Electrocardiogram

The Electrocardiogram (aka EKG or ECG) is a measure of overall sequential electrical activity of the heart. It stems from potential differences of many cells synchronously undergoing electrical changes, with the magnitude of potentials usually less than 1mV.

ECG waves

An ECG complex showing the major wave patterns

A normal ECG has several waves. The P wave represents atrial depolarization. The QRS wave represents ventricular depolarization (80-120ms). The T wave represents ventricular repolarization.

The P-R interval (120-200ms) is the time from the onset of the P-wave to the onset of ventricular activation (R). If a Q wave is recorded, this becomes the P-Q wave. The impulse is conducted through the atria, AV node, His bundle (and its branches), and Purkinje fibers. A P-R interval of > 0.2s usually indicates conduction abnormality, most often in the AV junctional region.

The Q-T interval represents the electrical activity of the ventricles. A normal Q-T interval should be ≤440ms in males and ≤460ms in females, a change that begins at around puberty. The heart rate affects the Q-T interval by decreasing the plateau phase, and therefore the Q-T interval time. However, the QRS complex is unaffected by heart rate changes.

During conduction through the A-V node and other specialized conducting tissues, the ECG is "isoelectric". The isoelectric nature of these measurements is due to the fact that the surface leads are not sensitive enough to pick them up. However, new techniques in clinical cardiac electrophysiology are allowing these conductions to be observed.

On a normal ECG, an upright T-wave is visible, which outwardly appears counterintuitive (because it is repolarization). The question then becomes: why is it not in the opposite direction of the depolarization? The reason is that the electrical current moves in an opposite direction to the depolarization (inverts wave), and is an opposite charge (inverts again). The net result is therefore an upright T wave.

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There are two major types of leads - unipolar and bipolar. Leads 1, 2, 3, Augmented Voltage Right (AVR), AVL, and AVF are bipolar leads, meaning that they have a positive (+) and negative (-) lead to detect movement from in a certain direction. Leads V1-V6 are unipolar leads, meaning that they only detect what's going towards or away from them [1]. To get a good look at the heart, six leads are placed around the heart.

By convention, 12 electrodes (leads) are placed in standard positions on the body [2]. The positions of the electrodes will determine the output visualized on the ECG graphs. Also by convention, whenever a wave front is moving towards the positive electrode, it is scored as upwards movement. Electrical movement at right angles to electrodes does not produce a reading.

Abnormalities

Cardiac arrhythmias can be caused by abnormalities in automaticity, conduction and refractoriness, among other things. Consequences of cardiac arrhythmia can be fibrillation and improper pumping.

Interpretation

Graph Paper

The graph paper used to record ECGs has has voltage on the x axis (1mV/cm), and time on the y axis (25mm/second).

Heart Rate

The first step of interpreting an ECG is to determine the heart rate. This is done by finding the rate at which R waves repeat. Once an R wave is found, the first three heavy black lines are counted "300, 150, 100", and the next three lines are counted "75, 60, 50". So, an R wave lying between four and five heavy black lines from the next R wave represents a heart rate of 60-75 bpm. The minor marks can be approximated to represent increments in between the minor changes, so an R wave lying at four heavy (major) and three light (minor) lines would represent a heart rate of $60 + 3 \left ( \frac{15}{5} \right )$

Diagram of the EKG axes

Axis

The interpretation of an ECG reading may involve an assessment of the axis in which an anomaly is happening. In this case, the vectors of six leads should be remembered: I 0°; II +60°; III +120°; aVR -150°; aVL -30°; aVF +90°. The axis can be determined with two methods [3]:

Method 1

A. Check lead I (0 degrees)
1. QRS positive (predominately up)
1. Vector points to patient's Left
2. Correlates with right half of axis circle
2. QRS negative (predominately down)
1. Vector points to patient's Right
2. Correlates with left half of axis circle
B. Check lead aVF (90 degrees)
1. QRS positive (predominately up)
1. Vector points to bottom half of axis circle
2. QRS negative (predominately down)
1. Vector points to upper half of axis circle
C. Interpretation
1. Indeterminate: Extreme Right Axis Deviation
2. Right Axis Deviation
3. Normal range
4. Left Axis Deviation


Method 2

1. Select Isoelectric lead from limb and augmented leads
1. Isoelectric lead averages to baseline
2. Positive deflection equals negative deflection
2. Identify isoelectric lead on axis circle