In developed countries, the number of maternal deaths associated with heart disease, including myocardial disorders, has been increasing during the past two decades.1,2 However, myocardial disorders represent a set of underrecognized, underreported, and poorly understood obstetric problems.3 Peripartum cardiomyopathy, a distinct clinical entity, has received the most attention in the obstetric literature because of its unknown etiology, unpredictable course, and frequently poor outcomes.4,5 The four diagnostic criteria for peripartum cardiomyopathy were adopted at a workshop on this disorder organized by the National Heart, Lung, and Blood Institute and the Office of Rare Diseases of the National Institutes of Health in April 1997.6 For a definitive diagnosis of peripartum cardiomyopathy, all four criteria must be present, and they include 1) the development of congestive heart failure in the past month of pregnancy or within 5 months after delivery, 2) the absence of preexisting cardiac dysfunction, 3) the absence of a determinable cause of cardiomyopathy, and 4) documented left ventricular systolic dysfunction, such as depressed shortening fraction or ejection fraction.6
The estimated prevalence of peripartum cardiomyopathy varies widely depending on the population, with the reported prevalence of this condition in the United States ranging from 1 per 1,300 live births in Little Rock, Arkansas, in 1963 to 1 per 15,000 live births in Dallas, Texas, in 1986.7 To improve the ascertainment and understanding of morbidity associated with peripartum cardiomyopathy among pregnant women, in October 2003 a specific International Classification of Diseases, 9th Revision, Clinical Modification (ICD-9-CM) code for peripartum cardiomyopathy (674.5) was separated from the nonspecific code 678.4, “Other complications of the puerperium, not elsewhere classified.”8
Peripartum cardiomyopathy is not the only myocardial disorder that complicates pregnancy. Because an increase in the prevalence of preexisting cardiac and noncardiac disease that can present as myocardial disorders has been observed in the obstetric population during the past 10 years,9,10 we hypothesized that many hospitalizations likely involved myocardial disorders other than peripartum cardiomyopathy. In addition, with the notable improvements in the treatment of congenital heart disease over the past few decades, congenital heart disease has become an important cause of heart disease (including myocardial disorders) in pregnancy in the developed world.11,12 However, population-level data on the prevalence of myocardial disorders in obstetric population are limited. Moreover, the risks of severe obstetric complications among women with myocardial disorders are not well understood. The objectives of the present report were to 1) evaluate the feasibility of using the ICD-9-CM code for peripartum cardiomyopathy for surveillance purposes, 2) estimate the rate of pregnancy hospitalizations with myocardial disorders, and 3) report the rate of severe obstetric complications by myocardial disorders status among delivery and postpartum hospitalizations using data from the 2004–2006 Nationwide Inpatient Sample of the Healthcare Cost and Utilization Project.
MATERIALS AND METHODS
The Healthcare Cost and Utilization Project is a federal–state–industry partnership sponsored by the Agency for Healthcare Research and Quality.13 The Nationwide Inpatient Sample is the largest all-payer inpatient care database in the United States. All of the nonfederal community hospitals from the states participating in the annual Healthcare Cost and Utilization Project data collection are stratified by rural/urban location, number of beds, region, teaching status, and ownership. Within each stratum, a systematic random 20% sample of hospitals is drawn. The sample size of this large cross-sectional study was predetermined by sampling methodology that allows using discharges from the sampled hospitals to produce nationwide estimates. A full description of the Nationwide Inpatient Sample methodology can be found on the Healthcare Cost and Utilization Project Web site.13 Because the Nationwide Inpatient Sample excludes data elements that could directly or indirectly identify individuals, this research was considered exempt from review by the institutional review board of the Centers for Disease Control and Prevention.14
For the myocardial disorders prevalence analysis, we included all 2004–2006 pregnancy-related hospitalizations, which were identified by ICD-9-CM diagnostic codes 630–677, V22, V23, V24, V28, and 792.3; ICD-9-CM obstetric procedure codes 72.xx, 73.xx, 74.xx, and 75.xx; and diagnosis related group (DRG) codes 370–384. The delivery, postpartum, and antenatal hospitalizations were identified hierarchically. The specific details about ICD-9-CM and DRG coding to identify delivery hospitalizations are described in detail elsewhere.15 Postpartum hospitalizations were identified by a fifth digit of 4 in ICD-9-CM codes for primary or secondary diagnosis; ICD-9-CM code V24 for any listed diagnosis; or postpartum DRG codes 376–377. Antenatal hospitalizations were identified by a fifth digit of 3 in ICD-9-CM codes for primary or secondary diagnosis; ICD-9-CM code V22, V23, V28, or 792.3 for any listed diagnosis; or antenatal DRG codes 378–384.
