American Journal of Emergency Medicine (2012) 30, 657-664

Original Contribution

Hospital burden of unintentional carbon monoxide poisoning in the United States, 2007^?,??

Shahed Iqbal PhD ^a,?, Huay-Zong Law BS ^a, Jacquelyn H. Clower MPH ^b,

Fuyuen Y. Yip PhD ^a, Anne Elixhauser PhD ^c

^aair pollution and Respiratory Health Branch, Division of environmental hazards and health effects,

National Center for Environmental Health, Centers for Disease Control and Prevention, Atlanta, GA 30341, USA

^bCazador, contracted to Air Pollution and Respiratory Health Branch, Division of Environmental Hazards and Health Effects,

National Center for Environmental Health, Centers for Disease Control and Prevention, Atlanta, GA 30341, USA

^cAgency for Healthcare Research and Quality, Rockville, MD 20850, USA

Received 18 January 2011; revised 11 February 2011; accepted 2 March 2011

Abstract

Background: Unintentional, non-fire-related (UNFR) Carbon monoxide poisoning is a leading cause of poisoning in the United States, but the overall hospital burden is unknown. This study presents patient characteristics and the most recent comprehensive national estimates of UNFR CO-related emergency department (ED) visits and hospitalizations.

Methods: Data from the 2007 Nationwide Inpatient and Emergency Department Sample of the Hospitalization Cost and Utilization Project were analyzed. The Council of State and Territorial Epidemiologists’ CO poisoning case definition was used to classify confirmed, probable, and suspected cases. Results: In 2007, more than 230 000 ED visits (772 visits/million) and more than 22 000 hospitalizations (75 stays/million) were related to UNFR CO poisoning. Of these, 21 304 ED visits (71 visits/million) and 2302 hospitalizations (8 stays/million) were confirmed cases of UNFR CO poisoning. Among the confirmed cases, the highest ED visit rates were among persons aged 0 to 17 years (76 visits/million) and 18 to 44 years (87 visits/million); the highest hospitalization rate was among persons aged 85 years or older (18 stays/ million). Women visited EDs more frequently than men, but men were more likely to be hospitalized. Patients residing in a nonmetropolitan area and in the northeast and midwest regions of the country had higher ED visit and hospitalization rates. Carbon monoxide exposures occurred mostly (N60%) at home. The hospitalization cost for confirmed CO poisonings was more than $26 million.

Conclusion: Unintentional, non-fire-related CO poisonings pose significant economic and health burden; continuous monitoring and surveillance of CO poisoning are needed to guide Prevention efforts. Public health programs should emphasize CO alarm use at home as the main prevention strategy.

^? Funding Source: Not applicable.

^?? Disclaimer: The findings and conclusions in this article are those of the author(s) and do not necessarily represent the official position of the Centers for Disease Control and Prevention.

* Corresponding author. Air Pollution and Respiratory Health Branch, Centers for Disease Control and Prevention, Office of Noncommunicable diseases,

Injury and Environmental Health, Division of Environmental Hazards and Health Effects, Air Pollution and Respiratory Health Branch, Chamblee, GA 30341, USA. Tel.: +1 770 488 0787; fax: +1 770 488 1540.

E-mail address: [email protected] (S. Iqbal).

0735-6757/$ - see front matter. doi:10.1016/j.ajem.2011.03.003

Introduction

Unintentional and non-fire-related (UNFR) carbon monoxide poisoning is responsible for more than 20 000 emergency department (ED) visits and approximately

450 deaths annually in the United States [1-3]. Carbon monoxide is a colorless, odorless gas produced from incomplete combustion of hydrocarbons. Major nonoccupa- tional sources include poorly maintained or poorly ventilated home heating systems, cooking appliances, motor vehicle exhaust, and gasoline or other fuel-powered equipment (eg, portable generators and space heaters) [4]. Carbon monoxide poisoning is often misdiagnosed because of its nonspecific symptoms, which can range from minor flu-like symptoms such as fatigue, headache, dizziness, nausea, vomiting, and confusion to more severe effects such as disorientation, collapse, cardiac irregularities, coma, and death [1,5-7]. Patients having carbon monoxide poisoning may develop neurologic sequelae, including impaired memory and executive functions, some of which may be permanent in nature [7-10].

