Fig. 1.
Hypothetical photoplethysmographic traces demonstrating two types of modulation. (A) An example of amplitude modulation that might occur with stroke-volume variation. (B) Baseline modulation that would occur with the movement of venous blood with ventilation. Δvariation of the plethysmographic waveform of the pulse oximeter as described would measure type A modulations but completely miss type B. The spectral analysis can measure both type A and type B modulations.6 (C) Demonstrates the effect of combining the two types of modulation together (A and B), which occurs with patients who are significantly hypovolemic (fig. 2). PPG = photoplethysmographic.