PHOENIX — The season’s first heat wave is already baking the Southwest with triple-digit temperatures as firefighters in Phoenix — America’s hottest big city — employ new tactics in hopes of saving more lives in a county that saw 645 heat-related deaths last year.

Starting this season, the Phoenix Fire Department is immersing heatstroke victims in ice on the way to area hospitals. The medical technique, known as cold-water immersion, is familiar to marathon runners and military service members and has also recently been adopted by Phoenix hospitals as a go-to protocol, Fire Capt. John Prato said.

Prato demonstrated the method earlier this week outside the emergency department of Valleywise Health Medical Center in Phoenix, packing ice cubes inside an impermeable blue bag around a medical dummy representing a patient. He said the technique could dramatically lower body temperature in minutes.

“Just last week, we had a critical patient that we were able to bring back before we walked through the emergency room doors,” Prato said. “That’s our goal — to improve patient survivability.”

The heatstroke treatment has made ice and human-sized immersion bags standard equipment on all Phoenix fire department emergency vehicles. It is among measures the city adopted this year as temperatures and their human toll soar ever higher. Phoenix for the first time is also keeping two cooling stations open overnight this season.

Emergency responders in much of an area stretching from southeast California to central Arizona are preparing for what the National Weather Service said would be “easily their hottest” weather since last September. The high forecast for Wednesday in Las Vegas was just one degree below the June 5 record high of 109 degrees Fahrenheit (42.8 Celsius) set in 2016.

“There’s a very high-pressure system over the Southwest that’s bringing the first heat wave of the summer to the region,” said Sean Benedict, a lead meteorologist for the weather service based in Phoenix. He said in addition to Arizona, the extreme heat will bake areas of eastern California, northern California and Nevada and even parts of southern Texas over the next few days.

Excessive heat warnings were issued for Wednesday morning through Friday evening for parts of southern Nevada and Arizona, with highs in subsequent days expected to top 110 degrees F (43.3 C) in Las Vegas and Phoenix. The unseasonably hot weather was expected to spread northward and make its way into parts of the Pacific Northwest by the weekend.

Officials in Phoenix Wednesday morning closed the popular Camelback Mountain and Piestewa Peak Trails from 9 a.m. to 5 p.m. until the weekend.

In southern New Mexico, highs were expected to reach triple digits, prompting the city of Las Cruces on Wednesday to activate its cooling centers to provide residents with temporary shelter from the blistering heat. A heat advisory for the area will be in effect through Thursday.

The Albuquerque mayor on Wednesday was announcing this year’s “Operation Cooldown,” which includes plans for cooling centers and the use of sprinklers at city parks for kids to keep cool.

The City Council in Arizona’s second-largest city of Tucson this week adopted a heat protection ordinance to ensure that city employees have access to cool water, shade and extra breaks at their workplaces. Members also moved to better protect the community east of Phoenix from extreme heat by approving a plan to cool people’s homes, community centers and neighborhoods.

The action comes after Pima County, home to Tucson, last year saw 176 heat-related deaths and another 51 such deaths in the five additional rural counties that the medical examiner handles.

Officials in Maricopa County were stunned earlier this year when final numbers showed 645 heat-related deaths in Arizona’s largest county, a majority of them in Phoenix. The most brutal period was a heat wave with 31 subsequent days of temperatures of 110 degrees F (43.3 C) or higher, which claimed more than 400 lives.

“We’ve been seeing a severe uptick in the past three years in cases of severe heat illness,” said Dr. Paul Pugsley, medical director of emergency medicine with Valleywise Health. Of those, about 40% do not survive.

Cooling down patients long before they get to the emergency department could change the equation, he said.

The technique “is not very widely spread in non-military hospitals in the U.S., nor in the prehospital setting among fire departments or first responders,” Pugsley said. He said part of that may be a longstanding perception that the technique’s use for all cases of heatstroke by first responders or even hospitals was impractical or impossible.

Pugsley said he was aware of limited use of the technique in some places in California, including Stanford Medical Center in Palo Alto and Community Regional Medical Center in Fresno, and by the San Antonio Fire Department in Texas.

Banner University Medical Center in Phoenix embraced the protocol last summer, said Dr. Aneesh Narang, assistant medical director of emergency medicine there.

“This cold-water immersion therapy is really the standard of care to treat heatstroke patients,” he said.