Hayes 1994

JPM Vol. 32, No. 4
December 1994:201-207
Relationship Between QaT and RR Intervals in Rats,
Guinea Pigs, Rabbits, and Primates
E. Hayes, M. K. Pugsley, W. P. Penz, G. Adaikan,* and M. J. A. Walker
Department of Pharmacology and Therapeutics, Universityof British Columbia, Vancouver, British Columbia, Canada
*Department of Obstetrics and Gynaecology, Lower Kent Ridge Road, National Universityof Singapore, Singapore
The ECG is routinely used in many species to monitor effects of drugs. While it is relatively
easy to measure both PR and QRS, measurement of QT is complicated by the fact that this
interval can change with heart rate. In order to compensate for variations in QT due to
variations in heart rate, various correction factors have been used, including those of Bazett
and Hodges. Such corrections were devised for humans and may have limited applicability in
other species. We have systematically varied heart rate in anesthetized rats, guinea pigs,
rabbits, and primates using procedures such as vagal stimulation, direct atrial stimulation,
injection of cold saline and drugs, including anesthetics, and measured the resulting QT (as
QaT and related measures). Over a wide range of heart rates we tested various formulas for
their value in correcting for the variation in QT interval associated with changes in heart rate.
In rats the "QT" interval did not change appreciably with heart rate. In the other species QaT
intervals varied in the expected manner with heart rate in that they decreased with tachycar-
dia and increased with bradycardia. Various formulas were tested for their utility in correct-
ing measures of the QaT interval (QaTc) for changes in heart rate in guinea pigs, rabbits, and
primates. In species other than rats, there was little difference between the various formulas
in their ability to increase the precision of QaTc and the normality of its distribution,
although the best correction is that derived from the regression (either linear, square root, or
polynomial) equation relating RR and QaT.
Key Words: QaT correction; Vagal and atrial stimulation; Heart rate; Small animals
Introduction
The QT interval varies with heart rate in many spe-
cies (Kisch, 1953). Generally such changes involve wid-
ening of the QT at low heart rates and shortening at high
heart rates. These changes in rate can be usefully
corrected in humans using formulas such as those due to
Bazett (1920), Fredericia (1920), Hodges et al. (1983),
and others. (See Simonson et al., 1962, for a complete
list of additional formulas.) While these corrections are
useful in humans (Ashman, 1942), their utility may be
limited in other species (Browne et al., 1983). In nonpri-
mate species, various procedures have been suggested
Address reprint requests to M.LA. Walker, Department of Phar-
macology and Therapeutics, University of British Columbia, 2176
Health Sciences Mall, Vancouver, B.C., V6T 1Z3, Canada.
Received March 25, 1994; revised and accepted July 12, 1994.
Journal of Pharmacologicaland ToxicologicalMethods32, 201-207 (1994)
© 1994ElsevierScienceInc.
655 Avenueof the Americas,New York,NY 10010
for correcting rate-dependent changes in QT interval
while it is often assumed that the formulas for humans
apply equally well to primates. Carlsson et al. (1993)
have corrected (i.e., normalized) the QT in rabbits on
the basis of a linear regression relating QT interval and
heart rate as found in their own series of rabbits. QT
correction (QTc) has been attempted in a similar manner
for other common laboratory species such as the dog
(Van de Water et al., 1989), the rat (Bienfeld and Lehr,
1968), and primates (Adaikan et al., 1992).
Measurements of the ECG and QT interval are useful
for a number of purposes, and therefore the most precise
and normally distributed (Gaussian) derivative of QT is
to be preferred. The QT interval is used in the investiga-
tion of the physiological and pathological factors that
underlie QT duration and its prolongation (Taran and
Szilagyi, 1947) as well as evaluation of drugs, such as
class III antiarrhythmics, which lengthen ventricular ac-
1056-8719/94/$7.00

202 JPM Vol. 32, No. 4
December 1994:201-207
tion potentials and increase refractory periods. In order
to assess the actions of various class III antiarrhythmics
in various species, we have made use of various mea-
sures of the QT interval as an indicator of drug effects
on ventricular action potential duration in various labo-
ratory species, i.e., rat, guinea pig, rabbit, and primates,
and for cross-species comparison we sought the most
precise measure of QT. Interpretation of such QT data is
complicated by the fact that some class III drugs lower
heart rate and therefore require correction, and so we
have made a systematic analysis of the effect of varia-
tions in heart rate on measures of QT interval in these
species. In the following study, anesthetized rats, guinea
pigs, rabbits, and primates were subjected to various
procedures chosen for their ability to slow or increase
heart rate.
