The Heart Rate of the Developing Chick

After several attempts, records were obtained by the use of valve amplification which showed the gross form of the electrocardiogram of the embryo at a comparatively early stage. In later experiments a technique was evolved which showed the true form of the electrical response of developing hearts.

In this paper only the heart rate is discussed; the form of the electrocardiogram and the performance of the amplifier will be discussed in a later paper.

A single stage amplifier employing a Mazda AC/SG valve and a Cambridge pattern Einthoven String Galvanometer were used.

A warm chamber as shown in Fig. 1 was constructed so as to enable the research to be carried out under as nearly ideal conditions as possible. The chamber housed a stand for the egg, this consisting of two sheets of ebonite separated by four ebonite pillars. An egg-shaped aperture large enough to permit the whole of the embryo to be examined by means of a light was cut in the top of the stand. The light was supplied by means of a bulb operated by a 6-volt battery and mounted on a polished reflector.

The cover of the chamber was made of glass through which three holes were drilled, one for the thermometer and the wires to the light, and two for the universal joints of the electrodes. The electrodes consisted of two large steel ball bearings through which a hole had been drilled; a thick-walled fine-bore brass tube was secured in each ball, a length of copper wire was passed through the brass tube, to the distal ends of which the silver electrodes were soldered, the proximal ends of the wires being connected to the input of the amplifier. The balls were mounted in a brass case which was clamped on to the glass. The electrodes were secured in position by means of three clamping screws, and free movement was obtained by allowing the balls to rest upon a triangle of elastic with the screws slackened, the whole joint being well smeared with grease to permit easy movement and as a protection against moisture. A small glass vessel containing water was placed in the chamber so as to keep the air moist.

The oven could not be heated electrically owing to the interference from the heating element affecting the amplifier. The temperature was kept constant within 0-25° C. by means of a small flame.

The egg to be electrocardiographed was removed from the incubator and “candled” in order to find the position of the embryo. This was marked with a pencil on the shell, and two small holes were then drilled through the shell, one near the air sac and the other at the distal end of the embryo. The egg was then placed in the chamber and left for 15 min. ; then the light under the egg was switched on and the electrodes guided through the holes, piercing the shell membrane, care being taken not to puncture any blood vessels. It was possible to see the shadow of the electrodes through the shell. The universal joints were then clamped and the small light switched out, as there was a certain amount of heat radiated from the bulb.

A trial record was then taken to see if the electrodes were in a suitable position, and if this was satisfactory three records were taken on one plate at 5 min. intervals. In the case of embryos from 30 hours to the fourth day the two holes were drilled in the shell at each end of the embryo, and the electrodes were pushed through all the membranes so as to get as near to the embryo as possible without touching it.

In the early stages it was necessary to place the electrodes fairly close to the heart in order to obtain the response, but it was also necessary to avoid touching the heart or blood vessels, for the slightest touch caused injury and this altered the frequency. After the electrodes had been adjusted it was necessary, in the case of the older embryos, to wait until the embryos became quiet and then take a record ; this often involved waiting as long as an hour.

The results obtained are shown in Table I. Each measurement is the average value of three records at 5 min. intervals, but the figures show an extensive individual variation. This variation was also observed by Cohn (2); it may be due partly to experimental error, for irritation of the embryo by placing the electrodes too near it often caused an alteration in rate, and a variation in temperature of 0-5° C. was sufficient to cause a considerable change in rate.

The earliest age at which a satisfactory record was obtained was at 33 hours of incubation. The average rates, graphed in Fig. 2, show a steady rise from 33 hours until the tenth day of incubation. It is interesting to note that the upper limit of the rates observed reached the maximum (240 per min.) as early as the fifth day.

One of the original purposes of these investigations was to determine whether there was any difference between the heart frequency of the male and female embryos. For this purpose eggs from hens with sex-linked plumage were used; this enabled the sex of the chick to be determined as soon as plumage appeared, which was on the tenth day. The results were entirely negative, that is to say, no difference was observed between the average frequencies of the two sexes during incubation, and the same was found to be true in the case of chicks after hatching. (See section dealing with heart rate of the chick.)

