Pacemakers and Defibrillators

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Chapter 18 Pacemakers and Defibrillators

Scott C. Streckenbach, MD , Kenneth Shelton, MD

1 What are the general principles of cardiovascular implantable electronic device (CIED) management according to the Heart Rhythm Society (HRS)–American Society of Anesthesiologists Consensus Statement published in 2011?

The primary recommendation is that the best prescription for the perioperative care of a patient with a CIED will be realized when that patient’s CIED team (electrophysiologist or cardiologist) is asked for advice and that advice is effectively communicated to the procedural team. Thus the surgical or procedural team should communicate with the CIED team to identify the type of procedure and the likely risk of electromagnetic interference. The CIED team should then communicate with the procedure team to deliver information about the pacer or implantable cardiac defibrillator (ICD) and recommendations for the perioperative management of the patient and the device.

2 How can one differentiate a pacemaker from an ICD?

Occasionally a patient is seen initially without information about his or her CIED. A critical step is to determine whether the device is a pacemaker or an ICD. The patient may be able to tell you why the device was inserted, and this might provide a clue. However, if the patient’s history cannot help, the patient’s chest radiograph can. The ICD leads, unlike pacer leads, have one or two radiodense shocking coils. Because of the shocking coils, the ICD leads (Fig. 18-1) are much larger than the pacer leads (Fig. 18-2), and this can be appreciated radiographically.

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Figure 18-1 Chest radiographs (PA and lateral) of an ICD. A, The SVC shocking coil (arrowhead) and the RV shocking coil (arrow) are radiopaque and differentiate the ICD from a pacemaker radiographically. B, An atrial pace/sense lead is also noted in the lateral view.

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Figure 18-2 Chest radiographs (A and B, PA and lateral) of a pacemaker. The atrial pacing lead (arrowhead) and the ventricular pacing lead (arrow) are smaller in diameter and less radiopaque than the leads associated with an ICD.

3 Describe the five-letter pacemaker code

• First position of the code reflects the chamber(s) in which stimulation occurs.

image Second position refers to the chamber(s) in which sensing occurs.

image Third position refers to the mode of setting (or how the pacemaker responds to a sensed event).

image Fourth position reflects rate modulation.

image Fifth position of the code indicates whether multisite pacing is present.

See Table 18-1.

Table 18-1 The Generic Pacemaker Code

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4 What is cardiac resynchronization therapy (CRT)?

CRT is the term applied to reestablishing synchronous contraction between the left ventricular (LV) free wall and the ventricular septum in an attempt to improve LV efficiency and subsequently to improve functional class. The LV pacing lead is usually placed in the posterior lateral LV wall via the coronary sinus circulation. CRT may be used with a pacemaker or with an ICD. Indications typically include a low ejection fraction (< 35%), a prolonged QRS (> 120 ms), and class IV heart failure symptoms.

5 Explain the following pacemaker codes: DOO, VVI, DDD, and DDDRV

image DOO = asynchronous atrial-ventricular pacing with a constant A-V interval. Atrial and ventricular pacing pulses are emitted regardless of the underlying cardiac rhythm.

image VVI = ventricular-only antibradycardia pacing. Failure of the ventricle to produce an intrinsic event within the appropriate time window results in ventricular pacing. With no atrial sensing, there can be no atrioventricular (AV) synchrony in a patient with any intrinsic atrial activity.

image DDD = dual-chamber antibradycardia pacing. In the absence of intrinsic activity in the atrium, it will be paced. After any sensed or paced atrial event, an intrinsic ventricular event must occur before the expiration of the AV timer or the ventricle will be paced.

image DDDRV = dual-chamber antibradycardia pacing with rate response mode circuitry (to increase the paced rate in setting of increased metabolic demand) and biventricular pacing capability.

