Relief of pain in childbirth (1954)

In childbirth, four things are required of the perfect analesic agent. It should produce a maximum relief from pain, but leave the mother conscious and cooperative, not inhibit uterine contractions and thereby delay labor and predispose to an increase in interference rate and to postpartum hemorrhage. The toograph tracing shows the effect of a general anesthetic on the uterine contractions. Not depress the fetal respiratory center with risk of affixion neonatorum and be safe for both mother and child. Nothing perfectly fulfilling these requirements has yet been discovered. But nitrous oxide and triricloreththylene which we're now to discuss come as near to the desired standards as can be attained at present. Analesic agents can be introduced to the body by any route. Oral, rectal, parental, inhalational. The inhalational method has been developed primarily in Great Britain. The advantage of this route is rapidity of absorption and excretion. It is also particularly adaptable for self- administration. For analesia to be produced by nitrous oxide, the concentration in the brain has to be built up above a certain threshold. There is therefore a time lag between the mother's first inhalation of gas and the attainment of that threshold wherea after analesia will be present. Conversely, there is a prolongation of analesia when the mother stops breathing gas before concentration has fallen sufficiently for the brain cells to recover. Nitrous oxide is a gas at normal temperatures and pressures. It is stored in cylinders of various sizes which are colored French blue and have N2O stencled on the collar. In order to get sufficient gas into the cylinders, they are filled under pressure. When nitrous oxide is subjected to a pressure of 650 lb to the square in, it becomes a liquid. A full cylinder therefore has liquid N2O below and gaseous N2O above. As the gas is withdrawn, the liquid N2O gradually turns into a gaseous state, but the pressure remains the same until all the liquid has turned into gas. Only then, when all the liquid is vaporized, will the pressure gauge show a fall. For this reason, the sole method whereby an accurate assessment of the contents of a cylinder can be made is by weighing it. 50 gallons nitrous oxide weigh 15 o. Consequently, the full and empty weight are stamped on every cylinder. It is a colorless gas with a very faint smell and is one and a half times as heavy as air. Nitrous oxide may be given according to four principles. Gas and air mixture. Pure gas for a limited time followed by air. Pure gas followed by gas and air mixture, gas and oxygen mixture. The gas and air mixture principle is the only one of the four which an unsupervised midwife is allowed to use. It is involved in minutes various machines. Air is a mixture of roughly 20% oxygen and 80% nitrogen. Healthy people can maintain normal metabolic processes for a limited time when the amount of oxygen in the inspired air is reduced to 10%. Provided the mixture is inhaled for short intermittent periods. Because nitrous oxide is a very weak analesic agent to obtain adequate relief. It is necessary that the maximum possible amount of nitrous oxide should be introduced to the inhaled mixture. This maximum is however limited by the patients oxygen requirements. As the patient cannot manage safely with less than 10% of oxygen and if that oxygen is given in the form of air which is itself but 20% oxygen, the inhaled mixture must contain 50% of air. This obviously limits the N2O content of the mixture to 50%. And the figure of 50% nitrous oxide must not be exceeded even in healthy patients or serious harm will result from an oxy. Minute's machine consists of a cylinder of gas to which is attached to reducing valve which lowers the pressure to about 5 lb per square in at which it can be easily controlled. To allow the gas to come out, the main pin valve has to be opened. The gauge shows the pressure actually in the cylinder and the needle will not fall until all the N2O has been vaporized. Flow is stopped by means of a check valve which just counterbalances the pressure. The gas is led to the patient by a non-kinkable tube and a face piece with a finger hole safety device. If the mother becomes uncooperative, her finger will fall from this hole and she will breathe pure air. As soon as the mother takes an inspiration, negative pressure opens the check valve. The gas flows and is diluted with air to a 50% mixture by means of air inlet holes. This is known as the intermittent flow principle. There are other types of apparatus. This is the jector which fulfill the requirements of the central midwives board and here is the tally. The principle of pure gas for a limited time followed by air was incorporated in professor chassamir's apparatus. His object was quicker concentration of N2O in the brain and a certain reduction of time lag. Only a limited amount may be given in this way and then pure air is breathed. The disadvantage is that analesia tends correspondingly to wear off before a pain has ended. The principle of using pure gas followed by gas and air mixture is followed in the machine designed by Elim. A bag taking a certain length of time to fill with pure N2O is emptied by the mother's first breath and then the normal mixture of gas and air is breathed. By fitting the CM attachment to an ordinary minute, the same effect is achieved, though it may not then be used unsupervised. The bag is of 2 and 1/2 L capacity and fills at the rate of 1 liter per minute. Where the services of a trained anist are available, nitrous oxide and oxygen is probably the most efficient and safe form of analesia. Up to 80% of nitrous oxide can be given and still leave the normal 20% of oxygen. Any anesthetic apparatus may be used, but those which give an intermittent flow are the most economical. Triricchloreththylene is a colorless liquid of relatively low volatility. It has a characteristic smell and powerful analesic action. For anesthetic use, thyimol is added as a preservative and it's colored blue to distinguish it from chloroform. This mixture is supplied under the trade name of trilene. When inhaled in the concentration