The advance of El Niño, the phenomenon marked by abnormal warming of the waters of the equatorial Pacific Ocean, is prompting a health warning that goes beyond weather forecasts. The pattern drives episodes of high temperatures, low humidity and wildfires across multiple regions, and doctors say the resulting heat stress can overload vital organs including the heart and the brain.
El Niño is a naturally occurring climate pattern that periodically warms surface waters in the tropical Pacific, altering weather systems around the world and often pushing up temperatures in affected regions.
Why extreme heat strains the heart
To cool the body in high temperatures, the body widens blood vessels and increases blood flow to the skin, while also stepping up sweat production. That process forces the heart to work harder, speeding up the heartbeat to keep blood pressure at an adequate level.
Heavy sweating without enough fluid replacement causes the body to lose water and essential electrolytes such as sodium and potassium. This leads to dehydration and hemoconcentration, in which the blood becomes denser because of reduced circulating volume, and it can also alter the heart muscle's electrical activity.
Nilton Carneiro, a cardiologist at Hospital Santa Catarina in Sao Paulo, said that when the temperature rises, the heart has to work harder at the same time that dehydration can reduce circulating blood volume. He said the resulting loss of electrolytes can favor changes in heart rhythm.
The combination of vasodilation, a faster heartbeat and denser blood significantly raises the heart muscle's demand for oxygen. In people with pre-existing conditions such as atherosclerosis, that imbalance raises the risk of acute episodes including arrhythmias, tachycardia, fainting and heart attacks.
Julianny Freitas, a cardiologist at Casa de Saude Sao Jose, said the rise in medical visits linked to excessive heat is notable. She said the heart needs to work harder to maintain circulation when the body is dehydrated, creating a state of overload in the organ.
Heat exhaustion versus heat stroke
The effects of severe heat exposure can appear at different levels of severity, ranging from heat exhaustion to heat stroke.
With heat exhaustion, the body shows symptoms such as heavy sweating, weakness, dizziness, nausea and cold or clammy skin. When these signs appear, the advice is to move to a cool place, drink fluids, loosen clothing and lie down with the legs raised.
Heat stroke, also known as thermal shock, is considered a serious medical emergency. Body temperature exceeds 40 degrees Celsius, and the person shows hot, dry skin, mental confusion, loss of consciousness and even seizures.
In these cases, the recommendation is to call for urgent help and cool the body quickly with cold water and ventilation, without offering fluids if the person is unconscious or confused. A lack of immediate care in scenarios of dehydration and heat stress can lead to death.
Impact on the brain and stroke risk
The damage caused by heat waves is not limited to the circulatory system. The brain also suffers direct effects from rising temperatures and fluid loss.
Reduced cerebral blood flow, combined with electrolyte imbalance and hemoconcentration, increases the likelihood of blood clots forming.
Mauricio Hoshino, a neurologist at Hospital Santa Catarina in Sao Paulo, described heat stroke as the most serious neurological manifestation linked to high temperatures and said it constitutes a medical emergency. He said it can cause altered consciousness, seizures, brain swelling and other systemic complications.
Scientific studies point to an association between high temperatures and an increase in strokes, both ischemic, caused by a blocked artery, and hemorrhagic, caused by a ruptured blood vessel, as well as worse outcomes and higher mortality.
Other neurological conditions that can worsen include epileptic seizures, migraines, worsening of Parkinson's disease and multiple sclerosis symptoms, and episodes of confusion and cognitive decline in patients with dementia.
Wildfire smoke adds to the risk
The situation becomes even more critical during episodes accompanied by wildfires. Smoke releases fine particulate matter that penetrates deep into the respiratory system and triggers systemic inflammatory responses.
This process contributes to a pro-thrombotic state, in which blood tends to clot more easily, raising the risk of cerebrovascular and cardiovascular events in susceptible people.
How to protect yourself
To reduce health risks during El Niño periods and heat waves, specialists stress a set of key prevention measures.
These include drinking water constantly, avoiding intense physical exercise or direct sun exposure between 10am and 4pm, wearing light clothing, keeping spaces ventilated or air-conditioned, eating light meals and avoiding alcohol.
Symptoms such as dizziness, intense weakness, headache, nausea, cramps, heart palpitations and a drop in blood pressure indicate the body is out of balance with the heat and needs rest and fluids.
Severe symptoms, on the other hand, including acute confusion, seizures, fainting, chest pain, significant shortness of breath, loss of strength or altered speech, require immediate medical evaluation.
Carneiro said intense heat should not be seen only as a discomfort. He said that in vulnerable people it can trigger a decompensation of existing illnesses, and that hydration, controlling exposure and paying attention to warning signs are important prevention measures.
