Noncardiac Implantable Devices

Published on 08/04/2015 by admin

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Chapter 71

Noncardiac Implantable Devices

Paul Jhun and Eduardo Borquez

In addition to cardiac pacemakers and defibrillators, a number of noncardiac devices have been developed for electronic neuromodulation and drug delivery.1,2 Although these devices are placed by a variety of subspecialists for the treatment of chronic illnesses, if the devices malfunction, patients may arrive at the emergency department in an acute state, thereby necessitating intervention by the emergency physician.

Insulin Infusion Devices

Background

External insulin infusion devices are becoming increasingly popular since their introduction in 1974. As of 2007, more than 375,000 external insulin infusion pumps are in use.³

Anatomy

An external insulin infusion pump device consists of a portable, programmable infusion pump connected to a subcutaneously implanted catheter maintained in place with adhesive tape (Fig. 71-1). The implanted catheter site varies, but it is commonly placed on the abdomen in adults and on the buttocks in young children. The thighs, hips, and upper part of the arms are other sites. It is recommended that the implanted catheter be replaced every 2 to 3 days.

Figure 71-1 External insulin infusion pump. Components include a subcutaneously implanted catheter (A) and a programmable infusion pump (B) (OneTouch Ping, Animas Corporation, West Chester, PA). To discontinue the flow of insulin into the patient, simply peel off the adhesive and embedded catheter together. Also shown is a wireless continuous glucose monitoring receiver (C), along with its sensor and transmitter (D) (exCom SEVEN PLUS, DexCom, Inc., San Diego, CA).

In 2010, the Food and Drug Administration (FDA) published a panel report that highlighted problems associated with insulin infusion devices.³ From the top five device manufacturers, the FDA noted 16,640 adverse events, including 310 deaths, 12,093 injuries, and 4,294 malfunctions. Box 71-1 lists the most frequently reported problems with devices in descending frequency. Box 71-2 lists the most frequently reported patient-oriented adverse reactions, which include hyperglycemia and hospitalization.

Of the 310 deaths reportedly related to insulin infusion pumps, the vast majority of problems with the device were not known to the patient or providers at the time and the root cause of failure of the device was not identified by the manufacturer. In 29 deaths, problems with the device identified included overinfusion, bent cannulas, disconnection, pump alarming, failure to deliver, suspected electromagnetic interference, and display failure.

Procedure

As with all patients seen in the emergency department, consider device malfunction as part of the differential diagnosis in patients with insulin infusion devices. In addition to medical management, troubleshoot the device and remove it in cases of uncertainty or emergency.

To remove the catheter, simply peel off the adhesive and embedded catheter together to discontinue the flow of injected medication into the patient. There are a variety of proprietary pump manufacturers, each with their own device programming. Call the appropriate manufacturer to troubleshoot the device, visit the manufacturer’s website for an online troubleshooting manual, or simply instruct the patient to discontinue use of the device and return to a standard calculated insulin schedule by manual injection until the device-related complication is investigated and resolved.

Intrathecal Drug Delivery Systems

Background

Intrathecal drug delivery systems (IDDSs) have been in clinical use since the 1980s. Although data are accumulating, very few robust clinical studies of IDDSs have been published.⁴ The FDA approved the use of intrathecal baclofen in 1992 for severe spasticity secondary to spinal cord injury, multiple sclerosis, cerebral palsy, or stroke. In addition, the FDA approved the use of intrathecal morphine in 1995 for chronic pain refractory to traditional medical therapies. An IDDS is also used for the delivery of chemotherapeutic medications for specific oncologic conditions. Moreover, there are numerous off-label uses of various intrathecal medications.

Anatomy

Currently, Medtronic is the only IDDS manufacturer in the United States. Medtronic’s SynchroMed II Programmable Infusion Pump consists of an infusion pump connected to an intrathecal catheter (Fig. 71-2A). The infusion pump is available with a refillable reservoir of either 20 or 40 mL. The pump is powered by a permanent lithium battery that cannot be recharged. The battery must be surgically replaced every 4 to 7 years. Normal refill intervals are usually between 2 and 3 months.

Figure 71-2 A, The Medtronic SynchroMed II Programmable Infusion Pump. B, The pump is usually implanted in a subcutaneous pocket in the right lower quadrant, with the catheter tunneled subcutaneously to an appropriate lumbar interspace.

The pump is placed in a subcutaneous pocket, generally in the right lower abdominal region. The intrathecal catheter is inserted slightly lateral to the spinous process into an appropriate lower lumbar interspace and connects to the pump via a subcutaneous tract that wraps around the abdominal wall (see Fig. 71-2B).

Interrogation and programming of the device are conducted with a magnetic field–induced, external programming wand connected to a handheld computer.