Materials and Methods
Approval for experiments with rats, rabbits, and
guinea pigs was obtained from the Animal Care Com-
mittee of the University of British Columbia. Standard
laboratory animals were used for this part of the study.
Experiments on primates were performed at the Depart-
ment of Obstetrics and Gynaecology, National Univer-
sity of Singapore, Singapore.
General Surgery
Male Sprague-Dawley rats weighing between 250-
300 g, male Hartley guinea pigs weighing between
500-800 g, and male New Zealand white rabbits weigh-
ing between 1.5-2.5 kg were used in the studies. Two
different anesthetic regimens were used; pentobarbitone
was used to anesthetize both rats (60 mg/kg i.p.) and
rabbits (30 mg/kg i.v.), while guinea pigs were anesthe-
tized with urethane (1 g/kg i.p.). Additional anesthetic
was given when necessary. The anesthetic used in these
species were those conventionally used in our labora-
tory.
Primates were anesthetised with either halothane
(0.5-1.0%) or pentobarbitone (10-20 mg/kg i.v.) after
being initially tranquilized with ketamine (50 mg/kg
i.m.). The pentobarbitone anesthesia was used to induce
high heart rates whereas halothane anesthesia was used
for lower heart rates. The primates were baboons (Papio
anubis) or macaque monkeys (Macaque fascicularis).
Blood pressure was recorded from a transcutaneous
cannula placed in the femoral artery.
In the small species the left carotid artery was cannu-
lated for recording blood pressure on a Grass polygraph
(Model 79D) while the right external jugular vein was
cannulated for drug administration. The ECG was
recorded using a Lead II type of configuration along the
anatomical axis of the heart as determined by palpation.
(For a complete description, see Penz et al., 1992).
ECGs were recorded on Grass polygraph chart paper at
a standard chart speed of 100 mm/sec and on a Honey-
well E for M storage oscilloscope. Measurements of
intervals were made on the Grass polygraph chart re-
corder and from the memory trace of the monitor. Both
measurements were compared and did not differ signifi-
cantly in terms of ECG intervals.
Heart rate was altered by various procedures includ-
ing vagal stimulation, direct atrial stimulation and ad-
ministration of a bolus of cold saline or treatment with
various drugs and anaesthetics.
Vagal Stimulation
Vagal stimulation was accomplished by isolating
both the right branch of the vagal nerve as well as the
accompanying ascending cervical sympathetic nerves.
The nerves were separated by blunt dissection and cut
at the level of the submandibular gland. The cardiac end
of the vagus nerve was stimulated with bipolar elec-
trodes using square wave stimulation at twice threshold
current (it), pulse width of 0.5 msec, and at suitable
frequencies (25-60 Hz) to obtain a heart rate which
varied between 60 beats/min and sinus rhythm. In some
animals the resting heart rate was reduced by metoprolol
(1.0 mg/kg i.v.) administration. It was assumed that
vagal stimulation of the right branch of the nerve did not
result in the significant release of acetylcholine in the
ventricles. During vagal stimulation an attempt was
made to ensure that atrioventricular (AV) conduction
was still intact and that the heart rate was not of an AV
nodal origin.
Atrial Stimulation
For atrial stimulation studies, a specially constructed
stimulating electrode was prepared using polyethylene
(PE) tubing. Two teflon-coated silver wires were in-
serted into either PE50 (rats and guinea pigs) or PE90
(rabbits) tubing. In order to ensure that electrodes were
positioned equidistant apart, a second PE tubing was
inserted between the silver wires. For rats and guinea
pigs the second tubing was PE10, while for rabbits it
was PE50. All tubing was then filled with polyethyl-
ene glycol (PEG40o) to prevent blood from entering
the electrode and to electrically insulate the wires.
The Teflon coating of the wires was removed from the
stimulating end of the electrode to expose bare metal to
allow for pacing of the atria once the electrode was
threaded through the right jugular vein into the right
atria. The suitability of the electrode position was deter-
mined by ensuring that a minimum threshold current at
8 Hz and 1 msec gave a tachycardia with an ECG
recording close to that obtained with the animal in sinus

E. HAYES ET AL. 203
RELATIONSHIP BETWEEN QaT AND RR INTERVALS
rhythm. Tachycardia was induced over the range of
5.5-9.0 Hz.
Cold Saline and Drug Administration
In addition to electrical pacing, changes in heart rate
were also induced by the administration of ice-cold
saline and drugs including the beta adrenoceptor agonist
adrenaline or isopropylnoradrenaline (0.1-10.0 lxg/kg
i.v.). Ice-cold saline, at various volumes (1.0--5.0 ml),
was injected into the jugular vein, and the resulting
changes in heart rate monitored.
