Anatomy and Physiology.
Cardiac and Conduction System.
Aims and Objectives.
• To understand the process of the cardiac
cycle.
• To understand the basic physiology of
myocardial fibre contraction.
• To understand the physiology of the cardiac
conduction system.
• To understand the relationship between
electro-physiology and cardiac output.
• To understand the basis of ECG wave
formation.
The Heart.
• Anatomical position in
chest.
• Location of chambers.
• Location of major
vessels.
• Typical size.
• Arterial Territory.
• Electrical impulse
(vector).
Key Points Cardiac Cycle.
•
•
•
•
•
Diastolic filling (passive atria > ventricles).
Atrial contraction.
Ventricular contraction (systole).
Arterial flow.
Increased venous pressure (diastolic
filling).
• Process repeats.
Cardiac Conduction System.
• Two types of cardiac tissue:
– Ordinary myocardium.
– Specialised cardiac conduction system.
(sino-atrial node, anterior, middle and
posterior inter-nodal tracts, atrio-ventricular
node, His bundle, right and left bundle
branches, antero-superior and postero-inferior
divisions of left bundle, Purkinje network).
Conduction cont…
• Both tissue allow electrical conduction.
• Cells in specialised system depolarise
spontaneously.
• Inherent cardiac pacemaker.
• Decreased rate the further down the
conduction tree.
• Fastest is SA node (60-100bpm) dominant pacemaker.
Sinoatrial node
• Submyocardial structure at the lateral aspect of
the SVC & RA.
• Cardiac myocytes belonging to the right atrium.
• Its superficial aspect is covered by adipose
tissue.
• Innervated by the autonomic nervous system.
• Parasympathetic nervous system – slows rate
• Sympathetic nervous system – increases rate
Bachmann’s bundle
• One of four conduction tracts
• Conducts electrical stimulus to left atrium
• Anterior, middle & posterior are the other
three tracts.
• These join to the A-V node near to the
coronary sinus.
The Atrio-Ventricular Node
• Electrical control system of the heart.
• Found in the posterioinferior region of the
inter-atrial septum near the coronary sinus
opening.
• It is located at the center of Koch's
Triangle - a triangle enclosed by the septal
leaflet of the tricuspid valve, the coronary
sinus, and the membrane of the interatrial
septum
• The AV node receives two inputs from the atria:
posteriorly, via the crista terminalis, and
anteriorly, via the interatrial septum.
• AV conduction during normal cardiac rhythm
occurs through two different pathways:
• The first “pathway” has a slow conduction
velocity but shorter refractory period
• The second “pathway” has a faster conduction
velocity but longer refractory period.
AV Node cont…
• Usually the only place conduction can
pass from the atria to ventricles.
• Conduction slowed and delayed (0.120.20s).
• Delay :- Allows for Atria to complete
contraction and Ventricular filling to occur.
• Signal is then passed to the Bundle of His.
Bundle Of His
• The His bundle carries the signal from the AV node to
the inter-ventricular septum (1.5-4 m/s).
• Most proximal part of the His-Purkinje system.
• Bifurcation occurs into left & right bundle.
• The Left Bundle Branch carries the signal across the left
ventricle: The left bundle branch divides further in to:
– the left anterior superior fascicle
– the left inferior posterior fascicle
• The Right Bundle Branch carries the impulse across the
right ventricle
Purkinje Fibres
• Terminal purkinje fibres extend beneath
endocardium & divide into smaller &
smaller branches.
• Rapid depolarisation (4.0 m/s).
• Carries action potential to the cardiac
muscle.
• Initially to the Apex and then upwards to
the remainder of the cardiac muscle.
Cardiac Conduction System.
Action Potentials.
• Cardiac myocyte depolarisation and
repolarisation.
• Triggered by external or intra-cellular
spontaneous mechanisms
– cell to cell depolarisation.
– cardiac pacemaker cells.
Action Potentials.
• Non-pacemaker action potentials ('fast
response' - rapid depolarisation).
