Note:
1) P-P interval doesn't match R-R interval
2) Broad QRS
3) More no. of P waves. So no. of P waves doesn't match c no. of R wave.
Mobitz II: Slow conduction
3° Heart block: No conduction
Reference: ECG made Easy by Hampton, The ECG made practical, Harrison
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