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
Energy is measured in joules (J). External defibrillators may offer a range of energy
selections. So-called "low energy" defibrillators are those that limit their energy
selections to 200J or less. Escalating energy defibrillators offer a range of energies,
starting with low energy levels with the option to increase the energy levels for
subsequent shocks.
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
You can't tell if someone has high impedance simply by looking at him or her. If
impedance is high, the heart may not receive enough current for defibrillation to be
successful. More current may be delivered by increasing the voltage and by
increasing the energy selected (more joules) on the defibrillator.
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.
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
Energy is measured in joules (J). External defibrillators may offer a range of energy
selections. So-called "low energy" defibrillators are those that limit their energy
selections to 200J or less. Escalating energy defibrillators offer a range of energies,
starting with low energy levels with the option to increase the energy levels for
subsequent shocks.
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
You can't tell if someone has high impedance simply by looking at him or her. If
impedance is high, the heart may not receive enough current for defibrillation to be
successful. More current may be delivered by increasing the voltage and by
increasing the energy selected (more joules) on the defibrillator.
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.
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