Electrocardiogram and Phonogram of Adult and Newborn Mice in Normal Conditions and Under the Effect of Cooling, Hypoxia and Potassium’

A. G. RICHARDS, E. SIMONSON AND M. B. VISSCHER

From the Department oj Physiology and the Laboratory of Physiological Hygiene, Ulziversity oj Milznwota, Minneapolis, Minnesota

T HE ELECTROCARDIOGRAM Of SIlld ani- mals is of interest for problems of con- ductivity, time relationships between

heart rate and the various intervals of the cycle, and physiological correlations to body tem- perature and metabolic rate, but the literature is extremely small. Buchanan (I) found a P-R internal of .04 second in the dormouse, using a capillary electrometer. Agduhr and Stenstrijm (2) as well as Lombard (3) found no discernible T wave in the electrocardiogram of mice and other small mammals. While O’Bry- ant, et al. (4) give measurements for the T wave in the mouse electrocardiogram, the pub- lished tracings do not show clearly discernible T waves. The absence of a T wave in the mouse electrocardiogram is an intriguing problem. Lombard (3) suggested that this might be due to prolonged repolarization, but did not con- sider other possible mechanisms. No heart sounds on mice have yet been published al- though the relationship between mechanical and electrical systole at heart rates as high as 6oo/min. is of importance for the general validity of formulations suggested for a much lower range of h.eart rates.

METHOD

Mice were placed on their backs and fixed to a board with small strips of adhesive tape attached to the legs. Needle electrodes were placed just under the skin. Three standard leads and four precordial leads (VI, VE, V2 and V6) were taken in all animals, but additional unipolar leads were taken from many other positions for exploratory purposes.

Records were made by means of a Sanborn Stetho- cardiette, with a speed of the photographic paper at 75 mm/set. and a sensitivity of 2 cm deflection for I mv input. Simultaneous phonocardiograms were obtained from a shaved area of the chest.

In a study of the effects of cold, the animal was lightly anesthetized with ether, in order to prevent

Received for publication February 16, 1953.

l This work was supported in part by a research grant from the Minnesota Heart Association.

shivering, and surrounded by ice cubes. With this method, temperatures could be reduced from 37O to as low as 2o°C, and heart rates from about 6oo/min. to 200 or less.

Anoxia was induced by nitrogen breathing, but this method was unsatisfactory due to muscular activity. Impediment of respiration by opening the chest under anesthesia was more satisfactory.

Body temperatures of adult and newborn mice were recorded through a thermally sensitive resistor (ther- mistor) attached to a standard Wheatstone bridge. The thermistor was made up in a glass-coated bead of about 0.015 inches diameter. This was placed in the tip of a fine plastic catheter, with lead wires running through the length of the catheter to the bridge. The tip of the catheter was then advanced a few millimeters into the rectum. The bridge was calibrated to record temperatures from 27-40°C with an error of about O.OIOC within this range.

In order to vary the temperature of the newborn mouse, a wooden box (IO x IO x 4 inches) was con- structed, with one side covered with glass to permit a view of the interior. Six light bulbs of 15 watts were connected around the inside walls of the box to provide a source of heat. The mouse was taped down inside the box and ECG and thermistor leads were led through a hole in the wall. Temperatures inside the box were recorded through a mercury thermometer.

RESULTS AND DISCUSSION

Absence of T Wave. Like other authors (2,

3), we were not able to find any characteristic T waves in the electrocardiogram of IO normal adult mice with heart rates ranging from 540- Tzo/min. Figure I shows one typical example. The possibility that a T wave was present in another plane was excluded by taking unipolar leads from many other positions. However, on the downstroke of the R wave in lead I, II, III, on VF, VE and VS, and on the upstroke of the S wave in a VR, there is a marked notch, gener- ally of lesser amplitude than the R wave. It is noted that slowing of the movement of the string starts with this notch, which then sub- sides to the base line. This notch is present in all leads. In the precordial lead over the xiphoid process, in the example of figure I, but not always, it is even higher than the R wave.

