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

Electrocardiogram (ECG/EKG)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 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

Normal conduction pathway:

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

Types of ECG RecordingsBipolar leads record voltage between electrodes placed on wrists & legs (right leg is ground)Lead I records between right arm & left armLead II: right arm & left legLead III: left arm & left leg

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 (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

Ischemic Heart DiseaseIs most commonly due to atherosclerosis in coronary arteriesIschemia occurs when blood supply to tissue is deficientCauses increased lactic acid from anaerobic metabolismOften accompanied by angina pectoris (chest pain)

Ischemic Heart DiseaseDetectable 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)

Arrhythmias Detected on ECGArrhythmias are abnormal heart rhythmsHeart rate 100/min is tachycardia

Arrhythmias Detected on ECG In flutter contraction rates can be 200-300/minIn fibrillation contraction of myocardial cells is uncoordinated & pumping ineffectiveVentricular fibrillation is life-threateningElectrical defibrillation resynchronizes heart by depolarizing all cells at same time

AV node block occur when node is damagedFirstdegree AV node block is when conduction through AV node > 0.2 secCauses long P-R intervalSecond-degree AV node block is when only 1 out of 2-4 atrial APs can pass to ventriclesCauses P waves with no QRSIn third-degree or complete AV node block no atrial activity passes to ventriclesVentricles driven slowly by bundle of His or Purkinjes

Arrhythmias Detected on ECG

AV node block occurs when node is damagedFirstdegree AV node block is when conduction thru AV node > 0.2 secCauses long P-R intervalArrhythmias Detected on ECG

Second-degree AV node block is when only 1 out of 2-4 atrial APs can pass to ventriclesCauses P waves with no QRSArrhythmias Detected on ECG

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

Arrhythmias Detected on ECG

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*T: QRS T

*ST segment: QRS T . : 0.08-0.12 sec (80-120 msec)

QT QPS T QT 0.40sec . QT interval .

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