The heat was unrelenting. So were the calls requesting aid for maladies resulting from the 100-plus degree temperatures that have suffocated Washington, D.C., and much of the Eastern portion of the country, over the past few days.

For Sharon Moulton, a captain with the emergency medical services in Washington, that meant responding to about 30 calls on Thursday, crisscrossing the neighborhoods of Northwest Washington to render aid.

“You could feel it on your skin, it was almost like a sunburn starting,” Capt. Moulton said. “Even the breeze was hot.”

The heat, under a cloudless sky, rendered a man unconscious on the sidewalk in Columbia Heights.

Capt. Moulton slowly swiped a digital thermometer across the man’s forehead and tried to rouse him.

“How long have you been outside today?” the E.M.S. supervisor asked, and the question was repeated in Spanish by a police officer. Both attempts proved futile; medics suspected the man was suffering from the results of too much alcohol consumption, sky-high temperatures and attire that included long pants and a hoodie.

“Hey, hey we’re going to get this off of you,” Capt. Moulton tried to tell the man, who finally moved his head but showed no signs of comprehension.

Two thermometers read his body temperature as being close to normal, but in the extreme heat medics took no chances, placing cold packs under his T-shirt until an ambulance arrived to take him to a hospital.

The patient was one of dozens that city medics were called to examine for suspected heat-related maladies and the associated morbidity concerns that extreme heat can trigger.

“Heat drives a lot of other problems,” said John A. Donnelly, the chief of the city’s fire and emergency medical services department. “It may not be heat exhaustion, it can be the heat causes air quality problems that exacerbates an asthma problem.”

Medical emergencies had increased the department’s call volume by 20 percent on Wednesday, Chief Donnelly said.

“We predicted heat,” he said. “We didn’t predict this heat.”

On Thursday, the city responded to 748 calls, including 551 for emergency medical help, 145 for fire and 52 for other emergencies, officials said, noting it was the highest call volume day in seven years.

The rising heat index adds another layer of complexity as city leaders and emergency medical workers prepared for tens of thousands of visitors to descend on Washington to celebrate the 250th anniversary of the country’s founding.

“The Fourth of July is always the busiest day of the year for a fire department,” Chief Donnelly said. “One of the things we’re doing is making sure we have water everywhere as we get our teams ready to go.”

The chief said that organizers planned to provide extra water to spectators and had been erecting shade tents for the long lines expected at magnetometer security checkpoints along Constitution Avenue.

“Those things are potentially lifesaving changes,” he said.

Officials had rehearsed scenarios for any number of calamities, Chief Donnelly said, adding that he felt confident city and local officials could handle challenges in Washington’s neighborhoods as well as on the National Mall.

“The scariest thing would be a storm coming through right before the fireworks,” he said. “That’s probably going to be the biggest crowd. That would be scary. As a team, we have to work together to get them to safety.”