Unexpected sawtooth artifact in beat-to-beat pulse transit time measured from patient monitor data

TL;DR

This study reveals a consistent sawtooth artifact in pulse transit time signals derived from standard patient monitor data, emphasizing the need for calibration to prevent misinterpretation in clinical research.

Contribution

It identifies a previously unrecognized sawtooth artifact in PTT signals from common clinical monitors and demonstrates a method to visualize and validate this artifact.

Findings

Discovered a sawtooth artifact in PTT signals

Applied de-shape method for artifact visualization

Highlight importance of calibration for accurate PTT analysis

Abstract

Object: It is increasingly popular to collect as much data as possible in the hospital setting from clinical monitors for research purposes. However, in this setup the data calibration issue is often not discussed and, rather, implicitly assumed, while the clinical monitors might not be designed for the data analysis purpose. We hypothesize that this calibration issue for a secondary analysis may become an important source of artifacts in patient monitor data. We test an off-the-shelf integrated photoplethysmography (PPG) and electrocardiogram (ECG) monitoring device for its ability to yield a reliable pulse transit time (PTT) signal. Approach: This is a retrospective clinical study using two databases: one containing 35 subjects who underwent laparoscopic cholecystectomy, another containing 22 subjects who underwent spontaneous breathing test in the intensive care unit. All data sets include recordings of PPG and ECG using a commonly deployed patient monitor. We calculated the PTT signal offline. Main Results: We report a novel constant oscillatory pattern in the PTT signal and identify this pattern as a sawtooth artifact. We apply an approach based on the de-shape method to visualize, quantify and validate this sawtooth artifact. Significance: The PPG and ECG signals not designed for the PTT evaluation may contain unwanted artifacts. The PTT signal should be calibrated before analysis to avoid erroneous interpretation of its physiological meaning.