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

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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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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

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Normal conduction pathway:

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

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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.

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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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Fig. 13.22b

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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

ECG

Fig 13.2413-63

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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

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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

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Fig. 13.24b

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Fig. 13.24c

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Fig. 13.24d

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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

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•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

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• 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

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Fig. 13.24g

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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

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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

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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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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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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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Arrhythmias Detected on ECG 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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• 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

Arrhythmias Detected on ECG continued

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• 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

Arrhythmias Detected on ECG continued

Fig 13.36

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• 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

Arrhythmias Detected on ECG continued

Fig 13.36

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• In third-degree or complete AV node block, no atrial activity passes to ventricles– Ventricles are driven slowly by bundle of His or Purkinjes

Arrhythmias Detected on ECG continued

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.

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