An EKG machine detects the tiny electrical signals your heart produces with every beat, amplifies them, and converts them into the familiar wave pattern doctors use to assess heart health. The voltages involved are remarkably small, typically around 1 millivolt at the skin’s surface, so the machine’s core job is separating that faint cardiac signal from the much louder electrical noise your body and environment generate.
Your Heart Runs on Electricity
Every heartbeat begins as an electrical impulse. Heart muscle cells maintain a resting electrical charge by keeping certain ions (charged particles like sodium, potassium, and calcium) at different concentrations inside and outside the cell. When a cell is triggered to contract, positively charged ions rush inward, flipping the cell’s voltage from negative to positive. This flip is called depolarization, and it spreads across the heart in a predictable wave, causing each region to contract in sequence.
After contracting, each cell resets by pushing ions back out, restoring the original charge. This reset is called repolarization. The entire cycle, from the first electrical spark in the upper chambers to the final reset of the lower chambers, takes less than a second. Because this process involves millions of cells firing in coordinated waves, the combined electrical activity is strong enough to reach the skin’s surface, where electrodes can pick it up.
Electrodes vs. Leads: What’s on Your Body
A standard clinical EKG uses 10 physical electrodes: one on each arm, one on each leg, and six across the chest. These adhesive patches with conductive gel serve as antenna-like sensors. But you’ll hear doctors refer to a “12-lead EKG,” which can be confusing since there are only 10 electrodes.
The difference is that a “lead” isn’t a wire. It’s a specific viewpoint of the heart’s electrical activity, created by measuring the voltage difference between selected electrodes. Some leads are bipolar, meaning they compare one positive electrode against one negative electrode. Others are unipolar, using a single positive electrode compared against a calculated average from the remaining electrodes. By combining signals from 10 physical sensors in different mathematical arrangements, the machine produces 12 distinct perspectives of the heart’s electrical behavior, much like photographing an object from 12 different angles.
Inside the Machine: From Signal to Screen
The raw electrical signal arriving from the electrodes is tiny and buried in noise. The machine processes it through several stages to produce a clean, readable tracing.
Protection Circuits
The first component the signal encounters is a safety barrier. The electrical connections to a patient must not create a shock hazard or interfere with other medical equipment. These circuits also need to recover quickly after a defibrillation event, which can blast the machine’s front end with far more voltage than it’s designed to measure.
Lead Selector and Switch Matrix
A switch matrix routes signals from the correct combination of electrodes for each of the 12 leads. Averaging circuits calculate the composite negative reference needed for the unipolar leads. This happens electronically and automatically.
Amplifier
This is arguably the most critical component. The heart’s signal at the skin is roughly 1 millivolt, while electrical interference from power lines, nearby equipment, and even your own muscles can be thousands of times stronger. The machine uses a specialized amplifier called an instrumentation amplifier, which compares the signals from two electrodes and amplifies only the difference between them. Any noise that appears equally on both electrodes (called common-mode noise) gets canceled out. Clinical EKG machines must achieve a common-mode rejection ratio of at least 92 decibels, meaning they suppress shared interference by a factor of roughly 40,000 to 1.
Analog-to-Digital Converter
After amplification, the continuous electrical signal is converted into digital data. The machine samples the signal hundreds of times per second. The American Heart Association recommends a minimum sampling rate of 500 times per second for adult EKGs and notes that 1,000 samples per second is desirable for capturing fine details, particularly in pediatric recordings where waveforms can be narrower and higher in frequency. Each sample is recorded with enough precision (typically 12 bits or higher) to distinguish even subtle voltage changes.
How the Machine Filters Noise
Even with a high-quality amplifier, additional filtering is necessary. The biggest offender is power line interference, which creates a steady 60 Hz hum (50 Hz in some countries) that can distort the tracing. Digital filters selectively remove this frequency without affecting the heart signal. Muscle movement from trembling, shivering, or even breathing also introduces noise, and the machine applies baseline correction algorithms to compensate.
Some machines use a technique called right-leg drive, where a small processed signal is fed back to the electrode on the right leg. This actively cancels common-mode interference at its source rather than trying to filter it out after the fact, significantly improving signal quality.
Reading the Waves: P, QRS, and T
The cleaned-up signal produces a repeating waveform with distinct peaks and valleys, each corresponding to a specific event in the heart.
The P wave is the first small bump. It represents the electrical activation of the upper chambers (atria). The first half reflects the right atrium depolarizing; the second half reflects the left atrium.
The QRS complex is the tall, sharp spike that follows. This is ventricular depolarization, the electrical wave sweeping through the heart’s main pumping chambers. It unfolds in a specific sequence: first the wall between the ventricles, then the inner walls of both chambers, then progressively more of the thick left ventricle, and finally the base of the left ventricle. The whole process takes less than 120 milliseconds in a healthy heart.
The T wave is the broader, gentler bump after the QRS. It represents the ventricles resetting their electrical charge (repolarizing) in preparation for the next beat. The time from the start of the QRS to the end of the T wave, known as the QT interval, reflects how long the ventricles take to fire and recover. Abnormalities in potassium or sodium channels that delay this recovery show up as a prolonged QT interval on the tracing.
Calibration: A Universal Measuring Stick
Every clinical EKG uses the same calibration standard so that tracings from different machines and different hospitals can be compared. The universal setting is 10 millimeters of deflection per 1 millivolt of signal. On the printed grid, each small square is 1 millimeter (representing 0.1 millivolt vertically), and each large square is 5 millimeters (0.5 millivolt). Paper feeds at a standard speed of 25 millimeters per second, so each small square horizontally represents 0.04 seconds. This grid system lets clinicians measure precise intervals and voltages at a glance.
You’ll often see a small rectangular calibration pulse printed at the beginning or end of an EKG strip. It’s exactly 10 millimeters tall, confirming the machine is properly calibrated.
What an EKG Can Detect
Because the tracing reflects the precise timing, direction, and strength of electrical flow through every region of the heart, distortions in the pattern can reveal a wide range of problems. An EKG can identify abnormal heart rhythms (arrhythmias), poor blood flow to the heart muscle from blocked coronary arteries, evidence of a current or past heart attack, enlarged heart chambers, congenital heart defects, heart valve problems, and signs of heart failure.
What an EKG cannot do is see the heart’s physical structure or measure blood flow directly. It only captures electrical activity. That’s why it’s often paired with imaging tests like echocardiograms when structural problems are suspected. But as a quick, noninvasive, and inexpensive snapshot of cardiac function, it remains one of the most widely used diagnostic tools in medicine.