• Found throughout the heart except for
pacemaker cells.
• Pacemaker cells generate spontaneous
action potentials ('slow response' - slower
rate of depolarisation).
• Found in the sino-atrial node and atrioventricular node.
Pacemaker Cells.
• Regular spontaneous action potentials.
• Current carried into cell by slow Ca++ and
lesser extent K+.
• Divided into 3 phases:
– PHASE 4 - Spontaneous depolarisation (triggers
action potential at threshold between -30 and 40Mv)
– PHASE 0 - Depolaristation of action potential.
– PHASE 3 - Repolarisation at ~-60Mv - then
repeat.
Pacemaker cell Action Potential.
Non-pacemaker cells Action
Potential.
• Atrial, ventricular myocytes and Purkinje
Fibres.
• True resting membrane potential - Phase 4
• Rapidly depolarised to -70Mv - Phase 0
(adjacent cell action potential).
• Initial repolarisation with a plateau
(Phases 1 and 2).
• Complete repolarisation (Phase 3).
Non-pacemaker cell Action
Potentials.
Effective Refractory Period.
•
•
•
•
•
Period during Phases 0,1,2 and part of 3.
Cannot initiate new action potential.
Intrinsic protective mechanism.
Limits depolarisation and therefore HR.
More effective ventricular filling - improved
cardiac output.
• More important at higher heart rates
(increased excitability).
Sequence of Cardiac
Depolarisation.
• Action potentials generated by SA node.
• Spread (cell to cell conduction) through atria.
• Some evidence of specialised inter-nodal
tracts - controversial.
• Action potential enters ventricles through AV
node (slows impulse considerably).
• Travels through Bundle of His, left and right
branches and Purkinje Fibres.
• Action potential spreads to ventricular
myocytes.
Excitation-Contraction Coupling.
• Action potential triggers
myocyte to contract.
• Release of Ca in sarcoplasmic
reticulum.
• Binds to troponin - C.
• Exposes site on actin
molecule.
• Binding results in ATP
hypdrolysis.
• Movement 'ratcheting' between
actin / myosin heads.
• Filaments slide past each
other.
• Sarcomere shortening contraction.
ECG Waveforms.
• Summative measure of action potentials.
• Different waves represent atrial and
ventricular depolarisation and repolarisation.
• Standardised recording measures:
– speed 25mm/s
– 1Mv = 10mm vertically
Allows comparison, calculation of
normal and abnormal values.
ECG Waveform.
P wave.
• Atrial depolarisation.
• Impulse from SA node
- spread throughout
atria.
• Results in atrial
contraction.
• PR interval = impulse
in AV node.
• Adequate time for
ventricular filling.
QRS Complex.
• Ventricular
depolarisation.
• Rapid and powerful
contraction.
• Shape of trace
depends on:
– electrode position
– pathophysiology
– conduction
abnormality.
ST segment.
• Normally iso-electric.
• Point at which entire
ventricle is depolarised.
• Corresponds to the
plateau phase of
ventricular
depolarisation.
• Important in
recognising ventricular
ischaemia / hypoxia.
T wave.
• Ventricular
repolarisation.
• Longer in duration
than depolarisation.
• Sometimes 'U' wave.
• Additional
repolarisation wave.
• If very prominent then
sometimes pathology.
Q-T Interval.
• Total time for ventricular
depolarisation and
repolarisation.
• Length of ventricular
action potential.
• Can be diagnostic for
certain types of
arrhythmia.
• Changes depending on
heart rate:
– High HR shorter interval.
Conclusion.
• There are detailed physiological processes
involved in cardiac myocyte and pacemaker
stimulation.
• All electrophysiological events lead to mechanical
responses - 'excitation contraction coupling'.
• Electrophysiology is therefore a key regulator of
cardiac output.
• ECG waveforms are recordings of cardiac action
potentials 'as a whole'.
• These waveforms are standardised to allow for
consistent measurement worldwide.