Almost everything we know about the effects of cold temperatures on the cardiovascular (CV) system is derived from studies conducted in countries with temperate climates. A new Brazilian study published in the PLOS Global Public Health is the first to compare the association between ambient temperature and CV hospitalization risk in cities with tropical and temperate climates within the same country.

Researchers from the Universidade Federal do Paraná and the Pontifícia Universidade Católica do Paraná, both in Curitiba, Brazil, cross-referenced climate data with records of nearly 3 million CV hospitalizations in more than 100 Brazilian cities over 14.5 years.

The study found that temperatures below 19 °C were associated with an increased risk for CV hospitalization in both tropical and temperate regions, raising the question of whether individuals living in warmer climates respond differently when temperatures fall. Few recent studies have examined the relationship between CV hospitalization rates and temperature in Brazil.

The researchers compared hospitalizations for acute myocardial infarction, stroke, heart failure, and cardiac arrhythmias recorded in Brazil’s open-access health data system, the Department of Informatics of the Unified Health System (DATASUS), between January 2010 and June 2024, with average daily temperatures recorded by the National Institute of Meteorology (INMET), Brazil, for each city included in the analysis.

The Findings

The results showed no statistically significant difference in CV hospitalization risk between cities in tropical and temperate climates. A temperature threshold of 19 °C, below which the risk for hospitalization increased, was applied across Brazil, from Manaus to Porto Alegre.

Between 9 °C and 18 °C, the risk for CV hospitalization increased progressively with decreasing temperatures. At 18 °C, the relative risk (RR) was 1.01 (95% CI, 1.01-1.02); at 14 °C, it was 1.03 (95% CI, 1.02-1.04); and at 9 °C, it was 1.05 (95% CI, 1.01-1.09). Between 19 °C and 24 °C, the ambient temperature had a neutral association with hospitalization rates. Above 25 °C, the trend was reversed, with the risk for CV hospitalization decreasing with increasing temperatures. At 30 °C, the RR was 0.97 (95% CI, 0.93-1.02).

The analysis included nearly 3 million CV hospitalizations in 103 cities with complete climate data. The researchers noted that approximately 20 million CV-related hospitalizations were recorded nationally during the study period; however, these additional records were excluded because complete climate data were not available for all Brazilian cities.

According to Bruna Dóris, MD, cardiologist, and the study’s first author, the findings were consistent with what the researchers had observed in clinical practice. 

Speaking with Medscape’s Portuguese edition, shesaid,“We had already noticed that there were many more hospitalizations during cold weather, so that was precisely our question: whether this was just an observational bias or whether we would actually confirm this association in a study.” 

The study found an association between lower temperatures and a higher number of CV hospitalizations. However, Dóris cautioned that “since this is an ecological and observational study, it is not possible to conclude that low temperatures were the cause of the increase in the hospitalization rate.”

“The idea is to expand the analysis by breaking down the data by region, type of cardiovascular disease, and age group,” the cardiologist said. “This information is already available in DATASUS.”

Temperature and Other Factors

Speaking with Medscape’s Portuguese edition Glaucia Maria Moraes de Oliveira, MD, PhD, cardiologist, and member of the Department of Women’s Cardiology at the Brazilian Society of Cardiology (SBC) and serves as editor-in-chief of Arquivos Brasileiros de Cardiologia, the society’s official journal noted that temperature is only one of several factors that can affect the CV system. Pollution, socioeconomic conditions, population age distribution, and respiratory infections during winter may occur concurrently and influence hospitalization rates. Ecological study design could not isolate the individual effects of these factors. “It’s as if we were seeing just a piece of an iceberg.” Another concern, according to Glaucia, is the use of average temperatures, which “creates a very significant measurement bias.”

The study evaluated only ambient temperature, and humidity, dietary habits, and pollution were not included in the analysis. In southern Brazil, where Rio Grande do Sul had the highest rate of CV hospitalization in the country, the authors speculated that regional dietary habits may contribute to the higher hospitalization rate, although the study did not evaluate this factor. In the equatorial region, which had the lowest CV hospitalization rates in the country, the authors hypothesized that high humidity in the Amazon region might have a protective effect. However, the study did not assess humidity, and the available data do not permit either explanation to be confirmed.

The effects of cold on CV may not occur immediately. Previous research indicates that the effects of cold on CV can persist for several weeks. In this study, the distributed lag model evaluated the temperature effects over a maximum of 7 days.

According to Oliveira, extreme temperatures, rather than average temperatures, lead to CV hospitalization. Average temperatures, she said, are not the “triggers” for hospitalization.

In addition, the effects of extreme temperatures on CV can vary depending on the exposure history of an individual. “Another important factor is whether the body is accustomed to a certain temperature range. We saw this recently in the EU,” she said, referring to the more than 30,000 additional deaths recorded in the EU during extreme heat waves this past summer.

Important Precautions

What can be done to reduce the potential health effects of temperature? During cold weather, Dóris recommended campaigns targeting people experiencing homelessness and programs to improve access to heating systems in socioeconomically disadvantaged areas. Oliveira emphasized the importance of vaccination.

“We need adequate immunization against respiratory syncytial virus and influenza, vaccines that we recommend at the SBC because they prevent respiratory infections, which can trigger a heart attack or stroke,” she said, adding that, above all, the most important measure is to treat patients with CV disease. “If the patient is receiving appropriate pharmacological treatment, following the guidelines we recommend at the SBC and worldwide, we will see fewer adverse effects of these temperatures on the individual’s body,” she concluded.

Ilana Polistchuck is a Brazilian journalist and physician with a master’s degree in communication and Culture from the UFRJ. She is a specialist in Internal Medicine, having completed her residency at the Andaraí Federal Hospital in Rio de Janeiro, and in Family and Community Medicine, having completed her training through the Brazilian Society of Family and Community Medicine.

This story was translated from Medscape’s Portuguese edition.