HOW IT ALL WORKS
BEHIND THE DPA'S TECH
A photodiode detects changes in the amount of light absorbed by hemoglobin and its output waveform is termed photoplethysmography or PPG (sometimes also referred to as PTG).
PPG has been validated for calculating systemic arterial compliance (flexibility). The application of this technique in population studies confirms that early detection of vascular disease is possible.
The technique is underpublicized and many physicians are not aware of the great clinical impact of this technology.
Blood has a light absorption coefficient that is higher than surrounding tissue. This is a consequence of the Labert-Beer law relating light absorption to optical density. Therefore increases in the amount of blood give rise to decreased detected light. Erythrocytes and vessel walls also reflect light.
However, reflection heavily dominates, and arterial pulses produce merely small reductions in detected light (1% - 2%). Detected light variation is amplified and converted to a voltage signal.
There are many factors involved in the attenuation of light, including absorption, multiple scattering and reflection, but all technical issues have been resolved: small changes in a patient's profile are easily detectable!
Extracted from: Townsend Letter: Breakthrough in Clinical Cardiology: In-Office Assessment with Pulse Wave Velocity and Digital Pulse Analysis (DPA) by Brian Scott Peskin, BSEE, with Robert Jay Rowen,
MD - May 2010
ANALYSIS BY PHOTO-PLETHYSMOGRAPHY (PPG / PTG)
The PhotoPlethysmoGraph or PPG (also referred to as PTG) is a non-invasive analysis method that has proven to be extremely efficient. It relies on the measurement of volume and pressure of peripheral blood vessels, in this case, of the finger.
By using an LED emitter that reflects into a sensor, the DPA translates the measurement to electric signals (unit mV/V) represented in graphs to help detect a number of different irregularities.
- S (Starting Point)
Starting point of the Systolic phase of arterial pulse-wave. The aortic valve opens and the blood of the LV is ejected into the aorta.
- P (Percussion Wave)
Wave caused from LV ejection that increases the blood volume within artery.
- T (Tidal Wave)
Reflected wave from the small artery. Higher point means contraction and stiffness of small artery.
- C (Incisura Wave)
End-point of systolic phase, then aortic valve closed. Less drop from pulse height (PH) means larger resistance (arterial contradiction & tension).
- D (Dicrotic Wave)
Its location corresponds to the closure of the aortic valve and subsequent retrograde blood flow, and can be used to monitor cardiac function.
The PPG analysis reports include the following measurements:
- Circulation analysis
- Pulse recognition
- Ejection time
- Pulse rate
- PH-pulse rate
- APG Type–biological age of arteries
One of the many irregularities that the PPG analysis can detect is “arterial wall stiffness”. Although there are other methods that can detect the disorder, such procedures are cumbersome since a specialized operator is needed. The DPA allows the practitioner to perform such a test without the need of an operator and takes less than 1 minute to test, making the device a great tool for your clinical practice.
THE ELEMENTS OF APG
With the PPG wave as a basis, its second derivative, termed APG wave (Acceleration Plethysmogram) depends on slope changes at each point of the PPG. APG provides an excellent method to evaluate Arterial Aging and the "Biological Age" of the patient's cardiovascular system.
- b/a: LV ejection an compliance of the large artery.
Shorter negative peak with aging.
- c/a: Clear differentiation of P wave from PPG.
Slopes downward with aging.
- d/a: Contraction and tension of small artery.
Longer negative peak with aging.
- e/a: Decreases with aging.
AI (Aging Index) = (b/a) - (c/a) - (d/a) - (e/a)
Increases with age.