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The clinical indications for performing point-of-care ultrasound in the second and third trimesters of pregnancy are as follows:
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Determination of Gestational Age and Fetal Heart Rate
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Fetal assessment begins with measurement of fetal cardiac activity to establish fetal viability. This step is recommended early, not only in evaluation of the pregnant trauma patient7 but also in patients with vaginal bleeding8 and preterm labor.9 The normal fetal heart rate following the first trimester is 120–160 beats per minute (bpm). Sustained bradycardia is often associated with fetal hypoxia and acidemia. Historically, in emergency medicine, fetal cardiac activity was established by measuring fetal heart tones with a handheld Doppler stethoscope.
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Point-of-care ultrasound is an attractive alternative to the handheld Doppler stethoscope for initial detection of fetal cardiac activity. Using B-mode scanning of the gravid uterus, locating the fetal heart, and assessing the presence of cardiac motion is relatively straightforward. Using M-mode scanning, the waveform produced by cardiac motion can be recorded. Fetal heart rate is then determined rapidly and accurately with the aid of obstetrics software contained in most ultrasound machines.
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Knowledge of gestational age is critical in many clinical settings. This information may be used to guide the decision to perform an emergency cesarean section, such as in the setting of maternal cardiac arrest.10 In addition, gestational age and fetal maturity often influence the management of placenta previa, preterm labor, rupture of membranes, eclampsia, and other severe medical illnesses in late pregnancy. In the setting of trauma, sonographic assessment of gestational age, along with fetal heart rate, can be performed as an adjunct to the initial E-FAST examination. In a stable trauma patient, gestational age may influence the decision to proceed to exploratory laparotomy.11
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When assessing gestational age, consider the following general points. By current convention, obstetric dating begins with the first day of the last normal menstrual period, referred to as gestational age or menstrual age, and is equal to the fetal or conceptual age plus 14 days. Assume fetal viability when the gestational age is >24 weeks. The pregnancy is considered “term” at 38 weeks. Sonographic estimates of gestational age are progressively less accurate in later pregnancy due to natural variations in the size of the fetus (Table 15-1). A simple rule that reinforces this principle is that the variability (2 SD from the mean) of a gestational age estimate is equal to approximately 8% of the predicted age.12 An ultrasound measurement of BPD that yields a gestational age of 32 weeks has a variability of ±19 days. Nevertheless, estimates based on BPD, obtained as late as 20 weeks, still outperform menstrual history for predicting onset of labor.13 If possible, base gestational age on results of an ultrasound examination performed prior to 20 weeks or on a reliable menstrual history. Beyond 20 weeks, point-of-care ultrasound may be used to estimate gestational age if menstrual dates are unreliable or when the patient cannot provide a history.
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In the first trimester, crown-rump length is the preferred biometric measurement for establishing gestational age. In the second and third trimesters, measurements commonly used to estimate gestational age include BPD, head circumference (HC), and femur length. Modern ultrasound machines contain software that will automatically calculate gestational age based on any one of these parameters. In choosing which biometric parameter to measure, the established predictive validity of the parameter (Table 15-1) should be considered, and the ease and speed with which it can be obtained. A parameter that has excellent predictive validity according to the obstetric literature, but is difficult to measure and therefore prone to error, may not be well suited to the emergency setting.
