bubble_chart Overview

Respiratory acidosis is a hypercapnia characterized by a primary increase in PCO₂ and a decrease in pH.

bubble_chart Etiology

It is caused by impaired alveolar ventilation function. Common causes include: a. Respiratory center depression, such as overdose of anesthetics; b. Airway obstruction, such as laryngospasm, bronchospasm, respiratory tract burns and foreign bodies, drowning, neck hematoma or mass compressing the trachea, etc.; c. Pulmonary diseases, such as shock lung, pulmonary edema, atelectasis, pneumonia, etc.; d. Chest injuries: such as surgery, trauma, pneumothorax, pleural effusion, etc.

bubble_chart Pathogenesis

1. Respiratory center depression Some lesions of the central nervous system, such as medullary tumors, bulbar poliomyelitis, encephalitis, meningitis, vertebral artery embolism or thrombosis, increased intracranial pressure, and craniocerebral trauma, can inhibit the activity of the respiratory center, reducing ventilation and causing CO₂ accumulation. Additionally, certain drugs like anesthetics and sedatives (e.g., morphine, sodium barbital) can suppress respiration, and excessive doses may also lead to insufficient ventilation. As previously mentioned, carbonic anhydrase inhibitors like acetazolamide can induce metabolic acidosis. They also inhibit carbonic anhydrase in red blood cells, reducing the release of CO₂ from red blood cells in the lungs, thereby increasing arterial blood Pco₂. This medication should be used cautiously in patients prone to acidosis.

2. Respiratory nerve and muscle dysfunction Seen in conditions such as poliomyelitis, acute infectious polyneuritis (Guillain-Barré syndrome), botulism, myasthenia gravis, hypokalemia or familial periodic paralysis, and high spinal cord injuries. In severe cases, respiratory muscles may become paralyzed.

3. Thoracic abnormalities Thoracic abnormalities that affect respiratory movement commonly include kyphoscoliosis, flail chest, ankylosing spondylitis, and the obesity hypoventilation syndrome (Pickwickian syndrome).

4. Airway obstruction Common causes include foreign body obstruction, laryngeal edema, and aspiration of vomitus.

5. Diffuse pulmonary diseases These are the most common causes of respiratory acidosis, including chronic obstructive pulmonary disease, bronchial asthma, and severe interstitial lung diseases. These conditions can severely impair alveolar ventilation.

6. Excessive CO₂ inhalation This refers to inhaling air with excessively high CO₂ concentrations, such as in confined spaces with poor ventilation like tunnels or tanks. In such cases, alveolar ventilation is not reduced.

bubble_chart Clinical Manifestations

In respiratory acidosis, the increase of H₂CO₃ in the blood cannot be compensated by the lungs, so the regulation primarily relies on the buffer system and renal acid excretion and alkali retention. Therefore, the clinical manifestations mainly include: a. Respiratory distress, insufficient ventilation, shortness of breath, cyanosis, chest tightness, headache, etc. b. Worsening acidosis leading to changes in mental status, such as drowsiness, confusion, delirium, unconsciousness, etc. c. Excessive accumulation of CO₂ can cause a drop in blood pressure and may lead to sudden ventricular fibrillation (due to Na⁺ entering cells and K⁺ moving out of cells, resulting in acute hyperkalemia). d. Laboratory findings: - In acute or decompensated cases, blood pH decreases, PCO₂ increases, while CO₂CP, BE, SB, and BB are normal or slightly elevated. - In chronic respiratory acidosis or compensated cases, the pH decrease is not significant, PCO₂

is elevated, and CO₂CP, BE, SB, and BB are all increased. - Blood K⁺ may be elevated.

bubble_chart Diagnosis

The patient has a history of impaired respiratory function and presents with symptoms of respiratory acidosis, which should raise suspicion of respiratory fatigue-induced acidosis. In acute respiratory acidosis, blood gas analysis shows a significant decrease in blood pH, an increase in PCO₂, and normal plasma [HCO₃^-]. In chronic respiratory acidosis, the blood pH decrease is less pronounced, PCO₂ is elevated, and plasma [HCO₃^-] shows an increase.

bubble_chart Treatment Measures

1. Actively prevent and treat the primary disease causing respiratory acidosis.

2. Improve alveolar ventilation to eliminate excess CO₂. Depending on the situation, measures such as tracheotomy, artificial respiration, relieving bronchospasm, dispelling phlegm, and oxygen therapy may be taken. The oxygen concentration should not be too high to avoid suppressing respiration.

Artificial respiration should be moderate because, in respiratory acidosis, H₂CO₃ in the NaHCO₃/H₂CO₃ ratio primarily increases, while NaH₂CO₃ compensatorily increases secondarily. If ventilation is excessive, plasma Pco₂ rapidly decreases, while NaHCO₃ remains at a high level, leading the patient to develop extracellular alkalosis, and the same applies to cerebrospinal fluid. This can cause hypokalemia, decreased plasma Ca⁺⁺, extracellular alkalosis in the central nervous system, unconsciousness, or even death.

3. Generally, alkaline drugs are not administered unless the pH drops severely, as the use of sodium bicarbonate can only temporarily alleviate acidemia and is not suitable for prolonged use. In severe cases of acidosis, such as when the patient is unconscious or has arrhythmias, THAM can be used to neutralize excessive [H⁺

]. NaHCO₃ solution may also be used, but it must be ensured that adequate alveolar ventilation is maintained. This is because administering NaHCO₃ to correct the excessive [H⁺] in respiratory acidosis can generate CO_2, and if it cannot be fully expelled, the CO₂ concentration will rise.