PHOENIX (AP) — 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, said Fire Capt. John Prato.

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.