The ECG Tracing

In Part 1 of ‘The Basics of ECG Interpretation’, we learnt that the ECG is a simple diagnostic test that records the electrical activity of the heart over a set time period.

The ECG machine does this by creating a trace in which voltage is plotted on the vertical axis and time is plotted on the horizontal axis. The needle moves across the graph paper that the ECG is recorded on and is deflected a given distance depending upon the voltage that is measured. This tracing is recorded on graph paper that is divided into 1 mm² ‘small squares’ and 5 mm² ‘large squares’.

The standard ECG paper speed is 25 mm/sec and therefore:

• 1 mm (1 ‘small square’) = 0.04 seconds
• 5 mm (1 ‘large square’) = 0.2 seconds

On the vertical axis,10 mm (10 ‘small squares’) is equal to 1 mV when standard calibration is used.

Please refer to the ECG tracing below to familiarize yourself with the waves of the ECG and how they are labelled:

Figure 1. ECG Waves © Medical Exam Prep

These correspond to the following events:

• P wave – atrial depolarization
• QRS complex – ventricular depolarization
• T wave – ventricular repolarisation

We will return to the concept of ECG waves in more detail next month in Part 3 – Waves, Segments & Intervals.

When interpreting an ECG the first three things that should be assessed are:

1. The rate
2. The rhythm
3. The axis

ECG Rate 

The heart rate can be calculated from the ECG using a number of different methods and depending upon the circumstances.

In most circumstances when there is a regular rhythm the simplest way to calculate the rate is by counting the number of ‘large squares’ between the ‘spike’ of each complex. These are the R waves and this is called the ‘R-R interval’, this will be discussed in more detail later. By dividing 300 by this number, you will then have calculated the heart rate.

Figure 2. Calculating rate using the ‘R-R interval’. © Medical Exam Prep

In this case, there are 5 ‘large squares’ between each R wave:

Rate = 300/5 = 60 beats per minute

If the rhythm is very fast and there is less than 1 ‘large square’ between each R wave, then an alternative method is to count the number of ‘small squares’ between each consecutive R wave and then and then divide 1500 by this number.

Calculation of the rate becomes more difficult if there is an irregular rhythm, such as in atrial fibrillation. Under these circumstances, the rate can be calculated by counting the number of complexes on the rhythm strip provided across the bottom of the ECG and then multiplying this number by 6. This will give the average rate over a 10 second period.

In order to interpret the rate and its relevance, it is also important to know what the rate means. In an adult the rate can be interpreted as follows:

• < 60 beats per minute = bradycardia
• 60-100 beats per minute = normal
• > 100 beats per minute = tachycardia

ECG Rhythm 

The best way to assess the ECG rhythm is by inspecting the rhythm strip. This is usually a 10-second recording from lead II.

The first thing to look at is whether or not the QRS rhythm is regular or irregular. This can be done by inspecting the rhythm strip and looking at the R-R interval. One approach is to place a piece of paper across the top of the rhythm strip marking a small line next to the top of an R wave, then draw another small line next to the top of the next R wave, and then slide the paper along the rhythm strip. If the rhythm is regular, you should see that your two lines match the tops of the R waves throughout the entire rhythm strip.

If the rhythm is irregular, the next thing to be determined is if it is regularly irregular or irregularly irregular. An underlying regular rhythm can be made irregular by the presence of extrasystoles (ectopic beats).

Once the regularity of the rhythm has been assessed the QRS morphology should be inspected. The QRS complex is less than 0.12 seconds in duration (3 ‘small squares’) under normal circumstances. If the QRS duration is less than this, then the rhythm originates from above the bifurcation of the bundle of His (i.e. it is ‘supraventricular’) and originates in the SA node, atria or the AV node. If the QRS duration is greater than this then the rhythm is either coming from the ventricular myocardium or, alternatively, is supraventricular with aberrant conduction.

Having assessed the QRS duration the rhythm strip and ECG, in general, should then be inspected carefully for the presence of atrial activity. As P waves correspond with atrial depolarization, this can be done by looking for the presence or absence of P waves. If there are no P waves present and the baseline is irregular this is very suggestive of atrial fibrillation:

Figure 3. Rhythm strip demonstrating atrial fibrillation.

A ‘saw-toothed’ shaped baseline is suggestive of the flutter waves of atrial flutter. An absence of P waves entirely is suggestive of sinoatrial arrest.

If the rhythm is regular, the QRS duration and morphology is normal, and there is a P wave present before each QRS complex then ‘normal sinus rhythm’ is said to be present:

Figure 4. Rhythm strip demonstrating normal sinus rhythm.

The QRS Axis 

The axis of the ECG is the average direction of the overall electrical activity of the heart. When talking about to the ECG axis is it generally the QRS axis that is being referred to. The QRS axis is the most important to determine but it should be borne in mind that it is also possible to calculate the P wave and T wave axis.

The normal QRS axis is between -30 and +90 degrees.  Left axis deviation is said to be present if the major QRS vector is between -30 and -90 degrees. Right axis deviation is said to be present if the major QRS vector is between +90 and +180 degrees. If the QRS vector is between +/- 180 and -90 degrees the axis is referred to as either ‘extreme axis deviation’ or ‘indeterminate axis’.

Figure 5. The axis of the ECG. © Medical Exam Prep

Calculating the Axis 

There are several ways of calculating the QRS axis but the most efficient is the ‘quadrant method’.

The quadrant method works by looking at leads I and AVF. If we return to the ‘hexaxial system’ from Part 1 of ‘The Basics of ECG Interpretation’, we can see how this can be used to look at the relationship between the QRS axis and these two leads:

Figure 6. The relationship between leads I and AVF and the QRS axis. © Medical Exam Prep

It can be seen that lead I cuts the hexaxial system in half horizontally (at +90 degrees) and lead AVF cuts the hexaxial system in half vertically (at 0 degrees). Therefore these two leads can be used to divide the hexaxial system into four quadrants and the QRS axis can be placed in one of those quadrants:

One limitation of this method is that the QRS axis can be up to -30 degrees into the upper left quadrant and it still be considered as normal. When the QRS axis is in this region (between 0 and -30 degrees), it is sometimes referred to as physiological left axis deviation. One way around this problem is to look closely at the QRS complex in lead II. When the QRS complex in lead II is positive, the axis is generally normal (physiological left axis deviation) but when the QRS complex in lead II is negative there is left axis deviation present.

The QRS axis provides us with lots of useful information about the patient and it is useful to know some causes of both left axis deviation and right axis deviation when formulating a differential diagnosis based around the ECG findings. A list of a few of these causes is shown below:

After calculating the rate, rhythm and axis the next step in ECG interpretation should be the evaluation of the waves, segments and intervals. We shall explore this further in Part 3 next month.

Next: The Basics of ECG Interpretation (Part 3 – Waves, Segments & Intervals)

Thank you to the joint editorial team of www.mrcemexamprep.net for this article.