The basic definition of a breathing cycle, also known as a respiratory cycle, is the working together of the diaphragm and rib muscles to permit inhalation and exhalation, or breathing in and out. Contraction of numerous muscles increases the size of the chest cavity and is a process that takes place upon inhalation, during which the diaphragm — a muscular structure that separates the chest cavity from the abdominal cavity — moves downward, and the ribs move upward and outward. This expansion of the chest cavity is what causes air to flow into the lungs. After inhalation, the diaphragm and rib muscles relax. The diaphragm rises and the rib muscles move downward and inward, which causes a decrease in the size of the chest cavity and subsequently the flowing out of air from the lungs.
A breathing cycle strong enough to support life should occur 12 to 20 times per minute in adults. Rates of breathing for children and infants should be 15 to 30 times per minute and 25 to 50 times per minute, respectively. These are considered normal rates that, when maintained, should allow a person the ability to speak in full sentences without experiencing shortness of breath. He or she also should have a normal skin color, have a normal mental status and be oriented to person, place and time.
One aspect of determining adequate respiration involves evaluation of a person's breathing cycle. The presence, occurrences and quality of a breathing cycle along with adequate circulation are closely monitored by healthcare providers, because without adequate respiration and circulation, life cannot be maintained regardless of any other type of medical intervention. This importance often is referred to by emergency medical technicians (EMTs) as the ABCs: airway, breathing and circulation.
Whenever a breathing cycle is inadequate in quality or occurs too infrequently, a condition known as hypoxia develops. Hypoxia is a deficient amount of oxygen in the body's tissues, which leads to cellular damage and ultimately to death if medical intervention does not take place. Rescue breathing must be performed on a patient in such a condition in order to provide him or her with an adequate breathing cycle according to the established normal rates based on age. What is considered an appropriate breathing rate might vary slightly among health care providers, according to the medical systems in which they practice.