History and Term ECG (Electrocardiogram)
In 1787 Galvani was the first to discover the relationship between electrical currents and muscle contractions. In 1843 Carlo Matteucci detected that the heart’s activity is also based on electrical currents. The first graphic representation of this was made by E.J. Marey in 1876. The breakthrough came with the Dutch physiologist Willem Einthoven who was awarded the Nobel Prize in medicine for the invention of the electrocardiography. The deflections and curve descriptions developed by him are still in use today. These deflections were amended by the American cardiologist Emanuel Goldberger on limb leads and by Frank Wilson on precordial leads.
The electrocardiogram records the electrical activity of the heart. The heart is a muscular organ which beats in rhythm to pump the blood through the body. The signals that make the heart’s muscle fibres contract come from the sinoatrial node, which is the natural pacemaker of the heart.
In an ECG test, the electrical impulses made while the heart is beating, are recorded and usually shown on a piece of paper. This is known as an electrocardiogram, and provides information on the condition and performance of the heart.
Anatomy of the Heart
It is important to know the heart’s structure and blood flow to understand the ECG. The heart is a hollow muscle which is divided into four chambers. These are the right atrium, the right ventricle and the left atrium and the left ventricle.
The right atrium receives venous blood which passes via the tricuspid valve to the right ventricle, which propels it through the pulmonary artery to the lungs. In the lungs venous blood comes in contact with inhaled air, picks up oxygen, and loses carbon dioxide. Oxygenated blood is returned to the left atrium through the pulmonary veins. Passage of blood through the left atrium, bicuspid valve, into the left ventricle. Via the aortic valve the blood is pumped in the aorta and the arterial branches of the whole body.
Standard ECG - Records
Method of graphic tracing of the electric current generated by the heart and information on the condition and performance of the heart.
In the following the waveform of a normal ECG is explained. Any deviation from the norm in a particular electrocardiogram is indicative of a possible heart disorder. A selection of ECG recordings taken during various electrocardiographic tests will be explained.
The normal ECG shows characteristic waves in its course. It was Einthoven who assigned the letters P, Q, R, S, and T to the various deflections.
The P-Wave
The P-Wave is caused by atrial contraction. The first upward deflection corresponds with the right atrium and the second downward deflection corresponds with the left atrium.
Examples of deviations from the normal P-Wave indicate:
- Pointed, upright P-wave when the right atrium is overstrained, e.g. in case of cor pulmonale acutum or cor pulmonale chronicum , i.e. in Latin pulmonary heart – a pressure-loaded heart due to a risen pressure in the pulmonary circulation because of a pulmonary disease
- Bicuspidal , often spreadout P-wave, emphasizing the 2nd peak, e.g. indicating high blood pressure
- Both parts of the P-wave are changed, merged representation of the changed P-wave as mentioned before, e.g. in case of high blood pressure, right heart hypertrophy and severe organic heart defects
- Negative deflection of the P-wave occurs in cases of pacemaker actions in the atrioventricular area
ECG-Instruments measure the PR-Interval
The P-Q-time or PR-Interval extends from the start of the P-wave to the very start of the QRS-complex. The excitation is decreased by the AV-node and led via the bundle of His to the left and right bundle branch (thus, conduction time).
The normal duration is between 0.12 – 0.20 sec. A PR-interval of more than 0.20 sec may indicate a first degree an AV-block.
The QRS- Complex
Normally, the QRS interval is 0.07 to 0.10 sec.
The excitation is led via the left bundle branch and the ventricular septum and is visible as Q-wave n the ECG. During the R-phase most of the heart’s muscles are activated. For this reason the ECG shows the great wave.
Whereas during the S-phase the activation runs from the apex of heart to the base of the right and left ventricle.
Examples for Abnormalities of the QRS-Complexes
- Left-ventricular hypertrophy demonstrates thickening of the heart muscle (left ventricle). More cells are activated which leads to a taller R-wave.
- Right-ventricular hypertrophy demonstrates thickening of the heart muscle (right ventricle). However, the right ventricle still has less muscle mass than the left ventricle.
- Ventricular conduction abnormality. An abnormal QRS-complex can be a sign of disturbances in stimulus conduction. There is also an abnormal QRS-complex, which may indicate myocardial infarction.
The ECG-instrument also measures the QRS-duration from the beginning of the Q-wave to the end of the S-wave.
It demonstrates the duration of the depolarization of the heart’s ventricles. A normal duration lies between 0.08 and 0.12 sec. If the duration is longer this may indicate a conduction abnormality as described before.
ECG-instruments measure the QT/QTc Interval
The QT-interval is measured from the beginning of the Q-wave to the end of the T-wave. The QT-interval represents the duration of activation and recovery of the ventricular muscles. This duration is reciprocal to the pulse.
The ST-Segment
The ST-segment represents the period from the end of ventricular depolarization to the beginning of ventricular repolarization. Here all cells of the atria are depolarized. An isoelectric line is generated because in this segment there is no electrical current.
Examples of Abnormalities of the ST-segment
The T-Wave
The T wave reflects ventricular repolarization. It usually takes the same direction as the QRS complex (concordance); opposite polarity (discordance) may indicate past or current infarction. The T wave is usually smooth and rounded but may be of low amplitude in hypokalemia and hypomagnesemia and may be tall and peaked in hyperkalemia, hypocalcemia, and left ventricular hypertrophy.
The U-Wave
The U-wave is typically small and follows the T-wave. Its origin has not yet been understood completely.
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