Electrocardiography(ECG)

Recording and interpretation

Definition:

• The 12 lead ECG is a recording of the electrical activity of the heart, and is an essential diagnostic tool in the management and treatment of heart

disease - (Jevon 2000)

• ECG provides graphical representation of electrical forces which appears in graph as a series of positive and negative deflections

Indications of ECG recording:

• Chest pain

• Myocardial infarction

• Palpitations

• After successful CPR

• History of syncope (fainting)

• Screening due to multiple risk factors of CVD

Purposes of ECG

Measure the rate and regularity heartbeats

Size and position of the chambers, Presence of any damage to the heart Effects of drugs Conduction system of heart Effects of electrolyte imbalace

Points kept in mind before ECG recording:

Although recording an ECG is a relatively easy

procedure, it is vital that it is recorded

accurately to avoid misinterpretation to ensure

an accurate reading:

• It is important that the patient is as comfortable as possible .

Contd…• The temperature of the room should be

adequate .

• The patient should preferably be in a supine position

• In order to facilitate contact with the electrode pads, it is necessary to clean the skin with an alcohol swab to remove any body lotion or sweat

Equipments required for ECG recording:

• ECG machine

• Alcohol swabs

• Shaving set (optional)

• ECG jelly

• Disposable paper/ tissue

• Screen

Procedure ECG recording:

Procedure of ECG recording:

• Explain procedure to patient and confirm consent

• Wash hands as per protocol

• Ensure patient is comfortably positioned

• Prepare skin and electrode sites by cleaning with alcohol swabs

• Apply electrodes ensuring adequate adhesion

Limb leads:

• Red (RA) – inner right wrist • Yellow (LA) – inner left wrist • Green (LL) – inner left leg just above

ankle • Black (RL) – inner right leg just above

ankle • (Starting at right arm, in a clockwise

direction Ride Your Green Bike)

Chest leads:• V1 – fourth intercostal space, rt of sternum

• V2 – fourth intercostal space, lt of sternum

• V3 – midway between V2 and V4

• V4 – 5th intercostal space, mid clavicular

• V5 – 5th intercostal space , anterior axilla)

• V6 – 5th intercostal space, midpoint of armpit

Chest leads

Contd…

• switch on machine

• Check calibration is 10mm/millivolt

• Input patient/client data

• Ask patient/client to relax and refrain from movement

• Start recording 12 lead ECG

• Reassure patient/client throughout recording

After procedure:

• Detach recording and ensure labelling is correct

• Remove the electrodes

• Provide tissue paper to patient to clean jelly

• Clean ECG leads with tissue paper and spirit swabs

• Correctly file ECG recording & report to physician

ECG graph paper:

• In the ECG Graph Paper there are Horizontal axis and vertical axis.

• The horizontal axis represents time in milliseconds (ms) and vertical axis represents amplitude or voltage in millivolts (mV).

Interpretation of ECG recording

• In ECG graph there are small and large boxes. If we see ECG graph :

• Horizontally:

- One small box - 0.04 s, = 1mm– One large box - 0.20 s = 5mm

• Vertically :

- One large box - 0.5 mV– 2 large boxes – 1mV

Two 5-mm-divisions on the vertical axis are calibrated to represent 1 mV

Contd…

• Horizontally– One small box - 0.04 s– One large box - 0.20 s

• Vertically– One large box - 0.5 mV

Contd…

ECG waves and intervals

Certain important facts about the direction and magnitude of ECG waves:

Provides graphical depiction of electrical forces

Graph appears as a series of deflections Deflections above isoelectric line are positive Isoelectric line – period of electric inactivity,

during which no deflections are observedDeflection mainly depends- 2 factors spread of electric force location of recording electrode

Contd…Electrical impulses moving towards an

electrode- positive deflectionAway – negativeMagnitude of deflection- muscle massActivation of atria occur- longitudinally-

reflects atrial enlargementVentricles-transversely-hypertrophy

Contd… a current surging directly in

the direction -recording electrode-positive deflection

a current flowing in the direction but not directly toward the recording electrode -positive deflection of lower amplitude

running at right angle - recording electrode -no deflection or a biphasic deflection;

flowing away -recording electrode -negative deflection

Normal Impulse ConductionSinoatrial node

AV node

Bundle of His

Bundle Branches

Purkinje fibers

Impulse Conduction & the ECGSinoatrial node

AV node

Bundle of His

Bundle Branches

Purkinje fibers

The “PQRST”

• P wave - Atrial depolarization

• T wave - Ventricular repolarization

• QRS - Ventricular depolarization

The P Wave

• The first deflection is the P wave associated with right and left atrial depolarization. Wave of atrial repolarization is invisible because of low amplitude

PR interval

Interval: 0.12 to 0.20Prolonged PR Interval AV Node Block Hyperthyroidism Shortened PR Interval Wolf-Parkinson-

