In April 2015, 13-year-old Walker Johnson was on the mound for his baseball team when in the fifth inning a batter made contact with one of his pitches, striking him in the chest between heartbeats and sending him into sudden cardiac death. 

With his father, Carey Johnson, watching, Walker fumbled for the ball before dropping to his knees and falling over, unconscious. Another player’s father struggled to find his heartbeat as it had stopped beating. His heart resumed beating after about 30 seconds, and Walker was able to stand on his own.

“Carey's still struggling with it, and it’s definitely affected him seeing Walker like that,” Christy Johnson, Walker’s mother, who was not at the game, told FoxNews.com.

The odds of sudden cardiac death occurring among children are 6 in 100,000, resulting in between 100 to 250 fatalities among youth athletes each year.

Most cases of commotio cordis, or aborted sudden cardiac death, occur among people who have a previous heart problem— most commonly hypertrophic cardiac myotrophy, said Lisa D’Alessandro, staff pediatric cardiologist at Texas Children’s Hospital, who treated Walker. While Walker didn’t have a heart condition prior to his injury, he was young, and 75 percent of commotio cordis cases affect people ages 18 and younger.

“We think it’s because those [people] less than 18 years of age have muscles of the chest that are less mature, and their rib cages are more compliant, which means there’s more room to bend,” D’Alessandro told FoxNews.com.

D’Alessandro said for commotio cordis to occur, four things need to happen: The person must be young, the speed of the projectile must be in the 30- to 50-mile per hour range, the impact of injury must be right over the heart on the left side of the chest, and the projectile must hit the heart within a 10- to 30-millisecond window in the cardiac cycle to create the characteristic abnormal rhythm.

“In this case, you have to be struck with a projectile but not have a penetrating blow,” she said.

A national registry for commotio cordis that began in Minnesota in the mid-’90s shows survival rates for the condition were only 10 to 30 percent at the time, but rates have improved to about 60 percent due to the initiation of CPR and defibrillation at the scene of injury.

There wasn’t a defibrillator at the fields where he was playing, but an ambulance arrived within two minutes of a 911 call to take Walker to Houston Northwest Medical Center, after which he was transferred to Texas Children’s, where he stayed for about six days.

“Most of the time when this happens, people don’t regain consciousness spontaneously. There’s a high incidence of death for this condition, and most people who survive do because they get CPR and shock from the defibrillator. [Walker is] very, very lucky,” D’Alessandro said. 

Related: Star softball player suffers brain aneurysm, collapses on field

At the hospital, doctors learned Walker experienced commotio cordis after using an electrocardiogram (EKG) to monitor his abnormal heart rhythm, and took a blood sample to detect a biomarker for heart injury, troponin. He also had a bruise on his heart, which is healing.

Christy said she began journaling the first night at the hospital and tried to reflect on the significance of her family’s experience.

“I started writing stuff down and started thinking, ‘Why did this happen, and what is this supposed to teach us, and what can I do, and how can I make a difference?’” she said.

The second night in the hospital, Christy started a Go Fund Me account to buy defibrillators for athletic departments in the Johnsons’ school district. So far, the fund has raised about $7,000. The Johnsons also plan to buy defibrillators for the Memorial Spring Branch Sports Association, where Walker began playing baseball when he was 5, and the facility where Walker plays on his year-round team, the Houston Warriors. Christy is also looking into purchasing protective vests for athletic facilities.

D’Alessandro said 75 percent of commotio cordis cases occur while playing sports and that 50 percent occur during competitive sports. In the United States, baseball is involved in 38 percent of all reported cases, whereas cricket and hockey are the most commonly associated in countries where those sports are more popular than baseball.

To prevent his heart rate from climbing too much, Walker can’t play sports and has to limit physical activity until six months have passed since his injury. He can’t do much more than walk around the block, Christy said.

D’Alessandro has Walker on two medications: a beta blocker to protect his heart from an abnormal heart rhythm and an ACE inhibitor to help remodel his heart muscle.

“I expect that he is going to completely recover,” she said. “At his follow-up appointment, his EKG and the imaging we did of his heart looked really good.”

Meantime, Walker passes his free time by playing Madden and Call of Duty on Xbox, watching TV shows like “The Walking Dead,” and going to friends’ houses.

When asked if he’s afraid to return to the field, Walker gave a definitive no.

“The only thing I’m scared about is not being able to succeed,” Walker Johnson told FoxNews.com. “I’m not afraid of getting hit again; I’m just afraid of not being able to play what I want, or do what I want to.”

Walker will get the chance to play again June 15, his 14th birthday, when he’ll throw the first pitch at Minute Maid Park, where the Houston Astros will play the Colorado Rockies.

“I’ve always dreamed of being able to do that,” Walker said. “My dream as a kid was always to play baseball as much as I can, and I always wanted to become a player in the MLB (Major League Baseball), and I completely think that that’s possible. But I mean, I don’t really know the future, so I can’t really figure out what I want to do at the moment, but I just know that it’s gonna be great.”

Click to learn more about Walker's Heart Warriors.