ECG Interpretation and Calculation: A Clinical Scenario

An electrocardiogram (ECG) is a diagnostic tool used to assess the electrical activity of the heart. It helps healthcare professionals identify abnormalities in heart rate, rhythm, and conduction. Understanding how to calculate ECG measurements is essential for accurate patient assessment and timely intervention.

Clinical Scenario

A 65-year-old man presents to the emergency department complaining of dizziness and palpitations. A 12-lead ECG is performed to assess his cardiac rhythm. On examination of the ECG strip, the distance between two consecutive R waves is found to be 4 large squares.

Step 1: Calculate the Heart Rate

The patient's rhythm appears regular, so the 300 Rule can be used:

Heart Rate=300Number of Large Squares Between R Waves\text{Heart Rate} = \frac{300}{\text{Number of Large Squares Between R Waves}} Heart Rate=3004\text{Heart Rate} = \frac{300}{4} Heart Rate=75 beats per minute (bpm)\text{Heart Rate} = 75 , \text{beats per minute (bpm)}

Therefore, the patient's heart rate is 75 bpm, which falls within the normal adult range of 60 to 100 bpm.

Step 2: Assess the PR Interval

The ECG shows a PR interval measuring 4 small squares.

Since each small square represents 0.04 seconds:

4×0.04=0.16 seconds4 \times 0.04 = 0.16 , \text{seconds}

The normal PR interval ranges from 0.12 to 0.20 seconds, indicating that atrioventricular conduction is normal.

Step 3: Assess the QRS Duration

The QRS complex measures 2 small squares.

2×0.04=0.08 seconds2 \times 0.04 = 0.08 , \text{seconds}

A normal QRS duration is less than 0.12 seconds, suggesting normal ventricular conduction.

Step 4: Assess the QT Interval

The QT interval measures 10 small squares.

10×0.04=0.40 seconds10 \times 0.04 = 0.40 , \text{seconds}

The RR interval is 0.80 seconds (4 large squares × 0.20 seconds).

Using Bazett's Formula:

QTc=QTRRQTc = \frac{QT}{\sqrt{RR}} QTc=0.400.80QTc = \frac{0.40}{\sqrt{0.80}} QTc=0.447 seconds=447 msQTc = 0.447 , \text{seconds} = 447 , \text{ms}

A QTc of 447 ms is within normal limits for most adults.

Conclusion

In this scenario, the ECG reveals a regular sinus rhythm with a heart rate of 75 bpm, a normal PR interval of 0.16 seconds, a QRS duration of 0.08 seconds, and a QTc of 447 ms. These findings indicate normal cardiac conduction and ventricular repolarisation. Systematic ECG calculation and interpretation enable healthcare professionals to identify abnormalities quickly and make informed clinical decisions regarding patient care.