Alternativas de reforzamiento

In addition to the device failures highlighted by the Bilitch reports, periodic advisory or warning notices of device malfunction are also published by STIMAREC based in

France (Godin et ai, 1992). There have been 317 such reports in the past 5 years, of which over half were considered to be dangerous. The size of the population at risk from which these cases are drawn, however, is not known. Thus, although the data may serve to draw attention to emerging patterns of failure, they are of limited value in the numerical assessment of risk. From time to time manufacturers, distributors and other responsible parties also issue Medical Device Safety Alerts voluntarily or Medical Device Notifications at the request of the Food and Drug Administration (FDA) in the USA. Formal reporting systems also exist in a number o f European countries, including the issue of Hazard Notices in the UK. Tyers (1989) has reviewed FDA device recalls during a 13 year period from 1974 to 1987. Almost 250,000 devices were recalled during the period with considerable variation between different years and for different manufacturers. There was no evidence of improvement with time, with 40% o f the recalls during the 13 year period occurring in the last 30 months o f the study.

EXTRANEOUS CAUSES O F PACING SYSTEM MALFUNCTION:

Apart from the propensity to failure due to component malfunction, inherent in any electronic device, there may be special risks of failure due to exposure to physical elements or hostile environments. For the most part, these are unlikely to affect a pacemaker in situations when they would not, in any event, threaten the user. Thus, extremes of heat and humidity, for example, pose little threat. Direct mechanical trauma may damage the pacemaker or cause lead displacement or fracture and pacemaker users are generally advised to refrain" from contact sports or activities associated with a risk of direct impact or excessive movement at the site o f lead entry to the venous system (Hayes, 1989(c)). Discomfort from direct pressure has been described in association with the wearing o f seat belts in motor cars (Wallis et al,

1985). This is usually only mild but may affect as many as 28% of users. Susceptibility may be influenced by the amount of subcutaneous tissue, the pacemaker location and whether the user is in the driving seat or the passenger seat. Similar problems might be anticipated in aircraft particularly when using a shoulder harness. Pacemaker safety standards (BSI, 1990) require the devices to be unaffected by mechanical jolts and vibration but the latter may nonetheless cause inappropriate rate rises in activity-sensing rate-adaptive pacemakers as discussed above. This is considered further in the studies described in chapter 7. The principal extraneous threat to normal pacemaker function

is posed by electromagnetic interference which may arise either from endogenous sources, in the form of skeletal muscle myopotentials or from external electromagnetic radiation. The latter has the capacity to disturb the function of the pacing system at levels o f intensity below those that would otherwise affect the user and is therefore of particular importance.

ENDOGENOUS ELECTROM AGNETIC INTERFERENCE:

All demand pacemakers with a sensing function are potentially susceptible to interference from the effects of electrical impulses other than those intended to be sensed. In attempting to detect the R wave in the intracardiac electrogram, the sensing mechanism may be erroneously activated by the P wave, the T wave or by skeletal muscle myopotentials. The T wave spectral content is predominantly below 3Hz and it can be adequately distinguished from the P and R waves by appropriate filters (Kleinert et al, 1979) but myopotentials have spectral densities and amplitudes that overlap with those of the P and R waves and band-pass filters can only partially resolve the problem. The unipolar intracardiac electrogram has frequency components in the range 20-100Hz and myopotential signal frequency components have been reported in the range 30- 200Hz (Watson, 1985).

Inhibition of demand pacemakers by skeletal muscle myopotentials was first described by Wirtzfeld (1972) and remains a problem in unipolar systems today (Gross et al,

1992). The phenomenon may occur in as many as 50% o f patients and is only rarely recognised by the sensing circuitry with reversion to the interference mode. The majority o f patients are asymptomatic but symptoms are not uncommon and may include syncope (Wirtzfeld et al, 1972; Mymin et al, 1973; Barold, 1974). Problems may also arise in dual chamber systems, in which myopotential sensing in the atrial channel may result in rapid ventricular pacing with loss of AV synchrony, retrograde conduction and pacemaker-mediated tachycardia (Rozonski et al, 1983). Myopotential inhibition can be almost completely eliminated by bipolar pacing (Fetter et al, 1985; Gabry et al, 1987; Furman, 1989(e); Hayes, 1989) although inhibition of a ventricular- inhibited bipolar demand pacemaker by skeletal muscle activity has been reported (Widansky & Zipes, 1974)

EXOGENOUS ELECTROM AGNETIC INTERFERENCE:

The potential hazard of electromagnetic interference (EMI) is the principal subject of the experimental work described in this thesis. EMI may affect pacemakers in a number of ways. First, it may directly damage the components within the device, although the risk o f this is attenuated to some extent by the interposition of the subcutaneous tissue and the pacemaker casing. The risks of direct damage are most marked with high energy sources such as radiation therapy (Marbach et al, 1978; Adamec et al, 1982; Blamires & Myatt, 1982; Calfee, 1982; Katzenberg et al, 1982) and those which are conductively coupled such as diathermy, electrocautery and defibrillation (Giedwoyn, 1971; Levine et al, 1983). Secondly, EMI may affect the function of the device without damaging it. An example of this would be the closure of magnetic reed switches within the device. Thirdly, it may cause current flow in the pacemaker lead. With high energy sources, this may cause damage to the myocardium or induce arrhythmia, either directly or following low-frequency demodulation by Zener diodes in the pacemaker iiput circuit (Zoll & Linenthal, 1960; Burchell, 1961; Noordijket al, 1961; Weinberg et al, 1962; Lichter et al, 1965). More commonly, however, problems arise when current induced in the pacemaker lead is detected by the sensing amplifier of a pacemaker operating in a demand or triggered mode. The consequences of this will depend on whether the device recognises the signal to be of extraneous origin or falsely interprets it as being o f cardiac origin. When an input signal is recognised as being o f extraneous origin, most pacemakers revert to fixed rate pacing at a rate that is normally the same as, or slightly above, the programmed rate. This is commonly referred to as the ’interference' or ’reversion’ mode. If the input signal is falsely interpreted to be of cardiac origin, it may cause inappropriate inhibition or triggering o f the pacemaker, depending on the operating mode. This may be brief, with only one or two missed or extra pulses, or prolonged, with pacing that is erratic or at an abnormally slow or fast rate. In the worst case, it may cause total cessation of demand pacing for as long as exposure is continued (Imich, 1984). Finally, EMI may affect the pacemaker by altering the selected settings in a programmable device, a phenomenon sometimes referred to as ’phantom programming’ (Barold et al, 1978; Domino & Smith, 1983; Belott et al, 1984; Goldberg et al, 1984).

ELECTROM AGNETIC COUPLING:

There are a variety of ways in which electrical or electromagnetic energy from external sources may be coupled to an implanted pacing system (Irnich et al, 1978; Irnich, 1984). Some understanding of the mechanisms involved, facilitates recognition o f the circumstances in which problems may arise.