6 What is the effect of placing a magnet on a pacemaker?

Most pacemakers respond to a magnet by converting to an asynchronous pacing mode (e.g., DOO if pacer mode is DDD or VOO if VVI) at a rate dependent on the device manufacturer and the remaining battery life. For adequately charged pacemakers, the following is generally true:

image Medtronic pacemakers pace at 85 beats/min

image Biotronik, 90 beats/min

image St. Jude, 98 beats/min

image Boston Scientific, 100 beats/min

See Table 18-2.

Table 18-2 Magnet Effect on Pacemakers

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7 What is a rate response mode?

The normal heart rate response to increased physiologic demand is linearly related to oxygen demand and consumption. Circumstances requiring heart rate variation include exercise, emotion, anxiety, baroreflexes, vagal maneuvers, hypovolemia, fever, and anemia. The two more common types of rate response sensors are based on patient movement (accelerometer) and patient ventilation (transthoracic impedance monitor). Both of these sensors will increase the patient’s paced rate when increased patient activity is detected. The rate at which the heart rate changes is programmable. Typically the heart rate increase is slightly delayed and will return to baseline several minutes after the increased activity subsides. Because both of these sensors can be falsely activated in a hospital, knowledge of their presence is ideal for optimal patient care.

8 How should one manage a patient with a minute ventilation rate response mode?

Minute ventilation sensors should be disabled while the patient is in the hospital. These monitors measure the rate and depth of change in thoracic impedance measured between one of the intracardiac pacing leads and the pulse generator. Increased frequency and depth of change in transthoracic impedance increase the paced rate to match the patient’s increased minute ventilation. Electrocautery, ventilators, and patient monitors that measure respiratory rate can all cause significant changes in the paced rate if the minute ventilation rate response mode is not disabled. The rate response mode may be disabled with a device-specific programmer or a magnet.

9 What is the mode switch function?

The automatic mode switching function describes the capability of a pacemaker both to detect the presence of atrial tachyarrhythmias and to switch automatically from a triggering mode (e.g., DDD) to a nontriggering one (e.g., DDI) for the duration of the tachyarrhythmia. In this manner, mode switching prevents the pacemaker-mediated conduction of an atrial tachyarrhythmia to the ventricle. During a mode switch the lower rate limit of the pacer usually increases (to compensate for the loss of the atrial kick). This may be interpreted as a pacer malfunction if the clinician is not aware of this programmed function.

10 What is the rest or sleep mode?

During sleep the native heart rate decreases to provide a physiologic rest period for the heart. Absence of this nightly rest period can decrease overall LV function. Therefore pacemaker manufacturers have created algorithms that allow the lower rate limit to drop at night. For example, the St. Jude rest mode activates whenever perceived patient activity decreases sufficiently for a specified duration of time. If the patient takes a nap at noon, approximately 15 minutes into the nap, the pacer’s rate might drop from 60 to 50. The Medtronic sleep mode is time based, rather than activity based, and will activate at a preset time, e.g., from 9 PM to 5 AM. Both of these functions could confuse the clinician were he or she not aware of their presence.

11 What is the “R on T” phenomenon, and how is it related to commotio cordis?

“R on T” refers to a phenomenon in which the R wave of the QRS complex (ventricular depolarization) falls within the vulnerable part of ventricular repolarization (the 10- to 20-ms window immediately preceding the peak of the T wave) of the preceding electrical complex on the electrocardiogram (ECG). Clinically, this phenomenon may result in ventricular tachycardia or fibrillation, especially if the myocardium is ischemic or metabolically compromised. This was originally described in humans in 1949 by F. H. Smirk in an article reviewing 17 cases. Clinically, this explains the risk associated with using asynchronous pacing in a patient whose intrinsic heart rate exceeds the paced rate. If a ventricular pacing pulse were delivered in the high-risk window of the T wave, ventricular fibrillation (VF) can occur. A blow to the anterior chest in this similar vulnerable period can induce VF, an event known as commotio cordis.

