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Prevalence, trends, and outcomes of chronic hypertension: a nationwide sample of delivery admissions

Published:November 09, 2011DOI:https://doi.org/10.1016/j.ajog.2011.10.878

      Objective

      We sought to define the prevalence, trends, and outcomes of primary and secondary chronic hypertension in a population-based sample of deliveries.

      Study Design

      An estimated 56,494,634 deliveries were identified from the 1995 through 2008 Nationwide Inpatient Sample. The association of primary and secondary chronic hypertension with adverse fetal and maternal outcomes was evaluated using regression modeling and adjusted population-attributable fractions were calculated.

      Results

      During the study period, the prevalence of primary and secondary hypertension increased from 0.90% in 1995 through 1996 to 1.52% in 2007 through 2008 (P for trend < .001) and from 0.07% to 0.24% (P for trend < .001), respectively. The population-attributable fraction for chronic hypertension was considerable for many maternal adverse outcomes, including acute renal failure (21%), pulmonary edema (14%), preeclampsia (11%), and in-hospital mortality (10%).

      Conclusion

      Primary and secondary chronic hypertension were both strongly associated with adverse pregnancy outcomes and accounted for a substantial fraction of maternal morbidity. Prioritizing research efforts in this area is needed.

      Key words

      Chronic hypertension is a relatively common comorbidity in pregnancy
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      Risk factors for preeclampsia, abruption placentae, and adverse neonatal outcomes among women with chronic hypertension: National Institute of Child Health and Human Development Network of Maternal-Fetal Medicine Units.
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      Although the majority of chronic hypertension among pregnant women is due to primary hypertension, about 10% of cases occur secondary to other medical conditions, such as diabetes mellitus, chronic renal disease, thyroid disease, and collagen vascular disease.
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      • Sibai B.M.
      Chronic hypertension in pregnancy.
      For Editors' Commentary, see Table of Contents
      There are few population-based studies examining the impact of chronic hypertension on obstetric outcomes in the United States
      • Gilbert W.M.
      • Young A.L.
      • Danielsen B.
      Pregnancy outcomes in women with chronic hypertension: a population-based study.
      • Kuklina E.V.
      • Ayala C.
      • Callaghan W.M.
      Hypertensive disorders and severe obstetric morbidity in the United States.
      ; this is particularly true for chronic hypertension that is secondary to or associated with other conditions. As the prevalence of advanced maternal age
      • Martin J.A.
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      and obesity
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      Trends in pre-pregnancy obesity in nine states, 1993-2003.
      increase among childbearing women in the United States, both primary and secondary chronic hypertension are likely to become an increasingly common obstetric conditions.
      The purpose of this study is 3-fold: (1) to examine nationwide trends in the prevalence of primary and secondary chronic hypertension during delivery hospitalizations in the United States; (2) to assess the effect of primary and secondary chronic hypertension on fetal and maternal obstetric complications; and (3) during the most recent years in the study period, to estimate the contribution of primary and secondary chronic hypertension to the burden of select fetal and maternal complications in the United States.

