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Frequently Asked Questions – Heat Stress Hospitalization

What is heat stress and how does it occur?

Any individual, regardless of age, sex, or health status can develop heat stress if engaged in intense physical activity and/or exposed to environmental heat and humidity.  The “heat index” combines air temperature and relative humidity in an attempt to determine the human-perceived equivalent temperature.  For example, when the temperature is 90 °F (32 °C) with very high humidity, the heat index can be about 105 °F (41 °C). Physiologic mechanisms maintain the core body temperature (i.e., the operating temperature of vital organs in the head or trunk) in a narrow optimum range around 37 °C (98.6 °F). When core body temperature rises, the physiologic response is to sweat and circulate blood closer to the skin's surface to increase cooling. If heat exposure exceeds the physiologic capacity to cool, and core body temperature rises, then a range of heat-related symptoms and conditions can develop. Heat stress or heat-related illness ranges from mild heat edema and rash, heat syncope, and heat cramps to the most common type, heat exhaustion. Heat-related cramps, rash, and edema are relatively minor readily treatable conditions; however, they should be used as important warning signs to immediately remove the affected individual from the exposure situation.  

Why is the MA EPHT Program tracking heat stress hospitalization?

In 2002, Massachusetts was one of seven states across the U.S. to be awarded funds from the U.S. Centers for Disease Control and Prevention (CDC) to track health conditions thought to be impacted by the environment. In its Fourth Assessment Report (AR4), released in 2007, the Intergovernmental Panel on Climate Change (IPCC) projected with “virtual certainty” that climate change will cause more frequent, more intense, and longer heat waves. In 2010, the Massachusetts Department of Public Health received funds from CDC to evaluate the state’s vulnerabilities and readiness to address climate change-related health impacts. Tracking heat stress hospitalizations using a standardized method allows for the monitoring of trends over time and the identification of high risk groups that may be more susceptible to heat stress incidents.

What information can be obtained from tracking heat stress hospitalization?

The relationship between extreme heat and increased daily morbidity and mortality is well established. This indicator captures hospital admissions directly attributed to heat stress (e.g., heat illness, heat stroke, and hyperthermia). It is a measure that can be tracked easily and consistently across geography and time and acts as a sentinel for the broader range of heat-related illness that is not recognized and/or coded as such. Heat stress hospitalization data can be linked to temperature and humidity data to evaluate the health impact of hotter summers and more frequent heat waves. These data can also be used to document changes over place and time, to monitor vulnerable areas, and to evaluate the results of local climate-adaptation strategies.

What are some specific types and symptoms of heat stress?

Heat stress can manifest in a number of clinical outcomes, and people with chronic health problems (e.g., cardiovascular disease, diabetes, obesity) are more susceptible to the effects of heat than healthy individuals. For these reasons, heat stress may not be listed as the primary diagnosis. This indicator, therefore, includes all cases where heat stress is explicitly listed as the primary diagnosis or any other diagnosis. The following are some specific types and symptoms of heat stress:

Heat cramps are brief, intermittent, and often severe muscular cramps occurring typically in muscles that are fatigued by heavy work. Individuals with heat cramps can also exhibit hyponatremia (low serum sodium) and hypochloremia (low serum chloride).

Heat syncope is a temporary loss of consciousness as a result of prolonged heat exposure. Individuals adapt to hot, humid environments by dilation of cutaneous vessels in the skin to radiate heat. Peripheral vasodilation along with blood volume loss results in a lowering of blood pressure which can result in inadequate central venous return and cerebral perfusion, causing light-headedness and fainting.

Heat exhaustion is a consequence of extreme depletion of blood plasma volume, which may be coincident with hyponatremia and/or peripheral blood pooling. Heat exhaustion often does not present with definitive symptoms and may be misdiagnosed, often as an acute viral illness. Symptoms include mild disorientation, generalized malaise, weakness, nausea, vomiting, headache, tachycardia (rapid beating of the heart), and hypotension. Because untreated heat exhaustion can progress to heat stroke, the most serious form of heat-related illness, treatment should begin at the first signs of heat exhaustion.  

