Photoplethysmography

For physicians

What is measured

Digital photoplethysmographic waveform acquisition at finger and toe, at rest and during controlled respiratory maneuvers. The signal supports contour analysis, amplitude fluctuation analysis, derived stiffness indices, and — as a time series of interbeat intervals — the input to heart rate variability analysis.

Why the waveform, rather than the peak

Systolic pressure is a single scalar taken at one point in the arterial tree. The waveform carries the interaction between forward and reflected pressure waves, and that interaction is where vessel compliance shows up. Fluctuations in photoplethysmographic amplitude have been described as carrying prognostic information relating to coronary artery disease, and the acquisition is non-invasive and radiation-free.

Practical notes

• Peripheral vasoconstriction destroys the signal. A cold room or cold extremities is the most common cause of an uninterpretable trace.
• Nicotine and caffeine both alter vascular tone and must be excluded beforehand.
• Nail polish, artificial nails and peripheral edema all degrade the digital signal.
• Motion artifact is the second most common cause of a discarded recording; the rest period is not optional.

Where it fits

Acquired within the same study as the ankle-brachial index and pulse volume recording, and feeding the autonomic assessment. Interpreting the report.

Evidence and limitations

Written for a reader who wants the literature rather than the summary. Where the evidence is thin we say so and give both sides, and where a claim is mechanistic rather than demonstrated we label it as such.

Mechanism

Light directed into tissue is absorbed partly by hemoglobin, so the amount returning to a detector varies with the blood volume present. Each cardiac cycle produces a pulsatile component superimposed on a slowly varying baseline. The contour of that pulsatile waveform — upstroke, amplitude, and the dicrotic feature on the downslope — reflects the interaction of the forward pressure wave with waves reflected from the periphery, which is where vessel compliance becomes visible. The same interbeat interval series also supplies the input for heart rate variability analysis.

What the evidence shows

The waveform is a route to two separate quantities that do have outcome evidence behind them. Arterial stiffness measured as aortic pulse wave velocity predicts events: pooled relative risks of 2.26 for total cardiovascular events, 2.02 for cardiovascular mortality and 1.90 for all-cause mortality across 17 longitudinal studies and 15,877 subjects followed a mean of 7.7 years, with a 1 m/s increase corresponding to adjusted risk increases of 14–15%.

Autonomic function derived from the interval series is likewise prognostic within the graded framework used to define cardiac autonomic neuropathy, where the pooled relative risk of cardiovascular events is 3.16 (95% CI 2.42 to 4.13) and of all-cause mortality 3.17 (95% CI 2.11 to 4.78).

Limitations of that evidence

• Optical waveform analysis is not the method those outcome studies used. The stiffness meta-analysis is of carotid-femoral pulse wave velocity. Digital photoplethysmographic indices correlate with it to varying degrees and are not interchangeable, and quoting one study’s hazard ratio for the other method would be wrong.
• Signal quality is fragile. Peripheral vasoconstriction from a cold room, nicotine or caffeine, plus nail polish, artificial nails, peripheral edema and motion artifact, are the common causes of an uninterpretable trace.
• Contour indices are less standardized between devices than pressure-based stiffness measures, which limits comparison across published cut points.
• It is not an electrocardiogram and carries no information about rhythm origin or conduction.

What remains uncertain

How much independent prognostic information a digital pulse contour adds over blood pressure, age and conventional risk factors is not settled, and this site does not present optical waveform indices as validated risk scores. They are used for what they reliably provide: a non-invasive, radiation-free acquisition that supplies the interval series for autonomic analysis, a peripheral counterpart to the cuff measurements taken in the same sitting, and a repeatable within-person comparison.