      Materials and Methods

      Hospital discharge data were obtained from the Nationwide Inpatient Sample (NIS), part of the Healthcare Cost and Utilization Project, a federal-state-industry partnership sponsored by the Agency for Healthcare Research and Quality. The NIS is a 20% stratified sample of all US community hospitals as defined by the American Hospital Association: nonfederal, short-term, general, and specialty hospitals whose facilities are open to the public. To create a sample that is maximally representative of all US community hospital admissions, hospital are selected for inclusion in the NIS based on 5 characteristics: rural/urban location, number of beds, region of the country, teaching status, and ownership. The NIS includes all discharges from the sampled hospitals and includes between 5-8 million discharges from an average of 1000 hospitals each year.
      • Steiner C.
      • Elixhauser A.
      • Schnaier J.
      The healthcare cost and utilization project: an overview.
      Further information about the methodology used to create the dataset is available at http://www.hcup-us.ahrq.gov/nisoverview.jsp.
      Our analysis included all delivery hospitalizations of women aged ≥15 years from 1995 through 2008; those who had abortions, ectopic pregnancies, or molar pregnancies were excluded from the analyses. Delivery hospitalizations were identified using a validated approach that selects admissions with relevant diagnosis-related groups and International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) diagnosis/procedure codes, as previously described.
      • Kuklina E.V.
      • Whiteman M.K.
      • Hillis S.D.
      • et al.
      An enhanced method for identifying obstetric deliveries: implications for estimating maternal morbidity.
      In addition, ICD-9-CM codes were used to classify hospitalizations with chronic hypertension, associated comorbidities, and maternal and fetal outcomes (Appendix).
      AppendixList of International Classification of Diseases, Ninth Revision, Clinical Modification codes
      Disease/comorbidities/outcomeICD-9-CM code
      Acute renal failure669.3x, 584.x
      Cesarean delivery74.x
      Chronic hypertension642.0x-642.2x, 642.7x, 401.xx-405.xx
      Chronic renal disease581.xx-583.xx, 585.xx, 587, 646.2x
      Coarctation of aorta747.1x
      Collagen vascular disease710.xx
      Cushing syndrome255.0x
      Hyperplasia of renal artery447.3x
      Hypoaldosteronism255.1x
      Multiple births651.xx
      Nongestational diabetes249.xx-250.xx, 648.0x
      Pheochromocytoma255.6x
      Poor fetal growth656.5x
      Preeclampsia642.4x-642.7x
      Previous cesarean delivery654.2x
      Pulmonary edema518.4
      Spontaneous delivery <37 wk gestation644.2x
      Stillbirth656.4x, V27.1, V27.3, V27.4, V27.6, V27.7
      Stroke/other cerebrovascular complications325.xx, 346.6x, 348.1x, 348.3x, 348.4x, 348.5x, 430.xx- 434.xx, 436.xx-437.xx, 671.5x, 674.0x, 997.0, 997.02.
      Thyroid disorders242.xx-244.xx, 648.1x
      Ventilation96.7x
      ICD-9-CM, International Classification of Diseases, Ninth Revision, Clinical Modification.
      Bateman. Chronic hypertension: a nationwide sample of delivery admissions. Am J Obstet Gynecol 2012.
      Data management and statistical analyses were conducted using SAS (SAS Inc, Cary, NC) and SAS-callable SUDAAN software (version 9.2, RTI International, Research Triangle, NC) to account for the stratified sampling design used to collect the hospital discharge data. We used χ2 tests with a significance level of .05 to compare the distribution of deliveries with and without chronic hypertension by sociodemographic and hospital characteristics and maternal comorbidities. For purposes of analysis, secondary hypertension was defined as chronic hypertension in association with conditions that can cause hypertension through either vascular or endocrinologic mechanisms including pregestational diabetes, chronic renal disease, collagen vascular disease (including systemic lupus erythematous, scleroderma, and other diffuse diseases of connective tissue), thyroid disorders, pheochromocytoma, hyperplasia of the renal artery, Cushing syndrome, hyperaldosteronism, and maternal coarctation of the aorta. Primary hypertension was defined as chronic hypertension without any of these associated comorbidities. To examine trends, the age-standardized prevalence of overall, primary, and secondary chronic hypertension, as well as chronic hypertension with selected individual comorbidities, were computed for 2-year intervals. Estimates were standardized by the age distribution of delivery hospitalizations for the last 2 years (2007 through 2008) and applying this distribution to the former 2-year intervals.
      Given the increasing prevalence of chronic hypertension, we restricted our analyses of the effect of overall chronic hypertension, primary hypertension, and secondary hypertension on fetal and maternal outcomes to the last 2 years of the study period (2007 through 2008). Frequencies per 1000 deliveries were calculated for fetal outcomes (stillbirth, poor fetal growth, spontaneous delivery <37 weeks of gestation) and maternal outcomes (preeclampsia, stroke/cerebrovascular complications, acute renal failure, pulmonary edema, mechanical ventilation, cesarean delivery, length of hospital stay >6 days [corresponding to approximately the 98th percentile for length of stay], and in-hospital mortality), stratified by all-cause, primary, and secondary chronic hypertension status. Logistic regression was used to estimate odds ratios (ORs) and respective 95% confidence intervals of maternal and fetal delivery outcomes by maternal chronic hypertension status, while adjusting for multiple births, year of study, insurance status, region, and age; the models for cesarean delivery were additionally adjusted for previous cesarean delivery and the models for length of stay included adjustment for admission and disposition status. To estimate the burden of disease at the population level, population-attributable fractions for chronic hypertension (overall), primary hypertension, and secondary hypertension for each of the fetal and maternal outcomes of interest were calculated using adjusted OR as estimates of relative risk, as described elsewhere.
      • Rockhill B.
      • Newman B.
      • Weinberg C.
      Use and misuse of population attributable fractions.
      The population-attributable fraction estimates the proportion of disease that would be eliminated if the exposure, in this case chronic hypertension, could be eliminated.
      A similar analysis was conducted examining the association of chronic hypertension with the comorbidities most commonly associated with chronic hypertension–pregestational diabetes, chronic renal disease, collagen vascular disease, and thyroid disorders–and selected fetal/maternal outcomes. Because of the relatively small numbers of patients with chronic hypertension and the comorbidities considered, the NIS for the entire study period (1995 through 2008) was used for this analysis to ensure adequate power to make reliable estimates of risk.

