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Cardiac Diagnostic Testing in Pregnancy

Edlira Tam and Cynthia Taub

Introduction

Maternal heart disease, although it complicates a small percentage of all pregnancies in developed countries, is a major cause of non-obstetric maternal morbidity and mortality. Cardiac complications in pregnant women with heart disease occur in 16% of pregnancies and are primarily related to maternal arrhythmias and heart failure. The majority of cardiac complications are seen in the antepartum period, followed by the postpartum period, with the fewest occurring at the time of labor and delivery [1]. Maternal mortality continues to rise, and cardiovascular death is one of the main driving factors. Between 2011 and 2013, cardiovascular disease accounted for approximately 15% of maternal mortality in America [2]. Congenital heart disease is the most common form of heart disease complicating pregnancy in the United States, whereas rheumatic heart disease remains the most common form of heart disease in developing countries.

Pregnant women with known or suspected cardiovascular disease often require cardiovascular diagnostic testing during their pregnancy. There are multiple testing modalities available that can be used in pregnant women with suspected cardiac disease to confirm the diagnosis, risk stratify, treat, and assess response to therapy. The imaging modalities used for these purposes include electrocardiogram (ECG), x-ray (which encompasses chest radiography, computed tomographic pulmonary angiography [CTPA], coronary computed tomographic angiography [CCTA], fluoroscopy, and invasive angiography), echocardiography, cardiac magnetic resonance imaging (MRI), and nuclear techniques [3].

Special Considerations in Pregnancy

Normal cardiovascular changes in pregnancy such as increased cardiac output, volume overload, and reduced systemic vascular resistance may lead to significant changes in physical examination findings, laboratory changes, and imaging findings that often mimic cardiac pathology. A detailed exam should be performed in all women with signs and symptoms of cardiovascular disease, such as dyspnea, fatigue, palpitations, low oxygen level, and changes in blood pressure [2].

Imaging studies are important adjuncts in the diagnostic evaluation of cardiovascular disease in pregnancy. Poor understanding about the safety of imaging modalities in pregnancy and lactating women often results in unnecessary avoidance of useful diagnostic tests. Thus, it is important that the physician understand the indications for, limitations of, and potential harms or benefits of tests ordered during pregnancy. Appropriate counseling of patients before radiological studies are performed is critical, and informed consent should always be obtained. Risk to the embryo or fetus depends on the type and amount of radiation and gestational age of the fetus. Reports vary, but the most sensitive period for CNS teratogenicity seems to be between 8–17 weeks of gestation, with a higher threshold dose at more advanced gestational ages. Based on dose-response calculations, diagnostic procedures involving radiation do not pose a risk to the fetus unless the cumulative dose to the uterus is greater than 10 cGy; conservative guidelines suggest that non-urgent radiological testing should be avoided during this time and cumulative exposure to the uterus during pregnancy be kept below 5 cGy [3–5].

Pregnancy may limit the diagnostic accuracy of various imaging modalities. For instance, the appearance of cardiomegaly on chest x-ray, QRS axis changes in electrocardiogram, or exaggerated transvalvular gradient on echocardiogram can be due to the normal structural, physiological, and hemodynamic changes in pregnancy [2,6].

Radiation Exposure in Pregnancy

It is estimated that a fetus will be exposed to 1 mGy of background radiation in pregnancy [7]. The risk to the fetus from ionizing radiation is dependent on gestational age at the time of exposure and dose of radiation. The accepted cumulative dose of ionizing radiation during pregnancy is 50 mGy (equal to 50 mSv or 5 rads). If extremely high-dose exposure (in excess of 1 Gy) occurs during early embryogenesis, it will most likely be lethal to the embryo. The most common adverse effects seen with high-dose radiation exposure in humans beyond the period of early embryogenesis are growth restriction, microcephaly, and intellectual disability. However, for the development of these adverse effects the radiation exposure has to be sufficiently high. Fetal anomalies, growth restrictions, and miscarriages have not been reported with radiation exposure of less than 50 mGy (see Table 7.1) [4]. The estimated minimal threshold for an adverse effect is thought to be in the range of 60–310 mGy; however, the lowest clinically documented dose to produce severe intellectual disability has been reported to be 610 mGy [8]. Cardiovascular diagnostic imaging does not reach these levels, therefore these effects are rarely of concern for patients.

Laboratory tests should be the initial screening modality in women presenting with chest pain and shortness of breath out of proportion to pregnancy. Serum biomarkers can be used as screening tests to aid in diagnosis of cardiovascular disease. B-type natriuretic peptides (BNP or its inactive amino terminal fragment NT-proBNP) are important biomarkers used in diagnosis and management of heart failure. Levels of natriuretic peptides may be elevated based on stage of pregnancy, preeclampsia, preexisting cardiomyopathies, congenital heart disease, or peripartum cardiomyopathy. Furthermore, BNP can be elevated in noncardiac conditions such as sepsis, kidney disease, and anemia [9,10]. A complete blood count (CBC) would aid in diagnosing anemia or an active infection. High levels of natriuretic peptides are associated with increased risk of cardiovascular events and should raise suspicion for cardiac disease and lead to careful supervision during pregnancy and postpartum period [10]. Troponin is another serum biomarker used to diagnose cardiovascular disease, particularly acute coronary syndrome (ACS) and massive PE with hemodynamic compromise. Pregnant women with ACS present similar to their female counterparts. Coronary artery disease (CAD), and in particular ACS, during pregnancy are associated with high risk of mortality and significant morbidity. A low threshold to investigate women with chest pain is paramount. Elevated troponin level in pregnant women with chest pain should be investigated seriously [11].