The right wing and the left leg of the chick were cleaned with alcohol and ether, and a small piece of cotton-wool soaked in warm saline was wound round the leg and the base of the wing; only a small pad was used so as to ensure the minimum of inconvenience to the chick. The wool was kept in position by means of fine silver wire coated with silver chloride, these wires were led off to the string galvanometer. The chick was then placed in an oven which was kept at the same temperature as the brooder from which it had been removed and was allowed to rest for an hour on a pad of cotton-wool, the oven being kept dark and well ventilated. If the electrode pads had dried too much during the resting period, they were moistened again and the chick allowed to rest for another quarter of an hour. Any interference such as light or too much noise excited the chick and made an appreciable difference in the rate of the heart beat. It was also found that the chicks kept much quieter when several were placed in the oven at the same time with small cardboard partitions between them.

Records were obtained in this manner from the moment of hatching to 3 weeks after. Three records were taken on the same plate at 5 min. intervals to ensure that the average rate was as near normal as possible under these conditions.

In spite of the precautions taken the heart rates showed a considerable amount of variation. The lowest rate recorded was 157, but this was due to incorrect oven temperature. The highest rate was 560 and was due to excitement, it occurred immediately after a 24-hour-old chick had been accidentally dropped; 10 min. later the rate had dropped to 450 per min. and after 25 min. the rate was between 340 and 350 per min.

Inspection of the records showed that the young chick could not be said to have any definite pulse rate, because the rate varied from minute to minute in a single individual. Table II shows the variations observed in a single typical case. All these records were taken after the chick had been left undisturbed for an hour. The probable error of averages of figures varying as widely as those shown in Table II is so great that averages have little significance. The heart rate shows a slight rise during the first 12 hours after hatching, but no other certain change occurs during the first 21 days after hatching.

Records were taken from 59 chicks, 29 males and 30 females. The average rates for the two groups were identical, namely 295 beats per min. It was noticed that the male birds were more excitable than the females, and hence any excitement caused a greater increase in the heart rate of the males.

The contraction rate of the heart of the embryonic chick is easily altered either by slight mechanical injury or by exposure to cold. Most previous workers have failed to eliminate these errors and have given figures which are much too low.

Cohn (2) took adequate precautions and his figures for the embryonic heart rate agree very closely with mine, as is shown in Fig. 2, which gives the two sets of figures. The graph shows that the pulse rate rises rapidly during the first 9 days of incubation and then becomes remarkably uniform during the remainder of incubation. The only difference between the two sets of figures is that Cohn’ s figures show a somewhat slower rate of rise of the pulse rate during the first week of incubation. This difference is probably due to the fact that with the author’ s method it was not necessary to open the egg to obtain a record, and hence the interference was reduced to a minimum. Cohn measured his rates by visual observation and this necessitated exposing the embryo.

The following are a few figures of other workers :

Lewis (5) noted that the heart rate after opening the shell varied between 120 and 214 on the second day and between 140 and 240 on the third day.

The pulse rate of the chick embryo remained constant at nearly 240 per min. from the ninth day of incubation until hatching, and directly after hatching the average pulse rate rose to an average of 295 per min. This rate remains practically constant throughout life, for the average pulse rate of a fowl at rest is only slightly more than 300 per min.

The variation in the pulse rate of the developing and adult hen forms a striking contrast to the corresponding figures obtained with mammals which are shown in Table III. In the case of the hen the pulse rate rises during incubation, rises sharply at hatching and after hatching remains practically constant throughout life, whereas in mammals the foetal pulse rate is greater than that of the newborn animal, and the adult pulse rate is still lower.

No certain figures are available for the foetal pulse rate of mammals prior to the latter half of pregnancy, and therefore it is uncertain whether a steady rise in pulse rate such as observed during the first half of incubation in the embryonic chick occurs in other animals.