Device Complications

Several studies have shown a significant rate of adverse effects in IDDS patients ranging anywhere from 2% to 50%.5,6

The most frequent and serious adverse events related to the device and implant procedures are catheter dislodgement from the intrathecal space, catheter fracture, implant site infection, and meningitis. Internal device programming errors that cause underdosing or overdosing are less common. If the drug supply is depleted, systemic symptoms related to acute drug withdrawal will occur. This is characteristic for the specific drug being delivered and should be anticipated. Opioid and baclofen withdrawal syndromes are unlikely.

Medications currently in use via IDDSs include clonidine, bupivacaine, morphine, hydromorphone, fentanyl, ziconotide, and baclofen.⁷ Of these, bupivacaine and morphine have specific antidotes (Intralipid and naloxone, respectively). Medication overdoses, aside from standard emergency management, can also be treated by accessing the infusion pump reservoir.

A unique complication related to IDDSs that was identified by the FDA and issued as a warning is the formation of a granuloma at the tip of the intrathecal catheter. This can obstruct infusion of the medication and lead to withdrawal symptoms.8,9

Common adverse effects related to intrathecal baclofen include hypotonia, somnolence, headache, convulsions, dizziness, urinary retention, nausea, and paresthesias. Acute intrathecal baclofen withdrawal, which may resemble alcohol or benzodiazepine withdrawal, is also possible in cases of underdosing or inability to deliver medication.

With regard to intrathecal opioids, the degree of lipophilicity of the intrathecal opioid affects its pharmacokinetics. Fentanyl, with its high lipophilicity, is absorbed rapidly by the spinal cord and little is left to ascend in cerebrospinal fluid (CSF). Thus, it has a rapid onset and shorter duration with fewer adverse effects. Morphine, with its high hydrophilicity, penetrates the spinal cord slowly, which allows a considerable amount of the drug to ascend in CSF. This, in turn, results in a slower onset and longer duration with a higher incidence of adverse effects, including pruritus, nausea and vomiting, urinary retention, changes in mental status, and respiratory depression.10,11

Procedure

When there is concern for medication overdose secondary to malfunction of the device, in addition to emergency medical management, the emergency physician can empty the pump reservoir by using the steps outlined in Box 71-3.

Box 71-3 Drainage of the Infusion Pump Reservoir

Equipment

Antiseptic agent

22-gauge needle

20-mL Luer-Lok syringe or syringes

Procedure

1. Palpate and locate the pump on the patient (typically in the lower abdominal subcutaneous layer). The reservoir fill port is located in the center of the pump.

2. Cleanse the injection site with the antiseptic agent.

3. Attach the 20-mL syringe to the 22-gauge needle and insert the needle through the skin into the center of the reservoir fill port until the needle touches the metal needle stop (see Fig. 71-3).

4. Use negative pressure to withdraw fluid from the reservoir. Empty the reservoir until backflow has stopped. Depending on pump volume, more than one syringe may be needed. Intrathecal drug delivery systems vary in their reservoir capacity from 18 to 60 mL.⁷

5. Remove the needle from the reservoir fill port.

6. Record the amount of medication fluid removed from the reservoir.

Access the Medtronic online manual and review the steps necessary to remove the contents of the pump reservoir and thus prevent further infusion of intrathecal medication (Fig. 71-3).¹² Assuming no contraindications, withdraw an additional 30 to 40 mL of CSF by lumbar puncture to reduce the drug’s concentration in CSF.

Figure 71-3 The contents of the intrathecal pump reservoir can be removed with a syringe and 22-gauge needle (see Box 71-3).

VNS

Background

Since the 1930s, numerous studies have shown the effects of vagal nerve stimulation on cerebral activity. In 1985, Zabara demonstrated the anticonvulsant effect of vagal nerve stimulation through animal studies.¹³ In 1997 the FDA approved use of the vagal nerve stimulator (VNS) as an adjunctive treatment of medically refractory partial-onset epilepsy.¹⁴ Subsequently, in 2005 the FDA approved use of the VNS as an adjunctive therapy for treatment-resistant depression.^15,16

Anatomy

Currently, only one company in the United States, Cyberonics, manufactures the VNS under the brand name VNS Therapy System (formerly known as the NeuroCybernetic Prosthesis).

The device is composed of a generator attached to a bipolar VNS lead. The generator is approximately 4 cm long, 6 cm wide, and 7 mm thick and weighs 25 g. Figure 71-4 shows how the generator is implanted into subcutaneous tissue in the left upper part of the chest, with the electrode lead being attached to the left cervical vagus nerve trunk. Interrogation and programming of the device are conducted with a magnetic field–induced, external programming wand connected to a handheld computer. The variable settings that can be adjusted include current output, signal frequency, pulse width, and on/off stimulation times. When reset, the generator is on for 30 seconds, followed by 60 minutes off.^17,18

Figure 71-4 Vagal nerve stimulator (Aspire HC, Cyberonics, Inc., Houston).

