function updateEcgLabel() {
var method = document.getElementById('ecgMethod').value;
var label = document.getElementById('inputLabel');
var input = document.getElementById('rrInterval');
if (method === '1500') {
label.innerText = "Number of Small Squares between R-R";
input.placeholder = "e.g., 20";
} else {
label.innerText = "Number of Large Squares between R-R";
input.placeholder = "e.g., 4";
}
}
function calculateHeartRate() {
var method = document.getElementById('ecgMethod').value;
var inputVal = document.getElementById('rrInterval').value;
var resultDiv = document.getElementById('ecgResult');
// Validation
if (inputVal === "" || isNaN(inputVal) || inputVal <= 0) {
resultDiv.style.display = "block";
resultDiv.innerHTML = "Please enter a valid positive number for the squares.";
return;
}
var squares = parseFloat(inputVal);
var bpm = 0;
// Logic Implementation
if (method === '1500') {
// Formula: Rate = 1500 / Small Squares
bpm = 1500 / squares;
} else {
// Formula: Rate = 300 / Large Squares
bpm = 300 / squares;
}
// Rounding
var roundedBpm = Math.round(bpm);
// Interpretation Logic
var status = "";
var badgeClass = "";
var explanation = "";
if (roundedBpm < 60) {
status = "Sinus Bradycardia";
badgeClass = "status-brady";
explanation = "Heart rate is slower than normal (= 60 && roundedBpm 100 BPM).";
}
// Specific warning for impossible values
if (roundedBpm > 350 || roundedBpm < 10) {
explanation = "Warning: This result is physiologically unlikely. Please check your count of squares.";
badgeClass = "status-tachy"; // Alert color
status = "Review Input";
}
// Construct Result HTML
var htmlOutput = '' + roundedBpm + '';
htmlOutput += 'Beats Per Minute (BPM)';
htmlOutput += '' + status + '';
htmlOutput += " + explanation + ";
resultDiv.style.display = "block";
resultDiv.innerHTML = htmlOutput;
}
How Do You Calculate Rate on an ECG?
Interpreting an Electrocardiogram (ECG or EKG) often begins with determining the heart rate. While modern machines provide automated readings, understanding how to calculate the rate manually is a fundamental skill for medical professionals, students, and technicians. This ensures accuracy when automated artifacts occur or when quickly glancing at a rhythm strip.
The standard ECG paper speed is 25 mm/second. The paper is printed with a grid of small and large squares:
Small Square: 1mm wide, representing 0.04 seconds.
Large Square: 5mm wide (5 small squares), representing 0.20 seconds.
Clinical Note: Manual calculation methods assume a regular rhythm (the distance between R waves is constant). If the rhythm is irregular (like Atrial Fibrillation), the "6-Second Method" is required instead.
Method 1: The 1500 Method (Most Accurate)
The 1500 method is considered the most precise way to calculate heart rate for regular rhythms, specifically for fast heart rates.
The Formula:Heart Rate = 1500 / Number of Small Squares
How to use it:
Identify two consecutive R waves (the tall peaks in the QRS complex).
Count the number of small squares between these two peaks.
Divide 1500 by this number.
Example: If there are 20 small squares between R-R intervals, the calculation is 1500 ÷ 20 = 75 BPM.
Method 2: The 300 Method (Quick Estimation)
The 300 method is a faster sequence used for quick estimation without needing to count every tiny millimeter line.
The Formula:Heart Rate = 300 / Number of Large Squares
How to use it:
Find an R wave that falls on a thick line (start of a large square).
Count the number of large squares until the next R wave.
Divide 300 by this number.
Example: If there are 4 large squares between R waves, the calculation is 300 ÷ 4 = 75 BPM.
Medical professionals often memorize the sequence based on large squares: 300, 150, 100, 75, 60, 50. If the next R wave lands on the first heavy line, the rate is 300. If it lands on the second, it is 150, and so on.
Interpreting the Results
Once you have calculated the rate, you can classify the ventricular rate:
Normal Sinus Rhythm: 60 – 100 BPM
Sinus Bradycardia: Less than 60 BPM
Sinus Tachycardia: Greater than 100 BPM
Why 1500 and 300?
These constants are derived from the paper speed. At 25mm/second:
There are 60 seconds in a minute.
25 mm/sec × 60 sec/min = 1500 mm/minute (1500 small squares per minute).
Since there are 5 small squares in a large square: 1500 ÷ 5 = 300 large squares per minute.
Therefore, dividing the total number of squares available in one minute by the number of squares in one beat cycle gives you the Beats Per Minute (BPM).