Generally speaking, the most important thing to look for is a model that you can set to go off/beep once a certain heart rate threshold has been reached. Typically the models of pulse oximeters with Bluetooth (to connect to your smart phone) will have the alarm functionality. Some examples would be:
Wellue Bluetooth Pulse Oximeter
LPOW Bluetooth Pulse Oximeter,
For a pulse oximeter without a built-in alarm, the FaceLake FL400 Pulse Oximeter is one that is frequently recommended by Certified Gottman Therapists.
Some other options:
Zacurate Pro Series 500DL Fingertip Pulse Oximeter
Innovo Deluxe iP900AP Fingertip Pulse Oximeter
During a Q&A, one of our Certified Gottman Therapists also mentioned:
"There is a free app available for iPhone and Android called Instant Heart Rate which allows you to use your phone to check your heart rate. Consider downloading it, since your phone is likely to be handy most of the time."
We hope this helps make a decision!