Cardiac Excitation

Question 1

What two properties allow normal, ordered excitation of the heart?

Answer 1

Ability of the heart to initiate its own beat - automaticity

Regularity of pacemaking activity - Rhythmicity

Question 2

What is the sinoatrial node?

Answer 2

Initiator of electrical activation of the heart, pacemaker

APs originating here spread to the surrounding atrial myocytes and reach the AV node

Question 3

What are the atrial internodal tracts?

Answer 3

Connect SA to the AV node: anterior, middle (Wenchkebach), and posterior (Thorel)

Connect right and left atria: Bachmann bundle - branch of the anterior internodal tract

Question 4

What occurs at the AV node?

Answer 4

Delay in conduction

Important to allow complete atrial depolarization, contraction, and emptying of atrial blood

Question 5

What is the Bundle of His?

Answer 5

Continuous with the AV node, gives off left bundle branch and continues as the right bundle branch

Question 6

What are the purkinje fibers?

Answer 6

Carry signal from the right and left bundle branches throughout the ventricular myocardium

Once the impulse reaches the ends of the purkinje system it is transmitted through the muscle fibers themselves

Question 7

How does sympathetic nervous activity raise heart rate?

Answer 7

NE activates B1 receptors

Produces an increase in funny current, therefore increasing the rate of phase 4 depolarization

SA node is depolarized more frequently and fires more APs per time

Question 8

How does parasympathetic nervous activity decrease heart rate?

Answer 8

ACh activates M2 receptors in the SA node

Decreases funny current, decreasing the rate of phase 4 depolarization

Alsow enhances K efflux, allowing for hyperpolarization of the max diastolic potential

Question 9

How does temp affect the rate of discharge by the SA node?

Answer 9

Increases discharge frequency

Question 10

How does digitalis influence the heart?

Answer 10

Strenghten contractility

Can also decrease AV nodal conduction velocity

Question 11

What are the latent pacemakers present in the conduction system?

Answer 11

AV node and bundle of HIS have intrinsic firing rates of 40-60 bpm

Pukinje system has rates of 15-40 bpm

Question 12

What is the significance of the latent pacemaker sites?

Answer 12

These sites may initiate impulses and take over for the SA node when it slows or fails

Or if conduction between the SA node and AV node is prevented

Question 13

What is an escape beat?

Answer 13

An impulse initiated by a latent pacemaker

Question 14

What is an escape rhythm?

Answer 14

A continued series of escape beats

Question 15

What is junctional escape?

Answer 15

Beats arise from the AV node or proximal bundle of His

E.g. moderate parasymp. stimulation slows the SA rate and allows pacemaker shift

Question 16

What is ventricular escape?

Answer 16

Beats arise from a more distal point in the conduction system

E.g. Very strong parasymp. stimulation suppresses excitability at both the SA and AV nodes

Question 17

What is an ectopic beat?

Answer 17

A latent pacemaker develops an intrinsic rate of depolarization faster than that of the SA node

A sequence is called an ectopic rhythm

Question 18

What are some causes of ectopic beats?

Answer 18

High catecholamine concentration

Hypoxemia, ischemia, electrolyte disturbances and certain drug toxicities

Question 19

What is abnormal automaticity?

Answer 19

Cardiac tissue injury can lead to myocardial cells outside the conduction system acquiring automaticity

Question 20

What are early afterdepolarizations?

Answer 20

Changes of the membrane potential in the positive direction that interrupt normal repolarization

more likely to develop in conditions that prolong the action potential duration

Can lead to tachyarrhythmia

Question 21

What are delayed afterdepolarizations?

Answer 21

Appear shortly after repolalrization is complete

Commonly develop in states of high intracellular calcium

Question 22

What is conduction block?

Answer 22

When an impulse can’t propagate because it encounters a region of the hear that is electrically unexcitable

Can be transient or permanent

Question 23

What results from AV block?

Answer 23

Prevents impusle propagation from the SA node to distal sites

Removes overdrive suppression, resulting in the emergence of escape beats from the distal sites

Question 24

What is Reentry?

Answer 24

Reentry occurs when a conduction pathway is stimulated prematurely be a previously conducted action potential leading to a rapid, cyclical reactivation

Caused by a unidirectional block

Question 25

What is the difference between anatomical and functional reentry?

Answer 25

Anatomical reentry occurs of an anatomically fixed path

Functional reentry does not require a fixed path, occurs in electrically heterogenous medium

Question 26

What are some examples of anatomical reentry?

Answer 26

AV Nodal Reentrant tachycardia (AVNRT)

Atrioventricular Reentrant tachycardia (AVRT) (includes accessory pathway)

Atrial flutter

Question 27

What are some examples of Functional reentry?

Answer 27

Polymorphic ventricular tachycardia

Atrial fibrillation

Ventricular fibrillation