1 Explain the letters in the NBG coding system for pacemakers

The first three letters refer to chamber paced, chamber sensed, and response to sensed events. The fourth letter generally refers to programmability and rate responsiveness. The fifth letter is rarely used (Table 64-1).

2 What does AOO, VOO, or DOO mode mean?

AOO means pacing in the atrium alone, VOO means pacing in the ventricle only, and DOO means pacing in the atrium and ventricle without sensing or response. Such a device would simply pace at a set lower rate regardless of underlying rhythm.

These modes (ending in OO) are known as asynchronous modes and are useful in the operating room because they allow pacing to be maintained and prevent interference of electrocautery caused by the absence of sensing.

3 What is the result of VVI pacing?

This mode results in ventricular pacing, with sensing in the ventricle alone, and the response is inhibition. This means that the device will release a pacing stimulus unless a sensed event occurs before the expiration of a timer based on the lower rate. If a sensed event occurs, the only response by the device is to inhibit pacing, allowing for the intrinsic rhythm.

4 What is the result of DDI pacing?

DDI means pacing in the atrium and ventricle, sensing in the atrium and ventricle, and the response is constrained to inhibition. The device will thus pace at a set rate in the atrium unless a sensed event occurs at a faster rate in the atrium, which will result in inhibition. The device will then pace in the ventricle at a set rate unless sensing in that chamber occurs at a faster rate, leading to inhibition. This allows for some atrial and ventricular synchrony to be maintained. However, because the response is only inhibition, the pacemaker will not pace in the ventricle as a result of a sensed event in the atrium, which is known as a tracking or triggered response. Thus competitive atrial pacing and variable atrioventricular (AV) timing can occur. This mode is helpful when atrial arrhythmias would lead to rapid heart rates if tracked and is a frequent default mode.

5 What does DDD pacing mean?

DDD pacing means pacing in atria and ventricle, sensing in atria and ventricle, and a dual response (both triggered and inhibited). This is the most common pacing mode. A sensed atrial event occurring before the lower rate timer has expired will inhibit atrial pacing. If no event is sensed in the atrium above the lower rate, an atrial stimulus is released. Both events will initiate a timer expiring before ventricular pacing. If a ventricular-sensed event occurs before this timer expiring, ventricular pacing is inhibited, and intrinsic conduction is maintained. If the AV timer expires, a ventricular pacing stimulus is delivered and is triggered from the previous atrial paced or sensed event. This is known as tracking. If no underlying sensed events occur, atrial and ventricular sequential pacing occur at the lower programmed rate.

6 Describe the difference between a unipolar and bipolar pacemaker

Both scenarios detect electrical signals between two electrodes. In the case of unipolar pacemakers, one electrode is in the heart at the lead-tissue interface, and the other is comprised of the pacemaker generator. Sensing occurs from both electrodes. Thus sensing from the generator may lead to sensing extracardiac signals, muscle myopotentials, external noise, and interference. In the case of bipolar leads, the electrodes are closely spaced at the lead tip within the heart. Sensing between the electrodes is limited mainly to cardiac signals with much less oversensing or external interference.

7 What is an implantable cardioverter defibrillator?

Implantable cardioverter defibrillators (ICDs) are devices much like pacemakers that can pace and support the heart rate in bradycardia. However, these devices have leads with shocking coils, usually one or two, that allow the device to administer internal shocks for tachyarrhythmias (which may be lifesaving). These devices also have complex programming. They allow rapid pacing to terminate regular, fast, more organized arrhythmias in a painless fashion. Algorithms in these devices may even allow differentiation of supraventricular arrhythmias from more serious arrhythmias from the ventricle. These devices are implanted for the treatment of life-threatening arrhythmias and prevention of sudden death.

8 What are common indications for permanent pacing?

In general patients with symptoms from slow heart rate of any type that are not reversible require pacing.

Patients with AV block located below the AV node require pacing. These patients have wide escape complexes, and their escape mechanisms are unreliable and can lead to asystole or death.

Patients with arrhythmias caused by bradycardia such as torsades de pointes responsive to pacing require pacing.

