The human heart coordinates its pumping action through specialized cardiac muscle cells that generate and carry electrical impulses independently of the nervous system. This pathway begins at the sinoatrial (SA) node, located in the upper wall of the right atrium near the superior vena cava. Known as the natural pacemaker of the heart, the SA node fires electrical impulses fastest, setting the normal rhythm.
From the SA node, the impulse spreads through the atrial walls, causing them to contract and push blood into the ventricles. It then converges at the atrioventricular (AV) node, situated at the junction between the atria and ventricles. The AV node briefly delays the signal, allowing the ventricles to fill completely with blood before they contract.
After this brief pause, the impulse moves rapidly down the bundle of His (atrioventricular bundle), which divides into the right and left bundle branches traveling along the interventricular septum. Finally, the signal fans out into the Purkinje fibers, spreading upward through the ventricular walls and triggering a powerful, synchronized contraction that ejects blood into the lungs and systemic circulation.
Cardiac muscle cells maintain an electrical charge across their cell membranes when resting, known as the resting membrane potential. Inside the cell is relatively negative compared to the outside. When an electrical impulse arrives, tiny gates called ion channels open, allowing charged particles to flow across the cell membrane.
An electrocardiogram (ECG or EKG) is a graphical recording of the heart's electrical activity captured from electrodes placed on the skin. It does not measure the mechanical pumping of blood directly; rather, it records the electrical currents generated as the heart depolarizes and repolarizes with every beat.
Midwives and nurses use ECGs to quickly assess heart rate, identify abnormal rhythms (arrhythmias), and detect signs of heart muscle damage or electrolyte imbalances. Because body tissues conduct electricity well, small electrical signals produced by the myocardium travel to the skin surface, where metal electrodes detect them and transmit the data to the monitoring machine.
A single normal heartbeat on an ECG tracing appears as a series of distinct waves and deflections, traditionally labeled with the letters P, Q, R, S, and T.
ECG paper is specialized graph paper marked with a grid of small and large boxes designed to measure time and voltage accurately. Standard calibration runs at a paper speed of 25 millimeters per second ($mm/s$) and a voltage calibration of 10 millimeters per millivolt ($mm/mV$).
An ECG lead is not a physical wire attached to the patient, but rather an electrical view of the heart from a specific angle. A standard 12-lead ECG uses 10 physical electrodes to create 12 different perspectives (leads I, II, III, aVR, aVL, aVF, and V1 through V6), allowing clinicians to view the heart's electrical activity from the front, side, and bottom.
Interpreting a basic rhythm strip requires a systematic, step-by-step approach to avoid missing critical abnormalities. Midwives and nurses should follow a consistent sequence every time they evaluate a strip:
Obtaining a clean, accurate ECG tracing requires careful patient preparation and attention to technical details. Poor electrode contact or patient movement introduces artifact—unwanted electrical interference that distorts the waveform.
Explain the procedure clearly to the patient, reassuring them that the machine only records electricity and delivers no electrical shock. Clean oily skin with alcohol wipes and clip chest hair if necessary to ensure firm electrode adhesion. Ask the patient to lie completely still, breathe normally, and avoid talking while the machine records.
If the baseline wanders or erratic spikes appear, check for loose electrode cables, trembling muscles, or nearby electrical equipment causing interference.