Lecture 3: The Electrocardiogram

Question 1

Describe the phylogeny of channel proteins

Answer 1

• T-type Ca2+ channels are the most closely related to sodium channels-> because Ca shares a most recent common ancestor with sodium that is more recent than K channels
• Shortest distance between T-type Ca channel to Na Channel

Question 2

State what an ECG is

Answer 2

• Measures heart rhythm by setting up circuit with your body for conductive connection

Question 3

Explain the basis of an ECG trace

Answer 3

• Movement of a wave of electrical activity as it travels across the skin
• Waves relate to anatomical features of the heart + change of voltage
• y-axis= change in electrical activity +the direction of that wave of change
• baseline of ECG= 0 -> reference electrode (usually black) on the patient
• Lead 2 used-> parallel to wave of depolarisation of heart by pacemaker cells

Question 4

Interpret and explain a normal ECG trace

Answer 4

P Wave
- SAN initiates depolarisation + transferred to AVN
- =Depolarisation
- =Atrial contractions
- After depolarisation reaches AVN-> conduction slows down
->because: diameter of AVN nodal cells smaller= slower conduction
: t-type Ca2+ channels are slow to open= less flow of Ca2+= slower conduction
: delay of depolarisation-> allows atria to relax after + ventricle to fill while relaxed
QRS Complex
- Wave of depolarisation travels from AVN to bundle of His
- Bundle of His ->left + right bundle branches-> Purkinje fibres =>fast conduction
- Depolarisation through this= leads to mass depolarisation of ventricular cells
- Fast conduction-> ventricular cells have regular Na+ channels= fast acting
->Sharp slope + ventricular cells act together

• Direction of depolarisation starts off with left septum-> away from direction of lead 2= neg. value= Q wave
• Then goes right + down-> in direction of lead 2= pos. value
• Simultaneously-> atrial repolarisation = atrial relaxation
• Not shown on ECG-> mass of atrial cells > mass of ventricular cells
  ST Segment
• Initial ventricular repolarisation
• No net current:
• Active, but membrane potentials of cardiomyocytes steady ->due to L-type Ca2+ channels remaining open
  = ECG trace records this as zero
  -> because it is zero change in membrane potential of the tissues that are important in this phase of heart activity
• Ventricular contraction-> blood goes to aorta + pulmonary artery
  T Wave
• Ventricular repolarisation
• Wave flatter + longer than QRS complex= repolarisation takes longer than depolarisation
• Pos. because:
  ->endocardium depolarised first, then epicardium
  ->epicardium repolarised quicker than endocardium
  = wave of repolarisation in opposite direction of lead 2 = direction of ventricular depolarisation

Question 5

Describe the difference in membrane potential profile for the different groups of pace maker cells

Answer 5

• membrane potential profile of SAN differs a bit from of other pace making cells
• AVN +bundles of His =act as pacemakers for the heart-> membrane potential profiles look more like cardiomyocytes

Question 6

What does an ECG trace not show?

Answer 6

Electrical activity across membrane of 1 cell

Question 7

Explain how the ANS can affect conduction of the heart

Answer 7

• ANS= sympathetic (fight or flight) + Parasympathetic (rest and digest)
• Sympathetic stimulation ->efferent fibres
  = speed up the pace +velocity of AP waves+ increase rate of APs
• Parasympathetic stimulation ->vagus nerve(s)
  ->affect the SA and AV nodes
  = slow down the pace of AP waves+ decrease rate of Aps

Question 8

Define and explain the different types of AV blocks

Answer 8

• Blocks: areas of abnormal (or absent) conduction= tissue downstream will not be excited by the AP wave
   AVN + other pace making tissues downstream of SAN can generate its own APs
• Block occurs between the SAN and AVN
  -> atria will be excited at the pace of the SAN pacemaker cells
  -> ventricles will be excited at the pace (slow) of the AVN’s AP wave myogenesis
  = not ideal - better than the ventricles getting no signal at all due to a block
• Re-entries: areas where the conduction turns back on itself= loop of conduction -> not good
• Can occur in localised areas ->loop in the left atrium
• Common re-entries = Wolff-Parkinson-White syndrome
  -> Bundle of Kent = piece of conductive tissue from the atrium to the ventricle->allowing the signal to pass along the ventricle
  = causing parts of ventricle to contract before other parts + not synchronised
  ->wave passes along ventricles + some of the muscle contract earlier
  = reduced pause after atrial contraction + beginning of ventricular contraction
  = earlier in and the refractory period of ventricular contraction - when the wave from the Purkinje fibres finally arrives

Question 9

Describe the different degrees of AV blocks and how they affect the ECG trace

Answer 9

1st degree AV block
- conduction through the AVN is slowed, but not interrupted
- 1:1 correspondence between P and QRS complexes
- longer PR interval
2nd degree AV block
- Conduction through the AVN is partially interrupted, but proceeds some of the time
- may see several P waves without ever seeing a QRS complex

3rd degree AV block

• no conduction through the AVN
• no correspondence between P and QRS complexes
• QRS complexes very infrequent (< 40 bpm)