Review Question - QID 216526

This man with emphysema, a 50-pack-year smoking history, new-onset dyspnea, hypoxia, and cough in the absence of fever is likely experiencing a COPD exacerbation. His arterial blood gas will therefore reflect acute-on-chronic respiratory acidosis: low pH, high carbon dioxide levels (pCO2), high bicarbonate levels (HCO3-), and low oxygen levels (pO2).

Emphysema is a term for a subset of disease within the spectrum of chronic obstructive pulmonary diseases (COPD), usually caused by prolonged exposure to inhaled pollutants such as tobacco smoke. In emphysema, the alveolar walls become damaged, creating enlarged & weak air sacs in the lungs which cannot support the alveolar ducts. As a result, the alveolar ducts collapse during exhalation, trapping air inside the lungs. This is often accompanied by some degree of chronic bronchitis (the other disease element of COPD), which further obstructs exhalation. This air trapping leads to a state of chronic respiratory acidosis with metabolic compensation. Exacerbations of COPD can occur secondary to illness or environmental triggers. During the exacerbation, the patient will experience an acute worsening of symptoms, such as dyspnea at rest or increased oxygen demand. Patients present with profound respiratory acidosis on top of their existing baseline acidosis with metabolic compensation. While stable COPD patients are usually managed with a combination of short- and long-acting bronchodilators, patients experiencing a COPD exacerbation require immediate evaluation and treatment to ensure adequate oxygenation and prevent worsening of the metabolic derangement.

Mirza et al. provide further detail on the management of COPD and COPD exacerbations. Mild exacerbations require only treatment with short-acting bronchodilators, whereas moderate exacerbations require steroids and antibiotics. Severe exacerbations require additional emergency or inpatient care, or even ventilatory support.

Figure A shows a range of possible blood gas values. As described above, a patient with a COPD exacerbation will present with acute-on-chronic respiratory acidosis. On an arterial blood gas, this can be identified by low pH, increased pCO2 levels, increased HCO3- (representing the chronic compensation), and low PO2 levels.

Incorrect Answers:
Answer 1: Low pH, high PCO2, normal HCO3- and low PO2 represent a case of acute respiratory acidosis. This might be the case in acute alcohol or opioid intoxication, for instance. However since the bicarbonate is too low relative to the level of acidosis, this is unlikely to represent the blood gas of a chronic COPD patient.

Answer 2: Normal pH, elevated PCO2, elevated HCO3- and normal PO2 indicate a state of chronic respiratory acidosis with metabolic compensation. This would likely reflect this patients’ blood gas when he is not having an exacerbation. However, during an exacerbation like the one presented in this case, we would expect the patient’s pH to be more markedly decreased and the CO2 to be more markedly increased.

Answer 3: Low pH, low pCO2, low HCO3-, and normal PO2 represent a state of metabolic acidosis with incomplete respiratory compensation. This might be the case during severe sepsis, for instance. Since this type of acidosis is not driven by the lungs, it is not consistent with this patient’s presentation.

Answer 5: High pH, low pCO2, normal HCO3-, and normal PO2 represent acute respiratory alkalosis. This might be the case in a hyperventilating patient, such as someone experiencing a panic attack. This would not represent a chronic obstructive respiratory pattern and therefore is inconsistent with this patient's presentation.

Bullet Summary:
COPD exacerbations lead to acute-on-chronic respiratory acidosis, defined by low pH, increased PCO2, increased HCO3-, and low PO2.