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In the second trimester, BPD and HC are the most widely used measurements to determine gestational age.12,14 Although in expert hands, HC has a somewhat better predictive validity than BPD, measurement of BPD is preferable in the emergency setting because of the relative ease with which it is obtained. While HC must be calculated in a particular plane, BPD can be measured in any plane, provided the line of measurement intersects the thalamus and third ventricle.12,15 Of particular relevance to emergency medicine providers is a study that directly correlated neonatal survival in premature infants with various biometric measurements obtained by ultrasound shortly before birth. Based on analysis of receiver operator curves, a BPD of >54 mm was the single best predictor of survival.16
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In the third trimester, femur length is a frequently used alternative to BPD for estimating gestational age.12,17 In late pregnancy, measurement of BPD may be difficult because the fetal skull is frequently located within the maternal pelvis and can be obscured by acoustic shadowing. The predictive validity of femur length is slightly better than BPD at this stage.14 Femur length is relatively easy to measure because the transducer need only be parallel to the long axis of the femur.12,18
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In the late third trimester, identification of an ossified distal femoral epiphysis represents a potentially rapid means of estimating gestational age, and may be useful in the emergency setting. The appearance of this ossification center indicates a gestational age of 29 weeks or greater, whereas its absence means that the gestational age is <34 weeks.19 Similarly, the appearance of an ossified proximal tibial epiphysis suggests a gestational age of at least 35 weeks indicating the fetus is at or very near term.12
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Assessment of Obstetrical Causes of Abdominal Pain and Vaginal Bleeding
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The primary pregnancy-related causes of abdominal pain in the second and third trimesters are preterm labor, placental abruption, and chorioamnionitis.20–24 Chorioamnionitis refers to an infection of the amniotic fluid, typically following a rupture of membranes, or rarely as a complication of diagnostic amniocentesis. In rare cases, infection may occur without membrane rupture, causing pain, preterm labor, and systemic signs of infection. The diagnosis can be made by ultrasound-guided amniocentesis.25
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There are several disorders unique to pregnancy that may cause abdominal pain. Severe pregnancy-induced hypertension can be complicated by HELLP syndrome in 5–10% of cases, which is characterized by hemolysis, elevations in liver function tests, and low platelets. Midepigastric or right upper quadrant pain is present in 25% of cases.25 Spontaneous liver or spleen subcapsular hematomas can also develop, usually, but not always, in association with pregnancy-induced hypertension. Patients may experience right or left upper quadrant pain and mild coagulopathy, but have normal liver function tests. Hematoma rupture results in peritonitis and hemorrhagic shock.24 The diagnosis is often difficult, since this condition may resemble uterine rupture or abruption, and ultrasound may be helpful in differentiating these conditions.
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Patients with vaginal bleeding in the second and third trimesters are at high risk, with fetal mortality or adverse outcomes in up to one-third of all cases.25,26,27 Vaginal bleeding beyond 20 weeks' gestational age complicates 5% of all pregnancies. In 13% of those with bleeding, placental abruption is the cause, and in 7% placenta previa is the cause.28,29 Placenta previa and abruption account for the vast majority of cases requiring transfusion or cesarean section, as opposed to cases of vaginal bleeding caused by early labor, lower genital lesions, or other diagnoses.
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The term placenta previa refers to a placenta that completely covers the internal cervical os. Placenta previa has traditionally been subdivided into complete, meaning that the entire os is covered by placenta, and partial, meaning the os is partially covered. When the placental edge is located within 3 cm of the internal os, it is termed marginal placenta previa. The term low-lying placenta is useful for describing the case of a placenta located in the lower portion of the uterus in which the exact os–placenta relationship cannot be defined, or for describing an apparent placenta previa when seen in the second trimester (Figure 15-3).
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Placenta previa is present at term in only approximately 0.5% of pregnancies. Yet, routine ultrasound in early second trimester has found low-lying placenta in up to 45% of patients and an apparent placenta previa in 5%.30–32 The explanation for this paradox is widely referred to as placental migration, the relatively rapid elongation of the lower uterine segment during the third trimester, which effectively moves the placenta away from the os.33
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Maternal risk factors for placenta previa include advanced age, multiparity, non-Caucasian race, previous cesarean section, and prior history of placenta previa.33,34 Placenta previa usually presents as painless vaginal bleeding. However, pain from contractions sometimes accompanies the hemorrhage. The first episode of bleeding typically occurs in the third trimester, but may not occur until after the 36th week in up to one-third of cases.33
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The evaluation of possible placenta previa begins with transabdominal scanning as it is rapid, noninvasive, and reliable for locating the placenta (Figure 15-4) A digital vaginal examination can precipitate severe hemorrhage in the presence of placenta previa. Ultrasound can be used to locate the placenta and exclude placenta previa prior to vaginal examination. The sensitivity for ultrasound diagnosing placenta previa is 92–98%.35,36 When the placenta is visualized at or near the fundus by transabdominal ultrasound, placenta previa is effectively excluded. After excluding the diagnosis of placenta previa, the clinician can then proceed to evaluate the patient for placental abruption. However, if the placenta is clearly seen covering the entire cervical os, particularly in the third trimester, the diagnosis of placenta previa is confirmed. When the placenta appears to be low lying or partially covering the os, or when an adequate view cannot be obtained with transabdominal ultrasound, further evaluation with transvaginal or translabial ultrasound is generally indicated. The diagnosis of placenta previa by transabdominal ultrasound has a high false-positive rate, up to 17% in one large study.37 Placenta previa is often overdiagnosed in the second trimester because it is mimicked by two conditions: (1) an overdistended bladder that compresses the lower uterine segments and (2) focal contractions.38 With the transabdominal approach, the relationship of the inferior edge of the cervix to the internal os is frequently obscured by patient obesity, an overdistended bladder, myometrial contractions, a posterior placenta, or the ossified fetal skull.39,40 In one study of patients with suspected previa, assessment of the placenta–os relationship was impossible in 31% of transabdominal studies.41 Nevertheless, in the case of severe hemorrhage, if findings on transabdominal ultrasound appear to be consistent with placenta previa, the patient should proceed directly to the operating room. A double setup examination may then be performed in the operating room at the obstetrician's discretion.
