6. The electrical Conducting System of the Heart

structure of electrical conducting system of heart

function of elements of conducting system

• SAN→ generates electrical impulse

• AVN→ mode of passage of impulse from atria to ventricles

• bundle of his, right bundle branch, left bundle branch→ specialised conducting system to transmit impulse

action potential

• transient depolarisation of a cell as a result of activity of ion channels

types of cardiac action potentials

• pacemaker action potential

• non pacemaker action potential

pacemaker action potentials

• autorhythmic

• controlled by I_f (funny) channels

stages in pacemaker action potential generation

1. funny channels allow Na⁺ influx into membrane

2. depolarises membrane

3. as membrane potential reached until threshold reached→ action potential generated

   1. Ca²⁺ channel open

4. at top of depolarisation, K+ channels open and Ca²⁺channels close

5. K+ leave→ causes repolarisation

non-pacemaker action potentials

0. sodium ions influx into cell membranes, rapidly depolarising membrane

1. sodium channels close, potassium channels open so K+ moves out of cell

2. calcium channels open, so lots of calcium moves in, little bit of potassium leaving to allow electrochemical balance

3. calcium channels close and potassium channels open→ lots of potassium exits to repolarise

4. potassium leaves channel slowly from potassium channel

pacemaker activity

• intrinsic, spontaneous time dependant depolarisation of a cell membrane that leads to an action potential

hierarchy of pacemakers

• primary pacemaker→ highest firing frequency (fastest pacemaker):

  • SAN→ 100bpm

• secondary pacemaker:

  • AVN→ 40bpm

• tertiary pacemaker:

  • purkinje fibres→ 20bpm

• SAN under constant vagal stimulation which suppresses its intrinsic frequency to approx. 60 bpm

drugs affecting cardiac action potential

• Class 1→ Na⁺ channel blocker

• class 2→ Beta blocker

• Class 3→ K⁺ channel blockers

• Class 4→ Ca²⁺ channel blocker

examples of class 1 channel blockers

• 1a→ moderate:

  • quinidine

  • procainamide

• 1b→ weak:

  • lidocaine

  • phenytoin

• 1c→ strong:

  • flecainide

  • propafenone

examples of class 2 heart drugs

• propranolol

• metoprolol

examples of class 3 heart drugs

• amiodarone

• sotalol

examples of class 4 heart drugs

• verapamil

• diltiazem

conduction of action potentials

• pass through gap junctions located at intercalated disks

components of ECG

• P wave→ atrial depolarisation

• PQ interval→ AV nodal delay

• QRS→ ventricular depolarisation

• ST→ interval between depolarisation and repolarisation

• T wave→ repolarisation

normal sinus rhythm

atrial fibrillation

• no ordinated atrial contraction

• sinus node no longer in charge

• ventricular rate is irregular

• AVN important in regulation of atrial signals

atrial flutter

• abnormal rhythm in atrium

• regular but fast→ 300ppm

• AV node is vital

ventricular fibrillation

• cardiac arrest rhythm

• no regularity

• shockable

ventricular tachycardia

• associated w cardiac arrest

• shockable

1st degree heart block

• long time between P wave and QRS

• aka 1st degree AV block

• AVN takes longer to produce ventricular depolarisation

• always QRS after p wave

Second degree AV block→ Mobitz I)

• progressive lengthening of PR interval, then missed QRS complex

secondary degree AV block→ Mobitz II

• PR interval normal, then absent QRS after P wave

second degree AV block→ 2:1 block

• absent QRS after alternate P waves

third degree block with junctional escape

• no relationship between P waves and QRS complexes