In photoplethysmography tracing, when is the toe systolic pressure determined?

• A. First appearance of pulsatile signal
• B. When pulsatile signal reaches maximum
• C. First appearance of pulsatile signal followed by incremental pulsations
• D. At the trough of the waveform

Answer: (C)

In photoplethysmography toe pressures, you read the systolic value at the moment blood flow resumes enough to produce a pulsatile trace, and you confirm it by the signal continuing to increase in amplitude as you deflate further. The first appearance of pulsations shows the opening of arterial flow, and the subsequent incremental pulsations verify a true arterial signal rather than noise. The trough reflects diastole and the maximum amplitude isn’t used to define systole, so the combination of initial pulsatility with its progressive rise is the correct criterion.