Emergency department visits and hospitalization data are needed to understand the magnitude, distribution, and trends in CO exposure. Although previous studies have character- ized ED visits, hospitalizations, and fatal cases of CO poisoning [1,2,11], the overall hospital burden of UNFR CO poisoning is unknown. Analysis of hospital data can provide quantitative assessment of the burden of CO poisoning on health care services; quantify health care service needs; and help determine priorities in public health prevention, research, and surveillance [12,13].

The objective of this study was to generate the most recent and comprehensive national estimates of UNFR CO-related ED visits and hospitalizations and characterize patients who had received treatment of CO poisoning. The 2007 Nationwide Inpatient Sample (NIS) and Nationwide Emer- gency Department Sample (NEDS) data from the Hospital- ization Cost and Utilization Project (HCUP) were used for this analysis [14].

Methods

The Hospitalization Cost and Utilization Project

The HCUP is a nationwide information resource for patient care data developed through federal state partner- ships. It is sponsored by the Agency for Healthcare Research and Quality. The basis of HCUP data is all- payer, discharge information from community hospitals in partner states. Community hospitals are defined as non- federal, short-term general hospitals, and other specialty hospitals (eg, obstetrics and gynecology, orthopedic, pediatric, and academic medical centers), excluding federal, psychiatric, rehabilitation, long-term, and tuberculosis

hospitals. Prison hospitals and institutional hospitals such as college infirmaries [14] are also excluded. Hospitals in participating states send billing information and additional data elements to their respective data organizations that perform quality checks on the data received. State-level data are sent to the Agency for Healthcare Research and Quality where they undergo further standardization and internal consistency checks before being stored in state-specific databases. Data on hospital stays and ED visits are stored in separate databases [14]. The NIS is a stratified probability sample of hospitals drawn from a combined pool of the HCUP state-specific inpatient databases. The NIS approx- imates a 20% sample of all US hospitals, and all discharges from each hospital are included. The 2007 NIS contains data from more than 1000 hospitals from 40 participating states and 8 million unweighted discharge records or 39 million weighted discharges [15].

First made available in 2006, the NEDS is a stratified probability sample of US EDs approximating a 20% sample of hospital-based EDs. The NEDS uses records from both the HCUP state-specific ED databases (which include ED visits that are “treat-and-release” [ie, do not result in admission to the hospital in which the ED visit occurred]) and the inpatient databases (which include records for patients initially seen in the ED and then admitted to the same hospital). The 2007 NEDS contains more than 950 EDs from 27 participating states and approximately 26 million unweighted records or more than 122 million weighted ED visits [16].

Case definition

The Council of State and Territorial Epidemiologists’ definition of CO poisoning was used to classify con- firmed, probable, and suspected case types [17]. This definition was based on the International Classification of Disease, Ninth Revision, Clinical Modification (ICD-9- CM), code. Because the public health prevention approach to intentional and fire-related CO exposures differs from that of unintentional CO exposures, we excluded inten- tional and fire-related ICD-9-CM codes for CO poisoning from our analysis. Cases were included and categorized into specific case types if any of the following codes were listed in any diagnosis (eg, primary, secondary, tertiary, and other) fields.

1. Confirmed cases: “986, toxic effect of carbon monoxide” or external cause-of-injury codes (E-codes) indicating CO exposure (E868.3, E868.8, E868.9, E982.1);
2. Probable cases: E-codes indicating Acute CO poisoning inferred from exposure to motor vehicle exhaust (E868.2,

E982.0); and

1. Suspected cases: E-codes that mention CO exposure (E818.0-818.9, E825.0-825.9, E844.0-844.9, E867,

E868.0, E868.1) or E-codes where CO exposure is plausible (E838.0-838.9, E869.9, E981.0, E981.1, E981.8).

