Pacing Capture Threshold Research Articles

Safe magnetic resonance image scanning of the pacemaker patient: current technologies and future directions

Magnetic resonance imaging (MRI) is the imaging modality of choice in many clinical situations, and its use is likely to grow due to expanding indications and an ageing population. Many patients with implantable devices are denied MRI except in cases of urgent need, and when scans must be performed they are complicated by the need for burdensome and costly personnel and monitoring requirements that have the net effect of restricting access to scans. Several small studies, enrolling a total of 344 patients, suggest that some patients with conventional systems may undergo MR examinations without clinically overt adverse events. However, a number of potential interactions exist between implantable cardiac devices and the static and gradient magnetic fields and modulated radio frequency (RF) fields generated during MR scans; nearly all studies have reported pacing capture threshold changes, troponin elevations, ectopy, unpredictable reed switch behaviour, and other 'subclinical' issues with pacemakers and implantable cardioverter-defibrillators (ICDs) in patients who have undergone MRI. Attention has turned to devices that are specifically designed to be safe in the MRI environment. A clinical study of one such device documented its ability to be exposed to MRI in a 1.5 T scanner without adverse impact on patient outcomes or pacemaker system function. Such new technologies may enable scanning of pacemaker and ICD patients with reduced concerns regarding the short- and long-term effects of MRI. As importantly, these devices may increase the number of centres that are able to safely perform MRI and, thus, expand access to scans for patients with these devices.

MRI and cardiac pacing devices — beware the rules are changing

We have been following the development of MRI-compatible cardiac permanent pacemakers (PPM) with great interest. Recently, we have been asked to perform clinical MRI studies in patients with a MRI-compatible PPM. We would like to share our experience in performing one such study and highlight important safety measures that radiologists should undertake prior to imaging. Conventional PPM has always been regarded as an absolute contraindication for MRI. This has prevented a large number of patients from undergoing clinically important MRI studies. MRI-compatible PPM is now available for clinical use following a prospective randomised controlled unblinded multicenter study, involving 464 patients [1,2]. Our first patient had an Advisa DR MRI SureScan (Medtronic, Minneapolis, MN) system, which is one of the most widely used MRI-compatible systems. These devices have been designed to minimise thermal damage and limit induced voltages preventing unintended stimulation of the heart. Performing an MRI study in these patients requires robust preparation and liaison between physicist, radiographer, electrophysiology technicians, consultant radiologist and referring clinician. Informed consent should be obtained from the patient after discussing the benefits of the MRI study and potential complications. The following radiology specific pre-requisites should be fulfilled and adhered to and these may change depending on the manufacturer of the device: Cylindrical bore, clinical MR systems with a static magnetic field of 1.5 T. Gradient systems with maximum gradient slew rate performance per axis of ≤200 T m−1per second. Whole body averaged specific absorption rate (SAR) must be ≤2 W kg−1 and for head <3.2 W kg−1. Implant must consist of an MRI-compatible device as well as the lead. Any other leads or broken leads remain a contraindication. The pacing system should be implanted in either the right or left pectoral region and should have been in place for more than 6 weeks. The patient should not be positioned on his or her side within the scanner. Local transmit/receive coils should not be placed over the pacing system. Prior to taking the patient into the scanner the PPM is programmed to MRI safe mode by electrophysiologists outside the MR safety zone. Once in the scanner, continuous monitoring using electrocardiography (ECG), pulse oximetry and blood pressure is essential. External defibrillator must be readily available and the procedure should be abandoned if patient's haemodynamic function is compromised. After completing the scan, the PPM is turned back on to normal mode and the correct pacing capture threshold is ensured prior to discharging the patient. Our patient was a young man who underwent a cardiac MRI study for cardiomyopathy. He was involved in discussions regarding the safety of the procedure and consented prior to the study. He was extremely anxious when he went into the scanner, but relaxed as the procedure went on. He did not experience any untoward sensations or arrhythmia during the 40 min study. Standard TrueFISP (true fast imaging with steady state precession) images were degraded owing to artefacts from the leads; this was rectified by switching over to FLASH (fast low angle shot) cine images (Figure 1). Overall the image quality was good. Figure 1 Cardiac MRI in standard planes in (a) TrueFISP (true fast imaging with steady state precession) and (b–d) FLASH (fast low angle shot) sequences. Note the clear delineation of the pacing leads (arrow) on the latter sequence with no significant ... We foresee that more patients will undergo MRI scanning with compatible cardiac pacing systems. Currently there is limited evidence regarding its safety and long-term effects. We, therefore, profess extreme prudence and a multidisciplinary approach prior to scanning a patient with an MRI-compatible pacemaker.