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294 RICHARDS, SIMONSON AND VISSCHER Volume I74

Exami.nation of the tracings of other authors (2, 3) also reveals this notch.

the QRS is unlikely from the point of compara-

In the absence of any comment in the previ- tive physiology, since none of the larger ani-

ous literature, the notch was obviously con- mals, starting from the guinea pig shows this

sidered to be a part of the QRS complex phenomenon. A notch would indicate a delay of ventricular conduction in some part of the

assuming absence of the T wave (3). However, such assumptions (notched QRS or absence of

myocardium. The probability of such desyn- chronization would increase with the mass of

T waves) are very hard t.o accept. A T wave muscle to be activated. may be absent in a given location when its Therefore, it appears more likely that the axis is perpendicular t.o that point, but then notch constitutes the T wave. It is assumed? it should be present in other locations, in fact then, that the duration of excitation is so short

ETHER

+COLD

420/min.

FIG. I. First row: ECG of adult mouse, in normal condition. There is a notch on the descending limb of the R wave or ascending limb of the S wave. Second TOW: ether anesthesia and cold decreases heart rate and separates the notch into a distinct wave in leads I II III aVR, and aVF. Third mu: in- traperitoneal administr;tio; o;

KC1 produces, with further slow- ing of the heart rate, a still more

second wave especially in lead II

and aVF*

GUlff EA PIG RAT MOUSE

300hin. 48 O/min. 63Ohin.

FIG. 2. Transition of the pattern from a notch on the descending limb of the R wave in the mouse to a separate T wave in the guinea pig.

over the largest part of the body surface. As mentioned already, no distinct T wave could be found in any location. Also, in case of an equiphasic QRS complex the T wave may be nearly isoelectric, since the slower repolariza- tion of segments of opposite polarity may over- lap over a much greater interval of time, while the faster depolarization of the two different segments is separated. However, a separate T wave was missing in tracings with essentially monophasic QRS as well as in tracings with diphasic QRS.

The presence of a notch as normal feature of

in each fiber, that depolarization and repolari- zation overlap to a greater extent than in larger animals. In other words, repolarization starts in some parts of the heart before the depolarization is complete in other parts. There is, therefore, no measurable S-T segment, but one might postulate and even measure a nega- tive S-T segment from projections of the de- scending limb of the R wave above the notch, and the ascending limb of the notch, to the baseline.

This assumption is strongly supported by Rotschuh’s (5) investigation of monophasic action currents in man, rabbit and rat. The duration of excitation decreases sharply in this order. The monophasic action current of the rat shows a sharp peak, as contrasted to the slow decrease or plateau in larger animals.

For a more direct experimental proof, we expected that by slowing down the heart rate,

the notch might become more clearly separated from the QRS complex. We also hoped that it

might be possible to change the direction of the notch by procedures which invert the T

waves in larger animals.

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Azlgust 1953 ELECTROCARDIOGRAM OF MICE 295

Experimental Variables. a) Cold-Figure I due to a differential effect on different parts of shows the effects of ether and cold on the elec- the heart. It is possible that in the small heart trocardiogram of an adult mouse. Ten animals of the mouse the effect of anoxia is more uni- were subjected to this procedure and this figure form. is a representative sample. Comparison of the ECG of Guinea Pig,

Here the rate has been slowed from 630/ Rat and Mouse. For the question of the T min. to 4zo/min. The notch of the ventricular wave, comparison of the electrocardiogram of complex has become separated from the QRS, the guinea pig, rat and mouse is of interest with the string reaching the isoelectric line (fig. 2). The guinea pig shows a definite S-T between the two peaks. Although the separa- segment, while the electrocardiogram of the tion of the peaks was definitely more distinct, rat has an intermediate position between that it was not possible to produce a measurable of the guinea pig and that of the mouse. The S-T segment in the adult mouse by use of cold electrocardiogram of the rat is similar to that alone. It is of interest that the amplitude of all of the mouse under effect of ether and cold waves increased during exposure to cold. (fig. I). In the mouse, the T wave occurs on

b) HypoxiaThe production of hypoxia by the downstroke of the R wave, that is, repolari- opening the chest under ether anesthesia pro- zation has begun before depolarization is com- duced a profound drop in heart rate with great plete. increase of the P-R interval up to the point of In the rat, depolarization and repolarization complete heart block. The second wave of the are nearly, but not quite, separated. In the ventricular complex did not become inverted guinea pig, depolarization and repolarization but became separated from the first wave, are completely separated. In fact, the electro- simi.lar to the effect of cold in figure I. The cardiogram of the guinea pig is identical in

