Swiss author, Gertrud Hirschi, recommends using a yogic hand position, Apan Vayu Mudra, as an emergency measure in the case of a heart attack. She says, in her book, Mudras: Yoga in Your Hands, that this mudra regulates many heart complications and can even have a quicker effect than using nitroglycerin under the tongue.
To perform apan vayu mudra, curl the pad of the index finger to the heel of the thumb and then take the pads of the middle and ring finger to the pad of the thumb. Extend the little finger. Use both hands if possible. Then relax everywhere else as much as possible and check in with your breath. Let the breath flow naturally.
Use as needed, or for 15 minutes three times per day.
While I cannot personally vouch for its efficacy in the case of life-threatening cardiac arrest, I can say that I feel much calmer when I practice it.
Hirschi also recommends that you imagine a red rosebud in your heart and have a petal open each time you exhale. Then the petals form an ever-enlarging rosette that you can even feel rising and falling with the breath and whose scent you may even be able to discern. Suggested affirmation while practicing is “I have the time and the leisure to see beauty and enjoy the silence.”