Module 2: Heart Rhythm Interpretation

Question 1

Sinus node dysfunction affects what characteristics of a cardiac cell?

Answer 1

Automaticity, conduction

Question 2

Sinus node dysfunctions

Answer 2

Sinus tachcardias, sinus bradycardias, sinus arrhthymias, sinus pause

Question 3

Symptoms of SSS

Answer 3

• Dizziness
• Mental confusion
• Shortness of breath
• Excercise intolerance
• Syncope/ pre-syncope

Question 4

Sinus Tachycardia-Rate, regularity, rhythm?

Answer 4

Rate: 100-180 bpm
Regularity: PR + R-R intervals are regular
Rhythm: One P-wave for each QRS complex

Mechanism: Abnormally fast automaticity of sinus node

Question 5

Sinus Bradycardia-Rate, regularity, rhythm?

Answer 5

Rate: 40-60 bpm
Regularity: PR + R-R intervals are regular
Rhythm: One P-wave for each QRS complex

prolonged time btw each complex. Mechanism: Abnormally slow automaticity of the sinus node

Question 6

Sinus pause or arrest. Rate, regularity, rhythm?

Answer 6

Rate: 60-100 bpm
Regularity: PR intervals are regular, with missing beats causing irregularity (one beat to several secs)
Underlying rhythm does not resume on time after pause
Rhythm: One P-wave for each QRS complex, with a one beat-to-several-second pause.

Sinus arrest occurs when the SA node fails to fire > loss of entire ECG cycle > disorder of automaticity

Mechanism: Irregular automaticity of the sinus node

Question 7

Characteristics of sinus arrest

Answer 7

Rate within the normal, tachy, or brady range when the sinus node activates the impulse
Pause will cause a slower rate to occur
Sinus beat or escape beat (atrial or ventricular) usually occur at the end of the pause; the absence of a P-wave results in a pause on the ECG tracing
Pause can be anywhere from one beat to several seconds
Pause terminates with a sinus or escape beat from the atrium or ventricle
Sinus node typically resumes pacing after the escape beat occurs

Question 8

Sinus exit block. Rate, regularity, rhythm?

Answer 8

Rate: 60-100 bpm
Regularity: PR intervals are regular, with missing beats causing irregularity (one beat to several secs)
Underlying rhythm does not resume on time after pause
Rhythm: One P-wave for each QRS complex, with a one beat-to-several-second pause.

Electrical impulse is initiated by the SA node, but is blocked as it exits from the SA node > prevents conductions of the impulse into the atria

Mechanism: conduction issue

Question 9

Difference btw sinus exit block and sinus arrest?

Answer 9

In sinus arrest, the P-P or R-R intervals do not resume or “march-out” on time after the pause.
But in sinus exit they do.

Both have a sinus pause, but to tell the difference- compare the length of the pause with the underlying PP or R-R interval to determine if the underlying rhythm resumes on time following the pause.

If the underlying PP or R-R interval does resume on time, it is sinus exit block.
If the underlying PP or R-R interval does not resume on time, it is sinus arrest.

In most clinical situations, you should refer to a sinus arrest or a sinus block as a “sinus pause,” and let the physician determine what type of pause it actually is.

Question 10

Atrial arrhythmias vs Sinus arrhythmias

Answer 10

Atrial arrhythmia originates in the atrium outside the sinus node.

Question 11

Premature atrial contractions (PACs) Rate, regularity, rhythm?

Answer 11

Rate: 60-100 bpm
Regularity: PR intervals are regular, Irregular with PACs
Rhythm: One P-wave associated with the PAC is premature > sinus node to reset >underlying rhythm continues

Occurs when a specific site in the atrium initiates a depolarisation wave throughout the heart faster than the SA node in a normal conduction.

Question 12

What causes PACs?

Answer 12

Cardiac cells get irritated due to disease, blood chemistry imbalance + drug toxicity > Ectopic Foci (areas where this impulse originates outside conduction system) can spontaneously depolarise faster than the SA node.

Ectopic beats can initiate and maintain electrical impulses in emergency situations. For example, if the SA node is completely blocked, ectopic foci in the atria assume pacing responsibility. The beat produced by an ectopic focus is called an escape beat, and the rhythm produced by escape beats is called an escape rhythm.

