Seasonal Spikes in Death Occur in Warm and Cold Climates

November 07, 2012

LOS ANGELES — Data from a new study have thrown a wrench into the hypothesis that colder climates precipitate a spike in cardiac death rates when the weather turns frosty. In an analysis of the seasonal variation of total and cardiac deaths across different climates, the seasonal mortality curves in Arizona and Texas were nearly identical to those observed in cold-weather states Pennsylvania and Massachusetts.

Speaking with heartwire , lead investigator Dr Bryan Schwartz (Good Samaritan Hospital, Los Angeles, CA) said they are not discounting the association between mortality and temperature, noting that there is physiological evidence showing that temperature increases blood pressure and peripheral vascular resistance.

"If it were only temperature, these curves would have reflected that, but these curves were exactly the same," said Schwartz. "So if it is temperature, it's more complex than simply an absolute temperature change. It could be that cold temperatures don't affect people in Pennsylvania the way they do in Los Angeles, or maybe other factors are also involved."

To conduct the study, Schwartz, along with senior investigator Dr Robert Kloner (University of Southern California, Los Angeles), analyzed death-certificate data of individuals living in hot climates (Texas, Georgia, and Maricopa Country in Arizona), moderate climates (Los Angeles and western Washington), and cold climates (Massachusetts and Pennsylvania). After doing so, they calculated the seasonal variation of circulatory death around the mean for each location.

The average seasonal cardiac death rates showed a U-shaped curve with respect to temperature, with higher rates of death in the winter and lower rates in the summer. Using the raw daily death rates over an eight-day peak period, circulatory mortality was significantly greater during the winter months than in the summer. In an analysis that normalized the rate by dividing it by the average annual death rate for each location, the investigators observed a similar pattern in circulatory mortality between the highest and lowest temperatures in all locations regardless of climate.

"Each location in the wintertime had approximately an 18% increase over the average, and in each location in the summertime there was an approximate 10% to 12% decrease from the average yearly death rate," said Schwartz. "Beyond that, the lines in the springtime come down very close to each other, and in the fall they increase almost together, are almost superimposed on each other. We found this to be surprising. We thought that colder climates with a colder winter would have a greater increase in mortality in the wintertime or have a prolonged increase in the wintertime, but that's not what we found."

In addition to acclimatizing, Schwartz said that behavioral habits, including diet, exercise, and weight, change in the winter months, and this might have had an impact on the similar rates of mortality across the different climates. In addition, even hot-climate states like Arizona, Georgia, and Texas lose daylight during the winter months, and this might have an impact on the rates of depression and its subsequent link to mortality. They were unable to control for any of these variables, given the retrospective nature of the analysis of death-certificate data.

It is also possible that the seasonal spike in mortality might be related to respiratory infection. In a second abstract presented at the AHA meeting, Schwartz and Kloner showed that respiratory infection is significantly associated with total ischemic heart disease deaths throughout the year in Texas and Los Angeles Country, particularly during annual peaks.