Q-T Interval Determination
In view of the difficulty that was sometimes experi-
enced in determining when the T wave returned to the
isoelectric line, an alternative measure of QT (QaT) was
used for species other than rat. QaT is measured to the
peak of the T wave, which could be more clearly
defined in this study. Calculations of the QaT interval
were made directly from the surface of paper charts. In
cases where a clear positive or negative T wave was
seen, the QT interval was the time between the negative
peak of the Q wave and the peak (negative or positive)
of the T wave. In some circumstances when heart rate
changed, there were alterations in the shape of the T
wave. When this occurred, an attempt was made to use
the part of T-wave configuration that best related to the
T wave seen in the normal condition, that is, sinus rate.
In the cases where changing heart rate produced a
change in the configuration of the T wave, a note was
made of the change in such configuration. The use of
QaT was suggested originally by Lepeschkin (1955),
and its value substantiated by Beck and Marriott (1959)
and more recently by Nierenberg and Ransil (1979).
Chernoff (1972) has discussed QaT in relation to QT in
detail.
In the case of rat, it is difficult to detect a T wave that
corresponds exactly with the T wave seen in other
species (Beinfeld and Lehr, 1968; Driscoll, 1981). In
this species therefore, T-wave calculations were made
on the basis of the repolarization wave that followed the
QRS complex. In previous studies we have considered
the difficulties associated with measurement of QT and
as a result measure a surrogate of QT ("QT"). This
measurement is taken from the Q wave to the first major
inflection point on the repolaxization phase. Exact de-
tails of this measurement is illustrated in Figure 1 of
Penz et al. (1992). The RR interval was the mean of 7
beats.
In individual animals, QaT estimates were made over
a range of heart rates, and the regression between QaT
and heart rate was plotted graphically. If the same
regression line occurred between different individuals of
the same species, the individual regressions were accu-
mulated to give an overall regression for the species.
These regressions were used to determine a regression
formula for each species. The effects of using such
derived formulae to correct QaT intervals to a common
RR of 250 msec were compared with the effects of the
other common correction factors.
The utility of the different correction factors was
tested by correcting all rates to a basic 240 beats/min
and examining the distribution of QaTc intervals for
closeness to normality (Gaussian) in terms of heterosce-
dascity by means of D'Agostino's test and calculation of
kurtosis and skewness. In addition, the coefficient of
variation (mean/standard deviation) was calculated as an
index of precision.
Results
Figures I(A-C) show the relationships between un-
corrected QaT as the dependent variable and heart rate
(RR) as the independent variable in individual rabbits,
guinea pigs, and rats, regardless of the technique used to
alter heart rate. As can be seen in Figure 1, the relation-
ships between heart rate and QaT were similar for the
different individuals of the same species, except for
rabbit 2 (Figure l-A), which was statistically signifi-
cantly different from the other animals (p < 0.05 for
difference for the parameters of its regression line from
the group). In the case of rats (Figure l-C), QaT did not
change with rate. However, with both rabbits and guinea
pigs, there was a clear positive regression between RR
and QaT.
In order to determine whether the method of chang-
ing heart rate had an effect on QaT intervals indepen-
dent of rate, data for all members of a particular species
were accumulated and plotted in Figures 2 and 3. In
Figure 2 the data points obtained by the different tech-
niques for changing heart rate show that the relationship
between RR and QaT for the rabbit (a) and the guinea
pig (b) were the same regardless of the procedures used
to change heart rate. In the case of primates (Figure 3),
values for monkeys and baboons initially were plotted
individually, but in view of the fact that the data were
not statistically different, they were grouped together for
calculation of the regression lines of best fit.
An attempt was made to determine the line of best fit
for the data using linear, square root, and polynomial
(power 2) functions, and the resulting best fit lines are
indicated in Figures 2 and 3. In the case of rabbit data,
the regression coefficients were 0.97, 0.90, and 0.97,
respectively, for the three types of regression. In the
case of guinea pig data, the corresponding coefficients
were 0.84, 0.84, and 0.85. For the accumulated pri-
mates, the values were 0.75, 0.75, and 0.75.
