Overview
Pleth shows the pulse-oximetry-style waveform, saturation and respiratory context. Its contour/amplitude responds to modeled perfusion and the current cardiovascular state, rather than simply printing a lower saturation number.
High filling pressures can worsen the modeled oxygenation/congestion state. Reduced pulse flow or continuous-flow support can reduce pleth pulsatility. The trace is not a direct measurement of wedge pressure.
A small pleth does not by itself identify hypoxemia, and improved displayed SpO₂ does not prove that cardiac output or oxygen delivery is normal. Read the pressures, flow and support context together.