|TABLE 34.2 Standard American College of Cardiology/American Heart Association Classification for Recommendations and Indications|
|TABLE 34.3 Indications for Pacing in Acute Myocardial Infarction|
|TABLE 34.4 Common Pacing Modes Available for Temporary Pacing|
There are also multiple catheter designs for atrial pacing. Some pacing catheters are preformed to facilitate placement into the right atrial appendage or coronary sinus, thus allowing atrial pacing (Fig. 34.3). Other atrial pacing catheters use a delivery system consisting of a guiding catheter to position the pacing electrode catheter within the right atrium. A variety of new electrode catheters consist of several electrodes positioned 10 to 20 cm proximal to the distal-tip electrodes. These electrodes are positioned to lie along the lateral right atrial wall, allowing atrial sensing and pacing. An innovative modification of this technique allows for a small atrial J-wire to be placed through a dedicated lumen in the catheter into the right atrium, with distal electrodes already positioned in the right ventricular apex (Fig. 34.4). Both of these types of catheter adaptations have been developed to allow a “one venous stick” approach to AV pacing. However, the atrial electrodes provided by these catheters often do not reliably pace the atrium.
The vast majority of temporary pacing catheters are designed to lie against the ventricular myocardium once positioned (passive fix). However, newer catheter designs, especially those for right atrial pacing, have a deployable screw that is embedded in the myocardium (active fix) (Fig. 34.5). All leads placed in the heart carry a risk of migration and perforation (7), but active fix leads may reduce the risk of dislodgement (8). Therefore, if the use of a temporary pacing catheter is anticipated to be longer than a few days, then consider placement of an active fix lead. The most common technique for placement of an active fix lead involves using an actual permanent ventricular pacing lead with a deployable screw provided by one of the manufacturers. The lead is inserted through a peel-away sheath placed in either the right internal jugular or subclavian veins via a modified Seldinger technique. The pacing lead is positioned in the right ventricle under fluoroscopy using straight and curved stylets. Once in position, the lead screw is deployed, the peel-away sheath is removed, and the lead is sutured to the skin using the lead sleave. The lead is then attached to an external pacing unit via pacing cable with alligator clips.
Temporary pacing catheters can be bipolar or unipolar. Bipolar catheters have both the negative (anode) and positive (cathode) electrodes in contact with the heart (Fig. 34.6A). In a unipolar catheter, the anode is in contact with the heart, but the cathode is elsewhere on the body (Fig. 34.6B). Bipolar electrodes are preferred because they are less susceptible to external electrical interference.
EXTERNAL PACEMAKER UNIT
The external temporary pacemaker unit controls the pacing mode, stimulus output, stimulus frequency, and threshold for sensing intrinsic activity (Fig. 34.7). Pacing modes can be synchronous (demand/inhibited) or asynchronous to pace the atrium, ventricle, or both. The range of output varies from 0 to 20 mA. The frequency can be adjusted from 30 to 180 beats/min. Sensing threshold can be varied from no sensing (asynchronous) to less than 1.5 mV. The function of these units varies widely by manufacturer, making it imperative that staff and physicians are familiar with the routine function of the particular unit in their hospital.
ASSESSMENT OF THE PATIENT
A complete patient assessment must be made prior to pacemaker placement. Bradycardia alone is not sufficient. Hemodynamic instability, symptoms, or evidence of significant conduction system disease on the electrocardiogram (ECG) (bundle branch block, high-degree heart block) favors therapy. Also, reversible causes, especially medications, should be sought. Glucagon may be effective for β-blocker overdose, calcium for calcium channel blocker overdose, and digoxin immune Fab (Digibind®) for digitalis glycoside overdose. Some bradycardias can be treated medically with agents such as isoproterenol, a β1-receptor agonist that increases heart rate. Electrolyte abnormalities such as hyperkalemia and other adverse metabolic states, such as severe acidosis, should be corrected, and in fact, if uncorrected, may make pacing ineffective.
