The aim of this study was to investigate the impact of morbid obesity (body mass index ≥40 kg/m 2 ) on in-hospital mortality and coronary revascularization outcomes in patients presenting with acute myocardial infarctions (AMI). The Nationwide Inpatient Sample of the Healthcare Cost and Utilization Project was used, and 413,673 patients hospitalized with AMIs in 2009 were reviewed. Morbidly obese patients constituted 3.7% of all patients with AMIs. Analysis of the unadjusted data revealed that morbidly obese patients compared with those not morbidly obese were more likely to undergo any invasive cardiac procedures when presenting with either ST-segment elevation myocardial infarction (97.4% vs 93.8%, p <0.0001) or non–ST-segment elevation myocardial infarction (85.5% vs 80.6%, p <0.0001). The unadjusted mortality rate for morbidly obese patients with AMIs was 3.5%, compared with 5.5% of those not obese (p <0.0001). After adjustment, lower odds of mortality in those morbidly obese compared to those not morbidly remained. In conclusion, patients with morbid obesity had lower odds of in-hospital mortality, compared to those not morbidly obese, consistent with the phenomenon of the “obesity paradox.”
Given previously reported worse short-term outcomes for morbidly obese patients presenting with acute coronary syndromes in registry studies, we explored whether this association was evident in a large national database. Using data from the Nationwide Inpatient Sample (NIS) of the Healthcare Cost and Utilization Project, we analyzed the associations among morbid obesity, treatment utilization, and mortality while adjusting for baseline characteristics, including co-morbidities, for 413,673 patients hospitalized with acute myocardial infarctions (AMIs).
Methods
This study involved a population-based sample of all patients admitted with AMIs to 1,045 hospitals in 44 states in 2009 whose admission and discharge data were included in the NIS. Our sample included those admitted with principal diagnoses of AMI (International Classification of Diseases, Ninth Revision, Clinical Modification [ICD-9-CM] codes 410.0 to 410.92). ST-segment elevation myocardial infarction (STEMI) was recorded when the principal diagnosis was billed with ICD-9-CM codes 410.0 to 410.62 or 410.81 to 410.82 and non-STEMI (NSTEMI) with ICD-9-CM codes 410.70 to 410.72 or 410.90 to 410.92. Patients were excluded if AMI was an in-hospital complication. Institutional review board approval was obtained from our university.
These data include ICD-9-CM-coded primary and secondary diagnoses; primary and secondary procedures; admission and discharge status; demographic information such as gender, age, race and ethnicity, and median income for ZIP code divided into quartiles; expected payment source; total charges; length of stay; and hospital region, teaching status, ownership type, and bed size. We used ICD-9-CM secondary diagnosis codes and a database-defined variable for morbid obesity (body mass index ≥40 kg/m 2 ) developed by the Agency for Healthcare Research and Quality. ICD-9-CM secondary codes were used to indicate the presence of up to 30 chronic co-morbidities likely to have been present on admission, using the Elixhauser co-morbidity adjustment method developed at the Agency for Healthcare Research and Quality. Variables in the risk adjustment algorithm include age, gender, peripheral vascular disease, paralysis, other neurologic disorders, chronic pulmonary disease, diabetes mellitus, diabetes mellitus with chronic complications, hypothyroidism, renal failure, liver disease, peptic ulcer disease, acquired immune deficiency syndrome, lymphoma, metastatic cancer, solid tumor without metastasis, rheumatoid arthritis, coagulopathy, weight loss, fluid and electrolyte disorders, chronic blood-loss anemia, iron deficiency anemia, alcohol abuse, drug abuse, psychoses, and hypertension.
The principal outcome measure was short-term all-cause mortality (in-hospital mortality), which was defined as death that occurred during the initial hospitalization, between the day of hospital admission and date before discharge, provided the length of stay was ≤30 days. Secondary outcomes included cardiac procedures and were defined using ICD-9-CM primary procedure codes to indicate whether diagnostic coronary angiography (ICD-9-CM codes 37.22, 37.23, 88.53, 88.54, 88.55, 88.56, and 88.57), percutaneous coronary intervention (PCI) (ICD-9-CM codes 36.04, 36.06, 36.07, and 00.66), or coronary artery bypass graft (CABG) surgery (ICD-9-CM codes 36.10 to 36.19) was performed during the hospitalization.
We used SAS version 9.1 (SAS Institute Inc., Cary, North Carolina) for all analyses. Univariate and distributional analysis included measures of central tendency, kurtosis, and skew. Differences between those morbidly obese patients and those morbidly obese were assessed using chi-square tests for categorical variables and Student’s t tests or 1-way analysis of variance as appropriate for continuous variables. Adjusted odds ratios (ORs) for in-hospital mortality as well as procedure use were estimated using unconditional logistic regression. To control for differential characteristics of morbidly obese patients and those not morbidly obese, covariates including age, gender, race, income, Elixhauser co-morbidities, and hospital characteristics such as hospital location, hospital control (for profit or nonprofit), hospital teaching status, and hospital volume were included in the models. All analyses were weighted using NIS-provided weights to create national estimates.
