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How to interpret the ECG

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Interpreting the ECG consists of a number of parameters important for an informed clinical decision. Let us take a look at the most vital aspects of ECG interpretation – from distances on the ECG paper to the intricacies of waves, waveforms, segments, and intervals. We will also discuss how to interpret duration and the data on cardiac axis.

A big role in ECG interpretation is also played by automatic algorithms. When an ECG is recorded using a machine equipped with the Glasgow Interpretation Algorithm, the collected data is processed and analyzed by it. The algorithm takes into account a variety of patient-specific factors, such as age, gender, race, and clinical history, to provide a more accurate and tailored interpretation of the ECG results.  

In this blog you will learn:

Distances on ECG paper or graph

The ECG paper, divided into squares, shows the passage of time during ECG measurement on the rhythm strip. 

The standard paper speed is 25 mm/s. The rhythm strip is comprised of: 

  • 1 SMALL square (1 mm) = 0.04 s (40 ms) 
  • 5 SMALL squares (5 mm) = 1 LARGE square = 0.2 s (200 ms) 
  • 5 LARGE squares = 1 second 
  • 250 SMALL squares = 50 LARGE squares = 10 seconds 


The electric activity of the heart, as captured by the ECG leads, is shown in the form of waves. One heartbeat (i.e. one cardiac cycle) consists of a number of waves. For a medical interpretation, the following waves and their combinations are considered. [3]

Individual waves

Individual waves in the ECG are the following: 

R wave: reflects depolarisation of the main mass of the ventricles.

P wave: represents atrial depolarisation.

Q wave: represents the normal left-to-right depolarisation of the interventricular septum.

S wave: signifies the final depolarisation of the ventricles.

T wave: represents ventricular repolarisation.

U wave: a small, rounded deflection sometimes detected after the T wave. It represents the last phase of ventricular repolarisation, but its exact source remains unclear.

Wave combinations

ECG waves can be joined into various combinations, each of which gives us additional information about the patient’s heart cycle:  

PR interval: the time from the beginning of atrial depolarisation to the beginning of ventricular depolarisation

QT interval: the time from the beginning of ventricular depolarisation to the end of ventricular repolarisation

QRS interval: the time from the beginning to the end of ventricular depolarisation

ST segment: the time from the end of ventricular depolarisation to the beginning of ventricular repolarisation

J point: where the QRS complex and the ST segment connect (important e.g. when distinguishing between acute cardiac ischemia from a normal variant)


Waves have a variety of characteristics. [2] They include: 


(Example: an unusually long P wave may be a sign of left atrial enlargement. Normal: =< 0,12 s) 


(Example: an unusually tall P wave may be a sign of right atrial enlargement. Normal: < 2,5 mm)


(Example: an inverted P wave may indicate an origin of a heart rhythm other than a sinus node)


Segments are the lines that connect the waves: 

  • PR segment connects P wave and QRS complex
  • ST segment connects QRS complex and T wave
  • TP segment line connects the T wave with the following P wave


PR interval is the sum of the duration of the P wave + the PR segment

QRS interval is the duration of the QRS complex

QT interval = duration of QRS complex + ST segment + T wave

RR interval = duration of all waveforms and segments within one cardiac cycle


The characteristics of the intervals include: 

Note: Tachycardia and bradycardia do not necessarily indicate pathology; a lot depends on the patient’s age and condition.


The cardiac axis gives us an idea of the overall direction of electrical activity, and its deviation to either left or right can indicate various conditions. [5]

Automatic interpretation Algorithm

Introduction to the Glasgow Interpretation Algorithm

The Glasgow ECG Interpretation Algorithm, developed at the University of Glasgow, UK, enables automatic ECG analysis. The algorithm has been in continuous development for over 30 years[6]

It has been adapted to meet the needs of different patients (the algorithm accounts for age, gender and race and compares the ECG with historic data to arrive at specific interpretations). It is applicable to neonates as well as adults and takes account of racial variation in wave amplitudes. 

How the algorithm works

The University of Glasgow (Uni-G) ECG interpretation program is based on an analysis of simultaneously recorded leads acquired at 500 samples per second. [7]

The first stage in analysis is to apply a 50 Hz or 60 Hz notch filter to remove AC interference. Then, it checks if there are any errors in the recording of any of the leads. If there are, it replaces that part of the recording with one continuous value, or the lead may be set as unavailable if too noisy. 

Next, the program identifies the QRS complex and determines its type. In simple terms, this is done by comparing the first complex in Lead I with the second complex in the same lead, and then comparing all the complexes in this lead. The process is repeated for four other leads, as usually just one or two leads will show aberrations in the conducting of the complex. 

A complex selection procedure then decides which heartbeat will be selected for averaging and subsequent interpretation.

The advantages of the Glasgow ECG Interpretation

While other products use only age and gender to a limited extent, the Glasgow Algorithm uses more clinically significant variables. They are gender, age, race, and clinical history. This is critical because ECG patterns for patients of various ages from different ethnic backgrounds can vary greatly. 

The interpretations help analyse QTc measurements and assesses cardiac risk

The algorithm is highly effective in interpreting and alerting on STEMI (ST Segment Elevation Myocardial Infarction). 

And how does automatic interpretation work in practice? See a case study of automatic ECG interpretation with the MESI mTABLET ECG!

Here is a user-friendly ECG e-book full of illustrations!