      Results

      From 1995 through 2008, an estimated 56,494,634 delivery hospitalizations were identified, of which 731,694 were classified as deliveries with chronic hypertension. Of the patients with chronic hypertension, 649,899 (88.8%) had primary hypertension and 81,795 (11.2%) had secondary hypertension; in the final 2 years of the study period (2007 through 2008), of the 153,570 patients with chronic hypertension, 132,808 (86.5%) had primary hypertension and 20,762 (13.5%) had secondary hypertension. Compared to women with delivery hospitalizations without chronic hypertension, those with chronic hypertension were older, and had higher rates of multiple birth, previous cesarean delivery, pregestational diabetes, chronic renal disease, collagen vascular disease, hyperaldosteronism, Cushing disease, thyroid disease, and maternal coarctation of the aorta (all P < .001) (Table 1). The most common comorbidities associated with chronic hypertension included pregestational diabetes (present in 6.6% of chronic hypertension admissions), thyroid disorders (present in 4.1%), chronic renal disease (present in 0.9%), and collagen vascular disease (present in 0.6%).
      TABLE 1Patient characteristics, comparing those with and without chronic hypertension
      CharacteristicWith chronic hypertension (N = 731,694), n (%)Without chronic hypertension (N = 55,762,940), n (%)P
      χ2;
      value
      Age, y
       <2023,094 (3.2)6,236,678 (11.2)< .001
       20–34478,006 (65.3)41,889,300 (75.1)
       ≥35230,594 (31.5)7,637,061 (13.7)
      Primary payer
       Public
      Including Medicaid and Medicare;
      258,620 (35.4)21,254,523 (38.2)< .001
       Private (including HMO)432,453 (59.3)30,752,962 (55.3)
       Other (including self-pay)38,693 (5.3)3,596,175 (6.5)
      Multiple birth
       Yes18,384 (2.5)919,225 (1.7)< .001
       No713,304 (97.5)54,843,715 (98.3)
      Previous cesarean delivery
       Yes156,303 (21.4)7,374,516 (13.2)< .001
       No575,391 (78.6)48,388,423 (86.8)
      Pregestational diabetes
       Yes48,263 (6.6)364,907 (0.7)< .001
       No683,431 (93.4)55,398,031 (99.3)
      Chronic renal disease
       Yes6614 (0.9)84,866 (0.2)< .001
       No725,080 (99.1)55,678,074 (99.8)
      Collagen vascular disease
       Yes4482 (0.6)49,520 (0.1)< .001
       No727,212 (99.4)55,713,420 (99.9)
      Pheochromocytoma
       Yes
      Number too small to report stable estimate (n ≤10 and/or RSE >0.30).
      Number too small to report stable estimate (n ≤10 and/or RSE >0.30).
      .32
       No
      Number too small to report stable estimate (n ≤10 and/or RSE >0.30).
      55,762,940 (100.0)
      Hyperplasia of renal artery
       Yes
      Number too small to report stable estimate (n ≤10 and/or RSE >0.30).
      Number too small to report stable estimate (n ≤10 and/or RSE >0.30).
      .05
       No
      Number too small to report stable estimate (n ≤10 and/or RSE >0.30).
      Number too small to report stable estimate (n ≤10 and/or RSE >0.30).
      Hyperaldosteronism
       Yes158 (0.02)558 (0.001)< .001
       No731,537 (99.9)55,762,382 (100.0)
      Cushing syndrome
       Yes
      Number too small to report stable estimate (n ≤10 and/or RSE >0.30).
      412 (0.001)< .001
       No
      Number too small to report stable estimate (n ≤10 and/or RSE >0.30).
      55,762,529 (100.0)
      Thyroid disorders
       Yes29,810 (4.1)772,032 (1.4)< .001
       No701,885 (95.9)54,990,908 (98.6)
      Maternal coarctation of aorta
       Yes130 (0.02)684 (0.001)< .001
       No731,565 (99.9)55,762,257 (100.0)
      HMO, health maintenance organization; RSE, residual standard error.
      Bateman. Chronic hypertension: a nationwide sample of delivery admissions. Am J Obstet Gynecol 2012.
      a χ2;
      b Including Medicaid and Medicare;
      c Number too small to report stable estimate (n ≤10 and/or RSE >0.30).
