The electrocardiogram (EKG) is a recording of the spread of electrical depolarization in the heart
The recording is made from twelve different sites called LEADS. These are divided into three main types:
1. Limb leads or Frontal plane leads (I, II and III).
›› Lead I: Right arm (negative) and Left arm (positive)
›› Lead II: Right arm (negative) and Left leg (positive)
›› Lead III: Right arm (negative) and Left leg (positive)
The three standard limb leads form a triangle over the body and have a mathematical relationship described by Einthoven:
The height or the depth of the recordings in lead I plus lead III equals the height or depth of the recordings in Lead II.
Lead I + Lead III = Lead II
2. Augmented Leads (aVR, aVL, AvF)
›› aVR-Augmented: Voltage of the right arm
The right arm is the positive electrode in reference to the left arm and the left leg, this lead records the electrical activity from the directions of the right arm to the heart.
›› aVL-Augmented: Voltage of the left arm
The left arm is the positive electrode in reference to the right arm and the left leg, this lead views the electrical activity from the direction of the left arm to the heart.
›› aVF-Augmented: Voltage of the left foot
The left foot or the left leg is the positive electrode in reference to the left arm and the right arm. This lead views the electrical activity from the direction of the inferior surface of the heart.
3. Precordial leads (V leads or Horizontal plane leads)
The six precordial leads are unipolar leads and views the electrical activity of the heart in a horizontal plane.
Placement of Precordial Leads
V-1-Fourth intercostal space at the right sternal border.
V-2- Fourth inter costal space at the left sternal border.
V-3-Midway between V2-V4.
V-4- Fifth or sixth, intercostal space at the mid clavicular line.
V-5- Left anterior axillary line.
V-6- Left mid axillary line at the same level as V4 to V5.
V-7- Posterior axillary line at the same level as V4-V6.
V-8- Below the angle of the left scapula.
V3R- On the right chest in comparable position to V3.
1. Lewis Leads:
A bipolar measurement of the atrial cavity. The negative right arm lead is connected to an electrode place in the 2nd intercostal space. The positive left arm wire is connected to an electrode located in the 4th intercostal space.
2. Esophageal leads:
A unipolar lead and an invasive procedure to identify atrial activity or this is done by the introduction of a temporary pacing lead to the patient's esophagus via the nose or throat. The optimal position of the lead is posterior to the right atrium near the SA node. The less invasive esophageal leads recording is done by Pill electrode; where the patient swallows a pill connected to an amplifier and to an EKG machine. The physician can control the height of the attached pill to get the best tracing.
3. Right Precordial Leads
Can be recorded by changing the left placed leads to the mirror image positions on the right side of the chest walls. These lead positions are always indicated if right ventricular infarction is suspected.
4. Position Placement Leads (V7-8-9)Are used as an aid in the diagnosis of a posterior wall myocardial infarction. These special leads show the postero-lateral area of the heart. V7, V8, V9: Located at the same horizontal level as V4, V5 and V6 in the posterior axillary line, the mid scapular line and 1 inch lateral from the spine respectively.
Remember to use double STD sensitivity when using V8, V9 due to the thickness of the chest wall in those areas.
Features of an EKG Strip
A) EKG paper:
›› One little square (1mm) is 0.04 seconds wide.
›› One big square (5mm) is 0.20 seconds wide.
B) Paper speed:
›› Normally at 25mm/second.
›› Be aware of double speed tracing at 50mm/second, which may simulate heart block and conduction disturbances.
›› Must be on tracing for proper interpretation.
›› The one millivolt standard must be 10mm tall.
›› The standardization must have sharp edges.
The Stepwise Approach To Reading an EKG
The P-wave to P-wave interval and the R-wave to the R-wave should be constant. Rate
The R-wave to R-wave interval determines the ventricular rate.
The P-wave to P-wave interval determines the atrial rate.
The inherent rate of the conduction system are as follows:
SA node 60-100 bpm
AV junction 40-60 bpm
Ventricles 20-40 bpm
|1.||Count the number of R-waves in 5 seconds strip and multiply by 12||Quick estimate less accurate|
|2.||Count the number of large squares between consecutive R-waves and divide by 300
alternatively memorize this scale
1 large square = 300 bpm
2 large squares= 150 bpm
3 large squares = 100 bpm
4 large squares = 75 bpm
5 large squares = 60 bpm
6 large squares = 50 bpm
|Quick estimate only with regular rhythms less accurate with fast heart rates|
|3.||Count the number of small squares between 2 consecutive R-waves and divide by 1500||The most accurate used with regular rhythms|
|4.||Count the duration in seconds between two R-waves and divide this number by 60|
Determination of the QRS Axis (Cardiac Vector)
›› Indicates the direction of the mean QRS vector within the heart.
