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OBSERVATION OF ELECTROMAGNETIC SIGNALS FROM IMPLANTABLE PACEMAKERS
OBSERVATION OF ELECTROMAGNETIC SIGNALS FROM IMPLANTABLE PACEMAKERS Perry Sprawls, Jr., MS., William B. Miller, Jr., B.S., and William D. Logan, Jr., M.D., Atlanta,
Ga.
B
implanted cardiac pacemakers by necessity have a limited functional duration. Some have ceased functioning earlier than anticipated because of various reasons. These may be: (a) battery failure, (b) broken elec trode, and (c) an apparent increase in electrical resistance at the electrodes. The Thoracic and Cardiovascular Surgery Service at Emory University has had experience with 45 patients with permanent implanted pacemakers over a period of 3 years. Ten of these have returned with malfunctions one or more times. Two types of pacemakers have been used (Medtronic and Electrodyne). With the increasing number of these patients who return, it has become apparent that a simple method of determining the exact cause of the malfunction would be desirable. All implanted cardiac pacemakers and leads produce electromagnetic sig nals with each electrical pulse. There are two sources of these signals. First, these particular pacemaker units generate a relatively strong electromagnetic field because of the presence of certain components which function as small electromagnets with each discharge. Second, the current pulses in the leads to the heart also give rise to a magnetic field, but with different characteristics. These electromagnetic signals can be observed and recorded without making physical contact with the patient. The following data, derived from preliminary laboratory investigations, show that the presence and characteristics of these lead signals may offer much as a diagnostic aid. ATTERY-CONTROLLED
APPARATUS
The electromagnetic signal transducer is a coil of approximately 25,000 turns of copper wire enclosed in an aluminum shield. The signal from the coil is fed through a high-gain audiofrequency amplifier into an oscilloscope. By placing the transducer near the pacemaker or its connected leads, the electro magnetic pulse can be observed on the oscilloscope. Prom the Department of Radiology and the Division of Thoracic and Cardiovascular Surgery, Emory University School of Medicine, Atlanta, Ga. This work was supported in part by U. S. Public Health Service Grant No. HTS-5494. Received for publication Nov. 5, 1964. 748
Vol. 49, No. S May, 1965
ELECTROMAGNETIC SIGNALS FROM PACEMAKER
749
Fig. 1.—Electromagnetic pulses observed over pacemaker leads when connected to normal heart-equivalent load (A), and normal load with added resistances of 470 ohms (B), 1,000 ohms (C), 2,200 ohms (D), 4,700 ohms (B), 10,000 ohms (F).
METHOD
Pacemakers by two different manufacturers* were used in this experiment. Each pacemaker was placed on the table with the leads extended and connected to a heart-equivalent load consisting of a 1,000 ohm resistor shunted by a 500 ohm resistor in series with a 0.5 microfarad capacitor. 1 A variable resistance was included in the circuit between one electrode and the heart-equivalent load. The signals were observed by placing the transducer near the pacemaker or the leads. *Electrodyne Company, Norwood, Mass., and Medtronic, Inc., Minneapolis 18, Minn.
Fig. 2.—Electromagnetic pulses observed over pacemaker leads when connected to normal heart-equivalent load (A), and normal load with added resistances of 470 ohms (B), 1,000 ohms (C), 2,200 ohms (D), 4,700 ohms (E). DISCUSSION
Although signals from implanted pacemakers have been observed, the work reported here used only exposed pacemakers so that the electrical conditions could be varied for experimental purposes. Chardack 2 has shown that the electrical resistance around the electrodes in the myocardium will often increase with time, which is probably due to the formation of scar tissue. The increased resistance decreases the current to the myocardium and can cause it to fall below the threshold level. I t was found that the shape of the electromagnetic pulse from the leads is a function of this increase in resistance. Various signals were observed over the leads of a Medtronic, Inc., pacemaker connected to a heart-equivalent load with
Vol. 49, No. 5
ELECTROMAGNETIC SIGNALS FROM PACEMAKER
May, 1965
751 , u ±
different values of series resistance (Fig. 1). The height of the initial positive spike appears to be quite indicative of the added series resistance. A similar experiment, performed with a pacemaker manufactured by the Electrodyne Co., produced a different pulse shape but the effect of added series resistance could also be observed (Fig. 2). The signal produced directly over the pacemaker originates in the oscil lator or timing circuit and appears to have little value other than to indicate that the circuit is functioning. It has not been possible to relate the shape of this pulse to the output voltage or current of the pacemaker. Further classification of these signals as related to various pacemakers and clinical situations appeal's to be indicated. REFERENCES 1. Davies, J . G., and Sowton, G. E . : Cardiac Pacemakers, Physiol. M. Biol. 9: 257, 1964. 2. Chardaek, W. M., Gage, A. A., and Greatbatch, W.: A Transistorized, Self-Contained, Implantable Pacemaker for the Long-Term Correction of Complete Heart Block, Sur gery 48: 643, 1960.
