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Hypercapnia means excess carbon dioxide in the body fluids.
One might suspect, on first thought, that any respi-ratory condition that causes hypoxia would also cause hypercapnia. However, hypercapnia usually occurs in association with hypoxia only when the hypoxia is caused by hypoventilation or circulatory deficiency. The reasons for this are the following.
Hypoxia caused by too little oxygen in the air, toolittle hemoglobin, or poisoning of the oxidative enzymes has to do only with the availability of oxygenor use of oxygen by the tissues. Therefore, it is readily understandable that hypercapnia is not a concomitant of these types of hypoxia.
In hypoxia resulting from poor diffusion through the pulmonary membrane or through the tissues, serious hypercapnia usually does not occur at the same time because carbon dioxide diffuses 20 times as rapidly as oxygen. If hypercapnia does begin to occur, this immediately stimulates pulmonary ventilation, which corrects the hypercapnia but not necessarily the hypoxia.
Conversely, in hypoxia caused by hypoventilation, carbon dioxide transfer between the alveoli and the atmosphere is affected as much as is oxygen transfer. Hypercapnia then occurs along with the hypoxia. And in circulatory deficiency, diminished flow of blood decreases carbon dioxide removal from the tissues, resulting in tissue hypercapnia in addition to tissue hypoxia. However, the transport capacity of the blood for carbon dioxide is more than three times that for oxygen, so that the resulting tissue hypercapnia is much less than the tissue hypoxia.
When the alveolar PCO2 rises above about 60 to 75 mm Hg, an otherwise normal person by then is breathing about as rapidly and deeply as he or she can, and “air hunger,” also called dyspnea, becomes severe.
If the PCO2 rises to 80 to 100 mm Hg, the person becomes lethargic and sometimes even semicomatose. Anesthesia and death can result when the PCO2 rises to 120 to 150 mm Hg. At these higher levels of PCO2, the excess carbon dioxide now begins to depress res-piration rather than stimulate it, thus causing a vicious circle: (1) more carbon dioxide, (2) further decrease in respiration, (3) then more carbon dioxide, and so forth—culminating rapidly in a respiratory death.
Dyspnea means mental anguish associated with inabil-ity to ventilate enough to satisfy the demand for air. A common synonym is air hunger.
At least three factors often enter into the develop-ment of the sensation of dyspnea. They are (1) abnor-mality of respiratory gases in the body fluids, especially hypercapnia and, to a much less extent, hypoxia; (2) the amount of work that must be performed by the res-piratory muscles to provide adequate ventilation; and (3) state of mind.
A person becomes very dyspneic especially from excess buildup of carbon dioxide in the body fluids. At times, however, the levels of both carbon dioxide and oxygen in the body fluids are normal, but to attain this normality of the respiratory gases, the person has to breathe forcefully. In these instances, the forceful activity of the respiratory muscles frequently gives the person a sensation of dyspnea.
Finally, the person’s respiratory functions may be normal and still dyspnea may be experienced because of an abnormal state of mind. This is called neurogenicdyspnea or emotional dyspnea. For instance, almostanyone momentarily thinking about the act of breath-ing may suddenly start taking breaths a little more deeply than ordinarily because of a feeling of mild dyspnea. This feeling is greatly enhanced in people who have a psychological fear of not being able to receive a sufficient quantity of air, such as on entering small or crowded rooms.
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