The importance of a magnet in a pre-hospital emergency situation

V. 01 | ISSUE 03 | OTTOBRE 2021


ISSN 2674-001X



  • Dr. Francesco Patrone; 118 Genoa Rescue Service San Martino General Hospital Genoa and Liguria Region Helicopter Service;
  • Dr. Lorenzo Borgo: 118 Genoa Rescue Service San Martino General Hospital Genoa and Liguria Region Helicopter Service;

July 2017, request for intervention in the doctor’s surgery, for tachycardia. On arrival of the advanced rescue personnel, the patient reported heart palpitations, appeared pale and diaphoretic. The colleague shows a tracing with wide complex tachycardia, which he believes is VT.

In fact, evaluating the morphology, if it were not for the clearly identifiable spikes, the suspicion of ventricular tachycardia would be more than legitimate. The patient is obviously a pacemaker (PM) carrier which inexplicably stimulates the myocardium with a very high frequency.

Tachycardia in a pacemaker patient is not an uncommon occurrence, but the presence of the device can sometimes cause difficulties for the non-cardiologist.

There are many reasons for the increased frequency:

  • Incorrect setting with too high response speed;
  • Electromagnetic interference (rare with latest generation devices);
  • Occurrence of a fast atrial rhythm (atrial tachycardia, atrial flutter, atrial fibrillation) which is detected by the PM (operating in atrioguided mode) and can be “followed” up to the set maximum frequency limit;


Pacemaker-mediated tachycardia (endless loop tachycardia), the most common cause of tachycardia in electrostimulated patients, can be defined as a condition in which the electro-induced rhythm is inappropriately elevated. It is caused by reentry, whereby stimulation of the ventricle is conducted backward through the atrioventricular node, resulting in depolarization of the atrium and stimulation of pacemaker activity.

The mechanism is very similar to the re-entry tachycardia (AVRT) seen in WPW, except that in this case the accessory circuit is formed by the pacemaker conduction system. Classically, the catheter forms the anterograde atrioventricular bundle and the AV node the retrograde one; the pacemaker should be programmed in DDD or VAT mode but not DDI or single-chamber mode.

The patient must have ventricular-atrial retrograde conduction with an atrial activation time longer than the PVARP (programmed post ventricular atrial refractory period). An electro-induced ventricular beat (or ventricular extrasystole) conducted retrograde through the AV node or through an accessory pathway if present, depolarises the atrium; if this occurs after the PVARP set, but before pacemaker-stimulated atrial depolarization, ventricular pacing is triggered.

To understand this mechanism, it should be remembered that more than a third of patients with atrioventricular block retain their ability to lead in the retrograde direction perfectly intact.

Tachycardia tends to have frequencies close to the programmed maximum limit and once established tends to persist as long as retrograde atrial pacing is present outside the PVARP. This type of tachycardia can in some cases resolve itself, but more often therapeutic intervention is required.

In emergency situations, with the patient being symptomatic, the treatment of pacemaker-mediated tachycardia is very simple.

Modern devices have advanced systems to protect against environmental electromagnetic stimuli, such as mobile phones, dental instruments, and muscular electro-stimulators, but a magnet placed on the skin at the generator can temporarily induce asynchronous stimulation until the clinical situation is resolved, or until the device can be reprogrammed in the event of a malfunction.

In general, the application of the magnet produces asynchronous pacing in all pacemakers, if this does not happen and specifically no change in the ECG pattern occurs, the pacemaker may have been programmed to ignore the magnet (very rare), may have a low battery or the magnetic field may not reach the device with sufficient intensity.

All pacemakers respond to magnet application by switching to asynchronous mode with the atrioventricular delay programmed by the manufacturer and with a frequency that also depends on the manufacturer’s settings as well as on the state of the battery.

Ultimately, the stimulation mode changes from DDD to DOO, from VVI to VOO and from AAI to AOO.

Let us briefly review the meaning of these letters:

  1. Site of stimulation: A atrium, V ventricle, D= A+V
  2. Autologous signal detection site: A atrium, V Ventricle, D=A+V
  3. Response type: I inhibition, T trigger, D=I+T
Europace, Volume 13, Issue 9, September 2011, Pages 1222–1230,
The content of this slide may be subject to copyright: please see the slide notes for details.

