Electrical function and ECG

Question 1

Where do action potentials in the heart originate from?

Answer 1

Nodes

Question 2

How is the action potential in cardiac muscle is propagated from cell to cell?

Answer 2

• branching of the cells

- intercalated discs which contain gap junctions

Question 3

What is automaticity?

Answer 3

Spontaneous depolarisation generating an action potential

Question 4

Where are the fastest pacemaker cells located?

Answer 4

sinoatrial node

Question 5

What are ectopic pacemakers and what is their function?

Answer 5

Other cells in the myocardium that develop automaticity after injury
– can be responsible for some clinically serious abnormal rhythms

Question 6

How do action potentials occur in the skeletal muscle surrounding the heart?

Answer 6

• the action potential is initiated by somatic motor neurones
• the skeletal muscle cells are insulated from each other
• each needs to receive neurological stimulation

Question 7

How does the ANS innervate the heart/for what reasons?

Answer 7

Affects the frequency and speed of action potentials to modulate the heartbeat

Question 8

Which type of muscle cell had a longer action potential duration?

Answer 8

Cardiac muscle cells

Question 9

Which ion channel in cardiac muscle causes the prolonged plateau phase?

Answer 9

Calcium ion channels

Question 10

What is the purpose of the refractory period?

Answer 10

Ensures a new action potential cannot be initiated before the previous one is completed.
Makes sure each contraction of the heart is distinct

Question 11

The time during which a new AP cannot be generated is called the … ?

Answer 11

Absolute refractory period

Question 12

What is temporal summation and why does it not occur in cardiac muscle?

Answer 12

• multiple action potentials within the space of one muscle contraction
• cardiac muscle needs to relax to allow for diastolic filling

Question 13

Why are atrial action potentials are shorter than ventricular action potentials?

Answer 13

• The slow Ca++ channels stay open for less time
• K+ channels close for less time
• shorter refractory period

Question 14

How is a pacemaker potential different to an action potential?

Answer 14

• slow rise to threshold level following an action potential
• no fast Na+ channels, so no sudden influx of sodium, have pacemaker Na channels instead
• K+ channels close
• Ca++ channels open
• slower depolarisation

Question 15

Describe the calcium excitation-contraction coupling that occurs in cardiac muscle

Answer 15

• extracellular Ca++ enters through the voltage-gated Ca++ channels of the T-tubule
• This stimulates calcium induced calcium release from the sarcoplasmic reticulum

Question 16

What does the ECG record and measure?

Answer 16

• Records the electrical activity of the heart by measuring the voltage

Question 17

What does the ECG plot a graph of?

Answer 17

Voltage against time

Question 18

If an action potential is travelling towards the +ve electrode what deflection will be shown on the graph?

Answer 18

An upwards deflection

Question 19

What do the height and width of a deflection reflect?

Answer 19

Height =  the size of the electrical potential difference perceived by the electrodes
Width =  the time taken for the potential difference to return to zero

Question 20

What do the following waves represent:
P wave?
Q wave?
R wave?
S wave?
T wave?

Answer 20

P = atrial depolarisation
Q = early ventricular depolarisation 
R = Full ventricular depolarisation 
S = late ventricular depolarisation
T = ventricular repolarisation

Question 21

What is occuring due to the P-R interval when the ECG graph returns to baseline?

Answer 21

Represents the delay in conduction which is due to the action potential travelling through the AV node

Question 22

What should the ratio of P:QRS be?

Answer 22

1:1

Question 23

What do leads 1-3 compare?

Answer 23

Lead I - compares the voltage at the left forelimb to the right forelimb
Lead II - compares the left hindlimb to the right forelimb
Lead III - compares the left hindlimb with the left forelimb

Question 24

What is the function of the augmented unipolar leads – AVR, AVL and AVF?

Answer 24

These measure the electrical potential between a positive electrode at one limb with the average voltage from the other two

Question 25

What does each augmented unipolar leads – AVR, AVL and AVF compare?

Answer 25

AVR = right forelimb to the average of the left forelimb and left hindlimb
AVL = left forelimb to the average of the right forelimb and left hindlimb
AVF (foot) = left hindlimb to the average of the two forelimbs

Question 26

How can heart rate be measured from an ECG?

Answer 26

• Measuring the R-R interval and dividing 60 (one minute) by this number
• Counting the number of PQRST complexes in 3 or 6 seconds and multiplying that by 20 or 10

Question 27

What is the normal heart rate for

1. dogs?
2. cats?

Answer 27

Dog =  between 70bpm-160bpm
Cat =  between 120-220bpm

Question 28

What abnormality does a tall p wave indicate?

Answer 28

Right atrial enlargement

Question 29

What abnormality does a wide P wave indicate?

Answer 29

Left atrial enlargement

Question 30

What is myocardial ischaemia?

Answer 30

Blood supply to the heart is inturrupted

Question 31

Define the following:

1. bradycardia

2. tachycardia

Answer 31

1. abnormally slow heart rate

2. abnormally fast heart rate