Gradually increase the length of your breath, but do not practice breath retention except under the careful supervision of a teacher. (yogainternational.com)
For example, bandhas should not be practiced with breath retention during a rapid heartbeat or a shortened breath. (sonima.com)
Starting with basic pranayama breath retentions, like alternate nostril breathing and full yogic breath, this encourages the stillness of mind that leads us in to meditation. (mindbodygreen.com)