TABLE I. ELECTROCARDIOGRAPHIC TIME INTERVALS IN IO ADULT

AND IO NEWBORN MICE (MEANS AND STANDARD DEVIATIONS)

Group Rate P-R QRS Q-T KQ-T

Newborn 286 zt 56.8 0.06 xt 0.003 0.01 * 0.00 0.08 A 0.018 oqg II= 0.036

Adult 632 k 51.3 0.038 h 0.1004 0.01 h 0.004 0.035 =I= 0.005 0,111 h 0.014

appearance of electrical alternans was .a common finding. It will be discussed in a separate paper.

c) Potassitim-Hyperpotassemia produced marked changes. Figure I (third row) shows the effect of an injection of I cc of 2% KC1 into the peritoneum of a mouse already show- ing the effects of ether anesthesia and cold. The rate drops from qao/min. to 18u. The P-R interval increases from 0.07 to about 0.14 and the second wave of the ventricular complex is heightened out of proportion to the R wave in lead II. This supports the view that the second wave is a T wave, since increase of the T wave is a known effect of hyperpotassemia. Similar effects were found in animals not sub- jected to anesthesia and cold. It should be noted that in lead III a definite S-T segment (elevated) has developed.

The failure to invert the direction of the T wave in anoxia is not necessarily a contradic- tion to our hypothesis. The inversion of the T wave in larger animals and man in anoxia is

pattern with that of man, with a rather uni- form shortening of all intervals. The differences between the species are not due to the differ- ences of the heart rates alone, since in the mouse a separation of depolarization and re- polarization does not occur at a heart rate of goo/min. A different duration of the excited state (5) in the different species is the more likely explanation.

Newborn Mice. The evolution of the ECG of the mouse as it develops from birth is also of interest for the problem of the T wave.

The record of a newborn mouse without special attention to temperature is strikingly different from that of the adult. Records taken from IO different mice (table I) within 3 days of birth showed an average heart rate of 286/ min. with a P-R interval of 0.06 second, com- pared with values of 632/min. and 0.038 second for IO normal adult mice. The most noticeable diflerence is in the second peak of the ventricular complex. In the 2-day old mouse this takes the form of a thick curve of

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296 RICHARDS, SIMONSON AND VISSCHER Vohm 174

far greater amplitude and duration than the QRS complex, with a corresponding increase in the Q-T interval. In a 7-day old white mouse the T wave appears as a notch on the downstroke of R (standard leads) or a second peak in VI, VE, Vs, similar to the pattern in adult mice under effect of cold.

Newborn mouse (lead II)

Age f 6 i2 20 days -

300 330 540 66Ohnin.

FIG. 3. The separate T wave of a newborn mouse (1st day) becomes a notch on the descending limb of the R wave on the 20th day.

gm. The second wave gradually diminishes in amplitude and duration until it appears to be represented merely by a slurring of the down- stroke of the R wave, so that the record takes on adult characteristics.

Since smaller animals generally have a more rapid heart rate than larger animals, it was surprising to find that a newborn mouse of 1.5

gm has a heart rate less than half that of an adult mouse 20 times its weight. This slow rate with prolonged P-R and Q-T was found, how- ever, to be due to the loss of heat from hairless body of the newborn mouse. While the rectal temperature of adult mice averaged about 37OC, that of the newborn under room temperature conditions ranged from 27-29°C and followed changes in room temperature closely.

FIG. 4. Effect of temperature on the ECG of a newborn mouse.

TABLE 2. VARIATION OF HEART RATE AND ELECTROCAR-

DIOGRAPHIC TIME INTERVALS WITH BODY TEMPERATURE

Rectal Temp. 23' 28.5” 33” 37” 33” 28.5’ -~- ~_ ~-

Heart rate 240 345 510 615 540 360 P-R 0.09 0.06 0.04 o-35 0.04 0.06

Q-T 0.08 0.07 0.06 0.03 0.05 0.06 -

Ether Ether and cold

4OWmin. 240/min.