Patients with syncope caused by carotid hypersensitivity require pacing.

9 What are some common indications for implantable cardioverter defibrillator implantation?

Cardiac arrest or ventricular fibrillation not from reversible causes

Sustained ventricular tachycardia with associated structural heart disease

Patients with left ventricular ejection fraction less than 35%, regardless of etiology

Primary electrical disorders such as Brugada syndrome or long QT syndrome

10 What is the effect of placing a magnet over a device?

The answer depends on the type of device and its programming. In general placing a magnet over a device prevents device sensing. In the case of pacemakers this usually leads to asynchronous pacing at a preprogrammed rate. regardless of underlying rhythm. If the device is an ICD, the default is usually to prevent sensing leading to arrhythmia detection and therapy. This may be used emergently when device malfunction is occurring, leading to inappropriate shocks, or when external noise (i.e., electrocautery) is sensed as a tachyarrhythmia. Be certain that the device is not functioning appropriately at the time of ICD discharge when magnet application may lead to withholding of lifesaving therapy! Application of a magnet over a device may also be useful when emergency surgery is required and the ICD therapies cannot be deactivated in a timely manner.

11 Do patients with devices need to avoid microwave ovens or other hospital electronics?

In general modern devices are shielded against routine electromagnetic interference. This shielding is inadequate for radiofrequency (intraoperative electrocautery) or transcutaneous electrical nerve stimulation units. This may lead to false detection of extracardiac noise, leading to inappropriate inhibition of pacing, or to defibrillation caused by inappropriate arrhythmia detection with ICDs. Other forms of strong magnetic fields such as arc welding and some short-wave radios may lead to device reprogramming or other malfunction. Cellular phones generally are not problematic unless in a pocket overlying the device; they should be used in the contralateral hand.

12 Are there other responses to electromagnetic interference by devices?

Some devices are rate responsive. In this mode the device is programmed to respond to various stimuli by increasing or decreasing heart rate to correct for various metabolic demands of exertion in patients. Some of these sensors function with crystals that deform with impact, increase pacing with changes in chest impedance, or increase pacing in response to changes in momentum. With these sensors lithotripsy, certain electromagnetic monitoring equipment, or simply chest motion may trigger inappropriate rapid pacing. Rate responsiveness should always be programmed off before surgery.

13 Can device leads be dislodged?

Generally after about 6 weeks the myocardium has begun to scar around the lead tip, which contributes to lead stability. However, new devices known as biventricular pacemakers and defibrillators have a special lead placed in a vein behind the left atrium and ventricle to improve symptoms of heart failure. These leads are placed in a vein with friction as the only mechanism of retention and may easily be displaced years after placement. The placement and withdrawal of intravascular monitoring catheters should be performed with extreme caution and probably with fluoroscopic guidance.

14 Can device leads be damaged with intravascular access?

Device leads are generally wound wires surrounded by silicone that can be torn or damaged with an introducer needle. In general, access should be avoided in the veins where leads are in place.

15 Do typical anesthetics and intraoperative medications affect the ability of implantable cardioverter defibrillators to defibrillate patients?

Fentanyl reduces the energy required from the device to convert a ventricular arrhythmia (defibrillation threshold). Most inhalational anesthetics do not appear to change the defibrillation threshold except for enflurane and pentobarbital, which appear to increase it.

16 Can changes in the patient’s clinical status affect pacemaker function?

In critically ill patients severe acidosis, hyperkalemia, and/or cardiac ischemia may lead to dramatic increases in pacemaker capture thresholds, resulting in apparent pacemaker malfunction.

17 Can programming patients to a different mode lead to hemodynamic compromise?

Certain patients may become hypotensive, diaphoretic and nauseated, or short of breath when programmed to modes that lead to AV dyssynchrony such as VVI setting in a patient with intact sinus mechanism. This may also occur when prolonged AV timing leads to atrial stimulation immediately following previous ventricular contraction. This is known as pacemaker syndrome and can on occasion be quite dramatic.