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The approach to management of placenta previa depends largely on the sonographic assessment of fetal well-being and gestational age. Although cesarean section is the definitive treatment, vaginal bleeding due to confirmed placenta previa is frequently managed in an expectant fashion. The rationales for expectant management are as follows: (1) bleeding prior to the third trimester is often self-limited and can be treated by transfusion, if necessary; (2) vaginal bleeding represents little direct risk to the fetus in the absence of significant abruption or maternal shock; and (3) delaying delivery, to maximize fetal maturity, improves perinatal outcome.33 Confirmation of fetal well-being is a prerequisite to expectant management. A rapid initial measurement of fetal cardiac activity can be performed in the emergency setting with transabdominal ultrasound, although cardiotocography is then required for ongoing fetal monitoring. Other measures of fetal well-being, such as amniotic fluid volume and biophysical profile, may have an impact on management.
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A determination of gestational age also is required to guide treatment decisions.8 The following general guidelines have been proposed with regard to management of previa based on gestational age. When gestational age is <24 weeks, delivery is indicated only for hemorrhage that is life threatening to the mother. Between 24 and 34 weeks, fetal distress and life-threatening hemorrhage are indications for delivery. Beyond 34–37 weeks, delivery is indicated for fetal distress, significant bleeding, or labor.33
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Any abnormal separation of the placenta occurring after 20 weeks' gestation is defined as a placental abruption. Prior to this date, placental separation is considered part of the process of a spontaneous abortion. While it affects <1% of all pregnancies, abruption accounts for more than a quarter of all perinatal mortality.8,33 The epidemiology of placental abruption suggests that a variety of risk factors contribute to its development, many of which relate to more general microvascular disease. One of the strongest associations is with maternal hypertension, both chronic and pregnancy induced.42 Cigarette smoking and cocaine abuse have also been linked to higher rates of abruption.43,44 Trauma is an uncommon but important cause of abruption. Particular attention must be given to victims of motor vehicle crashes, falls from height, and domestic violence.28,33,45
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Hemorrhage from abruption begins at the point of separation between the placenta and the uterus, or the placenta and the amnion. The timing and degree of subsequent bleeding from the cervix are dependent on the size of the hemorrhage and its location relative to the placenta. The amount of vaginal bleeding is not a reliable guide to the degree of placental abruption or the severity of hemorrhage. In some cases, patients may experience no vaginal bleeding despite significant placental separation. The amount of vaginal bleeding must never be taken as a guide to degree of internal hemorrhage. Also, the presence of abdominal pain, considered a hallmark symptom for abruption, is absent in nearly half of all cases.8,33 Therefore, when evaluating patients with painless vaginal bleeding in pregnancy, the possibility of abruption must be considered.7,33 The most consistent finding in abruption will be the presence of uterine irritability and contractions.7 These may be unappreciated by both patient and physician without the aid of cardiotocography.
