Cardiac cycle timing and heart-rate dependence
A cardiac cycle timing calculator helps estimate how one heartbeat is divided into systole and diastole and how those durations change as heart rate changes. The main computed quantities are total cycle duration, estimated systolic duration, estimated diastolic duration, and the fraction of the cycle spent in each phase.
This topic is important because the cardiac cycle does not shorten evenly as heart rate rises. Total cycle duration becomes shorter at higher heart rate, but diastole usually shortens more than systole in a teaching model. That means ventricular filling time becomes progressively more limited during tachycardia or exercise.
Core definitions and formulas
The total duration of one cardiac cycle is determined from heart rate:
\[
\begin{aligned}
T_{\text{cycle}} &= \frac{60000}{HR}
\end{aligned}
\]
Here, \(T_{\text{cycle}}\) is the duration of one beat in milliseconds and \(HR\) is heart rate in beats per minute. Once total cycle time is known, the cycle is partitioned into systole and diastole:
\[
\begin{aligned}
T_{\text{cycle}} &= T_{\text{systole}} + T_{\text{diastole}}
\end{aligned}
\]
\[
\begin{aligned}
T_{\text{diastole}} &= T_{\text{cycle}} - T_{\text{systole}}
\end{aligned}
\]
The fractions of the cycle spent in systole and diastole are then:
\[
\begin{aligned}
f_{\text{systole}} &= \frac{T_{\text{systole}}}{T_{\text{cycle}}}
\end{aligned}
\]
\[
\begin{aligned}
f_{\text{diastole}} &= \frac{T_{\text{diastole}}}{T_{\text{cycle}}}
\end{aligned}
\]
In a simplified teaching model, systole changes only modestly with increasing heart rate, while diastole shortens more strongly. Because of that, high heart rate reduces filling time much more than it reduces ejection time.
How to interpret systole and diastole
Systole is the part of the cardiac cycle associated mainly with ventricular contraction and ejection. Diastole is the part associated mainly with ventricular relaxation and filling. At resting heart rates, diastole usually occupies a larger portion of the cycle than systole.
As heart rate rises, total cycle duration falls. Since systole does not decrease as dramatically, diastole loses a larger absolute and relative share of the cycle. This helps explain why rapid heart rates can reduce ventricular filling and why cardiac output does not depend only on heart rate but also on filling time and stroke volume.
Valve-event and ECG timing markers
Valve event markers can be added to show approximate timing of atrioventricular valve closure, semilunar valve opening, semilunar valve closure, and atrioventricular valve reopening. These markers help connect mechanical events to cycle phases.
An optional ECG and simplified Wiggers-style timing view can also be used to relate electrical and pressure events to the cycle. In a teaching sense, ventricular systole begins shortly after the QRS complex, ventricular repolarization occurs around the T wave, and filling resumes after semilunar valve closure and atrioventricular valve reopening.
Common pitfalls
- Assuming systole and diastole shorten equally as heart rate increases.
- Forgetting that reduced diastolic time means reduced filling time.
- Treating a simplified timing model as a full patient-specific hemodynamic model.
- Interpreting cycle timing without relating it to valve events or ventricular filling.
Micro example: if heart rate is \(75\) bpm, then the total cycle duration is
\[
\begin{aligned}
T_{\text{cycle}} &= \frac{60000}{75} = 800\ \text{ms}
\end{aligned}
\]
If a teaching model estimates systole as \(320\) ms, then diastole is
\[
\begin{aligned}
T_{\text{diastole}} &= 800 - 320 = 480\ \text{ms}
\end{aligned}
\]
At a much higher heart rate, such as \(150\) bpm, total cycle duration becomes only \(400\) ms. Even if systole falls only modestly, diastole becomes much shorter, so filling time is markedly reduced.
This tool is useful for cardiac physiology, heart-rate timing analysis, and understanding why high heart rate limits filling. It is not a full clinical Wiggers diagram or patient-specific model; the next step for deeper analysis is often ECG interval tools, pressure-volume relationships, or stroke volume and cardiac output analysis.