A waveform is a graphical representation of an electrical current’s properties—such as its amplitude and direction—over a period of time. The biphasic waveform is a specific and widely adopted design in electrical stimulation devices. The term “biphasic” literally means “two phases,” indicating that the current direction reverses during a single pulse cycle.
Defining the Two Phases
A biphasic waveform is defined by a precisely sequenced flow of electricity that includes two distinct phases within a single pulse. The process begins with the first phase, where the electrical current flows in one direction, such as from the positive electrode to the negative electrode. This initial phase is responsible for delivering the primary therapeutic or stimulating charge to the target tissue.
Immediately following the first phase, the current flow reverses polarity to begin the second phase. The duration and amplitude of this second phase are engineered to balance the electrical charge delivered by the first phase. In a perfectly designed, or “charge-balanced,” biphasic pulse, the total electrical charge delivered in the positive direction is precisely equal to the total charge delivered in the negative direction. This careful balancing results in a zero net charge over the entire duration of the complete pulse cycle.
Functional Superiority Over Monophasic Current
The primary advantage of the biphasic design over monophasic currents, which flow in only one direction, lies in its ability to prevent the accumulation of electrical charge at the electrode-tissue interface. When charge flows continuously in one direction, it causes an electrochemical reaction that polarizes the tissue and generates potentially toxic byproducts. The reversing current of the biphasic waveform acts to neutralize or “clear” the charge buildup that occurred during the first phase.
This neutralization process significantly minimizes the risk of adverse tissue reactions, such as skin irritation, burns, or cellular damage, making the stimulation safer and more comfortable for patients. Symmetrical biphasic waveforms, where both phases are identical in magnitude but opposite in direction, are particularly effective at reducing the risk of skin reactions. The reversed phase helps reset the electrochemical state of the tissue membrane, which allows for more effective subsequent pulses. This improved efficiency means a biphasic shock can achieve the same therapeutic effect as a monophasic shock while often requiring less total energy.
Common Medical Applications
The safety and efficiency of the biphasic waveform have made it the standard in several high-stakes medical applications. In cardiac care, it is the preferred waveform for defibrillation and cardioversion devices, including both external defibrillators and implantable cardioverter-defibrillators (ICDs). Clinical studies have shown that biphasic shocks are more effective at terminating life-threatening arrhythmias, often requiring significantly lower energy levels—for example, a 130-Joule biphasic shock can be as effective as a 200-Joule monophasic shock. This lower energy requirement reduces the degree of damage to the heart muscle, or myocardium, following the shock.
The charge-balanced design is also used extensively in Transcutaneous Electrical Nerve Stimulation (TENS) units utilized for pain management. TENS therapy often involves long periods of low-level stimulation, where minimizing skin irritation and discomfort is paramount. The ability of the biphasic pulse to prevent residual charge accumulation is why it is preferred for these prolonged surface applications.
The truncated exponential biphasic waveform is also a standard design in implantable devices such as pacemakers. These devices deliver electrical pulses to internal tissues over many years, and the inherent safety of the biphasic design ensures long-term, reliable stimulation while preventing chronic tissue damage or electrode corrosion.