Heat stroke is an extreme medical emergency that if untreated can result in death or permanent neurological impairment. Heat stroke occurs when a person’s core body temperature rises above 40 °C (104 °F) as a result of impaired thermoregulation. High core body temperature and disseminated intravascular coagulation results in cell damage in vital organs, such as the brain, liver, and kidneys, which can lead to serious illness and death. Death may occur rapidly due to cardiac failure or hypoxia, or it can occur days later as a result of renal failure due to dehydration and/or rhabdomyolysis (the breakdown of muscle fibers into the circulation system). Heat stroke is typically divided into two types: “Exertional” heat stroke involves strenuous physical activity under high temperature conditions to which the heat stroke victim was not acclimatized and usually affects healthy young adults, such as athletes, outdoor laborers, and soldiers. “Classic” heat stroke does not involve exertion and usually affects susceptible individuals such as infants, young children, the elderly, or people with chronic illness. Because heat stroke, even when treated, can have a death rate as high as 33% and can cause permanent damage in up to 17% of survivors, measures should be taken to prevent heat-related illness, especially among vulnerable populations.

What is the difference between hospital inpatient admissions and emergency department (ED) visits and why does the MA EPHT Program track both?

Nearly all heat stress hospitalizations begin with an ED visit. After being examined and/or treated in the ED, some patients are then discharged while others require further monitoring or treatment and are admitted to the hospital as inpatients.

The MA EPHT Program tracks both types of hospital visits because they each provide useful, but different information about heat stress hospitalizations in the state. Looking at heat stress ED visits gives the best estimate of the total number of heat stress hospital visits. On the other hand, heat stress inpatient admissions represent the number of heat stress hospitalizations serious enough to require a hospital stay longer than 24 hours.

Since there is overlap between the two measures in that many patients are included in both the inpatient and ED hospitalization datasets, heat stress counts and rates should not be summed across the two measures of hospitalization.

How do I interpret a rate, and what is the difference between age-specific, crude, and age-adjusted rates?

A rate tells us how frequently a disease or disease-related event (in this case heat stress hospitalization) is occurring in a population. An age-specific rate is calculated for each age group to show how the rate of heat stress hospitalization changes with age.  A crude rate is the number of heat stress admissions over a specified period of time, divided by the total population. An age-adjusted rate enables comparisons to be made between populations which have different age structures (e.g. between two counties or between one community and the state as a whole).

What is a confidence interval (CI)?

To determine if a community’s heat stress hospitalization rate is significantly different from the state rate or if the difference may be due solely to chance, a 95% confidence interval (CI) is calculated for each rate.  A 95% CI assesses the magnitude and stability of a measure.  Specifically, a 95% CI is the range of estimated values that has a 95% probability of including the true rate for the population.

A method for determining if one hospitalization rate estimate is statistically significantly different from another is by comparing the CIs.  If the 95% CI for the rate of one community or population does not overlap the CI of another, then it can be concluded that the two populations are statistically significantly different from each other.  If the 95% CIs do overlap, then the hospitalization rates of the two populations are likely not statistically significantly different from one another.

“Statistically significantly different” means that the difference observed between the two rates will occur by chance less than 5 percent of the time.  For example, if the rate of heat stress hospitalization in community A is 5.6 with a 95% CI of 4.8-6.4 and the state rate of heat stress hospitalization is 10.2 with a CI of 10.0-10.4, then the two 95% CIs do not overlap. In other words, no part of the 95% CI for community A falls within the range of the state’s 95% CI.  Therefore, it is concluded that community A’s heat stress hospitalization rate of 5.6 is statistically significantly different than the state estimate of 10.2.  It can also be said, since community A’s rate is lower than the state’s, that community A’s rate is statistically significantly lower than the state rate.

What are some limitations of the data?

  • Data may only be presented to the public if confidentiality guidelines of the MDPH and CHIA are followed through data aggregation and/or suppression in order to protect privacy. Access to restricted data must follow the application procedure specified on the MDPH website.
  • Hospitalization data, by definition, do not include individuals who do not receive medical care or who are not hospitalized, including those who die in emergency rooms, in nursing homes, or at home without being admitted to a hospital, and those treated in outpatient settings.
  • Data may exclude admissions from specialty hospitals (e.g. psychiatric), long-term care facilities, and federal hospitals which are exempt from state reporting requirements.
  • Transfers from one hospital to another may be included in the dataset as separate hospitalization events.
  • Reporting rates at the state and/or county level will not show the true disease burden at a more local level (i.e., neighborhood).
  • Reporting rates at the state and/or county level will not be geographically resolved enough to be linked with many types of environmental data.
  • When comparing rates across geographic areas, factors such as the percentage of persons with diabetes, heart disease, or elderly living alone can impact the likelihood of persons hospitalized for heat stress.
  • When looking at small geographic levels (e.g., zip code), users must take into consideration appropriate cell suppression rules imposed by the data providers or individual state programs.
  • Rates are based on the residential location of cases and not necessarily the location where incidents occur.
  • Numbers and rates may differ slightly from those contained in other publications. These differences may be due to file updates, differences in calculating rates and updates in population estimates.
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