Therefore, placing a magnet over the pacemaker is sufficient to inhibit sensing, causing it to operate asynchronously in the atrium and ventricle, thus stopping the tachycardia by blocking the anterograde bundle of the reentry circuit.

Vagal maneuvers and drugs that block conduction through the atrioventricular node (adenosine, verapamil, beta-blockers) can also theoretically work by blocking retrograde conduction, provided there is no accessory pathway bypassing the atrioventricular (AV) node.

Finally, any fast atrial rhythm, such as atrial tachycardia, atrial flutter or atrial fibrillation, detected by the atrial lead, can be followed by the pacemaker up to the set maximum speed limit, even though the reentry mechanism seen above is missing. In this case, it is the pacemaker that sustains the high frequency and the magnet positioned on the pacemaker can bring the frequency back to the set standard.

In this case, it was decided to place a magnet over the device, which immediately stopped the tachycardia and reduced the symptoms.


The management of a patient with an implanted cardioverter defibrillator in an out-of-hospital setting or in an intensive care unit without cardiology can be very complicated.

But let’s take it step by step and try to understand something more about this precious device that emergency doctors are increasingly coming across.


In Italy, the number of patients with this device is constantly increasing, with new implants ranging from 3630 in 2002 to 17116 in 2014, so there are currently around 160,000 ICD patients, obviously with all the associated advantages and disadvantages.


Basically, ICDs function both as a Pace Maker (VVI, VDD, DDD R modes) and as an anti-tachycardia / Ventricular Fibrillation system, acting with Beats Sequences ( BURST – RAMP – SCAN ) or DC shocks.

Let’s look at some examples:

In this patient, you can see how a BURST sequence of 8 beats brings the patient back into rhythm.
In this case, however, there is a progressive increase in the overdrive frequency (RAMP) with interruption of the arrhythmia.

Another example: an extrasystole with the phenomenon of R on T induces ventricular tachycardia.
The ICD tries to interrupt it with 6 beats in BURST to no avail.
At this point the ICD ”decides” to defibrillate successfully….
Several large-scale studies have shown that implanted defibrillators (ICDs) in appropriately selected patients unequivocally reduce mortality.


One of the largest studies of ICD wearers, about 200,000 patients, found that the occurrence of shock at 1 year after implantation was 14% and at 5 years 38% (Saxon LA, Hayes DL, Gilliam FR, et al. Long-term outcome after ICD and CRT implantation and influence of remote device follow-up: the ALTITUDE survival study. Circulation. 2010).

It should not be forgotten that there is a strong association between the number of shocks and increased mortality in ICD patients. It is unclear whether shocks are simply an indicator of more severe cardiovascular disease or contribute directly to increased mortality.

Although the delivery of the shock is often essential to save the patient’s life, the negative consequences of this procedure, which produce physical and psychological consequences with a negative impact on quality of life, should not be overlooked, particularly when the shock is inappropriate.

High-voltage internal shocks delivered in a fully awake patient are a non-negligible problem, as they are associated with pain, anxiety, decreased quality of life and even increased mortality.

There are many studies in the literature on shock delivery by implanted defibrillators, the results are similar, IN 4-5 YEARS ABOUT 1/3 of ICD PATIENTS HAVE SHOCKED AND ABOUT 16-18% HAVE RECEIVED AN UNAPPROPRIATED SHOCK.

Relationship between shock and mortality

The publication of favourable data on the effectiveness of the ICD in primary prevention, following analysis of the Multicenter Automatic Defibrillator Implantation Trial (MADIT-II) and the Sudden Cardiac Death in Heart Failure Trial (SCD-HeFT), has led to an increase in implantations.

Poole et al. analysed the types of shocks in 811 ICD patients for 46 months, 33% of patients experienced at least one shock, only 47.6% to terminate ventricular fibrillation or ventricular tachycardia.

Interestingly, both appropriate (HR 5.68, 95 % CI 3.97-8.12, p<0.001) and inappropriate shocks (HR 1.98, 95 % CI 1.29-3.05, p=0.002) were associated with increased mortality.

