MRI in patients with pacemakers and ICDs

There has been a powerful incentive to investigate the safety of MRI scanning in patients with cardiac rhythm management devices and to develop new systems that can be used more safely

 

Christopher Plummer BSc PhD BM BCh FRCP

Consultant Cardiologist,

The Newcastle upon Tyne Hospitals NHS Foundation Trust, UK

 

Since the first implantation of pacemakers in 1958, implantable cardioverter defibrillators (ICDs) in 1980 and cardiac resynchronisation therapy in 1998, these cardiac rhythm management (CRM) devices have revolutionised the treatment of patients with heart rhythm disorders and heart failure, improving quality of life and reducing sudden death. Although each has undergone dramatic changes in technology over the past decade, they all include one or more wires connecting the inner or outer surface of the heart to a device placed under the skin to monitor intrinsic cardiac rhythm and electrically stimulate contraction. In addition, ICDs can automatically deliver a controlled electric shock to terminate life-threatening fast heart rhythms, ventricular tachycardia and ventricular fibrillation, in a cardiac arrest.

 

The clinical use of magnetic resonance imaging (MRI) has developed over a similar timescale, with the first MRI images produced in 1952, the first human MRI scan in 1977 and, in the last decade, the development of more powerful computer reconstruction and 3 Tesla (3T) scanners with improved image quality. MRI is now the imaging modality of choice in a wide variety of clinical situations because of its excellent soft-tissue image quality and the avoidance of ionising radiation.

 

Inevitably, as the indications for CRM devices and MRI have widened, increasing numbers of people have indications for both. It has been estimated that 50–75% of patients with a pacemaker or ICDs will have an indication for an MRI scan at some point during their lives. This estimate is quoted in multiple publications but the data on which it is based are unknown – it appears to have originated in calculations by one of the CRM device manufacturers and the data on which it is based are quoted as ‘on file’ in Kalin et al.(1) 

 

Magnetic resonance imaging applies a strong static magnetic field to the body, powerful enough to align protons in water molecules. This static field is then varied across the body to give spatial information using pulses of a field gradient. Short radiofrequency (RF) impulses are then applied, which disturb the axis of protons, causing them to emit a radiofrequency signal as they return to their original spin orientation. These signals differ between tissues and can be detected and localised in space allowing a three-dimensional image to be constructed.

 

There are clear theoretical reasons to avoid MRI scanning in patients with pacemakers or ICD. Pacemaker and ICD systems contain multiple ferromagnetic components, both in the leads and generators. This produces four categories of risk when these devices are placed in MRI scanners:

 

• macroscopic dislodgement of the device or lead tip due to the strong static magnetic field
• direct effect of the static, gradient and RF fields on the magnetic reed switch which is used to control certain functions in pacemakers and ICDs – altering mode and pacing rate in pacemakers and inhibiting tachycardia therapy in ICDs. This can result in unpredictable behaviours, especially if a power-on reset to factory settings occurs, resulting in an inappropriate pacing mode or inappropriate therapy delivery from an ICD
• inappropriate pacing or inhibition of pacing can occur due to over-sensing of the RF pulses as intrinsic heart beats. This can result in bradycardia or asystole in pacing dependent patients or tachycardia if triggered pacing occurs in the ventricle due to over-sensing in the atrial channel
• heating of the lead tip can result from aerial properties of leads in the RF field. This can result in thermal injury to the myocardium causing direct stimulation, or tissue damage, which can increase pacing threshold.

 

These risks were recognised very early in the development of human MRI scanning and pacemakers and ICDs were regarded as absolute contraindications to this form of imaging. These risks are not merely theoretical, as deaths have been reported in unmonitored patients not known to have pacemakers by the radiology staff. Worryingly, some of the deaths occurred in patients who were not pacing dependent, suggesting that inhibition of pacing was not the cause and that induction of a ventricular tachyarrhythmia by inappropriate pacing or direct stimulation was the aetiology.(2)

 

There are situations where only MRI scanning can provide life-saving information. Where this occurs in a patient with a CRM device, removal of the device is seldom safe or practical, especially in those dependent on pacing. There has therefore been a powerful incentive to investigate the safety of MRI scanning in CRM device patients and to develop new devices that are safer in MRI scanners.

 

Safety in MRI scanning

Multiple small  in vitro studies of pacemaker behaviour during MRI scanning have been published since 1983,(3) with more clinical studies published since 1987.(4) The first reported case of inadvertent MRI scanning of a patient with an ICD was published in 2002,(5) with the first prospective series published in 2004.(6) Pauses, rapid pacing, increased pacing thresholds, decreased battery voltage, inappropriate sensing and power-on reset have all been observed, although no adverse event was seen in the majority of patients. The enormous variety of device and lead combinations, orientations, patient body shape, indications and programming make it impossible to draw truly evidence-based conclusions from multiple small series and attempts to do so have resulted in over-complicated and over-cautious protocols.

 

Guidelines and position statements have been published in the US(7) and by the European Society of Cardiology(8) which state that, on the basis of published data, ‘on a case-by-case basis’, the diagnostic benefit from MR imaging outweighs the presumed risks for some pacemaker and ICD patients’.