Safety of serial MRI in patients with implantable cardioverter defibrillators

ObjectiveWhile patients with cardiac implantable electronic devices could benefit from magnetic resonance (MR) imaging, the presence of such devices has been designated as an absolute contraindication to MR. Although scanning...

Safety, feasibility, and diagnostic value of cardiac magnetic resonance imaging in patients with cardiac pacemakers and implantable cardioverters/defibrillators at 1.5 T

Recent studies suggest that magnetic resonance (MR) imaging of the brain and spine may safely be performed in patients with pacemakers (PMs) and implantable cardioverter/defibrillators (ICDs), when taking adequate precautions. The aim of this study was to investigate safety, feasibility, and diagnostic value (DV) of MR imaging in cardiac applications (cardiac MR [CMR]) in patients with PMs and ICDs for the first time. Thirty-two PM/ICD patients with a clinical need for CMR were examined. The specific absorption rate was limited to 1.5 W/kg. Devices were reprogrammed pre-CMR to minimize interference with the electromagnetic fields. Devices were interrogated pre-CMR and post-CMR and after 3 months. Troponin I levels were measured pre-CMR and post-CMR; image quality (IQ) and DV of CMR were assessed. All devices could be reprogrammed normally post-CMR. No significant changes of pacing capture threshold, lead impedance, and troponin I were observed. Image quality in patients with right-sided devices (RSD) was better compared with that in patients with left-sided devices (LSD) (P < .05), and less myocardial segments were affected by device-related artefacts (P < .05). Diagnostic value was rated as sufficiently high, allowing for diagnosis, or better in 12 (100%) of 12 patients with RSD, and only in 7 (35%) of 20 patients with LSD. Cardiac MR may be performed safely when limiting specific absorption rate, appropriately monitoring patients, and following device reprogramming. Cardiac MR delivers good IQ and DV in patients with RSD. Cardiac MR in patients with RSD may therefore be performed with an acceptable risk/benefit ratio, whereas the risk/benefit ratio is rather unfavorable in patients with LSD.

Magnetic resonance imaging in patients with a pacemaker system designed for the magnetic resonance environment

Magnetic resonance imaging (MRI) of pacemaker patients is contraindicated due to documented potential risks to the patient from hazardous interactions between the MRI and pacemaker system. The purpose of this prospective, randomized, controlled, worldwide clinical trial was to evaluate the safety and effectiveness of a pacemaker system designed for safe use in MRI for any bradycardia indicated patient. Patients (n = 464) were randomized to undergo an MRI scan between 9 and 12 weeks postimplant (MRI group, n = 258) or not to undergo MRI (control group, n = 206) after successful implantation of the specially designed dual-chamber pacemaker and leads. Patients were monitored for arrhythmias, symptoms, and pacemaker system function during 14 nonclinically indicated relevant brain and lumbar MRI sequences. Sequences were performed at 1.5 T and included scans with high radiofrequency power deposition and/or high gradient dB/dt exposure. Clinical evaluation of the pacemaker system function occurred immediately before and after MRI, 1 week and 1 month post-MRI, and at corresponding times for the control group. Primary endpoints for safety analyzed the MRI procedure complication-free rate and for effectiveness compared capture and sensing performance between MRI and control groups. No MRI-related complications occurred during or after MRI, including sustained ventricular arrhythmias, pacemaker inhibition or output failures, electrical resets, or other pacemaker malfunctions. Pacing capture threshold and sensed electrogram amplitude changes were minimal and similar between study groups. This trial documented the ability of this pacemaker system to be exposed in a controlled fashion to MRI in a 1.5 T scanner without adverse impact on patient outcomes or pacemaker system function.