Table 1 Carbon monoxide-related hospitalizations and ED visits, United States, 2007
	Case type	ED visits			Hospitalizations
		n (%)	Rate (95% CI) per million		n (%)	Rate (95% CI) per million
	Principal diagnosis	16 840 (7.2)	56 (54-57)		1565 (6.9)	5(4-7)
	Confirmed case (including principal diagnosis)	21 304 (9.1)	71 (69-72)		2302 (10.1)	8 (6-9)
Probable case		208 (0.1)	1 (1-1)	14 (0.1)		a
Suspected case		2 11 362 (90.8)	701 (693-709)	20 401 (89.8)		68 (60-76)
Total		2 32 875 (100.0)	772 (764-781)	22 718 (100.0)		75 (67-84)
CI indicates confidence interval. a Value suppressed as relative SE was greater than 0.5.

We reported estimates for all case types but analyzed only the confirmed cases further, assuming that they most accurately represent the CO-related hospital burden.

Statistical analyses

Residences of patients having CO poisoning were categorized into metropolitan (includes large central metro, large fringe metro, medium metro, and small metro) and nonmetropolitan (includes micropolitan and noncore) areas [18]. Payer information was categorized as Medicare/ Medicaid, privately insured, other insurance, and uninsured (self-pay and no charge) [19]. Place of CO poisoning occurrence was determined using ICD-9-CM E-code E849. Places of occurrences were categorized into residential (home or residential institution), occupational (farm, mine and quarry, and industrial place and premises), recreational (place for recreation and sports), public building (airport, bank, office buildings, hotel, cafe, and other), and other (other specified or unspecified places).

Analyses were conducted using SAS software (version 9.2; SAS Institute, Inc, Cary, NC), and SUDAAN software (version 9.0; Research Triangle Institute, Research Triangle Park, NC) was used to generate SE estimates. Variances were calculated using appropriate methods [19,20]. Estimates with a relative SE of 0 or 0.30 to 0.50 were deemed unreliable and were reported with annotation where applicable; estimates with relative SE of greater than 0.5 were suppressed. Missing values (b2% for all but 1 variable) were not reported. The US Census Bureau’s American Community Survey population estimates were used to calculate the CO-related ED visit and hospitalization rates for 2007 [21,22]. The US Census Bureau population estimates for 1993 to 2007 were used for hospitalization trends over this period [23,24]. Average cost of CO-related hospitalizations was calculated using 2007 HCUP cost-to-charge ratio method [25].

Results

In 2007, a total of 232 875 ED visits (772 ED visits per million population) and 22 718 hospitalizations (75 stays per

million population) occurred in the United States that were UNFR CO related (Table 1). Of these, 9% of ED visits (71 per million population) and 10% of hospitalizations (8 per million population) were confirmed cases of CO poisoning. Among the confirmed cases, CO poisoning was listed as the principal diagnosis for 79% (56 per million population) of ED visits and 68% (5 per million population) of hospital- ization cases (Table 1).

Among the confirmed cases, the highest rates of ED visits were among persons aged 0 to 17 years (76 per million population) and 18 to 44 years (87 per million population), whereas the highest rate of hospitalization was among persons aged 85 years or older (18 per million population) (Table 2). Women visited EDs more frequently than men for CO poisoning, but men were more likely to be hospitalized. Rates of both CO-related ED visits and hospitalization were higher among persons residing in a nonmetropolitan area than persons residing in a metropolitan area. The highest rates of ED visits and hospitalization for confirmed cases of CO poisoning were reported in the northeast and midwest regions of the United States. Uninsured patients comprised 17% of CO-related ED visits and 16% of CO-related hospital stays.

Most ED visits resulted in release after treatment (87%); 10% were either admitted to the hospital or transferred to another facility. Only 0.3% of ED visit patients who were released after treatment received Hyperbaric oxygen (HBO) therapy in the ED compared with 22% of hospitalized patients. Death was a relatively Rare occurrence among CO- related patients: 46 patients (0.2%) died in the ED and 49 patients (2.1%) died during their hospitalization. The mean length of a hospital stay was 4.9 days, and the average cost of a hospitalization was $11 381 (SE, 2522).

Fig. 1 illustrates higher rates of CO-related ED visits and hospitalizations during the winter months (November- March). Relatively higher rates of hospitalization, compared with other nonwinter months, were also observed in April and June.

