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NOTE: This content is currently being rewritten by our editors, but we have included the original article from OpenAnesthesia’s encyclopedia section before our March 2023 site update.


The following is intended to provide a brief introduction to the world of pacemakers and automated implantable cardioverter defibrillators (AICDs). While the information below may be of use and is hopefully applicable, nothing that follows should trump, supercede, or replace manufacturer recommendations regarding specific devices, which should ALWAYS be consulted, when available, prior to administering an anesthetic in a patient with a pacemaker or AICD.

How Pacemakers Work

All pacemakers are capable of triggering a heartbeat (where they initiate activity is determined by position one) at a given rate. Some (most) pacemakers have sensory functions as well (position two) – an “inhibiting” pacemaker will normally fire at a given rate (ex. 80), stopping only if electrical activity is sensed, whereas a “triggering” device will normally not fire, but will only fire if an electrical event is not sensed. Triggered mode is extremely rare, but is often combined with inhibited mode (to form “dual”) so that the pacemaker can fire if an atrial beat does not produce ventricular contraction (ex. normally the pacemaker beats at 80, but if the atria contract 1.1s after the previous contraction, a dual mode pacemaker would be able to trigger the ventricles.

How ICDs Work

Continually detect the R-R interval. If an ICD detects too many short R-R intervals, it decides between antitachycardia pacing (less battery use, more tolerable for the patient) or shock. Charging time is 6-15 seconds, after which many ICDs will reconfirm an arrhythmia prior to actually shocking. 10% of shocks are due to a non-VF non-VT rhythm, most commonly due to SVT. Many ICDs also have antibradycardia pacing functions.

What Can Go Wrong?

When thinking about pacemakers, it is useful to think of them in terms of what can go wrong. There are really only three disadvantageous events that can happen to a pacemaker during surgery – 1) the pacemaker can be destroyed 2) the pacemaker/AICD can fire when it shouldn’t 3) the pacemaker/AICD can fail to fire when it should. Thinking of pacemakers in these terms simplifies management significantly. For instance – a pacemaker with no sensing capability (ex. VOO) cannot become bradycardic, because there is no way to inhibit it (unless it is bovie’d or otherwise destroyed. Note, however, that as it has no sensing capabilities, it IS possible to produce R-on-T phenomena, which is why VOO is not commonly used in the non-operative arena). A VVI pacemaker, if it is “oversensing” ESI, will not fire when it should, thus possibly causing bradycardia or asystole.

Note that biventricular pacing can lengthen the Q-T interval in patients, thus if a bi-ventricular pacer is present, defibrillation pads are particularly important

Synchronous vs. Asynchronous Modes

Asynchronous modes are those in which no sensing occurs, which allows for the possibility of competitive rhythm generation. There are three asynchronous modes – AOO, VOO, and DOO. Modern pacemakers are never programmed in this manner. The advantage of these pacemakers/modes, from a surgical standpoint, is that they won’t be inhibited by electrosurgical interference (ESI), which theoretically could lead to prolonged bradycardia/asystole, although with modern pacemakers and proper placement of grounding pads, this is increasingly unlikely.

AV synchrony can also be lost from other modes – VDD loses AV synchrony during sinus bradycardia, during which the device does not detect an atrial p-wave (because one has not occurred yet) and thus fires the ventricle without a preceding atrial beat.


NEVER put a magnet on a “pacemaker” (i.e. a device you’re not sure of) because if you place a magnet on a Guidant ICD, you may end up turning it off – magnets should be used with extreme caution around ICDs

Magnets were not designed to “turn off” pacemakers, but rather, to test battery life. Only 60% of pacemakers will convert to a high-rate (80-100 bpm) asynchronous mode with magnet application – 25% will convert to asynchronous mode at the programmed rate, and 15% will respond with 60-100 asynchronous beats. The only way to determine what actually occurs with magnet placement is to consult the manufacturer

Magnets should also be taken seriously as magnet removal can cause pacemaker mediated tachycardia (PMT), which results from retrograde P waves that occur during asynchronous slower-than-intrinsic pacing – these retrograde P waves can be picked up by dual sensing pacemakers (DDD, VDD). To correct, re-apply the magnet and remove again

Magnet placement on ICDs usually removes their tachydysrhythmia detection capabilities

Pacemaking Modes

Most Common Pacemaking Modes


DDD (synchronous): Detection of a paced (VP) or sensed ventricular (VS) resets its clock an also initiates the ventricular refractory period (VRP) to prevent T-wave oversensing and initiates the post-ventricular atrial refractory period (PVARP) which helps prevent oversensing of retrograde P waves. VA interval (time from VP/VS to atrial firing) plus AVI (atrioventricular interval, roughly ~ PR interval) equals the LRI (VA + AVI = LRI).

