Hypercapnia with hypoxemia

Hypercapnic respiratory failure is due to failure of the ventilatory pump caused by acute (drug overdose, acute neuromuscular diseases) or chronic (chest wall abnormalities, chronic neuromuscular diseases) disorders. It is characterized by alveolar hypoventilation, which leads to hypercapnia with coexistent, usually mild, hypoxemia. The central drive may be globally reduced with the fall in Pa02 resulting from the increase in alveolar CO2. More commonly, the drive remains high, but the mechanical load on the respiratory systan is too great or the capacity of the muscles too low to ensure efficient CO2 elimination (Fig. 1). 

Nocturnal hypoventilation, which persists during the day in obese patients with nocturnal apneas and hypoventilation may be associated with COPD, defining the overlap syndrome (3). In 264 OSAS patients (32), 30 had coexistent COPD, with hypoxemia in 57%, hypercapnia in 27%, and PH in 37% of them. In the other 234 pure OSAS patients, only 26% were hypoxic, 8.5% were hypercapnic, and 11 % had PH at baseline. The same schedule may be proposed for OHS, but long-term studies are also warranted to confirm the initial choice and continuation of nCPAP or bi-level NIV. 

In advanced COPD, caution should be used since overly aggressive administration of oxygen to patients with chronic hypercapnia may result in respiratory depression and respiratory failure. In these patients, mild hypoxemia, rather than carbon dioxide accumulation, triggers their drive to breathe. 

Significant changes in arterial blood gases are not usually present until the FEV is less than 1 L. At this stage, hypoxemia and hypercapnia may become chronic problems. Hypoxemia usually occurs initially with exercise but develops at rest as the disease progresses. 

The most common cause of acute respiratory failure in COPD is acute exacerbation of bronchitis with an increase in sputum volume and viscosity. This serves to worsen obstruction and further impair alveolar ventilation, resulting in worsening hypoxemia and hypercapnia. Additional causes are pneumonia, pulmonary embolism, left ventricular failure, pneumothorax, and CNS depressants. 

Chronic bronchitis is a persistent inflammation of the bronchi because of excess mucous production that irritates the bronchial and results in a persistent productive cough. Smoking is a common cause of chronic bronchitis and is aggravated by air pollution, infection, and allergies. Patients with chronic bronchitis have rhonchi (a gurgling sound) on inspiration and expiration, caused by airway blockage from excess mucus. This excess results in hypercapnia (buildup of carbon dioxide in the blood) and hypoxemia (decreased oxygen in the blood). The patient experiences respiratory acidosis. 

Respiratory acidosis may produce neuromuscular symptoms, including altered mental status, abnormal behavior, seizures, stupor, and coma. Hypercapnia may mimic stroke or CNS tumors by producing headache, papilledema, focal paresis, and abnormal reflexes. Carbon dioxide acts as a vasodilator in the brain, thus causing an increase in cerebral blood flow. This increase in cerebral blood flow is thought to be partially responsible for the CNS symptoms. The CNS response to hypercapnia is extremely variable between patients and is also influenced by the acuity of presentation. Chronic hypercapnia blunts the usual respiratory stimulus resulting from increased PaC02. In patients with severe chronic respiratory acidosis, hypoxemia rather than hypercapnia provides the primary ventilatory stimulus. ... 

Respiratory failure may be classified as hypoxemic (type 1) or hypercapnic (type II or ventilatory failure) (3), either of which may be acute and chronic. Hypoxemic respiratory failure is due to failure of the lungs, caused by acute (cardiogenic pulmonary edema, pneumonia, acute respiratory distress syndrome) or chronic (emphysema, interstitial limg disorders) diseases (Tables 1 and 2). It is characterized by hypoxemia with normocapnia or hypocapnia. In these conditions central respiratory drive is high and there is sufficient alveolar ventilation (VA) to eliminate CO2 and prevent hypercapnia. 

In individual patients, however, both types of respiratory failure may coexist, as one respiratory problem leads to another with a cascade of interaction (3). For example, patients with cardiogenic pulmonary edema or status asthmaticus first develop hypoxemia due to lung failure if the disease persists or progresses, pump failure and hypercapnia appear because of several mechanisms (increased work of breathing, reduced oxygen delivery, hyperinflation). 

Table 2 Causes of Chronic Hypoxemia with Hypercapnia...

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