Assuming normal parameters and output, the VNS generator has a projected battery life of approximately 10 years. Of note, VNS devices are implanted only on the left vagus nerve because the right vagus nerve directly innervates cardiac tissue and may have undesired cardiac effects if stimulated.

Device Complications

The device is typically turned on 10 to 14 days after implantation to allow adequate wound healing. Be aware that patients seen postoperatively during the first 2 weeks after implantation may not have had their device activated yet.

In addition to surgery-related risks and complications, if the patient complains of severe neck pain, worsening hoarseness, choking, or difficulty breathing, always consider device-specific complications. Voice alteration or hoarseness is the most common device-specific adverse effect, with more than 50% of patients being affected. Other common side effects include increased coughing, shortness of breath, and pharyngitis. Less common device-specific adverse effects reported during clinical studies include ataxia, dyspepsia, dysphagia, hypoesthesia, infection, insomnia, laryngismus, nausea, pain, paresthesia, and vomiting.17,18

Procedure

In cases of emergency, diagnostic uncertainty, or significant adverse effects, turn off the pulse generator temporarily by holding a magnet over the generator (Box 71-4). Pass the external VNS magnet over the generator to independently trigger it. This on-demand stimulation, if initiated at the onset of an aura or seizure, may abort the attack or, if initiated during a seizure, may halt its progression. Holding a magnet over the pulse generator causes a reed switch inside the generator to close. When the magnet closes the reed switch, the signal cannot be conducted and the pulse generator is temporarily turned off. When the magnet is removed, the switch opens and the generator is turned back on.^17,18

Box 71-4 Vagal Nerve Magnet Applications

To Deliver on-Demand Stimulation

Place the magnet over the generator for longer than 1 second and then quickly remove it. The generator turns off and then immediately on again, thereby delivering a burst of vagal nerve stimulation based on the preprogrammed settings.

This maneuver can be repeated as needed.

To Turn Off the Stimulation

Hold and maintain the magnet over the pulse generator. If the device is properly turned off, the patient should notice loss of episodic adverse effects, such as stimulation-induced voice alteration or pain.18,19

Unlike the typical donut-shaped magnet of a cardiac pacemaker, the VNS magnet is bar shaped, as shown in Figure 71-5. Be aware that patients with VNSs are typically provided with two magnets: one fitted with a wristband and the other fitted with a belt clip.

Figure 71-5 Vagal nerve stimulator magnet.

Additional Implantable Devices

There are several additional noncardiac implantable devices that the emergency physician may encounter, which are outlined in Table 71-1. However, should these devices malfunction, there are no specific procedures to be aware of in the emergency setting beyond supportive treatment.

TABLE 71-1

Additional Noncardiac Implantable Devices

DEVICE	DESCRIPTION	MALFUNCTION
Bladder stimulator or sacral nerve stimulator	Stimulator wire inserted into the S3 sacral foramen adjacent to the sacral nerve for the treatment of urinary incontinence in the setting of severe neurologic disease²⁰	Hardware complications include infection, skin irritation, and wire migration²⁰
Deep brain stimulator	Stimulator implanted into the thalamus for treatment of Parkinson’s disease, tremor, and dystonia¹	Hardware complications include infection and lead migration²¹
Gastric pacemaker	Neurostimulator with implanted leads into the gastric musculature to improve gastroparesis²²	Gastric wall perforation, infection, and lead migration²²
Phrenic nerve stimulator or diaphragmatic pacemaker	Electrodes implanted into each phrenic nerve with a receiver implanted into subcutaneous tissue. An external transmitter controls the receiver. Used to treat respiratory insufficiency secondary to upper motor neuron paralysis or respiratory drive dysfunction^23,24	Pneumothoraces have been described immediately after implantation. Infection and wire migration have been described.^23,24 Failure of the pacemaker is treated by ventilatory support

MRI and Implantable Devices

For patients with electrically, magnetically, or mechanically activated implants, the FDA requires labeling that magnetic resonance imaging (MRI) is contraindicated,²⁵ and indeed, manufacturers of gastric, urinary, diaphragmatic, and spinal cord stimulators list these devices as absolute contraindications to MRI.²¹ However, there are a number of clinical series and case reports indicating the compatibility of certain devices with MRI. For VNSs, a 1.5-T magnet has been studied and deemed compatible with MRI by the manufacturer.^21,26,27 However, this compatibility is limited to MRI machines with transmit and receive head coils. MRI machines with transmit body coils and receive-only head coils may cause heat injury and damage the device. For deep brain stimulation, some studies have suggested that MRI is safe in certain circumstances,²⁸ but case reports of patient injury can be found in the literature.²⁹ For IDDSs, pumps have been shown to stop operating while in the magnetic field but resume functioning once removed.²⁵ Insulin pumps may easily be removed in the event that MRI is required.

In summary, the issue of MRI compatibility with implantable devices is very complex. In the event that MRI appears to be necessary, a case-by-case risk-benefit analysis involving the relevant subspecialist and radiologist is recommended.