Ga.
B
implanted cardiac pacemakers by necessity have a limited functional duration. Some have ceased functioning earlier than anticipated because of various reasons. These may be: (a) battery failure, (b) broken elec trode, and (c) an apparent increase in electrical resistance at the electrodes. The Thoracic and Cardiovascular Surgery Service at Emory University has had experience with 45 patients with permanent implanted pacemakers over a period of 3 years. Ten of these have returned with malfunctions one or more times. Two types of pacemakers have been used (Medtronic and Electrodyne). With the increasing number of these patients who return, it has become apparent that a simple method of determining the exact cause of the malfunction would be desirable. All implanted cardiac pacemakers and leads produce electromagnetic sig nals with each electrical pulse. There are two sources of these signals. First, these particular pacemaker units generate a relatively strong electromagnetic field because of the presence of certain components which function as small electromagnets with each discharge. Second, the current pulses in the leads to the heart also give rise to a magnetic field, but with different characteristics. These electromagnetic signals can be observed and recorded without making physical contact with the patient. The following data, derived from preliminary laboratory investigations, show that the presence and characteristics of these lead signals may offer much as a diagnostic aid. ATTERY-CONTROLLED
APPARATUS
The electromagnetic signal transducer is a coil of approximately 25,000 turns of copper wire enclosed in an aluminum shield. The signal from the coil is fed through a high-gain audiofrequency amplifier into an oscilloscope. By placing the transducer near the pacemaker or its connected leads, the electro magnetic pulse can be observed on the oscilloscope. Prom the Department of Radiology and the Division of Thoracic and Cardiovascular Surgery, Emory University School of Medicine, Atlanta, Ga. This work was supported in part by U. S. Public Health Service Grant No. HTS-5494. Received for publication Nov. 5, 1964. 748
Vol. 49, No. S May, 1965
ELECTROMAGNETIC SIGNALS FROM PACEMAKER
749
Fig. 1.—Electromagnetic pulses observed over pacemaker leads when connected to normal heart-equivalent load (A), and normal load with added resistances of 470 ohms (B), 1,000 ohms (C), 2,200 ohms (D), 4,700 ohms (B), 10,000 ohms (F).
METHOD
Pacemakers by two different manufacturers* were used in this experiment. Each pacemaker was placed on the table with the leads extended and connected to a heart-equivalent load consisting of a 1,000 ohm resistor shunted by a 500 ohm resistor in series with a 0.5 microfarad capacitor. 1 A variable resistance was included in the circuit between one electrode and the heart-equivalent load. The signals were observed by placing the transducer near the pacemaker or the leads. *Electrodyne Company, Norwood, Mass., and Medtronic, Inc., Minneapolis 18, Minn.
Fig. 2.—Electromagnetic pulses observed over pacemaker leads when connected to normal heart-equivalent load (A), and normal load with added resistances of 470 ohms (B), 1,000 ohms (C), 2,200 ohms (D), 4,700 ohms (E). DISCUSSION
Although signals from implanted pacemakers have been observed, the work reported here used only exposed pacemakers so that the electrical conditions could be varied for experimental purposes. Chardack 2 has shown that the electrical resistance around the electrodes in the myocardium will often increase with time, which is probably due to the formation of scar tissue. The increased resistance decreases the current to the myocardium and can cause it to fall below the threshold level. I t was found that the shape of the electromagnetic pulse from the leads is a function of this increase in resistance. Various signals were observed over the leads of a Medtronic, Inc., pacemaker connected to a heart-equivalent load with
Vol. 49, No. 5
ELECTROMAGNETIC SIGNALS FROM PACEMAKER
May, 1965
751 , u ±
different values of series resistance (Fig. 1). The height of the initial positive spike appears to be quite indicative of the added series resistance. A similar experiment, performed with a pacemaker manufactured by the Electrodyne Co., produced a different pulse shape but the effect of added series resistance could also be observed (Fig. 2). The signal produced directly over the pacemaker originates in the oscil lator or timing circuit and appears to have little value other than to indicate that the circuit is functioning. It has not been possible to relate the shape of this pulse to the output voltage or current of the pacemaker. Further classification of these signals as related to various pacemakers and clinical situations appeal's to be indicated. REFERENCES 1. Davies, J . G., and Sowton, G. E . : Cardiac Pacemakers, Physiol. M. Biol. 9: 257, 1964. 2. Chardaek, W. M., Gage, A. A., and Greatbatch, W.: A Transistorized, Self-Contained, Implantable Pacemaker for the Long-Term Correction of Complete Heart Block, Sur gery 48: 643, 1960.