Chapter 34
Respiratory Diseases

Obstructive sleep apnea (OSA) and obesity hypoventilation syndrome are addressed in Part 2 of this text. This section focuses on chronic obstructive pulmonary disease (COPD) and the nuances of respiratory monitoring in the perioperative period.

COPD and Anesthesia

Patients with COPD present several perioperative challenges, including an increased risk for bronchospasm, air-trapping, and delayed return to spontaneous ventilation. However, it’s important not to overstate the anesthetic risk. In reality, the greater threat comes from the nature of surgery and postsurgical factors, not the anesthetic itself.

Most modern anesthetics, particularly for noninvasive procedures like imaging, endoscopy, and biopsies, have a relatively modest impact, even in advanced COPD. In contrast, major surgeries, especially those involving the chest or abdomen, carry significantly higher risk due to postoperative factors: splinting, atelectasis, impaired secretion clearance, and increased risk for pneumonia. Orthopedic surgeries can pose similar risks due to immobility in the postoperative period.

Optimal anesthetic management includes:

minimizing airway instrumentation (e.g., using neuraxial or regional techniques when appropriate)
preemptive bronchodilation with inhaled agents
effective pain control
facilitating early pulmonary toilet, early ambulation, and incentive spirometry

Pitfalls of Respiratory Monitoring

As anesthesiologists, we can become overly reliant on capnography and pulse oximetry and lose sight of their limitations. It’s important to periodically revisit what these monitors are actually telling us:

Capnography measures end-tidal CO₂ (EtCO₂), which is a surrogate—not a direct measure—of PaCO₂. In many patients, particularly those under monitored anesthesia care(MAC), the EtCO₂ may substantially underestimate true arterial CO₂.
Pulse oximetry reflects oxygen saturation (SpO₂), a surrogate for PaO₂, and says nothing about ventilation or CO₂ clearance.

This disconnect becomes especially dangerous in hypoventilating patients. The alveolar gas equation demonstrates that elevated PaCO₂ can be masked by even modest supplemental oxygen, such as a few liters via nasal cannula. This can allow SpO₂ to remain in the 90s while PaCO₂ climbs to dangerous levels, delaying recognition of critical hypercarbia and impending respiratory failure.