Kent Ueland

Maternal cardiovascular dynamics: IV. The influence of gestational age on the maternal cardiovascular response to posture and exercise

Abstract Serial hemodynamic evaluations were carried out in 11 patients during pregnancy and the postpartum period. The patients were studied at rest in the supine, lateral, and sitting positions as well as during and after exercise on a bicycle ergometer. The cardiac output was found to be elevated to peak levels as early as 20 to 24 weeks of gestation and was maintained at this high level through the thirty-second week of gestation. At 38 to 40 weeks there was a decline in cardiac output in all positions. This fall was most pronounced in the supine position, in which the cardiac output was found to be lower than in the same position during the postpartum period. The heart rate increased only slightly during early gestation but continued to rise throughout pregnancy in the supine and lateral positions. Accordingly, The stroke volume showed a progressive decline from 20 to 24 weeks of gestation to term. The cardiovascular response to mild exercise was constant throughout pregnancy and similar to that encountered in nonpregnant individuals. However, moderate exercise indicated there is a progressive decline in circulatory reserve as pregnancy advances. This is not attributed to any change in cardiac function but rather to peripheral pooling of blood and obstruction to venous return by the large gravid uterus. Following exercise cardiovascular function returned to resting levels with equal rapidity during pregnancy and post partum.

Maternal cardiovascular dynamics

Abstract A hemodynamic evaluation was performed in 12 patients undergoing repeat cesarean section at term under subarachnoid block anesthesia. It was found that posture exerted a profound influence on maternal cardiodynamics in late pregnancy. Following spinal anesthesia, a decline in blood pressure invariably occurred in the supine position. This hypotension could be completely reversed by turning the patient on her side. The degree of maternal hypotension could not be related to anesthetic dose or level of anesthesia. Delivery resulted in a 52 per cent increase in cardiac output. 11 per cent decrease in heart rate, and a 67 per cent increase in stroke volume. Cesarean section under spinal anesthesia seems contraindicated in the pregnant patient with heart disease because of the extensive cardiovascular changes encountered.

Maternal cardiovascular dynamics: VII. Intrapartum blood volume changes

Blood volume was measured in 75 normal pregnant women at term and serially at 10 and 60 minutes and on Days 1 and 3 post partum in women delivered vaginally and on Days 1, 3, and 5 in women who underwent cesarean section. The mean blood volume at term was 5.95 L. and 83.3 c.c. per kilogram, an increase of 44 and 16%, respectively, over nonpregnant values. The increment appeared to be related to lean body mass and the weight of the neonate alone and to the combined weights of the neonate and placenta, but the wide range of values precluded statistical significance. The blood volume loss at 60 minutes post partum was 610 c.c. (10.2%) for patients delivered vaginally and 1,030 c.c. (17.4%) for those delivered by cesarean section. The blood volume showed a steady decline to Day 3 post partum in the vaginal delivery group, whereas it remained fairly stable from 60 minutes to Day 5 post partum in the surgically delivered group. Remarkably, the volume had declined by the amount, 16.2%, in both groups on the third postpartum day. On the other hand, the hematocrit showed a rise of 5.2% in the former group and a decline of 5.8% in the latter. A remarkable tolerance for blood loss at delivery was demonstrated. A more realistic definition of postpartum hemorrhage is strongly recommended.

Maternal Cardiovascular Adjustments to Pregnancy

T HERE ARE several reasons for interest in the maternal hemodynamic adjustments that accompany pregnancy. An understanding of these reversible changes and the mechanisms regulating them will lead to increased knowledge concerning the physiology of the circulation and its control. Moreover, as physicians we have additional concerns. In a few women with preexisting disease of the heart or blood vessels, the circulatory changes of pregnancy are hazardous. In a few others, previously normal, events occur in association with pregnancy that result in important diseases of the circulatory system. Finally, from the standpoint of preventive medicine, an inadequate maternal hemodynamic response to pregnancy may provide an intrauterine environment that jeopardizes fetal development: congenital defects, including congenital heart diseases, begin during pregnancy, and mea-

Maternal cardiovascular dynamics: II. Posture and uterine contractions

Maternal hemodynamic changes were evaluated in 23 normal patients undergoing induction of labor at term. It was found that maternal posture exerted a significant influence on cardiodynamics. A change in position from supine to lateral produced an increase in cardiac output of 21.7 per cent, a decrease in heart rate of 5.6 per cent, and an increase in stroke volume of 26.5 per cent. Similar cardiovascular alterations were produced by effective uterine contractions in the supine position during early first stage of labor. The cardiac output increased by 15.3 per cent, the heart rate declined by 7.6 per cent, and the stroke volume increased by 21.5 per cent. Uterine contractions with patients in the lateral position produced only a 7.6 per cent rise in cardiac output and stroke volume. The heart rate remained unchanged. The vascular obstruction produced by the gravid uterus at term must be considered when evaluating maternal hemodynamics.

