Novel Cutaneous Use of Implantable Loop Recorders in Two Dogs with Unexplained Episodes of Collapse

Introduction

Determination of the cause of collapsing episodes or episodes of weakness can be difficult because there are many possible causes, including cardiac, neurologic, and metabolic diseases. Sometimes, in patients suspected to have episodes associated with cardiac disease, the history and physical examination combined with a brief (i.e., <5 min) in-hospital surface electrocardiographic (ECG) evaluation, echocardiogram, or ambulatory ECG monitoring (usually 24 hr monitoring in veterinary patients) can provide a diagnosis. Unfortunately, this approach may not provide clinically useful information in patients with infrequent episodes, especially if the episode does not occur while the patient is being monitored.¹ Multiple lead loop recorders or event monitors can be placed and worn for many weeks, but these devices have relatively short recording periods when activated (<20 sec), and loss of lead contact can cause result in malfunction.^2,3 Implantable leadless loop recorders (ILRs) have been developed that can be implanted subcutaneously for long periods of time (up to 3 yr). ILRs are small devices that can store ECG data surrounding a clinical event. ECG data can be stored either automatically (triggered by certain heart rate parameters recognized by the device) or manually (using a small, hand-held activator). Data can be retrieved using a specialized programmer. Depending on the model of ILR used, up to 48 min of ECG data can be stored in a variety of configurations. The use of ILRs is useful in the investigation of collapsing episodes in dogs and cats.^4–6 Device implantation is a simple procedure, but requires either heavy sedation or general anesthesia in veterinary patients. Recent studies in veterinary medicine report the rate of complications requiring removal of the ILR to range from 3% to 8%, and a recent study in children with implanted ILRs reported a 15.8% complication rate requiring removal of the device. Complications in that study included infection and implant erosion.^5,7,8 Despite the relatively simple nature of the implantation procedure, some owners may decline implantation based on cost, risk of sedation or anesthesia, and possible complications associated with device implantation.

The purpose of this study was to describe the cutaneous placement of ILRs on two patients with episodes of collapse/weakness. To the authors’ knowledge, the use of an ILR in this fashion has not been previously described in veterinary literature. Cutaneous application of an ILR device may be a useful diagnostic tool in the evaluation of collapsing patients.

Case 1

A 2 yr old spayed female boxer presented to the teaching hospital at Michigan State University college of Veterinary Medicine for evaluation of a single episode of collapse that occurred 1 wk prior to referral. The owner reported that the episode occurred while running. The dog collapsed suddenly and was unresponsive, but recovered abruptly and fully within 2 min. There were no significant abnormalities noted on physical examination. A surface ECG demonstrated a normal sinus rhythm with no indications of chamber enlargement. Normal cardiac structure and function were confirmed with transthoracic echocardiography. Boxer dogs are predisposed to ventricular arrhythmias, and standard Holter monitoring or event monitoring (either standard surface event or an ILR) were recommended, but the owner declined due to financial concerns and concerns regarding the risk of ILR implantation. An ILR^a was therefore placed by external skin contact as an alternative.

With the dog in right lateral recumbency, an area of the thorax approximately 2.5 cm × 2.5 cm over the precordial impulse was clipped on the left side. The contact points on an ILR were placed against the skin using ECG electrode gel to facilitate conductance (Figure 1). A clear ECG tracing was obtained with obvious, identifiable P, QRS, and T waveforms (Figure 2). Placement in other locations resulted in waveforms with a lower signal strength. Once the optimal location was found, the skin was scrubbed with alcohol to remove cutaneous oils. Two conductive adhesive gel discs^b were placed over the contact points of the ILR, which was then placed onto the skin. An indelible marker was used to trace the outline of the device on the patient to mark the placement location if the device needed to be reattached for any reason. A transparent dressing^c was placed over the device to keep it from slipping, and a tight fitting shirt or conforming bandage^d was placed to keep the ILR from being dislodged by the patient. The device was programmed for manual activation (allowing three separate 14 min recordings when activated by the owner using a remote activation device) via radiotelemetry using a pacemaker programmer^e. The owners were taught how to replace the ILR if it was dislodged and were supplied with spare adhesive gel discs and five spare ILRs. To evaluate signal quality, the owners were asked to return an ILR after using the device for 72 hr. If no events had occurred during the initial 72 hr period, they were asked to activate the ILR three times prior to removing and returning the ILR (in the latter case, the owner could return the ILR by mail). The owners were instructed to place another ILR device 24 hr after removing the previous ILR to minimize skin reactions. The patient wore the device over a period of 6 wk, and the ILR was dislodged only twice. The owners replaced the device as instructed without complications or loss of signal quality. No collapsing episodes were noted during the 6 wk period of ILR use. A total of 27 manual activations were triggered during that time period. For each of those recordings, a clear ECG tracing confirmed sinus rhythm. Because the patient experienced no further episodes of collapse, the owners decided to stop using the ILR and declined further evaluation of noncardiac causes of collapse. Over the following 1 yr, the patient did not experience another episode of collapse, and the cause of the initial collapse remains unknown.

