By careful placement of surface electrodes on the body, it is possible to record the complex, compound electrical signal of the heart. This tracing of the electrical signal is the electrocardiogram (ECG), also commonly abbreviated EKG (K coming kardiology, from the German term for cardiology). Careful analysis of the ECG reveals a detailed picture of both normal and abnormal heart function, and is an indispensable clinical diagnostic tool.
Sample 3 Lead ECG
Electrodes and Leads
An electrode is the actual conductive pad attached to the body surface. In comparison, a lead is a measure of the electrical potential difference between an exploring (positive) electrode and a reference (negative) electrode
Lead = Vexploring electrode – Vreference electrode
Electrodes can be real (physical) or virtual. A virtual electrode is the calculated average potential of voltages produced by two or three real electrodes.
For example, Wilson’s Central Terminal (WCT or Vw) is a calculation of the voltage at the center of the thoracic cavity, which is made using the right arm electrode (RA), left arm electrode (LA), and left leg electrode.
Vw = 1/3 (RA + LA + LL)
A lead provides a view of the electrical activity of the heart at a vector (angle) formed between exploring and reference electrodes. Regardless of how the exploring electrodes and the reference electrodes are arranged, the vectors have the same impact on the ECG curve. A vector heading towards the exploring electrode produces a positive wave (defection) and a vector heading away from the exploring electrode produces a negative wave (deflection).
ECGs are most commonly produced using 3 leads, 5 leads, or 12 leads. A 12 lead ECG combines information collected from 10 electrodes and 12 different vectors (viewpoints) to give the most comprehensive view of the electrical activity of the heart. The signals detected by the electrodes are transmitted to an electrocardiograph and displayed on a monitor as ECG tracings.
12 Lead (10 Electrode) Arrangement
12 Lead Arrangement Vectors
ECG Waves, Segments, and Intervals
A printed ECG tracing is a graph of voltage versus time of the electrical activity of the heart. Electrodes placed on the skin detect the small electrical changes produced by the depolarization and repolarization of cardiac muscle during each heartbeat (cardiac cycle). Whenever the direction of electrical activity moves towards a recording lead, a positive deflection is produced (a deflection above the baseline line or isoelectric line). Whenever the direction of electrical activity moves away from a recording lead a negative deflection is produced.
The electrical current generated by a single heartbeat produces three prominent defection points on an ECG tracing. (a) The first defection is a small P wave, which is produced by the depolarization of the atria. (b) After a brief delay, a much larger second deflection, called the QRS complex, occurs due to the depolarization of the ventricles. The amplitude of this deflection is greater because the muscle mass of the ventricles is larger and gives off a stronger signal. Also, during this time, the atria repolarize. The resulting signal, however, is masked by the ongoing depolarization of the ventricle. (c) The last deflection point, called the T wave, is caused by the repolarization of the ventricles.
An ECG tracing is also divided into segments and intervals. A segment is a region between two waveforms, and an interval is a segment plus one or more waveforms.
|What is Represented
|P – R segment
|End of the P wave to the start of the QRS complex
|S – T segment
|End of the QRS complex to the start of the T wave
|P – R interval
|Start of the P wave to the start of the QRS complex
|The time between the onset of atrial depolarization and the onset of ventricular depolarization
|Q – T interval
|Start of the QRS complex to the end of the T wave
|The time it takes for the ventricles of the heart to depolarize and repolarize, or to contract and relax
|R – R interval
|Distance between two consecutive R waves
|Time between heartbeats
The electrical currents produced by the cardiac tissues cause the heart muscles to contract and relax, although this mechanical activity is not displayed on an ECG. Approximately 25 ms after the start of the P wave, the atria begin contracting. The ventricles begin contracting as the QRS reaches its peak.
Measuring the Duration of ECG Events
ECG tracings are typically recorded and measured on standardized graph paper, moving at a standardized rate. The horizontal axis measures signal amplitude (in millivolts), and the horizontal axis measures signal duration (in milliseconds).
Dark (thicker) and light (thin) lines divided the graph paper into large and small boxes.
- The small boxes represent 1 square millimeter (mm2). Horizontally, one small box equals a duration of 0.04 seconds when the paper moves at a speed of 25 mm per second. Vertically, one small box equals 0.1 millivolts (mV) of electrical current.
- The large boxes consist of 25 small boxes. Therefore, the horizontal axis of one large box equals 0.20 seconds, and the vertical axis equals 0.5 mV (or 1.0 mV for two large boxes).
Normal ECG Values for Waves and Intervals
|0.08 – 0.10 seconds
|P – R interval
|0.12 – 0.20 seconds
|P – R segment
|0.5 – 1.2 seconds
|0.06 to 0.10 seconds
|S – T segment
|0.80 – 0.12 seconds
|0.10 – 0.25 seconds
|Q- T interval
|4.0 seconds (shorter with increasing heart rate)
|R – R interval
|0.6 – 1.2 seconds
Clinical Relevance of ECGs
It is clinically relevant if an ECG tracing show waves, segments, and intervals that deviate from their normal values,.
For example, a long P – R interval (over 0.20 sec) indicates a slowing of conduction between the atria and ventricles, usually due to slow conduction through the atrioventricular node (AV node). This is known as first-degree AV nodal block.
A second-degree or incomplete AV nodal block occurs when some impulses from the SA node reach the AV node and continue, while others do not. In this instance, the ECG would reveal some P waves not followed by a QRS complex, while other tracings would appear normal.
In the third-degree or complete block, there is no correlation between atrial activity (the P wave) and ventricular activity (the QRS complex). Even in the event of a total SA block, the AV node will assume the role of pacemaker and continue initiating contractions at 40–60 contractions per minute, which is adequate to maintain consciousness.
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Reference: “Cardiac Muscle and Electrical Activity“
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