Differential Diagnosis

The patient's lab results show a pH of 7.35, pCO2 of 50 mmHg, pO2 of 52 mmHg, HCO3- of 36 mmol/L, and an O2 saturation of 85%. Given these results and the patient's history, the differential diagnosis can be categorized as follows:

• Single Most Likely Diagnosis

  • Chronic Respiratory Acidosis: The patient's high pCO2 level (50 mmHg) and slightly acidic to normal pH (7.35) in the context of a long history of smoking and home oxygen use suggest chronic respiratory acidosis. This condition often results from chronic obstructive pulmonary disease (COPD), which is likely given the patient's significant smoking history.

• Other Likely Diagnoses

  • Compensated Respiratory Acidosis: Although the pH is within the normal range, the elevated pCO2 and bicarbonate (HCO3-) levels suggest a compensatory mechanism for chronic respiratory acidosis. The kidneys compensate for the increased CO2 by retaining more bicarbonate, thus maintaining a relatively normal pH.
  • Mixed Disorder: The presence of both an elevated pCO2 (indicative of respiratory acidosis) and an elevated bicarbonate level could also suggest a mixed acid-base disorder, where there is both a respiratory and metabolic component.

• Do Not Miss Diagnoses

  • Severe Pneumonia or Acute Respiratory Distress Syndrome (ARDS): Although less likely given the chronic nature of the patient's symptoms and lab results, conditions like severe pneumonia or ARDS could lead to acute respiratory acidosis and would be critical to identify due to their high mortality rates if not promptly treated.
  • Carbon Monoxide Poisoning: This could lead to an elevated carboxyhemoglobin level, not directly measured by the provided lab results, but could present with similar symptoms and would be crucial to diagnose due to its potential for severe morbidity and mortality.

• Rare Diagnoses

  • Cystic Fibrosis: Although rare in a 67-year-old, cystic fibrosis could lead to chronic respiratory issues, including acid-base disturbances. However, given the patient's age and the information provided, this would be less likely.
  • Neuromuscular Disorders: Conditions affecting the respiratory muscles, such as amyotrophic lateral sclerosis (ALS), could lead to respiratory acidosis. These conditions would be rare in this context but should be considered if other explanations are ruled out.