Thoracic Outlet Syndrome: Fingertip Cannot Replace Forearm Photoplethysmography in the Evaluation of Positional Venous Outflow Impairments

Abstract

Objective: Fingertip photoplethysmography (PPG) resulting from high-pass filtered raw PPG signal is often used to record arterial pulse changes in patients with suspected thoracic outlet syndrome (TOS). Results from venous (low-pass filtered raw signal) forearm PPG (V-PPG) during the Candlestick-Prayer (Ca + Pra) maneuver were recently classified into four different patterns in patients with suspected TOS, two of which are suggestive of the presence of outflow impairment. We aimed to test the effect of probe position (fingertip vs. forearm) and of red (R) vs. infrared (IR) light wavelength on V-PPG classification and compared pattern classifications with the results of ultrasound (US). Methods: In patients with suspected TOS, we routinely performed US imaging (US + being the presence of a positional compression) and Ca + Pra tests with forearm V-PPG _IR . We recruited patients for a Ca + Pra maneuver with the simultaneous fingertip and forearm V-PPG _R . The correlation of each V-PPG recording to each of the published pattern profiles was calculated. Each record was classified according to the patterns for which the coefficient of correlation was the highest. Cohen's kappa test was used to determine the reliability of classification among forearm V-PPG _IR , fingertip V-PPG _R , and forearm V-PPG _R . Results: We obtained 40 measurements from 20 patients (40.2 ± 11.3 years old, 11 males). We found 13 limbs with US + results, while V-PPG suggested the presence of venous outflow impairment in 27 and 20 limbs with forearm V-PPG _IR and forearm V-PPG _R , respectively. Fingertip V-PPG _R provided no patterns suggesting outflow impairment. Conclusion: We found more V-PPG patterns suggesting venous outflow impairment than US + results. Probe position is essential if aiming to perform upper-limb V-PPG during the Ca + Pra maneuver in patients with suspected TOS. V-PPG during the Ca + Pra maneuver is of low cost and easy and provides reliable, recordable, and objective evidence of forearm swelling. It should be performed on the forearm (close to the elbow) with either PPG _R or PPG _IR but not at the fingertip level.

Keywords: fingertip; forearm; movement (MeSH); pathophysiology; photoplethysmography (PPG); thoracic outlet syndrome (TOS); veins.

FIGURE 1

Schematic representation of the methods.

FIGURE 2

Illustration of the location of probes. The left panel shows the IR light V-PPG probe of the ELCAT device on the forearm close to the elbow crease; the right panel shows the probes at the fingertip and forearm level used during the red light V-PPG recordings.

FIGURE 3

Example of recordings in a patient with bilateral symptoms (patient A) and unilateral left symptoms (patient B) during the candlestick (light gray arrow) and prayer (dark gray arrow) maneuvers. Zero is resting state with filled veins, and 100% is the maximal volume observed until the end of the prayer maneuver. Since a small overshoot was sometimes observed when moving the arms down after the prayer maneuver, some values may be in excess of 100%. It is noted that the system used for VPP_IR does not allow recordings in excess of 60 s.

FIGURE 4

Schematic representation of the hypotheses of the fingertip (light blue) and forearm (dark blue) volume changes during the Ca + Pra maneuver. Normally, the forearm and fingertip volumes decrease with arm elevation, and refilling will only occur during arm lowering (upper figure). In case of ischemia (isolated inflow impairment), it is expected that the decrease in adrenergic tone due to postischemic vasodilation would be better observed distally (at the fingertip level). If simultaneous inflow and outflow impairments occur, emptying of the forearm is stopped while fingertip veins may drain into the forearm veins due to hydrostatic pressure. Emptying of the forearm will be completed during the prayer attitude. In cases of isolated venous outflow impairment, arterial inflow during the candlestick attitude will fill the forearm from bottom to top (as in a filling bottle) and will increase the volume at the forearm far before the fingertip volume increases. It is noted that fingertip volume would increase too if the duration of the candlestick attitude was long enough.