1. Electrocardiography
A recording of the electrical activity of the heart over time
Gold standard for diagnosis of cardiac arrhythmias
Helps detect electrolyte disturbances (hyper- & hypokalemia)
Allows for detection of conduction abnormalities
Screening tool for ischemic heart disease during stress tests
Helpful with non-cardiac diseases (e.g. pulmonary embolism or hypothermia

2. Electrocardiogram (ECG/EKG)
Is a recording of electrical activity of heart conducted thru ions in body to surface
Fig 13.22a
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3. ECG Graph Paper Runs at a paper speed of 25 mm/sec
Each small block of ECG paper is 1 mm2
At a paper speed of 25 mm/s, one small block equals 0.04 s
Five small blocks make up 1 large block which translates into 0.20 s (200 msec)
Hence, there are 5 large blocks per second
Voltage: 1 mm = 0.1 mV between each individual block vertically

4. Normal conduction pathway:
SA node -> atrial muscle -> AV node -> bundle of His -> Left and Right Bundle Branches -> Ventricular muscle

5. Recording of the ECG:
Leads used:
Limb leads are I, II, II. So called because at one time subjects had to literally place arms and legs in buckets of salt water.
Each of the leads are bipolar; i.e., it requires two sensors on the skin to make a lead.
If one connects a line between two sensors, one has a vector.
There will be a positive end at one electrode and negative at the other.
The positioning for leads I, II, and III were first given by Einthoven. Form the basis of Einthoven’s triangle.

6. Types of ECG Recordings
Bipolar leads record voltage between electrodes placed on wrists & legs (right leg is ground)
Lead I records between right arm & left arm
Lead II: right arm & left leg
Lead III: left arm & left leg
Fig 13.23
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7. Fig b

8. ECG 3 distinct waves are produced during cardiac cycle
P wave caused by atrial depolarization
QRS complex caused by ventricular depolarization
T wave results from ventricular repolarization
Fig 13.24
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9. Elements of the ECG: P wave: Depolarization of both atria;
Relationship between P and QRS helps distinguish various cardiac arrhythmias
Shape and duration of P may indicate atrial enlargement
PR interval: from onset of P wave to onset of QRS
Normal duration = sec ( ms) (3-4 horizontal boxes)
Represents atria to ventricular conduction time (through His bundle)
Prolonged PR interval may indicate a 1st degree heart block
QRS complex: Ventricular depolarization
Larger than P wave because of greater muscle mass of ventricles
Normal duration = seconds
Its duration, amplitude, and morphology are useful in diagnosing cardiac arrhythmias, ventricular hypertrophy, MI, electrolyte derangement, etc.
Q wave greater than 1/3 the height of the R wave, greater than 0.04 sec are abnormal and may represent MI

10. ST segment:
Connects the QRS complex and T wave
Duration of sec ( msec
T wave:
Represents repolarization or recovery of ventricles
Interval from beginning of QRS to apex of T is referred to as the absolute refractory period
QT Interval
Measured from beginning of QRS to the end of the T wave
Normal QT is usually about 0.40 sec
QT interval varies based on heart rate

12. Fig b

13. Fig c

14. Fig d

15. P wave Elements of the ECG:
Depolarization of both atria;
Relationship between P and QRS helps distinguish various cardiac arrhythmias
Shape and duration of P may indicate atrial enlargement

17. QRS complex: Represents ventricular depolarization
Larger than P wave because of greater muscle mass of ventricles
Normal duration = seconds
Its duration, amplitude, and morphology are useful in diagnosing cardiac arrhythmias, ventricular hypertrophy, MI, electrolyte derangement, etc.
Q wave greater than 1/3 the height of the R wave, greater than 0.04 sec are abnormal and may represent MI

19. PR interval: From onset of P wave to onset of QRS
Normal duration = sec ( ms) (3-4 horizontal boxes)
Represents atria to ventricular conduction time (through His bundle)
Prolonged PR interval may indicate a 1st degree heart block

20. Fig g

21. T wave: Represents repolarization or recovery of ventricles
Interval from beginning of QRS to apex of T is referred to as the absolute refractory period

22. ST segment: QT Interval Connects the QRS complex and T wave
Duration of sec ( msec
QT Interval
Measured from beginning of QRS to the end of the T wave
Normal QT is usually about 0.40 sec
QT interval varies based on heart rate

23. Ischemic Heart Disease
Is most commonly due to atherosclerosis in coronary arteries
Ischemia occurs when blood supply to tissue is deficient
Causes increased lactic acid from anaerobic metabolism
Often accompanied by angina pectoris (chest pain)
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Myocardial Infarction
RealMedia Movie
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24. Ischemic Heart Disease
Detectable by changes in S-T segment of ECG
Myocardial infarction (MI) is a heart attack
Diagnosed by high levels of creatine phosphate (CPK) & lactate dehydrogenase (LDH)
Fig 13.34
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25. Arrhythmias Detected on ECG
Arrhythmias are abnormal heart rhythms
Heart rate <60/min is bradycardia; >100/min is tachycardia
Fig 13.35
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26. Arrhythmias Detected on ECG continued
In flutter contraction rates can be /min
In fibrillation contraction of myocardial cells is uncoordinated & pumping ineffective
Ventricular fibrillation is life-threatening
Electrical defibrillation resynchronizes heart by depolarizing all cells at same time
Fig 13.35
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27. Arrhythmias Detected on ECG continued
AV node block occur when node is damaged
First–degree AV node block is when conduction through AV node > 0.2 sec
Causes long P-R interval
Second-degree AV node block is when only 1 out of 2-4 atrial APs can pass to ventricles
Causes P waves with no QRS
In third-degree or complete AV node block no atrial activity passes to ventricles
Ventricles driven slowly by bundle of His or Purkinjes
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28. Arrhythmias Detected on ECG continued
AV node block occurs when node is damaged
First–degree AV node block is when conduction thru AV node > 0.2 sec
Causes long P-R interval
Fig 13.36
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29. Arrhythmias Detected on ECG continued
Second-degree AV node block is when only 1 out of 2-4 atrial APs can pass to ventricles
Causes P waves with no QRS
Fig 13.36
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30. Arrhythmias Detected on ECG continued
In third-degree or complete AV node block, no atrial activity passes to ventricles
Ventricles are driven slowly by bundle of His or Purkinjes
Fig 13.36
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31. Representation in culture
In TV medical dramas, an isoelectric ECG (no cardiac electrical activity, aka, flatline, is used as a symbol of death or extreme medical peril.
Technically, this is known as asystole, a form of cardiac arrest, with a partcularly bad prognosis.
Defibrillation, which can be used to correct arrythmias such as ventricular fibrillation and pulseless ventricular tachycardia, cannot correct asystole.