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PubMed · 8123767

[Magnetomyography with the SQUID].

M Reincke
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Abstract

The results of investigations of the neuromuscular system in man using a SQUID magnetometer with a second-order gradiometer detector are presented. After electrical stimulation of the tibial nerve in the hollow of the knee and the triggering of a monosynaptic proprioceptic reflex (Hoffmann-(H-)reflex), the biomagnetic field was recorded from the dorsal surface of the lower leg (soleus muscle of the m. triceps surae). The results obtained indicate that, over and beyond the data provided by electromyography, diagnostic information is provided on the state of the neuronal reflex pathways including muscle effector function. The biomagnetic field was evaluated in terms of latency, intensity, composition, characteristic behaviour and site of generation of the signal. In the geometry used, this noninvasive measuring technique can be employed for office observation of the spread of excitation through the muscle, as demonstrated by the response obtained following supramaximal stimulation of the supply nerve.

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Electric StimulationElectromagnetic FieldsElectromyographyFourier AnalysisH-ReflexHumansMicrocomputersMusclesNeuromuscular JunctionReference ValuesSignal Processing, Computer-AssistedTibial NerveEnglish AbstractJournal Article

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M Reincke. 1993. [Magnetomyography with the SQUID].. https://doi.org/10.1515/bmte.1993.38.11.276

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PubMed1999

Cardiovascular collapse caused by electrocardiographically silent 60-Hz intracardiac leakage current. Implications for electrical safety.

BACKGROUND: The national standard for safe 60-Hz intracardiac leakage current under a single-fault condition is 50 microA. This standard is intended to protect patients from alternating current (AC) at levels below the threshold for sensation, but the minimum unsafe level for AC in closed-chest humans is not known. To determine this value, we studied 40 patients at testing of implantable cardioverter-defibrillators using a programmable source of 60-Hz AC. METHODS AND RESULTS: We applied AC for 5-second test periods in increasing strengths until ventricular fibrillation (VF) was induced or 1 mA was reached. Two current paths were tested: bipolar, between tip and ring electrodes of a right ventricular pacing catheter, and unipolar, from tip to a remote electrode. We observed a characteristic sequence of 3 responses as AC was increased: (1) intermittent ventricular capture with QRS morphology identical to pacing through the electrodes (minimum value, 20 microA); (2) continuous capture at cycle length 282+/-88 ms (minimum value, 32 microA); and (3) VF persisting after AC termination (minimum value, 49 microA). Continuous capture caused loss of pulsatile arterial pressure and cardiovascular collapse (mean arterial pressure, 32+/-8 mm Hg) for the duration of AC with no ECG evidence of AC stimulation. Thus, the clinical picture was that of hypotensive ventricular tachycardia (VT). The continuous-capture threshold was </=50 microA in 9 patients (22%) for bipolar AC and in 5 (12%) for unipolar AC. All patients showed continuous capture over a wide range for both bipolar AC (68+/-18 to 216+/-238 microA) and unipolar AC (84+/-27 to 278+/-226 microA). CONCLUSIONS: Leakage current causes cardiovascular collapse at levels below the VF threshold. Stimulation by silent AC that is neither felt nor visible on the ECG presents as hypotensive VT. In patients with intracardiac electrodes, leakage current less than or equal to the present standard of 50 microA may cause VT or VF. The safety standard for leakage current lasting >/=5 seconds should be </=20 microA. This standard should be based on the continuous-capture threshold.

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