Named AliveCor, it is a smartphone cover with two metal plates on the back.
It can be used with an app on the phone to conduct an electrocardiogram (ECG) wirelessly which can be sent from the smartphone to the doctor.
The device, however, is not a substitute for a full ECG because it only tests the heart's rhythm and rate.
But the device makes it easy for parents to read the results and alert the doctor if there is an anomaly.
"It is so easy for them to upload that ECG and send it to me and just like they are sending it from their smartphone, I am reading on my smartphone," Jennifer Silva, a pediatric cardiologist at St Louis Children's Hospital, was quoted as saying in a fox2now report.
The device, approved by the US Food and Drug Administration (FDA), is being used to monitor 37 children ranging in age from two months to 18 years old.
"We can take care of them faster because as soon as parents can upload the data, I can give them the answer on how to best care for themselves," Silva added.