A defibrillator is a medical device designed to deliver a controlled electrical shock to the heart. Its purpose is to interrupt and correct certain life-threatening abnormal heart rhythms, thereby allowing the heart to restore a normal and effective pumping beat.
The Heart’s Electrical Activity and Life-Threatening Rhythms
The heart operates through a precise electrical system that coordinates its contractions. Electrical impulses originate in the sinoatrial (SA) node, the heart’s natural pacemaker, and then spread through the upper chambers (atria) and lower chambers (ventricles), prompting them to contract in a synchronized manner. This coordinated action ensures efficient blood circulation throughout the body.
When this electrical coordination falters, it can lead to dangerous arrhythmias, particularly Ventricular Fibrillation (VF) and Ventricular Tachycardia (VT). In Ventricular Fibrillation, the heart’s electrical signals become chaotic and disorganized, causing the ventricles to merely quiver rather than contract effectively. This prevents the heart from pumping blood, leading to immediate cardiac arrest.
Ventricular Tachycardia involves a very rapid heart rate that begins in the ventricles. While the heart still contracts, the beats are so fast that the chambers do not have enough time to fill properly with blood, significantly reducing the heart’s pumping efficiency. If sustained and without a pulse, VT can quickly degenerate into VF and lead to cardiac arrest.
How Defibrillators Identify Abnormal Rhythms
Defibrillators detect life-threatening rhythms by continuously monitoring the heart’s electrical activity. Automated External Defibrillators (AEDs) use adhesive pads placed on the patient’s chest, while Implantable Cardioverter-Defibrillators (ICDs) utilize leads surgically positioned within or around the heart. These components act as sensors, transmitting the heart’s electrical signals, ECG, to the device.
An internal computer within the defibrillator analyzes these incoming ECG signals. It employs algorithms to differentiate between normal heart rhythms and dangerous, shockable ones like VF and pulseless VT. These algorithms assess characteristics of the electrical waveforms, including their amplitude, frequency, and overall morphology.
The device then makes a “shock” or “no-shock” determination based on this analysis. This process is highly accurate, ensuring that a shock is only delivered when medically appropriate.
Primary Triggers for Defibrillator Activation
The primary conditions that trigger a defibrillator to deliver an electrical shock are Ventricular Fibrillation (VF) and pulseless Ventricular Tachycardia (VT). When the device identifies these rhythms, it initiates a sequence of actions to restore normal heart function.
In the case of Ventricular Fibrillation, the heart’s electrical activity is completely chaotic, causing the ventricles to quiver ineffectively and stopping blood circulation. Upon detecting this disorganized electrical pattern, the defibrillator charges and delivers a controlled electrical current. This shock aims to momentarily depolarize a large portion of the heart muscle, effectively “resetting” its electrical system. The goal is to allow the heart’s natural pacemaker to regain control and re-establish a coordinated, effective rhythm.
Similarly, if the device detects pulseless Ventricular Tachycardia, where the ventricles are beating too rapidly to pump blood adequately, it will also deliver a shock. The electrical discharge interrupts the rapid, abnormal electrical circuit within the ventricles, providing an opportunity for the heart to return to a more stable and functional rhythm.
When Defibrillators Do Not Activate
A defibrillator is specifically designed to treat only certain types of life-threatening electrical problems in the heart. Therefore, it will not deliver a shock if it detects a normal heart rhythm, even if a person has collapsed due to other medical issues. The device’s algorithms are programmed to distinguish between rhythms that can be corrected by a shock and those that cannot.
The device will not activate for asystole, commonly known as a “flatline.” In asystole, there is a complete absence of electrical activity in the heart. Since there is no disorganized electrical activity to interrupt or reset, an electrical shock would be ineffective in this scenario.
Similarly, defibrillators typically do not shock pulseless electrical activity (PEA). In PEA, the heart’s electrical system shows organized activity on the ECG, but the heart muscle is not contracting effectively enough to produce a pulse or pump blood. Defibrillation does not address the underlying mechanical failure that characterizes PEA. For asystole and PEA, other medical interventions, such as chest compressions and specific medications, are necessary.