Flashcards in Lecture 9: Cardiac Arrhythmias Deck (17)
Give the definition and cause for tachycardia
* Definition: Fast heart rate (> 100 beats/min)
- 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
Describe endogenously mediated tachycardia
- 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.
Describe pathologically mediated tachycardia
- 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.
Give the definition and cause for bradycardia
Definition: Slow heart rate (
Describe the respiratory type of sinus arrhythmia
- 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
Describe the characteristics of sinoatrial blocks
- 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
What conditions can cause atrioventricular block?
- 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
Describe a First-Degree Incomplete Atrioventricular Block
- 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
Describe a Second-Degree Incomplete Atrioventricular Block
- 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
Describe a Complete Atrioventricular Block
- 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
Describe a partial intraventricular block
- 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
Describe a premature contraction
- 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
Describe paroxysmal tachycardia
- 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.
Describe the basis for fibrillation
- 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.
What three conditions can cause the impulse to travel around in a circle?
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.
Describe Circus Movements
- 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.