A hot day can become a heart emergency. Here's how to stay safe.

Summer often means hiking trails, youth sports, backyard barbecues, and long days outdoors. As summer temperatures climb, the heart begins working harder long before physical activity even begins.

"Our body is working overtime just to maintain a safe core temperature," says Aishwarya Verma, MD, a clinical cardiac electrophysiology fellow at Loma Linda University Health. "For healthy individuals, that's usually manageable. But for people with underlying heart conditions, that extra demand can push the cardiovascular system beyond its limits."

When the body heats up, blood vessels widen and the heart pumps faster to move blood toward the skin, allowing heat to escape. That natural cooling process forces the heart to work harder, making hot weather especially challenging for older adults, people with heart disease, diabetes, high blood pressure, and those taking certain heart medications, including diuretics and beta blockers.

Why is dehydration hard on the heart?

One of the biggest mistakes people make during the summer is waiting until they feel thirsty before drinking water.

"If you're feeling thirsty, that means you're already mildly dehydrated," Verma says. "Be proactive rather than reactive and hydrate yourself before stepping outside."

Dehydration reduces blood volume, forcing the heart to beat faster and work harder to circulate blood throughout the body. It can also disrupt electrolyte levels that help regulate the heart's electrical system, increasing the risk of abnormal heart rhythms.

Early warning signs of dehydration include:

• dry mouth

• dark-colored urine

• headache or lightheadedness

• muscle cramps

• rapid or irregular heartbeat

• fatigue that's unusual for your activity level

Hydration isn't just about drinking more water. During prolonged outdoor activities, replacing electrolytes is equally important.

Verma recommends limiting excess caffeine and abstaining alcohol, both of which can contribute to dehydration, and choosing lower-sugar electrolyte drinks that replenish sodium, potassium, and magnesium without excessive added sugar.

Why do seemingly healthy people sometimes collapse in the summer?

Every summer, stories emerge of someone collapsing during a hike, recreational sports game, or community event. While these incidents may seem sudden, heat and dehydration can expose underlying heart conditions that have gone unnoticed.

"I think of heat like a stress test," Verma says. "It increases the heart's workload and can unmask an underlying cardiac condition that someone may not even know they have."