  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.
