Heart Failure (HF) is a challenging condition to manage, with hospital readmission for HF exacerbation having negative impacts on patient outcomes and financial burden to both patient and health system \[Lloyd-Jones, 2010; Yancy, 2017; Ross, 2009; Chaudhry, 2007\]. An intuitive need for more sensitive predictors of HF exacerbations has led researchers to explore remote monitoring as a possible answer. Consumer-owned sensors have become more accurate in their recording of vital signs, and thus could hold potential for remote monitoring \[Dickinson, 2018\]. The combined measure of heart rate (HR) and respiratory rate (RR) has been shown to predict New York Heart Association (NYHA) HF class, an indicator of severity of heart disease, in implantable cardiac devices with multi-sensor monitoring capabilities \[Auricchio, 2014; Prasun, 2019; Boehmer, 2015; Boehmer, 2017\]. Heart rate variability (HRV), a measure of sympathetic autonomic function, has also shown potential in prediction of adverse cardiac events \[Al-Zaiti, 2019; Shaffer, 2017; Bullinga, 2005; Tsuji, 1996\].

The WHOOP device, a wearable strap similar to a Fitbit, allows for real-time HR monitoring and can determine RR using respiratory sinus arrhythmia \[www.whoop.com/experience; Berryhill, 2020\]. It is one of the few devices on the market that can accurately track heart rate as well as respiratory rate in real-time (during activity) and is equipped with a multidirectional accelerometer for activity tracking. The WHOOP device was recently externally validated against polysomnography and continuous electroencephalogram (EEG) for sleep tracking, and continuous electrocardiogram (ECG) for HR and HRV (with less than 5% error) \[Berryhill, 2020\]. HRV, which represents the balance of the sympathetic and parasympathetic nervous systems, is a known predictor of cardiac events. It is especially useful in HF, which is a chronically elevated catecholamine state leading to depressed HRV and is tied to NYHA HF class, an indicator of severity of disease \[Bullinga, 2005; Tsuji, 1996\].

Data so far regarding the efficacy of remote physiologic monitoring using cardiac implantable electronic devices (CIEDs), although promising in theory, has not yet proved sensitive in the detection of HF exacerbation. The aim of the CLEPSYDRA study was to use data extracted from implanted cardiac resynchronization therapy with defibrillation (CRT-D) devices in HF patients to predict heart failure events; although the main variables used in the novel algorithm, minute ventilation and patient activity, would intuitively seem to be predictors of poor outcome/HF exacerbation, the sensitivity of the algorithm to predict an event was only 34% \[6\]. It would appear that this combination of variables is not sufficient to predict adverse HF events. However, the HOME-CARE (HOME Monitoring in CArdiac REsynchronization Therapy) study showed more promising results, as their enhanced predictor, utilizing seven diagnostic variables from implanted CRT-Ds, boasted a sensitivity of 65.4% \[Sack, 2011\]. While the data from these studies is helpful, no study has been able to adequately identify and assess accurate predictors of HF class.

Current efficacious management strategies are limited to hemodynamic or multisensor monitoring systems. However, these are only available in implanted cardioverter-defibrillator (ICD) or cardiac resynchronization therapy-defibrillator (CRT-D) devices. These are not implanted in every HF patient \[Al-Zaiti, 2019\]. Non-invasive monitoring that provides similar data, such as wearable device monitoring, would expand the cohort of patients that would benefit from remote monitoring and would avoid the risks of having implanted hardware. Furthermore, better prediction of HF severity could help guide follow-up care and predict HF events \[Boehmer, 2015; Boehmer 2017\]. This would lead to more efficient management, less hospital readmission, and improve outcomes for HF patients overall \[Dickinson, 2018\].

The investigators propose a feasibility study in HF patients to better assess HF disease state, which can aid in management and improve outcomes. Subjects will wear the WHOOP device, which measures both activity and HR parameters and can derive RR using respiratory sinus arrhythmia, for 90 days. During this period, their HR and RR will be recorded at rest, during activity and post-activity recovery phases. This combined measure of HR/RR has been shown to predict NYHA HF class, an indicator of severity of disease, in implantable devices with multi-sensor monitoring capabilities; thus, it represents a useful management strategy in HF patients \[Bullinga, 2005; Tsuji, 1996\]. A continuous external monitoring device worn on the wrist, such as the WHOOP device, would provide valuable physiologic data for a cohort of HF patients who were previously unable to be monitored in this fashion. Secondary analysis of this study will investigate the use of intra- and post-activity HR and RR as predictors of hospitalization rates, a common problem in HF patients that correlate with worse mortality outcomes.