Place-of-occurrence data for CO-related ED visits and hospitalizations were available for 43% and 36% cases, respectively. Most CO exposures occurred at home for both ED visit (61%) and hospitalization (63%) cases, and 10% to 11% were occupational (Fig. 2). Trends for CO-related

Table 2 Sociodemographic and other characteristics of hospitalizations and ED visits for confirmed cases of CO poisoning, United States, 2007

Variable	ED visits			Hospitalizations
	n (%)	Rate (95% CI) per million		n (%)	Rate (95% CI) per million

Total	21 304 (100.0)	71 (64-77)	2302 (100.0)	8 (6-9)
Age (y)
0-17	5587 (26.2)	76 (66-86)	182 (7.9)	3 (1-4)
18-44	9879 (46.4)	87 (79-96)	664 (28.9)	6 (4-8)
45-64	4259 (20.0)	56 (50-62)	903 (39.3)	12 (9-15)
65-84	1367 (6.4)	42 (36-48)	457 (19.9)	14 (10-18)
>=85	212 (1.0)	41 (28-55)	92 (4.0)	18 (9-27)
Sex
Men	9879 (46.4)	67 (60-73)	1306 (56.8)	9 (6-11)
Women	11 426 (53.6)	75 (67-83)	992 (43.2)	7 (5-8)
Residence
Metropolitan	16 245 (76.6)	65 (60-69)	1771 (78.6)	7 (6-9)
Nonmetropolitan	4963 (23.4)	100 (83-117)	483(21.4)	10 (7-12)
Region
Northeast	7259 (34.1)	133 (110-155)	544 (23.6) a	10 (2-18)
Midwest	7454 (35.0)	112 (95-130)	814 (35.3)	12 (9-16)
South	4421 (20.8)	40 (31-49)	634 (27.5)	6 (4-7)
West	2170 (10.2)	31 (23-39)	311 (13.5)	4 (3-6)
Payer information
Medicare/Medicaid	5718 (27.2)	c	991 (43.1)	c
Private/HMO	8707 (41.4)	c	761 (33.1)	c
Other insurance	2968 (14.1)	c	180 (7.8)	c
Uninsured	3619 (17.2)	c	366 (15.9)	c
HBO	60 (0.3) b	c	498 (21.6)	c
ED event
Treated and released	18 466 (86.7)	c	d
Admitted	1652 (7.8)	c	d
Transferred	556 (2.6)	c	d
Died in ED	46 (0.2) a	c	d
Not admitted, unknown	584 (2.7) a	c	d
Admitted from ED	d		1576 (68.5)
Inhospital deaths	d		49 (2.1)
Mean length of stay	d		4.9 d
Average charge (SE)	d		$25 716 (2710)
Average cost (SE)	d		$11 381 (2522)
HMO indicates health maintenance Organization. HBO indicates hyperbaric oxygen therapy. a Relative SE is between 0.3 and 0.5. b Treat and release cases only. c Denominator data are not available. d Data were not collected/not applicable.

hospitalizations revealed a decline from 1993 to 2000 (Fig. 3). The rate appears to have plateaued from 2001 to 2005; a slight upward trend in rate occurred in 2006 and 2007.

Discussion

Previous national estimates reported more than 20 000 CO-related ED visits annually during 2004 to 2006 and more than 4000 confirmed CO poisoning hospitalizations during

2005 [1,11]. However, the number of hospitalizations may have been an overestimate due to an inability to exclude fire- related and, in some instances, duplicate cases [11]. In this study, 21 304 ED visits and 2302 hospitalizations were classified as confirmed cases of CO poisoning in 2007. Although these national figures of confirmed cases of CO poisoning are consistent with previous estimates of uninten- tional, nonfatal CO poisoning, the actual burden of CO poisoning may be orders of magnitude greater in that the fact that confirmed cases of CO poisoning comprised only approximately 10% of total CO-related ED visits and hospitalizations; probable and suspected cases were not

Fig. 1 Daily ED visit and hospitalization rate per million population per day by month for confirmed cases of CO poisoning, United States, 2007.

included in these estimates. Although probable and suspected cases may not be the primary reason for ED visits or hospitalization, CO poisoning remains an important comorbid condition that contributes to the health outcome and Costs of care [11]. In addition, underdiagnosis and misdiagnosis of CO poisoning are quite common; and patients seen in hospitals that were not part of HCUP (specifically, hospitals that are part of the Indian Health Service, Veterans Administration, and Department of Defense) were not included in these analyses [1,5,14].