      The age-adjusted prevalence of all-cause chronic hypertension increased significantly throughout the seven 2-year intervals, from 1.01% in 1995 through 1996 to 1.76% in 2007 through 2008 (P for trend < .001) (Figure 1) . The prevalence of primary hypertension increased from 0.90% to 1.52% (P < .001) and of secondary hypertension from 0.07% to 0.24% (P < .001). Significant increasing trends were also observed for chronic hypertension with pregestational diabetes, chronic renal disease, collagen vascular disease, and thyroid disorders (all P < .001) (Figure 2) .
      Figure thumbnail gr1
      FIGURE 1Trends in the rate of chronic hypertension: 1995 to 2008
      Bateman. Chronic hypertension: a nationwide sample of delivery admissions. Am J Obstet Gynecol 2012.
      Figure thumbnail gr2
      FIGURE 2Trends in the rate of chronic hypertension with comorbidities: 1995 to 2008
      Bateman. Chronic hypertension: a nationwide sample of delivery admissions. Am J Obstet Gynecol 2012.
      The frequency of considered adverse fetal and maternal outcomes were uniformly higher for delivery hospitalizations with overall, primary, and secondary hypertension (Table 2). As compared to delivery hospitalizations without chronic hypertension, those with primary hypertension had an OR of 2.2 for stillbirth, 3.0 for poor fetal growth, and 2.9 for spontaneous delivery <37 weeks' gestation (Table 3). For adverse maternal outcomes, the OR associated with primary hypertension were >5 in all categories with the exception of cesarean delivery. For secondary hypertension, the effects on outcomes were even more profound, particularly adverse maternal outcomes; the OR for most adverse maternal outcome were >10, including maternal mortality which was 13.2 (Table 3). The population-attributable fractions for overall chronic hypertension were substantial for many maternal adverse outcomes; principal among these were acute renal failure (21%), pulmonary edema (14%), preeclampsia (11%), and in-hospital mortality (10%) (Table 3).
      TABLE 2Frequency (SE) per 1000 deliveries, of fetal and maternal complications: 2007-2008
      VariableWithout chronic hypertensionWith chronic hypertension (overall)Primary hypertensionSecondary hypertension
      Fetal outcomes
       Stillbirth6.0 (0.01)15.1 (0.7)14.1 (0.7)21.1 (2.3)
       Poor fetal growth18.8 (0.4)53.9 (1.9)54.3 (2.0)51.5 (3.7)
       Spontaneous delivery <37 wk gestation74.0 (0.1)193.1 (5.0)183.8 (4.6)252.2 (9.6)
      Maternal outcomes
       Preeclampsia35.4 (0.7)268.3 (5.0)264.0 (5.0)295.8 (8.9)
       Stroke/cerebrovascular complications0.4 (0.02)2.7 (0.3)2.2 (0.3)2.8 (0.8)
       Acute renal failure0.4 (0.02)5.9 (0.5)4.2 (0.4)16.7 (2.2)
       Pulmonary edema0.2 (0.01)1.5 (0.3)1.4 (0.2)
      Number too small to report stable estimate (n ≤10 and/or RSE >0.30).
       Mechanical ventilation0.5 (0.02)3.8 (0.4)3.5 (0.4)6.3 (1.3)
       Cesarean delivery323.3 (0.3)561.7 (4.9)543.1 (5.1)680.2 (7.7)
       Length of stay >6 d13.6 (0.5)95.9 (3.8)86.7 (3.3)154.6 (9.1)
       In-hospital mortality0.1 (0.01)0.4 (0.1)
      Number too small to report stable estimate (n ≤10 and/or RSE >0.30).
      Number too small to report stable estimate (n ≤10 and/or RSE >0.30).
      RSE, residual standard error.
      Bateman. Chronic hypertension: a nationwide sample of delivery admissions. Am J Obstet Gynecol 2012.
      a Number too small to report stable estimate (n ≤10 and/or RSE >0.30).
      TABLE 3Estimated ORs and PAFs of fetal and maternal complications: 2007-2008
      VariableWith chronic hypertension (overall)Primary hypertensionSecondary hypertension
      OR
      Referent group: without chronic hypertension;
      (95% CI)
      PAF (95% CI)OR
      Referent group: without chronic hypertension and select comorbidities;
      (95% CI)
      PAF (95% CI)OR
      Referent group: without chronic hypertension and select comorbidities;
      (95% CI)
      PAF (95% CI)
       Fetal outcomes
       Stillbirth
      Adjusted for multiple birth, year of study, insurance status, region, and age;
      2.31 (2.11–2.53)2.44 (1.99–2.90)2.23 (2.02–2.48)1.93 (1.52–2.35)3.23 (2.57–4.06)0.56 (0.36–0.78)
       Poor fetal growth
