Electrical conducting system of the heart Flashcards
How does the cardiac system work?
Heart beats in an orderly sequence. Contraction of the atrium in systole, followed by contraction in the ventricles in ventricular systole. Heart beat originates in a specialised cardiac conduction system.
Start with SAN and then internodal pathways to atrial ventricular node to bundle of His to left and right bundle branch then to terminal of Purkinje fibres.
AP defintion
Transient depolarization of a cell as a result of activity of ion channels
What are the 2 types of cardiac AP’s
Non-pacemaker (contractile cells) and pacemaker action potentials
What are the phases of non-pacemaker APs
Phase 0 Na+ channels open
Phase 1 Na+ channels close
Phase 2 Ca2+ channels open; fast K+ channels close
Phase 3 Ca2+ channels close; fast K+ channels open
Phase 4 Resting potential
Phase 1- repolarisation- when sodium channels close, membrane cells are repolarised as potassium leaves through K channels. Phase 2- initial repolarisation is brief but get a flattening due to decrease in K permeability and increase in Ca permeability. This is due to voltage gated Ca channels that are activated by the depolarisation and Ca enters the cell. Phase 3- rapid repolarisation- Ca channels close- activated K channels and K exits rapidly repolarising back to resting potential. This is in myocardial contractile cells- non pacemaker cells.
How does pacemaker activity occur normally?
Cardiac pacemaker cells are mostly found in SAN node and then followed downwards and upwards- these cells beat naturally.
Definition of pacemaker activity
The intrinsic, spontaneous time dependent depolarisation of a cell membrane that leads to an action potential.
What is the hierarchy of pacemakers?
The Primary Pacemaker is defined as the tissue with the highest ‘firing’ frequency, in other words the fastest pacemaker sets heart rate and overrides all slower pacemaker tissues.
SAN> AVN> Purkinje fibres
Pacemaker action potentials how does it occur?
Autorhythmic cells- generate AP spontaneously, they do this without input from NS due to having an unstable membrane potential. Never get a constant value as always fluctuating.
Whenever AP reaches threshold causes cell to depolarise. Have a funny current and when current that flows, pacemaker becomes less negative until it reaches a threshold. Voltage gated Ca channels open- influx in Ca channels. Ca channels then close and K channels open- K moves out of cell and repolarises membrane. The decay of pacemaker potential determines the HR. The faster it is, the faster the HR.
Differences between contractile and auto rhythmic myocardium
Membrane potential- contractile myocardium stable, auto-rhythmic myocardium unstable
Events leading to threshold potential- contractile- depolarisation enters via gap junctions, autorythmic- net sodium entry through channels and calcium entry
Rising phase of AP- Contractile- extended plateau caused by calcium and rapid phase of K reflux and auto-rhythmic- rapid by K flux
Hyperpolarisation- none for both myocardium
Refractory period- Long in contrail myocardium and not significant in autorhythmic myocardium
Drugs that affect cardia action potential
Class 1- Na channel blockers- phase 0
Class 4- Ca channel blockers
Class 3- Phase 3- K channel blockers
Class 2-Phase 4- Beta blockers
How are AP conducted?
Depolarizing currents pass through gap junctions located at intercalated disks
What are the stages for initiation and condition of an electrical impulse?
① Action potential initiated in the SA node
-Propagates to the AV node via intermodal pathways in the atria
② Cells of the AV node transmit AP more slowly
-Delay impulse by 100ms
③ Impulse spreads down to ventricles along the Bundle of His
-To the apex of the heart
④ AV bundle divides into left and right bundle branches
-Supplies left and right ventricles
⑤ Impulses spread to the contractile cells of ventricles
-As it spreads upward through the ventricular muscle
-Through network of Purkinje fibres
ECG- where are the leads placed?
Chest
V1- fourth intercostal space
V2- fourth intercostal space left of the sternum
V4- Fifth intercostal space
V3- in between V2 and V4
V5- Fifth intercostal space- anterior axillary line
V6- Fifth intercostal space in midline
Limb
Einthoven triangle
Put them on shoulders and top part of pelvis.
Can normally put on arm and legs.
3 limb leads- Einthoven’s triangle
How do ECG’s work?
- Electrical activity towards an electrode results in a positive deflection on ECG
- Electrical activity away from an electrode results in a negative deflection on ECG
Components of the ECG
P wave- represents atrial depolarisation
PR segment- conduction through AVN- get delay
QRS complex- Depolarisation at ventricles, atria repolarising
T wave- Ventricles repolarising
ST- wave elevation- if goes up have myocardial ischemia