Question 13

Important characteristics of PACs

Answer 13

They originate in parts of the atrium other than the sinus node.
The morphology (or shape) of the P-wave is different than a P-wave originating from the SA node.
These impulses occur before the sinus node depolarizes.
They appear as an extra complex on a normal ECG waveform; the extra P-wave may have an unusual morphology or shape.
These beats are usually ectopic, originating outside the normal pathway, conducting cell-to-cell in the atrium.
PACs are often, but not always, conducted down into the ventricles. When a PAC is conducted, the impulse reaches the AV node and is slowed down, just like a normal sinus beat. Once through the AV node, PACs are then conducted through the ventricle in the same fashion as a normal sinus beat.
A PAC also depolarizes the SA node, resetting its pacemaker activity for that beat; this creates the “compensatory pause” that is observed after a PAC.
PACs are very common, and can be completely undetected by the patient; they are sometimes perceived as a “skip” or “pause.”
The underlying mechanism is abnormal automaticity.

Question 14

Atrial tachycardia. Rate, regularity, rhythm?

Answer 14

Rate: 100-250 bpm
Regularity: R-R intervals are regular
Rhythm: One P-wave precedes each QRS complex, P waves abnormal, sometimes hidden in QRS

Tachycardias > the heart beats too rapidly, there is less time for the ventricles to fill completely. This causes a drop in cardiac output.

Atrial tachycardia is defined as a series of three or more consecutive atrial premature beats occurring at a rate greater than 100 beats per minute.

Atrial tachycardia is usually paroxysmal; that is, it starts and ends abruptly >paroxysmal atrial tachycardia (PAT). This tachycardia can occur in healthy as well as diseased hearts, and may result from emotional stress or excessive use of alcohol, tobacco, or caffeine.

The underlying mechanism is abnormal automaticity.

Question 15

Atrial Flutter. Rate, regularity, rhythm?

Answer 15

Rate: 250-400 bpm
Regularity: R-R intervals regular/ irregular, depending on AV conduction ratios
Rhythm: Multiple P-waves per QRS complex. P-waves appear as “saw tooth” deflection.

During atrial flutter, atrial impulses are conducted to the ventricles in various ratios. The saw tooth pattern seen in the ECG is due to the reexcitation of a region of cardiac tissue by a single impulse that continues for one or more cycles, a process known as reentry.

Conduction ratios such as 2:1 and 4:1 are more common than odd ratios such 3:1 and 5:1. In a 2:1 ratio, there are two flutter waves for every QRS complex.

For example, a constant conduction ratio such as 2:1 results in a regular ventricular rhythm, which is the most common. A variable ratio, such as 4:1–2:1–5:1, results in an irregular ventricular rhythm.

The underlying mechanism is abnormal automaticity reentry.

Question 16

Atrial Fibrillation. Rate, regularity, rhythm?

Answer 16

Rate: Atrial rate-400 bpm, Ventricular rate varies depending on AV conduction
Regularity: Irregular
Rhythm: P-waves are erratic & multifocal, QRS will not be preceded by a discrete P-wave.

Characterized by random, chaotic electrical activity of the atrial myocardium, this state is due to many cells firing all over the heart chamber.

Patients with AF have an atrial rate of 400 beats per minute or more, which is often too fast to measure on an ECG. The chaotic activity does not allow the atria to reorganize. The ectopic foci in this state are said to be located around or within the pulmonary veins.

A surface ECG shows atrial fibrillation as irregular, wavy deflections called fibrillatory waves, between narrow QRS complexes. Fibrillatory waves vary in shape, amplitude, and direction.

The chaotic nature of atrial fibrillation results in a grossly irregular ventricular rhythm. The rhythm is considered controlled if the ventricular rate is less than 100 beats per minute, and uncontrolled if the ventricular rate conducts to greater than 100 beats per minute.

The underlying mechanism is abnormal automaticity–chaos.

Question 17

SVTs vs in the ventricles

Answer 17

Supraventricular tachycardia or SVT refers to any tachyarrhythmia that occurs above the ventricle. SVTs include:

Sinus tachycardia
Atrial tachycardia
Atrial flutter
Atrial fibrillation
Tachyarrhythmias that originate within the ventricles are classified as:

Ventricular tachycardia
Fast ventricular tachycardia (FVT), or
Ventricular fibrillation

Question 18

What are Junctional arrhythmias?

Answer 18

Arrhythmias whose focus is within the tissue of the AV node (rather than the SA node).

Question 19

Junctional rhythm. Rate, regularity, rhythm?