FIG. 5. Phonocardiogram and ECG in adult mouse, under the effect of ether anesthesia and cold.

ECG’s of three littermates were recorded at intervals from birth up to the age of 20 days. These show a profound change at the age of about 6 or 7 days, as illustrated in figure 3, when the animal is showing early hair develop- ment. At about this time the rate increases markedly and the P-R interval diminishes.

Figure 3 shows records of lead II from a white mouse at the age of 2,6, 12 and 20 days during which time its weight increased from q-10

Serial ECG’s were recorded on 6 newborn mice (2 litters) while their rectal temperatures were raised from about 2 7’C to the adult level of 37OC and then returned to 27’C. Figure 4 and table 2 illustrate the result. There is a steady increase in heart rate with increasing temperature until at the adult temperature the rate is roughly that of the adult animal. As body temperature is increased there is a progressive diminution in amplitude of all waves, along with shortening of the P-R and Q-T interval to the adult values. However, the large T wave still dominates the QRST complex, so that the record is not identical with that of the adult. As the animal’s tem- perature is returned to its previous level, the ECG returns to the newborn pattern in all

respects,

While the ECG pattern of the newborn mouse leaves little doubt as to the identity of

the second wave as a T wave, this is due to its greater duration, and not to a better separa-

tion in the beginning. It starts on the down grade limb of the R wave, similar to that of

the adult mouse. It seems, therefore, that the

duration of excitation in each fiber is also very short in the newborn mouse, so that repolariza- tion develops before depolarization is complete, but that repolarization is slower.

Phonocardiograms. In an attempt to relate

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A ugzcst 1953 ELECTROCARDIOGRAM OF MICE 297

mechanical and electrical events, simultane- ously recorded phonocardiograms and ECG’s were studied in the mouse.

Figure 5 shows such a record obtained from a mouse under ether anesthesia, with a heart rate of 4oS/min. The first heart sound occurs about .OI~ second after the beginning of the R wave, and amplitude, jus

second t before

heart sou the onset

nd of of the

smaller P wave

and newborn mouse under normal conditions. It is known that the P-R interval varies

of the next cycle. This relationship is slightly inversely with the heart rate and this is borne altered when the heart rate is reduced to about out by the present study (fig. 8). The QRS half the rate (zbo/min.) by cooling the animal interval in the adult mouse is about 0.01

terval to 0.14 second in conditions of cold or anoxia, is extreme in view of the small object and short distances involved. It must represent nearly exclusively delay of conduction-in the A-V node.

Table I shows the mean values and standard deviations of various intervals in the adult

with ice. The second sound now is synchronous with the P wave of the next cycle. Figure 6 shows another example of heart sound record- mg.

Time Relationships. Figure 7 shows a plot of the Q-T interval versus heart rate. The values of adult and newborn mice, rats and guinea pigs follow reasonably well a linear relationship, which extends also to the experi- ments with slow heart rates under hypoxia. In small animals, therefore, the Bazett (6)

formula (Q-T = KdR-R) does not seem to express the best fit for the relationship be- tween Q-T interval and heart rate. In the graph of figure 7, the Q-T interval in mice and rats was measured from the beginning of the QRS complex to the end of the notch on the descending limb. The good correlation is further evidence for the hypothesis that this notch is the T wave, since the values from guinea pigs with a distinct T wave follow the same trend. On the other hand, the values for the Q-T interval in the experiments with cold - and papaverine fall outside the linear relation- ship. These conditions affect the heart rate to a greater extent than the Q-T interval.

Figure 8 shows the increase of the P-R in- terval with the cycle length, which follows a curvilinear, approximately logarithmic rela- tionship. The scatter diagram includes all experiments. The prolongation of the P-R in-

8 0 0

0.10 L Q 5

/

% c cl06 uo

00 0

Y oca 0

a Cl06 0

I 200

Heart rclte/min. l I me t 1 I I i

300 400 500 600 700

FIG. 7. Heart rate vs. Q-T interval; includes all experiments.

second. The length of the ventricles is about 0~75 cm. Using Schgfer’s (7) value of 90 cm/ sec. for conduction speed in cardiac muscle, it is seen that 0.008 second is approximately the time required for the impulse to travel the length of the ven tricle This suggests that

by muscular specialized

conduction. conduction

the propagation of heart. Histological

tissue is not the impulse

necessary for in the mouse

studies are at present being carried out t-6 try tissue actuall) to determine whether such

exists. In table I, the Bazett formula was used to

calculate KQBT. The values of adult and new-

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298 RICHARDS, SIMONSON AND VISSCHER Volu/lPze 174

born mice are drastically different, and both are far outside the range in man. This shows again that the square-root relationship is not valid for small animals.

