Conduction Disorders Part One

Question 1

Diagnostic Characteristics of Normal Sinus Rhythm

Answer 1

Normal: rate 60-100 Sinus Rhythm: p waves are regular with normal upright configuration, PR interval is normal, QRS is regular and normal width, P to QRS relationship is 1:1

Question 2

Diagnostic Characteristics of Sinus Bradycardia

Answer 2

slowing of discharge rate s, normal PR, 1:1 AV conduction

Question 3

Cause of Sinus Bradycardia

Answer 3

Physiologic: well conditioned athlete, sleep, vagal stimulation Pharmacologic: digoxin, beta-blockers, calcium channel blockers Pathologic: inferior MI, increased intracranial pressure, hypothyroidism

Question 4

Clinical Significance of Sinus Bradycardia

Answer 4

depends on cause; most causes are benign

Question 5

Diagnostic Characteristics of Sinus Tachycardia

Answer 5

Acceleration of sinus discharge rate (usually 100 - 160) Normal sinus P’s, normal PR, 1:1 AV conduction

Question 6

Cause of Sinus Tachycardia

Answer 6

Acceleration of sinus rate may be: Physiologic: infants/children, exertion, anxiety Pharmacologic: atropine, epinephrine, nicotine, caffeine, cocaine Pathologic: fever, hypoxia, anemia, pulmonary embolus

Question 7

Clinical Significance of Sinus Tachycardia

Answer 7

depends on cause; most causes are benign

Question 8

Diagnostic Characteristics of Sinus Arrhythmia

Answer 8

Variation in sinus node discharge rate Normal P waves, normal PR interval, 1:1 AV conduction

Question 9

Cause of Sinus Arrhythmia

Answer 9

Most common in children, young adults Usually results from change in vagal tone during respiration

Question 10

Clinical Significance of Sinus Arrhythmia

Answer 10

Benign, usually asymptomatic

Question 11

Describe the characteristics of first degree AV block.

Answer 11

Characteristics: Delay in AV conduction; each impulse is conducted to the ventricles but slower than normal; PR interval > 0.2 seconds

Question 12

Describe the causes of first degree AV block.

Answer 12

Cause: increased vagal tone, Digoxin or Digitalis toxicity, inferior MI, myocarditis

Question 13

Describe the clinical significance of first degree AV block.

Answer 13

Clinical Significance: The delay is typically at the level of the AV node; usually benign.

Question 14

Describe the characteristics of second degree AV block, type I (Wenckebach or Mobitz Type I).

Answer 14

Characteristics: progressive prolongation of PR interval until a P wave is blocked or not conducted; cycle usually repeats itself; conduction ratio describes the number of atrial depolarizations to ventricular depolarizations.

Question 15

Describe the causes of second degree AV block, type I (Wenckebach or Mobitz Type I).

Answer 15

Cause: acute inferior MI, Digoxin or Digitalis toxicity, myocarditis, cardiac surgery, rheumatic heart disease, increased parasympathetic tone

Question 16

Describe the clinical significance of second degree AV block, type I (Wenckebach or Mobitz Type I).

Answer 16

Clinical Significance: Occurs at the level of the AV node; may be due to progressive prolongation of AV node refractory period until the AV node is blocked; then the AV node resets itself; generally does not progress to complete heart block.

Question 17

Describe the characteristics of second degree AV block, type II (Mobitz Type II).

Answer 17

Characteristics: constant PR interval, then non-conducted P wave; QRS complexes may be narrow or wide, depending on whether a bundle branch block is present.

Question 18

Describe the causes of second degree AV block, type II (Mobitz Type II).

Answer 18

Cause: acute anteroseptal MI, cardiomyopathy

Question 19

Describe the clinical significance of second degree AV block, type II (Mobitz Type II).

Answer 19

Clinical Significance: block usually occurs in infranodal conduction system; prognosis worse than Mobitz Type I; usually permanent and may progress to complete heart block; pacemaker is likely needed.

