Читать книгу Obstetrical Nursing - Carolyn Conant Van Blarcom - Страница 15

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Although the fetus at term is in many respects simply a diminutive, immature man, or woman, its anatomy and physiology present certain characteristics which have adapted it to a protected existence in a sac of fluid. Some of the fetal structures and functions become increasingly active after birth, while others subside and disappear.

We have seen that after the first month of pregnancy the placenta serves the fetus as a combined respiratory and digestive apparatus, not alone in supplying the oxygen and nourishment requisite for life and growth, but also in excreting the broken-down products of fetal life. It apparently acts somewhat as a liver, too, in performing something akin to a glycogenic function.

Obviously, then, the fetus must possess a circulatory mechanism which is peculiar to itself alone, and not found in the independently existing human body, in which the lungs and alimentary tract are functioning as intended. This mechanism is provided by means of certain structures which exist in the fetal circulatory system and which automatically disappear shortly after birth. The nurse must be aware of these anatomical changes that take place, in addition to growth, if she is to have an intelligent grasp of her tiny patient’s possible needs.

The structures which change or disappear after birth are the foramen ovale, a direct opening between the right and left auricles, and four blood vessels: the ductus arteriosus, ductus venosus and the two hypogastric arteries. An understanding of the functions of these vessels involves an understanding of the course followed by the fetal blood currents, as indicated in Fig. 28, page 85.

We see that there are three vessels within the umbilical cord: the umbilical vein and two arteries. In spite of its name, the vein conveys arterial blood from the placenta to the fetus. After piercing the baby’s abdominal wall, it divides into two vessels; the larger one, called the ductus venosus, empties into the inferior or ascending vena cava, while the smaller branch joins the portal vein, which enters the liver. The relatively large amount of arterial blood sent directly to the liver may in part account for the large size of this organ in the fetus. Upon its emergence from the liver, this blood stream flows into the inferior vena cava.

Fig. 28.—Diagram showing course of fetal circulation through hypogastric arteries, ductus venosus, ductus arteriosus and the foramen ovale. (From The American Text Book on Obstetrics.)

The ascending vena cava, then, pours into the right auricle a mixture of arterial blood, which has come directly from the placenta, and venous blood returned from the liver, intestines and lower extremities. There is a difference of opinion concerning the course of the blood stream after reaching the right auricle. The general teaching, however, is that the eustachian valve, guarding the foramen ovale, deflects the current through this opening from the right into the left auricle. It then pours into the left ventricle, is pumped into the arch of the aorta, from which most of the blood is sent to the head and upper extremities, though a small part carries nourishment to other parts of the body.

The descending, or superior, vena cava, carrying blood returning from the head and arms also empties into the right auricle; this stream presumably crosses the stream which is directed toward the foramen ovale, flows into the right ventricle by which it is pumped into the pulmonary artery. The circulation of blood through the lungs, however, is for their own nourishment, and not for aëration as with the adult. For this reason most of the contents of the fetal pulmonary artery empties into the aorta through the ductus venosus, one of the temporary fetal structures already referred to. From the aorta the stream is directed in part to the lower extremities and the pelvic and abdominal viscera, but most of it flows into the hypogastric arteries. These are also temporary arteries. They lead to the umbilical cord and, as the umbilical arteries, carry the venous or vitiated blood through the cord to the placenta where it is oxygenated, freed of its waste in the chorionic villi and returned to the fetus through the umbilical vein.

As soon as the child is born and it is obliged to obtain its oxygen from the surrounding air, its pulmonary circulation of necessity becomes immediately more important and is greatly increased in volume. In fact, the entire fetal circulation is readjusted to meet the needs of the new and independent functions which the little body now assumes. The temporary structures are obliterated, since they are no longer needed, and the lungs and intestines become more active in compensation.

Fig. 29.—Diagram showing circulation of the blood after birth, with hypogastric arteries, ductus venosus, ductus arteriosus and foramen ovale in process of obliteration and pulmonary circulation greatly increased. (From The American Textbook on Obstetrics.)

As the ductus venosus and hypogastric arteries terminate in blind ends and become useless as soon as the umbilical cord is cut, they soon begin to atrophy and are obliterated within a few days after birth. This means that less blood is poured into the right auricle, which naturally results in relatively less tension in the right heart and an increased pressure in the left, which tends to close the foramen ovale. The foramen ovale does not entirely disappear at once, however, but closes gradually, sometimes remaining open for months. Occasionally it remains open permanently, and though some people have gone through life comfortably with a patent foramen ovale, its ultimate failure to close usually results in serious circulatory trouble. This is also true of the ductus arteriosus, which sometimes, but not often, fails to close.

The rule is that as the lungs expand and an increased amount of blood is carried to them for aëration, the ductus arteriosus deflects a steadily diminishing stream from the right ventricle to the arch of the aorta. Thus it gradually ceases functioning in most cases and disappears in the course of a few weeks. The abandoned vessels may degenerate and disappear in time or they may persist in the form of small fibrous cords. (Fig. 29.)

Although the circulatory system shows the most elaborate adjustments to the protection afforded by intra-uterine life, there are also other adaptations made by the fetal organism.

