LETTERS TO THE EDITOR
Reply to the letter of Naehle et al.
Department of Biomedical Electronics
University Hospital
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E-mail address: werner{at}irni.ch
Naehle et al. want to give ‘substantial clarification’ in their letter to the editor regarding our article on ‘Do we need pacemakers resistant to magnetic resonance imaging?’1
Instead of clarifying, they repeat ‘previously published experience’, which is, in our opinion, not proved by hard data, but rather delivered since the beginning of the discussion on pacemakers and MRI. Pacemaker technology teaches that repetitive signals with a rate of higher than 300 ppm cause all pacemakers to switch to their asynchronous ‘interference mode’. The attached figure of Naehle et al. (without any description of how and which model was tested) is no evidence against it, as it shows most probably the reaction of an implantable DDD defibrillator in which noisy signals of about 500 ppm cause it to switch to the interference-mode ‘inhibition’, which is typical for implantable cardioverter-defibrillators (ICDs). But we did not discuss ICD behaviour in our paper. We still believe the statement to be correct that prolonged inhibition is unlikely to occur in MRI examinations of pacemaker patients unless proof is given of the condition that ‘the scan duration Tscan is smaller than the refractory period Tref followed by pauses so that Tscan period larger than Tref really exists. Nevertheless, it is an important item that pacemaker-dependent ICD patients need special attention as they may suffer from prolonged inhibition.
Our calculations of power density around ball-shaped electrodes as a function of radius are correct in any case, though this is only one of the several parameters in heat balance (see point 3 of the reply to Luechinger et al.). Temporary loss of capture for 12 h, as reported by Roguin et al.2 in one animal, must not conclusively be related to thermal destruction. Tissue heated to 50°C or more is irreversibly damaged by denaturization. Why should the threshold return to normal hours later if excitable tissue around the electrode is denaturized by heat? The authors interpret this case as ‘some oedema occurred at the lead tip-tissue interface, which subsequently resolved’. Are oedema and threshold elevations of screw-in leads only to be expected when RF fields are present?
Martin et al.3 did not specify the accuracy of their threshold measurements. Threshold changes of 9.4% can also be explained by a lack in reproducibility of threshold measurements with implanted pacemakers. It is scientifically dubious to claim something (alterations at the lead tip) and then demand that this claim be considered correct unless disproved otherwise (see also the remarks to point 3 in our reply to Luechinger et al.).
We omitted to say in our paper that the pacemaker status before and after MRI examination was always identical. Our report on only 8 patients with 12 examinations should demonstrate the feasibility of triggered gating of the MRI scan, which was successfully performed in all cases; all pacemakers functioned in their synchronous mode. Thus, if asynchronous pacing and inhibition can be excluded and heat production is reduced to 40% or less by our proposed triggering procedure, then avoiding these risks increases safety to a large degree, in our opinion. The authors are, however, invited to use this technique to augment the number of cases. This is more necessary as they have published a study with only 51 patients from which they concluded that non-pacemaker-dependent patient can safely be examined in a 0.5 T MRI device.4 This statement must be questioned because three of our six reported patients who died during MRI examination were non-pacemaker dependent and in a 0.5 T device.
We have taken other published data into account as our reference list proves. If we contradict published opinions, which we deem to be physically incorrect, will it be unusual or even impertinent? We think it is science.
Of course, ‘to ensure patient safety and to minimize the risk of medicolegal consequences’ is a goal that we fully support. However, to deny about 44% of all pacemaker patients5 a possible life-saving or life-improving MRI examination only because they are pacemaker dependent is absolutely intolerable, as it is based on the false assumption that pacemakers could massively be inhibited during MRI procedures.
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[1] Irnich W, Irnich B, Bartsch C, et al. Do we need pacemakers resistant to magnetic resonance imaging? Europace 2005; 7: 353–65.
[2] Roguin A, Zviman MM, Meininger GR, et al. Modern pacemaker and implantable cardioverter/defibrillator systems can be magnetic resonance imaging safe: in vitro and in vivo assessment of safety and function at 1.5 T. Circulation 2004; 110: 475–82.
[3] Martin ET, Coman JA, Shellock JA, et al. Magnetic resonance imaging and cardiac pacemaker safety at 1.5-Tesla. J Am Coll Cardiol 2004; 43: 1315–24.
[4] Sommer T, Vahlhaus Ch, Lauck G, et al. MR imaging and cardiac pacemakers: in-vitro evaluation and in-vivo studies in 51 patients at 0.5 T. Radiology 2000; 215: 869–79.
[5] Irnich W, Stertmann WA, Batz L. Jahresbericht 1999 des Deutschen Zentralregisters Herzschrittmacher. Herzschrittmacher 2000; 20: 390–401.
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