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Case 2

A 9 yr old spayed female Chihuahua presented to the teaching hospital at Michigan State University College of Veterinary Medicine for evaluation of multiple episodes of collapse. The collapsing episodes occurred frequently, with five episodes observed by the owner each day during the week prior to referral. The episodes appeared to be associated with excitement, eating, or drinking. During an episode the dog was unresponsive, but would recover quickly. Each episode lasted no longer than 1 min. No abnormal behaviors or mentation abnormalities were noted either prior to or following the collapsing episodes. Physical examination revealed a grade 2/6 left apical systolic murmur. A surface ECG demonstrated a sinus rhythm with no evidence of chamber enlargement. Transthoracic echocardiography revealed thickening of the mitral valve consistent with myxomatous valvular degeneration resulting in moderate mitral regurgitation. Mild left atrial and ventricular dilation with hyperkinetic systolic function was noted. Standard Holter monitoring for 24 hr or event monitoring (either standard event monitor or ILR) were recommended, but the owner declined due to financial concerns. As an alternative, an ILR was placed cutaneously as described above. In this case, the ILR was programmed for both manual and autoactivations. The criteria used for automatic activation was detection of bradycardia (<30 beats/min), tachycardia (>230 beats/min), and/or asystole (>4.5 sec).

The dog experienced an episode the next day. The owner activated the ILR and returned the device by mail. Data obtained from autoactivations during that initial period did not provided any useful information because autoactivation subsequently occurred during shipping, clearing any previously stored autoactivated data. This might have occurred because the ILR was programmed to save only the last three autoactivations and motion during shipping was presumed to have activated the device. In contrast, the data obtained from the manual activation associated with the collapsing episode revealed an inappropriate bradycardia followed by periods of sinus arrest (longest duration was >5 sec, which was terminated by what appeared to be ventricular escape beats, Figure 3). The owner declined further evaluation and treatment due to financial reasons.

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Discussion

This case report describes the novel application of an implantable loop recorder to evaluate two patients with unexplained episodes of collapse. Cutaneous placement of the ILR was quick and simple, requiring no anesthesia or sedation. In the second case, the cutaneously placed ILR provided high quality ECG data suitable for definitive rate and rhythm analysis surrounding the moments of a witnessed event. The devices are self-contained and very durable. At the time of writing this report, the cost of the device from the Canpacers organization (www.canpacers.com) was $100. The devices are reusable with long battery lives (ranging from 14 mo to 3 yr depending on make and model), making ILRs an economical choice for repetitive, multi-patient use. Although programming requires a specialized programmer, devices can be programmed and sent out to either referring veterinarians or clients with placement instructions.

No complications were observed following the cutaneous placement of an ILR. Although neither patient wore the device for >72 consecutive hours, no irritation of the skin occurred in either patient. An electrocardiographic explanation for collapse was not identified in case 1; however, that case demonstrates the ability to use the device over an extended period of time with only client intervention. The owners were able to replace the device without returning to the clinic, and high quality ECG data were still obtained. The possibility of extended use of cutaneously placed ILRs requires further evaluation, but long-term use of these devices may be limited by the willingness of the owner to maintain the device on the patient.

ILRs can be programmed to autoactivate under certain limited heart rate parameters, but they are designed for use in human patients. As such, the parameters are often inappropriate for veterinary patients, and physiologically appropriate heart rates (both bradycardias and tachycardias) in veterinary patients might result in autoactivation. Additionally, over- and under-sensing resulting in inappropriate autoactivation and lack of appropriate activation respectively has been reported in both human and veterinary patients with implanted ILRs.^4,9,10 Manual activation of the device requires the owner to be present when the event occurs and may limit the overall clinical usefulness of the device compared to using ILRs in human patients.

Conclusion

ILRs are easily placed cutaneously without risk to the patient. Both cutaneously placed ILRs described in this report were well tolerated, and no complications were observed in either patient. The authors suggest that cutaneously placed ILRs can be used effectively in veterinary medicine to evaluate patients with unexplained episodes of collapse, exercise intolerance, or weakness that are suspected to have a cardiac cause.