In view of the fact that a good regression relationship
between RR and QaT was obtained for the guinea pig,

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R-R Interval (msec)
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Figure 1 (A-C). Relationship between heart rate (as RR interval in msec) and QaT interval duration (in msec) in individual rabbits (a),
guinea pigs (b), and rats (c). Data points are shown for individual members of the three species anesthetised and prepared as indicated in
the Methods section. Data points for individual animals are indicated by the following symbols: animal 1 (O), animal 2 (A), animal 3
(©), animal 4 (+), animal 5 (A), animal 6 (O), animal 7 (V). From 8-10 data points were obtained for each animal over a range of RR
intervals using the variety of procedures for manipulating heart rate described above.

E. HAYES ET AL. 205
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Figure 2. Regression between heart rate (as RR interval in msec) and QaT interval duration for rabbits (a) and guinea pigs (b). Data in
Figure l for rabbits and guinea pigs are replotted and the lines of best fit shown for the linear relationship QaT = aL + bE * RR + a (solid
line), square-root relationship QaT = bs * RR 1/2 (dotted line), and the polynomial relationship QaT = ae + be * RR + Ca * RR 2 (dashed
line), where QaT is the QaT interval in msec and RR in msec. No regression lines or data are shown for the rat because in this species
there was no relationship between QaT and RR (Figure lc). As discussed in the text, rabbit 2 was excluded from Figure 2a since it was
a statistically significant outlier and therefore can be excluded on statistical grounds according to the well-accepted Chauvenet's criteria.
The range of RR values over which various techniques were used to change heart rate are shown in the figure and denoted by VS for
vagal stimulation, AS for direct atrial stimulation, CS for cold saline, and DS for drug stimulation.
rabbit, and primates--whereas there was no such rela-
tionship for the rat--an attempt was made to determine
the most useful correction formulas for normalizing QaT
for changes in heart rate in guinea pigs, rabbits, and
primates (Table 1). The underlying premise for this
aspect of the study was that the best correction of QaT
would result in data that was normally (Gaussian) dis-
tributed, had the least heteroscedascity (by skewness
and kurtosis), and had the lowest coefficient of variation
(mean/standard deviation). The results of such correc-
tions are summarized in Table 2. This table compares
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Figure 3. Regression relationship between heart rate (as RR
interval in msec) and QaT interval duration for primates. Values
are accumulated data for both baboons and monkeys because no
significant difference between the two species was found. As in
Figure 2a and b, the lines of best fit are shown for linear (solid
line), square root (dotted line), and polynomial (dashed line)
formulas using the same equations as those found in the Figure 2
legend.
the correction using the above regression formulas with
a number of standard correction formulas that have been
used in human and other species. Examination of Table
2 shows that the best correction utilized the linear re-
gression equations found in Table 1 for the rabbit and
guinea pig, but not those for the rat because there was
no regression between QaT and heart rate in rats.
Discussion
Variations of QT interval with heart rate in humans
and other species has been noted many times. A com-
plete explanation, in terms of the underlying ionic
currents, is not available, but current hypotheses center
on the various potassium channels responsible for repo-
larization (Hume and Uehara, 1985; Carmeliet, 1993). It
is recognized that, at shorter diastolic intervals, a portion
of the repolarizing channels, such as those responsible
Table 1. Fitting Parameters for the Equations Used in Calculation
of Lines of Best Fit According to the Linear and Nonlinear
Formulas
Square
Linear root Polynomial
Species bL aL bs ap bp ce
G Pig 0.21 58.9 7.0 41.4 0.38 -0.00034
Rabbits 0.16 59.0 6.3 41.3 0.28 -0.00014
Primate 0.27 98.7 10.3 40.6 0.49 -0.00020
The equations used for the above fitting parameters were Linear:
QaT = aL + bL * RR; Square root: QaT = bs * VR'R; Polynomial: QaT =
ap + bp * RR + ce * RR2.
The fitting parameters in the above table are indicated by a, b, and c
with the appropriate subscript for the different functions.

