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Demand pacemaker
Editor
D‘niversity
_a-
of Pennsylvania Philadelphia,
December
Pa. 22, 1966
To the E&h: Once again, the snark-like, supposedly unique zero of potential has been introduced into electrocardiography. This time it has been discussed in eonncction with maps showing equipotential contours on the body surface during the cardiac cycle (“Body Surface Isopotential Maps in Normal Children, Ages 4 to 14 vears,” by M. S. Spach, W. P. Silberberg, 3~ I?. Bdineau, R. C. Barr, E. C. Long, T. M. Gallie, J, B. Gabor, and A. G. Wallace, AMERICAX HEART JOURXAL 72:640, 1966). It is particularly ironic that the subject should be reintroduced in this contest because isopotential maps provide a striking example of the uselessness of identifying a so-called zero line or any other absolute potential in electrocardiography. Consider a map drawn for any instant during the cardiac cycle. I3y definition, an equipotential contour is a line drawn through points between which no difference in potential is developed at that instant. Clearly, the construction of such a line has nothi:;g to do with the reference point chosen by the investigator in some intermediate step that he may have used to obtain his map. Differences among maps from the same subject at the same instant of the cardiac cycle could involve only the particular iines one chooses to draw and the numbers with which the lines are labeled. The more lines used, the closer all such maps will be to each other in appearance. For example, if the maps are all drawn showing equipotentials at intervals of 0.01 mv., the level of quantization, the contours of the maps obtained would be virtually indistinguishable, no matter what the reference. When the level of quantization is increased and fewer lines are used, detail is lost and maps may look different because they will not necessarily show identical contour lines. Under these conditions, it is conceivable that uncertainties in the interpolation of contour lines between those actually shown may impede aomparison of such maps. As long as a sufficient number of lines are drawn to convev the essential pattern, all maps provide equivalent informatlon no matter what is used as a reference. Altering the reference causes all numbers on a particular map to change by the same amount. ,9lthough adoption of a common reference appears 10 facilitate comparison of electrocardiographic maps among investigators, it has resulted, unfortunately, in the assignment of signilicance to the absolute value of a line rather than to its contour and association with neighboring lines. The electrocardio293
graphic data from which the map ‘was coi:-rructed, as well as any conventional or ilOnCailvf~~ti~.:il.ai SLITface lead, can be reconstituted from the ma;, (v;ilhin . . . rhe lumts of precls~on and accuracy ;vail.lbiei, regardless of the reference used. For this rezon, ibe concept of an absolute potential---rather l-h&in differences in potential-is without inearning. LIs50ciale
ProSpssor
David 5. Gaselowi%, Ph.D., of Elecfricai Eqineerin~ Stanley A Rriliir, XB.,
To the Edztor: iX:e are indebted to Dr. Geselowitz and t Ir. Briiie? for pointing out the implication they note
To the Ed&w: 3.~ desci-ib,d by i&-s. The demand pacemaker, 1,emberg and Castellanos and their asaoziates’,Z has certainly provided a dramatic new method of treatment of A-V block, particrrlar!y of the intermittent type. It has been emphasized that tl;is pacemaker will escape whenever a preset asystolic rnterval has been exceeded and will automa tidly stop when a natural or artificial beat occurs at a faster rate. In 1965, I had a chance to use this pacemaker* when I was in the United Statrs at the I-icart Station of the Philadelphia Genera! Xospita i, It worked
294
Fig.
Letters
to the Editor
1
almost completely on demand. 1 experienced, however, as shown in Fig. 1, a very interesting case in which the pacemaker functioned as iatrogenic parasystole which continuously stimulated the heart when the direction of the main QRS complexes of natural beats suddenly changed. This experience shows that iatrogenic parasystole can occur, even when a demand pacemaker is used, if the directions of the main QRS complexes of natural extrasysto!es or the natural rhythm are opposed to those of the preceding beats. I do not know whether the pacemaker has been improved to guard against such a dysfunction since I left the United States. I am interested in this point, and I would like to know the present status of the pacemaker, and of any improvements. Makoto Takagi, M.D. Cardiology branch Department of Medicine Kyoto City Hospital Kyoto, J@an *American
Optical
Co., Chelsea,
Mass.
REFERENCES 1.
2.
Lemberg, L., Castellanos, A., Jr., and Berkovits, B. V.: Pacemaking on demand in A-V block, J. A. M. A. 191:12, 1965. Castellanos, A., Jr., Lemberg, L., and Jude, J. R.: Depression of artificial pacemakers by extraneous impulses, AH. HEAKT J 73:24,1 967.
Reply To the Editor: Thank you very much for allowing me to conment on Dr. Takagi’s letter to the editor. Dr. Takagi has properly analyzed the electrocardiographic trace. It is evident that, with a change in direction of the QRS complex to a negative deflection, the demand pacemaker previously set to sense positive deflections now performs as a continuous pacemaker. A pacemaker-induced ventricular parasystole results. The third stimulus artefact in the top trace fuses with but does not effect the QRS complex. The original demand pacemakers built into the bedside cardiac monitors sense the first derivative of either the positive or negative deflection of the QRS complex but not both. A switch on the front panel of the unit can change the sensing from positive to negative. Dr. Takagi may be interested to know that the prototype units of the new implantable miniaturized demand pacemaker now in clinical use in our hospital sense both positive and negative deflections. Louis Lemberg, M.D. Associate Professor of Medicine Chief, Division of Electrophysiology School of Medicine University of Miami il/iami, Fla.
