Electrocardiography

Bradyarrhythmias and blocks

384

Tachyarrhythmias > 100 beats/min

Supraventricular tachyarrhythmias

Ventricular tachyarrhythmias

Bradyarrhythmias < 60 beats/min

385

Sinus bradycardia

Physiological causes Sleep, increase in vagal tone (sport, sinus carotis, eyeball

pressure)

Pathological causes

sinoatrial and AV node), obstructive jaundice, increased

intracranial pressure, sinus node dysfunction, hypothermia,

hypothyroidism

Drugs: digoxin (SA toxicity), amiodarone -

antagonists, morphine, quinine, sedato- hypnotics

386

Sinus bradycardia (~36 beats/min) with prominent U waves. Please determine QTc

387

Sinoatrial (SA) block

SA node depolarization is present, but conduction of impulses

to atrial tissue is impaired or blocked 1st-degree SA block

Type I 2nd-degree SA block (SA Wenckebach or SA Mobitz-type-

1)

Type II 2nd-degree SA block (SA Mobitz-Type-2)

3rd-degree SA block

388

Sinus arrest/pause

Transient sinus pause for seconds to minutes (P waves

disappear). The pause usually triggers escape activity in lower

pacemakers (e.g., atrial, junctional or ventricular) Escape activity: atrial, junctional or ventricular escape beats or

rhythm Junctional: narrow QRS and 40-60 beats/min

Ventricular: wide QRS and 15-40 beats/min

Long pauses cause dizziness and syncope

389

390

Sinus node dysfunction (formerly: sick sinus syndrome)

Sinoatrial (SA) dysfunction affects mainly the elderly and

develops as a result of chronic dysfunction of the sinoatrial

node

Patients are usually present with lightheadedness and/or

syncope or dyspnea, and palpitations. Some degree of AV node dysfunction is seen in ~ 50% of people

with sinus node dysfunction

Sinus node dysfunction can appear in different forms Inappropriate sinus bradycardia, sinoatrial exit block, sinus pause

or arrest, junctional or ventricular escape beats or rhythm,

tachycardia bardycardia syndrome (atrial tachycardia alternating

with bradycardia), chronic atrial fibrillation or atrial flutter

391

First-degree AV block

Consistently prolonged PR (PQ) interval: > 0.18 sec (child) >

0.2 sec (adult), > 0.22 (elderly)

Every P wave is followed by QRS (not a dysrhythmia)

First-degree AV block may be a normal finding in individuals

with no history of cardiac disease, especially in athletes

Other causes Increased vagal tone

Ischemia or injury to the AV node or junction (acute inferior wall

MI)

Infiltrative diseases: amyloidosis, sarcoidosis, hemochromatosis

Drugs: digitalis, -receptor antagonists

Inflammatory diseases: rheumatic heart disease, diphtheria,

Hyperkalemia

392

PR interval of 580 ms duration in the presence of a heart rate of 45 beats/min.

PR interval of 360 ms duration in the presence of a heart rate of 62 beats/min.

393

Second-degree AV block Mobitz-I or Wenckebach

PR (PQ) interval is increased gradually, then one QRS is

missing and then the cycle starts again (Wenckebach

periodicity) The RR interval of the pause is less than the two

preceding RR intervals, and the RR interval after the pause is

greater than the RR interval before the pause.

Block: at the level of AV node and often associated with AV

nodal ischemia secondary to occlusion of the right coronary

artery (acute inferior wall MI) or due to increased

parasympathetic tone or the effects of medication

Risk of progression to this type of AV block to complete AV

block is low

394

RR ~ 60 beats/min; PR interval 280, 380, 440 ms;

RR ~ 80 beats/min; PR interval 120, 260 ms;

395

Second-degree AV block Mobitz-II

Not as frequent as Mobitz type-I, but more serious and

frequently progresses to complete AV block (prophylactic

pacemaker)

PR interval is constant (normal or increased); P wave is not

followed by QRS and QRS may show wide morphology (bundle

branch block)

If ventricular rate is Within normal limits the patient may be asymptomatic

More commonly significantly slowed (symptoms: due to

decreased cardiac output)

Block: below the AV node at the bundle of His or bundle

branches (more commonly)

396

Causes: disease of the left coronary artery or an anterior

myocardial infarction or acute myocarditis or other types of

organic heart disease

Second-degree AV block Mobitz-II 4:3

Regular sinus rhythm at a rate of 56 beats/min (bradycardia), constant PR interval of 188 ms, and nonconducted P

waves.

Wider s wawes may indicate incomplete right bundle branch block

397

Mobitz type II second-degree atrioventricular block (3:2, 2:1) with narrow QRS complex. The patient had

recurrent episodes of syncope. Because of the narrow QRS complex, the block is probably at the level of

the His bundle

398

Mobitz type II second-degree atrioventricular block. There is 3:2 conduction. Normal PR. The

QRS complex has left bundle branch block morphology.

399

Third-degree AV block

The atria and ventricles beat independently of each other (atrio-

ventricular dissociation) Cannon a waves: contraction of the

right atrium against a closed tricuspid valve (jugular pulse)

Impulses are blocked at the AV node, bundle of His, or bundle

branches. If the QRS is narrow, block is higher in the junction

Rate: 40-60 bpm If the patient is symptomatic: atropine and/or transcutaneous pacing

If the QRS is wide (> 0.1 sec), block is very low in His bundle or

confined to bundle branches Potentially lethal (Stokes-Adams-Morgagni syndrome: circulatory

collapse [no pulse and blood pressure], unconsciousness,

convulsions, cyanosis)

If the patient is symptomatic: transcutaneous pacing should be instituted

and permanent transvenous pacemaker should be inserted

Causes Acute MI

Open heart surgery (valve replacement)

Congenital or acquired abnormality of the intraventricular

conducting system

Cardiomyopathy (CMP)

400

401

Atrio-ventricular dissociation: atrial tachycardia (~115/min), ventricular bradycardia (~36/min), QRS is narrow

Atrio-ventricular dissociation: atrial tachycardia (~83/min), ventricular bradycardia (~50/min), QRS is wide

402

AV dissociation Atrial rhythm (~72 beats/min); Bidirectional ventricular tachycardia (~160-170 beats/min)

V1

403

AV blocks (summary)

Block degree Site Result

1st degree AV node

Impulses from the AV node to the ventricles are

consistently delayed. Every P wave is followed by

QRS complex

2nd degree type I

Mobitz-I or

Wenckebach

AV node

Gradual increase in PR (PQ) interval followed by one

missing QRS and then the cycle starts again

(Wenckebach periodicity)

2nd degree type II

Mobitz-II

bundle of His

or bundle

branches

PR interval is constant (normal or increased); P wave

is not followed by QRS; QRS may show wide

morphology (bundle branch block)

3rd degree

AV node or

bundle of

His

AV junction does not conduct any impulses between

the atria and ventricles (atrio-ventricular dissociation)

Potentially lethal (Stokes-Adams-Morgagni syndrome)

404