PT15 LAB: Conduction System of the Heart / Electrophysiology

Intrinsic conduction system (nodal system)

"drummers" that sets basic rhythm of heart

SINUS node / Sinoatrial node (SA node)

- pacemaker; generate normal rhythmical impulse of 75 beats/min
- small, flat, crescent- shaped
- establishes sinus rhythm
- highest inherent rate of depolarization

INTERNODAL pathways

ensures AP spreads from SA node thru entire atrial muscle fiber to AV node

AV node

- delays the conduction of impulse from atria to ventricles
- ensures ventricles will not contract until all blood from atrial chamber is emptied
- takes over if SA node fails

AV bundle (bundle of His)

one-way conduction of impulse from atria to ventricles; near bifurcation where interventricular septum separates

R & L Bundle Branches

conduct impulse from atria into ventricles in the interventricular septum to the apex of the heart

Purkinje fibers

conduct impulse to all remaining parts of ventricles towards the base; AV bundle terminates here

sequence of excitation

- SA node fires AP
- depolarization spreads thru atrial myocardium
- AP travels thru AV node then apex via conduction pathway
- impulses passes from atria to ventricles via AV bundle of His
- Purkinje fibers distribute excitation thru ventricles
- cells of ventricles depolarize

autonomic nervous system

extrinsic innervation of the heart

Sympathetic nervous system

the "accelerator"; inc both heart rate & force/strength of heart contraction

Parasympathetic nervous system

the "brakes"; slows heart rate

medulla oblongata

- cardiac & vasomotor center
- also respiratory center

Cardio-acceleratory center

via sympathetic nervous system

postganglionic fibers of cardio-acceleratory center terminate in:

- SA/AV nodes
- atrial/ventricular myocardium
- aorta/pulmonary trunk
- coronary arteries

Cardio-inhibitory center

via parasympathetic nervous system

preganglionic fibers of cardio-inhibitory center extend through

vagus nerves to cardiac plexus

postganglionic fibers of cardio-inhibitory center lead to

- from right vagus nerve: SA node
- from left vagus nerve: AV node

cardiac plexus

- contains both sympathetic & parasympathetic efferent fibers
- transmits sensation of pain receptors when there is a problem with the heart

electric currents that spread through the heart are produced by three components:

- cardiac pacemaker cells
- specialized intrinsic conduction tissue
- myocardium

electrocardiograph

records the impulses / electrical currents in the heart as detected on the body surface by means of electrodes (leads) applied to the skin

electrocardiogram (ECG/EKG)

records overall (summed) electrical activity of heart; composite of all action potentials generated by nodal & myocardial cells

P wave

- small
- corresponds to depolarization of SA node & atrial depolarization (before atrial contraction)
- upward deflection & gradual slide

QRS complex

- corresponds to
ventricular depolarization & precedes the contraction of the ventricles (ventricular systole)
- largest muscle mass & greatest elec current
- sharp upward & downward deflection

Q

small downward deflection

R

tall sharp peak

S

final downward, negative deflection

T wave

corresponds to ventricular repolarization immediately before ventricular diastole

atrial repolarization

masked by QRS complex; seen in acute pericarditis & atrial infarction

J point

junction between the end of the QRS complex & the beginning of the S-T segment

P-Q interval

- represents the time required for impulses to travel from the SA node to the AV node
- beginning of atrial systole to ventricular contraction

- atrial depolarization & contraction

S-T segment

- after QRS complex; corresponds to ventricular systole
- AP plateau

Q-T interval

- beginning of ventricular depolarization through repolarization
- period of electrical systole of ventricles
- varies inversely with heart rate (the shorter, the faster)

Electrocardiography

Recorded in a special graph paper (electrocardiogram) divided into 1-mm2 grid like boxes

R-R interval

- peak to peak interval
- 0.8 seconds between 2 R peaks

P-R interval

between atrial & ventricular depolarization

Arrhythmia

caused by any deviation from the regular, SA node-driven sinus rhythm of the heartbeat

Ventricular fibrillation

most common & widely known (& most feared) arrhythmia; hallmark of heart attack

Defibrillation

is an emergency procedure in which the heart is given a strong electrical shock with a pair of paddle electrodes

Atrial Fibrillation

is a weak rippling contraction in the atria; common in elderly

Heart Block

a failure of any part of the cardiac conduction system to conduct signals, usually as the result of disease & degeneration of the conduction system fibers

Bundle Branch Heart Block

is a heart block resulting from damage to one or both branches of the AV bundle

Total Heart Block

damage to the AV node

Premature Ventricular Contraction (PVC)

a result of a ventricular focus firing & setting off an extra beat (extrasystole) before the normal signal from the SA node arrives

bradycardia

< 60 bpm

tachycardia

> 110 bpm