Where you live in the United States may affect how quickly an ambulance shows up after a 911 call, a large new study finds.

Looking back at a year's worth of data on patients who had cardiac arrests in 46 states, researchers found that it took an average of nearly four more minutes in poorer neighborhoods compared to wealthy ones for a patient to get to the hospital, according to the study published in JAMA Network Open.

"We now have a situation where public services, including ambulance services, may not be equally accessible for all patients and may depend on income," said the study's lead author, Dr. Renee Hsia, a professor of emergency medicine and health policy at the University of California, San Francisco. "This is a very sobering finding. Most of us would hope that if we call 911, regardless of our income we would receive the same response time and transport time to the hospital. But that is not the case."

Hsia suspects that part of the reason for the disparity could be the growing trend for ambulance services to be run by private companies rather than local governments. "So along with the mission to provide excellent pre-hospital care, these services now have to answer to shareholders and must try to make a profit," Hsia said.

To take a closer look at how one's neighborhood might affect how long it takes for help to arrive, Hsia and her colleagues examined 911 response data from 2014 from the National Emergency Medical Services Information System, a voluntary national registry of EMS activations funded by the National Highway Traffic Safety Administration.

The study included information on 63,600 instances in which an EMS team was dispatched to help a person whose heart had stopped suddenly. The researchers chose to focus on cardiac arrests because they are life-or-death situations in which every additional minute your heart is stopped matters.

After accounting for factors such as urban setting, time of week and time of day, the researchers found that the average time it took for a patient to be transported to a hospital after a 911 call, was 10 percent, or 3.8 minutes, longer in poor areas, compared to wealthy ones.

Whether a neighborhood was rich or poor, few ambulances arrived within the amount of time - four minutes - presumed necessary to give the best chance at preserving brain function and life after cardiac arrest. Ambulances arrived quickly enough in just 31.4 percent of cases in wealthy neighborhoods versus 30.0 percent of the time in poorer areas. Neither is good, but it's still a difference of 4.4 percent in favor of the wealthy neighborhoods.

"Here we're talking about life or death," Hsia said. "It's hard to wrap your head around the idea that shifts happening in the system may influence our likelihood of survival - not just quality of life, but survival."

The new results don't surprise Dr. Albert Wu, an internist and professor of health policy & management at the Johns Hopkins Bloomberg School of Public Health in Baltimore. "This is one more in a growing series of studies that suggest that geography is destiny and living in a lower income location tends to go with having a shortened lifespan," Wu, who was not affiliated with the new research, told Reuters Health. "It's depressing."