Prehospital Academy Lesson 16.3
The ECG tracing is a mathematical representation of cardiac electrical activity plotted against time and voltage. To interpret a rhythm, the paramedic must look past the "lines on the screen" and understand the standardized grid that makes clinical diagnosis possible. This guide explores the calibration of ECG paper, the morphology of individual waveforms, and the clinical significance of the intervals that define the cardiac cycle.
The Standardized Grid: Time and Amplitude
ECG paper is a universal coordinate system. By convention (the ISO standard), the monitor prints at a speed of 25 mm per second. This creates a specific relationship between distance and time:
The X-Axis (Time): One small box (1 mm) represents 0.04 seconds. Five small boxes form one large box (5 mm), representing 0.20 seconds.
The Y-Axis (Amplitude): This measures voltage. Standard calibration is 10 mm per 1 mV. One small box is 0.1 mV, and one large box is 0.5 mV.
The Calibration Box: Every diagnostic ECG begins with a "calibration pulse"—a rectangular wave typically 10 mm high and 5 mm wide. If this box is distorted, the subsequent waveforms (like ST-elevation or QRS width) cannot be accurately measured.
Waveform Morphology and the "Normal" Baseline
The Isoelectric Line is the baseline of the ECG, representing periods of electrical silence. Deviations from this line form waves and complexes:
P-Wave: Represents atrial depolarization originating from the SA node. It should be smooth, rounded, and upright, lasting <0.11 seconds.
QRS Complex: Represents ventricular depolarization. While it can vary in nomenclature (e.g., qRs, rS, or RSR’), a normal complex must be narrow (<0.12 seconds). A wide QRS indicates a delay in conduction, often originating from the ventricles or a bundle branch block.
T-Wave: Represents ventricular repolarization. It is asymmetrical, with a slow upslope and a fast downslope.
The J-Point and the ST-Segment
The J-Point (Junction Point) is the exact location where the QRS complex ends and the ST-segment begins. In academic cardiology, identifying the J-Point is the "gold standard" for diagnosing myocardial injury.
The ST-Segment should ideally be isoelectric. Elevation or depression is measured 0.04 seconds after the J-point. Elevation often signifies acute injury (STEMI), while depression may indicate ischemia or reciprocal changes.
Critical Intervals: PR and QT
Intervals measure the duration of specific phases of the conduction cycle:
PR Interval (0.12–0.20s): Measures the time from the start of atrial depolarization to the start of ventricular depolarization. It includes the critical AV-nodal delay.
QT Interval: Measures the total time for ventricular depolarization and repolarization. Because the QT interval changes with heart rate, clinicians use the QTc (Corrected QT), often calculated via Bazett’s Formula. A prolonged QTc (>440ms in men, >460ms in women) increases the risk of Torsades de Pointes.
Critical Thinking Questions
If an ECG is inadvertently printed at a speed of 50 mm/sec instead of the standard 25 mm/sec, how would this affect the measured width of the QRS complex and the resulting diagnosis?
A P-wave is noted to be "peaked" and taller than 2.5 mm in Lead II. Which chamber of the heart is likely enlarged or stressed, and what physiological condition might cause this?
Why is it critical to identify the J-Point when a patient has a "slurring" S-wave (as seen in some bundle branch blocks)? How does this impact the measurement of ST-elevation?
Relate the "Absolute Refractory Period" to the ST-segment. Why is the heart unable to respond to a new electrical stimulus during this specific isoelectric period?
A patient has a heart rate of 120 bpm and a QT interval that appears "normal" on the grid. Why is it still necessary to calculate a QTc for this patient before administering QT-prolonging medications like Ondansetron?