Answer 19

Rate: 40-60 bpm
Regularity: R-R intervals are regular,
Rhythm: The P wave is immediately before, after, or hidden in the QRS complex. P wave is inverted.

In a junctional rhythm, electrical impulses originate in the AV node and travel through the atrium and ventricle at about the same time.

Junctional rates are usually slow—from 40 to 60 beats per minute—but regular. The ECG shows normal QRS complexes and inverted P waves. The P waves may immediately precede, follow, or be hidden within the QRS complex.

Recall that the normal SA rates are between 60 and 100 beats per minute and that the normal AV rates are from 40 to 60 beats per minute, which is the junctional rate.

Note that junctional rhythms may be accelerated to rates between 60 and 100 beats per minute—or even more rapid, with rates over 100 beats per minute (called junctional tachycardia).

If retrograde P waves are hidden in the QRS complex, it may be difficult to distinguish between junctional tachycardia and atrial tachycardia. As a consequence, the slow rate and loss of atrial kick associated with junctional rhythm may cause a decrease in cardiac output.

The QRS complex width is “normal or narrow.” This is because, as the AV junction fires, the impulse follows the conduction pathway into the ventricles to efficiently cause a ventricular contraction.

The underlying mechanism is abnormal automaticity.

Question 20

Junctional tachycardia

Answer 20

Rate: > 100 bpm
Regularity: R-R intervals are regular,
Rhythm: The P wave is immediately before, after, or hidden in the QRS complex.

Junctional tachycardia is present when the automaticity of the AV junctional tissue is increased, sending out impulses between 100 and 200 per minute, most commonly at 140 per minute. Junctional tachycardia is usually treated with drugs, while atrial tachycardia usually requires cardioversion.

Note that the QRS complex is “normal or narrow” because as the AV junction fires, the impulse follows the conduction pathway into the ventricles to efficiently cause a ventricular contraction.

The underlying mechanism is increased automaticity.

Question 21

Accelerated Junctional rhythm. Rate, regularity, rhythm?

Answer 21

Rate: 60-100 bpm
Regularity: R-R intervals are regular,
Rhythm: The P wave is immediately before, after, or hidden in the QRS complex.

Accelerated junctional rhythm occurs when the AV nodal rate accelerates to a rate faster than that of the sinus node and takes over the rhythm. The rate is usually between 60 and 100 beats per minute.

The underlying mechanism is abnormal automaticity.

Question 22

Conduction issues

Answer 22

Conduction issues can present either as an impulse slowing down inappropriately in the conduction system or as a block issue where the electrical impulse is prevented from going any further in the conduction system.

These issues can occur for a number of reasons, including:

• Aging of the heart
• Myocardial infarction in the area, causing the death of the conductive tissue
• Attendant coronary artery disease
• Medications taken by patient
• Idiopathic source or cause of the block; that is, for an unknown reason
  Regardless of the cause, conduction issues within the heart are often serious, and must be addressed.

Question 23

Slow Conduction

Answer 23

A conduction issue within the heart is the failure to propagate electrical impulses appropriately. Slowed conduction or blocks can occur at any point within the conduction system, including the SA node, AV node, bundle of His, or distal conduction system.

Impulses that travel along the conduction pathway most efficiently and effectively cause the heart to depolarize. When these pathways are blocked or damaged, cardiac depolarization is slow, inefficient, and in some instances may not cause a chamber of the heart to depolarize at all.

Question 24

Slow conduction- Heart block?

Answer 24

Focus on slow conduction: While internodal pathways in the atrium are very fast, cell-to-cell conduction is slower.

The failure to propagate electrical impulses appropriately is also known as a heart block. Impulses in a patient with diseased heart tissue may be:

Intermittent
Irregular
Not generated at all
Occurring at an inappropriate rate for the patient’s metabolic demand
Heart block can occur at any point—within the SA node, AV node, bundle of His, or distal conduction system.

Question 25

First degree AV Block

Answer 25

Rate: 60-100 bpm
Regularity: PR intervals >200 ms
R-R intervals are regular,
Rhythm: One P-wave for each QRS complex, but inappropriately long PR interval

AV block can be described as a prolongation of the PR interval.

First-degree AV block is defined by a PR interval greater than 200 milliseconds. This kind of AV block can be thought of as a delay in AV conduction, but each atrial signal is conducted to the ventricles in a 1:1 ratio.