Results
The 2009 NIS database included 413,673 admissions for AMI, with 32% due to STEMI and 68% due to NSTEMI ( Table 1 ). Morbidly obese (body mass index ≥40 kg/m 2 ) patients constituted 3.7% of all patients with AMIs. Morbidly obese patients presenting with AMIs were more likely to be female (45.8% vs 36.7%, p <0.0001), younger (age 59.6 vs 65.3 years, p <0.0001), and black (11.7% vs 9.2%, p <0.0001) and had a higher incidence of co-morbid conditions such as diabetes mellitus (63.4% vs 33.0%, p <0.0001), hypertension (77.3% vs 67.6%, p <0.0001), and renal failure (21.6% vs 16.7%, p <0.0001).
| Variable | Patients With AMIs ∗ | Morbid Obesity ‖ | p Value § | |
|---|---|---|---|---|
| Overall Sample | Yes | No | ||
| Overall | 413,673 | 3.7 (15,254) | 96.3 (398,419) | |
| Women | 37.0 (153,147) | 45.8 (6,993) | 36.7 (146,154) | <0.0001 |
| Men | 63.0 (260,526) | 54.2 (8,262) | 63.3 (252,265) | <0.0001 |
| STEMI † | 32.4 (134,032) | 23.8 (3,623) | 32.7 (130,409) | 0.0001 |
| NSTEMI ‡ | 67.6 (279,641) | 76.3 (11,631) | 67.3 (268,010) | 0.0001 |
| In-hospital death | 5.4 (22,315) | 3.5 (529) | 5.5 (21,786) | <0.0001 |
| Diagnostic catheterization | 18.7 (77,157) | 21.1 (3,215) | 18.6 (73,942) | <0.0001 |
| PCI | 48.7 (201,291) | 41.5 (6,327) | 48.9 (194,964) | <0.0001 |
| CABG | 9.6 (39,551) | 16.2 (2,470) | 9.3 (37,081) | <0.0001 |
| Mean age (yrs) | ||||
| Overall | 65.6 ± 31.1 | 59.6 ± 26.2 | 65.9 ± 31.2 | <0.0001 |
| STEMI | 62.3 ± 30.6 | 57.3 ± 26.6 | 62.5 ± 30.6 | <0.0001 |
| NSTEMI | 67.1 ± 30.7 | 60.4 ± 25.9 | 67.5 ± 30.1 | <0.0001 |
| Race | ||||
| White | 76.7 (317,413) | 77.0 (11,742) | 76.7 (305,671) | |
| Black | 9.3 (38,361) | 11.7 (1,780) | 9.2 (36,581) | |
| Hispanic | 7.3 (29,991) | 7.0 (1,057) | 7.3 (28,934) | <0.0001 |
| Asian | 2.2 (8,873) | 0.9 (135) | 2.2 (8,738) | |
| Others | 4.6 (19,035) | 3.5 (540) | 4.6 (18,495) | |
| Median household Income | ||||
| $1–$39,999 | 26.9 (111,165) | 31.3 (4,771) | 26.7 (106,393) | |
| $40,000–$49,999 | 26.7 (110,471) | 27.0 (4,117) | 26.7 (106,354) | <0.0001 |
| $50,000–$65,000 | 25.0 (103,238) | 25.0 (3,813) | 25.0 (99,424) | |
| ≥$66,000 | 21.5 (88,799) | 16.7 (2,553) | 21.7 (86,246) | |
| Co-morbidities | ||||
| Diabetes mellitus | 34.1 (140,916) | 63.4 (9,677) | 33.0 (131,239) | <0.0001 |
| Hypertension | 68.0 (281,044) | 77.3 (11,787) | 67.6 (269,258) | <0.0001 |
| Perivascular disorders | 11.8 (48,868) | 10.9 (1,664) | 11.9 (47,204) | 0.0004 |
| Renal failure | 17.0 (69,841) | 21.6 (3,292) | 16.7 (66,550) | <0.0001 |
∗ ICD-9-CM codes 41000, 41001, 41002, 41010, 41011, 41012, 41020, 41021, 41022, 41030, 41031, 41032, 41040, 41041, 41042, 41050, 41051, 41052, 41060, 41061, 41062, 41070, 41071, 41072, 41080, 41081, 41082, 41090, 41091, and 41092.
† ICD-9-CM codes 41000, 41001, 41002, 41010, 41011, 41012, 41020, 41021, 41022, 41030, 41031, 41032, 41040, 41041, 41042, 41050, 41051, 41052, 41060, 41061, 41062, and 41080.
‡ ICD-9-CM codes 41070, 41071, 41072, 41090, 41091, and 41092.
§ Obtained by chi-square tests of independence for categorical variables.
Analysis of the unadjusted data revealed that morbidly obese patients compared with those not morbidly obese were more likely to undergo any invasive cardiac procedures when presenting with either STEMI (97.4% vs 93.8%, p <0.0001) or NSTEMI (85.5% vs 80.6%, p <0.0001) ( Table 2 ). In the STEMI and NSTEMI subgroups, morbidly obese patients were slightly more likely to undergo only diagnostic catheterization with no further revascularization (10.3% vs 9.01%, p <0.0001, and 29.2% vs 27.1%, p <0.001, respectively). Regardless of the type of AMI, morbidly obese patients were less likely to undergo PCI (45.1% vs 52.9%, p <0.0001) and more likely to undergo CABG surgery (18.6% vs 10.9%, p <0.0001) than those not morbidly obese.
