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. 2021 Oct 1;3(10):e0539. doi: 10.1097/CCE.0000000000000539

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Copyright © 2021 The Authors. Published by Wolters Kluwer Health, Inc. on behalf of the Society of Critical Care Medicine.

This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-No Derivatives License 4.0 (CCBY-NC-ND), where it is permissible to download and share the work provided it is properly cited. The work cannot be changed in any way or used commercially without permission from the journal.

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Figure 3. — Representative examples of fundamental frequency of the respiratory rate (f_R0) amplitude changes in the frequency domain recorded in coronavirus disease 2019–positive (COVID-19+) subjects. The fast Fourier transform (FFT) of venous waveform recorded using non-invasive venous waveform analysis in COVID-19+ subjects admitted requiring oxygen support therapy (A) and at time of discharge when oxygen support was no longer required (B). A representative FFT of a COVID-19+ subject who did not require any oxygen support therapy (C) and, for comparison, the FFT of a coronavirus disease 2019–negative (COVID-19–) healthy control subject (D). f₀ = fundamental frequency of the pulse rate.