Chapter 22: Conduction System of the Heart and Electrocardiography

what does heart contraction result from?

• a series of depolarization waves that travel through the heart preliminary to each beat

what does the entire myocardium behave like

• a single unit, a functional syncytium, because cardiac muscle cells are electrically connected by gap junctions

what does it mean that the cardiac muscle has the ability to beat intrinsically

• does not depend on impulses from the nervous system to initiate its contraction and will continue to contract rhythmically even if all nerve connections are severed

what does the intrinsic conduction system of the heart consist of?

• cardiac pacemaker cells

what two things does that intrinsic conduction system ensure?

• that the heart muscle depolarizes in an orderly and sequential manner, from atria to ventricles

• that the heart beats as a coordinated unit

what are the components of the intrinsic conduction system

• sinoatrial (SA) node

• atrioventricular (AV) node

• AV bundle (bundle of His)

• right and left bundle branches

• subendocardial conducting network

where is the sinoatrial (SA) node located

• in the right atrium just inferior to the entrance to the superior vena cava

where is the atrioventricular (AV) node located

• in the lower atrial septum at the junction of the atria and ventricles

where are the AV bundles (bundle of His) and the right and left bundle branches located?

• interventricular septum

what is the subendocardial conducting network also called

• Purkinje fibers

why can the depolarization wave only be transmitted to the ventricles via the tract between the AV node and AV bundles

• because the atria and ventricles arc separated from one another by a region of electrically inert connective tissue

any damage to the AV node-bundle pathway . . .

• partially or totally insulates the ventricles from the influence of the SA node

the conduction of impulses through the heart generates what?

• electrical currents that eventually spread throughout the body

how can the electrical currents/impulses be detected on the body’s surfaces

• can be recorded with an instrument called an electrocardiograph

what is an electrocardiogram and what are the abbreviations

• the graphic recording of the electrical changes occurring during the cardiac cycle

• ECG

• EKG

a typical ECG (electrocardiogram) has how many recognizable waves and what are these waves also called

• 3 recognizable waves

• called deflections

what are the 3 recognizable waves of the ECG

• P wave

• QRS complex

• T wave

what is the ECG divided into for analysis

• segments

• intervals

what is an ECG segment

• region between two waves

so what would an S-T segment be

• the region between the end of the S deflection and the start of the T wave

what is an ECG interval

• a region that contains a segment and one or more waves

so what would the Q-T interval be like?

• includes the S-T segment as well as the QRS complex and the T wave

what do the deflection waves of an EGC correlate to?

• the depolarization and repolarization of the heart’s chambers

what is the P wave

• start of P deflection to return to baseline

what is the P wave caused by

• atrial depolarization (contraction), initiated by the SA node

what is the P-R interval

start of P deflection to start of Q deflection

what does the P-R interval relate to?

• when atrial depolarization or contraction is complete, the impulse is delayed at the AV node

QRS complex

• start of Q deflection to S return to baseline

what happens during the QRS complex?

• ventricular depolarization or contraction begins at the apex, causing the QRS complex, and then atrial repolarization or relaxation happens

S-T segement

• end of S deflection to start of T wave

what happens during the S-T segment

• ventricular depolarization or contraction is complete

Q-T interval

• start of Q deflection to end of T wave

T wave

• start of T deflection to return to baseline

what happens during the T wave

• ventricular repolarization (relaxation) begins at the apex, causing the T wave

T-P segment

end of T wave to start of next P wave

what happens during the T-P segment

• ventricular repolarization is complete

R-R interval

• peak of R wave to peak of next R wave

what are the abnormalities of the deflection waves and changes in the time intervals of the ECG useful in detecting

• myocardial infracts - heart attacks

• problems with the conduction system of the heart

what is referred to as tachycardia

• a heart rate of over 100 beats per minute

what is referred to as bradycardia

• a heart rate below 60 beats/minute

are either tachycardia or bradycardia pathological?

• no

prolonged tachycardia may progress to

• fibrillation: a condition of rapid uncoordinated heart contractions

what is stroke volume?