206 JPM Vol. 32, No. 4
December 1994:201-207
Table 2. StatisticalAnalysisof Effectsof Various CorrectionFormulason the QaT Interval
QaT Linear SqRt Poly Bazett Hodges Driscoll
Rabbit
Mean 108.9 99.0 102.0 102.4 102.7 113.7 6.5
SD 34.3 8.9 8.2 7.9 7.6 17.8 0.5
C.V.(%) 49.0 8.9 8.0 7.7 7.4 16.0 7.4
K -0.66 2.6 -0.23 -0.09 0.23 0.92 0.23
S 0.90 -0.26 -0.23 -0.08 0.23 0.92 0.23
P for Fit <0.01 <0.01 >0.20 <0.20 0.1--0.2 <0.01 0.1-0.2
Guinea Pig
Mean 101.3 112.4 113.8 116.8 115.2 118.6 7.3
SD 13.1 7.1 6.9 7.0 8.2 8.2 0.52
C.V.(%) 13 6.3 6.1 6.0 7.1 6.9 7.1
K 0.83 -0.35 -0.40 -0.50 -0.37 0.54 -0.37
S 0.32 -0.47 -0.38 -0.44 -0.25 0.10 -0.25
Fit 0.02-.05 >0.20 >0.20 >0.20 >0.20 >0.20 >0.20
Primate
Mean 223.8 163.7 165.7 149.0 164.4 202.6 10.4
SD 44.2 29.7 29.8 29.4 22.0 38.5 1.39
C.V.(%) 19 18 18 20 13 19 13
K 0.44 -0.27 -0.35 -0.56 0.16 0.30 0.16
S -0.14 -0.13 -0.06 0.08 -0.27 -0.08 -0.27
Fit >0.20 >0.20 >0.20 >0.20 >0.20 >0.20 >0.20
The above table shows the results of the following formulas used to correct the QaT interval for changes in
heart rate in rabbits, guinea pigs, and primates. For the linear correction the equation used was QaTc = QaT -
bE * (RR-250), while for the square root (SqRt) correction the equation used was QaTc = QaT + bs * (2~/~-0 -
~). Finally, the equation QaTc = QaT - be * (RR-250) - cp * ((250)2-(RR)2) was used as the polynomial
(Poly) correction. For each corrected mean, the standard deviation (SD) and coefficient of variation (CV) were
calculated. As well, the shape of the distribution of values for each species or kurtosis (K) and heteroscedasti-
city (S) can be seen, and the level of significance for the fit (P for fit) completes the table.
for iK~, would still remain activated from the previous
action potential and thereby shorten the action potential
(Hauswirth et al., 1972; Boyett and Fedida, 1984). In
addition, over time, changes in rate can be expected to
change the intracellular concentration of sodium and
calcium ions (Nierenberg and Ransil, 1979) as well as
extracellular potassium (Kunze, 1977), and, thus, influ-
ence repolarization. Such changes will, less directly,
result in corresponding changes in the repolarization
currents. The importance of such processes varies with
species, and, thus, rate dependent changes in QT could
be expected to vary with species. The lack of iK~ and
predominant ito in the rat (Josephson et al., 1984) may
account completely for the lack of any major effect on
rate on "QT" in this species.
It is necessary to recognize that changes in QT are
relatively poor indices in how repolarization might be
influenced by heart rate. The QT interval reflects, prob-
ably in a manner proportional to the number of each
type of cell present in the ventricle, repolarization pro-
cesses in the different types of cells found in ventricles.
Thus the repolarization process and its sensitivity to rate
could probably differ between ventricular and Purkinje
cells, between endocardial and epicardial cells, and be-
tween Spike and Dome cells, etc. Unfortunately the
possible differential effects of heart rate changes on the
shape and configuration of the QT interval has not been
systematically investigated. Regardless of such compli-
cations, the QT interval probably still represents a useful
approximation of action potential duration in the ventri-
cle and therefore is worth measuring.
If the QT is to be measured, there is a need to correct
for any concomitant changes in heart rate. This problem
has been studied many times for humans and even for
common laboratory species but without any consensus
as to the most useful and appropriate correction factor. It
is, however, apparent that there cannot be any one
correction formula that applies equally well to all spe-
cies.
In the case of the rat, there appeared to be little
variation in QT, as measured in this study, and heart
rate. Other investigators have previously investigated
this problem in rats and found some variability in QT
with rate (Beinfeld and Lehr, 1956).
In the guinea pig, a positive relationship between
heart rate and QaT was apparent and best corrected by
formulas derived from various regression equations.
Corrections according to the procedures of Bazett, Dri-
scoll, and Hodges were also beneficial in correcting
QaT for heart rate in this species.
In the rabbit, the various regression-corrected QaT
distributions are much better than the uncorrected data.
Corrections proposed by Bazett and Driscoll were of
some value, whereas the correction proposed by Hodges

E. HAYES ET AL. 207
appeared to be of lesser use. In primates, the regression
corrected QaT distributions were not a great improve-
ment over the uncorrected data.
In conclusion, it appears that no one correction for-
mula has marked advantage over other correction for-
mulas for any species.
The BCHCRF and the BCYI-ISF are acknowledged for funding parts of
the above studies. MKP was a recipient of the B.C. Medical Services
Foundation Pre-Doctoral Scholarship.
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Hayes 1994

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