      Adjusted for multiple birth, year of study, insurance status, region, and age;
      3.00 (2.83–3.19)3.26 (2.89–3.64)3.05 (2.87–3.25)2.86 (2.54–3.20)2.92 (2.52–3.38)0.42 (0.31–0.53)
       Spontaneous delivery <37 wk gestation
      Adjusted for multiple birth, year of study, insurance status, region, and age;
      3.01 (2.88–3.14)2.99 (2.72–3.25)2.89 (2.77–3.02)2.41 (2.20–2.62)4.36 (3.98–4.79)0.61 (0.52–0.69)
      Maternal outcomes
       Preeclampsia
      Adjusted for multiple birth, year of study, insurance status, region, and age;
      10.07 (9.68–10.48)10.78 (10.15–11.41)10.18 (9.77–10.60)9.19 (8.67–9.71)11.92 (10.98–12.95)1.63 (1.45–1.82)
       Stroke/cerebrovascular complications
      Adjusted for multiple birth, year of study, insurance status, region, and age;
      5.41 (4.27–6.86)7.49 (5.30–9.77)5.37 (4.10–7.04)6.24 (4.16–8.42)6.71 (3.88–11.61)1.29 (0.42–2.26)
       Acute renal failure
      Adjusted for multiple birth, year of study, insurance status, region, and age;
      14.62 (12.06–17.73)21.02 (17.13–24.95)13.14 (10.57–16.33)12.90 (10.12–15.72)51.07 (37.60–69.38)8.43 (5.80–11.08)
       Pulmonary edema
      Adjusted for multiple birth, year of study, insurance status, region, and age;
      9.26 (6.67–12.85)14.12 (9.34–19.06)8.99 (6.40–12.64)10.89 (6.96–14.97)16.20 (8.82–29.75)3.36 (1.09–5.73)
       Ventilation
      Adjusted for multiple birth, year of study, insurance status, region, and age;
      6.33 (5.12–7.83)9.76 (7.33–12.28)6.22 (4.90–7.88)7.56 (5.43–9.76)10.67 (7.13–15.98)2.34 (1.21–3.53)
       Cesarean delivery
      Adjusted for previous cesarean delivery, multiple birth, year of study, insurance status, region, and age;
      2.54 (2.46–2.64)1.83 (1.69–1.98)2.39 (2.31–2.48)1.47 (1.36–1.59)4.37 (4.03–4.74)0.38 (0.34–0.42)
       Length of stay >6 d
      Adjusted for disposition status, admission status, multiple birth, year of study, insurance status, region, and age.
      6.73 (6.21–7.29)9.55 (8.78–10.33)6.49 (5.96–7.07)7.42 (6.80–8.05)11.83 (10.28–13.62)2.24 (1.92–2.56)
       In-hospital mortality
      Adjusted for multiple birth, year of study, insurance status, region, and age;
      6.20 (3.33–11.54)10.44 (3.43–18.23)5.49 (2.60–11.58)7.23 (1.34–14.15)13.21 (4.92–35.43)3.34 (–0.28 to 7.40)
      Secondary hypertension includes chronic hypertension with pregestational diabetes mellitus, chronic renal disease, collagen vascular disease, pheochromocytoma, thyroid disorders, Cushing syndrome, hyperaldosteronism, hyperplasia of the renal artery, coarctation of aorta.
      CI, confidence interval; OR, odds ratio; PAF, population-attributable fraction [PAF = pd (RR – 1/RR), where pd = proportion of cases (hospitalizations with maternal and fetal outcomes) exposed to risk factor (chronic hypertension status) and RR = relative risk for outcome for exposed compared with unexposed].
      Bateman. Chronic hypertension: a nationwide sample of delivery admissions. Am J Obstet Gynecol 2012.
      a Referent group: without chronic hypertension;
      b Referent group: without chronic hypertension and select comorbidities;
      c Adjusted for multiple birth, year of study, insurance status, region, and age;
      d Adjusted for previous cesarean delivery, multiple birth, year of study, insurance status, region, and age;
      e Adjusted for disposition status, admission status, multiple birth, year of study, insurance status, region, and age.
      In Table 4, an analysis of the interaction of chronic hypertension and selected associated comorbidities is presented (compared to a reference group that included delivery hospitalizations without chronic hypertension and without the comorbidity of interest). The combinations of chronic hypertension with chronic renal disease, collagen vascular disease, or pregestational diabetes were particularly detrimental. The combination of thyroid disease and chronic hypertension generally did not result in greatly elevated risk above what was observed for hypertension alone.
      TABLE 4ORs for fetal and maternal complications: 1995-2008
