It depends how you define respiratory impairment. Nerves supplying muscles of breathing come from differing areas of the spine. Notably as another person mentioned the C3, C4, C4 muscles supply the diaphragm. The diaphragm accounts for roughly 60% of your respiratory effort. Other muscle of respiration include accessory muscles of breathing such as intercostal muscles, neck muscles and upper thoracic wall muscles. A lower cervical spine injury ie. C6 would impair all of those muscle but leave the diaphragm. A person with a spinal injury but intact C6 and below has significant impairment of their breathing. An important consideration is the ability to cough, without being able to effectively cough secretions build up and recurrent lower respiratory tract infections can occur. The main muscle to support coughing are abdominal muscles supplied by thoracic nerves. Other factors include the ability to regulate our sympathetic nervous system. A paraplegic above T6 may have no sensation below T6 such that a painful stimulus will result in crazy amplification of the sympathetic nervous system below T6. This can result in this like acute pulmonary oedema from severe pulmonary hypertension. You’ll be fine cracking your back though, lol.