 

Larger series(9) concluded that MRI scanning was safe for patients with conventional CRM devices when these were programmed to an asynchronous pacing mode in pacemaker-dependent patients, to effectively disable pacing for others, and to disable all tachyarrhythmia functions. All patients required continuous monitoring of blood pressure, electrocardiogram and pulse oximetry, and symptoms were monitored by a nurse or physiologist with experience in cardiac life support and device programming, with immediate backup from an electrophysiologist. Three devices out of a total of 237 pacemakers (0.7%) reverted to a transient back-up programming mode without long-term effect. Small mean reductions were seen in sensing and lead impedances but no device revision or programming changes were required.

 

A large ongoing multi-centre registry (www.magnasafe.org/) is recruiting patients with thoracic devices implanted since 2001, which are not already labelled as ‘MRI-conditional’, where there is a ‘strong’ indication for non-thoracic MRI at 1.5T. Patients with other contraindications to MRI scanning (an abdominal diameter >60cm, an ICD and is pacing dependent, pregnancy, battery voltage at early response imaging or abandoned leads) are excluded. Preliminary results from the first 600 patients (447 with pacemakers, 153 with ICDs, 20% pacing- dependent) (www.magnasafe.org/images/AHA_Abract_2012.pdf) show no deaths, device failure, generator or lead replacement, loss of capture or ventricular arrhythmia. A clinically significant device parameter change has been seen in 13% of pacemakers and 31% of ICDs. This is the most robust estimate of risk available and should be weighed and discussed with individual patients in the decision whether to undertake MRI scanning. Recruitment is continuing and further publications are expected in 2014.

 

Small series of 3T MRI scanning in device patients have been published(10) without serious adverse events but experience is very limited compared to 1.5T MRI.

 

Abandoned leads are widely reported to be an absolute contraindication to MRI scanning(11) but the data on which this statement is based are theoretical and recent reviews suggest that ‘abandoned leads are no contraindication for magnetic resonance imaging (MRI)’.(12)

 

MRI-conditional pacemakers and ICDs

It would be very attractive to have pacemakers and ICDs specifically designed for MRI, which reduce or eliminate risks, allow scanning of all body regions and with all scanning protocols and simplify or eliminate the reprogramming required. This requires the complete redesign of all components and all major device manufacturers are developing devices. 

 

Modifications have included:

• redesign of leads to reduce heating in radiofrequency pulsed magnetic fields
• reduction in ferromagnetic components in the device
• changes in the device circuitry to reduce the risk of inappropriate pacing or power-on reset
• replacement of the magnetic reed switch with a Hall sensor.

All the major pacemaker  manufacturers now supply ‘MRI conditional’ devices and leads (Biotronik, Boston Scientific, Medtronic, St Jude Medical and Sorin). None of these systems is described as ‘MRI-‘safe’ and all have restrictions on scanning protocols and device programming that are very similar to published guidance for non-MRI compatible devices.

 

Studies of the safety of implantation and follow-up with MRI-compatible devices have been reassuring. A study of 464 patients implanted with MRI-compatible pacemakers showed no difference in sensing, capture threshold or impedance after non-thoracic 1.5T MRI.(13) The incidence of electrical reset and changes are lower than would have been expected in pacing systems not specifically designed for MRI compatibility, but no head-to-head trial has been published. This study was conducted with the Medtronic 5086 pacemaker leads designed for MRI compatibility. In May 2013, Medtronic announced CE marking for MRI scanning with its older 5076 lead launched in 2000. A recent report comparing the leads has shown superior stability, sensing and threshold characteristics of the 5076, emphasising the need to formally evaluate the advantages and disadvantages of new MRI-compatible technologies.(14)

 

ICDs are physically larger than pacemakers, potentially resulting in greater forces in the magnetic fields. They also have additional treatment algorithms and more complex high-voltage circuitry making behaviour less predictable. An additional concern is that a power-on reset could result in switching on tachycardia therapy and inappropriate shocks or anti-tachycardia because or interference from the scanner.

Recently, one manufacturer (Biotronik) has classified a number of its current devices as MRI conditional, including a range of ICDs. This is the first ICD system to receive CE approval for MRI scanning but significant exclusions remain, including field strengths and exclusion of thoracic scanning. 

 

Another manufacturer (Sorin) has designed a pacemaker which automatically reprogrammes itself to an MRI-compatible mode when exposed to a strong magnetic field.

 

Conclusions

MRI scanning is now the imaging modality of choice in multiple clinical situations and, in some of these, no other imaging can give the information required to guide medical treatment. Increasing numbers of patients around the world are benefiting from implantable cardiac rhythm devices and it is important that these patients are not unnecessarily excluded from the benefits of MRI.

 

Significant developments have been made in the understanding of device programming, scanning protocols and patient monitoring during MRI scanning over the past 20 years. In experienced centres with appropriate personnel, we are now able to scan the majority of device patients in the majority of clinical situations at low risk. The manufacturers are developing devices that are less affected by MRI to improve patient safety and reduce the complexity of the device reprogramming required. In the next decade, all new devices and leads will become MRI conditional and we will have much greater certainty about the risks associated with scans.

 

However, we must be mindful that every new technology carries its own uncertainty and risk – new devices cannot have long-term performance data by definition. Even when all new devices and leads are MRI conditional, we will still have to be vigilant not to miss patients with old leads, abandoned leads or old devices. Just because MRI scanning can be done safely in patients with CRM devices does not mean that it cannot be done unsafely. We must remain aware of the risks and weigh these carefully in decision making which must include the patient. National and international bodies need to reassess guidance in this field and frequent revisions will be required as more data accumulate.

 

This is an exciting time for MRI scanning in patients with CRM devices and practice is set to change around the world over the next five to ten years. 

 

References

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