Implantable Loop Recorders (ILRs)

What is an Implantable Loop Recorder?

An Implantable Loop Recorder (ILR) is a medical device used to detect and record abnormal electrical activity in patients suspected to have arrhythmia.

Cardiac arrhythmia is when the heart beats irregularly, too fast or too slow. Many types of arrhythmia do not present symptoms other than heart palpitations or feeling a pause between heartbeats. Unfortunately, great apes cannot communicate to veterinarians when these symptoms occur. While it is not always a serious condition, arrhythmia may be lead to stroke or heart failure.

Which ILRs are used in great apes in zoos?

There are two devices available in the human market that have been used in great apes. The Medtronic Reveal XT 9529 device is about the size of a flash drive, weights 15 grams and is able to store up to 27 minutes of data. This device has been used in over a dozen great apes. In 2014, Medtronic introduced the new Linq device to the market. The Linq device has several advantages over the Reveal; the device is smaller and easier to place, has improved tissue contact, and is harder for great apes patients to detect and remove. Both implant models record the daytime and nighttime heart rates, heart rate variability and activity level, and have a three year battery lifespan. At the time of surgical placement, each device is programmed with detection thresholds for arrhythmia, including tachycardia, bradycardia and asystole.


Why do we use ILRs – what can they tell us, what do they actually tell us?

In apes that are suspected of having arrhythmias, it is unlikely to observe the exact moment at which an episode is taking place during routine health assessments. An ILR device monitors the heart’s electrical activity 24-hours a day, and so it is far more effective in determining if and when arrhythmias occur.

What are things to consider before using an ILR on an ape?

ILR monitors produce continuous EKG recordings and other relevant heart information that need to be thoroughly reviewed and interpreted by a cardiologist and/or veterinarian that is familiar with the technology. In addition to the time spent interpreting the data, the monitors themselves may need to be routinely calibrated. The location of where the ILR is implanted is very important in determining whether or not quality data will be transmitted. In humans, the device is placed under the skin on the upper left chest. This is not a feasible option for great apes as it makes it very easy for an ape to extract the device or dislodge it through chest beating. In several ape implantations, gorillas and chimpanzees have extracted the ILR devices, which retail between $6,000-$12,000. Typically, ILRs have been implanted on the backs of great apes under muscle.

If your institution is planning on using an ILR in a great ape, we highly recommend discussing the procedure with other zoo veterinarians who have already performed the procedure.

How many zoos are using ILRs?

As of 2020, the GAHP is aware of seven US zoos that are using Medtronics ILR devices with gorillas and chimpanzees. Among the zoos are Detroit Zoo, Smithsonian National Zoo, and Lincoln Park Zoo.