We confined the analysis of severe obstetric complications associated with myocardial disorders to delivery hospitalizations and postpartum hospitalizations. Severe obstetric complications were identified by using condition-specific ICD-9-CM codes and additional information obtained from hospital discharge records, such as mortality, transfer from or to another health care facility, and duration of stay. The specific ICD-9-CM codes for severe complications and details of the method to identify delivery hospitalizations with these conditions can be found elsewhere.9,10 In addition to severe complications previously published, this study also included acute myocardial infarction (ICD-9-CM code 410.x), disorders of fluid, electrolyte, and acid–base balance (ICD-9-CM code 276.x), and cardiac arrest/ventricular fibrillation (ICD-9-CM codes 427.5, 427.41, and 427.42).
The hospitalizations with myocardial disorders were identified by ICD-9-CM codes 674.5x (peripartum cardiomyopathy) or 425.x (other cardiomyopathies). In the classification of cardiomyopathy proposed by the American Heart Association in 2006, myocardial disorders associated with coronary artery disease, hypertension, valvular disease, and congenital heart disease were excluded.16 However, in our 2004–2006 sample these hospitalizations would still have ICD-9-CM codes for cardiomyopathy. Thus, in this analysis we classified hospital records that simultaneously had ICD-9-CM codes for cardiomyopathy and heart disease, such as coronary artery disease, hypertension, valvular disease, and congenital heart disease, as records with other myocardial disorders. The remaining hospitalizations with ICD-9-CM code 425.x or 674.5x were classified as hospitalizations with cardiomyopathy.
As a next step of our analysis, we evaluated the feasibility of using the specific ICD-9-CM code 674.5x for surveillance of peripartum cardiomyopathy. Some hospital records with codes for peripartum cardiomyopathy also had ICD-9-CM codes for preexisting cardiac or cardiomyopathy-associated noncardiac disease (eg, diabetes mellitus) at the same time. We considered these records to be misclassified as peripartum cardiomyopathy because there was evidence of specific etiologies for cardiomyopathy. To describe a distribution of misclassified and correctly classified records with ICD-9-CM code 674.5x, we hierarchically categorized all these records into three mutually exclusive groups: 1) records with ICD-9-CM codes for heart disease (Table 1): valvular disease, congenital heart disease, coronary artery disease, hypertensive heart disease, conduction disorders and cardiac dysrhythmias, and pulmonary circulation disease; 2) records with ICD-9-CM codes for potential underlying conditions for cardiomyopathy (Table 2): myocarditis, diabetes (excluding gestational), thyroid disease, chronic hypertension (excluding hypertensive heart disease), chronic renal failure, rheumatoid arthritis/collagen vascular disease, rheumatic heart failure, severe anemia, human immunodeficiency virus, drug abuse, alcohol abuse, and nutrient deficiencies; and 3) records with ICD-9-CM codes for peripartum cardiomyopathy only.
The unit of analysis was a hospitalization, not an individual; these data did not allow us to account for multiple pregnancy hospitalizations of the same women during the study period. However, because a woman can only deliver once, the analysis of delivery hospitalizations can be considered an analysis of individuals. We reported overall rates of pregnancy hospitalizations with cardiomyopathy and other myocardial disorders and rates by age, payer, and hospital region per 1,000 deliveries. Rates of severe complications per 1,000 deliveries or 1,000 postpartum hospitalizations were also calculated. We compared these rates using χ2 tests with adjustment of P values for multiple comparisons using the Bonferroni method. Because many hospitals do not collect or report race/ethnicity, we did not include this variable in the analysis.13 We used SAS 9.1 (SAS Institute Inc., Cary, NC) to manage data and SAS-callable SUDAAN 9.0 (RTI International, Research Triangle, NC) to account for the multistage probability sampling design. Thus, all results are based on the weighted estimates of pregnancy hospitalizations in the United States during the study period.