White Syndrome (WPW Syndrome) Hypertension

QRS Complex• Normal findings: DurationLimb leads (I, II, III): 0.05

to 0.10• Precordial leads (V1 to V6): 0.06 to 0.12 Wide QRS or Prolonged QRS -Left Bundle Branch

Block• Medications ( Toxin Ingestion) Low QRS amplitude (<5 mm in limb leads)Diffuse

Coronary Artery Disease• Congestive Heart Failure• Pericardial Effusion High QRS amplitude- Left Ventricular Hypertrophy

ST segment

• Measurement– Measure at 0.04 sec (1 mm) after the J-

Point

• Causes( ST elevation)– Acute Myocardial Infarction– Pericarditis– Left Bundle Branch Block

- Left Ventricular Hypertrophy– Early Repolarization

T wave• Findings: Normal

– Upright: I, II, V3, V4, V5, V6– Inverted: aVR, V1– Increased Amplitude: aVL and aVF

• Findings: T Wave Shape– Smooth: Normal– Notched: Pericarditis– Pointed: Myocardial Infarction

Contd…• Findings: T Wave Height

– Normal• Limb leads: <5 mm• Precordial leads: < 10 mm

– Tall T Wave Causes• Hyperkalemia• Myocardial Infarction• Myocardial Ischemia• Cerebrovascular Accident

Contd…• Causes: T Wave Inversion in anterior

leads (V2 to V4)– Anterior Myocardial Ischemia– Posterior Myocardial Infarction– Pulmonary Embolism

U- wave

Deflections in different leads:

Intervals Atrial and ventricular depolarization and

repolarization are represented on the ECG

Contd…

Feature Description Duration

RR intervalThe interval between an R wave and the next R wave

0.6 to 1.2s(3-6 large boxes)

P wave

SA node towards the AV node, and spreads from the right atrium to the left atrium

80-120ms( 2-3small box)

PR interval

reflects the time the electrical impulse takes to travel from the sinus node through the AV node and entering the ventricles

120 to 200ms(1 large box)

Contd…feature description duration

PR segment

The impulse vector is from the AV node to the bundle of His to the bundle branches and then to the Purkinje fibers

50 to 120ms(1-3 small boxes)

QRS complex

The QRS complex reflects the rapid depolarization of the right and left ventricles

80 to 120ms(2-3 small boxes)

J-point

point at which the QRS complex finishes & ST segment begins, used to measure ST elevation / depression

N/A

Features Description Duration

ST segment

represents the period when the ventricles are depolarized. It is isoelectric.

80 to 120ms(2- 3 smll boxes)

T waveThe T wave represents the repolarization of the ventricles

160ms (4 small boxes)

ST interval

The ST interval is measured from the J point to the end of the T wave.

320ms( 1 large box & 3 small boxes)

QT interval

measured from the beginning of the QRS complex to the end of the T wave .It varies with heart rate ,for clinical relevance requires a correction for this, giving the QTc.

Up to 420ms in heart rate of 60 bpm

Calculation of heart rate

Method 1• Ecg strip of 6 sec• Count QRS

complexes• To get 1min HR

multiply it by 10

Method 2• Paper speed=

25mm/ sec• means 25 small

boxes / sec• Small boxes in 1

min = 25multiply 60= 1500

• 1500/no. of small boxes in P-P interval & R-R interval

Abnormal ECG findings

SA node dysrhythmiasSinus bradycardia- HR- less than 60b/m

Venrtricular & atrial rhythm - regular

Sinus tachycardia

• HR- more than 100 & less than 120b/m

• Ventricular & atrial rhythm - regular

Contd…

Sinus aarhythmias• HR- b/w 60-100b/m

• Ventricular & atrial rhythm – irregular

Atrial dysrhythymias• Premature atrial complex:early p wave & shorter

Ppintetval

Atrial flutter

Contd…

Atrial fibrillation

Ventricular tachycardia

Electrolyte abnormalities

• Serum potassium - major intracellular ion participates in- depolarization and repolarization of myocardial cells

• serum concentration- effect on the QRS and ST-T complex.

Hyperkalemia Peaked T wave

QRS wide

prolonged PR

QT short

Hypokalemia T wave -flattened or inverted Appearance of a prominent- U wave ST segment - depressed

Calcium hypercalcemia- is associated with short

QT interval

hypocalcemia- with long QT interval

Drug effects

• At toxic levels digoxin- causes sinus bradycardia

• Amiodarone – increases PR,QRS,QT intervals

• Quinidine , procainamide- prolong QRS duration & QT interval

References:

•  http://www. lifehugger.com. ECG- simplified. Aswini Kumar M.D. Retrieved 2013-11-11.

• Bazett HC. (1920). "An analysis of the time-relations of electrocardiograms". Heart (7): 353–370

• http://library.med.utah.edu/kw/ecg Einthoven's Triangle .Retrieved 2013-11-11

Contd…

• Luthra A. ECG for nurses. Japee brothers.p-3-127

• Bazett HC. (1920). "An analysis of the time-relations of electrocardiograms". Heart (7): 353–370.