›› Refers to the average direction of depolarization that spreads through the ventricles.
›› In addition to the hexaxial system to determine the QRS axis, there are three quick ways to calculate the QRS axis on an EKG using leads I, II, III, aVR, aVL and aVF:
1. The tallest QRS is found in the EKG lead that is pointing towards the QRS axis.
2. The most negative QRS that is seen in the EKG lead that points directly away from the QRS axis.
3. An equiphasic QRS is in the EKG lead that is at right angle t o the QRS axis.
N.B: You may find an EKG in which all of the six frontal plane leads are equiphasic. This arrangement makes it impossible to determine the correct axis and is known as the indetermine axis.
›› The Hexaxial system for determination of the cardiac axis:
›› Memorize the positive poles of the six limb leads ( standard and augmented leads)
›› Most frequently used to determine the axis
›› It can also be used to determine the axis of the P-wave and T-wave.
The P-wave:›› The P-wave represents atrial depolarization.
›› The P-wave duration or width: reflects the time required for depolarization to spread through the atria to the atrioventricular node.
›› In normal adults P-wave duration is < 0.10 seconds
›› The normal P-wave axis (vector) is normally directed anteriorly and leftward (+45° to +60°). If the P-wave is +60° to +90°, consider right atrial overload.
›› If the axis is to the left of +30°, consider left atrial overload or abnormal heart rhythm.
›› Normal P-wave amplitude is 0.5 - 2.5 mm.
The P-R Interval›› The P-R interval represents the time required for an impulse to depolarize the atria, pass the AV node and enter the ventricular conduction system.
›› It is measured from the start of the P-wave to the initial deflection of the QRS complex (Q or R).
›› A normal P-R interval is 0.12 - 0.20 seconds in adults in sinus rhythm. Shorter than 0.12 seconds is termed as accelerated conduction and longer than 0.20 seconds first degree heart block.
›› The P-R interval normally shortens as the heart rate increases and lengthens at slower heart rates.
The QRS Complex›› The QRS complex represents ventricular depolarization.
›› The QRS interval or duration is an indication of intraventricular time.
›› The normal value in adults is <0.10 seconds.
›› RSR in V1 is seen as a normal variant in young people.
›› The mean frontal plane QRS vector in the normal adult is -30°to 110°
›› The axis undergoes changes with aging as demonstrated below:
Age Normal Axis
Birth to 1 year +90° to +150°
1 year to 8years +45° to +105°
above 8 years 0° to +90°
Causes of Poor R-Wave Progression in the Precordial Leads are:
The normal transition to a positive QRS complex usually occurs in V4. Poor R-wave progression refers to a reduced rate of R-wave development as the tracing progresses from V1 to V6. This can be seen with the following:
›› Anteroseptal / anterior infarction
›› Left anterior hemiblock.
›› Right ventricular dilatation/hypertrophy.
›› Incomplete LBBB.
›› Left ventricular hypertrophy (hypertension, cardiomyopathy).
›› Ventricular pacing.
›› Double speed tracing.
Causes of Low QRS Voltage are:
The criteria for low QRS voltage are:
The QRS excursion is less than 5mm in all standard limb leads, less than 10mm in all of the precordial chest leads with the following:
›› Chronic obstructive airway disease
›› Pericardial disease (effusion) and transit in electrical alternans.
›› Poor left ventricular function.
›› Obesity or muscular chest.
›› Myocardial infiltrate diseases (amyloidosis, sarcoidosis, carcinoid heart disease etc.
›› Difuse myocardial disease (myocarditis).
Causes of Prominent R-wave in V1 are:
›› Right ventricular hypertrophy
›› True posterior myocardial infarction
›› WPW Syndrome type A
›› Right bundle branch block
›› Hypertrophic cardiomyopathy
›› Duchene muscular dystrophy
›› Left ventricular diastolic overload
›› Physiological in third trimester of pregnancy
The ST segment
›› A ST segment is the isoelectric segment following ventricular depolarization and preceeds ventricular repolarization
›› It is measured from the end of the QRS complex to the beginning of the T-wave.
›› The interval from the end of the T-wave to the beginning of the P-wave TP interval is usually taken as the isoelectric reference point
›› Although elevation or depression of the ST segment by +/- 1mm from the isoelectric baseline is considered abnormal, ST elevation upto 2mm in V1-V2 is still acceptable
Types of ST segment depression
›› A) Junctional: Normal in conditions with high heart rate.
