Fitbit Data to Assess Functional Capacity in Patients Before Elective Surgery: Pilot Prospective Observational Study

Abstract

Background: Preoperative assessment is crucial to prevent the risk of complications of surgical operations and is usually focused on functional capacity. The increasing availability of wearable devices (smartwatches, trackers, rings, etc) can provide less intrusive assessment methods, reduce costs, and improve accuracy.

Objective: The aim of this study was to present and evaluate the possibility of using commercial smartwatch data, such as those retrieved from the Fitbit Inspire 2 device, to assess functional capacity before elective surgery and correlate such data with the current gold standard measure, the 6-Minute Walk Test (6MWT) distance.

Methods: During the hospital visit, patients were evaluated in terms of functional capacity using the 6MWT. Patients were asked to wear the Fitbit Inspire 2 for 7 days (with flexibility of -2 to +2 days) after the hospital visit, before their surgical operation. Resting heart rate and daily steps data were retrieved directly from the smartwatch. Feature engineering techniques allowed the extraction of heart rate over steps (HROS) and a modified version of Non-Exercise Testing Cardiorespiratory Fitness. All measures were correlated with 6MWT.

Results: In total, 31 patients were enrolled in the study (n=22, 71% men; n=9, 29% women; mean age 76.06, SD 4.75 years). Data were collected between June 2021 and May 2022. The parameter that correlated best with the 6MWT was the Non-Exercise Testing Cardiorespiratory Fitness index (r=0.68; P<.001). The average resting heart rate over the whole acquisition period for each participant had r=-0.39 (P=.03), even if some patients did not wear the device at night. The correlation of the 6MWT distance with the HROS evaluated at 1% quantile was significant, with Pearson coefficient of -0.39 (P=.04). Fitbit step count had a fair correlation of 0.59 with 6MWT (P<.001).

Conclusions: Our study is a promising starting point for the adoption of wearable technology in the evaluation of functional capacity of patients, which was strongly correlated with the gold standard. The study also identified limitations in the availability of metrics, variability of devices, accuracy and quality of data, and accessibility as crucial areas of focus for future studies.

Keywords: ethics in wearables; mobile phone; preoperative risk assessment; smartwatch data; wearable devices.

Figure 1

(A) Correlation between the 6-Minute Walk Test (6MWT) distance and Non-Exercise Testing Cardiorespiratory Fitness (NET-F), (B) correlation between the 6MWT distance and resting heart rate (RHR), (C) correlation between the 6MWT distance and heart rate over steps (HROS) at 1% quantile, and (D) correlation between the 6MWT distance and daily steps. bpm: beats per minute.

Figure 2

Daily step count with patients, ordered by 6-Minute Walk Test (6MWT) result.

Figure 3

(A) Receiver operating characteristic (ROC) curve for 1% quantile heart rate over steps (HROS) threshold, (B) ROC curve for Non-Exercise Testing Cardiorespiratory Fitness (NET-F) threshold, (C) 6-Minute Walk Test (6MWT) distributions with chosen HROS threshold of 1.214, (D) 6MWT distributions with chosen NET-F threshold of 7.78, (E) contingency table of the 6MWT distance threshold of 350 m versus HROS 1% quantile threshold of 1.214, and (F) contingency table of the 6MWT distance threshold of 350 m versus NET-F threshold of 7.78. FPR: false positive rate; TPR: true positive rate.

Figure 4

(A) Time distance of successive heart rate samples in a patient who was compliant with the protocol and (B) focus on the distribution of low distances.