Low-field magnetic resonance imaging: increased safety for pacemaker patients?

The number of low-field (<0.5 T) magnetic resonance (MR) scanners installed worldwide is increasing due to a favourable cost and safety profile and improved patient comfort using an open-scanner design. Therefore, the aim of our study was to evaluate a strategy for the safe performance of magnetic resonance imaging (MRI) at a field strength of 0.2 T, in pacemaker (PM) patients without limitations on scan region, PM dependency, or the presence of abandoned leads. One hundred and fourteen PM patients, including PM-dependent patients and patients with abandoned leads, examined at a 0.2 T MR scanner due to an urgent clinical need for an MRI examination, were evaluated. All PMs were reprogrammed before MRI: if heart rate was <60 bpm, the asynchronous mode (with a rate of 80 bpm) was programmed to avoid MR-induced inhibition; if heart rate was >60 bpm, sense-only mode (ODO/OVO/OAO) was used to avoid MR-induced competitive pacing and potential proarrhythmia. Patients were monitored with electrocardiogram (ECG) and pulse oximetry. All PMs were interrogated before and after MRI, including measurement of lead impedance, pacing capture threshold (PCT), and battery voltage. All MRI scans were completed safely. No induction of arrhythmias or inhibition of PM output occurred. There were no statistically significant changes in lead impedance, PCT, or battery voltage (P>0.05). Low-field MRI of PM patients, including high-risk PM patients and MRI scan regions, can be performed with an acceptable risk-benefit ratio under controlled conditions.

The Effects of Temperature on Cardiac Pacing Thresholds

Human core body temperature can fluctuate between 36 degrees C (sleep) and 42 degrees C (intense exercise). Also, efforts are underway to develop implantable pacing systems that minimize heating during magnetic resonance imaging (MRI) scans (i.e., MRI safe). Concerns exist that ventricular pacing capture thresholds (VPCT) are modified by changing cardiac temperatures. This project was designed to assess the effects of temperature on VPCT of the mammalian heart. Fresh ventricular specimens were obtained from healthy canine, healthy swine, and diseased human hearts. Isolated trabeculae were suspended in temperature-controlled tissue baths containing oxygenated Krebs buffer. Small active fixation pacing leads delivered pacing pulses to each specimen. Baseline strength-duration curves were determined at 37 degrees C, then at randomized temperatures ranging from 35 degrees C to 42 degrees C. Final thresholds were repeated at 37 degrees C to confirm baseline responses. All threshold data were normalized to a baseline average. Both canine and swine trabeculae elicited significant decreases in thresholds (10-14%) at pacing stimulus durations (pulsewidths) of 0.02 ms (P < 0.01) and 0.10 ms (P < 0.05) between the temperatures of 38 degrees C and 41 degrees C, compared to baseline. Thresholds at 42 degrees C trended back to baseline for both canine (NS) and swine trabeculae (P < 0.05 compared to 38 degrees C-41 degrees C). Human trabeculae thresholds increased >35% (P < 0.05) at 42 degrees C relative to baseline with no significant differences at other temperatures. Temperature is a significant factor on pacing thresholds for mammalian ventricular myocardium. Our data for the diseased human trabeculae indicate that cases where cardiac heating may occur (e.g., radiofrequency energy due to MRI scans, febrile events), patients without adequate VPCT safety margin may be at higher risk of loss of proper function of an implanted pacing or defibrillation system.

