10.3.1 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

A Diagram of the Structure of the Heart

The diagram shows the left and right sides of the heart with the atrium and ventricle on each side

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
  • 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
• 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

Patient undergoing an ECG

Electrodes are attached in pairs to a patient's chest in the six positions shown and all four limbs. The variation in potential difference between them is detected

• 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

A Normal ECG Waveform

A normal electrocardiogram (ECG) waveform contains P, QRS and T components 

• 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