ECG Basics

What is an ECG? The electrocardiogram (ECG) is a representation of the electrical events of the cardiac cycle. Each event has a distinctive waveform, the study of which can lead to greater insight into a patient’s cardiac pathophysiology.

Electrocardiogram (ECG/EKG) 13-60

ECG Leads Leads are electrodes which measure the difference in electrical potential. They can be of two types: 1. Two different points on the body (bipolar leads) 2. One point on the body and a zero electrical potential.

EKG Leads The standard EKG has 12 leads: 3 Standard Limb Leads 3 Augmented Limb Leads 6 Precordial Leads The axis of a particular lead represents the viewpoint from which it looks at the heart.

Standard Limb Leads

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 13-61

aVR, aVL, aVF (augmented limb leads) Summary of Leads Limb Leads Precordial Leads Bipolar I, II, III (standard limb leads) - Unipolar aVR, aVL, aVF (augmented limb leads) V1-V6

Arrangement of Leads on the EKG

Precordial Leads Adapted from: www.numed.co.uk/electrodepl.html

Precordial Leads

one small block equals 0.04 s ECG Graph Paper one small block equals 0.04 s Five small blocks make up 1 large block which translates into 0.20 s (200 msec)

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

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

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

Waveforms and Intervals

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

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

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

Determining the Heart Rate Rule of 300 10 Second Rule

Rule of 300 Count the number of “big boxes” between neighboring QRS complexes, and divide 300 by this number. The result will be approximately equal to the rate Although fast, this method only works for regular rhythms.

What is the heart rate? www.uptodate.com (300 / 6) = 50 bpm

What is the heart rate? www.uptodate.com (300 / ~ 4) = ~ 75 bpm

What is the heart rate? (300 / 1.5) = 200 bpm

The Rule of 300 It may be easiest to memorize the following table: # of big boxes Rate 1 300 2 150 3 100 4 75 5 60 6 50

10 Second Rule As most EKGs record 10 seconds of rhythm per page, one can simply count the number of beats present on the EKG and multiply by 6 to get the number of beats per 60 seconds. This method works well for irregular rhythms.

The QRS Axis The QRS axis represents the net overall direction of the heart’s electrical activity. Abnormalities of axis can hint at: Ventricular enlargement Conduction blocks (i.e. hemiblocks)