10.3.1 Simple ECG machines and the normal ECG waveform

The Heart

The heart functions as a double pump, moving blood through the lungs and the rest of the body. The heart is divided into two sides, each with an atrium and a ventricle separated by a valve. Blood flows from the atria to the ventricles, which contract to pump blood through the body.

• Electrical Signals in the Heart: The contractions are controlled by electrical impulses produced at the sinoatrial (SA) node, which causes the atria to contract. A short delay allows the signal to travel to the atrioventricular (AV) node, which then causes the ventricles to contract.
  • Electrocardiogram (ECG): These electrical signals can be measured using an ECG. An ECG records the change in potential difference over time, providing a graphical representation of the heart's electrical activity.

Steps to Obtain an ECG Signal

1. Placement of Electrodes: Electrodes are attached to the patient's skin (near arteries where potential differences can be best detected).

• To improve the signal, the following precautions are taken:
• Attach electrodes firmly to prevent movement.
• Remove dead skin cells and hair from the electrode site using sandpaper.
• Use a conductive gel to ensure good electrical contact and minimise air gaps.

2. Reducing Electrical Noise: Electrical noise, which could distort the ECG reading, is minimised by:

• Ensuring the patient remains relaxed and still.
• Shielding the ECG machine and leads from external electrical interference.
• Using non-reactive electrode materials to prevent interference from chemical reactions at the skin surface.

Amplification of the Signal

The electrical signal detected by the electrodes is weak, so it needs to be amplified before being displayed on the ECG machine. The amplifier used must have:

• High gain to boost the signal strength.
• Low noise to prevent distortion.
• High input resistance to avoid distortion from electrical interaction with the skin.

Interpreting the ECG Waveform

The ECG waveform has three key components:

3. P Wave: This is when the SA node produces an electrical impulse, causing the atria to contract.
4. QRS Complex: Occurs about 0.2 seconds after the P wave; this is when the AV node triggers the ventricles to contract.
5. T Wave: Appears about 0.2 seconds after the QRS complex as the ventricles relax in preparation for the next heartbeat. Each segment on the ECG represents a specific phase of the heart's electrical activity.

• R-R Interval: The time between consecutive R waves, which can be used to calculate the heart's pulse rate per minute:

$$\text{Pulse rate per minute} = \frac{60}{\text{Period in seconds}}$$

Practical Application of ECG Measurements

• Amplitude of the ECG Waveform: The height of the waveform can show the potential difference at the skin, typically around 1 mV.
• Effect of Exercise: During exercise, the heart rate increases, shortening the R-R interval and reducing the duration of the flat section of the trace.