Calculating Electrical Axis

and Alwyn Scott2



(1)
School of Computer Science, University of Manchester, Manchester, UK

(2)
Cardiology High Dependency Unit, Papworth Hospital NHS Foundation Trust, Cambridge, UK

 



Keywords
AxisHexaxialVectorEinthovenDeviationBipolarUnipolarDeflectionDepolarization



Background


As electricity is conducted through the ventricles during depolarization, the mean or overall net direction the electricity travels in can be measured. This is often referred to as the cardiac axis or the mean vector (Fig. 3.1). This vector can be calculated and represented in degrees. A vector can be described as a directed quantity of something, in this instance electricity. In most cases when practitioners talk about cardiac axis, they are referring specifically to the mean frontal axis of the QRS complex. It is worth noting however that the axis can be calculated for any part of the waveform.

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Fig. 3.1
The mean vector of depolarisation


How Is This Useful?


Knowing the cardiac axis is useful in two main ways: It helps to confirm certain ECG interpretations and it provides certain supporting evidence for the diagnosis of other conditions as summarised in Table 3.1.


Table 3.1
Why knowing cardiac axis can be useful

















Identification of chamber enlargements such as ventricular hypertrophies

Helping to determine if a broad complex tachycardia is ventricular in origin

Helping to Identify septal congenital defects

Helping to Identify certain conduction defects, such as hemiblocks

Assists with the identification of pulmonary embolism

Can help identify pre-excitation conduction conditions


Normal Axis


The normal cardiac axis is the normal direction electricity takes through the heart. As the apex of the heart is angled to the bottom left and the electrical impulse originates from the top right of the heart, the impulse travels diagonally from the top right to the bottom left in a normal heart (Fig. 3.2).

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Fig. 3.2
Direction of depolarisation in a normal heart parallel to lead II

This incidentally is why lead II is one of the most widely used monitoring leads and is often used in textbooks and other examples. Lead II most closely matches the natural direction of the electrical impulse, which runs parallel to lead II. As the wave of depolarisation moves from top right to the bottom left, from a negative to a positive pole it produces the positively deflected classic PQRST waveform (Fig. 3.3).

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Fig. 3.3
Lead II and the classic positively deflected waveform seen in that lead


Axis Deviation


Simply put axis deviation is a deviation or departure from the expected route of electrical activity. This deviation alters the direction so it is either more to the left or the right of normal. This can be caused by certain conditions, for example MI or hemiblock. It can also be caused by physically moving the heart in the chest (a mechanical shift) for example: pregnancy, ascites or trauma. Different conditions can cause the axis to deviate from the norm. Some examples of these conditions can be seen in Table 3.2.


Table 3.2
Causes of left and right axis deviation





































Left axis deviation

Right axis deviation

Left bundle branch block (sometimes)

Right bundle branch block (sometimes)

Left ventricular hypertrophy

Right ventricular hypertrophy

Wolff-Parkinson-White syndrome (sometimes)

Left posterior hemiblock

Aortic stenosis

Pulmonary stenosis/hypertension

Aging

Emphysema

Hyperkalaemia

Lateral wall MI

Inferior wall MI

Dextrocardia

Mechanical shift

Mechanical shift

Normal variation

Normal variation


Categories of Axis Deviation


Normal axis is considered to be between 0° and 90°. Axis deviation can be split into three further sub categories: left, right and extreme right axis deviation (Table 3.3). It is worth mentioning that some experts classify normal cardiac axis to range from −30° to 90° with left axis deviation being −30° to −90°. Extreme right axis is also often referred to by various other names in different texts, including: extreme left axis, no-mans land and right superior axis deviation among others.


Table 3.3
Axis categorization


















Normal

0° to 90° or −30° to 90°

Left axis deviation

0° to −90° or −30° to −90°

Right axis deviation

90° to 180°

Extreme right axis deviation

−180° to −90°


Calculating the Cardiac Axis


There are many different methods that can be used to calculate the cardiac axis. These methods differ in their complexity and accuracy. As such, we present three methods that can be used in clinical settings. Before giving examples of these methods it is worth understanding how the leads work, so that these methods may be easier to understand.


Einthoven’s Law


Willem Einthoven, the Dutch physiologist who won the Nobel prize in 1924 for inventing the ECG determined a law that stated:



$$ I+\left(-II\right)+III=0 $$
Also known as Einthoven’s law. It basically means that if you add the waveform amplitudes from the three leads together they cancel each other out and equal zero. The polarity in lead II is switched. Polarity is flow of electrons from one pole to the other (negative to positive). Reversing the polarity changes the direction of the flow of electrons. It is speculated that Einthoven did this because he prefered to view waveforms upright.

To prove that this law works we look in the limb leads (leads I, II and III). We then take away the height of the S wave from the height of the R wave in all three of the leads. Figure 3.4 shows the waveforms in leads I to III. For example the R wave in lead I is positively deflected above the baseline by 5 mm in height, the S wave however is negatively deflected 2 mm below the baseline. Now we take the S wave away from the R wave 5 − 2 = 3. Lead II has a very small R wave just 1 mm in height. The S wave in lead II is 3 mm. 1 − 3 = −2. Finally the R wave in lead III is just ½ a mm in height (0.5 mm), but has a deep S wave measuring 5.5 mm. 0.5 − 5.5 = −5
May 29, 2017 | Posted by in CARDIOLOGY | Comments Off on Calculating Electrical Axis
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