During the 3-year study period, there were an estimated 14,323,731 pregnancy hospitalizations in the United States. Among the 16,824 records with myocardial disorders, 11,134 had codes for peripartum cardiomyopathy. Among 11,134 records with a code for peripartum cardiomyopathy, 6,097 (54.8%) also had ICD-9-CM codes for heart disease, and 2,704 (24.3%) also had codes for cardiomyopathy-associated noncardiac conditions (total 79.1%). Thus, only 2,332 records (20.9%) with the specific peripartum cardiomyopathy code had no codes for cardiac or noncardiac conditions. The overall rate of hospitalizations with any use of the ICD-9-CM code for peripartum cardiomyopathy was 0.88 per 1,000 deliveries (1 case per 1,136 deliveries), and the rate of hospitalizations with the ICD-9-CM code for peripartum cardiomyopathy without any code for cardiac and noncardiac conditions was 0.18 per 1,000 deliveries (1 case per 5,556 deliveries). For this analysis all hospitalizations with the ICD-9-CM code for peripartum cardiomyopathy were reclassified into two groups: cardiomyopathy (n=5,037) and other myocardial disorders (n=6,097).
Among all pregnancy hospitalizations (Table 3), the overall prevalence of hospitalizations with myocardial disorders was 1.33 per 1,000 deliveries. The rate of pregnancy hospitalizations with cardiomyopathy was 0.46 per 1,000 deliveries (0.18 for apparent peripartum cardiomyopathy and 0.28 for other cardiomyopathies). The rate of pregnancy hospitalizations with other myocardial disorders was 0.87 per 1,000 deliveries. Myocardial disorders were rare during delivery hospitalizations (0.01%) but not uncommon among postpartum hospitalizations (4.2%). The rates of myocardial disorders per 1,000 deliveries by age groups, payer status, and region were also calculated (Table 4). The significantly (P<.01) higher rates of myocardial disorders were observed for women aged 35 years or older compared with women aged 15–24 years, hospitalizations covered by public payer compared with hospitalizations covered by private insurance, and in the South compared with other regions. The same pattern was observed when rates of cardiomyopathy and other myocardial disorders were estimated separately.
The rate (per 1,000 deliveries) of severe complications among delivery hospitalizations without myocardial disorders ranged from 0.07 (95% confidence interval [CI] 0.06–0.08) for cardiac arrest to 1.88 (95% CI 951.77–1.99) for fluid and electrolyte disorders (Table 5). In contrast, among hospitalizations with myocardial disorders this rate ranged from 13.2 (95% CI 6.3–20.1) for acute myocardial infarction to 128.6 (95% CI 107.2–150.0) for adult respiratory distress syndrome. The rate (per 1,000 hospitalizations) of severe complications among postpartum hospitalizations without myocardial disorders ranged from 0.4 (95% CI 0.2–0.6) for cardiac arrest to 65.5 (95% CI 64.4–68.7) for fluid and electrolyte disorders (Table 6). The rate of severe complications among hospitalizations with myocardial disorders was significantly higher than among hospitalizations without myocardial disorders (P<.05) and ranged from 10.7 (95% CI 5.7–15.7) for pulmonary edema to 193.0 (95% CI 173.7–212.4) for fluid and electrolyte disorders.
Our study reports the prevalence of cardiomyopathy and other myocardial disorders during pregnancy hospitalizations in the United States after the implementation of the specific ICD-9-CM code for peripartum cardiomyopathy at the end of 2003. The recently implemented ICD-9-CM code for peripartum cardiomyopathy was frequently used simultaneously with codes for other heart disease and comorbidities, and hence it is unlikely to be specific enough to track peripartum cardiomyopathy as a distinct entity in the United States. Thus, our results underlined the importance of being more precise about language and diagnostic criteria when documenting hospital course among pregnant women. In addition, our results support the continuation of efforts to modify ICD-9-CM codes to include more granularity about conditions that occur during pregnancy.
The exact prevalence of cardiomyopathy during pregnancy hospitalization in the United States has not been known. Consistent with the existing obstetric literature focusing mostly on peripartum cardiomyopathy, recent reports have attempted to estimate the prevalence of this entity. A study attempting to estimate the national prevalence of peripartum cardiomyopathy in the United States from the 1990–2002 National Hospital Discharge Survey identified one hospitalization per 3,189 live births.17 Similarly, in a 1996–2005 study of 241,497 deliveries within the Southern California Kaiser health care system, the prevalence of peripartum cardiomyopathy based on the chart review of delivery hospitalizations with an ICD-9-CM code for heart failure was 1 in 4,025 deliveries.18 The results from these studies are difficult to compare, however, because of the difference in methodology applied to identify the hospitalizations with peripartum cardiomyopathy. In our study, approximately 1 in 1,136 delivery hospitalizations had the recently implemented ICD-9-CM code for peripartum cardiomyopathy. However, approximately 80% of these hospitalizations also had an ICD-9-CM code for heart disease or chronic conditions, potential underlying causes for dilated cardiomyopathy, and thus they could not fulfill the diagnostic criteria for peripartum cardiomyopathy. Moreover, a low sensitivity of ICD-9-CM codes for chronic conditions, resulting in their underreporting in obstetric hospital records, has been documented before.19 Accordingly, although we have reasonable confidence in using our data to estimate the prevalence of cardiomyopathy in a generic sense, our results highlight the limitations of using hospital discharge data to obtain reliable estimates for the prevalence of true peripartum cardiomyopathy.
