Cardiology Teaching Package
A Beginners Guide to Normal Heart Function, Sinus Rhythm & Common Cardiac Arrhythmias
Role of the ECG Machine
The ECG machine is designed to recognise and record any electrical activity within the heart. It prints out this information on ECG paper made up of small squares 1mm squared.
Image: ECG paper
Each electrical stimulus takes the form of a wave and so patterns emerge made up of a number of connected waves. A standard ECG is printed at 25mm per second or 25 small squares per second (see above). In this way it is possible to calculate the duration of individual waves.
10 small squares vertically is equal to 1 millivolt. So it is possible to calculate the amount of voltage being released within the heart. If the line is flat at any time in the duration of a series of waves, it indicates no electrical activity at that particular moment.
The direction in which the waves point indicates whether electricity is moving towards or away from a particular lead.
The general direction in which electricity normally travels through the heart is a downward diagonal line from the right shoulder to the left lower abdomen. This is because the electrical stimulus originates in the SA node (upper right side of the heart), travels through the AV node and bundle of His, and finishes mainly in the left ventricle. (remember that there is more conduction in the left ventricle).
So different leads may have waves pointing in different directions. Eg. Lead AVR (right shoulder/right arm/wrist) will always see the electrical stimulus travelling away from it, therefore the waves expressed in AVR for sinus rhythm, pqrst, will all point downwards.
Image: Rhythm strip of sinus rhythm recorded from ECG lead AVR
Similarly, lead V6 (mid-left axilla, 5th intercostal space), will always see the electrical stimulus coming towards it and therefore the waves expressed in V6 for sinus rhythm, pqrst, will always be point upwards.
Image: Rhythm strip of sinus rhythm recorded from ECG lead V6