      Pregestational diabetesChronic renal diseaseCollagen vascular diseaseThyroid disorders
      VariableWith chronic hypertensionWithout chronic hypertensionWith chronic hypertensionWithout chronic hypertensionWith chronic hypertensionWithout chronic hypertensionWith chronic hypertensionWithout chronic hypertension
      Fetal outcomes
       Stillbirth
      Adjusted for multiple birth, year of study, insurance status, region, and age;
      4.30 (3.81–4.85)3.05 (2.88–3.23)7.29 (5.59–9.52)1.74 (1.51–2.02)7.42 (5.37–10.25)2.74 (2.35–3.20)1.86 (1.48–2.33)0.98 (0.92–1.05)
       Poor fetal growth
      Adjusted for multiple birth, year of study, insurance status, region, and age;
      2.66 (2.40–2.94)1.20 (1.14–1.27)7.94 (6.67–9.44)2.29 (2.12–2.49)7.99 (6.44–9.91)3.87 (3.55–4.22)3.59 (3.20–4.02)1.29 (1.25–1.34)
       Spontaneous delivery <37 wk gestation
      Adjusted for multiple birth, year of study, insurance status, region, and age;
      4.88 (4.63–5.15)2.90 (2.83–2.98)8.60 (7.64–9.67)2.25 (2.15–2.35)7.19 (6.22–8.30)3.15 (2.98–3.33)3.24 (3.02–3.48)1.24 (1.21–1.27)
      Maternal outcomes
       Preeclampsia
      Adjusted for multiple birth, year of study, insurance status, region, and age;
      13.96 (13.29–14.66)3.80 (3.69–3.91)27.87 (24.85–31.25)3.28 (3.10–3.47)17.41 (15.09–20.09)2.96 (2.76–3.18)9.74 (9.15–10.35)1.38 (1.35–1.42)
       Stroke/cerebrovascular complications
      Adjusted for multiple birth, year of study, insurance status, region, and age;
      7.14 (4.90–10.40)1.85 (1.41–2.44)13.73 (6.63–28.44)3.52 (2.34–5.31)23.00 (11.47–46.14)7.60 (5.26–10.97)3.87 (2.07–7.23)1.58 (1.29–1.94)
       Acute renal failure
      Adjusted for multiple birth, year of study, insurance status, region, and age;
      35.41 (28.39–44.16)4.43 (3.57–5.48)253.4 (199.5–321.9)62.40 (54.37–71.63)191.5 (141.4–259.4)12.60 (8.88–17.88)14.17 (9.65–20.82)1.27 (0.97–1.65)
       Pulmonary edema
      Adjusted for multiple birth, year of study, insurance status, region, and age;
      11.97 (7.86–18.24)4.01 (3.07–5.25)23.29 (10.32–52.56)9.06 (5.84–14.06)15.52 (4.92–48.95)6.08 (3.46–10.69)9.85 (5.64–17.19)1.54 (1.16–2.05)
       Ventilation
      Adjusted for multiple birth, year of study, insurance status, region, and age;
      11.87 (9.22–15.26)3.34 (2.80–4.00)19.29 (11.36–32.76)8.25 (6.43–10.60)26.20 (15.04–45.63)11.09 (8.46–14.52)5.71 (3.69–8.86)1.84 (1.55–2.18)
       Cesarean delivery
      Adjusted for previous cesarean delivery, multiple birth, year of study, insurance status, region, and age;
      5.75 (5.46–6.05)3.33 (3.26–3.41)5.73 (5.03–6.53)1.74 (1.68–1.81)4.38 (3.74–5.12)1.89 (1.80–1.98)3.16 (2.97–3.36)1.27 (1.25–1.29)
       Length of stay >6 d
      Adjusted for disposition status, admission status, multiple birth, year of study, insurance status, region, and age.
      14.74 (13.68–15.89)5.34 (5.09–5.60)42.16 (36.78–48.32)6.52 (6.12–6.95)30.29 (25.45–36.04)6.18 (5.69–6.71)8.40 (7.60–9.28)1.77 (1.71–1.84)
       In-hospital mortality
      Adjusted for multiple birth, year of study, insurance status, region, and age;
      6.02 (2.71–13.40)2.58 (1.59–4.17)27.02 (8.72–83.73)6.88 (3.56–13.29)88.81 (41.90–188.2)23.81 (14.67–38.66)1.74 (0.24–12.40)1.72 (1.06–2.77)
      For each analysis, reference group was delivery admissions without chronic hypertension and without comorbidity of interest. Admissions with chronic hypertension but without comorbidity of interest was included as a group in each analysis. Because of similarity of estimates of association in these groups to those obtained when analyzing effect of overall chronic hypertension (Table 3), results are not shown.
      Bateman. Chronic hypertension: a nationwide sample of delivery admissions. Am J Obstet Gynecol 2012.
      a Adjusted for multiple birth, year of study, insurance status, region, and age;
      b Adjusted for previous cesarean delivery, multiple birth, year of study, insurance status, region, and age;
      c Adjusted for disposition status, admission status, multiple birth, year of study, insurance status, region, and age.