12 What are the options for establishing temporary cardiac pacing?

The least invasive method is transcutaneous pacing with external defibrillation pads. This method typically provides ventricular pacing (either demand or asynchronous depending on the clinician’s choice). It is uncomfortable for the awake patient because the energy required to pace is high. The next option is transesophageal pacing. This is typically limited to patients who have an endotracheal tube in place. Transesophageal pacing typically provides atrial pacing unless the pacing probe is inserted as far as possible. Transesophageal pacing uses long pulse wave duration to minimize the amount of pacing energy (and therefore prevent esophageal injury). This eventuates in significant ECG artifact. The last and most invasive method of pacing is transvenous pacing either directly through an introducer or through a pacing pulmonary artery (PA) catheter. This method is the most reliable and most flexible, that is, it allows the clinician to A-pace, V-pace, or AV-pace. This method should be used by only those with significant experience.

13 How does an ICD work?

An ICD detects the QRS signal of the patient. It measures the time between each QRS event. If the time interval is short (e.g., less than 300 ms, which corresponds to a heart rate of 200), the event counter starts. If there is a predetermined number of short intervals (e.g., eight out of the next 12), the device detects VF. Once detected, the ICD will charge its capacitors to 34 to 40 J. Most devices will reconfirm the presence of the dysrhythmia and then shock the patient. If the dysrhythmia has resolved during the reconfirmation period, the shock will be discharged slowly rather than shocking the patient, but battery life will be diminished.

14 What is the meaning of primary and secondary prevention with respect to ICD therapy?

Primary prevention refers to the use of an ICD to protect a patient from sudden cardiac death (SCD) who has already had a documented episode of VF, syncope. Secondary prevention refers to use of an ICD to prevent SCD in a patient at high risk for the same, for example, patients with hypertrophic obstructive cardiomyopathy and a family history of SCD, with an ejection fraction less than 35%, with arrhythmogenic right ventricular dysplasia, and with congenital long QT syndrome.

15 How will a magnet affect an ICD?

Magnets usually inhibit the antitachycardic functions (defibrillation, cardioversion, antitachy pacing) of an ICD. Because each manufacturer’s device has some idiosyncrasies, the clinician must be sure to understand how the magnet will affect the ICD before magnet application. Removal of the magnet usually returns the ICD to the active mode. The rare exception may be the Boston Scientific Prizm series. Boston Scientific devices emit a tone for as long as the magnet is on the ICD. Medtronic devices emit a tone for approximately 30 seconds. The other devices do not emit a tone. A few devices in a specific scenario might not respond to a magnet. See Table 18-3.

Table 18-3 Magnet Effect on ICDS

image

image

16 Why is it important to know when pacemaker or ICD leads were inserted when considering placement of a central line?

Pacer or ICD leads take time to become secure after implantation. Active fixation leads, which are screwed into the endocardium, usually become secure sooner than passive fixation leads, which rely more on fibrosis for fixation. Coronary sinus leads used for cardiac resynchronization therapy are the least likely to become fixed. Insertion of a central line, and particularly a PA line, may dislodge the newly implanted lead. As a general rule of thumb, the risk for lead dislodgment is highest in the first 3 months after lead implantation. If a central line or PA catheter must be inserted in the first 3 months it should be done with fluoroscopic guidance, and backup pacing or defibrillation should be immediately available. All pacemakers and ICDs should be evaluated by a qualified specialist after the line has been inserted to ensure proper function.

17 What are the considerations for placement of a PA line in a patient with an active ICD?

Caution is required whenever a guidewire is inserted into a heart in the presence of an active ICD. Contact between the guidewire and a sensing electrode (atrial or ventricular) can trigger antitachycardia therapy. If the therapy is a defibrillation, the guidewire can short the proximal coil to the distal coil and cause serious myocardial injury. To prevent this, ensure that the guidewire does not enter the ventricle or temporarily inhibit the ICD.

