Respiratory failure, acute

14 Respiratory failure, acute




Overview/pathophysiology


Acute respiratory failure (ARF) develops when the lungs are unable to exchange O2 and CO2 adequately. Clinically, respiratory failure exists when Pao2 is less than 50 mm Hg with the patient at rest and breathing room air. Paco2 of 50 mm Hg or more or pH less than 7.35 is significant for respiratory acidosis, which is the common precursor to ARF.


Although a variety of disease processes can lead to development of respiratory failure, four basic mechanisms are involved.




Alveolar hypoventilation:


Occurs secondary to reduction in alveolar minute ventilation. Because differential indicators (cyanosis, somnolence) occur late in the process, the condition may go unnoticed until tissue hypoxia is severe.



Ventilation-perfusion mismatch:


Considered the most common cause of hypoxemia. Normal alveolar ventilation occurs at a rate of 4 L/min, with normal pulmonary vascular blood flow occurring at a rate of 5 L/min. Normal ventilation/perfusion ratio is 0.8:1. Any disease process that interferes with either side of the equation upsets physiologic balance and can lead to respiratory failure as a result of reduction in arterial O2 levels.



Diffusion disturbances:


Processes that physically impair gas exchange across the alveolar-capillary membrane. Diffusion is impaired because of the increase in anatomic distance the gas must travel from alveoli to capillary and capillary to alveoli.



Right-to-left shunt:


Occurs when the previously mentioned processes go untreated. Large amounts of blood pass from the right side of the heart to the left and out into the general circulation without adequate ventilation; therefore blood is poorly oxygenated. This mechanism occurs when alveoli are atelectatic or fluid filled, inasmuch as these conditions interfere with gas exchange. Unlike the first three responses, hypoxemia secondary to right-to-left shunting does not improve with O2 administration because the additional Fio2 is unable to cross the alveolar-capillary membrane.

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Jul 18, 2016 | Posted by in NURSING | Comments Off on Respiratory failure, acute

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