The risk of death increased further with repeated appropriate shocks, which were associated with an eight-fold increased risk of death; the occurrence of inappropriate shocks in addition to appropriate shocks further increased the risk by almost 16-fold.


After electrical discharge from an ICD, cardiac cytonecrosis enzymes are released into the circulation, but the significance of this finding is unclear, and there is also an acute decrease in ejection fraction after current delivery, which is more pronounced in subjects with FE < 40% and usually takes up to 4 hours to return to pre-shock levels.

These two phenomena may have an effect on mortality but the only certain fact is that the final mechanism leading to patient death is post-shock PEA.

The ALTITUDE mentioned above is a prospective observational study of patients with ICDs or cardiac resynchronisation therapy defibrillators (CRT-D), aimed at differentiating the risk of death after shock. Studying 3,809 patients who had received 1 or more shocks over a 3-year follow-up, it showed that 41% of patients received shocks for non- VT / VF rhythms. Atrial arrhythmias were the most common cause, accounting for 44%, followed by other supraventricular arrhythmias (41%) and electromagnetic interference or oversensing (11%). Compared to the group where no shocks were delivered, the risk of death was no different if an inappropriate shock was delivered due to supraventricular arrhythmias (HR 0.97, CI 95% 0.68-1.37, p = 0.86) or interference/oversensing (HR 0.91, CI 95% 0.50-1.67, p = 0.76). In contrast, shocks delivered for AF/atrial flutter were associated with an increased risk of death (HR 1.61, CI 95% 1.17-2.21, p = 0.003).


There is a low but possible risk of a proarrhythmic effect. A clinical case of fatal shock is described (Fatal inappropriate ICD shock. Veltmann C. J Cardiovasc Electrophysiol 2007). The unfortunate patient suffered the consequences of displacement of the ventricular lead at the tricuspid ring, which led to erroneous signal detection, causing the ICD to treat sinus tachycardia as if it were ventricular fibrillation. The fourth improperly delivered shock induced a true low-voltage VF that was not recognised as such and was only treated with the antibradycardia function with fatal consequences.

But what does it mean to receive one or more shocks while the patient is conscious?

Managing a patient in such a condition may not be easy…first of all the patient needs to be monitored and checked whether defibrillator intervention is warranted. If this is the case, the patient should be sedated and, if necessary, antiarrhythmic drugs administered.

And if the shock is unfounded….


Inappropriate therapies (shock or antitachycardia pacing), as we have seen, occur in a fair proportion of patients with implantable cardioverter defibrillators (ICDs) and represent one of the most challenging aspects of emergency medical management. Electrical therapies are generally very poorly tolerated by the patient, because they occur while the patient is conscious.

The increasing indications for implanting pm and icd have greatly increased the number of patients with these devices, who may occasionally behave inappropriately.

We reiterate that the causes of abnormal icd responses are essentially alterations in sensing, e.g. following lead rupture, programming defects, supraventricular tachycardias incorrectly recognised as ventricular, and electromagnetic disturbances. Reprogramming the device can prevent abnormal responses, but this requires special tools and highly qualified personnel.

The use of an arrhythmologist may take too long in clinical emergency/urgency situations.

Fortunately, magnets can induce pacing in asynchronous mode in pacemakers if appropriately positioned… but also inhibit the anti-arrhythmic function in ICDs.

Curiously, although the magnet is inexpensive and readily available, it is often not included in the equipment of advanced rescue and DEA vehicles.

Many healthcare professionals are unfamiliar with the functions of cardiac rhythm management devices (CRMDs), yet magnet use requires no special training, making it an excellent option for repurposing CRMDs in urgent situations.

The earliest devices had a circuit of magnetic material inside, the shape of which changed when a magnetic field was applied, acting as a switch (see figure). More modern devices have more complex mechanisms, but an electromagnetic field perpendicular to the conduction of current in the device is able to deactivate certain functions.

The magnet must be placed over the insertion site of the CRMD, Medtronic even markets a magnet with a LED that lights up when the appropriate location is obtained.

