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)

• Is a recording of electrical activity of heart conducted thru ions in body to surface

ECG Graph Paper
• Runs at a paper speed of 25 mm/sec
• Each small block of ECG paper is 1 mm 2
• 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
QuickTime™ and a
TIFF (Uncompressed) decompressor are needed to see this picture.

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

Recording of the ECG :
Leads used:
• Limb leads are I, II, III. 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.

• 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
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• 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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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 = 0.12-2.0 sec (120-200 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 = 0.08-0.12 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

ST segment:
• Connects the QRS complex and T wave
• Duration of 0.08-0.12 sec (80-120 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

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

• QRS complex:
• Represents ventricular depolarization
• Larger than P wave because of greater muscle mass of ventricles
• Normal duration = 0.08-0.12 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

• PR interval
:
• From onset of P wave to onset of QRS
• Normal duration = 0.12-2.0 sec (120-200 ms) (3-4 horizontal boxes)
• Represents atria to ventricular conduction time (through His bundle)
• Prolonged PR interval may indicate a 1st degree heart block

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

ST segment:
• Connects the QRS complex and T wave
• Duration of 0.08-0.12 sec (80-120 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

• 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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• 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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• Arrhythmias are abnormal heart rhythms
• Heart rate <60/min is bradycardia ; >100/min is tachycardia
Fig 13.35
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continued
• In flutter contraction rates can be 200-300/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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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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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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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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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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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.