Learning Objective

After completing this section, learners will be able to interpret ECG tracings using standardized voltage and time scales, identify the size of small and big boxes, and estimate heart rate from a single-lead ECG.

ECG Scales and Boxes

Y-Axis (Voltage)

• One big box = 0.5 mV
• Total height example: 7 big boxes × 0.5 mV = 3.5 mV

X-Axis (Time)

• One big box = 0.2 sec (200 msec)
• Each big box has 5 small boxes → 1 small box = 0.04 sec (40 msec)
• 5 big boxes = 1 second

Visual Representation

• Heavy lines: “Big” boxes
• Light lines: “Small” boxes within big boxes
• Used to measure time intervals and amplitude on ECG

Heart Rate Estimation

• Count the number of big boxes between consecutive R waves
• Heart rate (bpm) = 300 ÷ number of big boxes
  • Example: 5 big boxes between R waves → 300 ÷ 5 = 60 bpm

Activity