ECG Basics: Parts of the ECG

The ECG Basics Course:

Part 1: Parts of the ECG

The ECG is basically an electrical picture of the heart as it cycles through consecutive beats. The ECG is printed on graph paper to allow for easy measurement. The ECG tracing shows the electrical conduction of the heart as the cells depolarize and repolarize through each beat.

The different leads of the ECG can be thought of as the same picture taken from different angles. Each lead is like a puzzle piece, it shows a part of the picture, but the puzzle has to be put together in order to see the big picture. Lets look at an ECG in order to start orienting ourselves:

At the extreme left of the ECG you will see the 1mV calibration box. It is important to make sure the box looks squared off, and is 2 large boxes tall before you spend time interpreting the ECG.

At the bottom of the ECG is a long tracing of lead II. This is called the rhythm strip and is generally the part of the ECG to look at to determine rate and rhythm.

Above the rhythm strip are the 12 leads of the ECG, each of which looks at the depolarization and repolarization of the heart from a different angle. Leads I, II and III are called the bipolar leads. Bipolar leads record the electrical activity between a reference lead and a sensing lead. Leads aVR, aVL and aVF are unipolar leads, they record the electrical activity between a sensing lead on one limb and a reference lead that is a combination of readings from other leads. The precordial leads, V1 to V6, are also unipolar leads but instead of recording between limbs they are placed directly on the chest so the sensing electrode is sensing the heart from a very close and particular angle.

The angle at which each of these leads views the heart will become important when we think about the axis of the heart.

Another way to consider the leads of the ECG is from the perspective of what part of the heart they represent. The image below shows how the various ECG leads can be grouped by the portion of the heart whose electrical activity they capture.

 

So the ECG gives us information about rate and rhythm, mainly in the rhythm strip of lead II along the bottom of the tracing, and gives us information about individual areas of the heart. In order to start looking at the ECG in more detail we will need to look at all those spikes of depolarization and repolarization in more detail.

Each individual beat is represented on the ECG as the PQRST complex. Each sequential letter represents a sequential discrete part of the complex and each represents a different portion of the cardiac cycle. Each of the parts of the PQRST will be examined in detail in subsequent sections.

 

The first part of the complex is called the P wave. The P wave represents the initiation of depolarization in the sinus node and subsequent atrial contraction. It is relatively small due to the small muscle mass of the atrium. The normal P wave should be upright (positive) in lead II and less than 2.5mm tall. Finding the P wave (or the lack of P wave) is the first step in figuring out the rhythm.

 

The next part of the tracing is called the QRS complex. The QRS complex represents the conductance and sequential depolarization of the ventricles. It is a much larger wave form that the P wave due to the large muscle mass of the ventricles. An initial downwards (negative) deflection in the QRS is termed the Q wave. A Q wave may or may not be present depending on the lead and the pathology of the ECG. The R wave is large and upright (positive) and is easily seen on this example. The S wave follows the R wave and is a downwards (negative) deflection. A small S wave can also be seen in this example.

 

The ST segment follows the QRS complex. The ST segment represents the period of time in which the ventricles are isoelectric. The position of the ST segment relative to the baseline prior to the preceding P wave is considered, ie. is the ST segment elevated, depressed or normal relative to the previous baseline. Evaluation of the ST segment is particularly important in considering cardiac ischemia and myocardial infarction.

 

The T wave is the last portion of the PQRST complex. The T wave represents the repolarization of the ventricles. The T wave is normally upright in all leads except V1 and aVR (isolated T wave inversion in III can also be normal). The morphology and direction of the T wave can provide critical information about cardiac ischemia and electrolyte abnormalities.

 

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