– Explain purpose of ECG.
– Locate limb and chest leads placement.
– Explain procedure.
– Practice taking 12 lead ECG.
Electrocardiogram (ECG) is a graphic record of the voltage produced by the myocardium during the cardiac cycle.
Continuous monitoring of the heart action by ECG:
This has become an extremely important medical tool in the diagnosis of many heart disorders and provides vital information about
– The client’s condition and progress
– Heart action
– Response to therapy
Cardiac disorders detected by ECG
– Myocardial infarction
– Atrial and ventricular hypertrophy
– Drug effects such as digitalis and quinidine
– Congenital heart disease.
– An electric current is generated in the cardiac tissues by two processes – depolarization and repolarization.
– Depolarization: The excited state of the tissue. Positive sodium ions move across the cell membrane causing the inside of the cell to become positive and the outside negative. The cardiac muscle contracts in the process and is repolarized.
– Repolarization: Slow movement of ions across the cell membrane to restore the polarized state.
– Heart muscle polarize (charged) when it is at rest. When ventricles and atria contract they depolarize.
– Electrical activity generated in the heart muscle is transmitted to all parts of the body. This can be detected by electrodes placed on the skin and connected to the wire leads of the oscilloscope.
– Graph recorded is called an electrocardiogram.
Procedure is called electrocardiography.
– Typical sites for placement of the electrodes are the extremities (arms and legs) and the chest.
12 Lead ECG
The ECG consists of the following main leads:
– Three bipolar standard leads labeled as lead I, II, & III.
– Three augmented, unipolar leads: aVR, aVL & aVF.
– Six unipolar chest leads: V1-V6.
An ECG is constructed by measuring electrical potential between various points of the body using a galvanometer (electrocardiograph). Leads I, II and III are measured over the limbs: I is from the right to the left arm, II is from the right arm to the left leg and III is from the left arm to the left leg. From this, the imaginary point V is constructed, which is located centrally in the chest above the heart. The other nine leads are derived from potential between this point and the three limb leads (aVR, aVL and aVF) and the six precordial leads (V1-6)
Placement of leads
Locations of chest leads
– 1st electrode (V1): Fourth intercostal space to the right of the sternum.
– 2nd electrode (V2): Fourth intercostal space to the left of the sternum.
– 3rd electrode (V3): Directly between leads V2 and V4. (Laterally follow outline of the heart)
– 4th electrode (V4): Fifth intercostal space at midclavicular line.
– 5th electrode (V5): Level with V4 at left anterior axillary line.
– 6th electrode (V6): Level with V5 at left midaxillary line.
– 12 Lead (10 Electrode) Placement
1. Electrocardiograph: Device used to record the electrical activity of the myocardium to detect the abnormal transmission of the electric impulses through the conductive tissues of the muscle.
4. Alcohol swabs
Electrocardiogram: Graph taken by a nurse, physician or an ECG technician
– Explain the procedure to the client. Explain that the procedure is painless and takes about 10-15 min. Client may be anxious about the result.
– Provide privacy.
– Sometimes men’s chest hair may need to be shaved. For women, make sure skin is dry especially under breasts before putting electrodes on.
– Patient lie flat. Remove necklace, watches.
– Put jelly on skin sites according to policy. Place electrodes on chest.
Post procedure: Remove jelly from skin.
– Switch off machine.
Wave forms in an ECG
– Cardiac stimulus originates in a point, called sinoatrial (SA) node or sinus node, located in the posterior wall of the right atrium at a rate of 60-80
beats per minute.
– Specialized conductive fibers inside the myocardium transmit the cardiac stimulus to all the myocardial cells.
– Atria contract first, and then the ventricles.
– Depolarization wave produces a current wave of atrial contraction, which is called the P wave.
– After about 1/10 of a second, the atrial depolarization wave reaches the atrioventricular (AV) node, which is situated between the two sets of chambers.
– Electrical stimulus passes then from the atrioventricular node through the bundle of His to the atrioventricular bundle branches, and the Purkinje fibers, which initiate the ventricular depolarization.
– Ventricular depolarization is represented graphically on the electrocardiogram by a waveform called QRS complex.
– During the ventricular depolarization, the right and left ventricles contract. After the QRS complex, there is a pause, called ST segment.
– During the ventricular repolarization, the heart cells regain the negative charge which is represented by the T wave.
– Normal sinus rhythm
– P wave rate 60-100 bpm
Each P wave is followed by a QRS
– Normal shape
– Duration 0.11 s.
– Normal PR interval: 0.12 to 0.2 s.
– Normal QRS complex: less than 0.12 s (3 small squares)
– Normal QT interval: 0.42 s
– Normal ST segment: No elevation or depression
– Normal T wave
– Normal U wave