The most striking difference between the hen and mammals is that the pulse rate is the same in the new-born chick as in the adult hen, whereas the pulse rate of the new-born mammal is about twice that of the adult. This difference is probably due to the fact that the mammalian pulse rate is reduced by increasing vagal control whereas the vagus exercises very little tonic control of the heart of the hen.

Stübel(11) showed that in the adult hen vagal section only increased the pulse rate from 288 to 312. The writer found that injections of atropine into young chicks failed to show any rise of pulse rate that could be attributed to vagal control. On the other hand similar experiments on young ducklings showed a definite increase in frequency. Any excitement in the chick caused a remarkable acceleration of the heart; it has been mentioned that the dropping of a 24-hour-old chick increased the pulse rate from a normal rate of 300 up to 560 per min. The vagus therefore appears to exercise little tonic control over the pulse rate of the chick or of the hen. The pulse rate of the hen at all stages of development represents therefore the fundamental frequency of the pace-maker of the heart, which is but little affected by vagal control. Many observers have shown that the vagus exercises very little control in the new-born mammals. For example, Lhoták von Lhota (6) obtained the following figures with puppies:

Similarly in man the administration of atropine raises the pulse rate of the adult to a figure which is higher than that of the new-born infant and approaches that of the foetus.

Miss Buchanan showed that there was a fairly close correlation in adult mammals and birds between the pulse rate and the metabolic rate. This problem has been discussed by Clark (1). The general argument is, that, since the oxygen-carrying power of the blood does not vary greatly, the minute volume of blood flow varies as the oxygen used. The minute volume of blood flow equals the output per beat multiplied by pulse frequency. The output per beat is approximately proportional to the heart size and the relation between the heart weight and body weight varies within relatively narrow limits. The oxygen usage however varies as (body weight) 2÷3, and hence the metabolic rate (oxygen used per unit weight) is far greater in small animals than in large animals, and this difference in oxygen requirement is compensated for chiefly by a variation in the pulse rate. This correlation explains the difference between the pulse rates of the new-born and adult mammals, for the metabolic rate of the former is higher than that of the latter. There is however no correlation between the metabolic rates and pulse rates in the different stages of the hen’ s life.

The figures in Table IV show that in the second half of embryonic life the pulse rate increases while the metabolic rate falls. The two sets of figures agree in so far as both show an increase on hatching, but after hatching the pulse remains constant whereas the metabolic rate at first rises and then falls. This lack of correlation is remarkable, but it can be partly explained by variations in other factors. The blood pressure in the adult hen is about 150 mm. of mercury, whereas the blood pressure in the chick embryo is only 1-5 to 2-0 cm. of water (Hill and Azuma (3)). Moreover the heart ratios vary. The heart weight of the adult hen is 6-8 gm. per kg. body weight, whereas in four chick embryos of 5 days’ incubation, the average weight of which was o-2 gm., the average heart weight was 4 mg. This gives a heart ratio of 2o-o gm. per kg.

1. Records taken of the heart beat of the hen’ s egg during incubation, which were made without injury to the embryo, confirm the results obtained by Cohn.

2. The heart rate of the chick rises sharply during and immediately after hatching, but thereafter appears to remain nearly constant throughout life.

3. There is no obvious relation between the variations of the pulse rate and of the metabolic rate which are observed during the development of the hen.

4. No difference was found in the average heart frequencies of the two sexes, either during incubation or after hatching.

5. The newly hatched chick shows a remarkable accelerator action of the sympathetic. No evidence of vagal control has been found in the chick, but such control exists in the duckling.

6. Very slight alterations in temperature have a marked effect on the heart rate of the embryo and of the newly hatched chick.

I wish to take this opportunity of thanking Prof. Clark for his invaluable help and criticism throughout the course of this work.

Part of the expenses for this research were defrayed by grants from the Moray Fund and from the Ministry of Agriculture and Fisheries.