RESPIRATORY ACID-BASE ABNORMALITIES

26 RESPIRATORY ACID-BASE ABNORMALITIES



James H. Meinkoth, Rick L. Cowell, Karen Dorsey



1 Define hypercapnia and hypocapnia.


Hypercapnia is an increase in the partial pressure of carbon dioxide (PCO2) of the blood. Hypocapnia is a decrease in PCO2 of the blood. PCO2 reflects alveolar ventilation. Increasing ventilation will decrease PCO2.



2 What is the difference between hypoxia and hypoxemia?


Hypoxia denotes decreased oxygen delivery to tissues, whereas hypoxemia denotes a reduction in the amount of oxygen dissolved in the blood (decreased PaO2). Partial pressure of arterial oxygen (PaO2) reflects the ability of the pulmonary system to oxygenate the blood. Hypoxemia often leads to hypoxia, depending on the severity of the hypoxemia.



3 Is it possible to have hypoxia without having hypoxemia?


Hypoxia can occur without hypoxemia. With severe anemia (but normal pulmonary function), there is a marked decrease in the total amount of oxygen dissolved in the blood, but PaO2 and the percent saturation of hemoglobin will be normal. As the blood perfuses the tissues and the dissolved oxygen is used, there will be insufficient oxygen “reserves” because of the reduced amount of hemoglobin available to carry oxygen. Poor tissue perfusion can also lead to tissue hypoxia with a normal PaO2.



4 How is it possible to have hypoxemia without an increase in PCO2, since both hypoxemia and hypercapnia result from inadequate gas exchange?


First, oxygen does not diffuse across alveolar membranes as easily as does carbon dioxide. Therefore, during the development of pulmonary disease that interferes with gas exchange, O2 diffusion may be impaired while CO2 diffusion is normal. This will lead to hypoxemia with a normal Pco2. Also, respiration is normally controlled by Pco2-induced changes in hydrogen ion concentration [H+]. Therefore, in the case of mild hypoxemia with a normal Pco2, there is no stimulus to increase respiration, and the hypoxemia is allowed to remain.



5 What is a respiratory acidosis?


A respiratory acidosis is a blood gas abnormality caused by a primary hypercapnia.



6 What changes on a blood gas analysis characterize a respiratory acidosis?


Respiratory acidosis is characterized by a decreased pH along with an increased Pco2. PaO2 is also decreased (hypoxemia). There is usually a compensatory increase in bicarbonate concentration [HCO3−].



7 What general mechanisms result in respiratory acidosis?


Respiratory acidosis is ultimately the result of decreased alveolar ventilation (gas exchange across the alveolar wall). Decreased alveolar ventilation may result from the following:


a. Any factor that interferes with neurologic control of respiration (e.g., drugs, central nervous system disease)

b. Mechanical obstruction (e.g., foreign body)

c. Restrictive pulmonary disease (e.g., pneumothorax, pleural effusion, masses)

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  4. METABOLIC ACID-BASE ABNORMALITIES

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Aug 26, 2016 | Posted by in INTERNAL MEDICINE | Comments Off on RESPIRATORY ACID-BASE ABNORMALITIES

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