Are toe pressures measured by a portable photophlethysmograph equivalent to standard laboratory tests?

Abstract

Purpose: The purpose of this study was to determine if toe pressures (TPs) obtained by a registered nurse using a portable photophlethysmograph (PPG) were equivalent to TPs obtained by a registered vascular technologist (RVT) using standard laboratory equipment.

Design: A within-subjects, comparative design was used for the study.

Setting and subjects: Thirty subjects referred to a vascular laboratory for arterial studies were recruited. All tests were performed in the outpatient vascular laboratory of a large, urban medical center.

Methodology: Toe pressures were measured on subjects by the same RN and RVT during the same visit. Data were analyzed using the Bland-Altman method that compares bias (mean difference) and precision (limits of agreement) of measurements to previously determined criterion for clinically important limits of difference (±15 mm Hg) in order to assess equivalence and repeatability of measurements. Kappa statistic was calculated to assess RVT/RN agreement to detect lower extremity arterial disease (LEAD) (ie, TP<50 mm Hg). Sensitivity and specificity of the portable PPG measures were determined.

Results: Precision for RVT-RN TPs exceeded the previously determined cutoff criteria (±15 mm Hg), but precision for repeated RN PPG measures fell within the clinically important limits. Kappa statistic calculation (κ=0.76) revealed substantial agreement (90%) between the RVT and RN measures to detect LEAD (<50 mm Hg). The portable PPG technique had good sensitivity (79%) and high specificity (95%) for detection of LEAD.

Conclusion: Although TPs obtained by the portable PPG were not equivalent to standard laboratory tests, the portable technique agreed sufficiently with the RVT to detect LEAD. The good sensitivity and high specificity of the portable PPG make it suitable for nurses and other primary care providers to use for high-risk patients or patients with wounds, when the ankle brachial index either is elevated above 1.3 or cannot be performed. Photophlethysmograph is also suitable to assess healing potential and the need for referrals to the vascular laboratory, surgeon, or the need for adjunctive therapies.