The Dicrotic Notch in PPG: Physiology, Clinical Significance, and What Changes It

The dicrotic notch is the small downward deflection visible in the descending limb of a PPG waveform, appearing between the systolic peak and the diastolic peak. It's one of the most diagnostically rich features of the PPG signal — and also one of the most frequently misunderstood. Its presence, absence, timing, and depth all carry physiological information about aortic valve function, arterial compliance, and peripheral vascular resistance.

What Causes the Dicrotic Notch?

To understand the dicrotic notch, follow the mechanics of a single cardiac cycle:

1. Systole: The left ventricle contracts, forcing blood through the open aortic valve. Pressure rises rapidly in the aorta. The PPG sensor records the rising edge of the waveform — the systolic upstroke.

2. Systolic peak: Ventricular pressure peaks and begins to fall. Blood continues flowing forward but decelerating. The PPG waveform reaches its systolic peak.

3. Aortic valve closure: When ventricular pressure falls below aortic pressure, the aortic valve closes. This closure creates a brief backward pressure transient — a small reflected wave from the valve snapping shut. In the central aorta, this appears as the incisura: a sharp V-shaped notch.

4. Dicrotic notch in peripheral PPG: By the time this transient reaches a peripheral site (wrist, finger), the sharp V is smoothed into a gentler inflection — the dicrotic notch. It appears as a second, smaller hump on the downslope of the waveform.

5. Diastolic runoff: After the notch, pressure (and PPG) continues declining as blood runs off into the capillary bed during diastole.

The key point: the dicrotic notch in peripheral PPG represents the combined effect of aortic valve closure and the arrival of the reflected pressure wave from the peripheral vasculature. These two contributions overlap, which is why the notch's timing and shape encode information about both.

Where in the Waveform Is It?

On a typical PPG waveform from a finger or wrist:

• The systolic foot (onset) occurs at the start of each cardiac cycle
• The systolic peak (primary peak) is the tallest feature, occurring in early-to-mid systole
• The dicrotic notch appears on the descending limb, typically at 30-50% of the way down from systolic peak to the next foot
• The diastolic peak (secondary shoulder or hump) follows the notch in young, compliant arteries

In healthy young adults, the diastolic peak is clearly visible. With aging and arterial stiffening, the reflection wave arrives earlier and merges with the systolic peak — the dicrotic notch and diastolic peak become less distinct until they effectively disappear.

Clinical Significance of the Dicrotic Notch

Aortic Valve Function

An absent or severely blunted dicrotic notch in central waveforms can suggest aortic regurgitation. In that condition, the aortic valve doesn't close properly — eliminating or dampening the closure transient that creates the notch. Peripheral PPG is less specific for this than tonometric central pressure waveforms, but severe cases may be detectable.

Arterial Compliance

The depth and prominence of the dicrotic notch correlates inversely with arterial stiffness:

• Young, compliant arteries: Clear, prominent notch with a distinct diastolic hump
• Aging or stiffened arteries: Blunted or absent notch; reflection wave merges with systolic peak
• Very high stiffness: A single broad systolic peak with no discernible notch

This progression is why the notch depth is incorporated into composite arterial stiffness indices derived from PPG waveforms.

Peripheral Vascular Resistance

Peripheral vascular resistance (PVR) affects notch timing relative to the cardiac cycle. Higher PVR (as in hypertension, cold conditions, or vasoconstriction) tends to:

• Move the notch earlier in the cardiac cycle (shorter time from systolic foot to notch)
• Reduce notch depth
• Increase the relative height of the systolic peak versus diastolic

Lower PVR (vasodilation, sepsis, fever) tends to:

• Move the notch later or make it more prominent
• Increase diastolic runoff rate (steeper post-notch slope)

In septic shock, peripheral vasodilation can dramatically change PPG waveform morphology — the dicrotic notch may shift position substantially, a feature some algorithms use for hemodynamic state estimation.

Heart Rate Effects

At higher heart rates, diastole shortens faster than systole. The notch occurs at roughly the same absolute time after systolic onset, but represents a larger fraction of the now-compressed cardiac period. This shifts notch timing ratios even without any change in vascular properties — important to account for when comparing notch position across heart rate conditions.

The Dicrotic Notch as a Feature for Algorithms

Several computed features use the dicrotic notch position:

Dicrotic Time (DT): Time from systolic foot to dicrotic notch. Typically 250-400 ms at rest. Shorter DT with higher heart rate; affected by PVR and arterial compliance.

