This study provided the first national estimates of UNFR CO-related hospitalization. In 2005, there were 4,216 hospitalizations (1.42/100,000 population) classified as confirmed cases of CO poisoning and 24,891 hospitalizations (8.40/100,000 population) with at least one CO-related ICD-9-CM code listed as a diagnosis. These estimates suggest that a substantial number of hospitalized cases resulting from CO exposure are not currently accounted for, as there is no surveillance system in place that includes CO-related hospitalizations.
In addition, HCUPnet provided preliminary data on the economic burden of these hospitalizations, which has also gone unrecognized until now. Hospital charge data were available for cases with a principal diagnosis of CO poisoning. The total hospital charges for cases where the principal diagnosis was “986—Toxic effects of carbon monoxide” was nearly $27 million (mean = $15,168; median = $7,613) in 2005 (data not shown). These amounts do not include professional fees (e.g., physician) or reflect the total amount reimbursed. In this study, ICD-9-CM code 986 as the principal diagnosis comprised only 42% of all cases. Therefore, the overall cost burden of CO-related hospitalization is much higher than this estimate.
Overall, the rate of hospitalization increased with age and was higher among males. These population subgroups (males and older adults) are similar to those at the highest risk for CO-related mortality;2
however, they are different from the subgroups at highest risk for CO-related ED visits, in which females and children aged ≤4 years are more often affected.1
Children could be more susceptible to CO poisoning due to higher basal metabolic rate and tissue oxygen demand.31
However, they are expected to manifest symptoms early and to recover more quickly because of higher minute ventilation per unit of body mass.32
Women also manifest symptoms at lower levels of exposure because of lower red-blood-cell count.33
These factors may lead to earlier exposure recognition and, therefore, a lower exposure and shorter recovery time.
As with mortality risk, it could be assumed that a higher hospitalization rate among men may be due to engagement in high-risk behaviors, such as using fuel-burning tools or appliances. It has been suggested that severe CO exposures among the older adult population could be due to the misidentification of CO-related symptoms as fatigue or flu-like illnesses.2
However, whether older adults and male populations are more severely poisoned, leading to higher hospitalization and mortality rates, requires further investigation.
CO exposures typically follow a seasonal pattern in which both fatal and nonfatal rates peak during the winter season; this peak 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, and improper maintenance of home heating systems.1,2,34
Observed regional patterns may be explained by weather differences across regions. For example, long and severe winter seasons and the subsequent increase in the aforementioned high-risk behaviors may have contributed to the higher rates of CO-related hospitalization in the Midwest and Northeast regions of the U.S.
The proportion of uninsured patients was higher for CO-related hospitalizations (15.6%) than for overall hospitalizations (5.3%) in the 2005 NIS (data not shown). Whether severe CO exposures disproportionately affect those with limited access to health care or low socioeconomic status warrants further investigation. Also, we do not know whether there is a difference between urban and rural CO-poisoning rates in the U.S. In this study, 74% of the confirmed cases were residents of either large or small metropolitan areas. This distribution is similar to the U.S. population distribution (79% urban and 21% rural), according to the 2000 Census.35
Examination of CO-related hospitalization trends revealed a decline in the rate of confirmed cases from 1993 through 2001, followed by a plateau from 2001 through 2005. This trend is important from a public health perspective, as it suggests that existing public health initiatives and efforts need to be continued and reinforced to further decrease the number of UNFR CO exposures.
Some limitations should be taken into account while interpreting the results from this study. CO-related hospitalization estimates obtained from HCUPnet could be an underestimation of total hospitalizations because they do not account for cases seen in federal or other hospitals that are excluded from the NIS sampling frame. Additionally, some injury cases without any listed E-codes were not included in our analysis.24
On the contrary, limitations of the query system might have led to duplicate case counts and the inclusion of some intentional and fire-related cases, primarily because of the complex exclusion criteria of the case definition. Also, we were unable to determine the overall effect of variable E-coding rates and practices by different states and hospitals.
HCUP quality-control procedures exclude similar E-codes for the same injury event—i.e., if a case has multiple CO-related E-codes listed in the diagnosis, all but one E-code would be removed to avoid duplicate counts (Personal communication, HCUP user support, November 2007); however, CO-related E-codes, including fire-related and intentional E-codes, may still be recorded for some cases when ICD-9-CM code 986 is listed as the principal diagnosis because 986 is not an E-code. This is a major limitation of using HCUPnet for UNFR CO-poisoning hospitalization surveillance.
Finally, some relevant data elements, such as dates of hospitalization and length of stay, are not available for any diagnoses other than the principal diagnosis. Information on place of CO exposure occurrence provides an opportunity for the development of public health prevention strategies. HCUP data do not include the place-of-occurrence data. Also, they do not include information on workers' compensation; this limits analysis of work-related CO-poisoning cases. Including additional queries and data elements, and expanding HCUP partner states will make HCUPnet a formidable public health data repository. Further, given that adequate resources are available, many of these limitations can easily be overcome by analysis of commercially available HCUP data.