According to the National Fire Protection Agency estimates, the municipal fire departments around the country responded to more than 51 000 (excluding responses to false

alarms) non-fire-related CO incidents in 2005; in compar- ison, the annual estimate of CO-related ED visits for the same year was reported to be more than 21 000 [2,26]. These findings indicate that many persons who are exposed to CO do not necessarily receive hospital care; however, some persons likely receive on-site health assessment or treatment. Response to these incidents and treatment of the affected persons contribute to the overall national CO burden but were not captured in this analysis. The average length of hospitalization for confirmed cases of CO poisoning was 5 days, resulting in more than 11 000 hospital days in 2007. Cost data were available only for the hospitalization of confirmed CO poisoning cases; the average cost for each

Fig. 2 Emergency department visits (n = 9142) and hospitalization (n = 829) by place of occurrence for confirmed cases of CO poisoning, United States, 2007.

Fig. 3 Trends in hospitalization rates for confirmed cases of CO poisoning, United States, 1993 to 2007.

case was more than $11 000, resulting in a cumulative Total cost of more than $26 million. Therefore, it is evident that UNFR CO poisoning poses a significant economic and public health burden. The necessity of preventing CO poisoning is further emphasized by the fact that CO poisoning is often undiagnosed or misdiagnosed because of the nonspecificity of its symptoms [2,10]. In 1 study, approximately 30% of the CO poisoning cases were diagnosed incorrectly; and 43% were misdiagnosed as food poisoning [27,28].

Hyperbaric oxygen therapy is a treatment option for severe cases of CO poisoning. The Undersea and Hyperbaric Medical Society recommends HBO therapy for patients with certain signs and symptoms (eg, transient or prolonged unconsciousness, abnormal neurologic signs or cardiovas- cular findings, or severe acidosis), severe exposure (eg, patients aged >=36 years with CO exposure history of >=24 hours including intermittent exposure), or elevated carbox- yHemoglobin levels (>=25%) [10,29]. In this study, very few (0.3%) ED patients received HBO therapy in the ED, compared with 22% of the hospitalized patients. This reflects that HBO therapy is more commonly administered to hospitalized cases compared with those who were released from the ED after treatment. The hospitalization rate increased with age and was higher among men than women. These population subgroups (male, older adults) are similar to subgroups at the highest risk for CO-related mortality; these findings are consistent with previous reports [3,11]. As with mortality risk, the higher hospitalization rate among men has been assumedly a result of engaging in high- risk behaviors such as using fuel-burning tools or appliances [3,11]. In addition, severe CO exposures among the older adult population could be due to the misidentification of nonspecific CO-related symptoms such as fatigue or flu- like illnesses [3,11]. Previous studies reported higher rates of ED visits among children 4 years or younger [1], but

persons aged 18 to 44 years had the highest rate of ED visits in this study. Biologically, children are more susceptible to CO poisoning because they have higher basal metabolic rates and tissue oxygen demands [30]; but a broad age categorization (0-17 years) may have diluted the high rates among very young children and infants. The higher ED visit rates observed among women are likely because they, similar to children, manifest symptoms at lower levels of CO exposure because they have a lower Red blood cell count [11,31]. These factors may lead to earlier exposure recognition and, therefore, a lower exposure and shorter Recovery time [11]. Identifying these high-risk subgroups is important because they can help guide targeted public health prevention efforts.

