Emergency cardiac pacing may be instituted either prophylactically or therapeutically. Prophylactic indications include those situations where there is high risk of atrioventricular block. Therapeutic indications include symptomatic bradyarrhythmias, asystole, and overdrive pacing. Since it can be instituted quickly and noninvasively, transcutaneous pacing is the technique of choice in the emergency department. Transvenous pacing should be used in patients who require prolonged pacing or who have a very high (>30 percent) risk of heart block. Detailed guidelines are available in C.h.ap,...2.4 and Chap 4.8..
Transcutaneous pacing has become the emergency technique of choice because of its easy application. It uses externally applied electrodes to deliver an electric impulse directly across the intact chest wall to stimulate the myocardium. Transcutaneous pacers differ from standard pulse generators in several important ways. The pulse duration of the stimulating impulse is longer and the current output higher than for standard internal leads. Muscle contraction (usually the chest wall or diaphragm) is notable during pacing, especially at higher outputs. This results in a twitching or bucking activity that can make assessment of cardiac output by palpation of the radial, carotid, or femoral pulse unreliable during transcutaneous pacing. The higher current outputs used make cardiac monitoring with standard electrocardiogram (ECG) monitors problematic due to interference from the large-amplitude pacing spike. Most transcutaneous pacing units come equipped with a monitor that automatically filters the pacing spike so that simultaneous monitoring is possible.
The external pacing electrodes are quickly and easily applied to the chest and back. If separate defibrillator pads or paddles are used, they should be placed at least 2 to 3 cm away from the pacing pads. There is little risk of electrical injury to health care providers during transcutaneous pacing. The electrodes are insulated, and chest compressions [cardiopulmonary resuscitation (CPR)] can be administered directly over them while pacing, although it is recommended that pacing be discontinued during CPR to minimize inappropriate stimulation of the patient due to electrical artifacts. Inadvertent contact with the active pacing surface results only in a mild shock. In the setting of bradyasystolic arrest, it is reasonable to turn the stimulating current to maximum output and then decrease the output if capture is achieved. In a patient with a hemodynamically compromising bradycardia (but not in cardiac arrest), the operator should slowly increase the output from the minimum setting until capture is achieved. Capture is assessed by following the ECG on the filtered monitor of the pacing unit and palpation of peripheral pulses. The hemodynamic response to pacing must also be assessed, either by blood pressure cuff or arterial catheter. Ideally, pacing should be continued at 1.25 times the threshold of initial electrical capture.
As with other pacing systems, transcutaneous pacing may be fixed rate (asynchronous) or demand (synchronous). Asynchronous pacing delivers an electrical impulse at a regular interval without regard to intrinsic cardiac pacemaker activity. This creates the risk of precipitating dysrhythmias if a pacing stimulus is given during the vulnerable period of ventricular repolarization. Synchronous pacing is therefore safer, since the pacing impulse is delivered only if an intrinsic electrical complex is not sensed within a preset interval. An increasing number of defibrillators include a built-in transcutaneous pacemaker. These units are equipped with multifunctional electrodes that allow defibrillation, pacing, and ECG monitoring through one set of pads. This development ensures that pacing will be available as soon as the defibrillator reaches the patient in cardiac arrest.
Failure to capture with transcutaneous pacing may be related to electrode placement or the patient's size. Patients who are conscious or who regain consciousness during transcutaneous pacing may experience discomfort due to muscle contraction. Analgesia with incremental doses of morphine or sedation with a benzodiazepine makes this discomfort tolerable. There is no evidence of clinically significant myocardial damage from properly performed transcutaneous pacing. 12 Nonetheless, transcutaneous pacing should be used for temporary stabilization only and should always be followed as soon as feasible by an internal pacing technique if there is a prolonged need for pacing.
Transvenous pacing consists of endocardial stimulation of the right ventricle by an electrode introduced into a central vein. The most commonly encountered difficulties with transvenous pacing are securing venous access and obtaining proper placement of the stimulating electrode, both of which can be time consuming. Venous access routes most commonly used include the subclavian, internal or external jugular, femoral, and brachial. Transvenous pacing catheters can be inserted through a variety of venous introducers. A soft, flexible, semifloating bipolar catheter is preferred. This type of pacer is safest to use and takes advantage of any forward blood flow that may be present.
Placement of the catheter tip into the apex of the right ventricle is the key to successful transvenous pacing. Several techniques can aid successful placement. Fluoroscopic guidance is the surest method of right ventricular placement but is rarely available in the emergency department. Electrocardiographic guidance is useful in patients with narrow complexes and/or P waves when fluoroscopy is unavailable. Balloon-tipped floating catheters may aid placement when used in conjunction with ECG and fluoroscopic guidance or when used alone. The balloon is inflated after catheter insertion into a central vein. Forward blood flow then directs the catheter tip toward the ventricle as the operator slowly advances the catheter. As with all balloon-tipped catheters, the balloon should always be deflated prior to withdrawal; the catheter should never be pulled back with the balloon inflated.
When patients have decreased or no forward blood flow (including many circumstances in which transvenous pacing would be used in the emergency department), positioning of the pacer tip within the right ventricle is difficult. Balloon-tipped catheters are not much of an aid in placement during low- or no-flow states. In a true emergency, the pacemaker electrodes are connected to the power source and the catheter advanced blindly in hopes that the tip will encounter the endocardium of the right ventricle and that capture will result. In this setting a right internal jugular venous access route should be used. From this approach, the catheter traverses a straight line into the right ventricle and rarely curls in the atrium or deflects into the inferior vena cava.
Pacer settings vary with the clinical situation. An initial rate of 80 to 100 impulses/min is appropriate for most patients. Asynchronous mode (sensitivity off) is used initially in patients requiring emergency pacing for hemodynamically unstable bradycardias. The ECG should be followed to determine the presence or absence of capture (Fig 18-5). Output should initially be set at maximum (usually 20 mA) and then decreased after capture is achieved. With optimal tip position, capture should occur at less than 2 mA. Pacing should be continued at 1.5 to 2 times the threshold output required for capture. Subsequent rate and sensitivity settings should be adjusted as clinically indicated by the patient's hemodynamic status and underlying rhythm disturbance.
FIG. 18-5. Pacing with intermittent capture. "P" indicates paced beats and "A," pacer artifact without capture.
Chest radiographs should be obtained after the patient is stabilized to ensure proper tip placement and to evaluate the possibility of pneumothorax from the preceding central venous line placement. Finally, care should be taken to firmly affix the pacing catheter to the insertion site prior to transferring the patient. Transvenous pacing is best used in urgent rather than emergent situations, particularly when there is adequate time to utilize fluoroscopy. In the setting of cardiac arrest, transcutaneous pacing is preferred.
Transthoracic pacing is mentioned here largely for historic reasons, since external pacing techniques have largely replaced its use. However, whenever transvenous pacing is tried but unsuccessful, transthoracic pacing may still be attempted. When the latter technique is performed blindly, the likelihood of successful placement is low, with risk of liver, pulmonary artery, diaphragm, lung, or coronary artery puncture.13 Although it has not been studied, use of ultrasound-guided placement may improve placement.
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