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Computational and Mathematical Methods in Medicine
Volume 2015, Article ID 576340, 6 pages
Research Article

On Better Estimating and Normalizing the Relationship between Clinical Parameters: Comparing Respiratory Modulations in the Photoplethysmogram and Blood Pressure Signal (DPOP versus PPV)

1Covidien Respiratory & Monitoring Solutions, Edinburgh EH26 0PJ, UK
2Department of Anesthesiology, The Ohio State University Wexner Medical Center, Columbus, OH 43210, USA
3Department of Neurological Surgery, The Ohio State University Wexner Medical Center, Columbus, OH 43210, USA

Received 22 September 2014; Accepted 11 December 2014

Academic Editor: Luca Faes

Copyright © 2015 Paul S. Addison et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


DPOP (ΔPOP or Delta-POP) is a noninvasive parameter which measures the strength of respiratory modulations present in the pulse oximeter waveform. It has been proposed as a noninvasive alternative to pulse pressure variation (PPV) used in the prediction of the response to volume expansion in hypovolemic patients. We considered a number of simple techniques for better determining the underlying relationship between the two parameters. It was shown numerically that baseline-induced signal errors were asymmetric in nature, which corresponded to observation, and we proposed a method which combines a least-median-of-squares estimator with the requirement that the relationship passes through the origin (the LMSO method). We further developed a method of normalization of the parameters through rescaling DPOP using the inverse gradient of the linear fitted relationship. We propose that this normalization method (LMSO-N) is applicable to the matching of a wide range of clinical parameters. It is also generally applicable to the self-normalizing of parameters whose behaviour may change slightly due to algorithmic improvements.