      Comment

      Chronic hypertension is a well-recognized risk factor for many adverse pregnancy outcomes and has been previously reported to be increasing in prevalence among delivery hospitalizations in the United States.
      • Kuklina E.V.
      • Ayala C.
      • Callaghan W.M.
      Hypertensive disorders and severe obstetric morbidity in the United States.
      Our 14-year study using the largest discharge data set available in the United States, the NIS, finds that the overall prevalence of chronic hypertension among delivery hospitalizations shows sustained increase (approximately 80% across the study period) such that 1.8% of delivery hospitalizations were complicated by chronic hypertension in 2007 through 2008, and confirms and quantifies the role of chronic hypertension in mediating a range of adverse fetal and maternal outcomes. Our study also demonstrates that secondary hypertension has increased in prevalence from 0.07% in 1995 through 1996 to 0.24% in 2007 through 2008 (approximately 300% increase) and has a particularly strong, detrimental effect on pregnancy outcomes.
      Consistent with previously published studies,
      • Flenady V.
      • Koopmans L.
      • Middleton P.
      • et al.
      Major risk factors for stillbirth in high-income countries: a systematic review and meta-analysis.
      • Allen V.M.
      • Joseph K.
      • Murphy K.E.
      • Magee L.A.
      • Ohlsson A.
      The effect of hypertensive disorders in pregnancy on small for gestational age and stillbirth: a population based study.
      • Canterino J.C.
      • Ananth C.V.
      • Smulian J.
      • Harrigan J.T.
      • Vintzileos A.M.
      Maternal age and risk of fetal death in singleton gestations: USA, 1995-2000.
      • Zetterstrom K.
      • Lindeberg S.N.
      • Haglund B.
      • Hanson U.
      The association of maternal chronic hypertension with perinatal death in male and female offspring: a record linkage study of 866,188 women.
      we found that chronic hypertension increased the risk of stillbirth by approximately 2.3-fold. In our study, the OR for stillbirth increased dramatically when chronic hypertension occurred in the setting of certain comorbid conditions–it was >4 for chronic hypertension with diabetes and >7 for chronic renal disease and collagen vascular disease. Similar patterns of associations were observed for poor fetal growth, also in a line with previously published reaserch.
      • Sibai B.M.
      • Lindheimer M.
      • Hauth J.
      • et al.
      Risk factors for preeclampsia, abruption placentae, and adverse neonatal outcomes among women with chronic hypertension: National Institute of Child Health and Human Development Network of Maternal-Fetal Medicine Units.
      • Rey E.
      • Couturier A.
      The prognosis of pregnancy in women with chronic hypertension.
      • Allen V.M.
      • Joseph K.
      • Murphy K.E.
      • Magee L.A.
      • Ohlsson A.
      The effect of hypertensive disorders in pregnancy on small for gestational age and stillbirth: a population based study.
      The synergistic effects of chronic hypertension and these comorbid conditions may be explained by vasculopathy leading to a poorly perfused placenta and/or accelerated placenta aging. Alternatively, it may be an iatrogenic effect of treatment of the hypertension; a metaanalysis of treatment of mild to moderate hypertension suggests that treatment-associated decrease in mean arterial pressure is associated with compromised fetal growth.
      • von Dadelszen P.
      • Ornstein M.P.
      • Bull S.B.
      • Logan A.G.
      • Koren G.
      • Magee L.A.
      Fall in mean arterial pressure and fetal growth restriction in pregnancy hypertension: a meta-analysis.
      Interestingly, pregestational diabetes mitigated the effect of hypertension on poor fetal growth; it is well known that diabetes increases fetal size and predisposes to macrosomia
      • Persson M.
      • Norman M.
      • Hanson U.
      Obstetric and perinatal outcomes in type 1 diabetic pregnancies: a large, population-based study.
      and this may be the mechanism.
