An electrical waveform is a graphical representation of how an electrical current or voltage changes over time. In medicine, controlled electrical pulses are delivered to interact with biological tissues. The precision of this pulse is paramount, as the goal is to stimulate or reset cell function without causing unintended harm. The biphasic waveform is a modern advancement that has become the standard in numerous medical applications requiring precise energy delivery.
Defining the Two Phases of the Waveform
The term “biphasic” describes an electrical pulse that has two distinct phases of current flow, unlike older monophasic designs. The pulse begins with the first phase, where current flows from one electrode to the other in a single direction, often called the positive polarity. This initial flow achieves the primary therapeutic effect, such as depolarizing the heart muscle during defibrillation or stimulating a nerve.
The current then crosses the baseline, which represents a brief moment of zero current, before immediately initiating the second phase. In this second phase, the current flow reverses direction, traveling back toward the originating electrode with a negative polarity. This directional reversal is the defining characteristic of a biphasic pulse. The entire pulse, encompassing both the positive and negative phases, is carefully managed to ensure the total electrical charge delivered is balanced.
In most therapeutic applications, such as nerve stimulation, the second phase mirrors the first to ensure a net zero charge is left on the tissue. Achieving this charge balance is important because a prolonged, unidirectional current can accumulate charge, potentially leading to harmful electrochemical changes or skin irritation. The controlled reversal of the second phase effectively neutralizes the charge delivered by the first phase.
Why Biphasic Waveforms Are Superior
The two-phase structure of the biphasic waveform provides distinct physiological and technical advantages over its single-direction predecessor. A primary benefit is the reduced energy requirement needed to achieve the same therapeutic effect, particularly in defibrillation. Studies show that biphasic pulses achieve successful defibrillation at significantly lower energy levels than monophasic waveforms, often reducing the necessary energy by half or more. This lower energy delivery is directly linked to a decrease in cellular damage and tissue injury.
When high-energy, single-direction shocks are delivered, they can cause an effect called electroporation, where the cell membranes are damaged by the intense electrical field, potentially leading to cell death or post-shock complications like myocardial dysfunction. The reversed current flow of the second phase is thought to help “repolarize” the cells, effectively reversing some of the negative cellular effects of the initial pulse.
The biphasic design allows the device to better compensate for the patient’s body impedance, which is the natural resistance of the chest to electrical current flow. Impedance is highly variable based on factors like size, fat content, and electrode placement, and high impedance can block the current from reaching the target tissue. Modern biphasic devices sense this resistance and automatically adjust the waveform’s characteristics, such as duration or voltage. This ensures sufficient current is delivered to the heart, ensuring a consistent and effective treatment compared to fixed-energy monophasic shocks.
Essential Medical Uses
The biphasic waveform is now the global standard across several medical devices that rely on external electrical intervention. Its most prominent application is in defibrillation, which includes both manual hospital-based defibrillators and automated external defibrillators (AEDs) found in public spaces. The efficacy and safety of the biphasic shock have made older monophasic devices largely obsolete, improving the chances of a successful return to a normal heart rhythm with less risk of injury.
The technology was initially developed and refined for use in implantable cardioverter-defibrillators (ICDs) and internal pacemakers, which require extremely precise and low-energy pulses for long-term use within the body. In cardiac pacing, biphasic pulses are used to stimulate the heart to maintain a regular rhythm, where the low energy and charge-neutral design are paramount for battery life and minimizing tissue irritation over many years of operation.
Beyond cardiac care, biphasic waveforms are widely used in transcutaneous electrical nerve stimulation (TENS) devices for pain management and therapeutic stimulation. These devices use the balanced biphasic current to stimulate nerves or muscles across the skin. A symmetrical biphasic waveform prevents net charge accumulation under the skin, which increases patient comfort and reduces the risk of burns or electrochemical reactions at the electrode site.