Paradoxical breathing is seen when inhalation results in a decreasing, or asymmetrical, chest movement rather than a normal chest expansion. This causes poor ventilation of the lungs, carbon dioxide buildup and oxygen depletion in the blood. Certain kinds of chest injury, thoracic surgery and lung disease can cause paradoxical breathing. It is not always treatable and may in part define a late stage of chronic obstructive pulmonary disease, often called COPD. 
Normal Breathing 
Breathing is largely an automatic act, controlled in the brainstem and mediated by the muscles of respiration. The dome-shaped diaphragm is the primary muscle of inspiration. When it contracts, it descends in the chest and enlarges the thoracic cavity. At the same time, it compresses the abdominal contents, pushing the abdominal wall outward. Muscles in the rib cage and neck expand the thorax during inspiration, especially the parasternals, which run obliquely from sternum to ribs, and the scalenes, which run from the cervical vertebrae to the first two ribs.
During inspiration, as these muscles contract, the thorax expands. Intrathoracic pressure decreases, drawing air through the tracheobronchial tree into the alveoli, or distal air sacs, and expanding the lungs. Oxygen diffuses into the blood of adjacent pulmonary capillaries, and carbon dioxide diffuses from the blood into the alveoli.
After inspiratory effort stops, the expiratory phase begins. The chest wall and lungs recoil, the diaphragm relaxes and rises passively, air flows outward, and the chest and abdomen return to their resting positions.
Normal breathing is quiet and easy—barely audible near the open mouth as a faint whish. When a healthy person lies supine, the breathing movements of the thorax are relatively slight. In contrast, the abdominal movements are usually easy to see. In the sitting position, movements of the thorax become more prominent.
During exercise and in certain diseases, extra work is required to breathe, and accessory muscles join the inspiratory effort. The sternomastoids are the most important of these, and the scalenes may become visible. Abdominal muscles assist in expiration.
When airways are obstructed, it is hard to move air in and out of your lungs. A strong inhalation attempts to lower your diaphragm and raise your ribs. But if you are choking, inward air flow meets a solid blockage. If air cannot enter your lungs, the diaphragm pulls down and bulges the belly outward, while the rib cage is sucked inward.
On exhalation, the belly muscles contract and push up under the lungs. But if the air in the lungs cannot escape, the chest simply expands. The net result is little or no breathing–suffocation.
Chronic Obstructive Pulmonary Disease
Chronic obstructive pulmonary disease, COPD, partially obstructs the airways, but does allow some air in and out of the lungs, but it’s like breathing through a soda straw. A strong effort to inhale or exhale must be made for a second or two before much air actually moves. In emphysema, one kind of COPD, the obstruction is worse on exhalation, while in chronic bronchitis, another kind of COPD, both inhalation and exhalation may be hard.
The result is that air flow is out of phase with effort, and chest movement is unsynchronized, or paradoxical, to breathing efforts.
Crushed Chest Injury
If many ribs are broken on both sides of the chest, the rib cage lacks its normal, rigid structure. This is called a flail chest. An effort to inhale collapses the chest in on itself before air can enter the lungs. On exhalation, the rib cage may be pushed outward instead of contracting to push the air out. This paradoxical breathing movement causes poor ventilation and can lead to death by suffocation.
One-Sided Crushed Chest
An injury that crushes only one side of the chest causes a special kind of paradoxical breathing called “pendeluft,” German for “pendulum air.” The intact side of the chest, on inhalation, pulls air out of the flailing side and exhalation pushes the healthy side’s stale air back into the flailing lung. The paradoxical air “pendulum” only switches stale air from one lung to the other.
In severe cases of both one-sided and whole-chest crush injury, only a mechanical respirator can internally splint the broken ribs–by inflating the lungs–and force a more normal air movement pattern. This is called “internal pneumatic fixation.”
When a surgeon opens a chest and exposes both lungs to atmospheric pressure, the mild suction that normally holds the lungs open inside the rib cage is lost. Both lungs collapse. The anesthesiologist is then responsible for inflating and deflating both lungs for each breath.
But if only one side of the chest is opened, one lung collapses and the other remains inflated inside the rib cage. This can cause surgical pendeluft, paradoxical air exchange between the lungs, unless controlled by the anesthesiologist’s respirator.
Obstructive Sleep Apnea
In obstructive sleep apnea (OSA), a person’s tongue and portions of the throat collapse during sleep–very much like choking on a foreign object. The person cannot inhale until repeated efforts become strong enough to create a life-saving gasp, sometimes only two or three times per minute. The efforts pull the diaphragm down and suck the ribs inward, the opposite of normal respiration. This paradoxical breathing helps diagnose OSA. Continuous positive airway pressure, called CPAP, applied by face mask, forces the throat open to overcome paradoxical breathing.
- Bates’ Guide to Physical Examination and History Taking, 10th Edition