Quantitative MRI studies for assessment of multiple sclerosis.

Although magnetic resonance imaging (MRI) is a valuable aid in the initial diagnosis of multiple sclerosis (MS), quantitatively MRI has been disappointing in staging and evaluating therapy protocols by means of serial examinations. In this study, image processing algorithms were developed for the global analysis of MR images of the cerebrum. Limited three-dimensional segmentation was achieved through histogram analysis by these algorithms, which are essentially operator independent. The effects of coil response and tip angles, patient positioning, and interslice gap thicknesses were examined for 10 MS patients with repeated examinations for a total of 72 images. Effects of technique and instrumentation errors were approximately 6%, and agreement between two independent operators for measuring the total MR pixel sum from periventricular effusions and intense MS plaques was better than 97% with a standard deviation of 2.9%.

Automatic methods for detection of tachyarrhythmias by antitachycardia devices.

Electrical devices play an increasingly important role in the control of tachyarrhythmias. Antitachycardia pacing and automatic defibrillation have been severely limited by the poor specificity of tachycardia discrimination in commercially available devices. Although absolute heart rate has been the principal means of automatic diagnosis, several new detection algorithms and methods are being investigated. Multiple electrode timing comparison, signal processing and pattern recognition are employed in these newer techniques. Although each offers some improvement over present technology, none is capable of identifying all arrhythmias. The methods employing comparison of atrial and ventricular rates, without additional criteria, are unable to detect ventricular tachycardia in the presence of 1:1 retrograde conduction. Electrographic analysis techniques require very stable electrodes and may not tolerate normal morphologic variations. A combination of two or more approaches may ultimately be required. All techniques will require that certain critical variables be programmable to allow for individualization in each clinical situation. Soft-ware-controllable devices and those capable of sensing from both the atria and the ventricles will provide the sophistication necessary for the implementation of complex tachycardia detection algorithms. This report reviews automatic tachycardia detection techniques in current use and under investigation.

Discrimination of antegrade and retrograde atrial depolarization by electrogram analysis.

The inability of cardiac pacemakers to selectively reject retrograde P waves limits the usefulness of dual-chamber pacemakers (because of the possibility of endless loop tachycardias) and of antitachycardia devices which use a dual-chamber sensing algorithm. In order to determine selective sensing parameters, amplitude, slew rate, and configuration of antegrade and retrograde atrial electrograms were measured in 34 patients undergoing dual-chamber pacemaker implant--31 with unipolar and three with bipolar units. All antegrade and retrograde pairs were measurably different. All 34 cases had measurable antegrade/retrograde amplitude differences; 30 of the unipolar cases (96.8%) and all bipolar cases displayed antegrade/retrograde amplitude differences of at least 0.25 mV. Thirty of the unipolar cases (96.8%) and two bipolar cases had measurable slew rate differences. Configuration differed in 14 of 31 (45.2%) of unipolar and in two bipolar cases. A combined criterion with 0.25 mV sensitivity steps (available in at least two presently available pacemakers) and 0.5 V/sec slew rate gradations (through the use of externally programmable filters) would allow the discrimination of retrograde from antegrade depolarizations in all 34 cases. With the use of amplitude and slew rate differences, it is therefore possible to reject retrograde P waves while sensing antegrade P waves with current technology.

Effect of magnetic resonance imaging on DDD pacemakers.

A previous study suggested the safety of exposing patients with certain pacemakers models to magnetic resonance imaging (MRI). However, the function of a variety of more advanced DDD pacemakers and the effect of higher magnetic and radio-frequency (rf) field strengths has not been reported. In the present study 4 different DDD pacemakers (Cordis 233F, Intermedic 283-01, Medtronic 7000A, and Pacesetter 283) were tested in a saline phantom under several conditions and with various imaging sequences. Pacemaker output was monitored using electrocardiographic telemetry. All units paced normally in the static magnetic field. However, during imaging, all units malfunctioned, with total inhibition of atrial and ventricular output in 3 of the pacemakers. In the fourth pacemaker, ventricular backup pacing was activated at high rf pulse repetition rates. However, the MRI scanner could trigger atrial output in this pacemaker at rates of up to 800/minute. All malfunctions were a result of rf interference, whereas gradient and static magnetic fields had no effect. Thus, despite magnetic fields had no effect. Thus, despite magnetic field strengths adequate to close pacemaker reed switches, rf interference during MRI may cause total inhibition of atrial and ventricular output in DDD pacemakers, and can also cause dangerous atrial pacing at high rates. MRI should be avoided in patients with these DDD pacemakers.

The role of implantable pacemakers in the therapy of tachycardias.

Antitachycardia electrical stimulation involves significant problems of sensing the cardiac rhythm and determining whether a tachycardia exists. The simple algorithms developed for bradycardia pacing are inadequate for tachycardia pacing. Electrogram signal analysis and cardiac activation analysis with at least two leads will be required to make a device diagnosis of a tachycardia. Termination of a tachycardia reliably and consistently is difficult and has only been accomplished uniformly by defibrillation. Further development of devices which are multimodal in termination capability is necessary. A registry for tachycardias and for tachycardia management is required to provide concentrated and analyzable data from a modality as yet infrequently used.

Unipolar and bipolar right atrial appendage electrodes: comparison of sensing characteristics.