Diastolic Time (DiasT): Time from dicrotic notch to next systolic foot. Represents diastole duration. Shortens more than systole at higher heart rates.

Notch Position Ratio: DT / Total pulse period. Normalized for heart rate. Increases with vasodilation, decreases with vasoconstriction.

Notch Depth: Amplitude of the notch relative to systolic peak and subsequent diastolic hump. Decreases with arterial stiffening.

Reflection Index (RI): Height of the diastolic peak relative to systolic peak. Depends on notch depth and diastolic hump height.

A 2015 study by Elgendi et al. in PLOS ONE (doi: 10.1371/journal.pone.0121622) systematically characterized how these features change with age, finding significant progressive changes in notch depth and diastolic peak prominence across decades, consistent with known vascular aging patterns.

Why the Dicrotic Notch Disappears

The notch is most commonly absent or indistinct in:

Older adults (>60): Arterial stiffness advances to the point where the reflection wave arrives so early it merges with the systolic peak. The descending limb of the waveform becomes a smooth, single-humped shape.

Hypertension: Chronically elevated pressure accelerates arterial stiffening and raises pulse wave velocity, bringing the reflection wave forward in time.

Vasoconstrictive states: Severe vasoconstriction (cold, sympathetic activation, vasopressor medications) can reduce peripheral compliance enough to blunt the notch.

Artifact and signal quality issues: Poor sensor contact, motion, or insufficient optical coupling can create PPG waveforms where the notch is lost in noise — a potential source of false clinical interpretation. Signal quality assessment should precede waveform feature extraction.

Reduced cardiac output states: In heart failure or severe hypovolemia, reduced stroke volume produces lower-amplitude waveforms where the notch may fall below noise threshold.

Detecting the Dicrotic Notch Algorithmically

Finding the notch reliably in noisy clinical or wearable PPG is non-trivial. Common approaches:

Second derivative (SDPPG / APG): The second derivative of the PPG waveform produces characteristic peaks and troughs that correspond to waveform inflection points. The 'c' and 'd' waves of the APG align with the systolic deceleration region and dicrotic notch area. See the PPG second derivative SDPPG article for details.

Threshold methods: Detect the notch as a local minimum between systolic and diastolic peaks, with amplitude constraints to exclude noise.

Template matching: Correlate each PPG cycle against a template that encodes expected notch position. Robust to noise but can fail when actual waveform shape deviates from the template (e.g., in arrhythmias).

First derivative zero-crossing: The notch corresponds to a local extremum where the first derivative crosses zero. The zero between the systolic peak slope and diastolic runoff slope is the notch.

The ChatPPG algorithm library includes dicrotic notch detection implementations with configurable sensitivity and quality thresholds for both clinical and wearable signal conditions.

Frequently Asked Questions

What does the dicrotic notch represent in PPG? The dicrotic notch represents the combined effect of aortic valve closure (which creates a brief backward pressure transient) and the arrival of the reflected pressure wave from peripheral arteries. It appears as a small deflection on the descending limb of each PPG pulse cycle.

Why does the dicrotic notch disappear with age? With aging, arterial walls stiffen progressively. This increases pulse wave velocity, so the peripheral reflection wave returns to the measurement site earlier in the cardiac cycle — merging with the systolic peak rather than appearing as a separate notch. The result is a smoother, single-humped waveform typical of older adults.

Is the dicrotic notch the same as the incisura? No, though they're related. The incisura is the sharp notch seen in the central aortic pressure waveform at aortic valve closure. The dicrotic notch is the smoothed version of this feature as it appears in peripheral PPG recordings after the wave has traveled to the measurement site.

What does a prominent dicrotic notch indicate? A well-defined dicrotic notch suggests compliant, healthy arteries and adequate cardiac function. It's generally a favorable sign, particularly in older adults where blunting is expected. A very deep or late notch may indicate low peripheral resistance (vasodilation).

How is the dicrotic notch used in clinical algorithms? Its timing relative to the cardiac cycle estimates peripheral vascular resistance and arterial compliance. Its depth correlates with arterial stiffness indices. Ensemble features including notch position, depth, and diastolic-to-systolic timing ratios are used in algorithms for blood pressure estimation and hemodynamic monitoring.

Can you detect the dicrotic notch with a wrist PPG sensor? Yes, though it's more challenging at the wrist than at the finger due to a generally lower signal-to-noise ratio and tissue differences. Algorithm sensitivity needs adjustment, and signal quality assessment should precede notch detection.

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