850-Arrhythmias (Section 1)

Question 1

Normal Conduction

Answer 1

• The conduction of the heart normally begins at the SA-node, located in the right atria
• The SA-node initializes depolarization of atrial muscle cells
• The AV-Node, located between the atria and ventricles, is activated and, after a delay, initiates the conduction impulse through the ventricles
• The Purkinje Fibers carry the signal through the septum to the apex and the rest of the ventricular mass

Question 2

Interval P-R

-Normal Ranges (per ms) & Pathology

Answer 2

1. Normal range (per ms): 120-200

2. Pathology: Long-heart block (drugs, electrolytes)

Question 3

Interval QRS

-Normal Ranges (per ms) & Pathology

Answer 3

1. Normal range: 80-120

2. Pathology: Long-conduction abnormalities e.g. bundle branch block

Question 4

Interval QT

-Normal Ranges (per ms) & Pathology

Answer 4

1. Normal range: (varies with heart value) >450 can lead to ventricular tachycardia
2. Pathology: Long-repolarisation abnormalities, ion channel-pathies e.g. long QT syndrome

Question 5

PR Interval

Answer 5

• A time interval that represent AV-node conduction (time impulse travels through the AV-node)
• Drugs are often used to slow impulses through the AV node.
• Drugs and disease states may increase time interval or action potential conduction that may lead to blockage of the impulse
• Need to monitor PR interval with certain drugs

Question 6

QT and QTc interval

Answer 6

-QT interval represents time for ventricular depolarization and repolarization.
-QT interval is rate dependent (i.e., depends on heart rate)
-Should correct with Bazett formula (corrects for HR):
QTc interval = [(QT)/(square root R-R)]

> > > (square root of the RR interval represents heart rate. Most ECG’s that are done will have the QTc calculated for you)«<

• In general, prolonged QTc interval is considered to be >0.45 secs (450 msec)
• NEED TO MONITOR FOR DRUGS THAT PROLONG QT INTERVALS TO PREVENT PROARRHYTHMIC EVENTS.

Question 7

Action potential (blank 1)&raquo_space; Electrical Impulses&raquo_space; Heart Beat&raquo_space; ECG

Answer 7

Blank 1: usually generated from SA node

Question 8

Phase 0

Answer 8

depolarization

Question 9

Phase 1

Answer 9

repolarization

Question 10

Phase 2

Answer 10

plateau phase

Question 11

Phase 3

Answer 11

rapid depolarization phase

Question 12

Phase 4

Answer 12

depolarization

Question 13

Phase 0 (Na+ Dependent & Ca++ Dependent):

Answer 13

• Na+ Dependent: Atrium, Ventricles
• Ca++ Dependent: SA and AV node

NEED TO KNOW (and how it relates to anti arrhythmic therapy)

Question 14

Impulse Generation “Leak”

Answer 14

• During Phase 4, a gradual slope (caused by background K+ and Na+ currents) in the potential leads to activation of voltage-gated sodium channels –THRESHOLD POTENTIAL and Phase 0!!! – electrical energy and start of one heart beat.
• Altering the characteristics of this slope or the sensitivity of sodium channels can alter cardiac pacing (i.e, heart rate)

Question 15

Slope of Phase 4

Answer 15

• Increase slope, increase automaticity (heart rate)
• Sympathetic activity increases slope of phase 4 and thus HR.
• Cholinergic activity decreases slope of phase 4 and thus HR
• Some antiarrhythmic drugs can change slope of phase 4 and change heart rate

Question 16

Impulse Generation

Answer 16

• SA node: 60 to 100 beats/minute (most leaky ion currents thus SA node is the primary pacemaker cell in the heart)
• AV node: 30 to 50 beats/minute
• Ventricles: < 30 beats/minute

Question 1: Which one has the highest Phase 4 slope?
Question 2: If the SA node does not generate an electrical impulse what would be the heart rate?

Question 17

Effective or Absolute, (phase 2) (ERP) is the

Answer 17

• Longest amount of time when class cannot be depolarized again. The longest input that fails to conduct
  1. Atria 200-270 msecs
  2. Ventricles 200-270 msecs
  3. AV node 280-450 msecs

Question 18

Relative Refractory Periods (RRP)

Answer 18

Relative (phase 3, 4) (RRP)
-Time when cells can be depolarized again with a strong enough stimulus. The longest premature coupling interval at which delay in conduction (prolongation of conduction) occurs