ChatPPG Editorial

The Cardiac Cycle in the PPG Signal: Systole, Diastole, and What Each Phase Reveals

How the cardiac cycle maps onto the PPG waveform. A complete guide to systolic upstroke, systolic peak, dicrotic notch, and diastolic runoff — and what each phase tells you about cardiac and vascular physiology.

ChatPPG Research Team

2026-03-21

8 min read

The Cardiac Cycle in the PPG Signal: Systole, Diastole, and What Each Phase Reveals

Every PPG waveform is a compressed diary of one heartbeat. The rising edge, the peak, the notch, and the falling tail each correspond to distinct mechanical events in the cardiac cycle. Understanding this mapping — precisely and quantitatively — is the foundation for extracting clinical information from PPG beyond simple heart rate.

The Cardiac Cycle: A Brief Overview

The cardiac cycle is the complete sequence of mechanical events that constitutes one heartbeat:

Diastole (heart filling):

Isovolumetric relaxation: ventricular pressure falls, valves closed
Early rapid filling: mitral/tricuspid valves open, blood rushes into ventricles
Slow filling (diastasis): gradual ventricular filling as atrial-ventricular pressure gradient equalizes
Atrial systole: atrial contraction provides final 15-25% of ventricular filling

Systole (heart ejecting):

Isovolumetric contraction: ventricular pressure rises rapidly, all valves closed
Rapid ejection: aortic valve opens, blood flows rapidly into aorta (~70% of stroke volume)
Reduced ejection: ejection slows as ventricular and aortic pressures equalize
Aortic valve closure: ventricular pressure falls below aortic pressure, valve snaps shut

At a resting heart rate of 60 bpm, the cycle lasts 1000 ms — roughly 330 ms systole, 670 ms diastole. At 120 bpm, the cycle compresses to 500 ms, with diastole disproportionately shortened to about 200 ms.

How the Cardiac Cycle Maps to the PPG Waveform

The PPG sensor doesn't measure pressure directly. It measures changes in light absorption in tissue, which correlates with blood volume changes in the microvessels. Here's the precise mapping:

Phase 1: Systolic Upstroke (0 to ~100 ms)

Cardiac event: Rapid ejection phase. Left ventricular pressure drives blood rapidly into the aorta. The pressure pulse propagates distally at pulse wave velocity (typically 6-10 m/s at the aorta, slowing in peripheral vessels).

PPG appearance: Steep rising slope from the waveform foot (onset) to the systolic peak. This is the fastest-changing portion of the waveform.

Physiological determinants:

Rise rate correlates with maximum rate of left ventricular pressure rise (dp/dt max) — a contractility index
Rise time from foot to peak correlates inversely with pulse wave velocity and arterial stiffness
Amplitude of rise reflects stroke volume and arterial compliance combined

Clinical value: Reduced rise rate can indicate reduced cardiac contractility. Shortened rise time suggests stiffer arteries (wave travels faster). The maximum slope of the upstroke is used in some algorithms as a cardiac output proxy.

Phase 2: Systolic Peak (~100 ms)

Cardiac event: Near the end of rapid ejection. Ventricular and aortic pressures are near their maxima. Ejection velocity is decreasing.

PPG appearance: The highest point of the waveform in each cycle, labeled P1 or systolic peak.

Physiological determinants: Peak amplitude (AC component) reflects stroke volume modulated by arterial compliance and peripheral impedance. The AC/DC ratio is derived from this peak.

Clinical value: Respiratory variation in peak amplitude (pulse pressure variation, PPV) predicts fluid responsiveness in ventilated patients. Cycle-to-cycle amplitude variation reflects both respiratory mechanics and cardiac preload.

Phase 3: Systolic Deceleration (~100-300 ms)

Cardiac event: Reduced ejection. Ventricular pressure is falling, ejection slowing. The pressure wave reflected from peripheral vascular beds is traveling back toward the heart.

PPG appearance: Descending slope after the systolic peak. May show an inflection or shoulder corresponding to the arriving reflected wave. This is the region where the SDPPG b, c, d waves occur.

Physiological determinants: The slope of descent and any secondary peaks depend on the timing and amplitude of the reflected wave — which reflects PVR and arterial stiffness. In young, compliant arteries, a distinct re-acceleration shoulder (wave 'c' in SDPPG) is visible here.

Clinical value: Waveform shape in this region encodes augmentation index information. The presence and timing of the secondary shoulder differentiates compliant from stiff arteries.

Phase 4: Dicrotic Notch (~300-400 ms at rest)

Cardiac event: Aortic valve closure. Ventricular pressure has fallen below aortic pressure. The valve snaps shut, creating a brief retrograde pressure transient.

PPG appearance: A small notch (deflection downward) on the descending limb, typically at 30-60% of the way from systolic peak to the next foot. Followed by the diastolic peak or shoulder.

Physiological determinants: Notch timing depends on aortic valve closure time, which relates to ejection duration and heart rate. Notch depth and position encode arterial compliance and PVR. See Dicrotic Notch Physiology for full detail.

