body {
font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
line-height: 1.6;
color: #333;
max-width: 1200px;
margin: 0 auto;
padding: 20px;
}
.calculator-container {
background: #f8f9fa;
border: 1px solid #e9ecef;
border-radius: 8px;
padding: 30px;
margin-bottom: 40px;
box-shadow: 0 4px 6px rgba(0,0,0,0.05);
}
.calculator-title {
color: #2c3e50;
margin-top: 0;
margin-bottom: 20px;
font-size: 1.5rem;
border-bottom: 2px solid #3498db;
padding-bottom: 10px;
display: inline-block;
}
.input-group {
margin-bottom: 20px;
}
.input-group label {
display: block;
margin-bottom: 8px;
font-weight: 600;
color: #2c3e50;
}
.input-group input {
width: 100%;
padding: 12px;
border: 1px solid #ced4da;
border-radius: 4px;
font-size: 16px;
box-sizing: border-box;
transition: border-color 0.2s;
}
.input-group input:focus {
border-color: #3498db;
outline: none;
box-shadow: 0 0 0 3px rgba(52, 152, 219, 0.2);
}
.calc-btn {
background-color: #3498db;
color: white;
border: none;
padding: 14px 24px;
border-radius: 4px;
font-size: 16px;
font-weight: 600;
cursor: pointer;
width: 100%;
transition: background-color 0.2s;
}
.calc-btn:hover {
background-color: #2980b9;
}
.results-area {
margin-top: 25px;
padding: 20px;
background-color: #ffffff;
border-radius: 6px;
border-left: 5px solid #3498db;
display: none;
}
.result-row {
display: flex;
justify-content: space-between;
align-items: center;
padding: 10px 0;
border-bottom: 1px solid #eee;
}
.result-row:last-child {
border-bottom: none;
}
.result-label {
color: #7f8c8d;
font-weight: 500;
}
.result-value {
font-weight: 700;
color: #2c3e50;
font-size: 1.1rem;
}
.normal-range-note {
font-size: 0.85rem;
color: #7f8c8d;
margin-top: 15px;
font-style: italic;
}
.content-section {
background: #fff;
padding: 20px 0;
}
h2 {
color: #2c3e50;
margin-top: 30px;
}
h3 {
color: #34495e;
margin-top: 25px;
}
p {
margin-bottom: 15px;
}
.info-box {
background-color: #e8f4f8;
padding: 15px;
border-radius: 4px;
border-left: 4px solid #3498db;
margin: 20px 0;
}
ul {
margin-bottom: 20px;
}
li {
margin-bottom: 8px;
}
@media (min-width: 768px) {
.calculator-container {
max-width: 600px;
margin-left: auto;
margin-right: auto;
}
}
function calculatePRContext() {
var hrInput = document.getElementById('inputHeartRate').value;
var resultDiv = document.getElementById('resultsArea');
var rrDisplay = document.getElementById('resultRR');
var maxPrDisplay = document.getElementById('resultMaxPR');
var minPrDisplay = document.getElementById('resultMinPR');
var ratioDisplay = document.getElementById('resultRatio');
// Validation
var hr = parseFloat(hrInput);
if (isNaN(hr) || hr 60.
// For HR < 60 (Bradycardia), limit often stays around 200-220ms.
var maxPR;
if (hr 220) maxPR = 220; // Cap at 220ms
} else {
maxPR = 200 – ((hr – 60) * 0.4);
// Cap minimum realistic physiological delay around 140ms for very fast rates before it merges?
// Actually, let's keep the floor reasonable.
if (maxPR < 140) maxPR = 140;
}
// Standard Lower Limit
// Usually 120ms (0.12s). In severe tachycardia, can be slightly shorter, but 120 is the standard cutoff for pre-excitation.
var minPR = 120;
// 3. Ratio of PR to RR (Duty Cycle of conduction)
// Using the Max PR for this ratio to show "at most X% of cycle"
var ratio = (maxPR / rrInterval) * 100;
// Display Results
rrDisplay.innerText = Math.round(rrInterval) + " ms";
maxPrDisplay.innerText = Math.round(maxPR) + " ms";
minPrDisplay.innerText = minPR + " ms";
ratioDisplay.innerText = "Max " + ratio.toFixed(1) + "%";
resultDiv.style.display = 'block';
}
How to Calculate PR Interval from Heart Rate
The PR interval is a critical measurement on an electrocardiogram (ECG) representing the time it takes for the electrical impulse to travel from the sinus node, through the atria, to the ventricles. While the PR interval is typically measured directly from the ECG trace, it has a physiological dependency on Heart Rate.
Key Formula:
RR Interval (ms) = 60,000 รท Heart Rate (BPM)
The Relationship Between PR Interval and Heart Rate
There is an inverse relationship between heart rate and the PR interval. Under normal physiological conditions, as the heart rate increases (tachycardia), the sympathetic nervous system facilitates faster conduction through the AV node, thereby shortening the PR interval.
Conversely, when the heart rate slows down (bradycardia), parasympathetic tone increases, slowing AV conduction and lengthening the PR interval. Understanding this relationship is vital for distinguishing between normal physiological responses and conduction blocks.
Standard Normal Ranges
In healthy adults, the normal PR interval is traditionally cited as:
- Duration: 120 ms to 200 ms (0.12s to 0.20s).
- ECG Grid: 3 to 5 small squares (at standard 25mm/s paper speed).
However, these fixed limits do not account for rate adaptation. The calculator above estimates the rate-adjusted upper limit of normal. For example, at a heart rate of 120 BPM, a PR interval of 190 ms might be technically within the "200 ms" absolute limit, but is physiologically prolonged for that specific rate.
Calculating from ECG Boxes
If you are looking to calculate the PR interval manually from an ECG strip rather than estimating it from the heart rate, use the standard time-voltage calculation:
1 Small Box = 40 ms (0.04s)
To calculate:
- Count the number of small squares between the onset of the P wave and the onset of the QRS complex.
- Multiply the count by 40 to get the duration in milliseconds.
- Compare this value to the expected range for the patient's heart rate calculated above.
Clinical Significance of Abnormal Values
Short PR Interval (< 120 ms): May indicate pre-excitation syndromes like Wolff-Parkinson-White (WPW) or a junctional rhythm where the impulse originates closer to the ventricles.
Prolonged PR Interval (> 200 ms or > adjusted max): Indicates First-Degree AV Block. While often benign, it signifies a delay in conduction between the atria and ventricles. If the PR interval varies significantly beat-to-beat, it may indicate Second or Third-Degree AV block.