How rising temperatures cause distress to foetuses, by study

•Extreme hot, cold temperatures increase risk of death among individuals with heart disease, stroke
Rising temperatures driven by climate breakdown are causing distress to the foetuses of pregnant farmers, who are among the worst affected by global heating.

A study revealed that the foetuses of women working in fields in The Gambia showed concerning rises in heart rates and reductions in the blood flow to the placenta as conditions became hotter. The women, who do much of the agricultural labour and work throughout pregnancy, told the scientists that temperatures had noticeably increased in the past decade.

There is already strong evidence that extreme heat leads to increases in stillbirths, premature births and low birth weights but this data is from rich, temperate countries. The new study for the first time focuses on subsistence farmers in a tropical country, where increasing extreme heat is a serious concern.

Around the world, hundreds of millions of people, including mothers, are expected to be exposed to extreme heat, even if the global temperature is kept below the internationally agreed limit of 1.5C above pre-industrial levels.

The research is the first step to understanding why foetuses suffer when expectant mothers are stressed by heat. Possible reasons include sweating that leads to dehydration and the diversion of blood and oxygen from the placenta to the mother’s skin to cool the body. The scientists aim to provide evidence for measures to protect expectant mothers and foetuses, such as growing trees to shade women as well as crops.

Dr. Ana Bonell at the Medical Research Council Unit in The Gambia and the London School of Hygiene & Tropical Medicine, who led the research, said: “Our study found that pregnant subsistence farmers commonly experience levels of extreme heat above recommended outdoor working limits, and that this can have significant effects on their health and the health of their babies.

“What we were very shocked to find was that in 34 per cent of the visits [to the fields], there was this impact on the foetus.”

A series of studies published in January found that the climate crisis was damaging the health of foetuses, babies and infants across the world. Scientists discovered increased heat was linked to fast weight gain in babies, which increases the risk of obesity in later life. Higher temperatures were also linked to premature births, which can have lifelong health effects, and to increased hospital admissions of young children.

The research, published in the journal Lancet Planetary Health, involved 92 pregnant subsistence farmers in a rural district of The Gambia. During the seven-month study period, the average air temperature during working hours was 33.5C (92.3F).

The scientists also measured humidity, the women’s temperature and the heart rate of expectant mothers and foetuses. The researchers found that when the woman’s body temperature and heart rate rose by one category in a heat strain index, the risk of foetal distress rose by 20 per cent. Foetal distress was indicated by a heart rate over 160 beats per minute, or reduced blood flow to the placenta, as measured by an ultrasound scan.

The team also found that when a measure of heat stress rose by 1C, the risk of foetal distress rose by 17 per cent. It rose by 12 per cent even when the rise in the woman’s temperature and heart rate was accounted for, indicating other factors affecting the foetus. These may include dehydration, low placental blood flow or heat-related inflammation.

Heat illness was found to be common among the female workers, with almost 60 per cent reporting at least one symptom during field assessments. The symptoms included headache, dizziness, weakness, muscle cramps, vomiting and dry mouth.

To tackle the growing problem of heat stress, Bonell said: “Firstly, I would just recommend stopping burning fossil fuels – that’s the big picture.”

Also, almost one in 100 heart disease-related deaths linked to extreme hot and cold days.

A multinational analysis of more than 32 million deaths from cardiovascular disease found that more people died on days with extreme temperatures— both hot and cold.

For every 1,000 cardiovascular deaths, 2.2 excess deaths were associated with extreme hot days, and 9.1 were associated with extreme cold days.

The researchers now want to see guidelines developed to help mitigate the impact of extreme temperatures.

The planet is enduring more frequent intense heat waves due to climate change, which is mostly caused by humans burning fossil fuels such as coal, oil, and gas. The last eight years have been the hottest on human record, according to the World Meteorological Organisation.

Some research studies indicate that climate change may also be linked to severe winter weather, like that experienced in Texas in February of 2021.

