HP CODEMASTER

Hewlett Packard Codemaster Defibrillator

Overview

The heart is the vital organ for pumping blood around the body. It works due to electrical signals being generated in the heart’s pacemaker the Sino Atrial node, which cause contraction of the various chambers, which in turn pumps blood around the body. A heart attack is a common reason for these signals to become chaotic and prevent the heart from pumping. This chaotic heart rhythm, Ventricular Fibrillation (VF), causes the heart’s main pumping chambers to quiver uncontrollably – the result being that blood is not pumped around the body. This is one kind of cardiac arrest. A defibrillator can apply a high-energy electric shock to the body that will stop the heart quivering & allow the heart’s natural pacemaker to regain control of the heart. Of all the various causes of cardiac arrest, the heart going into VF and then being successfully defibrillated, results in the greatest number of survivors.

By Mark Callaghan & Graham Eyre

Use

A defibrillator is used in an emergency life-threatening situation after a patient has suffered a cardiac arrest to revert the heart back to its normal rhythm. They may also be used in other emergency and non-emergency situations where the patient has an abnormally fast or irregular heart rhythm. The patient is given a general anaesthetic and the defibrillator is then used in a special mode that reverts the heart rhythm back to normal sinus rhythm. This procedure is called synchronous cardioversion.

Where are they found?

Due to the importance of a speedy response to cardiac arrest, defibrillators are located all over the Trust. They are kept in most ward areas, but in critical areas there is a higher concentration. For example in the Emergency Department and Coronary Care the need is higher so more defibrillators are available. They may also be found on ambulances & in various doctors’ surgeries. Simple automatic defibrillators (AED’s) can now be found in shopping centres and sports complexes.

How it works

A defibrillator consists of a high-energy electrical source. It also has a built in ECG monitor to display the electrical activity of the heart. A paper recorder may record this signal for later analysis. The energy is delivered to the patient through large metal pads called paddles. These paddles will pick up the ECG signal in a cardiac arrest situation.

More modern defibrillators are able to monitor and defibrillate through large stick on pads. This is safer, as accidental discharge is less likely, and a spark and accidental ignition of the oxygen used by the anaesthetist is also less likely.

Some defibrillators may have advanced functions like:

• Advisory (Automatic) operation – the defibrillator can automatically analyse the heart rhythm & advise whether a shock is required. This is useful so that staff who cannot recognise heart rhythms can still safely defibrillate, even before the cardiac arrest team arrives.
• In-built pacemakers - which can apply an electrical pacemaker signal that will regularly trigger the heart to beat faster when it is beating too slowly.
• Pulse oximetry (SpO2) – which may be useful to measure oxygen in the blood.

High capacity batteries are required to operate the defibrillator. It is kept plugged in when not in use, continuously charging, so that it may be used without any need to find a plug socket at a cardiac arrest. The energy is selected on a rotary dial & this may be from 2 Joules up to 360 joules (which can be equivalent to in excess of 5000 volts). Upon selecting an energy level, a high voltage circuit board converts battery energy to charge up a capacitor & a coil. Then a relay (electromagnetic switch) will control the release of energy into the patient.

The built in ECG monitor allows the ECG waveform – the hearts electrical activity - to be viewed on the screen to see if defibrillation has been successful. If someone has an irregular heart rhythm then this can be reverted back to a normal rhythm by performing cardioversion (a smaller timed shock). The delivery of this shock needs to be synchronised to the R wave of the QRS complex of the ECG. In this case an ECG cable must be attached to the patient to ensure a good quality signal is obtained. Internal circuitry in the defibrillator will detect the R wave & then only permit the delivery of a shock a short time after this.

Defibrillators are extremely dangerous medical devices & they must only be used by personnel trained in their use.