The plasma level of D-dimer is nearly always increased in the presence of acute pulmonary embolism (PE). It is a commonly used test with high sensitivity for ruling out venous thromboembolism (VTE) in nonpregnant patients [12]. The diagnosis of PE in pregnancy poses a challenge to physicians. Signs and symptoms of PE such as dyspnea, lower extremity edema, and tachycardia occur as part of normal pregnancy, and diagnostic tests such as the D-dimer can be unreliable because the level steadily rises throughout pregnancy [13]. A negative D-dimer is still reliable in pregnant patients with low pretest probability, and especially in the first trimester. However, if suspicion for VTE and pretest probability is high, further workup with imaging modalities is necessary. See Table 7.2.

Chest Radiograph

The chest radiograph is a commonly used diagnostic modality in pregnancy and it provides important information about the lungs, airways, blood vessels, and size of the heart and bones of the spine and chest.

Indications for obtaining a chest radiograph in pregnancy are no different from those in nonpregnant patients and should be considered in any pregnant patient who presents with new-onset dyspnea to evaluate for pulmonary edema, cardiomegaly, and atrial enlargement [8,14]. A review of 200 chest radiographs from pregnant women analyzed lung parenchyma, cardiac contour, and vascular markings and found no characteristic changes in pregnancy [15]. Thus, abnormal findings in pregnancy should be investigated just as they are in nonpregnant patients. Medically indicated chest radiograph can be safely performed in pregnancy provided that fetal (abdominal) shielding is used.

Electrocardiogram

The electrocardiogram (ECG) is often one of the initial diagnostic modalities performed in pregnant women with suspected heart disease. The ECG is helpful in identifying conditions such as myocardial infarction, arrhythmia, and PE. During normal pregnancy there may be some subtle changes in the ECG, such as shortening of the PR and QT intervals, left axis deviation [5], and nonspecific ST-T wave changes in the left precordial leads [16].

Rhythm Monitors

Palpitations, dizziness, and syncope are common complaints in pregnancy and are among the most frequent causes of referral to high-risk obstetric clinics [17]. According to the 2018 guidelines by the American College of Cardiology, American Heart Association, and Heart Rhythm Society, the presence of unexplained palpitations, syncope, or dizziness is considered a class 1 indication for ambulatory electrocardiogram monitoring [18].

Several types of ambulatory cardiac rhythm monitoring are available. For symptoms frequent enough to be detected within a short period, Holter monitoring can be used, which is a continuous recording for 24–72 hours but can be as long as 2 weeks in the newest models. External patch recorders continuously record and store rhythms for up to 14 days and have patient-trigger capability to allow for symptom rhythm correlation. The patch is leadless, can be easily self-applied, and is largely water resistant, making it more comfortable and less cumbersome, potentially improving patient compliance. In high-risk patients, mobile cardiac outpatient monitoring should be considered. This device records and transmits data for up to 30 days. If significant arrhythmias are detected, the data is automatically transmitted to a live central monitoring station, allowing for real-time and immediate feedback to a health care provider [18].

Heart rate increases by 25% in pregnancy, making sinus tachycardia the most common benign arrhythmia, especially in the third trimester. Furthermore, ectopic beats and non-sustained arrhythmia are encountered in more than 50% of pregnant women [19]. Sustained supraventricular tachycardia is the most common serious arrhythmia diagnosed in pregnancy. Interestingly, obese women (BMI >30) have a fourfold increased risk of experiencing a serious rhythm disturbance during gestation, and women with a prior history of arrhythmia have an eightfold increased risk [17].

Echocardiography

Transthoracic echocardiography can be used to evaluate ventricular function, valvular abnormalities, and pericardial disease. It uses high-frequency sound waves to image cardiac structures. Ultrasound waves are harmless to the tissues at the intensities used in diagnostic imaging. Echocardiography should be obtained in pregnant women who complain of chest pain, syncope, shortness of breath out of proportion to pregnancy, and palpitations. Furthermore, an echocardiogram should be performed on women with documented arrhythmia during pregnancy and those with known heart disease, stroke, or prior history of chemotherapy or radiation [20]. Serial echocardiography may be indicated during pregnancy based on the underlying cardiac disease.

It is important to note that significant cardiovascular hemodynamic changes occur during pregnancy, making clinical diagnosis of heart disease challenging. Normal pregnancy is associated with increased blood volume and cardiac output that lead to increases in left and right ventricular chamber size, an increase in left atrial size, and increase in left ventricular wall thickness; however, no change in left ventricular ejection fraction is seen. Other normal cardiac structural changes that can be seen in pregnancy include an increase in aortic root, increase in mitral and tricuspid annular size, and increase in stroke volume (see Table 7.3) [21,24]. In addition, asymptomatic pericardial effusions during the third trimester occur in approximately 40% of pregnant women [22].

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