Question 20

Describe the characteristics of third degree AV block (complete).

Answer 20

Characteristics: atrial and ventricular depolarizations are independent of each other; P waves and QRS complexes have no consistent relation to each other; PR interval varies; QRS may be either narrow or wide.

Question 21

Describe the causes of third degree AV block (complete).

Answer 21

Cause: usually acute MI; drug effect (digoxin, beta-blocker); may be transient or permanent.

Question 22

Describe the clinical significance of third degree AV block (complete).

Answer 22

Clinical Significance: no atrioventricular conduction; for a nodal block, the ventricular pacemaker is above the bundle of His and produces narrow QRS complexes; for an infranodal block, the ventricular pacemaker is at or below the bundle of His and produces a wide QRS complex (more unstable); pacemaker is required.

Question 23

What are the diagnostic characteristics of PAC’s?

Answer 23

Ectopic P-wave occurs sooner than expected

Ectopic P-wave has a different shape than other P’s

Ectopic P-wave may or may not be conducted through AV node

Question 24

What are common causes of PAC’s

Answer 24

Common in all ages - even without heart disease

Possibly precipitated by stress, fatigue, alcohol, tobacco, caffeine, sympathomimetics

Frequent PAC’s may be seen in chronic lung disease heart disease, or digoxin toxicity

Question 25

What is/are the clinical significance of PAC’s

Answer 25

Usually benign

Occasionally PAC’s may trigger other atrial arrhythmias (i.e., a-fib/flutter)

Question 26

What are the characteristics of SVT/atrial tachycardia?

Answer 26

Regular, rapid atrial rhythmn >160

May not see P waves

QRS are usually normal width

Question 27

What are the common causes of SVT?

Answer 27

Fairly common in 2/1000

Can occur in an otherwise normal heart

No age or sex predisposition

Possibly in association with MI, Rheumatic Heart Disease, Pericarditis, Mitral Valve Prolapse

Due to reentry of depolarization

Question 28

What is the clinical significance of SVT?

Answer 28

SVT is an abrupt onset of tachycardia caused by reentry of electrical impulse in the atria via closed loop of conducting tissue

Usually tolerated in young, healthy people

May cause palpitaitons, lightheadedness, dizziness, and shortness of breath

In elderly patients, or those with pre-existing heart disease, may cause syncope, pulmonary edema and myocardial ischemia

Question 29

What are the characteristics of atrial flutter

Answer 29

Regular atrial rate 250-350 bpm

Sawtooth flutter waves

AV block (usually 2:1 and ventricular rate of 150)

Question 30

What are the causes/clinical significance of Atrial Flutter?

Answer 30

Mostly (60%) due to underlying heart disease, ischemic heart disease, acute MI, hypertension

30% have no cause

Pulmonary embolus

Digoxin toxicity

May be a transitional arrhythmia between sinus rhythm and atrial fibrillation

May result in palpitations, fatigue, dyspnea

Question 31

What are the characteristics of Atrial Fibrillation?

Answer 31

No organized P-waves - has a shimmering baseline

Irregular ventricular rhythm, usually rapid 160-180

Normal QRS complex width

Question 32

What are the causes of Atrial Fibrillation?

Answer 32

**COMMON: **Rheumatic heart disease

Hypertension

Ischemic Heart Disease

Thyrotoxins

Other: COPD

Pulmonary Embolus

Pericarditis

ETOH

Question 33

What is the clinical significance of Atrial Fibrillation?

Answer 33

Multiple areas of atria are continuously depolarizing

No uniform depolarization or contraction of atria - “quivering bag of worms”

Atrial depolarization rate >400 but the refractory period of AV node limits ventricular response

Loss of atrial contraction may lead to heart failure in patients with underlying left ventricular dysfunciton (due to loss of ‘atrial kick’)

Rapid ventricular response may lead to myocardial ischemia, hypotension and shock

Predisposes thrombus formation in atria = risk of stroke/CVA