An international consortium of researchers recently joined forces to study how climate change impacts cardiovascular health. A paper about their efforts, which is published in Circulation, a flagship journal of the American Heart Association (AHA), explains that the researchers found more deaths on days when temperatures were at their highest and lowest.

The study suggests both extreme hot and extreme cold temperatures increase the risk of death among individuals with cardiovascular disease (CVD), like ischemic heart disease, stroke, heart failure, and arrhythmia.

On extremely hot days, the heart shifts blood from major organs to underneath the skin, where it is cooler.

“Sweating also happens,” Dr. Barrak Alahmad, lead author of the study as well as a research fellow at the Harvard T.H. Chan School of Public Health and mission scholar at the College of Public Health at Kuwait University in Kuwait City, explained to Medical News Today.

“This could lead to volume depletion that increases heart rate. Increased core body temperature will also increase the metabolic state and oxygen consumption. Then you might also [experience] fluid shifts and electrolytes imbalance (specifically, potassium, magnesium). All of these [cascade] in susceptible individuals…” he further explained.

In hot weather, people who are older, obese, have high blood pressure, or have a history of heart disease need to take precautions like avoiding the outdoors in the early afternoon and staying hydrated, according to the AHA.

“Blood vessels will constrict, and skeletal muscles will increase tone to preserve and generate heat. This will increase blood pressure. Some researchers suggested that cold makes cholesterol crystals deposit in blood vessels and causes heart attacks. Other researchers showed that cold makes your blood more sticky, and that also increases the risk of heart attacks,” said Dr. Alahmad.
Multinational effort

For this study, the team of researchers led by D. Alahmad used data collected by the Multi-County Multi-City Collaborative Research Network, an international collaboration of scientists looking for epidemiological evidence of associations “between environmental stressors, climate, and health.”

They analyzed more than 32 million cardiovascular deaths that occurred in 567 cities in 27 countries on five continents between 1979 and 2019. Researchers obtained city-specific daily temperatures from weather stations and climate reanalysis models.

For their analysis, researchers compared cardiovascular deaths on the hottest and coldest 2.5 per cent of days for each city with cardiovascular deaths on the days that had the optimal temperature (the temperature associated with the least rates of deaths) in the same city.

For every 1,000 deaths from CVD, the researchers found that extreme hot days resulted in 2.2 additional deaths, and extreme cold days accounted for 9.1 additional deaths.

“We now learned that for every 100 cardiovascular deaths, at least 1 additional death is from extreme cold and hot days. Considering that cardiovascular disease is the leading cause of death globally, this essentially translates into a very large burden,” Dr. Alahmad said.

Of all the types of heart disease, heart failure caused the greatest number of deaths from extreme temperatures, the researchers found.

Meanwhile, ischemic heart disease accounted for 37% of CVD deaths studied by researchers. They estimate that about one per cent of all ischemic heart diseases “are attributed to extreme temperatures alone.”

The researchers did not find a significant association between extreme heat and death from arrhythmia. The researchers write that this may be caused by arrhythmia being misclassified as the cause of death when the death was actually caused by ischemia or cardiomyopathy.

Dr. Don Pham, interventional cardiologist at Memorial Hermann in Houston, Texas applauded the study for providing “a new and interesting approach to prevention.”

“I think this serves as a good reminder to patients during the winter or summer months [that] when the weather has drastically [changed] in your region to pay attention to any new symptoms that occur,” he told MNT.

Although the study looked at cities in many countries, some parts of the world received less attention.

“Readers must be cautioned when interpreting our findings as global estimates since some regions were underrepresented in our data such as South Asia, the Middle East and Africa,” Dr. Alahmad told MNT.

“Extreme temperatures might have a larger impact in these underrepresented regions,” he added.

Future research needs to look at the social determinants of health and climate change, said Dr. Martha Gulati, director of preventive cardiology at the Smidt Heart Institute at Cedars-Sinai in Los Angeles, California, and president of the American Society for Preventive Cardiology.