Supraventricular Tachycardia (SVT), Narrow QRS Complex, Wide QRS Complex, Accessory Pathway, Heart Conduction, ECG Analysis
Explore how Atrioventricular Reentrant Tachycardia (AVRT) occurs. This guide explains the normal heart conduction "highway" vs. the "AVRT hijack" caused by an accessory pathway, Understanding Supraventricular Tachycardia
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| Understanding AVRT: How an Accessory Pathway Causes Supraventricular Tachycardia (SVT) |
Understanding AVRT: How an Accessory Pathway Causes Supraventricular Tachycardia (SVT)
Tachycardia, defined as a HR >100 beats per minute, can be separated into (a) those that arise above the ventricles, termed supraventricular tachycardias, and (b) those that arise within the ventricles, termed ventricular tachycardias (VTs). Tachycardias can generally be distinguished on the basis of the HR, the width and morphology of the QRS complex, and the length of the PR interval.
Generally, SVT (Supraventricular Tachycardia) means a Narrow QRS Complex with absent P waves. But there is a famous exception—if a patient has SVT along with a Bundle Branch Block, the QRS complex will appear wide.
In atrial tachycardia, the atria depolarize faster than 150/min. The AV node cannot conduct atrial rates of discharge greater than about 200/min. If the atrial rate is faster than this, ‘atrioventricular block’ occurs, with some P waves not followed by QRS complexes. The difference between this sort of atrioventricular block and second degree heart block is that in an atrioventricular block associated with tachycardia the AV node is functioning properly – it is preventing the ventricles from being activated at a fast (and therefore inefficient) rate. In first, second or third degree block associated with sinus rhythm, the AV node and/or the His bundle are not conducting normally.
Causes:
Mitral valve disease
Hypertension
Pulmonary embolism
Pericarditis
Post cardiac surgery
Hyperthyroidism
Obstructive lung disease
Atypical atrial flutter usually arises from atrial scars
CAD/ CHF
Myocardial infarction
Hypoxia
Hypokalemia
Digitalis toxicity
Prolonged QT interval (congenital or drug-related)
Myocarditis
Pathophysiology:
To understand how re-entry works, let's look at how a normal heart conducts electricity versus what goes wrong in AVRT (Atrioventricular Reentrant Tachycardia).
The Normal One-Way Highway:
In a healthy heart, electricity follows a strict, one-way system:
The signal initiates at the SA Node in the right atrium.
It spreads across both atria (using Bachmann's bundle to reach the left atrium), causing them to contract.
Then signal reaches the AV Node, where it is delayed for a fraction of a second to let the ventricles fill up with blood.
Finally, it passes through the Bundle of His and Purkinje fibers, causing the ventricles to contract.
Normally, the AV node is the only physical gateway between the atria and ventricles. Once the signal reaches the end of the Purkinje fibers, it dies out, and the heart waits for the next SA node signal.
The AVRT Hijack:
In patients with AVRT, there is an anatomical anomaly_an extra physical bridge of tissue called an Accessory Pathway. This pathway directly connects the ventricles back to the atria.
Loop: The signal goes down the normal AV node path to the ventricles, but instead of dying out, it travels backward (retrograde) up this accessory pathway back into the atria.
Hijack: This retrograde signal triggers the atria to contract way before the SA node can fire its next natural impulse.
Vicious Cycle: This premature atrial signal goes right back down the AV node to the ventricles, creating an endless, rapid loop.
Because this electrical loop spins continuously without a resting period, it drives the heart rate up to 200 to 300 beats per minute, presenting clinically as a severe supraventricular tachycardia.
Differential diagnosis:
Atrioventricular nodal reentry tachycardia
Typical atrioventricular reentrant tachycardia
Orthodromic atrioventricular reentrant tachycardia
Atrial tachycardia with 1st degree AV delay
Junctional tachycardia
Atrial tachycardia
Sinus tachycardia
Atypical atrioventricular nodal reentry tachycardia
Reference: washington manual of critical care:
Chapter 20 cardiac Arrhythmia and conduction abnormalities,
Harrison manual of medicine: chapter 125 Tachyarrythmias
ECG made Easy by Hampton: chapter 3 The rhythm of the heart
Current essential of medicine by Mary A. Whooley, MD
Lawrence M. Tierney, Jr., MD
Sanjay Saint, MD, MPH:
chapter 1 Cardiovascular Diseases
Today case:
A 56 years old male patient came in emergency department in drowsy state with k/c/o DM/HTN and with history with severe vomiting and loose motions since 3 days. On examination Bp: 80/50 mmHg, Heart rate 196/min, spo2 89% on RA, Hgt 68mg/dl.
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