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Y3 Medicine: Cardiovascular > AV Block > Flashcards

AV Block Flashcards

1
Q

What is AV block?

A

Cardiac electrical disorder defined as impaired (delayed or absent) conduction from the atria to the ventricles.

The severity of the conduction abnormality is described in degrees: first-degree; second-degree, type I (Wenckebach or Mobitz I) or type II (Mobitz II); and third-degree (complete) AV block.

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2
Q

What is first degree heart block?

A

First-degree heart block occurs where there is delayed atrioventricular conduction through the AV node. Despite this, every atrial impulse leads to a ventricular contraction, meaning every p waves results in a QRS complex. On an ECG this presents as a PR interval greater than 0.20 seconds (5 small or 1 big square).

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3
Q

How does first degree heart block appear on an ECG?

A

The PR interval is prolonged and unchanging; no missed beats.

PR interval >0.2 seconds (or >200 milliseconds).

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4
Q

What is second degree heart block?

A

Second-degree heart block is where some of the atrial impulses do not make it through the AV node to the ventricles. This means that there are instances where p waves do not lead to QRS complexes.

There are 2 types of second degree heart block:

  • Mobitz type 1 (Wenckebach’s phenomenon)
  • Mobitz type 2
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5
Q

How does Mobitz type 1 (second degree heart block) appear on an ECG?

A

The PR interval becomes longer and longer until a QRS is missed, then the pattern resets.

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6
Q

How does Mobitz type 2 (second degree heart block) appear on ECG?

A

Fixed, unchanging PR intervals with regular loss of QRS.

Example:

P-QRS-P–P-QRS-P–

This would be an example of Mobitz type 2 with 2:1 conduction block. This is a dangerous rhythm as it can progress to complete heart block.

Then, occasional loss of AV conduction for 1 beat (during sinus rhythm, excluding premature atrial beats).
Finally, fixed, unchanging PR intervals.

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7
Q

What is third degree heart block?

A

This is referred to as complete heart block. This is no observable relationship between P waves and QRS complexes. There is a significant risk of asystole with third-degree heart block.

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8
Q

How does third degree heart block appear on an ECG?

A

No impulses are passed from atria to ventricles so P waves and QRS appear independently of each other.

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9
Q

Why is third degree heart block a medical emergency?

A

As tissue below the AVN paces slowly, the patient becomes very bradycardic, and may develop haemodynamic compromise. Therefore, urgent treatment is required.

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10
Q

What are the causes of first degree and second degree heart block?

A
  • Normal variant
  • Athlete
  • Sick sinus syndrome
  • IHD (esp. inferior MI)
  • Acute myocarditis
  • Drugs (digoxin and beta-blockers)
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11
Q

What are the causes of third degree heart block?

A
  • IHD (esp. inferior MI)
  • Idiopathic (fibrosis)
  • Congenital
  • Aortic valve calcification
  • Cardiac surgery or trauma
  • Digoxin toxicity
  • Infiltration (abscess, granulomas or tumours)
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12
Q

What are the signs of heart block?

A
  • HR <40 bpm
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13
Q

What are the symptoms of heart block?

A
  • Syncope
  • Pre-syncope
  • Episodic lightheadedness
  • Progressive exertional fatigue and/or dyspnoea
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14
Q

What investigations should be ordered for heart block?

A
  • 12 lead ECG
  • Serum troponin
  • Serum potassium
  • Serum calcium
  • Serum pH
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15
Q

Why investigate using ECG?

A

Diagnosis of the various degrees of AV block is frequently made incidental to the work-up for any number of presenting conditions.

On ECG:

  • First-degree AV block: fixed PR interval >0.210 seconds (or >210 milliseconds)
  • Second-degree AV block:
    • Type I: progressive PR interval prolongation, eventual loss of AV conduction for 1 beat, return to normal PR interval, then progressive PR prolongation with eventual loss of AV conduction; second-degree AV block,
    • Type II: occasional loss of AV conduction for 1 beat preceded and followed by fixed, unchanging PR intervals
  • Third-degree AV block: no consistent PR relationship
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16
Q

Why investigate using serum troponin?

A

Because acute ischaemia should be aggressively managed and presents a potentially reversible cause of AV block, an indicative history and physical examination should prompt the obtaining of serum cardiac enzymes.

May be elevated.

17
Q

Why investigate serum potassium?

A

Severely abnormal values may represent reversible causes of AV block.

May be very low or very high.

18
Q

Why investigate serum calcium?

A

Severely abnormal values may represent reversible causes of AV block.

May be very low or very high.

19
Q

Why investigate serum pH?