Spontaneous pericardial hematoma with familial amyloid polyneuropathy

There are more than a few risks of hemorrhage complication in patients with amyloidosis. Although most cases with amyloidosis exhibit minor bleeding manifestations, they can be occasionally associated with life-threatening problems. To our knowledge, there are only a few cases of spontaneous pericardial hematoma associated with amyloidosis. We here report a patient who suddenly died of cardiac tamponade with massive pericardial hematoma 7 years after the diagnosis of familial amyloid polyneuropathy (FAP). A 69-year-old female with FAP with cardiogenic shock was admitted to our emergency room. Although she previously underwent permanent pacemaker implantation for atrial fibrillation with slow ventricular response, electrocardiogram showed a critical pacing failure. Emergent telemetry check revealed a sudden extreme increase of pacing capture threshold in the right ventricle. Maximum pacing voltage could not improve the critical condition, and she died 7 h after arrival. Autopsy showed a massive pericardial hematoma in the right ventricular free wall, and microscopic examination revealed typical amyloid deposition in the arterial wall of the pericardium. In this case, it is assumed that a sudden rupture of fragile pericardial vessels with amyloid deposition led to the lethal pericardial hematoma.

Evaluation of Cumulative Effects of MR Imaging on Pacemaker Systems at 1.5 Tesla

The purpose of this study was to evaluate possible cumulative effects of repeated magnetic resonance imaging (MRI) examinations on pacemaker systems in patients with cardiac pacemakers. The records of pacemaker patients who underwent repetitive MRI examinations in our institution were reviewed to identify patients who underwent two or more MRI examinations at 1.5T of any anatomical region. Using these criteria, a total of 47 patients who underwent a total 171 MRI examinations were identified and included in this study. Institutional Review Board approval for all pacemaker investigations was obtained. Written informed consent was obtained from all patients. Pacemakers were interrogated immediately before and after MR imaging, and after 3 months, including measurement of pacing capture threshold (PCT), lead impedance (LI), and battery voltage (BV). PCT, LI, and BV were analyzed for changes dependant on the number of MRI exams performed. Mean changes over time and changes between first and last pacemaker interrogation of PCT, LI, and BV were calculated. A statistically significant (P < 0.05), but clinically irrelevant trend for decrease in PCT and BV was found. No significant or clinically relevant changes in LI were observed. In this first study, no clinically relevant, cumulative changes in PCT, LI, or BV could be detected in PM patients who underwent two or more MRI examinations. However, a careful benefit/risk evaluation, among other MRI- and pacemaker-related safety precautions, remains mandatory, as clinically relevant alterations to the PM system cannot be excluded by all means.

Magnetic Resonance Imaging at 1.5-T in Patients With Implantable Cardioverter-Defibrillators

Our aim was to establish and evaluate a strategy for safe performance of magnetic resonance imaging (MRI) at 1.5-T in patients with implantable cardioverter-defibrillators (ICDs). Expanding indications for ICD placement and MRI becoming the imaging modality of choice for many indications has created a growing demand for MRI in ICD patients, which is still considered an absolute contraindication. Non-pacemaker-dependent ICD patients with a clinical need for MRI were included in the study. To minimize radiofrequency-related lead heating, the specific absorption rate was limited to 2 W/kg. ICDs were reprogrammed pre-MRI to avoid competitive pacing and potential pro-arrhythmia: 1) the lower rate limit was programmed as low as reasonably achievable; and 2) arrhythmia detection was programmed on, but therapy delivery was programmed off. Patients were monitored using electrocardiography and pulse oximetry. All ICDs were interrogated before and after the MRI examination and after 3 months, including measurement of pacing capture threshold, lead impedance, battery voltage, and serum troponin I. Eighteen ICD patients underwent a total of 18 MRI examinations at 1.5-T; all examinations were completed safely. All ICDs could be interrogated and reprogrammed normally post-MRI. No significant changes of pacing capture threshold, lead impedance, and serum troponin I were observed. Battery voltage decreased significantly from pre- to post-MRI. In 2 MRI examinations, oversensing of radiofrequency noise as ventricular fibrillation occurred. However, no attempt at therapy delivery was made. MRI of non-pacemaker-dependent ICD patients can be performed with an acceptable risk/benefit ratio under controlled conditions by taking both MRI- and pacemaker-related precautions. (Implantable Cardioverter Defibrillators and Magnetic Resonance Imaging of the Heart at 1.5-Tesla; NCT00356239).

