Overview
This lecture explains the five-step method for interpreting ECG/EKG rhythm strips, covering ECG basics, normal sinus rhythm characteristics, and manual heart rate calculation techniques.
ECG Paper & Measurement Basics
- ECG paper has small boxes (0.04 seconds) and large boxes (5 small boxes = 0.2 seconds).
- Time is measured horizontally using these boxes to analyze ECG intervals.
- A standard six-second strip has 30 large boxes, often marked by dashed lines at the top.
Cardiac Conduction System & Normal ECG Components
- Electrical conduction passes through SA node, Bachmann’s bundle, AV node, Bundle of His, bundle branches, and Purkinje fibers.
- The isoelectric line represents baseline electrical activity (neutral charge).
- P wave: atrial depolarization (SA node activation).
- QRS complex: ventricular depolarization (bundle branches and Purkinje fibers).
- T wave: ventricular repolarization.
- Atrial repolarization is hidden within the QRS complex.
Key ECG Intervals
- PR interval: start of P wave to start of QRS complex; normal range 0.12–0.20 seconds.
- QRS duration: start of Q wave to end of S wave; normal <0.12 seconds.
- ST segment: from end of QRS to start of T wave, may indicate ischemia or infarction if abnormal.
The Five-Step ECG Interpretation Method
- Step 1: Identify and assess P wave and QRS complex; check for upright P wave and 1:1 P:QRS ratio.
- Step 2: Measure PR interval (0.12–0.20 s) and QRS duration (<0.12 s) using box counts.
- Step 3: Assess rhythm regularity by comparing R-R intervals across the strip.
- Step 4: Determine heart rate using:
- Six-second method (QRS count x 10, works for regular/irregular rhythms).
- Big box method (300 / large boxes between R waves, regular only).
- Small box method (1500 / small boxes between R waves, best for fast/regular rhythms).
- Sequence method (count large boxes using sequence 300-150-100-75-60-50, regular only).
- Step 5: Interpret the rhythm using the above findings; normal sinus rhythm has all normal findings and HR 60–100 bpm.
Key Terms & Definitions
- Isoelectric Line — Baseline of the ECG, representing no net electrical charge.
- P Wave — Electrical activity from atrial depolarization.
- QRS Complex — Electrical activity from ventricular depolarization.
- T Wave — Ventricular repolarization phase.
- PR Interval — Time from atrial depolarization to ventricular depolarization (0.12–0.20 s normal).
- QRS Duration — Time for ventricular depolarization (<0.12 s normal).
- Regular Rhythm — Equal spacing between R waves.
- Six-Second Method — Multiply QRS count in 6 seconds by 10 for HR.
Action Items / Next Steps
- Memorize normal PR interval (0.12–0.20 s) and QRS duration (<0.12 s) values.
- Practice measuring intervals and calculating heart rate on sample ECG strips.
- Watch or review upcoming lesson covering abnormal ECG rhythms/arrhythmias.