Cardiac Anatomy, Conduction System & EKG Machine Basics

Cardiac Anatomy and the Conduction System

Cardiac muscle cells possess a unique property called automaticity — the ability to generate an electrical impulse on their own, without a signal from the brain or nervous system. This property is concentrated in a specialized pathway of conduction tissue that fires in a precise sequence to produce a coordinated heartbeat. Because every part of this pathway can independently generate an impulse, the heart always has a built-in backup pacemaker if the primary one fails, though backup sites fire at progressively slower rates.

The normal conduction sequence begins in the sinoatrial (SA) node, a cluster of pacemaker cells in the upper wall of the right atrium near the entrance of the superior vena cava. The SA node is the heart's dominant pacemaker because it has the fastest intrinsic firing rate, 60-100 beats per minute, and its impulse depolarizes both atria — producing the P wave on the EKG — before any other site can fire. The impulse then travels through internodal pathways to the atrioventricular (AV) node, located in the lower interatrial septum. The AV node deliberately slows conduction for a fraction of a second; this delay allows the atria to finish contracting and empty their blood into the ventricles before ventricular contraction begins. If the SA node fails, the AV node can take over as a backup pacemaker at 40-60 bpm.

From the AV node, the impulse enters the Bundle of His, which splits into the right and left bundle branches running down either side of the interventricular septum, and finally reaches the Purkinje fibers, which fan out through the ventricular walls. The Purkinje network conducts extremely rapidly so both ventricles contract together; it can also serve as a last-resort backup pacemaker at only 20-40 bpm.

EKG Machine Functions and Settings

An electrocardiograph amplifies the heart's tiny electrical signals, filters artifact, and prints them as a permanent, standardized tracing so any clinician can interpret it consistently. Two settings must be correct on every tracing: paper speed and sensitivity (gain).

Paper speed is standardized at 25 mm/second. At this speed, the printed grid becomes a timing tool:

• 1 small box (1 mm) = 0.04 second
• 1 large box (5 small boxes, 5 mm) = 0.20 second
• 5 large boxes = 1 full second

When a patient's heart rate is very fast, waveforms can overlap and become difficult to separate at standard speed. In that situation the technologist may run the paper at 50 mm/second, which stretches the tracing horizontally so individual P waves and QRS complexes can be measured accurately. Any change from standard speed must be documented directly on the tracing so the interpreting clinician does not misread interval measurements.

Sensitivity (gain) controls the height of the waveform relative to voltage and is standardized at 10 mm/mV — meaning a 1-millivolt electrical signal produces a 10 mm vertical deflection. If QRS complexes are so tall they run into adjacent tracings, the machine may be switched to half standard (5 mm/mV); if complexes are too small to interpret, double standard (20 mm/mV) may be used instead. Either change must always be labeled on the tracing so voltage-based findings, such as criteria for chamber enlargement, are not misread.

Before recording, every EKG machine runs a calibration pulse — a square wave that, at standard settings, should measure exactly 10 mm tall and 0.2 second wide. This confirms the machine is recording amplitude and time accurately. If the calibration box does not measure correctly, the machine must be recalibrated or serviced before use, and the tracing should not be relied upon for clinical decision-making.

Equipment Maintenance and Cleaning

Proper care of the EKG machine and its leads protects both patient safety and tracing accuracy:

• Disinfect lead wires, cables, and the machine housing between patients according to facility infection-control policy and standard precautions.
• Inspect cables and lead wires before each use for fraying, cracking, or exposed wire; remove any damaged cable from service immediately.
• Store cables coiled loosely rather than tightly wound or kinked, since tight kinking can fracture the internal wires over time.
• Keep disposable electrodes sealed until use and check the expiration date — expired electrodes have dried gel and cause poor contact and artifact.
• Follow the manufacturer's cleaning instructions for the machine surface, cart, and printer components.
• Report any malfunction, such as a blank tracing, error code, or failed calibration, to the appropriate staff and remove the unit from use until it is serviced.

A well-maintained machine, run at the correct paper speed and sensitivity with a passing calibration check, is the foundation for every accurate EKG a Patient Care Technician records.