What is overdrive suppression?
This is the dominant pacemaker’s (usually SA node) capability of suppressing all other slower pacing rates. The inherent rate is 60 to 100 b/min
If the pacemaker is at the AV node or bundle of his, what is the inherent rate?
40-60 b/min
If the pacemaker is in the bundle branches (i.e., ventricles) or purkinje fibers, what is the inherent rate?
20-40 b/min
If the SA node‘s discharge is too slow, it is referred to as what?
A latent pacemaker
When the other foci (AV node, bundle of his, bundle branches, purkinje fibers) become the pacemaker, this is referred to as:
Escape pacemakers
What is an escape rhythm?
The automaticity focus escapes the overdrive suppression of the SA node (due to a malfunctioning SA node) to become the inherent pacemaker.
What is an escape beat?
This is an acute or transient escape from the SA node to emit ONE beat from another foci (atrial, junctional, or ventricular).
During a sinus rhythm (originating at SA node), a momentary (transient) sinus block makes the SA node miss a cycle; therefore there is a pause and then another foci (perhaps in the atria) will emit an impulse or atrial escape beat. When the SA node resumes, the atrial focus will be overdrive-suppressed again.
What origins of depolarization are considered supraventricular?
Sinoatrial (SA Node), atria, and atrioventricular (AV Node)
Which rhythms are more stable: Supraventricular or ventricular rhythms?
Supraventricular
What is the differential criteria between a supraventricular and ventricular rhythm?
The duration of the QRS. If the QRS is less that or equal to 0.10 (borderline 0.11-0.12) the impulse has originated from a supraventricular forcus (i.e., the SA node, the atrium, or the AV node or junction)
What does the WIDE QRS indicate?
The electrical activation required considerable time for ventricular depolarization and thus did not use the ventricular conduction system.
So if a wide QRS is present, does this mean there is some type of defect present in the ventricular conduction system?
Yes
With some ventricular rhythms will there be a normal P wave and PR interval preceding the QRS?
Yes
What is the Wolff-Parkinson-White (WPW) preexcitation syndrome?
The conduction from the atria to the ventricles bypasses the AV node. A regular P wave will be present but the PR interval may be less then 0.12 second (normal is 0.12 to 2.0 seconds). A delta wave is often seen.
What is the delta wave?
That is the slurred upstroke onto the QRS.
Let’s review WPW’s conduction. This is pictured nicely on page 29
The impulse originates in the SA node
There is depolarization of the atria (P wave present)
An impulse BYPASSES the delay at the AV node (shorter PR Interval)
Thus, a small portion of the ventricles are PREEXCITED
This preexcitation creates the slurred upstroke (delta wave onto the QRS)
Also, another impulse is delayed (normally) at AV node
The combination of the preexcitation impulse of the ventricles with the normal impulse prolongs the QRS (wider QRS)
What is the idioventricular rhythm?
A ventricular rhythm with a wide QRS, generally bizarre QRS complexes and T waves that are opposite in direction.
The rate is generally 20 to 40 b/min since this the ventricles are now the pacemakers.
No P waves are present
What is an accelerated idioventricular rhythm?
Rate is 40 to 120 b/min
No P waves are present
Describe ventricular tachycardia?
Rapid, broad, and often bizarre QRS complexes, with T waves usually opposite in direction to main QRS direction. The text notes that it is a rate of at least 120 b/min but most times it is greater than 150 beats/min
What is ventricular fibrillation?
Chaotic depolarization in the ventricles causing a loss of organized QRS complexes. This is the cause of most sudden cardiac deaths.
What is a ventricular pacemaker?
The origin of the cardiac rhythm is in the ventricles. There is a wide QRS. Look for the "spike," which depicts the pacemaker discharge
What is meant by the term "asystole?"
The heart is not beating. There are no electrical complexes on the electrocardiogram.
EKG is fun!
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