The clinical presentation of peripartum or dilated cardiomyopathy is similar to that for systolic congestive heart failure. However, dilated cardiomyopathy may be difficult to diagnose because of the tendency to mimic the physiologic alterations that take place during pregnancy, especially when early signs of congestive heart failure are subtle.6 Hence, a high index of suspicion is warranted. Unfortunately, no specific diagnostic test for peripartum cardiomyopathy is available currently. Regardless, peripartum cardiomyopathy should be diagnosed after ruling out cardiac and noncardiac causes for dilated cardiomyopathy.6 Because dilatation of the left ventricle can be a normal echocardiographic finding in pregnancy, a documentation of decreased left ventricular systolic function is necessary for the definitive diagnosis of dilated cardiomyopathy.20
Myocardial disorders are conditions that often result in cardiovascular and other major system complications.16,21 Among hospitalizations with myocardial disorders, the rate of severe complications ranged from 13.2 for acute myocardial infarction to 128.6 for adult respiratory distress syndrome and from 10.7 for pulmonary edema to 193.0 for fluid and electrolyte disorders per 1,000 delivery and postpartum hospitalizations, respectively. Among hospitalizations without myocardial disorders, the rate of severe complications ranged from 0.07 to 1.9 and from 0.4 to 65.5 for cardiac arrest and for fluid and electrolyte disorders per 1,000 hospitalizations in delivery and postpartum periods, respectively. Unfortunately, the cross-sectional study design and a lack of clinical details in hospital discharge data did not allow us to determine whether severe obstetric morbidity developed as a complication of myocardial disorders after admission or was a comorbid condition that existed before admission. Nevertheless, our surveillance data indicated that delivery and postpartum hospitalizations with myocardial disorders have extremely high rates of severe obstetric complications.
Our study has several limitations that should be considered when interpreting the results. Our identification of severe complications is based solely on ICD-9-CM codes and data-driven criteria, such as mortality, transfer from or to other health care facilities, and duration of stay. There are potential risk factors for cardiomyopathy and other forms of myocardial disorders, such as race, body mass index, and use of tocolytic agents,21 for which we had no information. In addition, ICD-9-CM codes for myocardial disorders among pregnancy hospitalizations were not validated, and it is not known how practicing physicians used diagnostic criteria to diagnose cardiomyopathy. Thus, misclassification of exposure, outcomes, or both is possible in our study. The estimates for myocardial disorders reported in this study may be affected by several sources of error. Because the unit of our analysis was hospitalization and we could not identify the repeat hospitalizations for same patient, our rates may be overestimated. However, underestimation is also possible. Some cases of myocardial disorders with the clinical presentation ranging from mild heart failure to sudden cardiac death may be seen only in emergency departments20 and hence are not associated with an hospital admission. Finally, hospitalizations for peripartum cardiomyopathy that occurred after the 6-week time frame that is defined as the postpartum period and hence were not coded as postpartum hospitalizations may be not captured in our data set.22
In conclusion, the results of this nationwide study have shown that approximately 0.04% of delivery hospitalizations and approximately 4% of postpartum hospitalizations had ICD-9-CM codes for cardiomyopathy and other myocardial disorders. Although only a minority of hospitalizations for cardiomyopathy are consistent with peripartum cardiomyopathy, cardiomyopathy and other myocardial disorders are important contributors to severe obstetric complications. Because recent reports have indicated alarming increases in both maternal mortality from heart disease and the prevalence of risk factors for heart disease among women of reproductive age during the past decade, trends in hospitalizations with myocardial disorders need to be monitored. However, because of the inherent limitations of hospital discharge data, if we are to offer better diagnosis and treatment of this uncommon but potentially devastating condition, the collection of clinically detailed data are essential for making progress in understanding the differences and similarities between the entities currently designated as peripartum cardiomyopathy and other forms of myocardial disorders.
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