Abstract

Implantable cardiac monitors (ICMs), also called implantable loop recorders, have been used widely in human medicine for the diagnosis of transient life-threatening cardiac abnormalities, and are increasingly being used as a diagnostic tool in veterinary medicine.^1-3 ICMs capture electrophysiological recordings for a longer duration than a standard electrocardiogram (ECG), are essentially waterproof, and can be surgically implanted in subcutaneous tissue similar to data loggers and transmitters previously implanted in sea lions. ^2,4 These devices may be valuable tools to diagnose intermittent cardiac events in marine mammals, such as arrhythmias hypothesized to occur secondary to domoic acid toxicosis.^5,6 St. Jude Medical Confirm^TM Implantable Cardiac Monitors were surgically implanted in nine California sea lions (Zalophus californianus) between July 2010 and December 2010 at The Marine Mammal Center in Sausalito, California. All sea lions were anesthetized with isoflurane and an ICM inserted into a small pocket created within the subcutaneous tissues, on the left lateral thorax dorsocaudal to the axilla. The programmable algorithm for automatic activation was set to trigger a 40 second recording (20 seconds before, 20 seconds after) when heart rate registered below 30 bpm (bradycardia), above 190 bpm for greater than 40 R-R intervals (tachycardia), or cardiac arrest occurred for greater than 5 seconds (asystole). Recorded events were categorized by the device as bradycardia, tachycardia, or asystole based on the aforementioned criteria. Each device was interrogated using St. Jude Medical Merlin^TM Patient Care System and the surgical site inspected under manual restraint between 1 and 36 days after implantation (mean interval between interrogations, 11.7 days). ICMs remained implanted for 9 to 37 days (mean 22.6 days), depending on patient disposition and treatment protocol. Two devices were found explanted on the pen floor secondary to local inflammation. The remaining seven devices were surgically removed before the animals were released or euthanized. Nine hundred twenty-two ICM ECG recordings were obtained. Usable rhythm strips were obtained from all implanted devices with variable P and T wave amplitudes and clear QRS wave formations. No true asystole or cardiac arrests were recorded; of the 655 triggered "asystole" labeled events, 652 were due to under-sensing by the device and 3 triggered once contact with the device was disrupted following removal from the animal. Artifact baseline oscillation and noise from muscle or seizure activity triggered falsely labeled "tachycardia" events. These inappropriately labeled events were easily identified by examination of the recorded rhythm strip and provided additional data available for review. One 4 second episode of ventricular flutter (240 bpm) was recorded 22 days after implantation in a sea lion suspected of acute domoic acid toxicosis. Capture of this event illustrates the use of ICMs to detect, record, and diagnose transient cardiac arrhythmias in California sea lions. Surgical implantation of ICMs was well tolerated by the sea lions and created minimal inflammation. Results suggest ICMs are a promising tool for the extended monitoring of cardiac electrophysiological function and detection of arrhythmias in California sea lions.

Acknowledgements

The authors wish to thank St. Jude Medical for their generous donation of the Confirm^TM Implantable Cardiac Monitors, the use of Merlin^TM PCS pacemaker programming system, and specifically Chris and Stephanie Pirrone for their time and assistance during device interrogation. Thanks also to Marjorie Boor for her assistance in facilitating the donation of ICMs, the staff and volunteers of The Marine Mammal Center for their dedication to patient care, and Drs. Kittleson and Griffiths of the UC Davis Cardiology Service.

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