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Because neither the character of the bleeding nor the presence of pain can be relied upon to differentiate placental abruption from placenta previa, it is recommended that the evaluation of vaginal bleeding in the second and third trimesters begins with an ultrasound examination to exclude placenta previa. Once the diagnosis of placenta previa is excluded, abruption becomes the major diagnostic consideration.
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During placental abruption, hemorrhage occurs within a layer of the placenta causing separation from the adjacent uterine wall.33 This hemorrhage and separation may remain partial and self-limited or progress on to complete abruption. The clinical manifestations of abruption and its prognosis are directly determined by the extent to which placental circulation is compromised, which in turn depends on both its size and location. Hemorrhage confined to the edge of the placenta is referred to as a marginal abruption. The degree or size of placental-uterine separation is graded as mild (grade 1), partial (grade 2), or complete (grade 3) (Figure 15-5).27,46 These categories correlate reasonably well with the clinical presentation and prognosis. Grade 1 separations are usually marginal, involve less than a few centimeters of the placental border, and are usually not clinically significant. In contrast, grade 3 abruption can be fatal to both the fetus and the mother. Abruption is further categorized by the anatomic location of the hematoma relative to the placenta: retroplacental (in the decidua basalis, between placenta and uterine wall), subchorionic (between decidua and the membranes), and preplacental (between placenta and amniotic fluid, immediately beneath the amnionic membrane) (Figure 15-6).47 These distinctions also have significant prognostic implications, but primarily for fetal outcome. A retroplacental hemorrhage of 60 mL or more results in a 50% fetal mortality rate, but a similar-sized subchorionic hemorrhage results in a 10% fetal mortality rate. Preplacental hemorrhage is often self-limited and clinically silent, with 30% detected only after delivery.27,47 They appear sonographically as an irregular bulge along the inner border of the placenta. Rupture of these hematomas results in the classic “port-wine” staining of the amniotic fluid.
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Ultrasound is not a sensitive test for placental abruption and the diagnosis remains largely clinical, beginning with a meticulous search for evidence of vaginal bleeding, uterine tenderness, labor, or fetal distress. Signs of labor and fetal distress are the keys to making the diagnosis. Uterine contractions are present in nearly all cases of abruption, although they may be difficult to appreciate by either the physician or the patient. Contractions are characteristically of high frequency but low amplitude. It has been demonstrated that 6 hours of cardiotocographic monitoring after trauma is 100% sensitive for predicting all subsequent complications.7 Even rare cases of late-onset abruption or fetal distress after trauma are heralded by early abnormalities on cardiotocography.48 Hence, the absence of uterine irritability or fetal distress remains an excellent indicator of maternal–placental well-being, suggesting that abruption is either absent or clinically insignificant. Cardiotocography should be a routine part of the initial evaluation where there is a concern for abruption, even in those who are asymptomatic. The stable patient without evidence of uterine irritability for 6 hours may be discharged home with appropriate instructions and follow-up.
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Once placental abruption is diagnosed, an immediate cesarean section remains the definitive treatment. Decisions regarding the manner and timing of any intervention depend on an overall assessment of maternal–fetal well-being. If the fetus is immature and the abruption is judged to be mild, an expectant approach may be attempted. Signs of preterm labor may be difficult to distinguish from mild abruption.33 A term fetus or evidence of uterine irritability refractory to medical management should prompt expedited delivery. Similarly, fetal distress or maternal signs of abruption indicate a need for immediate cesarean section.25,33
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Approximately 7% of newborns are premature at birth, which may result in mental or physical impairment.49 Preterm labor is defined as regular uterine contractions accompanied by characteristic changes in the cervix, occurring prior to 37 weeks' gestation. Assessing the potential for premature delivery is the main goal of the emergency evaluation for preterm labor.
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Ultrasound is a safe, rapid, and accurate method of evaluating the cervix for signs of labor. As labor begins, the cervix undergoes effacement followed by dilation. While the digital examination has traditionally been used to evaluate such cervical changes, ultrasound has emerged as a safer and more accurate means to evaluate cervical changes.50 Sonographic measurement of cervical length represents an objective way to quantify effacement.
