Cardiac ElectrophysiologyElectrocardiogram – Part II

April Strader Ph.D

Lecture 4

Electrical Properties of Different Cardiac Tissues

Tissue Name Function Major Currents

B-adrenergic

Effects

Cholinergic

Effects

SA Node Primary

Pacemaker

Ica, Ik, If ↑ conduction velocity and pacemaker rate

↓ conduction velocity and pacemaker rate

AV node Secondary

Pacemaker

Ica, Ik, If ↑ conduction velocity and pacemaker rate

↓ conduction velocity and pacemaker rate

Purkinje

fibers

Tertiary

Pacemaker

Rapid conduction of AP

Ica, Ik, If, Ina ↑ pacemaker rate

↓ pacemaker rate

Atrial Muscle Expel blood from Atria

Ica, Ik, Ina ↑ strength of contraction

Little effect

Ventricular Muscle

Expel blood from

Ventricles

Ica, Ik, Ina ↑ strength of contraction

Little effect

From Table 20-3

Electrical system of the heart

Purkinje fibers

Bundle of His

Sinoatrialnode

Atrioventricular node

3 possible Pacemakers – Primary – Sinoatrial node, Secondary – Atrioventricular nodeTertiary – Purkinje fibers

Direction of cardiac muscle depolarization

P wave – depolarization of atriaQRS complex – depolarization of ventricular muscleT wave – repolarization of ventricular muscle*repolarization of atria lies under QRS

The Electrocardiogram

What information does the EKG give you?

Fig. 20-6

rate, rhythm, tissue health

Two cell model of EKG activity

Positive deflection

Negative deflection

isoelectric

When wave of depolarization moves towards apositive electrode, the deflection is positive.

Fig. 20-9

6 limb leads – define electrical activity in frontal plane

6 precordial leads –define electrical activityin transverse plane

Each lead is a single axisin one of the planes

12 Lead EKG

The 3 augmented leads compareone limb electrode to the average of the other two. (aVR, aVL, aVF)

Leads are made of a combination of electrodes that form imaginary lines

in the body along which the electrical signals are measured.

Fig. 20-7

Electrical axis of the heart is normally between -30 and 90 degrees.

Einthoven’s Triangle(6 limb leads)

Fig. 20-8

Willem Einthoven Nobel prize in 1924 for electrocardiogram

(discovered in 1903)

Using Einthoven’s TriangleWe can estimate the electrical axis of the heart

Read p. 498

Fig. 20-11

What does the axis tell us?

Left axis deviation(between -90 and -30 degrees)-Short and/or obese persons-parallels amount of conductance over tissue

Right axis deviation(between 180 and 90 degrees)-Tall and thin persons-parallels amount of conductance over tissue

Conduction abnormalitiesand arrhythmias

Conduction Blocks

• First degree AV block– Slowing of conduction from SA to AV – Longer P-R intervals

• Second degree AV block– Partial block/intermittent– P wave intermittently dissociated from QRS (Mobitz type I and II)

(long P-R or absent QRS)

• Third degree AV block– Complete block of impulse – complete AV dissociation. – Atria and Ventricles are electrically separate, Purkinje take over– P waves and QRS have no relationship– Usually requires artificial pacemaker

First degree block

Second degree block

Third degree block

Normal

WPW

Conduction Arrhythmias

Re-entry – defect is a unidirectional block

-Can cause continuous excitation called “circus movement”-Wave of depolarization travels in an endless circle

Requirements

1) Closed conduction loop

2) A region of unidirectionalblock

3) Slow conduction of action potentials aroundthe loop (to allow for ERPto pass)

Wolff-Parkinson-White Syndrome

-Common example of accessory conduction pathway (Bundle of Kent)

-A common route for a re-entry pathway-Often results in supraventricular tachycardia -Occurs in ~ 0.3-1% of population

(Bundle of Kent)

Delta wave

Cation distubances and the EKG

• Look at how potassium and calcium regulate the action potential and imagine what effect they have on the EKG pattern. – Hypokalemia– Hypercalcemia

http://www.gwc.maricopa.edu/class/bio202/cyberheart/ekgqzr2.htm

Good EKG quiz website

- decreases amplitude or inversion of the T wave-          increases amplitude of the U wave-          prolongation of the Q-T interval-          Increased amplitude of the P wave, prolongation of the P-R interval-          Widening of the QRS complex

HYPOKALEMIA

Hyperkalemia