›› B) Downsloping: Due to ventricular hypertrophy or to digoxin effect (isoelectric J point).
›› C) Scooping: When the ST segment dips below the baseline indicative of digoxin effect.
›› D) J-point Depression: with horizontal or downsloping ST segments indicates myocardial ischemia.
Early repolarization›› Early repolarization is a normal variant of ST elevation.
›› Thought to be caused by accelerated subepicardial repolarization.
›› It is most prominent in the lateral precordial leads (V4-V6).
›› The T-wave in these leads is characteristically broad based tall (usually >5mm) and upright.
›› Early repolarization has been reported in all age groups, more common in black males.
›› It is very important to differentiate early repolarization from pericarditis. It has been suggested that the ratio of the ST segment to the T-wave amplitude in lead V6 helps to distinguish the two conditions since the ST/T ratio in V6 is generally less than 0.25 in early repolarization, but is more than 0.25 in pericarditis.
The T-wave›› The T-wave represents ventricular repolarization.
›› A-T-wave vector is normally directed inferiorly and leftward.
›› The T-wave vector normally tracks with the QRS vector. If the QRS is predominantly negative in frontal plane lead, and inverted T-wave lead is usually seen and is not necessarily abnormal.
›› Normally inverted T-wave can be seen in V1, aVR. They can also be seen in patients under the age of 30 with back or chest wall abnormalities in leads V2 and V3.
›› Normal axis of a T-wave is 0° to +90°
›› Normal T-wave amplitude is more than 1mm in all extremity leads, or at least 10% of the preceeding R-wave.
Causes of Tall T-wave are:›› Hyperkalemia
›› Hyperacute phase of myocardial infarction
›› Inferior wall ischemia
›› Left ventricular diastolic overload
›› Double standard tracing
T-wave AlternansAlternating T-wave voltage, shape or axis can be associated with:
›› Advanced heart disease
›› Severe electrolytes disturbances
›› Following cardiac arrest
›› Acute pulmonary embolism
›› Idiopathic QT-prolongation syndrome and tachycardias.
Types of T-wave inversion:
There are primary or secondary types of T-wave inversions.
Primary T-wave inversion:
Are due to changes in the ventricular myocardium independent of the depolarization sequence. This can be due to physiological, pharmacological or pathological factors.
Features include:›› Symmetrical T-wave inversion
›› Peaked T-waves
›› J-Point is either isoelectric or elevated
Causes of Primary T-wave inversion:
Most times T-wave inversions are non-specific. However, these may become specific in the presence of QRS, ST or QT abnormalities due to:
›› After eating or after drinking ice water
›› Mitral valve prolapse
Secondary T-wave inversions:
Secondary T-wave inversions are the result of an abnormal sequence of the ventricular activation. It is independent of the basic alternation in the myocardium.
›› Asymmetric T-wave inversion
›› ST depression with upward convexity
›› J-Point depression
Causes of Secondary T-wave inversion are:›› Bundle branch block
›› VPBs or ventricular rhythm
›› Ventricular pacing
›› Ventricular hypertrophy
The QT interval is measured from the beginning of the QRS complex to the end of the T-wave.›› It represents the duration of electrical systole
›› The interval varies with heart rate
›› The normal corrected QT interval is usually less than 0.425 seconds in males and <0.43 seconds in females and is calculated by dividing the measured QT interval in seconds by the square root of the R-R interval
›› As a general rule, the QT interval should be less than 50% of the R-R interval at heart rate between 60-90 bpm
Causes of prolonged QT-interval include:›› Double paper speed
›› Cerebrovascular accidents (Stroke, SAH..etc)
›› Hypocalcemia & other electrolytes abnormalities
›› Anti-arrhythmic (class I.A & class III)
›› Anti depressants
›› Myocardial ischemia or infarction
›› Mitral valve prolapse
›› Congential long QT-syndromes - with deafness-Jervell syndrome
- without deafness: Romano -Ward syndrome
Causes of short QT- interval include:›› digitalis
›› tachycardias (sinus, supraventricular ..etc)
The U wave›› The U-wave is a positive deflection following the T-wave
›› Is of uncertain electrophysiological origin
›› Its amplitude is greatest in precordial leads V2-V4
›› It can be exaggerated with hypokalemia, digitalis, Amiodarone
›› A negative U-wave is rare (1%). It can be the first manifestation of myocardial involvement in hypertension, but can also be seen in conjunction with RVH, myocardial ischemia, aortic and mitral valve disease.