Clinical value: One of the most information-rich features of the PPG waveform. Timing used in BP estimation; depth used in stiffness assessment; presence/absence tracks acute hemodynamic changes.

Phase 5: Diastolic Peak (~350-500 ms)

Cardiac event: Early diastole. Aortic valve closed. The reflected pressure wave (which traveled to peripheral resistance sites and returned) is arriving at the measurement site.

PPG appearance: A secondary hump or peak after the dicrotic notch. Its prominence depends on the reflection wave amplitude and timing. Clear and distinct in young adults; absent or merged with systolic peak in older adults.

Physiological determinants: Reflection index (ratio of diastolic to systolic peak) encodes peripheral vascular tone and arterial stiffness. The stiffness index uses diastolic peak timing.

Clinical value: Diastolic peak timing enables stiffness index calculation. Its relative amplitude provides the reflection index for PVR estimation. Its presence and distinctness itself is an indirect indicator of vascular age.

Phase 6: Diastolic Runoff (~500 ms to next foot)

Cardiac event: Late diastole. The aortic valve is closed. Blood continues flowing through peripheral vascular beds driven by stored elastic energy in the aorta (Windkessel effect). Heart is in its filling phase (isovolumetric relaxation, then mitral valve opening).

PPG appearance: Declining waveform from diastolic peak to the foot of the next cycle. Slope and shape reflect the rate of peripheral runoff.

Physiological determinants: Diastolic decay rate depends on PVR (faster runoff = lower PVR) and arterial compliance (more compliant = stores more energy = sustains flow longer).

Clinical value: Diastolic decay time constant used in some algorithms for PVR estimation. Steep decay suggests vasodilation; shallow decay suggests vasoconstriction or high arterial compliance storing energy.

Timing Ratios and Their Physiological Meaning

Several timing ratios derived from these phases carry clinical significance:

Systolic Time Ratio (STR) = Systolic duration / Total cycle duration: Shortens at higher heart rates. Abnormal STR can indicate reduced ejection time from cardiac pathology.

Diastolic Time Ratio (DTR) = Diastolic duration / Total cycle duration: DTR falls disproportionately with heart rate increases. This is why tachycardia is poorly tolerated in coronary disease — coronary perfusion requires adequate diastolic time.

Notch-to-Peak Ratio: Time from systolic foot to dicrotic notch, divided by time from systolic foot to systolic peak. Varies with PVR and compliance, relatively less sensitive to heart rate.

Upslope-to-Cycle Ratio: Rise time as fraction of cycle period. Sensitive to arterial stiffness.

Practical Segmentation for Algorithms

Implementing accurate cardiac cycle phase segmentation from noisy PPG signals requires careful methodology:

Foot detection: Use the first-derivative maximum approach (rising zero-crossing after the diastolic decay) rather than simple minimum detection. More robust to waveform amplitude variation.
Systolic peak: Local maximum after the foot. Constrain to physiologically plausible time range (50-400 ms after foot at typical heart rates).
Dicrotic notch: Local minimum in the descending limb after systolic peak. Multiple methods exist; second-derivative-based approaches are most robust for noisy signals.
Diastolic peak: Local maximum after the dicrotic notch, before the next systolic foot.
Quality gating: Validate each cycle against morphological constraints — amplitude range, duration, peak ordering, minimum notch depth. Reject cycles failing quality criteria before feature extraction.

The ChatPPG algorithm library provides reference implementations for all segmentation steps with configurable quality thresholds.

Frequently Asked Questions

What part of the PPG waveform corresponds to systole? The systolic upstroke (foot to peak) and the immediate post-peak descent correspond to the ventricular ejection phase. The systolic upstroke captures rapid ejection; the post-peak slope captures reduced ejection and early reflected wave arrival.

What does the diastolic phase look like in PPG? After the dicrotic notch, the PPG waveform shows a diastolic peak (the reflected wave contribution) followed by a gradual decline (diastolic runoff) until the foot of the next cycle. The diastolic portion typically occupies 60-70% of the total waveform cycle at rest.

How does heart rate affect the PPG waveform shape? Higher heart rate compresses both systolic and diastolic durations, but diastole shortens more. The dicrotic notch moves proportionally earlier. The diastolic peak may merge with the systolic peak at very high heart rates as the cycle compresses.

Why is the systolic upstroke clinically useful? The steepness and duration of the systolic upstroke encode cardiac contractility (dp/dt) and arterial stiffness. A slow, prolonged upstroke may indicate reduced contractility or severe arterial stiffening. A very rapid upstroke suggests hyperkinetic states or compliant arteries with fast wave propagation.

How do I identify the cardiac cycle foot in PPG? The foot is the waveform onset — the lowest point just before the systolic rise. Use the maximum of the first derivative (peak slope) to locate the foot robustly, as simple minimum detection is sensitive to waveform noise and shape variations.

What happens to the PPG cardiac cycle in atrial fibrillation? In AF, there is no atrial systole and cycle lengths are irregular. PPG inter-beat intervals show characteristic irregularity (non-sinus rhythm pattern), which is detectable by algorithms analyzing IBI sequences. The waveform shape within each beat may also vary due to beat-to-beat preload differences.

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