Introduction
Acute respiratory failure occurs when the lungs do not release sufficient oxygen in the bloodstream, thus preventing the organs from functioning optimally. Acute respiratory failure also occurs due to insufficiency of the lungs to remove carbon dioxide from the blood, as a result of which carbon dioxide content increases in the blood. Thus respiratory failure occurs when the alveolar blood capillaries cannot exchange carbon dioxide and oxygen.
What Are the Types of Respiratory Failure?
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Hypoxemic Respiratory Failure - This occurs when the oxygen is less in the blood.
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Hypercapnic Respiratory Failure - This occurs when more carbon dioxide is in the blood.
What Is the Pathophysiology of Acute Respiratory Failure?
The respiratory components include the airways, alveoli (air sacs in the lungs), the central nervous system (CNS), peripheral nervous system (PNS), muscles of respiration, and the chest wall. Abnormality in any one of these respiratory components results in respiratory failure. Reduction in blood flow following cardiogenic, hypovolemic, and septic shock can result in respiratory failure. In healthy and normal conditions, ventilatory capacity or maximum breathing capacity (MBC) is much higher than the ventilatory demand. Respiratory failure can occur due to decreased ventilatory capacity or high ventilatory demand, or both. Ventilatory capacity is the maximum spontaneous breathing capacity that is maintained without causing fatigue to the respiratory muscles. Ventilatory demand is the ventilation amount that will result in stable partial pressure of carbon dioxide (PaCO2). Ventilatory capacity reduces due to diseases involving the respiratory pathway, and ventilatory demand is increased due to work involving high breathing.
Respiration involves three processes:
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Oxygen transfers across the alveolus (air sacs of the lungs).
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Oxygen transportation to the tissues.
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The alveolus removes carbon dioxide from the lungs into the external environment.
When one of these three processes does not function properly, it can lead to respiratory failure. Therefore, knowledge about pulmonary gas exchange is essential to understanding acute respiratory failure's pathophysiology.
Gas Exchange - The exchange of oxygen and carbon dioxide between the alveolus and the blood occurs at the alveolar-capillary unit of the lungs. Once the oxygen is mixed with the blood, it binds to the hemoglobin. Each hemoglobin contains four binding sites for oxygen. Normally one gram of hemoglobin binds with 1.36 ml of oxygen; however, partial pressure of oxygen in the blood determines the amount of oxygen that will bind with hemoglobin. Carbon dioxide in the blood is mainly present in three forms; as a simple solution, bicarbonates, and a carbamino compound in the hemoglobin. No alveolar-arterial oxygen tension (PO2) is seen in perfect gas exchange as a balance is maintained between blood flow and ventilation (airflow into and out of the alveoli). But such ideal conditions are not seen even in normal and healthy lungs. When blood flows into the alveolar capillaries (perfusion), a few alveoli are underventilated, and some are overventilated.
Similarly, in the case of alveolar ventilation, some alveolar-capillary units are under-perfused, while a few are over-perfused. Alveoli with optimal ventilation but improper perfusion have a high ventilation-to-perfusion ratio. They act like dead space (air volume that does not participate in gaseous exchange) and are named high V/Q units. Alveolus with normal perfusion but not properly ventilated act as shunts and are named low V/Q units.
Alveolar Ventilation - In normal conditions, carbon dioxide produced by the body equals the amount of carbon dioxide removed from the lungs. The lung efficiency for respiration is also measured by evaluating the alveolar-arterial partial pressure of oxygen (PO2) gradient. Even normal and healthy lungs have mismatched V/Q (ventilation-to-perfusion ratio). They also show a minor amount of right-to-left shunt (the deoxygenated blood passes from the right side of the heart to systemic circulation without participating in the gaseous exchange), and the partial pressure of oxygen in the alveolus is higher than the partial pressure of oxygen in the arterial blood. The alveolar-arterial partial pressure of oxygen (PO2) gradient of more than 15 to 20 mm Hg suggests pulmonary disease due to hypoxemia (less oxygen than normal in the arterial blood).
Hypoxemic Respiratory Failure - In hypoxemic respiratory failure, the V/Q (ventilation to perfusion ratio) is mismatched, and there is a right-to-left shunt. As a result, the alveolar-arterial partial pressure of oxygen (PO2) gradient increases by more than 15 mm Hg.
V/Q (Ventilation to Perfusion Ratio) Mismatch- V/Q mismatch is one of the most common reasons for hypoxemia, a low oxygen level in the arterial blood in patients. The alveoli may show low V/Q to high V/Q values in diseased conditions. Low V/Q values suggest hypoxemia and hypercapnia, and high V/Q values suggest an issue with the ventilation process of the lungs without affecting the gaseous exchange. A decrease in the V/Q ratio suggests a reduction in the ventilation process due to lung or airway disease. It may also be present due to over-perfusion, as seen in pulmonary embolism; however, the ventilation remains normal. Treating the patient with 100 percent oxygen removes low V/Q alveolar units, preventing hypoxemia. The partial pressure of carbon dioxide (PaCO2) is usually not affected.
Shunt - Here, the venous or deoxygenated blood mixes with oxygenated blood without passing through the alveolus. Hypoxemia remains even after inhalation with 100 percent oxygen. Shunts exist in normal and healthy lungs. A right-to-left shunt is seen in atrial and ventricular septal defects and arteriovenous lung malformation. Hypoxemia caused by shunts does not get corrected by oxygen treatment.
Hypercapnic Respiratory Failure - Hypercapnic respiratory failure is mainly seen in conditions that affect the central nervous system, like CNS depression, due to drugs and neuromuscular disorders affecting the muscles of respiration. It is corrected with oxygen treatment. Any reduction in alveolar ventilation, either due to a decrease in overall ventilation or higher dead space ventilation, causes an increase in partial pressure of carbon dioxide resulting in hypercapnic respiratory failure.
What Is the Treatment of Acute Respiratory Failure?
Acute respiratory failure is a medical emergency. Medications are prescribed to help with breathing. In addition, oxygen is provided to patients with troubled breathing. For patients on ventilators, a tracheostomy is performed to make an artificial airway for breathing.
Conclusion
Respiratory failure is a medical emergency in which the lungs fail to perform the gaseous exchange. As a result, it causes airway obstruction and can lead to difficulty breathing and drowsiness. The main treatment aims to support and stabilize the patient's breathing.
