A 65-year-old man arrives, again with a diverticular perforation, but this patient has an old median sternotomy incision and a lump just below his left clavicle. This patient’s BP is 130/90 with a heart rate of 100. By ECG, it looks like the ventricle is being paced, but the atrium is not.
You tell the operating room circulating nurse that you would like to use the electrocautery during surgery. She suggests putting a magnet over the pacemaker.
1. What triggers a ventricular demand pacemaker to fire?
2. What does the magnet do?
Pacemakers & Electrocautery
The early pacemakers were all “fixed rate” and were programmed to emit one pulse/second, for a ventricular rate of 60. However, occasionally, even patients with complete 3° heart block will exhibit a spontaneous ventricular beat. When the repolarization of this spontaneous QRS (upstroke of the T wave) coincides with a fixed-rate pacer stimulus, ventricular fibrillation may result.
So, the “demand” pacemaker was developed. This type of pacemaker is smart about how it fires. This can more easily be seen with an example. To make the math easier, we will pretend we have a demand pacer with a rate set at 60 beats/min. Every time this “demand” pacemaker senses a QRS complex it turns off and waits for one second. If, at the end of one second, it doesn’t sense another QRS, it fires. So, the patient’s ventricular rate can spontaneously rise above 60, but never below. The trick here is that the pacer can sense many electrical stimuli (like an electrocautery) as a QRS and will stop pacing, but only for as long as you use the cautery. So don’t blast away for long periods.
The ventricular demand pacer prevents the patient from dying; however, it does not increase its rate when the patient runs up stairs. Thus, an atrioventricular pacer coordinates electrodes in both the atrium and the ventricle. When this device “senses” an atrial P wave, it waits a short preset interval (say, 0.18 second), and if it senses a QRS, it turns off; if it doesn’t detect a QRS, it paces one. This device can therefore track faster atrial rates and create atrioventricular synchrony (adds atrial “kick”). But if the sensed atrial rate ever drops below 60, the A-V pacer paces both the atrium and the ventricle.
Pacemakers are switched into a “fixed-rate” mode whenever a magnet is placed over it. Typically, the fixed ventricular rate is set to 60. If you place a magnet over this man’s pacemaker to convert the pacer into its “fixed-rate” mode, his ventricular rate will actually slow down (from 100 to 60). This would probably be bad for his cardiac output.
Yes, but intelligently. The patient’s demand pacemaker will sense the electrocautery as the patient’s heartbeat and transiently turn itself off. If you ...
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