D‘niversity
_a-
of Pennsylvania Philadelphia,
December
Pa. 22, 1966
To the E&h: Once again, the snark-like, supposedly unique zero of potential has been introduced into electrocardiography. This time it has been discussed in eonncction with maps showing equipotential contours on the body surface during the cardiac cycle (“Body Surface Isopotential Maps in Normal Children, Ages 4 to 14 vears,” by M. S. Spach, W. P. Silberberg, 3~ I?. Bdineau, R. C. Barr, E. C. Long, T. M. Gallie, J, B. Gabor, and A. G. Wallace, AMERICAX HEART JOURXAL 72:640, 1966). It is particularly ironic that the subject should be reintroduced in this contest because isopotential maps provide a striking example of the uselessness of identifying a so-called zero line or any other absolute potential in electrocardiography. Consider a map drawn for any instant during the cardiac cycle. I3y definition, an equipotential contour is a line drawn through points between which no difference in potential is developed at that instant. Clearly, the construction of such a line has nothi:;g to do with the reference point chosen by the investigator in some intermediate step that he may have used to obtain his map. Differences among maps from the same subject at the same instant of the cardiac cycle could involve only the particular iines one chooses to draw and the numbers with which the lines are labeled. The more lines used, the closer all such maps will be to each other in appearance. For example, if the maps are all drawn showing equipotentials at intervals of 0.01 mv., the level of quantization, the contours of the maps obtained would be virtually indistinguishable, no matter what the reference. When the level of quantization is increased and fewer lines are used, detail is lost and maps may look different because they will not necessarily show identical contour lines. Under these conditions, it is conceivable that uncertainties in the interpolation of contour lines between those actually shown may impede aomparison of such maps. As long as a sufficient number of lines are drawn to convev the essential pattern, all maps provide equivalent informatlon no matter what is used as a reference. Altering the reference causes all numbers on a particular map to change by the same amount. ,9lthough adoption of a common reference appears 10 facilitate comparison of electrocardiographic maps among investigators, it has resulted, unfortunately, in the assignment of signilicance to the absolute value of a line rather than to its contour and association with neighboring lines. The electrocardio293
graphic data from which the map ‘was coi:-rructed, as well as any conventional or ilOnCailvf~~ti~.:il.ai SLITface lead, can be reconstituted from the ma;, (v;ilhin . . . rhe lumts of precls~on and accuracy ;vail.lbiei, regardless of the reference used. For this rezon, ibe concept of an absolute potential---rather l-h&in differences in potential-is without inearning. LIs50ciale
ProSpssor
David 5. Gaselowi%, Ph.D., of Elecfricai Eqineerin~ Stanley A Rriliir, XB.,
To the Edztor: iX:e are indebted to Dr. Geselowitz and t Ir. Briiie? for pointing out the implication they note
To the Ed&w: 3.~ desci-ib,d by i&-s. The demand pacemaker, 1,emberg and Castellanos and their asaoziates’,Z has certainly provided a dramatic new method of treatment of A-V block, particrrlar!y of the intermittent type. It has been emphasized that tl;is pacemaker will escape whenever a preset asystolic rnterval has been exceeded and will automa tidly stop when a natural or artificial beat occurs at a faster rate. In 1965, I had a chance to use this pacemaker* when I was in the United Statrs at the I-icart Station of the Philadelphia Genera! Xospita i, It worked
294
Fig.
Letters
to the Editor
1
almost completely on demand. 1 experienced, however, as shown in Fig. 1, a very interesting case in which the pacemaker functioned as iatrogenic parasystole which continuously stimulated the heart when the direction of the main QRS complexes of natural beats suddenly changed. This experience shows that iatrogenic parasystole can occur, even when a demand pacemaker is used, if the directions of the main QRS complexes of natural extrasysto!es or the natural rhythm are opposed to those of the preceding beats. I do not know whether the pacemaker has been improved to guard against such a dysfunction since I left the United States. I am interested in this point, and I would like to know the present status of the pacemaker, and of any improvements. Makoto Takagi, M.D. Cardiology branch Department of Medicine Kyoto City Hospital Kyoto, J@an *American
Optical
Co., Chelsea,
Mass.
REFERENCES 1.
2.
Lemberg, L., Castellanos, A., Jr., and Berkovits, B. V.: Pacemaking on demand in A-V block, J. A. M. A. 191:12, 1965. Castellanos, A., Jr., Lemberg, L., and Jude, J. R.: Depression of artificial pacemakers by extraneous impulses, AH. HEAKT J 73:24,1 967.
Reply To the Editor: Thank you very much for allowing me to conment on Dr. Takagi’s letter to the editor. Dr. Takagi has properly analyzed the electrocardiographic trace. It is evident that, with a change in direction of the QRS complex to a negative deflection, the demand pacemaker previously set to sense positive deflections now performs as a continuous pacemaker. A pacemaker-induced ventricular parasystole results. The third stimulus artefact in the top trace fuses with but does not effect the QRS complex. The original demand pacemakers built into the bedside cardiac monitors sense the first derivative of either the positive or negative deflection of the QRS complex but not both. A switch on the front panel of the unit can change the sensing from positive to negative. Dr. Takagi may be interested to know that the prototype units of the new implantable miniaturized demand pacemaker now in clinical use in our hospital sense both positive and negative deflections. Louis Lemberg, M.D. Associate Professor of Medicine Chief, Division of Electrophysiology School of Medicine University of Miami il/iami, Fla.