Brandon Lee Mason, 29, was arrested near the Von's grocery store on Eastlake Parkway around 7:20 a.m. on suspicion of attempted murder, according to Chula Vista police Lt. Fritz Reber.
Mason allegedly attacked his girlfriend while she showered in her apartment at 244 Palomar St. around 12:45 a.m. Police said he fled following the attack and neighbors rushed to the naked woman's aid after hearing her
The woman said that just prior to the stabbing, she confessed to her boyfriend to being unfaithful, police said.
One of the neighbors who came to her aid, Leonardo Becerra, said he was a National Guard medic and used his emergency first-aid kit to help slow the woman's bleeding before paramedics arrived.
She was stabbed in the chest and was in critical condition, police said, adding the knife used in the attack was found at the scene.
Paramedics took her to UCSD Medical Center with life-threatening injuries, a San Diego fire-rescue dispatcher said.