The proportion of uninsured patients in 2007 was higher among CO-related hospitalizations as compared with all hospitalizations (15.9% vs 5.8%) but was similar between CO-related ED visit cases and all ED visits (17.2% vs 18.4%). Studies have reported that CO poisoning dispropor- tionately affects ethnic minority, immigrant, and non- English-speaking populations [32,33]. Whether severe CO exposures disproportionately affect persons having limited access to health care or having a low socioeconomic status, which results in hospitalization, warrants further investiga- tion. Residents in nonmetropolitan areas had higher rates of CO-related ED visits and hospitalizations compared with residents in metropolitan areas. This is the first time such a difference has been observed, and the factors driving this difference remain unknown.

The seasonal pattern of CO-related ED visits and hospitalizations is consistent with previous findings regard- ing seasonal CO poisoning [1,2,11,26]. Both CO-related ED visits and hospitalization rates were higher in winter months. This increase is likely due to an increase in high-risk behaviors such as generator use, exposure to automobile exhaust by stranded motorists during and after winter storms,

indoor use of grills or stoves, warming up cars in garages, and improper maintenance of home heating systems [1,2,11]. Consequently, the higher rates observed in the northeastern and midwestern regions of the country may be attributed to the long and severe winter seasons in these regions and the potential for increased high-risk behavior [11]. When examining hospitalization rates for nonwinter months, we found higher rates in April and June. These rate increases do not correspond with the trends observed in fire department response to nonfire CO incidents, as reported by the National Fire Protection Agency [26]; the cause and source of these exposures need to be investigated further.

Understanding the place and source of exposure to CO may provide unique opportunities for prevention. It has been reported that 89% of the fire department responses to non- fire-related CO incidents occurred in residential settings in 2005 [26]. This finding is consistent with previous morbidity reports on UNFR CO poisoning [1,2]. Although many records did not mention place of occurrence in this study, most (N60%) of the ED visit and hospitalization cases were attributed to CO exposure in residential settings, similar to previous reports. Common sources of these exposures included using poorly maintained or unventilated heating and cooking appliances, operating motor vehicles in a garage, and using fuel-burning appliances such as portable heaters or charcoal grills or briquettes indoors [1,2]. Simple precautionary measures, such as proper installation of CO alarms, can effectively prevent many fatal and nonfatal CO poisoning cases [34-37]. Despite a steep decline in from 1993 to 2001, CO-related hospitalization rates seem to be increasing in recent years after a plateau from 2001 to 2005. Mott et al. [38] found that a decrease in deaths from motor vehicle-related unintentional CO poisoning from 1968 through 1998 coincided with the enforcement of the 1970 Clean Air Act and introduction of catalytic converters to automobiles in 1975. Whether the initial decrease in CO- related hospitalization can be attributed to the same to some extent needs further investigation. However, the recent increase is a reminder that this Public health problem needs continued attention and public health action to reduce the overall burden. It is also imperative that public health programs focus on education and the use of CO alarms at home as the main prevention strategy.

The HCUP data sets are the largest repository of hospital discharges in the United States. A large sample size provides more precise estimates, smaller sampling errors, and greater population representation. The 2007 NEDS sampling frame included 59.6% of all ED visits and 61.3% of the US population; and the 2007 NIS sampling frame included 78% of all US hospitals, representing nearly 90% of the US population [15,16]. Furthermore, E-code reporting for injury cases is more complete in HCUP databases than in other nationally representative data sets. For example, in the 2001 NIS, 86% of injury diagnoses had an E-code, compared with 68% in the 2001 National Hospital Discharge Sample data set [39]. The NIS weighted data for total hospital discharges

closely approximated that of the National Hospital Discharge Sample and American Hospital Association survey in hospital and patient characteristics, diagnoses, procedures, and length of stay [40,41].

To date, this is the first comprehensive review of UNFR CO poisoning-related hospitalizations and ED visits in the United States using a large, nationally representative data set. Hospital and Economic burden of CO poisoning reported in this study may be an underestimation; a substantial number of CO poisoning cases were unaccounted for because specific health care facilities were excluded in the sample and because probable and Suspected CO poisoning cases were excluded from the final estimates. Continuous and expanded surveillance efforts are needed to better understand the public health significance of UNFR CO poisoning. Public health programs must also focus on education and use of CO alarms at home as the main prevention strategy.

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