VIVA: Physiology - The Heart

Question 1

Describe the conducting system of the heart

Answer 1

SA node is located at the junction of the SVC and RA
AV node is in the right posterior interatrial system
Three bundles of atrial fibres connect the SA node to the AV node
AV node is continuous with the bundle of His, which gives off a left bundle branch at the top of the interventricular septum and continues as the right bundle branch
Left bundle branch divides into an anterior fascicle and a posterior fascicle
Branches and fascicles run subendocardially down either side of the septum and come into contact with the Purkinje system
Purkinje fibres spread to all parts of the ventricular myocardium

• SA, SV, RBB and LBB to be at standard

Question 2

Describe the phases of the cardiac cycle that produce the waves and segments in a normal ECG

Answer 2

3 to be standard:
- p wave: atrial depolarisation
- PR segment: AV conduction
- QRS: ventricular depolarisation
- ST segment: plateau portion of ventricular depolarisation
- QT: ventricular action potential
- T wave: ventricular repolarisation

Question 3

How do membrane changes in infarcted myocytes cause ST segment elevation?

Answer 3

1 to pass:
- Abnormally rapid repolarisation due to accelerated opening of K+ channels (seconds to few minutes)
- Decreased resting membrane potential due to loss of intracellular K+
- Slow depolarisation of the affected cells compared to surrounding normal cells

Question 4

Please draw or describe the Frank Starling law as it relates to cardiac muscle

Answer 4

Curve of SV against ventricular EDV (energy of contraction is proportional to initial length of cardiac muscle fibre, represented as EDV)

INSERT PICTURE

Question 5

What factors influence the Frank Starling curve?

Answer 5

Positive, shifting curve up and to the left:
- Circulating catecholamines
- Inotropes (e.g. caffeine, theophylline, digoxin)
- Sympathetic input

Negative, shifting curve down and to right:
- Acidosis
- Hypercarbia
- Hypoxia
- Vagal/parasympathetic stimulation
- Pharmacological depressants (e.g. quinidine, procainamide, barbiturates)
- Intrinsic depression (with heart failure)

The causes of this depression are not fully understood but may reflect down-regulation of B-adrenergic receptors and associated signalling pathways and impaired calcium liberation from the sarcoplasmic reticulum

• need two positive and two negative factors with correct influence to pass

INSERT PICTURE

Question 6

What two factors determine cardiac output?

Answer 6

CO = HR x SV*
SV is related to preload and afterload of the heart and the intrinsic contractility of the myocardial cells
HR is determined by sympathetic vs parasympathetic stimulation

• needed to pass