Fig. 1. Scatter plot of respiratory variations in the photoplethysmographic waveform amplitude (ΔPOP) and in the invasive arterial pulse pressure (ΔPP) in one intensive care unit patient (Patient 3 in the original article) before and after high-pass filtering.
A Butterworth high-pass filter (Labview, National Instruments Corp., Austin, TX) with a threshold of 0.15 Hz was used on signals from one patient who demonstrated large, slow oscillations. The average values and the variability of ΔPOP and ΔPP were only modestly reduced. ΔPOP = 19.3 (3.2) before and 17.2 (2.6) after filtration. ΔPP = 6.5 (0.8) before and 5.8 (0.7) after filtration. Data are given in mean (± SD) .