required to produce analesia, the vapor is non-erritant to the respiratory passages as a concentration of only.5% triclorithylene by volume is effective. The oxygen content of a triline air mixture need not be significantly different from that of air. as it is the most potent agent. However, the danger of overdose is present or anesthesia rather than analesia may ensue unless a very carefully controlled mixture is given. Anesthesia is undesirable in the second stage of labor for the following reasons. It may be dangerous to the mother who may vomit and inhale vomitus or whose airway may become obstructed. utterine contractions will be depressed and the mother will not cooperate. Toxic effects appear only in cases of overdos and show themselves as cardiac irregularities or a type of rapid shallow breathing known as tachypnea. But it must be remembered that since all anesthetics pass very rapidly into the fetal circulation, anesthesia of the mother gives a predisposition to asphixia neonatorum. It should be noted that in any case a closed circuit anesthetic must not be given immediately following triiline analesia as there is a danger of the soda lime decomposing tricclorthylene vapor from the patient. Unlike nitrous oxide, triclorethylene is not a gas at normal temperatures and pressures. It is a liquid which gives off from its surface a gas or vapor. The strength or pressure of this vapor depends on the temperature. If air passes over it, the amount of vapor picked up varies with the temperature and with the area of liquid exposed. Thus, changes of temperature or of the surface area of the liquid will alter the composition of such a mixture. Any machine which works on the draw over principle must therefore incorporate compensating devices if it is to give a constant mixture. The original type of apparatus designed for administration of triclorene is Freiedman's inhaler. There are no compensating devices in this machine and the strength of the mixture is variable. The Freriedman consists simply of a hollow tea tube down the center of which is a partition. The partition extends up into a cross piece which has a hole in it so that air may be drawn directly along. Some of the air passes down the centerpiece into the container, picks up triclorithylene and takes it back into the airream and to the patient. A one-way valve ensures that air can be drawn in only by inspiration and not driven back. Shaking a container causes liquid to splash and increase the surface area. Other draw over machines which include a thermostat are the Emitril, the Airlene, the CB Gorman and the Takakota. And these incorporate devices to minimize the effects of shaking. A method of obtaining a mixture which will not vary with temperature or shaking is to deliver a precise volume of liquid into the stream for each liter of air breathed. Provided this much liquid can be vaporized at the lowest temperature at which the machine is expected to work. No amount of shaking or raising of the temperature can produce any appreciable variation. This is the principle of the carburetor which gives a fairly constant mixture of petrol and air. It can be used to give a reasonably constant mixture of triline and air. The Burns Benson inhaler delivers a constant percentage of triline in air by delivering a fixed amount of liquid triline into every liter of air which is drawn through the machine. Trilene passes from the container via a jet into the main airream. The liquid is quickly vaporized here. Whatever the room temperature, no more than the correct amount can be vaporized as the quantity delivered is governed by the rate of air flow past the jet and not by temperature. As the rate of air flow increases, the mixture tends to become richer. This tendency is controlled by a gravity valve which admits more and more air and so dilutes the main mixture. The vapor concentration reaching the patient thus remains constant. Choice of analesic agent should if possible be made in the antiatal period. Gas and air is contraindicated though triline may be used in the following conditions. Severe heart disease Respiratory disease, toxmia of pregnancy with severe hypertension. But whichever type of analesia is decided upon and whichever machine is used, the actual conduct of all cases is identical. It is essential that the mother should fully understand how to use the selected apparatus well before she goes into labor. At some time during her antiatal period, preferably shortly before her confinement is due, she should be made acquainted with it. It should be explained that it will relieve pain, but that she will have to administer it herself. It is therefore important that she should fully understand the part she has to play. She will not be in a receptive state to learn these things when she's having pains every few minutes. And experience has shown that the previously educated mother gets very much better relief than one whose first knowledge of the machine is obtained when she's actually asked to use it during labor. She should learn to fit the face piece to her own contours. While it's explained that improperly fitted, it will give little result owing to air leaking in. She must have the finger hole explained, how it must be covered to achieve any effect, how it automatically prevents anesthesia. The importance of anticipating pains in order to overcome the time lag must be stressed. Lastly, she must be made to understand the difference between first and second stages of labor. in the first because she will not have to bear down. She may use the apparatus throughout a pain. Inhalational analesia should not be used over an indefinite period and should not therefore be started until well on in the first stage when pains are coming regularly and strongly and the cervix is dilating. Prior to that, sedatives are preferable. In the second stage of labor, the patient will have to inhale, then hold breath during the maximum pain intensity and bear down. It is therefore in this period that anticipation of pains is most important. Thus, with careful training and controlled use of inhalational analesics, we may obtain the ideal of perfect child birth with minimum suffering.