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There are several justifications for using an ultrasound examination rather than the digital examination in this setting. First, a direct contraindication to digital examination may exist, such as placenta previa or ruptured membranes, and digital examination in these settings can produce life-threatening bleeding or chorioamnionitis. A transabdominal or translabial ultrasound examination may negate the need for a digital examination. Second, an ultrasound examination has been shown in numerous studies to be more accurate than digital examination in estimating cervical length and predicting preterm labor.50
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An advantage of using ultrasound is that it can visualize the internal cervical os and detect funneling. Funneling is dilatation of the internal cervical os without dilation of the external cervical os, and it is one of the earliest signs of labor. A digital examination cannot palpate the internal os, so it cannot detect funneling. An ultrasound examination is the only practical way to detect this important finding.50–55
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There are three methods for imaging the cervix with ultrasound: transabdominal, transvaginal, and translabial. The transabdominal approach is the least reliable method, with successful imaging of the cervix in only 46% of patients without a full bladder and 86% with a full bladder.56 Presenting fetal parts and a large maternal habitus may obscure visualization of the cervix. The transvaginal technique produces the most consistent findings, with visualization of the cervix in up to 100% of patients.51 The translabial ultrasound is considered the most technically difficult method. In the hands of a skilled operator, however, this approach provides an adequate view of the cervix in up to 95% of patients.51,57 In the emergency setting, it is reasonable to begin with transabdominal ultrasound and then proceed to transvaginal or translabial imaging if the cervix cannot be well visualized.
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From studies of all three ultrasound methods, it is clear that cervical measurements can predict the risk of preterm delivery. In one study of nearly 3000 patients at 24–28 weeks' gestation, it was demonstrated that a positive correlation exists between short cervical length (<30 mm) and risk for preterm birth before 35 weeks (Figure 15-7).53 Another study of patients between 16 and 28 weeks' gestation found a 79% rate of preterm delivery in those with cervical funneling of >50%.58 Subsequent studies have confirmed the predictive value of both short cervical length and the presence of funneling, and have extended the findings to twin pregnancies.55,59 For the most part, patients in these studies were asymptomatic so applicability in the emergency setting is unclear. Also, the impact of such findings on clinical management, such as the need for cervical cerclage, remains uncertain.
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The use of ultrasound in determining the presence or absence of cervical changes can help the emergency physician risk stratify patients with symptoms suggestive of labor. The goal of such an evaluation is to identify patients who would benefit from admission and tocolysis versus candidates for outpatient follow-up. The utility of cervical sonography in the management of preterm labor was retrospectively evaluated at one facility. Women were hospitalized only if they had a cervical length <30 mm. This protocol produced a decrease in hospital days of 48% without affecting the rate of preterm births.60 The critical cervical length appears to be 30 mm, with preterm delivery much more likely in patients with a shorter cervical measurement by ultrasound.60 Apply cervical sonography in conjunction with clinical variables—such as the results of a biophysical profile (discussed below), the gestational age, and a history of preterm births—to influence management decisions.
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Lie refers to the relationship of the fetus to the long axis of the uterus, while presentation describes what fetal part is nearest the cervix. In normal deliveries, the fetal lie is longitudinal and the presentation is cephalic. Transverse lie and breech presentation—where the fetal sacrum and feet, respectively, are engaged in the pelvis—are referred to as malpresentations. The classification of breech presentations is demonstrated in Figure 15-8.
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In the emergency setting, it is absolutely essential to know the presenting fetal part. Even with good prenatal care, fetal presentation is often unknown because prior to 25 weeks the fetus frequently changes position. Breech presentations, which account for 3–4% of all deliveries, are fraught with complications such as asphyxia, cord prolapse, and spinal cord injuries to the fetus.61 Knowledge of the presentation allows the emergency physician to mobilize the appropriate equipment and support staff needed for delivery. In an emergency vaginal delivery, the presenting part is discovered easily by physical examination of the vagina or perineum. Prior to this point, determining fetal position by palpation may be difficult, particularly for a nonobstetrician. Moreover, vaginal examination is contraindicated when preterm labor is accompanied by vaginal bleeding. In such cases, an ultrasound examination can be used to establish fetal position.
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Just as important as fetal position is the determination of the number of fetuses prior to delivery. Ultrasound is invaluable for identifying the “surprise twin.” Perinatal death occurs seven times more frequently in twin deliveries compared to singletons.62 Like breech presentations, delivery of twins requires additional expertise, support, and equipment.