18 What are the considerations for cardioversion or defibrillation in a patient with a CIED?

Theoretically, high-voltage cardioversion or defibrillation can damage the pulse generator or the lead-myocardial interface. It appears that application of the defibrillation pads in the anterior-posterior orientation with the anterior pad > 8 cm away from the pulse generator minimizes this risk. The HRS recommends that if a patient with a CIED undergoes cardioversion or defibrillation, especially in an emergency setting, the patient’s device should be interrogated before the patient’s discharge from the intensive care unit.

19 How can electrocautery affect a pacemaker?

Electrocautery can affect a pacemaker in multiple ways. It can inhibit pacer output if the device is set in a demand mode. This is typically noticed if the electrocautery is sensed by the ventricular lead. The pulse generator presumes that the cautery signal represents native ventricular depolarization and inhibits the ventricular pacing output. Electrocautery may also be sensed by the atrial lead. This can cause rapid atrial tracking (rapid ventricular pacing in response to a sensed high intrinsic atrial rate) to rates up to the upper rate limit of the pacer if the pacemaker is in the DDD mode. Atrial oversensing can also trigger a mode switch if the atrial sensing rate exceeds the mode switch cutoff rate (usually 170-180 beats/min). Finally, prolonged electrocautery can temporarily convert the pacer to a noise reversion mode or cause permanent pacemaker reset.

20 How can electrocautery affect an ICD?

Electrocautery in close enough proximity to the ICD leads (usually anywhere above the umbilicus) can be detected by the ICD as VF. The device will charge and shock the patient shortly thereafter if the electrocautery persists during the reconfirmation period (at the end of the charging period). It takes only 3 to 4 seconds of electrocautery to fool the ICD into detecting VF. It takes another 4 to 10 seconds to charge before a shock can be delivered. If, during the reconfirmation period, electrocautery has stopped, the device will abort the charge. Although a shock is prevented, battery depletion is not. The electrocautery will also affect the ICD’s pacer function as described previously.

Key Points Pacemakers and Defibrillatorsimage

How will a magnet affect a pacemaker?

1. Magnets convert most pacemakers to an asynchronous pacing mode.

2. The magnet-induced paced rate is manufacturer specific.

3. No audible sound is emitted from a pacemaker when a magnet is applied.

4. Magnets will inhibit any programmed rate response mode in a pacemaker.

5. Magnets do not affect the pacemaker component of an ICD.

Websitesimage

www.biotronik.com

www.bostonscientific.com

www.hrsonline.org (Heart Rhythm Society)

www.medtronic.com

www.PacerICD.com (go to Fundamentals of Pacing, Fundamentals of ICDs, or IBHRE Exam Study Materials for video lectures)

www.sjm.com

Bibliography

1 American Society of Anesthesiologists. practice advisory for the perioperative management of patients with cardiac implantable electronic devices: pacemakers and implantable cardioverter-defibrillators. Anesthesiology. 2011;114:247–261.

2 Crossley G.H., Poole J.E., Rozner M.A., et al. Heart Rhythm Society/American Society of Anesthesiologists expert consensus statement on the perioperative management of patients with implantable defibrillators, pacemakers and arrhythmia monitors: facilities and patient management. Heart Rhythm. 2011;8:1114–1152.

3 Lau W., Corcoran S., Mond H. Pacemaker tachycardia in a minute ventilation rate-adaptive pacemaker induced by electrocardiographic monitoring. Pacing Clin Electrophysiol. 2006;29:438–440.

4 Manegold J.C., Israel C.W., Erlich J.R., et al. External cardioversion of atrial fibrillation in patients with implanted pacemaker or cardioverter-defibrillator systems: a randomized comparison of monophasic and biphasic shock energy application. Eur Heart J. 2007;28:1731–1738.

5 Maron B.J., Estes M. Medical progress: commotio cordis. N Engl J Med. 2010;362:917–927.

6 Smirk F.H. R waves interrupting T waves. Br Heart J. 1949;11:23–36.

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