The variation of heart rate, the P-R and Q-T interval with the body temperature in the adult mouse is shown in table 2. A comparison of the changes of various time intefvals with the heart rate is shown in table 3. Mechanical systole duration was calculated as the interval between first and second heart sound. The experiments were performed on adult mice; the slower heart rates were obtained under conditions of anoxia or cold. All intervals in-

I l adult mice o newborn I x guinea pig 8 rot I

P-R in seconds M

I1 ' . " " " I 0.04 a06 Cl08 Cl00 0.02 0.14

FIG. 8. P-R interval vs. cycle length.

TABLE 3. CHANGES OF VAICIOUS TIME INTERVALS

WITH HEART RATE IN ADULT MICE (MEAN VALUES)

&fech. Syst.

.04 1.010 1.04 ;o.or25 .055

.055 .0145 ,041o.oq -75

. 124 .022 I ,057 0.022 I * 130

crease with the slower heart rate. It is of in- terest that the increase of the mechanical systole duration is proportionately greater than that of the electrical systole (Q-T), and com- parable to the increase of the P-R interval. The latent period (L.P.), measured as the interval between beginning of the QRS de- flection and first heart sound, also shows marked prolongation with slowing of the heart rate.

SUMMARY

The electrocardiogram of adult mice does not show a distinct T wave in leads from any part of the body surface, but the terminal segment of the QRS complex is notched. Slow-

ing down of the heart rate from about 600 to approximately 200 beats/min., by cooling the animal or hypoxia, separates the notch into a distinct wave, although no definite S-T seg- ment is produced. Potassium administration increases the amplitude of the notch. It is as- sumed that the notch represents the T wave, and that depolarization and repolarization overlap due to the short duration of excitation.

The QRS duration measured from the begin- ning of the QRS complex to the notch (0.01

second) is long enough to be accounted for by muscular conduction alone, but when measured to the end of the notch it is far too long (0.08 second) to be accounted for on this basis. There is an approximately linear relationship between the Q-T intervals, measured from beginning of the QRS deflection to the end of the notch, and the heart rates of newborn and adult mice, rats and guinea pigs. The values of KQBT, calculated according to Bazett’s formula, fall far outside the range for man. Comparison of the ECG of mice, rats, and guinea pigs shows a transition from a notched QRS to a separate T wave. The ECG of newborn mice, taken at room temperature, shows a slower heart rate and a more distinct T wave than that of adult mice. The slower heart rate is due to the lower body temperature; warming the animal in- creases the heart rate, and the ECG approaches the pattern of adult mice. The first heart sound in adult mice occurs about o.oq second after the beginning of the QRS complex, and the second just before the onset of the P wave. Decrease of the heart rate by various agents lengthens all intervals (P-R, QRS, latent period, Q-T and mechanical systole duration, measured as interval between the first and second heart sound). However, the lengthening of the mechanical systole far exceeds that of the Q-T interval.

REFERENCES

I. BUCHANAN, F. J. Physiol., 40 : Proc. xliii, 1910.

2. AGDUHR, E. AND N. STENSTR~M. Acta paediat. 8:

493, 1928/29* 3. LOMBARD, E. A. Am. J. Plzysiol. 171: I&, 1952.

4. O’BRYANT, J. W., A. PACKCHANIAN, G. W. REIMER

AND R. H. VADHEIM. Texas Rep. Biol. & Med. 7:

66% 1949. 5. ROTHSCHUH, K. E. Elehtmphysiologie des Herzem.

Darmstadt: Steinkopff, 1952, p. 417. 6. BAZETT, H. C. Heart 7: 353, 1920.

7. SCBXPER, H. Das Elektrokardiogramm. Berlin: Springer, 1951, pp. 556.

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