• Main Ventricular Vector
• ECG Leads
• 12-Lead ECG Paper
• Square and Small Square
• Interval and Frequency
• Methods for Calculating Frequency on ECG
• Quick Method (300-150-100-75-60-50)
• Method 300
• Method 1500
• Calculating Heart Rate in Practice
• ECG Ruler
• 6-Second Method
• 3-Second Method

Main Ventricular Vector

• ECG leads record the electrical activity of the heart
• at different times during depolarization and repolarization
• The greater the myocardium depolarizes (or repolarizes)
• the greater the electrical activity (and the larger the vector)
• Ventricular depolarization occurs progressively:
• First, the thin ventricular septum is activated
• Generating a small septal vector (V_S)
• Then, the massive left ventricle is activated
• Generating a large main vector (V_H)
• Finally, the base of the left ventricle is activated
• Generating a small terminal vector (V_T)
  
  
• Heart Rate
• is the number of ventricular systoles per minute
• Main vector (V_H) produces the R wave on the ECG
• which is the ECG equivalent of ventricular systole

12-Lead ECG Paper

• The following ECG shows a sinus rhythm
• Heart vectors in the atria create the P wave
• Heart vectors in the ventricles create the QRS complex and the T wave
• The main heart vector creates the largest deflection (R wave, S wave)

Mechanism of 12-Lead ECG Recording

• ECG paper comes from the ECG machine progressively
• The ECG machine records the electrical activity of the heart gradually
• and writes it into individual leads
• Simultaneously, leads are recorded that are vertical on the ECG paper
• First, simultaneously recorded are leads (I, II, III, aVR, aVL, aVF, II)
• Then, simultaneously recorded are leads (V1-V6)
• The QRS complex (ventricular depolarization) or P waves (atrial depolarization) can be connected by a vertical line
• Because the same vectors are captured simultaneously by the ECG leads, which are stored vertically on the ECG paper
• Continuous Lead II Rhythm Strip
• Is located at the bottom of the ECG paper
• Records continuously from the start of the paper to the end
• Used for better diagnosis of arrhythmias
• The paper moves at a speed of 25 mm/s
• Sometimes 50 mm/s

Interval and Frequency

• 1 minute = 60 seconds = 60,000 ms
  
  
• Intervals are measured in ms (milliseconds)
• RR interval is the number of ms between RR waves
• Frequency is the number of systoles per minute
• Systole is equivalent to the QRS complex
  
  
• Frequency is calculated in 3 ways:
• 60,000/RR interval (ms)
• 300/number of squares in the RR interval
• 1,500/number of small squares in the RR interval

Quick Method (300-150-100-75-60-50)

• Frequency: (300-150-100-75-60-50-42-37-33-30...) beats per minute (bpm).
• Derived from the formula 300 divided by the number of large squares
• Frequency decreases as the number of large squares between RR intervals (SS intervals) increases

Method 1500

• 1500 divided by the number of small squares between RR intervals (SS intervals)

Heart Rate 150/min.

• 300-150-100-75-60-50
• 300/2 squares = 150/min.
• 1500/10 small squares = 150/min.
• 60000/400ms = 150/min.
• This is monomorphic ventricular tachycardia

Heart Rate 75/min.

• 300-150-100-75-60-50
• 300/4 squares = 75/min.
• 1500/20 small squares = 75/min.
• 60000/800ms = 75/min.
• This is sinus rhythm

Heart Rate 50/min.

• 300-150-100-75-60-50
• 300/6 squares = 50/min.
• 1500/30 small squares = 50/min.
• 60000/1200ms = 50/min.
• This is sinus bradycardia (borderline)

6-Second Method

• If the heart rhythm is irregular (e.g., atrial fibrillation)
• Then the above methods cannot be used to calculate the frequency
• Use the 6-second method
• This calculates the average frequency of QRS complexes over 6 seconds (30 squares)
• Frequency = number of QRS complexes in 6s x 10

Frequency 130/min.

• Number of QRS complexes in 6s (30 squares) is 13
• 13 x 10 = 130/min.
• This is indicative of atrial fibrillation