Electrical activity of the heart

Question 1

How do both sides of the heart beat in synchrony?

Answer 1

Electrically connected via gap junctions which go on to form intercalated discs. Physically connected via desmosomes.

Question 2

What is tetanic contraction?

Answer 2

Sustained contraction of a skeletal muscle evoked when the motor nerve innervating the muscle produces APs at a fast rate.

Question 3

What prevents cardiac muscle from exhibiting titanic contraction?

Answer 3

Cardiac muscle has a long AP (250ms) and therefore a long refractory period.

Question 4

How can calcium regulate heart contractions?

Answer 4

Ca2+ release does not saturate the troponin so can be used to regulate the strength of contraction?

Question 5

How does temperature modulate electrical activity of the heart?

Answer 5

Increases rhythm by around 10 beats per min per degree Celsius.

Question 6

How does Hyperkalemia modulate the activity of the heart?

Answer 6

High plasma K+ causes heart fibrillation and block.

Question 7

How does Hypokalemia modulate the activity of the heart?

Answer 7

Low plasma K+ causes heart fibrillation and block.

Question 8

How does Hypercalcemia modulate the electrical activity of the heart?

Answer 8

High plasma Ca2+ increases the force of contraction and heart rate.

Question 9

How does Hypocalcemia modulate the electrical activity of the heart?

Answer 9

Low plasma Ca2+ decreases heart rate and the force of contraction.

Question 10

Give some examples of drugs that affect the electrical activity of the heart?

Answer 10

Ca2+ channel blockers decrease the force of contraction.

Cardiac glycocides – increase force of contraction

Question 11

On an electrocardiogram what does the P wave correspond to?

Answer 11

Atrial depolarisation.

Question 12

On an ECG what does the QRS complex correspond to?

Answer 12

Ventricular depolarisation.

Question 13

On an ECG what does the T wave correspond to?

Answer 13

Ventricular repolarisation.

Question 14

What is the PR interval?

Answer 14

The time between atrial depolarisation and ventricular depolarisation through the AV node. Usually around 0.12-0.2secs.

Question 15

What is QRS?

Answer 15

The time taken for the whole of the ventricle to depolarise. Usually about 0.08 secs.

Question 16

Why isn’t atrial depolarisation represented on an ECG?

Answer 16

Because it coincides with ventricular depolarisation which involves much more tissue depolarising much faster, masking the signal from atrium.

Question 17

In what direction does the interventricular septum depolarise?

Answer 17

Left to right.

It is the first part of ventricle to depolarise.

Question 18

In what tissue order does the bulk of the ventricle depolarise?

Answer 18

From endocardial to epicardial surface.

Question 19

Why is the T wave positive going?

Answer 19

AP is longer in endocardial cells than epicardial so repolarisation wave runs in opposite direction to depolarisation. (moves away from electrode producing a positive blip.)

Question 20

What extra info do the augmented limb leads give you?

Answer 20

By recording from one limb lead with respect to the other two combined, it gives you 3 other perspectives on events in the heart.

Question 21

What extra info do the precordial leads give you?

Answer 21

Placed around the heart on the chest look at the same information but in the horizontal plane.

Question 22

At what speed should the paper in an ECG run?

Answer 22

25mm per second.

Question 23

How do you work out heart rate from an ECG?

Answer 23

Measure the R-R interval on the rhythm strip and then count how many occur in 60 secs.
Or count the R waves in 30 large squares of paper (=6secs) and then X by 10.

60-100 beats/min is normal

Question 24

What does elevation of the ST segment tell us?

Answer 24

Elevation of the ST segment indicates something serious has gone wrong. Uses to determine the severity of a myocardial infarction. STEMI.