      Consistent with its shared etiology of poor placentation, the risk of preeclampsia follows a similar pattern with what is observed in stillbirth and poor fetal growth–chronic hypertension by itself is significantly associated with preeclampsia, but markedly more so when concurrent with chronic renal disease or collagen vascular disease. At the most severe end of the spectrum of adverse maternal outcomes, stroke/cerebrovascular complication, acute renal failure, pulmonary edema, mechanical ventilation, and death, the synergistic detrimental effect of the combination of chronic hypertension and associated conditions becomes even more profound. This marked increase in risk of adverse maternal outcomes for patients with chronic hypertension and these associated illnesses should factor both into preconceptional counseling regarding the risks of pregnancy and the approach to the management of pregnancy and delivery. In many circumstances, patients with these conditions should be cared for by a high-risk obstetric specialist and delivered in hospital settings where intensity of care can be rapidly escalated if needed.
      We used standard methodology to calculate population-attributable fractions of chronic hypertension for a range of adverse pregnancy outcomes.
      • Rockhill B.
      • Newman B.
      • Weinberg C.
      Use and misuse of population attributable fractions.
      Assuming that the associations are causal and chronic hypertension removable, population-attributable fractions estimate the proportion of the adverse outcomes in the population that would be prevented if chronic hypertension or its effects could be negated. Although causality cannot be established in our cross-sectional study, the calculated population-attributable fraction estimates suggest that chronic hypertension may be responsible for >10% of the population burden of preeclampsia, acute renal failure, pulmonary edema, mechanical ventilation, prolonged hospitalization, and maternal death.
      Our study has certain limitations inherent in its methodology. The identification of comorbidities and fetal and maternal outcomes is largely dependent on ICD-9-CM codes collected for billing purposes and thus is susceptible to underascertainment or misclassification bias. Similarly, because we are only able to examine hospital admissions, our analysis of the role of chronic hypertension is confined to outcomes associated with delivery. If a patient had mild hypertension prior to pregnancy and it was no longer an active clinical issue at the time of delivery, it is likely that the patient would not be coded as having chronic hypertension, which would cause us to underestimate the incidence and overestimate the effect of hypertension. Further, we define secondary hypertension as chronic hypertension in the presence of comorbidities that can cause hypertension through vascular or endocrinologic mechanisms. However, we cannot definitively determine whether the hypertension is secondary to the comorbid condition or essential hypertension with associated comorbidity; in either case, the point remains that chronic hypertension associated with certain conditions identifies a group at remarkably high risk of adverse pregnancy outcomes. There are also certain potential confounders, including race and body mass index, associated with both chronic hypertension and adverse maternal and fetal outcomes, that we are unable to adjust for in our analysis because they are not well captured in our data set. Last, we do not have information on the severity of patients' chronic hypertension and what medications, if any, were used to control it–both of which are likely to have an important role in the risk of adverse outcomes. Despite these limitations, there are few population-based studies examining the role of chronic hypertension in pregnancy outcome and our findings help address this deficit. Further, the size of our cohort allows us to carefully examine the interaction of chronic hypertension and relatively rare associated comorbidities with a range of outcomes, including some that are infrequent and severe.
      In conclusion, our nationwide data show a rise in the prevalence of chronic hypertension (from 1.01 to 1.76%) and secondary hypertension (from 0.07 to 0.24%), in delivery hospitalizations in the United States from 1995 through 2008, suggesting that the clinical management of this problem will increasingly confront obstetricians and their colleagues. Although primary and secondary chronic hypertension account for a substantial fraction of maternal morbidity and mortality, current recommendations for the clinical management of pregnant women with chronic hypertension are based on a few small studies, with the benefits and risks of the available treatment strategies remaining uncertain. As the delivering population in the United States becomes older and more obese, chronic hypertension rates are likely to rise even further and this will therefore need to become a priority research area for obstetrics.

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