To optimize atrial sensing and reject far-field signals (i.e., ventricular potentials seen in the atrium), the atrial amplitudes of electrograms (EGMs) should be as high as possible and the ventricular amplitudes as low as possible. To compare clinical sensing results obtained with unipolar and bipolar electrodes, endocardial EGMs were recorded on a high-speed multichannel recorder with a paper speed of 200 mm/second and frequency cutoffs at 0.1 and 2,000 Hz. Forty acute unipolar and 18 acute bipolar electrodes (in different patients), three matched pairs of unipolar and bipolar electrodes, and seven coronary sinus electrodes were measured. Unipolar and bipolar right atrial appendage (RAA) electrode EGMs were compared for 1) amplitude of EGM, 2) slew rate (dv/dt); and 3) amplitude of the ventricular EGM as measured through the RAA electrode. Unipolar RAA EGMs were compared with unipolar coronary sinus EGMs. Three bipolar leads were measured as unipolar and bipolar simultaneously. Bipolar atrial EGMs had equal amplitudes (unipolar, 4.2 +/- 2.1 mV, versus bipolar, 5.9 +/- 2.5; NS), higher slew rates (unipolar, 2.6 +/- 1.6 V/second, versus bipolar, 4.4 +/- 2.1; P less than 0.005), and lower ventricular (far-field) amplitudes (unipolar, 1.1 +/- 1.1 mV, versus bipolar, 0.7 +/- 0.6; P less than 0.02) when compared with unipolar RAA electrodes. This observation was confirmed in the measurement of the matched pairs atrial/ventricular amplitude ratio (unipolar, 4.7 +/- 2.2, versus bipolar, 8.7 +/- 2.0; P less than 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)

Implantation techniques of antitachycardia devices.

Implantation of a defibrillator may be by a mixed technique of transvenous leads and subcostal thoracotomy. An alternative approach is placement of all defibrillator leads and possible pacemaker leads via median sternotomy. All sensing leads, either for a defibrillator or a pacemaker, should be bipolar to provide maximal rejection of interference signals. Interpretation of the electrogram requires analysis of the pathologic electrogram to distinguish it from the "normal" electrogram for that patient and may require several leads to determine the activation sequence which will allow diagnosis of a tachycardia in a fashion similar to that performed by a human observer.

Output programmability and reduction of secondary intervention after pacemaker implantation.

Six hundred one new or initial pacemaker implants between Jan. 1, 1972, and Dec. 31, 1979, consisted of 289 output-programmable pulse generators and 312 fixed-output generators. Standard leads were used in all instances. Of the 289 output-programmable pulse generators, 4.5% (13) required revision for failure to capture the heart in the postimplant period; of the 312 fixed-output units. 9.6% (30) required revision (p = 0.015). In 41 patients the early threshold was above standard output (5 V. 10 mamp, 0.5 msec), and in six of them approximately 6 months were required to return to stable threshold levels below standard output. In the remainder, threshold rose above standard output and was managed by increased output of the output-programmable pulse generators, either briefly or permanently. We conclude that output programmability allows reduction in secondary intervention after implantation and that threshold evolution may occupy a period as long as 6 months.

Comparison for active and passive leads for endocardial pacing--II.

Four commercially available transvenous pacing leads were evaluated in a series of 240 implants on a rotational order basis. Total (intra- and post-operative) failure rates for this series were: Cordis 1 mm, 1/60 (1.7%); Biotronik IE-65-I, 5/60 (8.3%); Medtronic 6961, 4/60 (6.7%); and CPI 4116, 6/60 (10.0%). Post-operative failure rates were: 1 mm, 1/60 (1.7%); IE-65-I, 4/59 (6.8%); 6961, 0/56 (0.0%), and 4116, 3/57 (5.3%). In a previous study conducted at this center with identical protocol, total failure rates were: Medtronic 6907, 7/76 (9.2%); Cordis CL, 7/76 (9.2%); Biotronik IE-65-I, 2/76 (2.6%); and the Vitatron MIP-2000, 9/45 (20.0%). Post-operative failure rates were: 6907, 4/73 (5.5%); CL, 6/75 (8.0%); IE-65-I, 2/76 (2.6%); and MIP-2000, 5/41 (12.2%). If the two IE-65-I series are combined, the total failure rate is 7/136 (5.1%), and the post-operative failure rate is 6/135 (4.4%). Significant differences exist between the highest and lowest failure rates in total and post-operative cases for each series. The final decision concerning use of a particular lead must, of course, be based on clinical criteria.

Pacemaker and patient response to the "point of sale" terminal as an actual and simulated electromagnetic interference source.

In response to two instances of pacemaker patient complaints of symptomatic episodes in the presence of a "point-of-sale" terminal, comprehensive tests were performed for possible electromagnetic interference effects. Power line frequency and radar transmitter tests in the laboratory indicated that no significant problems exist for recently manufactured pacemakers in the everyday environment. Laboratory and clinical tests with 271 patients and 54 pacer models demonstrated no hazard from "point-of-sale" terminals. Patients showed a generally benign attitude toward potential interference and very low occurrence of random intermittent pacer malfunction or effects from myopotentials.

Comparison of active and passive adhering leads for endocardial pacing.

The frequency of lead failure requiring invasive correction in a total of 276 implants of four different transvenous leads (6907, continuous lead, IE-65-I, and MIP 2000) was observed during a one-and-one-half year period with a minimum of two months follow-up post-implant. Implants were on a successive sequential basis, randomly distributed between the two surgeons normally performing implants, and unselected for presumed ease or difficulty of the procedure. Failure rates with the 6907 and continuous leads were 7 of 76, or 9.2%; with the IE-65-I, 2 of 76, or 2.6%; and with the MIP 2000, 8 of 45, or 17.8%. The difference between the IE-65-I and the two conventional leads was significant at the 5% level, and between the IE-65-I and the group of the other three at the 1.6% level. The MIP 2000 was significantly different from the other three leads at the 2.7% level. Previous clinical experience with 849 implants with continuous and 6807 leads indicated that the overall data was similar to that obtained in the present evaluation. No significant differences in failure rates between surgeons and no measurable "practice effect" could be detected. It was concluded that the design of the lead is a major factor in the differing need for early secondary intervention.