ECG Machines (AQA A Level Physics): Revision Note

Simple ECG machines

• Electrocardiography can be used to monitor and investigate the electrical activity of the heart

• Electrodes that are capable of detecting electric signals are placed on the skin

• These electrodes produce an electrocardiogram (ECG)

• An ECG shows several distinctive electrical waves produced by the activity of the heart

• A healthy heart produces a distinctive shape in an ECG

The structure of the heart

• The heart is controlled by electrical impulses in nerve cells

• These can be measured as weak electrical signals by detectors on the surface of the skin 

• The heart consists of four chambers with two chambers on both the left and right sides: 

  • atria in the upper parts

  • ventricles in the lower parts

• The left side of the heart is the right side of the diagram

Electrical signals of the heart

• During a heartbeat, the four chambers of the heart contract (this is depolarisation) and relax (this is repolarisation) in a sequence controlled by electrical signals:

  • First, the atria contract (depolarise), forcing blood into the ventricles 

  • This causes the ventricles to contract (depolarise) and the atria to relax (repolarise)

  • Blood is pumped into the lungs and body 

  • The ventricles then relax (repolarise) to complete one heartbeat

Obtaining an ECG trace

• Electrodes must be positioned to obtain the maximum electrical signal possible from the heart

• The reading measured by the ECG machine is the potential difference between pairs of electrodes

• To obtain a good trace, the following factors must be considered:

  • the optimum positioning of the electrodes

  • minimising contact resistance

  • improving the quality of the received signals

Positioning the electrodes

• Electrodes are placed strategically in pairs on the body to obtain the largest potential difference

• These are:

  • in six standard chest positions

  • one on each limb (close to the arteries)

Reducing contact resistance

• The point where the electrodes are attached to the skin is called a contact

• The contacts must be:

  • good conductors of electricity

  • able to provide a low resistance to the electrical signal

  • non-irritant

  • non-reactive to skin chemicals

  • all securely stuck in place

• As such, the contacts are:

  • made from a non-reactive material

  • secured in place with a conductive gel 

  • attached after removing hairs and dead skin cells (with sandpaper or a razor)

Improving the signal

• The electrical signal detected by the ECG is relatively small because it is easily absorbed by the body

• To improve the received signals:

  • The patient should remain relaxed and still

  • The machine and leads should be shielded from the electrical interference of other AC sources

• The signal needs to be amplified by a high-gain, low-noise, high-impedance amplifier:

  • High gain means the amplifier can increase the electrical signal without increasing the noise

  • Low noise means the amplifier does not reduce the quality of the electrical signal during the amplification process

  • High impedance means the amplifier increases the input signal enough so it can be interpreted by the machine

Normal ECG waveform

• ECG stands for electrocardiogram 

• It is a graph of the potential difference between the electrodes (in mV) against time (in s)

• A normal ECG, covering a single heartbeat, has three separate parts:

  • A P wave

  • A QRS wave

  • A T wave

• The heartbeat can be determined from the number of contractions (number of waveforms) present on the ECG in 1 minute

• The P wave:

  • Caused by the contraction (depolarisation) of the atria

• The QRS wave:

  • Corresponds to the contraction (depolarisation) of the ventricles

  • It occurs 0.2 seconds after the P wave

  • The signal is much greater than the P wave

  • It reaches its maximum at 1 mV

• The T wave:

  • Corresponds to the relaxation (repolarisation) of the ventricles

  • It occurs 0.2 seconds after the QRS wave