P: Cardiac muscle electrical activity

Question 1

What’s the speed of atrial conduction?

Answer 1

1 m/sec

Question 2

What conducts the impulse from right to left atrium?

Answer 2

Specialized fibres known as Bachmann’s bundle

Question 3

Where do impulses spread at the end of atrial conduction?

Answer 3

AV node (only route of conduction from atria to ventricles)

Question 4

What’s the speed of atrioventricular conduction and why?

Answer 4

0.05 m/sec
Delay between atrial and ventricular excitation/contraction allows optimal ventricular filling during atrial conduction

Question 5

What’s the speed of ventricular conduction in Purkinje fibres and why?

Answer 5

1-4 m/sec due to large cell size of Purkinje fibres compared to myocytes

Question 6

Explain route of ventricular conduction

Answer 6

Av node conducts to the bundle of His, then to bundle branches –> branches subdivide into the Purkinje fibres –> conduct impulses into ventricles

Question 7

Locations of autorhythmic cells? When is AP generated + speed in each?

Answer 7

1. SA node: APs generated here spread over entire cardiac tissue. 70-80 APs/min
2. AV node: only generated if SA node is destroyed. 40-60 APs/min
3. Purkinje fibres: APs generated by SA node inhibit their autorhythmic activity. 30-40 APs/min

Question 8

Types of cardiac APs and cells associated

Answer 8

Fast: myocytes in atria and ventricles
Slow: autorhythmic cells in SA and AV node

Question 9

Phases of cardiac AP

Answer 9

0: Upstroke
1: Early repolarization (only in fast response)
2: Plateau
3: Repolarization
4: Final repolarization

Question 10

Steps of AP in myocardial contractile cells

Answer 10

1. Arrival of AP at a contractile myocardial cell opens voltage-gated Na+ channels (upstroke/rapid depolarization)
2. Voltage-gated Ca2+ open more slowly
3. +20 mV: Na+ channels close, K+ channels open –> repolarization begins (early repolarization)
4. Slow inward diffusion of Ca2+ balances outward diffusion of K+ (plateau)
5. Ca+ channels close and K+ channels complete repolarization (repolarization + final repolarization)

Question 11

What does inward diffusion of extracellular Ca2+ allow?

Answer 11

It opens Ca2+ channels on SR and is used to initiate contraction in myocardial cells rather than intracellular stores

Question 12

What happens to Ca2+ during repolarization?

Answer 12

It’s transported out of the cell –> relaxation

Question 13

What does increase in intracellular Ca2+ trigger?

Answer 13

Triggers contraction in an identical mechanism to skeletal muscle

Question 14

How is myocardial cell contraction different to skeletal cell contraction?

Answer 14

• Length of AP in myocardial cell (250 msec) is much longer than in skeletal muscle (20 msec) due to plateau phase
• Myocardial: duration of AP is almost as long as associated contraction (skeletal: cell repolarised before contraction begins)
• Myocardial cells are refractory during almost entirety of contraction –> summation and tetany can’t occur in cardiac muscle

Question 15

Predominant type of Ca2+ channels

Answer 15

L-type (long-lasting)

Question 16

T/F: quantity of Ca2+ has no effect on strength of contraction

Answer 16

False: greater the influx of Ca2+ = stronger contraction

Question 17

2 examples of Ca2+ channel antagonists and their function

Answer 17

Diltiazem and Verapamil: Decrease duration of AP and diminish contractility of myocardial cells

(DVD –> Diltiazem Verapamil: Decrease)

Question 18

How is slow response AP different from fast response AP?

Answer 18

Slow:
- Generated spontaneously in pacemaker cells because resting membrane potential in phase 4 is less negative and unstable
- Depolarization (phase 0) is not as large or as rapid
- Early repolarisation (phase 1) is not apparent
- Plateau (phase 2) less prolonged and less flat

Question 19

Explain myocardial autorhythmic cells AP with Vm values

Answer 19

• Membrane potential (Vm) is unstable due to slow Na+ and Ca2+ channels (HCN) which open at -60 mV
• Slow drift in Vm from -60 to -50 mV
• At -50 mV (threshold): fast Ca2+ and Na+ channels open –> spontaneous depolarization (AP is generated)
• K+ channels open –> repolarization

Question 20

What happens during effective refractory period in fast and slow response?

Answer 20

Na+ channels (fast response) and Ca2+ channels (slow response) inactivate when cell is depolarized –> no AP can be generated –> effective refractory period

Question 21

What happens during relative refractive period?

Answer 21

Na+ channels (fast response) and Ca2+ channels (slow response) go back to a closed/activatable state as the cell is repolarizing (phase 3) and fully closed in phase 4 –> cell is fully excitable

Question 22

Pacemaker cells have a prolonged … and also have …

Answer 22

relative refractory period
post-repolarization refractoriness

Question 23

Name and describe the waves on ECG

Answer 23

P: atrial depolarization
QRS: ventricular depolarization
T: ventricular repolarization
(atrial repolarization wave obscured by QRS complex)

Question 24

Give the time of P-Q (P-R), Q-T and R-R intervals

Answer 24

P-Q (P-R): 0.16 sec (delay in conduction of impulse into ventricles)
Q-T: 0.35 sec (ventricular contraction)
R-R: 0.83 sec (72 bpm)

Question 25

What does ECG measure?

Answer 25

Overall electricity activity in the heart (NOT individual APs)

Question 26

When is QRS wave generated?

Answer 26

When ventricles are partially depolarized –> flow of current and QRS wave generated

Question 27

From where to where does the electricle current flow in the heart?

Answer 27

From depolarized area (base) to polarized area (apex)

Question 28

What is the mean electrical axis of the heart?

Answer 28

Orientation of cardiac vector during QRS is normally +60° from horizontal plane

Question 29

Where is the reference electrode placed?

Answer 29

Left leg

Question 30

What are the bipolar limb leads and where are they located? Give name of the triangle

Answer 30

Einthoven’s triangle:
Lead 1: right arm (-) to left arm (+)
Lead 2: right arm (-) to left leg (+)
Lead 3: left arm (-) to left leg (+)

Question 31

What happens if direction of cardiac vector is:
- towards positive electrode
- parallel to direction of lead

Answer 31

• upward deflection of ECG
• maximum deflection

Question 32

What are the 12 leads in 12-lead ECG?

Answer 32

• 3 bipolar limb leads
• 3 unipolar limb leads
• 6 chest leads

Question 33

Where does the axis shift in heart hypertrophy?

Answer 33

Towards hypertrophied side

Question 34

Name these cardiac arrhythmias and what causes them?

Answer 34

Sinus tachycardia = increase in heart rate
Sinus bradycardia = reduction in heart rate

Question 35

Name these and what causes them

Answer 35