An electrocardiogram, or ECG, is a medical test that records the tiny electrical signals that coordinate each heartbeat. ECG machines matter because they help doctors check heart rhythm, heart rate, and signs of problems such as reduced blood flow or damaged heart tissue. The test is fast, noninvasive, and widely used in clinics, ambulances, hospitals, and emergency rooms.
A modern ECG machine turns voltages measured at the skin into a clear waveform that can be read and compared over time.
Small adhesive electrodes placed on the chest and limbs detect voltage differences caused by the heart’s electrical activity. Colored leads carry these signals to the ECG machine, where amplifiers, filters, and software reduce noise and display the result as repeating P waves, QRS complexes, and T waves. Each part of the waveform relates to a step in the heart’s electrical cycle, from atrial contraction to ventricular recovery.
By measuring intervals and waveform shapes, clinicians can identify rhythm changes, conduction delays, and possible injury patterns.
Key Facts
- An ECG records voltage differences at the skin caused by electrical activity in the heart.
- Heart rate from ECG can be estimated by heart rate = 60 / R-R interval in seconds.
- The P wave represents atrial depolarization, which helps trigger atrial contraction.
- The QRS complex represents ventricular depolarization and is usually the tallest part of the tracing.
- The T wave represents ventricular repolarization, when ventricular cells reset electrically.
- A standard 12-lead ECG uses 10 electrodes to view the heart’s electrical activity from 12 different angles.
Vocabulary
- Electrode
- A sensor placed on the skin that detects small voltage changes produced by the heart.
- Lead
- A specific view of the heart’s electrical activity calculated from one or more electrodes.
- Depolarization
- The electrical change in heart muscle cells that starts contraction.
- Repolarization
- The electrical reset of heart muscle cells after they have been activated.
- QRS complex
- The sharp ECG waveform that shows ventricular depolarization.
Common Mistakes to Avoid
- Calling every wire a lead is wrong because the wires connect electrodes, while a lead is a measured electrical view calculated by the machine.
- Thinking the ECG directly shows the heart squeezing is wrong because it records electrical activity, not mechanical pumping force.
- Ignoring electrode placement is wrong because incorrect placement can change waveform size and shape, leading to misleading interpretations.
- Using only one heartbeat to judge rhythm is wrong because heart rhythm should be checked across multiple beats to see patterns and irregularities.
Practice Questions
- 1 An ECG shows an R-R interval of 0.80 s. Calculate the heart rate in beats per minute using heart rate = 60 / R-R interval.
- 2 A 12-lead ECG uses 10 electrodes. If each electrode cable has a resistance of 2.0 ohms, what is the total resistance of the 10 cables if they were connected in series?
- 3 Explain why an ECG machine needs good skin contact at the electrodes and why motion of the patient can create noise in the waveform.