Читать книгу Obstetrical Nursing - Carolyn Conant Van Blarcom - Страница 9

The present broad knowledge of the anatomy of the female pelvis has resulted in an enormous reduction in death and injury among obstetrical patients and their babies.

This knowledge of the pelvic anatomy, relating as it does, to both normal and malformed pelves, has made possible a system of taking measurements, termed pelvimetry, which gives the obstetrician a fair idea of the size and shape of his patient’s pelvis. Such information, coupled with observations upon the size of the child’s head, gives a foundation upon which to base some expectation of the ease or difficulty with which the approaching delivery is likely to be accomplished.

Since each patient’s pelvic measurements are considered from the standpoint of their comparison with normal dimensions, it is manifestly important that the obstetrical nurse have a clear idea of the structure of the normal female pelvis, and also of its commonest variations.

Viewed in its entirety, the pelvis is an irregularly constructed, two-storied, bony cavity, or canal, situated below and supporting the movable parts of the spinal column, and resting upon the femora or thigh bones. (Fig. 1, A. and B.).

Four bones enter into the construction of the pelvis: the two hip bones or ossa innominata, on the sides and in front with the sacrum and coccyx behind.

The innominate bones (ossa innominata), symmetrically placed on each side, are broad, flaring and scoop-shaped. Each bone consists of three main parts, which are separate bones in early life, but firmly welded together in adults: the ilium, ischium and pubis. The ilia are the broad, thin, plate-like sections above, their upper, anterior prominences, which may be felt as the hips, are the anterior superior spinous processes used in making pelvic measurements. The margins extending backward from these points are termed the iliac crests.

The ischii are below and it is upon their projections, known as the tuberosities, that the body rests when in the sitting position, and which also serve as landmarks in pelvimetry. The pubes form the front of the pelvic wall, the anterior rami uniting in the median line by means of heavy cartilage and forming the symphysis pubis.

The sacrum and coccyx behind are really the termination of the spinal column, the sacrum consisting, usually, of five rudimentary vertebrae which have fused into one bone. It sometimes consists of four bones, sometimes six, but more often of five. The sacrum completes the pelvic girdle behind by uniting on each side with the ossa innominata by means of strong cartilages, thus forming the sacro-iliac joints. The spinal column rests upon the upper surface of the sacrum. The coccyx, a little wedge-shaped, tail-like appendage, which ordinarily has but slight obstetrical importance, extends in a downward curve from the lower margin of the sacrum, to which it has a cartilaginous attachment, the sacro-coccygeal joint. This joint between the sacrum and coccyx is much more movable in the female than in the male pelvis.

We find, therefore, that although the pelvis constitutes a rigid, bony, ringlike structure, there are four joints: the symphysis pubis, the sacro-coccygeal, and the two sacro-iliac articulations. As the cartilages in these joints become somewhat softened and thickened during pregnancy, because of the increased blood supply, they all permit of a certain, though limited amount of motion at the time of labor. This provision is of considerable obstetrical importance, since the sacro-coccygeal joint allows the child’s head to push back the forward-protruding coccyx, as it passes down the birth canal, thus removing what otherwise might be a serious obstruction. And when, as is sometimes necessary, because of a constricted inlet, the pubic bone is cut through (the operation known as pubiotomy), the hingelike motion of the sacro-iliac joint permits of an appreciable spreading of the two hip bones and a consequent widening of the birth canal.

A. Normal female Pelvis.

B. Normal male Pelvis.

Fig. 1.—Normal Pelves. Note the broad, shallow, light construction of the female pelvis, A, as compared with the more massive male pelvis, B.

The pelvic cavity as a whole is divided into the true and false pelves by a constriction of the entire structure known as the brim or inlet. The inlet is not round, its antero-posterior diameter being shortened by the sacro-vertebral joint which protrudes forward and gives the opening something of a blunt, heart-shaped outline. (Fig. 2.)

Fig. 2.—Diagram of the pelvic inlet, seen from above, with most important diameters.

As the pelvis occupies an oblique position in the body, the plane of this brim is not horizontal, but slopes up and back from the symphysis-pubis to the promontory of the sacrum. Being swung upon the heads of the femora, the relation of the pelvis to the entire body differs in the sitting and standing positions. When a woman stands upright, her pelvis is so markedly oblique in its position that she would tip backward but for strong tendons attached to the pelvis and running down the front of the thighs. Added strain upon these tendons during pregnancy may account for some of the apparently undue fatigue experienced by the expectant mother.

The shallow, expanded portion of the pelvis above the brim is the large, or false pelvis, its walls being formed by the sacrum behind, the fan-like flares of the ilia on each side, with the incompleteness of the bony wall in front made up by abdominal muscles.

The false pelvis ordinarily serves simply as a support for the abdominal viscera, which do not occupy the true pelvis unless forced down by some such pressure as that caused by tight, or poorly fitting corsets. The false pelvis is of little obstetrical importance, its function during pregnancy being to support the enlarged uterus, while at the time of labor it acts as a funnel to direct the child’s body into the true pelvis below.

Fig. 3.—Diagram of pelvic outlet, seen from below, with most important diameters.

The true pelvis, on the other hand, is of greatest possible obstetrical importance since the child must pass through its narrow passage during birth. It lies below and somewhat behind the inlet; is an irregularly shaped, bottomless basin, and contains the generative organs, rectum and bladder. Its bony walls are more complete than those of the false pelvis, and are formed by the sacrum, coccyx and innominate bones. Its lower margin constitutes the outlet, or inferior strait, and being longer in its antero-posterior dimension than in its transverse measurement, its long axis is at right angles to the long axis of the inlet. (Fig. 3.) A baby’s head, accordingly, must twist or rotate in making its descent through this bony canal, for the long diameter of the head must first conform to one of the long diameters of the inlet, either transverse or oblique, and then turn so that the length of the head is lying antero-posteriorly, in conformity to the long diameter of the outlet, through which it next passes.

