Lecture 9: Cardiac Arrhythmias

Question 1

Give the definition and cause for tachycardia

Answer 1

• Definition: Fast heart rate (> 100 beats/min)
• Causes:
• Increased body temperature:
• • Heart rate increases about 10 beats per °F or 18 beats per °C.
• Stimulation of the heart by sympathetic nerves:
• • Due to loss of blood and/or state of shock
• Toxic conditions of the heart:
• • Results in weakening of myocardium
• See Figure 13-1 (Slide 5) for ECG depiction

Question 2

Describe endogenously mediated tachycardia

Answer 2

• Exercise
• Heart rate increases
• Cardiac output increases
• Filling time is reduced but stroke-volume does not fall…why? ->
• Sympathetic stimulation to the heart increases contractility and helps to maintain stroke-volume.
• Systolic interval is reduced allowing for more diastolic filling time.
• Sympathetic stimulation and skeletal muscle pump increase venous return to help maintain ventricular filling.

Question 3

Describe pathologically mediated tachycardia

Answer 3

• Heart rate increases
• Cardiac output decreases WHY?
• Mean atrial pressure decreases and activates the sympathetic nervous system, which occurs after the fact and is unable to compensate.
• There is no muscle pump to increase venous return.

Question 4

Give the definition and cause for bradycardia

Answer 4

Definition: Slow heart rate (

Question 5

Describe the respiratory type of sinus arrhythmia

Answer 5

Spillover signals:

• From medullary respiratory center into vasomotor center during inspiratory and expiratory respiratory cycles
• These signals alternately increase and decrease number of impulses transmitted through sympathetic and vagus nerves to the heart.
• See Figure 13-3 on slide 9

Question 6

Describe the characteristics of sinoatrial blocks

Answer 6

• Sudden cessation of P waves
• Resultant standstill of atria
• Ventricles pick up a new rhythm, usually originating in the AV node.
• Rate of QRS is slowed but not otherwise altered.
• See Figure 13-4 (Slide 11) for ECG

Question 7

What conditions can cause atrioventricular block?

Answer 7

• Ischemia of AV node or AV bundle fibers through coronary insufficiency
• Compression of AV bundle by scar tissue or calcified portions of the heart
• Inflammation of the AV node or bundle
• Extreme stimulation of the heart by the vagus nerves

Question 8

Describe a First-Degree Incomplete Atrioventricular Block

Answer 8

• Normal P-R time interval = 0.16 sec.
• Increases in length with slower heartbeat and decreases with faster heartbeat.
• When P-R interval increases to greater than 0.20 seconds, the P-R interval is prolonged and patient has a first degree incomplete heart block.
• See Slide 14

Question 9

Describe a Second-Degree Incomplete Atrioventricular Block

Answer 9

• P-R time interval increases to 0.25 to 0.45 sec.
• Atrial P wave is present but QRS-T wave may be missing, resulting in dropped beats of the ventricle
• 2:1 rhythm or other variations may develop.
• See Slide 15

Question 10

Describe a Complete Atrioventricular Block

Answer 10

• Ventricles establish their own signal (usually AV node).
• No relation between the rate of the P waves and the rate of the QRS-T complexes.
• Duration of the block is highly variable (seconds to weeks).
• After AV conduction ceases, ventricles may not start beating on their own for 5 to 30 seconds.
• Resumption of the ventricular beat (= ventricular escape) may be due to parts of the Purkinje system acting as an ectopic pacemaker.
• After a complete block occurs, patients will often faint due to lack of blood to the brain until the ventricles escape.
• These periodic fainting spells are referred to as Stokes-Adams syndrome.
• See Figure 13-7

Question 11

Describe a partial intraventricular block

Answer 11

• Referred to as “electrical alternans.”
• This term refers to an alternation in the amplitude of P waves, QRS complexes, or T waves.
• See Slide 18

Question 12

Describe a premature contraction

Answer 12

• As the name suggests, premature contractions occur before they should.
• Most are the result of ectopic foci:
• Local ischemic areas
• Calcified plaques
• Irritation of the conduction system or nodes
• See Slide 19

Question 13

Describe paroxysmal tachycardia

Answer 13

• Heart becomes rapid in paroxysms:
• Paroxysm begins suddenly and lasts for a few seconds, minutes, hours, or longer
• Paroxysm ends suddenly
  Refer to Figures 13-13 and 13-14. (Slide 20)
• Pacemaker of the heart instantly shifts back to the sinus node.

Question 14

Describe the basis for fibrillation

Answer 14

• The normal ventricular depolarization waves die out because the heart muscle that has already contracted is in the refractory period and cannot respond to being stimulated by the existing depolarization waves.
• Fibrillation is the twitching (usually slow) of individual muscle fibers in the atria or ventricles and also in recently denervated skeletal muscle fibers.
• Fibrillation occurs as a result of circus movements.
• (Refer to Figure 13-15 on Slide 25-26)
• Note: Second beat driven by an ectopic depolarization beat would occur. Don’t currently know what that means but hopefully I’ll get it after reading. Which you Will do.

Question 15

What three conditions can cause the impulse to travel around in a circle?

Answer 15

1. The pathway around the circle is too long.
2. The length of the pathway remains constant but the velocity of the conduction slows down.
3. The refractory period of the muscle might become greatly shortened.

Question 16

Describe Circus Movements

Answer 16

• Occur when the pathway around the circle (ventricles) is too long.
• • Normally, by the time the impulse returns to its starting point, the heart muscle will be in the refractory period.
• • If this pathway is too long, often because of a dilated heart, the impulse takes longer to get back to its starting point, and the muscle is no longer in its refractory period and can undergo another depolarization.
• Occur when the length of the pathway remains normal, but the conduction velocity of the impulse is slowed down.
• • Usually occurs because of the blockage of the Purkinje system, ischemia, or high potassium levels.
• Occur when the refractory period of the muscle is shortened.
• • May occur in response to drugs such as epinephrine.
• • May occur after repetitive electrical stimulation.

Question 17

Describe Atrial Fibrillation

Answer 17

• The atrial muscle fibers are separated from the ventricular muscle fibers by the cardiac fibrous skeleton.
• Ventricular and atrial fibrillation may occur either separately, or similtaneously from one another.
• Causes of atrial fibrillation:
  – Enlargement of the atria (i.e., due to valve lesions)
  – Inadequate emptying of the ventricles causing blood to back up into the atria
  Note: The diagram on slide 28 is missing any definable P-wave. So the ventricle works fine, the atria don’t. This isn’t necessarily life threatening since the ventricle sort of takes over.