It can be caused by:

Myocardial infarction (MI)
CAD
Rheumatic fever
Certain drugs

Question 26

Second degree AV Block Mobitz I (Wenckebach)

Answer 26

Rate: Irregular due to block
Regularity: PR intervals are irregular
R-R intervals are irregular,
Rhythm: Pattern of PR intervals progressively lengthen until P wave is not conducted at some ratio (in this case 4:3, where every 4 P waves yield 3 R waves and a dropped QRS complex). Progressive lengthening of PR interval until a P wave is not conducted (ratio 4:3). Cycle then starts over.

Second-degree AV block is characterized by intermittent failure of atrial depolarizations to reach the ventricle.

There are two patterns of second-degree AV block. The first, Type I, is marked by progressive prolongation of the PR interval in cycles preceding a dropped beat. This is also referred to as a Wenckebach or Mobitz Type I block.

The AV node is most commonly the site of Mobitz I block.

The QRS duration is usually normal.

Causes for a Mobitz I block include:

MI
CAD
Rheumatic fever
Certain drugs
The underlying mechanism is a conduction issue of slowed conduction at the AV node.

Question 27

Second-Degree AV Block Mobitz II

Answer 27

Rate: P waves 60-100 bpm, but R-R slower due to dropped beats
Regularity: PR intervals are regular
R-R intervals are regular,
Rhythm: Sinus node depolarises at expected rate. Due to conduction issue, more than one P-wave for each QRS complex ( usually in some ratio 2:1, 3:1). P waves do not conduct 1:1 (2:1, 3:1, etc). Conducted PR interval is normal.

Mobitz Type II second-degree AV block refers to intermittent dropped beats preceded by constant P-R intervals.

If the difference between these two P-R intervals is more than 20 milliseconds = Mobitz I. If the difference is less than 20 milliseconds = Mobitz II.

Mobitz Type II is usually caused by an MI affecting the septum.

The infranodal (bundle of His) tissue is most commonly the site of Mobitz II block.

Note that advanced second-degree block refers to the block of two or more consecutive P-waves (that is 3:1 block).

The underlying mechanism is a conduction issue of slowed conduction at the AV node.

Question 28

Third-Degree Block (“Complete” Heart Block)

Answer 28

Rate: P waves 60-100 bpm, but R waves 20-40 bpm
Regularity: No correlation btw P waves and R-waves
Rhythm: Sinus node depolarising at expected intrinsic rate, impulse is blocked from going any furthur than AV node, thus ventricles depolarise at their own instrinsic rate.

In third-degree or complete heart block, there is no correlation between the P-waves and R-waves, as a chamber is beating at its own intrinsic rate.

The underlying mechanism is a conduction issue in which there is no conduction at all.

Question 29

Bundle branches + blocks

Answer 29

We have three main bundle branches. Any one or any combination can be blocked, usually as a result of infarcted areas. When the blood supply to that area is obstructed, the bundle branches will begin to fail.

If there is a block in one of the bundle branches—for example, the left bundle branch—consider the electrical pathway of the impulse, how the ventricles will depolarize, and what this will look like on the ECG.

The electrical impulse in this case will travel down the right bundle branch to the Purkinje fibers, causing the right ventricle to contract. However, since the left bundle branch (LBB) was blocked, it takes the depolarization wave from the right side longer to reach the left ventricle and depolarize it; this extra time would not be taken if the LBB was not blocked.

Slower cell-to-cell conduction outside the conduction system is seen on an ECG as a wide QRS complex. This is because it takes longer for the ventricle to fully depolarize with a block than if the left bundle branch was intact.

Question 30

Right bundle branch block & left bundle branch block

Answer 30

Rate: 60-100 bpm
Regularity: PR intervals are regular
R-R intervals are regular,
Rhythm: One P wave for each QRS complex. Produces a wide QRS complex > 120 ms

In a bundle branch block, the conduction of impulses is delayed or blocked in the left or right bundle branch.

The blocked bundle branch delays depolarization to the ventricle that it supplies. Normally, both ventricles are depolarized rapidly and simultaneously. But if one of the bundle branches is blocked, that particular ventricle will depolarize later than the other ventricle. In this case the ECG will show a wide QRS complex.

Premature ventricular contractions (PVCs) appear as wide complexes because they are using cell-to-cell transfer versus the normally fast conduction pathway.

The underlying mechanism is a conduction issue in the ventricle that results in cell-to-cell depolarization to fully depolarize the ventricle.