Cardiac action potential and ECG

Question 1

What is autorhythmicity?

Answer 1

When the heart contracts rythmically as a result of APs it generates itself

Question 2

What are the two specialised types of cardiac cell?

Answer 2

1) Contractile cells (99%; normally dont initiate APs)

2) Autorhythmitic cells (do not contract; initiate or conduct APs)

Question 3

Compared to nerve or skeletal muscle cells, cardiac autorythmitic cells do not have a resting membrane potential. What do they display instead?

Answer 3

Pacemaker activity

Question 4

What is the pacemaker potential?

Answer 4

An autorythmitic cell membranes slow drift to threshold

Question 5

Autorythmitic cells cyclically initiate APs which then spread through the heart to trigger contraction without what?

Answer 5

Any nerve stimulation

Question 6

Specialised non-contractile cells that demonstarte autorythmicity are located at four specific sites. What are these sites?

Answer 6

1) The sinoatrial node (SV node)
2) The atrioventricular node (AV node)
3) The bundle of His (atrioventricular bundle)
4) Purkinje fibres

Question 7

What is the normal pacemaker of the heart?

Answer 7

The sinoatrial node

Question 8

What does it mean by the cells of the heart are linked electrically?

Answer 8

The rate of the fastest will be the rate of all

Question 9

The AV node forms the only conducting pathway between what two components?

Answer 9

The atrial muscle and Bundle of His and hence the ventricles

Question 10

When the AV node introduces a considerable delay to spread of excitation, what does this allow for?

Answer 10

Allows time for blood to move from the atria to the ventricles

Question 11

What do AV nodes have if impulses from the SA node fail to reach them?

Answer 11

Well developed latent powers of rythmciity and can take over pacemaking impulses

Question 12

The cycle of voltage change across cardiac myocytes occurs in 5 distinct phases. What are these phases?

Answer 12

1) Depolarisation
2) Early ropolarisition
3) Plateau phase
4) Late depolarisation
5) Resting potential

Question 13

SA nodes fire APs at a rate of what?

Answer 13

70-80 APs per min (at rest)

Question 14

Neighbouring cardiac muscle cells are linked by the presence of gap junctions which allow what?

Answer 14

Allow the rapid spread of APs from cell
e.g an AP originating in the SA node first spreads throughout both atria, stimulating the simultaneous contraction of R & L atrial muscle

Question 15

What is the ECG a record of?

Answer 15

The overall spread of electrical activity through the heart during polarisation and repolarisition.
It is not a recording of a single AP in a single cell at a single point in time

Question 16

Electrical currents generated by cardiac muscle are conducted through what?

Answer 16

Body fluids

Question 17

What do the orientation of the ECG electrodes determine?

Answer 17

Whether an upward or downward deflection is determined by the way the electrodes are orientated with respect to the current flow in the heart

Question 18

How many conventional electrode systems, or leads do ECG records routinely consist of? And what id the specific arrangement of each pair of connections called?

Answer 18

12

Lead

Question 19

The 12 different leads each record electrical activity in the heart from different locations. What are these locations?

Answer 19

6 different arrangements from the limbs

6 different chest leads at various sites around the heart

Question 20

What do the 6 limb leads include?

Answer 20

I, II, III, aVR, aVL and aVF

Question 21

Why are leads I, II and III called bipolar?

Answer 21

Two recording electrodes are used

Question 22

Where is the electrode that serves as a ground and not as a recording electrode positioned?

Answer 22

On the right leg

Question 23

Why are leads aVR, aVL and aVF called unipolar leads?

Answer 23

Even though two electrodes are used, only the actual potential under one electrode ], the exploring electrode is recorded. The other electrode is set at a zero potential and serves as a neutral refernece point

Question 24

What are the 6 chest leads called and are they unipolar or bipolar?

Answer 24

V1 to V6

Unipolar

Question 25

Where is lead I, II and III placed?

Answer 25

Lead I - Right arm and Left arm
Lead II- Right arm and Left leg
Lead III- Left arm and left leg

Question 26

A normal ECG has 3 distinct waveforms; The P wave, the QRS complex and the T wave. What does each wave represent?

Answer 26

P wave- Atrial depolarisation
QRS- Ventricular depolarisation
T wave- Ventricular repolarisiton

Question 27

True or False; No wave is recorded for SA nodal depolarisation?

Answer 27

True

Question 28

Why is the P wave smaller than the QRS complex?

Answer 28

The atria have a much smaller muscle mass than the ventricles

Question 29

What 3 points in time when no current flow is taking place in the heart musculature, the ECG remains at baseline?

Answer 29

1) During AV nodal delay
2) When the ventricles are completely depolarised and the cardiac contractile cells are undergoing the plateua phase of their AP
3) When the heart muscle is completely depolarised and at rest and ventricular filling is taking place

Question 30

When are waves recorded?

Answer 30

Only when the potential is changing across cell membranes

Question 31

The size of the potential depends omn what?

Answer 31

Depends on the size of the extracellular current