LETTERS TO THE EDITOR
Reply to the letter of Luechinger et al.
Department of Biomedical Electronics
University Hospital
Friedrichstr. 18
Giessen 35435
Germany
Tel: +49 641 99 41390
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E-mail address: werner{at}irni.ch
Luechinger et al., criticizing our article on ‘Do we need pacemakers resistant to magnetic resonance imaging?’,1
reproach us for underestimating the risk of (i) inadvertent changes in pacing mode, (ii) induced voltages due to gradient fields, and (iii) the potential heating effects around the lead tip. Our answers:
- Mode changes are exclusively caused by reed switches, which are unreliable in today's pacemakers as was found by Luechinger et al.2 If the ‘magnet mode’ can reliably be programmed, what we propose for synchronized scanning procedure, there is no mode change to be expected.
- Pacemakers generally possess a capacitance at the output stage of the order of 1–10 nF. Such a low capacitance is necessary to keep the outgoing pulse undistorted. This yields, in combination with a 10 k
input resistance, a cut-off frequency of 1.6–16 kHz, combined with a reduction in impedance of 70%. This modifies our calculation of voltage distribution only marginally in our paper. Rectification takes place above diode voltage of 0.5 V, which is hardly possible by induction of gradient fields.
- It is true that thermal behaviour around the lead tip is a complex physical system. In the immediate distance to the electrode surface, thermal conduction is the predominant cooling effect. In a distance of about twice the electrode radius, additional cooling by perfusion takes place, which is massive due to the blood stream within the heart.3 In any case, the measured temperature at the electrode surface is surely not that of excitable tissue distant from the electrode. The results of Martin et al.4 prove that 70% of thresholds remained unchanged or even become lower after MRI. They concluded, ‘threshold changes were unrelated to the peak whole-body-averaged specific absorption rates (SARs) associated with the MRI procedure for the patients in the present study’. ‘Despite the labelling of these changes as significant, they were of no clinical consequence. Changes of these magnitudes are commonly observed in daily practice of cardiology’.
If Luechinger et al. postulate in their letter to the editor that ‘an MRI-mode should have no sensing function to avoid inappropriate pacing because of the induced noise during MRI scanning’, they propagate fixed-rate pacing, thereby underestimating the risk of competitive rhythms, which is, in our opinion, the most serious risk in MRI scanning of pacemaker patients.
Our proposal of ECG-triggered scanning indeed carries the disadvantage of prolonging the MRI procedure. This is the price of having a safer MRI procedure that offers the possibility of also examining pacemaker-dependent patients who have been excluded in all studies published so far. If there are rare scanning protocols not compatible with our triggered scanning method, this must be tolerated, but is not an argument against our proposed procedure.
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[1] Irnich W, Irnich B, Bartsch C, Stertmann WA, Gufler H, Weiler G. Do we need pacemakers resistant to magnetic resonance imaging? Europace 2005; 7: 353–65.
[2] Luechinger R, Duru F, Zeijlemaker VA, Scheidegger MB, Boesiger P, Candinas R. Pacemaker reed switch behaviour in 0.5, 1.5, and 3.0 Tesla magnetic resonance imaging units: are reed switches always closed in strong magnetic fields? Pacing Clin Electrophysiol 2002; 25: 1419–23.[CrossRef][Medline]
[3] Tobisch R. Auswirkung elektromagnetischer Felder auf Herzschrittmacherelektroden 1996; Aachen; Shaker University Giessen Thesis ISBN 3-8265-1351-7.
[4] Martin ET, Coman JA, Shellock FG, Pulling CC, Fair R, Jenkins K. Magnetic resonance imaging and cardiac pacemaker safety at 1.5-Tesla. J Am Coll Cordiol 2004; 43: 1315–24.
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