Maternal cardiovascular dynamics: III. Labor and delivery under local and caudal analgesia

Maternal hemodynamic changes were evaluated in 23 normal patients who underwent induction of labor at term. Ten patients received paracervical and pudendal block analgesia during labor and delivery. Thirteen patients were given caudal analgesia. The latter modified some of the cardiovascular responses by preventing an accumulative increase in cardiac output during labor and limiting the absolute increase in cardiac output at delivery. The type of analgesia, however, did not modify the hemodynamic effects of uterine contractions or maternal posture. The magnitude of the maternal cardiovascular response was similar to changes in posture from supine to side, to uterine contractions, and to delivery, suggesting that the redistribution of blood volume was the common etiologic factor.

Reduction of the stretch modulus of human cervical tissue by prostaglandin E2

Samples of cervical tissue were obtained from women immediately following term delivery from spontaneous labor or oral PGE2-induced labor and from a small nunber of nonpregnant women in the reproductive years undergoing hysterectomy for benign gynecologic disease. The measured strips were placed in a well-oxygenated bath at 37 degrees C. containing Ringers solution and stretched at a constant rate while continuously recording length and tension. The data were converted to stress-strain diagrams. Each curve contained a long linear portion which allowed one to compute a stretch modulus for each sample. A total of 71 strips from 23 patients was used. The average stretch modulus from the oral PGE2-induced patients was significantly lower than the spontaneous labor group (p less than 0.005), which, in turn, was significantly lower than the stretch modulus of the nonpregnant cervical tissue. It was also noted that the yield point was lower in the PGE2 series when compared to the spontaneous labor series (p less than 0.05). The effect of the PGE2 at a bath concentration of 10(-5) to 10(-6)Gm. per cubic centimeter was to materially reduce the stretch modulus within 5 to 15 minutes of the drug addition in both the PGE2-induced and spontaneous labor series. The results of these experiments indicate that PGE2 has the effect of reducing cervial stiffness.

Cardiorespiratory responses to pregnancy and exercise in normal women and patients with heart disease

Abstract Serial measurements of blood oxygen transport were performed during pregnancy and post partum in 6 normal women and in 16 women with mild heart disease. During pregnancy, normal women have an increased rate of oxygen consumption ( V ˙ o 2 ) at rest regardless of posture. Early in pregnancy, the rise in cardiac output is proportionately greater than the increase in V ˙ o 2 , resulting in a decreased C a o 2 − C v ¯ o 2 . Late in pregnancy, the V ˙ o 2 continues to rise, but the resting cardiac output falls in the supine and sitting positions and the C a o 2 − C v ¯ o 2 widens. Standard sitting exercise costs more in terms of V ˙ o 2 during pregnancy, and the heart assumes a greater share of the burden for supplying the oxygen needs to the periphery in normal patients. In women with mild heart disease, the resting cardiac output and V ˙ o 2 during pregnancy is lower and they show a smaller increment in response to standard sitting exercise. The average C a o 2 − C v ¯ o 2 at rest throughout pregnancy and post partum is wider. Patients with mitral stenosis, in particular, have a low cardiac output and a consistently wider C a o 2 − C v ¯ o 2 . During exercise, the handicap in cardiac output and peripheral oxygen supply becomes even more apparent in this group of women. There is no accumulation in oxygen debt during exercise at any time in the study, and blood lactate and pyruvate concentrations during and after recovery from exercise show no significant differences attributable to either pregnancy or to heart disease.

Maternal cardiovascular dynamics VI. Cesarean section under epidural anesthesia without epinephrine

Abstract Hemodynamic measurements were carried out in 13 normal term pregnant women undergoing cesarean section under epidural anesthesia without epinephrine. Only minor alterations in maternal cardiovascular function were encountered. Following the administration of anesthesia, there was a transient decline in blood pressure, but it remained constant throughout the surgical procedure. The maximum rise in cardiac output was found immediately following delivery, but it was only 1.46 L. per minute (25 per cent above control values). No significant heart rate changes occurred, and the stroke volume rose by a maximum of only 19 c.c. (28 per cent above control values) at 10 minutes post partum. This hemodynamic stability has not been achieved previously with other anesthetic techniques.

The stretch modulus of human cervical tissue in spontaneous oxytocin-induced and prostaglandin E2-induced labor.

A total of 62 strips of cervical tissue from 28 patients at term were tested for stiffness (stretch modulus) by elongation and measuring the tension produced by a given stretch. The stretch modulus was taken as the slope of the linear regression curve derived from the linear portion of the stress-strain relationship. The data were obtained from three patient categories: (1) 17 strips from seven patients undergoing spontaneous labor, (2) 18 strips form 10 patients with labor induced by PGE2, and (3) 27 strips from 11 patients with labor induced by oxytocin. The stretch moduli of cervical tissue obtained from spontaneous and oxytocin-induced labor patients were similar. The stretch moduli of cervical tissue obtained from PGE2-induced labor patients were significantly lower than those from either the spontaneous or the oxytocin-induced labor groups. These results show that PGE2, when used for induction of labor at term, has the ability to lower the stiffness of cervical tissue. This property of prostaglandin may be useful therapeutically for the indicated induction of labor in patients with an unfavorable cervix.