ECG RATE AND RHYTHM: KEY INDICATORS FOR EMERGENCY CARDIAC CARE

The Rate of ECG Tracing

The rate of an ECG tracing can be determined by analyzing the intervals between specific waves, particularly the R waves. The standard paper speed for an ECG is 25 mm/sec, which means that:

• 1 small square (1 mm) = 0.04 seconds
• 5 small squares (5 mm) = 1 large square = 0.2 seconds
• 5 large squares = 1 second
• 1500 small squares or 300 large squares = 1 minute

How to Calculate the Rate from ECG Tracing

There are several methods to calculate the heart rate from an ECG tracing:

1. Large Square Method:
   • Identify two consecutive R waves.
   • Count the number of large squares between them.
   • Divide 300 by this number to get the heart rate in beats per minute (bpm).
   • Formula: Rate = 300 / number of large squares between consecutive R waves
2. Small Square Method:
   • Identify two consecutive R waves.
   • Count the number of small squares between them.
   • Divide 1500 by this number to get the heart rate in bpm.
   • Formula: Rate = 1500 / number of small squares between consecutive R waves
3. R Wave Method:
   • Count the number of R waves in a rhythm strip that is typically recorded over a ten-second period.
   • Multiply this number by six to get the average heart rate in bpm.
   • Formula: Rate = Number of R waves in a ten-second strip x 6

Interpreting Rule of 300

The Rule of 300 is a quick method for estimating heart rate from an ECG with regular rhythms:

• Count the number of large squares between two consecutive R waves.
• Divide this count into 300 to estimate the heart rate.

For example:

• If there are three large squares between two consecutive R waves, then: Heart Rate = 300 ÷ 3 = 100 bpm

This method is most accurate for regular rhythms and provides a rapid estimation.

Overview of Sinus Rhythm, Sinus Bradycardia, and Tachycardia

1. Sinus Rhythm:
   • Normal rhythm originating from the sinoatrial (SA) node.
   • Characterized by P waves before each QRS complex and consistent intervals between beats.
   • Heart rate typically ranges from 60 to 100 bpm.
2. Sinus Bradycardia:
   • A slower than normal sinus rhythm with a heart rate less than 60 bpm.
   • Can be normal in well-trained athletes or during sleep but may indicate underlying pathology if symptomatic.
3. Sinus Tachycardia:
   • A faster than normal sinus rhythm with a heart rate greater than 100 bpm.
   • Often occurs due to physiological responses such as exercise, stress, or fever but can also indicate medical conditions like hyperthyroidism or anemia.

Overdrive Suppression 

Overdrive suppression is a physiological mechanism where faster pacemaker cells suppress slower pacemaker cells within the heart:

• The SA node usually sets the pace because it has the highest intrinsic firing rate among cardiac pacemaker cells.
• When another part of the heart attempts to take over pacing at a slower rate, it is suppressed by continuous impulses from the SA node.
• This ensures coordinated contraction and efficient pumping action by maintaining hierarchical control over cardiac rhythms.