Together, the medullary centers, chemoreceptors, and cerebral inputs constitute the so-called controller, the physiologic control system that regulates breathing. Even in individuals whose lungs and chest wall are functioning normally, various influences on the controller may cause excessive suppression or excessive stimulation of the breathing apparatus with respect to the body's need to eliminate carbon dioxide. The output of a normal control system may be influenced by environmental stresses, metabolic disturbances, hormonal changes, and drugs. These tests include the ventilatory response to exercise, the ventilatory response to increased CO 2 concentrations in inspired gas, and the ventilatory response to decreased inspired oxygen levels. Ventilatory responsiveness to hypoxemia is usually tested by having a subject breathe a gas mixture containing progressively lower oxygen concentrations.
Total ventilation is the response that is measured.
Ventilatory responsiveness to carbon dioxide is usually measured by having the subject breathe an oxygen-rich gas mixture containing progressively higher carbon dioxide concentrations. However, there is a very wide variability in CO 2 responsiveness among normal persons and even in the same individual tested on two different occasions. That is, the recorded response is minute ventilation, which is determined not only by the drive to ventilation originating in the medulla but also by the capacity of the lungs and chest wall to respond to the medullary signal. For the purpose of clinical investigation, two techniques have been developed to assess the output of the control system, even in patients with obstructive lung disease or restrictive diseases of the lungs or thoracic cage. If neuromuscular function is normal, both the P 0.1 and the diaphragmatic EMG signal will indicate normal responsiveness when the control mechanism is functioning properly, even though abnormalities of the lungs, airways, or chest wall may exist.