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There are multiple noninvasive measures to evaluate the health of the unborn child. Traditionally, nonstress testing and amniotic fluid volume measurement have been used. The biophysical profile is a more sophisticated instrument that combines nonstress testing and amniotic fluid volume with three additional sonographic parameters—fetal tone, movement, and breathing—to derive an objective score that reflects overall fetal well-being (Table 15-2).63–65 The idea behind the biophysical profile is that the CNS, which is very sensitive to hypoxia, controls all of the measured parameters. Thus, a low biophysical profile score may indicate either acute or chronic fetal hypoxia.66,67 Results of the biophysical profile are always considered together with gestational age and maternal and fetal comorbidities, but in general, lower scores have increased risk of fetal hypoxia, and scores >8 are considered normal.65–72
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Evaluation of Traumatic Injuries
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Trauma occurs in 6% of pregnant patients73 and is the leading cause of nonobstetrical maternal mortality.74 Furthermore, fetal loss as a result of trauma far exceeds maternal mortality.75 Point-of-care ultrasound has the potential to play a critical role in the initial evaluation of pregnant trauma patients. Use of the E-FAST examination to evaluate maternal intra-abdominal hemorrhage has been shown to be beneficial.11 The sensitivity and specificity of the E-FAST examination in pregnant trauma patients are similar to the sensitivity and specificity in nonpregnant trauma patients.76 Because fetal well-being is dependent on adequate maternal circulation, a tenant of trauma care is that resuscitation of the mother and assessment of maternal injuries is the initial priority.
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Besides maternal shock, processes that contribute to fetal loss include direct fetal injury, uterine rupture, placental abruption, ruptured membranes, and premature labor. Factors associated with fetal loss after maternal trauma are listed in Table 15-3.77–82
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Placental abruption is the most common cause of fetal loss in pregnant trauma patients, and may occur after relatively minor trauma and in the absence of other injuries.7 While cardiotocography and observation for the occurrence of frequent contractions remain the cornerstone of the evaluation,73 ultrasound may sometimes be able to confirm the presence of significant abruption.7 In addition, point-of-care ultrasound may be used to rapidly demonstrate fetal cardiac activity and determine gestational age.11
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Perform the E-FAST examination either immediately following or, preferably, contemporaneously with the primary survey of the pregnant trauma victim. The sonographic evaluation of pregnant trauma victim is a combination of the E-FAST exam as well as three other rapid and goal-directed scanning protocols. In order to perform this combination of ultrasound studies, the preferred transducer is the curvilinear, due to its lower frequency range and obstetric preset calculations. First and foremost is the sonographic assessment of potential maternal life threats with the E-FAST examination. This will allow the emergency provider to rapidly determine the presence of intraperitoneal fluid, pericardial fluid, pleural fluid, or pneumothorax (see Chapter 5, “Trauma”). Following the E-FAST exam, change the machine settings to OB and answer four questions: (1) Does the patient have a sonographically visible intrauterine pregnancy?; (2) What is the gestational age?; (3) Is there fetal cardiac activity?; and (4) What is the fetal heart rate?
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Assessment of Nonobstetrical Causes of Abdominal Pain
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Abdominal pain in the second and third trimesters of pregnancy represents a significant challenge for clinicians. The differential diagnosis must include obstetrical and nonobstetrical causes of abdominal pain, and the impact of diagnostic and management decisions on both the mother and the fetus must be considered. The anatomic and physiologic changes of pregnancy may alter the traditional presentation of many disorders, and the symptomatology of pregnancy itself may overlap with that of nonobstetrical abdominal pathology. Concerns over ionizing radiation or medication-induced teratogenicity may limit choices of diagnostic imaging or therapy. All of these elements may confuse the clinical picture and delay definitive diagnosis, increasing the risk of morbidity to the mother and the fetus. Ultrasound offers a safe and effective first step for the evaluation of abdominal pain in pregnancy.
