Download - Cardiac electrophysiology part ii lecture 4
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