The baby acquires about 90 per cent of its weight during the latter half of pregnancy, as well as a steadily increasing proportion of solids and a decrease of fluids in its tissues, for in its early days the embryo consists largely of water. But for all of that, its existence and growth in utero, and the functioning of its heat producing centre require surprisingly little oxygen and nourishment. The amniotic fluid keeps the fetus at an equable temperature, about 1° above that of the mother, and as space within the uterine cavity permits of only limited movement, there is very little combustion for the liberation of heat and energy.

The kidneys assume functional form at a very early fetal age, probably about the seventh week, and the presence of albumen and urea in the amniotic fluid suggest that small amounts of urine may be voided, particularly during the latter part of pregnancy.

The bowels, on the other hand, are normally inactive, this is in spite of the fact that the baby evidently obtains fluid, and possibly some nutriment by swallowing amniotic fluid. But a discharge of meconium may be caused by pressure on the cord or by any condition which interferes with the umbilical circulation. For this reason, meconium stained fluid escaping during labor in a head presentation may be taken as an evidence of imminent asphyxiation, due to an interruption of the umbilical circulation.

The head is the most important part of the fetus, from an obstetrical standpoint, since the process of labor is virtually a series of adaptations of the size, shape and position of the fetal skull to the size and shape of the maternal pelvis. And since the pelvis is rigid and inflexible the adjustment must all be made by the fetal head, which is mouldable because of being incompletely ossified at birth. If the head passes through the inlet safely, the rest of the delivery will usually be accomplished with comparative safety. But a marked disproportion between the diameters of the head and pelvis, or limited mouldability of the head, constitutes a serious complication, which will be discussed later in connection with obstetrical operations.

A baby’s head is larger, in proportion to its body, than an adult’s, while the face forms a relatively smaller part of the baby’s than of the adult’s head. The major portion is the dome or vault-like structure forming the top, sides and back of the head, which in turn is made up of separate and as yet ununited bones. They are the two frontal, two parietal, two temporal and the occipital bone, with which the wings of the sphenoid bones, though less important, may be included.

These bones are not joined in the fetal skull, but are separate structures, with soft, membranous spaces between their margins, called sutures; while the irregular spaces formed by the intersection of two or more sutures are called fontanelles, possibly so called by the early observers because the pulsation of the soft tissues beneath these spaces suggests the spurting of a fountain.

The sutures are named and situated as follows: The frontal lies between the two frontal bones; the sagittal extends antero-posteriorly between the parietal bones; the coronal between the frontal bones and the anterior margins of the parietal, while the lambdoidal suture separates the posterior margin of the parietal from the upper margin of the occipital bone. There are also the temporal sutures between the upper margins of the temporal bones and the lower margins of the two parietals, but they are of no obstetrical importance, as they cannot be felt on vaginal examination. (Fig. 30.)

There are two fontanelles of obstetrical significance. The greater, or anterior fontanelle, also called the bregma or sinciput, is located at the meeting of the coronal, sagittal and frontal sutures. It is diamond or lozenge shaped, about an inch in diameter and is not obliterated during labor.

Fig. 30.—Side and top views of fetal skull giving average length of important diameters.

The smaller or posterior fontanelle is the triangular space at the inter-section of the sagittal and lambdoidal sutures, and may be obliterated as the surrounding bony margins approach each other during labor.

The coronal, frontal, lambdoid and sagittal sutures and the anterior and posterior fontanelles are of greatest diagnostic value as they can be felt through the vagina during labor. It is by recognizing and locating these sutures and fontanelles at this time that the accoucheur is enabled to determine the exact position and presentation of the fetus.

The fact that the skull is made up of separate bones, with soft membranous spaces interposed between them, permits of its being compressed or moulded to a considerable extent as it passes through the birth canal. Opposing margins may meet, or even overlap, to such a degree that the diameter of the head will be appreciably diminished and permit of its passage through a relatively narrow canal. This mouldability varies greatly, however, and the difference in the degree of compressibility of heads of approximately the same size may spell the difference between an easy and a difficult, or even an impossible labor.

A new-born baby’s head may be so distorted and elongated by the moulding process that it is unsightly and gives the young mother great concern. But the nurse can be quite confident in her assurances that the little head will assume its normal, rounded outline in a very few days.

The five most important diameters of the new-born baby’s head are:

1. The occipito-frontal (abbreviation, O.F.), measured from the root of the nose to the occipital protuberance, is 11.75 centimetres.

2. The biparietal (B.I.P.) is the longest transverse diameter, being the distance between the parietal protuberances, and measures 9.25 centimetres.

3. The bi-temporal (B.T.) is the greatest distance between the temporal bones and measures 8 centimetres.

4. The occipito-mental (O.M.) is the greatest distance from the lower margin of the chin to a point on the posterior extremity of the sagittal suture, and measures 13.5 centimetres.

5. The sub-occipito bregmatic (S.O.B.) is measured from the under surface of the occiput, where it joins the neck, to the centre of the anterior fontanelle, a distance of 9.5 centimetres.

The greatest circumference of the fetal head is at the plane of the occipito-mental and biparietal diameters and measures 38 centimetres. The smallest circumference is at the plane of the sub-occipito-bregmatic and biparietal diameters, and measures 28 centimetres.

These figures, however, like all of those which it is possible to give, simply represent averages taken from a large number of cases. Individual variations will be found among normal babies, for boys’ heads, for example, are usually larger than girls’ while the head of the first child is likely to be smaller than the heads of those born subsequently.