Defibrillation
When sudden cardiac arrest strikes, CPR alone doesn't save lives - it is merely a temporary measure that maintains minimal oxygen flow to the brain. Early defibrillation is required to re-establish a regular heartbeat.
A defibrillator can deliver a controlled electrical shock to a heart that has a life-threatening rhythm, such as ventricular fibrillation (VF). In VF, the heart's chaotic activity prevents blood from pumping to the body and brain. Voltage stored by the defibrillator conducts electrical current (a shock) through the chest by way of electrodes or paddles placed on the chest. This brief pulse of current halts the chaotic activity of heart, giving the heart a chance to re-start with a normal rhythm.
The defibrillator uses energy to deliver a shock. The amount of energy used depends on:
- How much voltage is used
- How much current is delivered
- The duration (length) of the shock
Many people confuse current and energy. This distinction is important in defibrillation, since defibrillators are often described in terms of energy (e.g., 200J) but it is their current - not the energy - that defibrillates. Successful defibrillation requires that enough current be delivered to the heart muscle during the shock.
A wave of electrical current has a shape that can be drawn as a "waveform". The waveform shows how the flow of current changes over time during the defibrillation shock. The highest part of the current waveform is called "peak current". Too much peak current during the shock can injure the heart. It's the peak current (not energy) that can injure the heart.
Defibrillation requires a true middle-of-the-road approach. You must have enough current reach the heart to defibrillate the heart (stop the lethal rhythm), but not so much peak current that you risk damaging the heart. In fact, low-energy shocks from some defibrillators deliver higher peak current than higher-energy shocks from other types of defibrillators.
Impedance is the body's resistance to the flow of current. Some people naturally have higher impedance than others.
Certain factors can also increase impedance, such as:
- A large and/or hairy chest
- Very dry skin
- Excess air in the lungs
- Improper application of the defibrillation electrodes
Biphasic waveforms adjust for impedance by varying the characteristics of their waveforms. How each waveform adjusts for impedance has important consequences - it may determine whether or not someone's life is saved.
It is important to know how each biphasic waveform adjusts for impedance to ensure that high-impedance persons will have the same chance for survival as those who are easily defibrillated.
Many clinical studies demonstrating the success of low-energy biphasic waveforms were conducted in electro-physiology labs under ideal conditions. In real life, cardiac emergencies are much less predictable. Many factors affect the chance of defibrillation success: time elapsed before the first shock is given, placement of the electrode pads, the person's impedance level and certain health conditions. Therefore, it may take more current, a longer shock duration, and/or increased voltage to ensure success. Current flow changes with time during a defibrillation shock. When drawn on a graph, this is known as a waveform. Hearts respond differently to different waveforms, which is why the introduction of biphasic waveforms to external defibrillators can have a positive impact.