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Nonobstetrical causes of abdominal pain in pregnancy occur at rates similar to that of the nonpregnant population. Similarly, the need for urgent abdominal surgery during pregnancy parallels that of the general population when ectopic pregnancies and cesarean sections are excluded.20,21 However, the morbidity associated with virtually every abdominal emergency is higher in pregnancy, mostly due to the difficulty in making an early diagnosis.22
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The evaluation of abdominal pain in the second and third trimesters demands that clinicians maintain a broad differential and an awareness of the altered presentation of common abdominal disorders in pregnancy. Table 15-4 lists the common obstetrical and nonobstetrical causes of abdominal pain. Every workup begins with a careful history and physical examination that should significantly narrow the differential. Laboratory tests must be interpreted in the context of pregnancy-induced changes (e.g., leukocytosis and relative anemia) and are frequently of limited diagnostic value.
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Sonography is a reasonable first diagnostic step in most cases and helps to substantially narrow the differential diagnosis, if not confirm the diagnosis. As an example, right upper quadrant pain in the third trimester engenders a broad differential that should include biliary disease, liver disease, nephrolithiasis, pyelonephritis, and appendicitis. Normal laboratory tests and urinalysis may help to reduce the list of likely possibilities to biliary colic and appendicitis. Ultrasound could then be used to confirm or exclude both of these disorders while providing additional information on the pregnancy itself.
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Ultrasound is the traditional first-line imaging modality for abdominal pain in pregnancy because it provides anatomic and functional information about the mother and fetus without exposing either to the effects of ionizing radiation. Intrauterine exposure to radiation may have both oncologic and teratogenic effects. Case-controlled studies of childhood cancer show a slight but significant increase in relative risk among the children of female radiologists exposed to 1000 mrems of radiation.21 The potential for teratogenicity and carcinogenesis appears to be greatest in the period of 2–15 weeks and decreases proportionally as the fetus nears term. In the first trimester, in utero radiation exposure is graded as low, moderate, or high depending on the total dose, with the clearest evidence of harm above a threshold level of 150 mGy (Table 15-5).23 Significant variability exists, however, depending on gestational age, body habitus, and the type of radiographic study. Exposure in the second and third trimesters is less critical with an estimated relative increase in cancer risk of 64% per rad (10 mGy), which corresponds to a 0.05% relative increase in the rate of childhood malignancies. Fortunately, the majority of radiographic tests fall well below the threshold level (150 mGy or 15 rads) and plain radiography and CT remain potential diagnostic tools in pregnant patients with abdominal pain.23 That said, the decision to use radiography in a pregnant patient must take into account a number of competing elements: the risk of radiation to the developing fetus, the risk of delayed diagnosis if available imaging techniques are not used, and the relative suitability of alternative imaging or management strategies. There are also the intangible concerns about radiation exposure that may not be supported by data, but nevertheless push providers toward using alternative imaging modalities. The use of diagnostic radiography in the pregnant patient is appropriate provided the risks and benefits are weighed in a manner that ensures the best outcome for the mother and fetus. Regardless of the relative risks and benefits of radiography, ultrasound is a valuable initial tool in the evaluation of the pregnant patient with abdominal pain, given its safety and unique ability to assess fetal well-being.
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As pregnancy advances into the second and third trimesters, a number of anatomic and physiologic changes occur. These changes alter the way a variety of common disease processes present. The primary sonographic approach to these disorders must change accordingly. As the uterus enlarges, the intestinal tract is displaced upward, backward, and to the sides. The appendix moves to the right upper quadrant and away from the omentum (Figure 15-9). Increased intra-abdominal pressure leads to increased gastroesophageal reflux, and the gravid uterus compresses the ureters, inferior vena cava, and bladder.20,21 Pregnant women commonly experience varying degrees of anorexia, nausea, vomiting, and back and flank pain, which are all related to the compressive and postural effects of the gravid uterus. Serious intra-abdominal processes can cause similar symptoms.
