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• AOO	Atrial pace; no sense, no inhibitions
• AAI	Atrial pace; atrial sense, inhibited by atrial beat
• VOO	Ventricular pace; no sense, no inhibitions
• VVI	Ventricular pace; ventricular sense, inhibited by ventricular beat
• DOO	Dual chamber pace; no sense, no inhibitions
• DDI	Dual chamber pace; ventricular sense, inhibited by ventricular beat
• DDD	Dual chamber pace; dual chamber sense, inhibited by either chamber

Sources: Mulpuru SK, Madhavan M, McLeod CJ, Cha YM, Friedman PA. Cardiac pacemakers: function, troubleshooting, and management: part 1 of a 2‐part series. J Am Coll Cardiol. 2017; 69:189–210; Kenny T. The Nuts and Bolts of Implantable Device Therapy Pacemakers. Hoboken, NJ: Wiley & Sons, Ltd. 2015. Chapter 13, Pacemaker modes and codes; pp 140–52.

The EMS physician who responds to a pacemaker patient with a clinical issue should determine if the device is the problem. Vital signs and cardiac monitoring are the best tools. The first determinant is the heart rate. If the patient is markedly bradycardic, the pacemaker is presumed to have failed (Figure 11.4). The patient will require hemodynamic support, which may include external cardiac pacing. If external pacing is indicated, care should be taken to not cover the implanted device with the external pads. If the patient is tachycardic, the physician will need to determine if the pacemaker is firing inappropriately or if there is another medical cause. The presence of pacer spikes prior to every tachycardic beat is the best indicator of a pacemaker issue.

Figure 11.4 The first ECG depicts failure of electrical capture with pacer spikes not associated with QRS complexes and a ventricular escape rhythm, while the second ECG shows the same patient with electrical capture with a QRS complex with every pacer spike.

The next step is to determine what therapy is needed. Optimally, the patient can be transported to a facility where the implanted device can be interrogated by an electrophysiologist, preferably at the hospital where the device was implanted. If the patient’s clinical condition requires more emergent intervention, a special magnet can be held over the pacemaker to suspend inappropriate pacing. The magnet will not turn the pacemaker off, but it will trigger the device to pace at an asynchronous (fixed) rate depending on the device and manufacturer [26]. A DDD pacemaker will pace at DOO, a VVI device will pace at VOO, and an AII device will pace as AOO [26]. Magnet therapy is only effective when the magnet is on the skin over the pacemaker. In the event that magnet therapy is ineffective, it is theoretically possible to cut the pacemaker wires. However, this would be difficult in the field, may permanently damage the device, and should not be performed unless as a last resort.

Temporary transvenous pacemakers may also be encountered by the EMS physician during interfacility transports. Transvenous pacemakers are placed in the hospital setting in patients with unstable bradycardic dysrhythmias unresponsive to medical therapy or transcutaneous pacing. The pacer wires are generally placed via the right internal jugular vein or the left subclavian vein and are attached to a pacing generator that generally allows for adjustment of the pacing rate, sensitivity, and energy output. Members of the transport team should be familiar with the specific pacing generator technology and how to troubleshoot it with regard to adjusting those settings. If the sensitivity is too low, the pacer may detect vibrations of transport, interpret them as R waves, and thus will not appropriately pace the patient. Consequently, if the sensitivity is set too high, it may not detect the underlying rhythm and will pace in an asynchronous mode.