Ganong 24e chapter 29 - Electrical Activity of the Heart (1)

Question 1

Name the components of the cardiac conduction system, in order of the pathway.

Answer 1

Sino-atrial node, Internodal atrial pathways (Bachmann’s, Wenckebach’s, Thorel’s), Atrioventricular node, Bundle of His, Left and Right Bundle Branches, Purkinje system

Question 2

Why is the SA node the normal origin of the impulse?

Answer 2

Because the SA node normally discharges the most rapidly.

Question 3

Where is the SA node located?

Answer 3

At the junction of the SVC and the Right Atrium

Question 4

What is the resting membrane potential of a ventricular myocyte?

Answer 4

Approx -90mV to -80mV

Question 5

What is the resting membrane potential of a cardiac pacemaker cell?

Answer 5

Approx -60mV (also called the “pacemaker potential”), but remember the membrane is undergoing slow depolarization at this point (Phase 4), with the inward sodium “funny current”.

Question 6

In an ECG, what are the bipolar leads called?

Answer 6

I, II and III

Question 7

What is the main ion movement that causes depolarization in ventricular myocytes?

Answer 7

Sodium influx.

Question 8

What is the main ion movement that causes the “funny current” of the pacemaker potential?

Answer 8

Gradual sodium influx.

Question 9

What is the main ion movement that causes depolarization in cardiac pacemaker cells?

Answer 9

Calcium influx.

Question 10

Explain the order in which different regions of the heart depolarize as the impulse is spread.

Answer 10

Atria –> AV node –> down the interventricular septum –> apex of the heart –> ventricular walls, from endocardial surface to epicardial surface

Question 11

What is Einthoven’s Triangle?

Answer 11

The triangle made up of the three points where the limb electrodes are attached: left arm, right arm, left leg. The heart is supposed to be in the centre of Einthoven’s triangle.

Question 12

What does axis deviation suggest about the patient’s heart pathophysiology?

Answer 12

Right axis deviation suggests RV hypertrophy. Left axis deviation may represent LV hypertrophy.

Question 13

In 3rd degree heart block where the block is in the AV-node, the remaining nodal tissue becomes the pacemaker. What is the rate of the idioventricular rhythm in this case, on average?

Answer 13

45 bpm

Question 14

In 3rd degree heart block where the block is in the Bundle of His (infranodal block), the ventricular pacemaker becomes activated. What is the rate, on average?

Answer 14

35 bpm

Question 15

What is a Stokes-Adams syndrome?

Answer 15

This occurs in infranodal 3rd degree heart block in individuals with an unusually low ventricular escape rate of about 15 bpm. There are periods of asystole that may last one minute, causing cerebral ischaemia, dizziness, and loss of consciousness.

Question 16

What is first-degree heart block?

Answer 16

The conduction between the atria and ventricles is slowed, but not completely interrupted, causing a prolonged PR interval.

Question 17

What is second-degree heart block?

Answer 17

Conduction between the atria and the ventricles is impaired such that not all atrial impulses are conducted to the ventricles. 2:1 block (or 3:1 block) means that a ventricular beat follows every second (or every third) atrial beat.

Question 18

What is meant by a circus movement? Give an example.

Answer 18

This means there is a re-entrant arrhythmia. Examples include atrial flutter, Wolff-Parkinson-White syndrome…

Question 19

What is the characteristic feature of the ECG in Torsade de pointes?

Answer 19

There is ventricular tachycardia with variable QRS morphology from beat to beat.

Question 20

What happens in Wolff-Parkinson-White syndrome?

Answer 20

These individuals have an additional aberrant muscular or nodal tissue connection (bundle of Kent) between the atria and the ventricles. There is accelerated AV conduction, (because the bundle of Kent transmits the impulse faster than the AV node does), and a circus movement is established. They are at risk of acute AF and SVT.

Question 21

What happens to the ECG when the plasma K rises to 7.0 ?

Answer 21

Tall, slender T-waves, but otherwise normal.

Question 22

What happens to the ECG when the plasma K rises to 8.5 and beyond?

Answer 22

Atria become paralysed and there is no evidence of atrial activity on ECG. QRS complex broadens. T-waves are tall and slender. Further elevation of plasma K level may result in VT or VF.

Question 23

What happens to the ECG in hypokalaemia of 3.5? And if it drops lower?

Answer 23

ST-depression. U-wave becomes evident, following the T-wave. If hypokalaemia worsens, there can be prolonged PR and T-wave inversion, with persistence of the U-wave. QT may appear prolonged if the U-wave merges with the T-wave, but the true QT is normal.

Question 24

What does the T-wave represent in a normal ECG?

Answer 24

Ventricular repolarization.

Question 25

Explain what is happening in this diagram.

Answer 25

Phase 0: Na⁺ influx. Phase 1: Na⁺ channels close, and there is slight repolarization due to K⁺ leakage, but L-type Ca²⁺ channels are open, allowing Ca²⁺ into the cell, keeping the membrane depolarized across the plateau phase (Phase 2). Phase 3: Ca²⁺ channels close, and there is K⁺ efflux, repolarizing the membrane. Phase 4: Resting membrane potential.

Question 26

Explain what is happening in this diagram.

Answer 26

Phase 4: Pacemaker potential, also called prepotential, there is a slow Na⁺ entry into the cell called the funny current. Phase 0: Depolarization, because voltage-gated Ca²⁺ channels open, causing Ca²⁺ influx. Phase 3: Repolarization with K⁺ efflux.

Question 27

What is the range of a normal axis?

Answer 27

-30 to 110 degrees

Question 28

With a heart rate of 75, what is the duration of:

1. the cardiac cycle?
2. action potential?
3. absolute refractory period?
4. relative refractory period?

Answer 28

1. 0.8 sec
2. 0.25 sec
3. 0.2 sec
4. 0.05 sec