Safety of Brain 3-T MR Imaging with Transmit-Receive Head Coil in Patients with Cardiac Pacemakers: Pilot Prospective Study with 51 Examinations

To evaluate the safety and feasibility of 3-T magnetic resonance (MR) imaging of the brain in patients with implanted cardiac pacemakers (PMs) by using a transmit-receive head coil. The study protocol was approved by the institutional review board. Signed informed consent was obtained from all subjects. In vitro testing at 3 T was performed with 32 PMs and 45 PM leads that were evaluated for force and torque (by using a floating platform) and radiofrequency (RF)-related heating by using a transmit-receive head coil (maximum specific absorption rate, 3.2 W/kg). Patient examinations at 3 T were performed in 44 patients with a cardiac PM and a strong clinical need; patients underwent a total of 51 MR examinations of the brain by using a transmit-receive head coil to minimize RF exposure of the PM system. An electrocardiograph and pulse oximetry were used for continuous monitoring during MR imaging. The technical and functional PM status was assessed prior to and immediately after MR imaging and at 3 months thereafter. Serum troponin I level was measured before and 12 hours after imaging to detect myocardial thermal injury. PM reprogramming was performed prior to MR imaging depending on the patient's intrinsic heart rate (< 60 beats per minute, asynchronous pacing; > or = 60 beats per minute, sense-only mode). For in vitro testing, the maximum translational force was 2150 mN (mean, 374.38 mN +/- 392.75 [standard deviation]), and maximum torque was 17.8 x 10(-3) N x m (mean, [2.29 +/- 4.08] x 10(-3) N x m). The maximum temperature increase was 2.98 degrees C (mean, 0.16 degrees C +/- 0.45). For patient examinations, all MR examinations (51 of 51) were completed safely. There were no significant (P < .05) changes in lead impedance, pacing capture threshold level, or serum troponin I level. MR imaging of the brain at 3 T in patients with a cardiac PM can be performed safely when dedicated safety precautions (including the use of a transmit-receive head coil) are taken.

Strategy for Safe Performance of Extrathoracic Magnetic Resonance Imaging at 1.5 Tesla in the Presence of Cardiac Pacemakers in Non–Pacemaker-Dependent Patients

The purpose of the present study was to evaluate a strategy for safe performance of extrathoracic magnetic resonance imaging (MRI) in non-pacemaker-dependent patients with cardiac pacemakers. Inclusion criteria were presence of a cardiac pacemaker and urgent clinical need for an MRI examination. Pacemaker-dependent patients and those requiring examinations of the thoracic region were excluded. The study group consisted of 82 pacemaker patients who underwent a total of 115 MRI examinations at 1.5T. To minimize radiofrequency-related lead heating, the specific absorption rate was limited to 1.5 W/kg. All pacemakers were reprogrammed before MRI: If heart rate was <60 bpm, the asynchronous mode was programmed to avoid magnetic resonance (MR)-induced inhibition; if heart rate was >60 bpm, sense-only mode was used to avoid MR-induced competitive pacing and potential proarrhythmia. Patients were monitored with ECG and pulse oximetry. All pacemakers were interrogated immediately before and after the MRI examination and after 3 months, including measurement of pacing capture threshold (PCT) and serum troponin I levels. All MR examinations were completed safely. Inhibition of pacemaker output or induction of arrhythmias was not observed. PCT increased significantly from pre- to post-MRI (P=0.017). In 2 of 195 leads, an increase in PCT was only detected at follow-up. In 4 of 114 examinations, troponin increased from a normal baseline value to above normal after MRI, and in 1 case (troponin pre-MRI 0.02 ng/mL, post-MRI 0.16 ng/mL), this increase was associated with a significant increase in PCT. Extrathoracic MRI of non-pacemaker-dependent patients can be performed with an acceptable risk-benefit ratio under controlled conditions and by taking both MR- and pacemaker-related precautions.