An engineering perspective on the development and evolution of implantable cardiac monitors in free-living animals

Abstract

The latest technologies associated with implantable physiological monitoring devices can record multiple channels of data (including: heart rates and rhythms, activity, temperature, impedance and posture), and coupled with powerful software applications, have provided novel insights into the physiology of animals in the wild. This perspective details past challenges and lessons learned from the uses and developments of implanted biologgers designed for human clinical application in our research on free-ranging American black bears (Ursus americanus). In addition, we reference other research by colleagues and collaborators who have leveraged these devices in their work, including: brown bears (Ursus arctos), grey wolves (Canis lupus), moose (Alces alces), maned wolves (Chrysocyon brachyurus) and southern elephant seals (Mirounga leonina). We also discuss the potentials for applications of such devices across a range of other species. To date, the devices described have been used in fifteen different wild species, with publications pending in many instances. We have focused our physiological research on the analyses of heart rates and rhythms and thus special attention will be paid to this topic. We then discuss some major expected step changes such as improvements in sensing algorithms, data storage, and the incorporation of next-generation short-range wireless telemetry. The latter provides new avenues for data transfer, and when combined with cloud-based computing, it not only provides means for big data storage but also the ability to readily leverage high-performance computing platforms using artificial intelligence and machine learning algorithms. These advances will dramatically increase both data quantity and quality and will facilitate the development of automated recognition of extreme physiological events or key behaviours of interest in a broad array of environments, thus further aiding wildlife monitoring and management. This article is part of the theme issue 'Measuring physiology in free-living animals (Part I)'.

Keywords: biologgers; biotelemetry; hibernation physiology; wildlife research.

Figure 1.

Inset image: The form factors of three generations of implantable monitors used by our research teams. (A) Generation 1 (GEN 1; 80 cc). (B) Generation 2 (Gen 2; 9 cc). (C) Generation 3 (GEN 3; 1.2 cc). The main image (adapted from [41]) shows the systems associated with the human clinical application of the GEN 3 device, including the implantation tools, device programmer (Model 2090), patient assistant, home monitor (Model 24950) and web-based repository (CareLink Network). All system components have been successfully used in wildlife applications, with the exception of the patient assistant, which requires patient interaction to capture episodes of interest.

Figure 2.

System configuration for a Generation 3 (GEN 3) device with a telemetry station. Antennas with a range of 3–5 m are buried 4–5 cm under the den floor. The antennas transfer the data from the implanted device to the home monitor on a 2 h interval for transmission over a cellular network to a web-based repository. The solar panel-powered system is duty-cycled (turned on for 20 min every 2 h) to enable transmissions while minimizing battery consumption. The sample electrocardiograms (ECGs) shown are from a female black bear in Minnesota hibernating with yearlings. Episodes shown include an episode with a 16 s asystole and a 222 bpm sinus tachycardia that was believed to be related to disturbance of the den by the landowners. The episode with the tachycardia includes two over-sensed beats due to skeletal myopotentials and/or motion artefact (marked with an *).

Figure 3.

Proposed configuration for the GEN 4 system. The implanted device transmits data to a radio collar-mounted transponder, which in turn communicates with either a cellular or Iridium network. This allows data transfer from an ambulatory/free-ranging animal to a web-based repository. The associated photo was captured by a trail camera at the den site during an active period where the mother and two yearlings had emerged from the den. The upper graph plots the heart rate over an 8 min interval occurring while this female was in the den hibernating. The variations in heart rate are due to a respiratory sinus arrhythmia, and thus the respiration rate can also be calculated (one heart rate cycle per respiration). The 30 s electrogram shown is for the same time period, allowing a detailed analysis of the rhythm. Although these exemplary data were captured by a stationary GEN 3 telemetry system, similar data capture and transfer are anticipated with GEN 4.