The posterior wall of the pelvis, consisting of the sacrum and coccyx, forms a vertical curve and is about three times as deep as the anterior wall formed by the narrow symphysis pubis. The structure as a whole, therefore, curves upon itself, resembling a bent tube with its concavity directed forward. (Fig. 4.)

Fig. 4.—Diagram of sagittal section of the pelvis showing curve of the bony canal, with most important diameters.

Thus it becomes apparent that the structure of the pelvis requires the child’s head, not only to rotate in its passage through the birth canal, but also to describe an arc, since the part of the head which passes down the posterior wall travels farther in a given time than the part which passes under the pubis.

This twisting and curving of the birth canal must be appreciated in order to understand the mechanism of labor.

In considering the question of pelvimetry, we find that there are both external and internal measurements to be taken, all for the purpose of estimating as accurately as possible the shortest diameter of the inlet through which the baby must pass. (Fig. 5.)

According to a common system of mensuration, the first external measurement is the inter-spinous, the distance between the anterior-superior spines, those bony points which are uppermost as the patient lies on her back. This distance is normally 26 centimetres. (Fig. 6.)

Fig. 5.—Two types of pelvimeters frequently used in taking measurements of the pelvic inlet and outlet.

The second measurement is the inter-crestal, or the distance between the iliac crests, and is normally 28 centimetres.

Baudelocque’s diameter is the third measurement and is taken with the patient lying on her side. (Fig. 7.) It is the distance from the top of the symphysis to a depression just below the last lumbar vertebra. This depression is easily located as it also marks the upper angle of a space just above the buttocks, which in normal pelves is quadrilateral. In malformed pelves this quadrangle may be so misshapen as to become almost a triangle with the apex directed either up or down. This dimension is sometimes called the external conjugate and ordinarily measures 21 centimetres.

The fourth measurement is the distance between the great trochanters, or heads of the femora, and normally is 32 centimetres.

All of these measurements, which after all are only approximate, relate to the top of the pelvis and are valuable in that they help in estimating the dimensions of the inlet, which are the important ones, and obviously cannot be measured on a live woman.

Fig. 6.—Diagram showing method of measuring distances between iliac crests and spines and the trochanters.

The inlet has four measurements of obstetrical importance: the antero-posterior, or true conjugate, which is the distance from the top of the symphysis pubis to the prominence of the sacrum, and is normally 11 centimetres; the transverse diameter, which is at right angles to the true conjugate and is the greatest width of the inlet, measuring from a point on one side of the brim to the corresponding point on the other, is normally 13.5 centimetres, and the two diagonal measurements, known respectively as the right and left oblique diameters, which are normally 12.75 centimetres.

Although it is very important to the expectant mother that all of these dimensions be of normal length, the length of the true conjugate, or conjugata vera, is of the gravest importance of all because it is the shortest diameter through which the child’s head must pass. If it is shorter than normal, the channel may be too constricted for the full-term baby’s head to pass through comfortably, thus making a spontaneous delivery extremely difficult, or even impossible.

Fig. 7.—Diagram showing method of measuring Baudelocque’s diameter.

The length of the all important, true conjugate is estimated by introducing the first two fingers of one hand into the vagina until the tip of the second finger touches the promontory of the sacrum. (Fig. 8.) The point at which the inner margin of the symphysis then rests upon the forefinger is measured, thus giving the length of the diagonal conjugate. This normally measures 12.5 centimetres or more, and is estimated as being 1.5 centimetres longer than the true conjugate.

The most important measurement of the outlet is the intertuberous diameter, the distance between the tuberosities of the ischii. This is the shortest diameter through which the child must pass in the inferior strait, and normally measures something more than 8 centimetres, usually about 11 centimetres. (Fig. 9.)

It is possible, by studying such measurements as these, made upon an expectant mother, and comparing them with dimensions which have been accepted as normal, to form a reasonably accurate estimate of the size and shape of her pelvis.

Fig. 8.—Diagram showing method of estimating the true conjugate by measuring the length of the diagonal conjugate.

A delivery may be, and frequently is, accomplished through a pelvis which is not entirely normal in size or shape. But the obstetrician of to-day is closely observant of the patient whose pelvic measurements depart from the normal by more than the accepted margin of safety, and he plans for labor in accordance with the indications in each case.

Disproportion between the measurements of the mother’s pelvis and the size of the child’s head must be considered in this connection. A small pelvis may permit of the spontaneous delivery of a small child, but be too narrow for the passage of a full-sized baby, while a woman with a normal pelvis may have an extremely difficult labor because of an unusually large child.

The size and shape of the pelvis is found to vary among different races and in different individuals. And the size and contour of the inlet may be so altered by rickets, lack of proper exercise during early life, or by growths upon the pelvic bones, as to seriously interfere with normal labor.

Fig. 9.—Diagram showing method of measuring the inter-tuberous diameter.

The various kinds of malformed pelves may be loosely classified as generally contracted or small; flat; simple funnel; generally contracted funnel; and the rachitic pelves, both flat and generally contracted. There may be a contracted inlet, or a contracted outlet, or both may occur in the same pelvis.^[1]

Rachitic pelves are common among negroes and not altogether rare among white women.

The normal male pelvis is deep, narrow, rough and massive as compared with the female structure (see Fig. 1.), and the angle of the pubic arch, formed by the two pubic bones, is deeper and more acute in the male than in the female skeleton.

The normal female pelvis, on the other hand, is light, broad, shallow, smooth and large, giving evidence of the infinite wisdom and skill that entered into constructing it for the high purpose it was designed to serve.