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Appendicitis is the most common surgical emergency in pregnancy, accounting for two-thirds of all laparotomies.25,83 In the first half of pregnancy, the clinical presentation of appendicitis is similar to that in nonpregnant patients. Thereafter, the clinical picture becomes more atypical.25 As mentioned above, early constitutional symptoms of appendicitis are often subtle and dismissed as normal symptoms of pregnancy. The abdominal pain is predominantly right-sided and corresponds to the location of the appendix in different stages of pregnancy (Figure 15-9). Leukocytosis is a common but unreliable finding, and 20% of pregnant patients with appendicitis have sterile pyuria.84 The differential diagnosis includes ovarian mass, ovarian torsion, and other sources of right upper quadrant pain, specifically cholecystitis, pyelonephritis, or hepatitis.20,21,85
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If appendicitis is suspected on clinical grounds, the diagnosis may be confirmed by graded compression ultrasound, which is highly accurate for the diagnosis of appendicitis during pregnancy. In one study of 45 pregnant women with abdominal pain, graded compression ultrasound demonstrated a sensitivity of 100% and specificity of 96% for the diagnosis of appendicitis.86 However, earlier studies found a considerably lower sensitivity of 75–89%, although with similar specificities.87,88 These results, along with its well-established diagnostic performance in nonpregnant patients, would suggest that ultrasound for appendicitis in pregnancy should be regarded as a diagnostic test that is specific, but may have limited sensitivity. A positive finding would allow surgical management without the need for other diagnostic testing, but a negative ultrasound exam, in the setting of an intermediate or high pretest probability, would require further testing.
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The sonographic evaluation for appendicitis is challenging and highly operator dependent. During pregnancy, this assessment is further complicated by the upward and outward displacement of the appendix by the growing uterus. A graded compression test is performed with special attention paid to the overall diameter of the appendix, wall thickening, or the presence of surrounding fluid and debris. The specific sonographic findings of appendicitis are discussed in Chapter 11, “General Surgery Applications.” In the third trimester, the size of the uterus may preclude adequate visualization of the appendix, despite a proper high-lateral approach.86 Placing the patient in the left lateral decubitus position may make visualization of the appendix more likely.
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Laparoscopy or laparotomy, without any diagnostic testing, may be indicated. It has been recommended that surgeons maintain a lower threshold for surgical exploration in pregnant patients with possible appendicitis, given the variability of clinical signs and the increased morbidity associated with diagnostic delay. Negative exploration rates as high as 40% in the third trimester are commonplace and may be acceptable.88
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The functional absence of the omentum in pregnancy means that a ruptured appendix is less likely to be walled off, resulting in early peritonitis. Perinatal mortality rises from 4.8% in nonperforated appendicitis to 27.8% when the appendix ruptures. The perforation rate prior to diagnosis is as high as 30% during the third trimester.83 Since the risk of fetal loss and maternal morbidity from a delayed diagnosis of appendicitis is considerable, clinical vigilance is required even in the absence of classic signs and symptoms.21
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Cholelithiasis and Cholecystitis
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Acute gallbladder disease is more common in pregnancy than in the nongravid population, which reflects a higher prevalence of gallstones in fertile women (3.5–11%).89,90 Signs and symptoms are essentially the same as in the general population: abrupt onset of stabbing or colicky right upper quadrant abdominal pain accompanied by nausea and vomiting. While the presence of fever and Murphy's sign suggests acute cholecystitis, mild elevations in the WBC, amylase, and alkaline phosphatase can be normal during pregnancy. The differential diagnosis includes appendicitis, pyelonephritis, nephrolithiasis, and rare entities such as the HELLP syndrome or subcapsular hematoma.
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Pregnant patients suspected of having biliary disease should invariably have a right upper quadrant ultrasound exam (see Chapter 10, “Hepatobiliary”; Videos 10-1 and 10-2). As in the nonpregnant population, sonography is the imaging modality of choice for biliary disease, and its technique and diagnostic performance are essentially unaltered by pregnancy. Oral cholecystograms and HIDA scans are effective but less attractive options in the pregnant patient due to the risks of radiation exposure. Ultrasound will identify nearly all gallstones when the exam is performed carefully.90 The presence of gallbladder wall thickening or pericholecystic fluid are strong indicators of gallbladder inflammation. Many patients with acute cholecystitis during pregnancy can be managed conservatively with IV hydration, analgesia, and antibiotics. The risk of fetal loss with cholecystectomy approaches 5%, but appears to be lowest when performed in the second trimester. Surgical intervention should not be delayed if the patient becomes toxic or develops pancreatitis, since the fetal loss rate may reach 50% in these patients.90