1. ©2012 Lippincott Williams & Wilkins. All rights reserved.
ECG Workout:
Chapter Review
Chapter 2
Electrophysiology
©2012 Lippincott Williams & Wilkins. All rights reserved.

2. Cardiac cells Myocardial cells Pacemaker cells
The heart is composed of thousands of cardiac cells. In general, cardiac cells have either a mechanical (contractile) or an electrical (pacemaker) function. Cardiac cells consist of two types: myocardial cells and pacemaker cells.
Myocardial cells
Found in muscular layer of heart wall
Contain contractile filaments, which cause muscle contraction
Called "working cells"
Pacemaker cells
Found in electrical conduction system of the heart
Spontaneously generate electrical impulses 2

3. Characteristics of cardiac cells
Cardiac cells have four primary characteristics.
Automaticity
Ability of cardiac cells to spontaneously generate electrical impulses
Characteristic specific to pacemaker cells
Normal concentrations of sodium, potassium, and calcium are important in maintaining automaticity
Excitability
Ability of cardiac cells to respond to an electrical stimulus
Characteristic shared by all cardiac cells
Conductivity
Ability of cardiac cells to conduct an electrical impulse to adjacent cardiac cells
Contractility
Ability of cardiac cells to cause cardiac muscle contraction
Characteristic specific to myocardial "working" cells 3

4. Depolarization and repolarization of cardiac cell
Electrical impulses are the result of the flow of ions – primarily sodium (Na+) and potassium (K+) – back and forth across the cardiac cell membrane. A series of events causes the electrical charge inside the cell to change from its resting state (negative) to its depolarized state (positive) and back to its resting state (negative).  
(continued) 4

5. Depolarization and repolarization of cardiac cell (continued)
Polarized cell (resting state)
Inside of cell is negative
No electrical activity occurring (straight line on ECG)
Depolarized cell (stimulated state)
Electrical stimulation of cardiac cell (depolarization) causes K+ to exit cell and Na+ to enter cell
Inside of cell becomes positive
Muscle contraction follows depolarization
Repolarized cell (recovery state)
Sodium-potassium pump activated to transport Na+ out of cell and K+ back into cell
Inside of cell returns to resting state (negative charge)
Waveforms on the ECG correlate with depolarization and repolarization.
Rate: Normal (60 to 100 beats/minute) or slow (less than 60 beats/minute) 5

6. Electrical conduction system of heart
The heart is supplied with an electrical conduction system that generates and conducts electrical impulses along a special pathway to the atria and ventricles.
Sinoatrial node (SA node)
Located in right upper atrium
Contains pacemaker cells, which discharge impulses at 60 to 100 beats/minute
Called "pacemaker of the heart" because firing rate is higher than that of other pacemaker sites
Interatrial tract (Bachmann’s bundle)
One conduction tract, which conducts impulse from SA node to left atrium
Internodal tracts
Three conduction tracts, which conduct impulse from SA node through right atrium to AV node 6

7. Electrical conduction system of heart (continued)
Atrioventricular node (AV node)
Located in lower right upper atrium near interatrial septum
Only normal conduction pathway between atria and ventricles
Three primary functions:
- Slows conduction of electrical impulse through AV node to allow time for atrial contraction (atrial kick); delay in AV node represented on ECG as flat line of PR interval
- Serves as backup pacemaker at firing rate of 40 to 60 beats/minute
- Blocks rapid atrial impulses from being conducted to ventricles, protecting ventricles from dangerously fast rates
Bundle of His
Connects AV node to bundle branches
Bundle branches
Consist of right bundle branch and left bundle branch, which conducts electrical impulses to the right and left ventricle
Purkinje fibers
Network of fibers located at end of bundle branches, which conduct electrical impulses to the ventricular myocardium
Ventricles are capable of serving as a backup pacemaker at firing rate of 30 to 40 beats/minute (sometimes less) 7

8. ECG waveforms
The heart's electrical activity is represented on the ECG tracing by three basic waveforms: the P wave, the QRS complex, and the T wave (a U wave is sometimes present)
Between the waveforms are segments and intervals: PR interval, PR segment, ST segment, and QT interval
(continued) 8

9. Cardiac cycle
• A cardiac cycle consists of one heartbeat or one PQRST sequence
- One cycle represents atrial contraction and relaxation followed by ventricular contraction and relaxation
- This basic cycle repeats itself
• Regularity of a cardiac rhythm can be measured from one heartbeat to the next (from one R wave to the next R wave, also called the R-R interval) 9

10. Waveforms and the isoelectric line
• Between cardiac cycles, the ECG records a straight line (isoelectric line or baseline)
• Any waveform above baseline is considered a positive deflection; a waveform below baseline is considered a negative deflection; a waveform that is both above and below baseline is considered a biphasic deflection 10

11. Waveforms and current flow
An ECG tracing provides a view of the heart's electrical activity between two poles (a positive and a negative pole).
• Current flowing toward a positive pole produces a positive deflection
• Current flowing toward a negative pole produces a negative deflection
• Current flowing away from both poles will produce a biphasic deflection; biphasic deflections may be equally positive and negative, more positive than negative, or more negative than positive. 11

12. Refractory periods of cardiac cycle
There is a period of time during the cardiac cycle when cardiac cells may be refractory (unable to respond) to a stimulus. Refractoriness is divided into three phases.
Absolute refractory period
Cardiac cells have not repolarized to their threshold potential and are unable to respond to a stimulus
Period extends from onset of QRS complex to peak of T wave
Relative refractory period
Cardiac cells have repolarized sufficiently to respond to a strong stimulus
Period extends from peak of T wave to end of T wave
Called "vulnerable period" of repolarization since a strong stimulus may take possession as pacemaker of the heart
Supernormal period
Cardiac cells have almost completely repolarized and will respond to a weak stimulus
Period occurs near end of T wave 12

13. QRS width of 2 squares = 0.08 second (0.04 second × 2 squares)
ECG graph paper
Waveforms are recorded on graph paper containing horizontal and vertical lines. The lines are used to measure width and height of deflections.
Width
• Horizontal lines are used to measure duration (width) of waveforms in seconds of time
• Each small square measured horizontally measures 0.04 second (½ square = 0.02 second)
QRS width of 2 squares = 0.08 second (0.04 second × 2 squares)
(continued) 13

14. ECG graph paper (continued)
Height
• Vertical lines are used to measure height (amplitude or voltage) of a waveform
• Each square measured vertically from baseline measures 1 millimeter (mm) in height
• QRS extending upward from baseline 16 small squares = 16 mm voltage (1 mm × 16 squares)
QRS width: 0.08 second
QRS height: 16 mm 14