Four rhythms are possible: 1. NSR 2. Atrial sensed, ventricle paced 3. Atrial paced, ventricle sensed 4. Atrial and ventricular pacing

Ensures that atrial events are followed by ventricular contraction, but inhibits itself if a native QRS is detected. Also ensures that an atrial contraction occurs, thus its advantage over VDD is that it guarantees atrial kick.


VVI (single-chamber demand): normally paces the ventricle at some predetermined interval called the lower rate interval (LRI, ex. 800 ms). Detection of a paced (VP) or sensed ventricular (VS) resets its clock. After a ventricular refractory period (VRP, 200-350 ms), in which no sensing occurs (to avoid pacing on detection of the T wave), sensation of VS prior to the LRI will inhibit the next VP, and will reset the clock. Essentially, if early, native ventricular electrical activity is sensed, the pacemaker will not fire, otherwise it fires continually at the LRI. The VVI is a step up from the VOO in that it prevents R-on-T phenomena. When the atria is beating above the LRI and producing a ventricular contraction, this pacemaker will inhibit itself and allow AV synchrony.

When the atria is not beating above the LRI, this pacemaker will fire after the LRI has occurred, leading to loss of AV synchrony – this loss of AV synchrony (i.e., the ventricle firing with no preceding atrial beat) can lead to pacemaker syndrome, a set of signs/symptoms (fatigue, palpitations, cough, chest fullness, cannon A waves, elevated venous pressure, rales…), due to decreased cardiac output, hypotension, VA conduction, or increased pulmonary venous / RA pressure (ex. can contract after the AV valve is closed), all of which can occur in a VVI pacemaker. Pacemaker syndrome occurs in ~ 25% of VVI patients, and produces severe symptoms in 5%

Less Common Pacemaking Modes


VDD (partially synchronous): used in patients with normal sinus function but an abnormal AV node (i.e. there is no need to stimulate the atrium). Essentially senses the atria in order to ensure a ventricular contraction (trigger function after a predetermined delay, roughly equal to the PR interval), but is also capable of inhibiting itself if it detects a QRS complex. Note that dual function (inhibit and trigger) pacemakers ALL have dual sensors (_DD), i.e. sense the atria for a trigger, and the ventricle to inhibit. Ensures that atrial events are followed by ventricular contraction, but cannot cause the atria to beat, thus a VDD pacemaker can be dangerous in patients who require atrial kick. Loses AV synchrony during sinus bradycardia, during which the device does not detect an atrial p-wave (because one has not occurred yet) and thus fires the ventricle without a preceding atrial beat.


DDI (partially synchronous): normally paces the atrium and ventricle at some predetermined rate (ex. 80), but can inhibit either impulse based on detection. Cannot “guarantee” a proper ventricular beat, thus is only partially synchronous.

Rare Pacemaking Modes


VOO (single chamber, asynchronous): generates a fixed-interval rate with no relationship to a spontaneous rhythm. Can produce R-on-T phenomena, which is VERY rare (although is more common during myocardial ischemia or infarction, electrolyte abnormalities)


AAI (): rare in the US, but sometimes used for sinus node disease in other countries

Proposed Pacer/AICD Algorithm

Proposed Pacer/AICD Algorithm 1. Determine device indication, including the patient’s underlying rhythm 2. Acquire magnet, pacing pads, atropine, and isoproteronol 3. Interrogate (model, setting). If voltage < 2.6 V or impedance > 3000, consult the manufacturer re: possibly needing to replace before surgery. An ICD with a charging time > 12 s may need a battery change 4. Turn off all ICD functions (using pacing pads and EKG instead) 5. Turn of all rate enhancements 6. Turn off minute ventilation rate responsiveness 7. Consider increasing the pacing rate (in the past, asynchronous modes were recommended, but now that many pacemakers have bipolar leads, and the risk of EMI is lower [depending on the surgery], availability of a magnet usually will suffice) 8. Determine magnet function 9. Defib pads on all ICD patients 10. Intraoperatively: arterial line, strategic ground placement, bipolar if possible, disable the “artifact filter” on the EKG

Indications for Reprogramming

According to Kaplan (Box 25-4), the indications for pacemaker reprogramming are in anyone who is pacer-dependent, rate-responsiveness, certain clinical conditions (dilated cardiomyopathy, HOCM, pediatric patients), procedures in the chest/abdomen, and others (see Boxes 25-4 and 25-6) [Kaplan, JA et al. Kaplan’s Cardiac Anesthesia: The Echo Era Saunders: Chapter 25. 2011.

Indications for Pacing (Adapted from Table 25-4 in Kaplan)

  • Pacer dependence
  • Rate responsiveness
  • Dilated cardiomyopathy, HOCM, pediatric patients
  • Procedures in the chest/abdomen

See Also

Pacemaker Designation in ABA Key Words.