Intraoperative Beat-to-Beat Pulse Transit Time (PTT) Monitoring via Non-Invasive Piezoelectric/Piezocapacitive Peripheral Sensors Can Predict Changes in Invasively Acquired Blood Pressure in High-Risk Surgical Patients

Abstract

Background: Non-invasive tracking of beat-to-beat pulse transit time (PTT) via piezoelectric/piezocapacitive sensors (PES/PCS) may expand perioperative hemodynamic monitoring. This study evaluated the ability for PTT via PES/PCS to correlate with systolic, diastolic, and mean invasive blood pressure (SBP_IBP, DBP_IBP, and MAP_IBP, respectively) and to detect SBP_IBP fluctuations.

Methods: PES/PCS and IBP measurements were performed in 20 patients undergoing abdominal, urological, and cardiac surgery. A Pearson's correlation analysis (r) between 1/PTT and IBP was performed. The predictive ability of 1/PTT with changes in SBP_IBP was determined by area under the curve (reported as AUC, sensitivity, specificity).

Results: Significant correlations between 1/PTT and SBP_IBP were found for PES (r = 0.64) and PCS (r = 0.55) (p < 0.01), as well as MAP_IBP/DBP_IBP for PES (r = 0.6/0.55) and PCS (r = 0.5/0.45) (p < 0.05). A 7% decrease in 1/PTT_PES predicted a 30% SBP_IBP decrease (0.82, 0.76, 0.76), while a 5.6% increase predicted a 30% SBP_IBP increase (0.75, 0.7, 0.68). A 6.6% decrease in 1/PTT_PCS detected a 30% SBP_IBP decrease (0.81, 0.72, 0.8), while a 4.8% 1/PTT_PCS increase detected a 30% SBP_IBP increase (0.73, 0.64, 0.68).

Conclusions: Non-invasive beat-to-beat PTT via PES/PCS demonstrated significant correlations with IBP and detected significant changes in SBP_IBP. Thus, PES/PCS as a novel sensor technology may augment intraoperative hemodynamic monitoring during major surgery.

Keywords: anesthesiology; intraoperative blood pressure; non-invasive hemodynamics; piezocapacitive; piezoelectric; pulse transit time.

Figure 1

Example of dual PES/PCS sensor affixed to the index finger.

Figure 2

Data collection device for the PES/PCS system.

Figure 3

Geometric schematic of the dual PES/PCS sensor system. Described are the main components that track and relay the signals from an incoming force. PVDF = polyvinylidene fluoride.

Figure 4

Flowchart of the PW detection pre-processing: (a) The R-peak in the ECG defines the PW onset. (b) The mean PW is identified as a robust template for physiologically plausible PWs in terms of morphology and magnitude. Each PW is then compared with a patient-specific template (green PW) for automatic deselection of non-optimal PWs (e.g., due to motion noise). A PW is rejected if either its Pearson correlation with the template is less than 0.85 or if the difference in normalized magnitude (i.e., area under the curve [AUC]), is greater than 0.5 (red PW). All remaining PWs are defined as valid. (c) For all valid PWs, PTT is calculated as the time from R-peak to the steepest rise in the PW. (d) For R-peaks where a PW is rejected, PTT values are inputted using a 4th order interpolation technique, WENO4. This interpolation is justified by the fact that 95% of all gaps in the PTT signal are less than 10 beats in length.

Figure 5

(a) Example of PW_PES. (b) Example of PW_PCS. (c) Example of PW_IBP from a cardiac surgery patient. PW_PES and PW_PCS are reported as arbitrary units (a.u.). PW_IBP is reported as mmHg.

Figure 6

(a) Example of PW_PES. (b) Example of PW_PCS. (c) Example of PW_IBP from a urological surgery patient. PW_PES and PW_PCS are reported as arbitrary units (a.u.). PW_IBP is reported as mmHg.

Figure 7

Example of (a) intraoperative SBP_IBP (red), DBP_IBP (green), and MAP_IBP (yellow), (b) 1/PTT_PES, (c) 1/PTT_PCS, from a cardiac surgery patient.

Figure 8

Example of (a) intraoperative SBP_IBP (red), DBP_IBP (green), and MAP_IBP (yellow), (b) 1/PTT_PES, (c) 1/PTT_PCS, from an abdominal surgery patient.

Figure 9

Example of (a) intraoperative SBP_IBP (red), DBP_IBP (green), and MAP_IBP (yellow), (b) 1/PTT_PES, (c) 1/PTT_PCS, from a urological surgery patient.

Figure 10

Boxplot of the Pearson correlation between SBP_IBP, MAP_IBP, and DBP_IBP with 1/PTT_PES and 1/PTT_PCS.

Figure 11

AUROC curves detailing 1/PTT changes detected with PES/PCS sensors with changes in SBP_IBP: (a) delta of 1/PTT_PCS with decrease in SBP_IBP, (b) delta of 1/PTT_PCS with increase in SBP_IBP, (c) delta of 1/PTT_PES with decrease in SBP_IBP, (d) delta of 1/PTT_PCS with increase in SBP_IBP.