14.5: ECG Interpretation of Rhythms

An electrocardiogram (ECG)graphically represents the heart's electrical activity on ECG paper or a monitor.

Components of the Electrocardiogram

The primary components of a normal ECG waveform in Normal sinus rhythm(NSR) include the P wave, PR interval, QRS complex, ST segment, T wave, and occasionally a U wave.

ECG waveforms are divided by vertical and horizontal lines at standard intervals.

The horizontal axis measures time and rate, and the vertical axis measures amplitude or voltage. When the ECG waveform trends upward on the graph, this movement is called a positive deflection. When it moves toward the bottom, it is a negative deflection.

P Wave: A slight, positive deflection before the QRS complex, typically ≤2.5 mm height and ≤0.11 seconds.

PR Interval: Fluctuates from 0.12 to 0.20 seconds, covering the P wave and PR segment.

QRS Complex: Includes a Q wave (negative deflection), R wave (positive deflection), and S wave (negative deflection), taking <0.12 seconds to complete.

ST Segment: Links the QRS complex to the T wave, usually isoelectric, indicating early ventricular repolarization.

T Wave: A smooth, positive deflection following the QRS complex.

QT Interval: Stretches from the start of the QRS complex to the conclusion of the T wave and varies with heart rate.

U Wave: An optional wave that may follow the T wave.

TP Interval: Refers to the isoelectric segment between the end of the T wave and the onset of the following P wave.

PP Interval: Measures atrial rate and rhythm from one P wave start to the next.

RR Interval: Determines ventricular rate and rhythm, measured from one QRS complex to the next.

Calculating Heart Rate from an ECG

One method to determine heart rate from an ECG strip involves dividing 1500 by the count of small boxes between two R waves to get beats per minute. For instance, 1500/25 = 60 beats per minute.

An electrocardiogram or ECG captures and analyzes the heart's electrical activity to assess its function, rhythm, and electrical conduction system.

Each phase of the cardiac cycle is represented by specific waveforms on the display of a cardiac monitor or a strip of ECG graph paper.

Key components of an ECG waveform include the P wave, PR interval, QRS complex, ST segment, T wave, and sometimes a U wave.

The P wave usually is 2.5 mm or less in height and 0.11 seconds or less in duration.

The PR interval fluctuates from 0.12 to 0.20 seconds.

The QRS complex usually lasts less than 0.12 seconds.

The ST segment is usually isoelectric.

The T wave appears as a smooth, positive deflection. The QT interval varies with heart rate.

A one-minute rhythm strip contains 300 large and 1500 small boxes.

Count the small boxes within an RR interval and divide 1500 by that number to determine heart rate with a regular rhythm.