Defibrillation is a time-sensitive medical procedure that uses a controlled electrical shock to correct a life-threatening heart rhythm disturbance. Modern emergency medical care relies on advanced biphasic defibrillation technology to deliver this treatment. This method represents the current standard for effectively resetting the heart’s electrical activity. It works by delivering a precise dose of energy that momentarily stuns the heart, allowing its natural pacemaker to regain control.
The Lethal Rhythms Biphasic Defibrillation Corrects
The primary rhythms that necessitate defibrillation are Ventricular Fibrillation (V-Fib) and pulseless Ventricular Tachycardia (V-Tach). These conditions are forms of sudden cardiac arrest where the ventricles are unable to pump blood effectively. In V-Fib, heart muscle cells fire chaotically, causing the ventricles to merely quiver instead of contracting forcefully. This state stops the circulation of blood to the brain and other organs.
Pulseless V-Tach involves rapid, ineffective electrical activity that prevents the ventricles from properly filling with blood. In both V-Fib and pulseless V-Tach, the heart fails its pumping function, leading to immediate collapse and loss of consciousness. Defibrillation is the only definitive treatment, as cardiopulmonary resuscitation (CPR) alone cannot stop the underlying electrical chaos. The intervention is intended to stop the chaotic electrical activity so a normal rhythm can resume.
How Biphasic Delivery Differs from Older Technology
The difference between modern biphasic technology and older monophasic devices lies in the electrical current’s waveform and direction of flow. Monophasic defibrillators delivered the current in a single direction, flowing from one electrode pad through the heart to the second. This single-direction shock required significantly higher energy levels, typically up to 360 Joules, to achieve a successful reset. This high energy output increased the risk of damaging the heart muscle and surrounding tissue.
Biphasic defibrillators utilize a bidirectional current flow. The device first sends the current in one direction, then immediately reverses the polarity and sends the current back in the opposite direction. This reversal allows for a much more efficient depolarization of the heart muscle cells. Biphasic devices achieve similar success rates at much lower energy settings, typically between 120 and 200 Joules, reducing the risk of post-shock complications like myocardial dysfunction and skin burns.
The Mechanism of Cellular Depolarization and Synchronization
The biphasic shock executes a momentary electrical reset of the entire heart muscle. When the heart is in a lethal rhythm, individual muscle cells (myocytes) fire electrical impulses randomly. The high-energy shock instantly depolarizes every cell in the myocardium simultaneously. This coordinated electrical event causes the heart to enter a brief period of electrical silence, known as transient asystole.
Following this induced silence, the heart’s intrinsic electrical system has a chance to reorganize. The entire myocardium enters a refractory period, where cells are temporarily unable to fire new impulses. This window of silence allows the heart’s natural pacemaker, the Sinoatrial (SA) node, to potentially take over. The SA node is a specialized cluster of cells that initiates the heart’s normal, synchronized rhythm.
As the refractory period ends, the SA node can successfully initiate a new, organized electrical impulse that is conducted normally through the heart muscle. This restores a functional heart rhythm capable of pumping blood effectively.
Delivering the Shock: Practical Considerations
Successful defibrillation depends on the precise delivery of electrical energy to the heart muscle. Pad placement is crucial, as the pads must bracket the heart to ensure the current flows directly through the target tissue. The standard adult placement is the antero-lateral position, with one pad below the right collarbone near the sternum and the second on the patient’s left side, below the armpit. Alternative placements, such as the anterior-posterior position, are sometimes used to optimize current flow.
The body’s natural resistance to electrical current, known as transthoracic impedance, influences the shock’s effectiveness. Factors like excessive chest hair, moisture on the skin, and poor pad contact increase this resistance. Biphasic defibrillators overcome this challenge by automatically sensing the patient’s impedance and adjusting the waveform to ensure the target current is delivered. This impedance compensation ensures the lower-energy shock remains effective across a wide range of patient conditions, minimizing the risk of burns and muscle injury.