• the amount of blood ejected by a ventricle with each contraction

why is bradycardia in athletes a positive finding

• it indicates an increased efficiency of cardiac functioning.

• Because stroke volume (the amount of blood ejected by a ventricle with each contraction) increases with physical conditioning. the heart can contract less often per minute and still meet circula- tory demands.

what is used to record an ECG for diagnostic purposes?

• 12 standard leads that together provide a fairly comprehensive picture of the electrical activity of the heart

what are three of the leads

• bipolar leads that measure voltage differences between the arms, or an arm and a leg

what are the rest of the nine leads

• unipolar leads

what is the possible clinical significance of an enlarged R wave

• enlarged ventricles

what is the possible clinical significance of a prolonged P-R interval

• first-degree heart block

• the signal from the SA node to the AV node is delayed longer than normal

what is the possible clinical significance of prolonged Q-T interval when compared to R-R interval

• increased risk of ventricular arrhythmias - fast heart rhythem

• this interval corresponds to the beginning of ventricular depolarization through ventricular repolarization

what is the possible clinical significance of S-T segment elevated from baseline

• myocardial infraction - heart attack

what does the electrical activity recorded by any lead depend on

• the location and orientation of the recording electrodes

what is assumed clinically for the recording versus in practice

• that the heart lies in the center of a triangle with sides of equal lengths (Einthoven’s triangle) and that the recording connections are made at the corners of that triangle.

• But in practice, the electrodes connected to each arm and to the left leg are considered to connect to the triangle corners.

what do standard limb leads record and explain

• the voltages generated between any two of the connections.

• A recording using lead I (RA-LA). which connects the right arm (RA) and the left arm (LA), is most sensitive to electrical activity spreading horizontally across the heart. Lead II (RA-LL) and lead III(LA-LL) record activity along the vertical axis (from the base of the heart to its apex) but from different orientations.

what is the significance of Einthoven’s triangle - Einthoven’s law

• the sum of the voltages of the leads connecting the leg to one of the arms equals that of the one connecting the two arms

• Hence, if the voltages of two of the standard leads are recorded, that of the third lead can be determined mathematically.

what are the heart sounds

• Lub Dub

what causes the heart sounds

Sounds of blood hitting SL and AV valves as they close

• Related to when ventricles are contracted or relaxed

when does lub happen

Lub (S1): closure of the AV

valves at the start of

ventricular systole

when does dub happen

Dub (S2): closure of the SL

valves at the end of

ventricular systole

systole is

contraction

diastole is

• relaxation

what does the Autonomic nervous system consist of

• consists of the sympathetic (SNS) and the

  parasympathetic NS (PNS)

what does the sympathetic nervous system do to heart rate

accelerates

what does the parasympathetic nervous system do to heart rate

decellerates

Electrocardiograms units

millivolts (mV)

what does each peak signal generally

Each peak signals depolarization in a

group of heart cells

1. Depolarization of SA node

2. Depolarization (and

contraction) of atrial muscle

3. Depolarization of AV node

4. Depolarization (and

contraction) of ventricular

muscle

Junctional rhythm

• P wave is absent

  • SA node is not acting as

  the pacemaker, leading

  to the AV node pacing

  the heart

Second degree heart block

• Not all P waves are

  followed by a QRS

  complex

  • Indicates damage to the

  AV node

Ventricular fibrillation

• Impulses generated in

  the atria do not pace

  ventricular contractions

  • Uncoordinated

  contractions of the

  myocardium

  • Typical in acute

  myocardial infarction

Atherosclerosis

Arteries are blocked by fat and cholesterol deposits, reducing blood flow

Normal cardiac muscle

Striated

• Uninucleate cells

Immediately following

infarction (within 12 hours) what happens to muscles

• Loss of striations

  • Loss of nuclei

Healing the tissue

(1-4 days) after infraction

• Necrosis of cardiac

  tissue

  • Invasion of neutrophils

  (phagocytize debris)

Healed tissue (6+ months) after infraction

• Extensive collagen

deposits (scarring)

• Scar tissue is

noncontractile