LETTERS TO THE EDITOR
reports showing changes in PEP and HR the directional results for
PEPI are generally the same as for PEP itself, probably owing to
disproportionate effects of the intervention on PEP and HR.
Despite these criticisms, which I hope Dr. Lewis and associates
will address in detail, this is an excellent paper that should be
welcomed by everyone interested in systolic time intervals. I have
already made it required reading among our noninvasive staff (as
are most of Dr. Lewis's previous contributions).
DAVID H. SPODICK, M.D., D.Sc.
St. Vincent Hospital
Worcester, Massachusetts 01604
References
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1. Lewis RP, Rittgers SE, Forester WR, Boudoulas H: A critical review of
systolic time intervals. Circulation 56: 146, 1977
2. Spodick DH, Kumar S, Flessas A, Sriratanaban AD, Sosler BS, Klints
RV, Muench HP: The relationship of paper speed to precision of pulse
wave measurements: A multiple observer study of left ventricular ejection
time. Aerospace Med 40: 707, 1969
3. Pigott VM, Ryan-Platt R, Elber E, Spodick DH: The influence of recording speed on apexcardiographic timing: a multi-observer study of
precision and performance utilizing randomized traces in multiple subjects. Cardiology 57: 232, 1972
4. Spodick DH, Ball HG, Pigott VM: Effects of registration speeds on the
precision of time-based polycardiographic measurements. (abstr) Proc
25th ACEMB, Oct 1972, p 453
5. Cokkinos DV, Heimonas ET, Demopoulos, Haralambakis A, Tsartsalis
G, Gardikas CD: Influence of heart rate increase on uncorrected preejection period/left ventricular ejection time (PEP/LVET) ratio in normal individuals. Br Heart J 38: 683, 1976
6. Talley RC, Meyer JF, McNay JL: Evaluation of the pre-ejection period
as an estimate of myocardial contractility in dogs. Am J Cardiol 27: 384,
1971
7. Knapp E, Aigner A, Baumgartl P, Raas E: Zur frage der frequenzabhangigkeit systolischer kreislaufzeiten und daraus berechneter indizes. Kreislaufforsch 61: 492, 1972
8. Higgins CB, Vatner SF, Franklin D, Braunwald E: Extent of regulation
of the heart's contractile state in the conscious dog by alteration in the
frequency of contraction. J Clin Invest 52: 1187, 1973
9. Harris WS: In Cardiac Mechanics. London, Wiley, 1974, p 238
10. Lance VQ, Spodick DH: Constant load versus rate targeted exercise:
Responses of systolic intervals. J Appl Physiol 38: 794, 1975
The author replies:
To the Editor:
We wish to thank Dr. Spodick for his comments on our
manuscript. Apparently he is in agreement with our results which
indicate 100 mm/sec is the optimum paper speed. He raised the
question of observer bias. We indeed conducted our study as he
suggested ... "completely coded and randomized to reduce
observer bias so that observers could never read data from the same
subject in sequence except by chance." All observers received individual copies of each taperecorded pulse arbitrarily coded and in
random order according to patients and paper recording speeds.
Apparently in Dr. Spodick's as yet unpublished study, he found
no statistically significant difference for the LVET while we noted
such a difference. It should be noted that we used a computer
analysis to achieve a standard against which to judge the LVET
measurements at each paper speed. This may account for our
different results. However, even if the mean values were not
statistically different, the greater observer variability at slower
speeds dictates that 100 mm/sec is the optimal recording speed. I
feel that this debate over paper speed has become somewhat
academic since modern technology has resulted in a number of
different recording devices with 100 mm/sec capability.
The argument over whether to correct the PEP for heart rate is
essentially an "empirical vs functional" argument. Until it can be
proved that PEP is functionally independent of heart rate, we must
rely on carefully collected empirical data. From such studies it is
clear that a significant, albeit weak, correlation exists. Naturally,
only studies with large numbers of subjects will reveal this
relationship. Atrial pacing data cannot be applied to this situation,
201
as this is an unnatural mode of increasing the heart rate (i.e., no increase in adrenergic tone).
RICHARD P. LEWIs, M.D.
Ohio State University Hospitals
Columbus, Ohio 43210
Small vs Large Surface Area Electrodes
To the Editor:
We were interested in the article by Hughes et al. (Circulation 54:
128, 1976) because the promise of lower long-term thresholds with
small-area electrodes has brought them into widespread use. If a
hazard exists in their failure to sense, it would seriously jeopardize
their use, as competitive pacing is still a highly unattractive and
potentially harmful long-term complication.
We would agree with the observations of the authors that the
combination of a small-area electrode and a low input impedance of
the detection device may lead to failure of sensing. However, we
would like to assert that small-area electrodes coupled with high impedance detection circuits such as are used in most modern
pacemakers do not constitute such a hazard. In other words, the
signal provided by a small-area electrode is determined by the combination of the interface impedance of the electrode and the input
impedance of the detection device, or pacemaker. Also, a small-area
electrode should produce a larger and sharper inherent signal than a
very large area (within limits) since the large area will average the
signal more than a smaller one.
The failure to clearly point out the unique and almost artificial
circumstances of their experiments is illustrated in their table 1. The
results demonstrate how the 1 KU load, in combination with the
electrode impedance, attenuates the signal strength. As pointed out
by the authors, the 1 KQ load is much lower than commercially
available pulse generators. The effects with actual pacemakers having impedances of 20 Kg will be about twenty times less severe. The
true "sensed" electrogram would be represented by an unloaded
measurement, but these values are unfortunately not provided.
Some other points are:
1) bipolar electrodes do not inherently sense smaller signals than
unipolar electrodes.
2) Fibrous tissue does not greatly affect the electrode-tissue impedance - the impedance is mainly at the metal-electrolyte interface. The fibrous tissue does, however, separate the electrode from
viable tissue and so reduces the sensed signal.
3) In figure 5, the electrode tip design for optimal sensing is surely
offered as an example of principle rather than as an actual electrode.
The geometry of the large band sensing electrode is such that it
would be very difficult to have it in intimate contact with excitable
tissue if used as an endocardial electrode.
In summary, small-area electrodes as they are currently used do
not constitute a hazard for sensing when the higher interface impedance associated with these electrodes is appropriately used with
a higher input impedance of the detecting pacemaker.
T. E. CUDDY, M.D.
M. B. RABER, P.ENG.
University of Manitoba
Winnipeg, Manitoba
The authors reply:
To the Editor:
We agree completely with Dr. Cuddy that bipolar electrodes are
not inherently inferior to unipolar electrodes. The ability of any type
electrode to sense R-wave potentials is based primarily on its surface area, not the mode in which sensing is done.' I hope we did not
leave that impression with anyone in our paper.
Secondly, we have only limited information of the chronically implanted electrodes' ability to sense. We would, however, expect the
sensing ability to be reduced by fibrous tissue ingrowth.
Small vs large surface area electrodes.
T E Cuddy and M B Raber
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Circulation. 1978;57:201-202
doi: 10.1161/01.CIR.57.1.201
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