18 Should patients with pacemakers or defibrillators be evaluated before and after surgery?

Yes—to ensure that normal device function and adequate battery life remains. The physician should be aware if the patient is pacemaker dependent. This implies that the patient has no underlying cardiac rhythm when the pacemaker is not functioning. If this is the case or if electrocautery is to be used, the device should be programmed to an asynchronous mode to avoid inadvertent device inhibition by electromagnetic interference (EMI). Patients with ICDs should have the arrhythmia therapy feature turned off. This mandates that an external defibrillator be available for external defibrillation should an arrhythmia occur. After surgery, ICDs and pacemakers should be investigated to ensure that no programming errors were caused by EMI.

19 Are there any precautions that can decrease the effect of electrocautery on devices?

Yes. The dispersive electrode should be close to the area of the proposed cautery and as far from the device as possible. Bursts of electrocautery should be as brief as possible to avoid prolonged inhibition. The cautery electrode and dispersive pad should not be parallel to the presumed lead configuration and are best if perpendicular to the lead to avoid current traveling down the device leads. Devices with near end-of-life battery voltage may be permanently inhibited with electrocautery.

20 If no underlying rhythm is seen when a pacemaker rate is rapidly decreased, does that mean that the patient is pacemaker dependent?

No. Many patients have their rhythm suppressed by pacing but when given time will have a hemodynamically tolerated underlying rhythm. To assess for this the pacing rate should be slowly reduced to allow for the patient’s underlying rhythm to recover. If, after slow reduction in pacing rate, no underlying rate is seen above 30 to 40 beats/min, the patient is likely pacemaker dependent.

21 No pacer spikes are seen on the monitoring system with your patient with a pacemaker. Does this mean that the pacemaker is not functioning properly?

Not necessarily. The pacemaker may simply be inhibited because of the patient’s underlying rhythm. However, it must be noted that with bipolar pacemakers the pacing spike is extremely small and may not be detected. In fact, with most digital recording systems the pacemaker spike seen is added to the strip by the recording system when electrical frequencies are detected that fall in the range of pacemaker stimuli. This sometimes leads to pacer spikes being seen on monitoring strips for patients without devices!

22 Does failure of a pacemaker stimulus to capture the heart necessarily imply pacemaker malfunction?

There are many reasons a pacemaker stimulus may not depolarize the tissue. Assess whether a stimulus is an atrial or ventricular stimulus (Figure 64-1). The tissue may have been refractory from a recent event such as a premature ventricular contraction, which is known as functional loss of capture. Occasionally the paced complex is not seen on a single-lead monitoring system because the axis of the paced rhythm is perpendicular to the monitoring lead. In this case the paced complex will not be visible on the lead viewed despite normal function and normal appearance on other leads if they were monitored. However, if repeated failure to capture is seen or the patient appears compromised, emergent evaluation of pacemaker function should be sought.

Figure 64-1 Normal atrioventricular pacing (sequential pacing). AV is Atrioventricular (paced) Interval, ID is Intrinsic Depolarization, LR is Lower Rate (of pacing), PVARP is Post Ventricular Atrial Refractory Period, TARP is Total Atrial Refractory Period.

(From Zaidan JR: Pacemakers. In Barash PG, editor: Refresher courses in anesthesiology, vol. 21, Philadelphia, 1993, Lippincott, with permission.)

23 If a pacemaker stimulus is superimposed on a native complex, is the pacemaker necessarily malfunctioning?

No. There are many explanations for the previous finding. Most commonly this is a case in which the pacemaker timer times out before sensing an intrinsic event. In this case the device will release a stimulus. The intrinsic event may have started earlier on a surface lead but was not sensed by the device at the site of the lead until well into the complex. This is a common finding with normal VVI pacing (Figure 64-2). Other programmable behavior may also explain this finding, although inappropriate sensing must also be ruled out.

Figure 64-2 Normal VVI pacing.LR is Lower Rate (of pacing), VRP is Ventricular Refractory Period.

(From Zaidan JR: Pacemakers. In Barash PG, editor: Refresher courses in anesthesiology, vol. 21, Philadelphia, 1993, Lippincott, with permission.)