Medtronic Smart MagnetTM.

Ideal positioning is recommended by various manufacturers.

The effect of the magnetic field is directly proportional to the strength of the clinical magnet and inversely proportional to the distance of the magnet from the CRMD.

A magnetic field greater than 10 Gauss aligned with the switch is required to activate the switch function; in obese patients, two magnets are often required.

This device would normally be placed directly in the centre of the top of the device (Medtronic, Boston Scientific, Biotronik) with the exception of the St. Jude ICD which would require the magnet to be placed at the poles, avoiding the centre.

In general, the application of a magnet to an ICD suspends the antitachycardia function of the device, without affecting the PM function.

Removal of the magnet normally restores the antiarrhythmic function of the ICD, although there are exceptions.

Let’s take a brief look at how the most popular ICD models perform:

BOSTON SCIENTIFIC respond to magnet positioning in a complex way.

In older models, magnet placement for more than 30 seconds deactivates the antitachycardia function (OFF mode), this inhibition persists even after magnet removal, this ”particular” behaviour is aimed at avoiding invasion of the sterile field in patients who are to undergo surgery (to avoid inappropriate shocks in response to electromedical interference).

ICD functionality can be restored by repositioning the lens for a further 30 seconds (ON mode).

While newer models have a more compliant response than other manufacturers and the deactivation of the anti-tachycardia function lasts only as long as the magnet remains

placed on the device. A continuous beep for a few seconds confirms deactivation and reactivation of the antitachycardia function.


There is no ON/OFF mode, the anti tachycardia function is inhibited only for as long as the magnet remains in place on the device. If the reset is unsuccessful, the device will emit a long beep requesting that the magnet be repositioned and removed. An alternating high and low tone is a warning sign of a serious malfunction requiring specialist technical assistance.


Inhibition of the anti-tachycardia function only lasts as long as the magnet is placed on the device and resumes automatically when the device is removed.


Inhibition of the anti-tachycardia function only lasts as long as the magnet is placed on the device and resumes automatically when the device is removed.

However, unlike other manufacturers, the positioning of the magnet on the ICD also changes the pace maker function, with fixed stimulation at a predetermined frequency that varies according to the state of charge of the battery.


In some models, the antitachycardia function is reactivated 8 hours after magnet placement, even if the magnet is not removed.

In conclusion, we can safely say that the magnet is indispensable for those who deal with emergencies on a daily basis, for the management of patients wearing cardiac rhythm management devices.


Poole JE, Johnson GW, Hellkamp AS, et al. Prognostic importance of defibrillator shocks in patients with heart failure. N Engl J Med. 2008

Daubert JP, et al. Inappropriate ICD shocks in MADIT II. JACC 2008; 51:1357-1365.

Bardy GH,et al. SCD-HeFT. NEJM 2005; 352;3:225-237.

Saxon, Leslie et al. Predictors of Sudden Cardiac Death and Appropriate Shock in the Comparison of Medical Therapy, Pacing, and Defibrillation in Heart Failure (COMPANION) Trial. Circulation 2006; 114; 2766-2772.

Saxon Leslie et al. The ALTITUDE Survival Study. Circulation 2010; 122:2359-2367. Clinical applications of magnets on cardiac rhythm management devices

Sony Jacob, Sidakpal S. Panaich, Rahul Maheshwari, John W. Haddad,Benzy J. Padanilam4, and Sinoj K. Europace (2011) 13, 1222-1230 doi:10.1093/europace/eur137

Inappropriate shocks in the subcutaneous ICD: Incidence, predictors and management Louise R.A.Olde NordkampaTom F.BrouweraCraigBarrbDominic A.M.J.TheunscLucas V.A.BoersmadJens B.JohansenePetrNeuzilfArthur A.M.WildeaNathanCartergMichaelHusbygPier D.LambiasehReinoud E. 15 September 2015

Fatal inappropriate ICD shock.

Veltmann C, Borggrefe M, Schimpf R, Wolpert C.

J Cardiovasc Electrophysiol. 2007 Mar;18(3):326-8.


Your email address will not be published. Required fields are marked *