A

If severe acidosis or alkalosis is suspected, a serum pH should be obtained. Severely abnormal values may represent reversible causes of AV block.

May be very low or very high.

20
Q

Give examples of common AV-node blocking drugs

A

The most common AV-nodal blocking medications include beta-blockers, non-dihydropyridine calcium-channel blockers and digoxin.

21
Q
A
22
Q

How is asymptomatic heart block treated?

A

No specific treatment is required. Patients are at low risk for progression to higher-degree AV block. ECGs may be re-checked if symptoms develop, but do not need to be re-checked on a routine basis.

23
Q

How is symptomatic heart block treated?

A

Medications blocking the AV node should be stopped first.

In the absence of a reversible cause, these patients should undergo permanent pacemaker (PPM) implantation. Biventricular pacemaker, with or without an implantable cardioverter-defibrillator (ICD) placement, may be considered when the left ventricular ejection fraction is <35%

24
Q

What are the complications of heart block?

A
  • Progression to complete heart block in type 2 second degree heart block
  • Pacemaker implantation sequelae
25
Q

What differentials should be considered in heart block?

A
  1. Supraventricular tachycardia
  2. Atrial fibrillation or multifocal atrial tachycardia
26
Q

How does heart block and supraventricular tachycardia differ?

A

Differentiating signs and symptoms:

  • In patients with profound first-degree AV block and very long PR intervals, the P wave may encroach so closely on the QRS complexes that they appear to be retrograde P waves, suggesting SVT

Differentiating investigations:

  • Variations in the heart rate and the PR and RP (QRS to P) intervals suggest against the diagnosis of SVT
  • Comparison of numerous ECGs may be very useful
27
Q

How does heart block and atrial fibrillation/ multifocal atrial tachycardia differ?

A

Differentiating signs and symptoms:

  • The irregularity of the RR intervals in type I second-degree AV block may lead to the incorrect diagnosis of atrial fibrillation or MAT
  • The presence of distinct P waves and the grouped pattern of the RR intervals are characteristic of type I second-degree AV block and are not seen in atrial fibrillation or MAT

Differentiating investigations:

  • Careful inspection of the ECG should allow distinction between these diagnoses
28
Q

How can the different types of heart blocks be remembered?

A

If the R is far from P, then you have a First Degree.

Longer, longer, longer, drop! Then you have a Wenkebach.

If some P’s don’t get through, then you have Mobitz II.

If P’s and Q’s don’t agree, then you have a Third Degree.

29
Q

Which heart block is shown in the picture?

A

First degree heart block.

PR interval > 300 ms, P waves are buried in the preceding T wave.

30
Q

Which heart block is shown in the picture?

A

Sinus bradycardia with 1st degree AV block.

PR interval > 300 ms.

31
Q

Which heart block is shown in the picture?

A

Normal sinus rhythm with 1st degree AV block.

PR interval 260 ms.

32
Q

Which heart block is shown in the picture?

A

Mobitz I AV block.

Progressive prolongation of PR interval, with a subsequent non-conducted P wave.

Repeating 5:4 conduction ratio of P waves to QRS complexes.

Relatively constant P-P interval despite irregularity of QRS complexes.

33
Q

Which heart block is shown in the picture?

A

Mobitz I AV block.

QRS complexes clustered in groups, separated by non-conducted P waves.

The P:QRS conduction ratio varies from 5:4 to 6:5.

Note the difference in PR interval between the first and last QRS complex of each group.

34
Q

Which heart block is shown in the picture?

A

Mobitz type 2.

Arrows indicate “dropped” QRS complexes (i.e. non-conducted P waves).

The PR interval in the conducted beats remains constant.

The P waves ‘march through’ at a constant rate.

The RR interval surrounding the dropped beat(s) is an exact multiple of the preceding RR interval (e.g. double the preceding RR interval for a single dropped beat, triple for two dropped beats, etc).

35
Q

Which heart block is shown in the picture?

A

Complete Heart Block.

Atrial rate is ~ 85 bpm.

Ventricular rate is ~ 38 bpm.

None of the atrial impulses appear to be conducted to the ventricles.

Rhythm is maintained by a junctional escape rhythm.

Marked inferior ST elevation indicates that the cause is an inferior STEMI.

36
Q

Which heart block is shown in the picture?

A

Complete Heart Block.

Atrial rate is ~ 60 bpm.

Ventricular rate is ~ 27 bpm.

None of the atrial impulses appear to be conducted to the ventricles.

There is a slow ventricular escape rhythm.

Y3 Medicine: Cardiovascular (31 decks)

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