All the necessary equipment should be available and inspected. The external pacing unit should be programmed to the desired settings and turned to the “On” position. A new battery should be installed. The lead should be examined for any defects. The connector cables should be inserted into the pacing generator to make sure that they are compatible (Fig. 34.8).
Continuous electrocardiographic monitoring and a defibrillator at the bedside are required. Fluoroscopy is preferred, but is not mandatory, if a balloon-tipped electrode catheter is being placed through the internal jugular or subclavian vein. Having the equipment ready before temporary catheter insertion is critical to avoid complications.
|TABLE 34.5 Common Sites for Central Venous Access When Placing a Temporary Pacing Catheter|
Access is obtained using the modified Seldinger technique for placement of an introducer sheath that is large enough to accommodate the size of the pacing catheter (usually 4 to 7 French). The most common sites used are the internal jugular, subclavian, or femoral veins. The advantages and disadvantages of various access sites are given in Table 34.5. For bedside placement, the right internal jugular vein or left subclavian vein is the preferred site. In the cardiac catheterization lab, the femoral vein is commonly used. Of note, if the patient is likely to require permanent transvenous pacing, then the subclavian vein should be avoided on the side of the planned permanent implantation. Other potential sites include the basilic or cephalic veins; however, the downside is that access of these vessels usually requires a surgical cut-down approach.
PLACEMENT OF THE TRANSVENOUS PACING CATHETER
Once the sheath is in place, the pacing catheter is advanced through the sheath into the venous system. The lead is attached to the connector cables, which have been inserted into the pacing unit. The lead is gradually advanced under continuous electrocardiographic monitoring, with care not to use excessive force, as this may lead to perforation. If performed under fluoroscopic guidance, an inferior or septal position in the distal third of the apex is preferred. Apical right ventricular capture is noted by a left bundle branch, superior axis pattern. Once ventricular capture is obtained, threshold testing is performed. If no fluoroscopy is available, a balloon-tipped catheter must be used. Placement into the right ventricle is confirmed by ventricular capture on the ECG with a left bundle branch block, superior axis pattern (Fig. 34.9). Once in place, the balloon is deflated to avoid advancement into the pulmonary artery.
Placement of a temporary atrial pacemaker should be performed by clinicians who implant permanent pacemakers, as the implant techniques are similar. Fluoroscopy is essential for this procedure.
Dual-chamber temporary pacing most commonly occurs after cardiac surgery when the patient has epicardial wires that were placed at the time of the surgery. Rarely is temporary endocardial dual-chamber pacing necessary. An example would be a patient with CHB and congestive heart failure who requires the hemodynamic benefit of AV synchrony and atrial contraction. In such a rare situation, separate catheters are placed, or if single access is preferred, a specialized dual-chamber pacing catheter is appropriate (see Fig. 34.4).
PACE SENSING AND THRESHOLD DETERMINANTS
After obtaining good anatomic positioning of the pacing catheter, a stimulation threshold should be determined. With continuous electrocardiographic monitoring, pacing should begin at a rate at least 10 beats/min faster than the patient’s intrinsic heart rate, with the output set at 5 mA. The output of the pacemaker is gradually decreased until the stimuli fail to produce ventricular (or atrial) capture (Fig. 34.10). The current setting at which capture fails to occur is called the pacing threshold and should be less than 1 mA. The pacemaker output should be set at three to five times the pacing threshold.
If the pacemaker is to be used in a demand mode, it is also important that there is adequate sensing of the endocardial electrogram. To ensure good sensing, the pacing rate is set lower than the patient’s intrinsic rate. The sensitivity of the pacing unit is set at its most sensitive level (lowest value), and is then decreased (higher values) gradually. The setting at which the pacemaker fails to sense and begins pacing competitively with the patient’s intrinsic rhythm is called the sensing threshold (Fig. 34.11). For demand pacing, the sensitivity should be set at a more sensitive level than the sensing threshold.