Electrocardiography (Wed 18th and Tues 24th)

Question 1

Electrocardiogram

Answer 1

Recording of the electrical activity of the heart made by placing electrodes on the surface of the skin. Only detects changes in electrical activity

Question 2

P Wave represents?

Answer 2

Atrial depolarisation/ contraction. Is the smallest peak.

Question 3

QRS Complex represents?

Answer 3

Ventricular depolarisation/ contraction. Tallest.
Q: the tiny deflection below isoelectric point at start
R: big peak above isoelectric point
S: last deflection below isoelectric point.

Question 4

T wave represents?

Answer 4

Ventricular repolarisation (The repolarization phase usually returns the membrane potential back to the resting membrane potential, resets for another contraction). In between the two.

Question 5

Why is atrial re-polarisation not picked up?

Answer 5

Happens during ventricular depolarisation/ QRS

complex.

Question 6

P-R/Q interval?

Answer 6

Should not be longer than 0.2 seconds clinically. The PR-interval is from the start of the P-wave to the start of the Q wave. It represents the time taken for electrical activity to move between the atria and ventricles.

Question 7

Arrhythmia

Answer 7

Abnormal or irregular heart rhythms

Question 8

Third degree block- a problem with impulse propogation

Answer 8

Impulse not passed to ventricles. So ventricles contract at their own slow rate. No QRS complex in ECG. Implant pacemaker

Question 9

Problems with impulse initiation

Answer 9

1. Atrial Fibrillation: Usually due to abnormal group of cells. Laser remove these cells.
2. Ventricular Fibrillation: Squiggly line. All cells randomly fire, so heart stops being an effective pump. Try defibrillation and shock to depolarise all heart cells, and hopefully SAN take over.

Question 10

S-T segment?

Answer 10

The ST-segment starts at the end of the S-wave and finishes at the start of the T-wave.

The ST segment is an isoelectric line that represents the time between depolarization and repolarization (repolarization refers to the change in membrane potential that returns it to a negative value just after the depolarization phase of an action potential has changed the membrane potential to a positive value. The repolarization phase usually returns the membrane potential back to the resting membrane potential, resets for another contraction) of the ventricles (i.e. contraction).

Question 11

Calculating HR from ECG

Answer 11

Heart rate can be calculated using the following method (if regular):
◦Count the number of large squares present within one R-R interval (qRs- qRs)
◦Divide 300 by this number to calculate the heart rate

If irregular count no. R-R peaks in 6 seconds and times by 10

Question 12

Conditions that can be identified from ECG?

Answer 12

1) In First Degree Heart Block, the impulse conducting from atria to ventricles through the atrioventricular node is delayed and travels slower than normal. This results in a prolonged P-R wave (greater than 0.2 seconds)
2) If p wave is short, this means the impulse is getting from the SAN to the AVN faster somehow. Simply, the P-wave is originating from somewhere closer to the AV node so the conduction takes less time (the SA node is not in a fixed place and some people’s atria are smaller than others!)
◦The atrial impulse is getting to the ventricle by a faster shortcut instead of conducting slowly across the atrial wall.