Defibrillator is an electronic device that sends a therapeutic dose of electrical energy to the heart to stop an extremely rapid, irregular heartbeat, and restore the normal heart.
Defibrillation is performed immediately after identifying that the patient is experiencing a cardiac emergency, has no pulse, and is unresponsive.
Defibrillation is a common treatment for life-threatening cardiac dysrhythmias which can lead to cardiac arrest, such as ventricular fibrillation (an irregular and uncoordinated contraction of the cardiac muscle of ventricles) and pulseless ventricular tachycardia (Abnormally rapid heartbeat).
The process of defibrillation depolarizes a critical mass of the heart muscle, terminates the dysrhythmia and allows normal sinus rhythm to be re-established by the body’s natural pacemaker, in the sinoatrial of the heart.
Normal (Electrocardiogram)ECG:
Ventricular Tachycardia ECG:
Ventricular Fibrillation/Tachycardia ECG:
Ventricular Fibrillation ECG:
Know different types of defibrillator:
Broadly speaking, It is divided into two following categories:
External Manual Defibrillator:
It is generally used in conjunction with electrocardiogram readers, which the clinicians uses to diagnose a cardiac condition.
The healthcare provider will then decide what charge/energy (in joules) to use, based on proven guidelines and experience, and will deliver the shock through paddles or pads on the patient’s chest.
Implantable cardioverter-defibrillator (ICD):
Also known as automatic internal cardiac defibrillator (AICD). These devices are implants, similar to pacemakers (and many can also perform the pacemaking function).
They are programmed to constantly monitor the patient’s heart rhythm, and automatically administer shocks for various life-threatening arrhythmias.
Circuitry behind Defibrillator:
1). Capacitor stores energy and hence used for defibrillation while discharge.
- Cap charged to ~1000V
- Delivers 100-200 J to patient
- Discharge time ~5-10ms
2). Inductor used to create a finite damped sine wave of finite duration (5-10 ms) from DC power source.
3). Body is modeled as impedance, electrode & Body contacts with approximately 25 lbs pressure is required to avoid static discharge.
4). Monophasic vs. biphasic waveforms:
In biphasic waveform an exponentially decaying DC voltage is reversed in polarity about halfway through the shock time, then continues to decay for some time after which the voltage is cut off, or truncated. The studies showed that the biphasic truncated waveform could be more efficacious while requiring the delivery of lower levels of energy to produce defibrillation.
Preparation for Defibrillation:
- After help is called for, cardiopulmonary resuscitation (CPR) is begun and continued until the caregivers arrive and set up the defibrillator.
- Electrocardiogram leads are attached to the patient’s chest.
- Gel or paste is applied to the defibrillator paddles, or two gel pads are placed on the patient’s chest.
- The caregivers verify lack of a pulse and proceeds with defibrillation.
References and Further reading:
- http://www.nhlbi.nih.gov/health/health-topics/topics/aed
- http://www.who.int/medical_devices/innovation/defibrillator_manual.pdf
- http://en.wikipedia.org/wiki/Defibrillation
- http://www.redcross.org/take-a-class/program-highlights/cpr-first-aid/workplaces-schools-individuals
