There are two types of breathing techniques that can be done to help you relax and lower your heart rate. They are the Pursed Lip Breathing technique & Abdominal Breathing.
Pursed Lip Breathing
Pursed lip breathing sends a signal to the body to relax and is a great way to quickly lower your heart and respiratory rate. In addition to helping you relax, it causes the tiny air sacs deep in your lungs to fully expand, which allows more oxygen to enter your bloodstream. Pursed lip breathing has been used in medicine to ease respiratory distress.
The Pursed Lip Breathing technique can be referenced at the link below from the American Lung Association.
Abdominal breathing is normal breathing done with the belly. You breathe with your diaphragm muscles and not with your chest muscles. The chest remains still.
For this method, put one hand on your chest and one hand just above your belly button. Concentrate your attention on your lower abdomen, just above your belly button.
Gently breathe in and feel your belly expand. Gently breathe out and feel your belly contract.
Feeling dizzy while breathing with the pacer?
Breathing with too much effort can make us feel dizzy. Breathing should be effortless and without unnatural pauses. Dizziness can mean that inhalation is too deep, or air intake is directed into the chest, rather than the abdomen.
Breathe naturally, easily and relaxed. Do not breathe too deeply or try too hard. If you overexert or breathe too deeply, you may hyperventilate. You may feel a little dizzy or light headed, and in that case, slow down your breaths. Try the pursed-lip and abdominal breathing techniques focusing on effortlessness.