Mild hypoxemia, airway obstruction, hypercapnia, atelectasis, and bronchospasm are so common during emergence from anesthesia that anesthesiologists and postanesthetic nurses routinely provide good prophylactic therapy in the modern surgical setting. These problems are often considered a natural consequence of giving drugs that depress central respiratory drive, temporarily decrease lung volume, impair protective airway reflexes, depress secretion mobilization, and eliminate sighing (auto-positive end-expiratory pressure [auto-PEEP], preventing atelectasis). Adverse pulmonary outcomes are often attributed to anesthesia care, but a significant component of perioperative risk derives from the surgical site, postoperative pain, and effects of pharmacologic pain management. These risks must be recognized by the perioperative staff so that more serious perioperative pulmonary complications, such as bronchitis, pneumonia, pulmonary edema, aspiration, and respiratory failure, do not evolve.

Problems with oxygenation, ventilation, and airway maintenance are cardinal signs of perioperative pulmonary complications and can be discussed within the context of hypoxia. Hypoxia is defined as decreased oxygen tension with concomitant decreased oxygen supply in the blood delivered to the tissues. The common postoperative causes of hypoxia fall into two categories: anemic hypoxia and hypoxemic hypoxia. Anemic hypoxia results from a reduction in hemoglobin concentration, either from diseases such as sickle cell disease, or from unreplaced surgical losses. Anemic hypoxia can also be due to the conditions that shift the oxyhemoglobin dissociation curve (acid base balance, temperature) and or change the binding capacity of hemoglobin such as HbF found in infants. Postoperative hypoxemic hypoxia results when oxygen exchange in the lungs is problematic and can be divided into two categories: ventilation/perfusion mismatch and hypoventilation.

When the ratio of ventilation ([V with dot above]) and perfusion ([Q with dot above]) of the lungs is not normal, the resulting hypoxemia is from [V with dot above]/[Q with dot above] mismatch. Acute changes in [V with dot above] and [Q with dot above] take place as a function of changes in chest wall configuration,
surgical positioning, intraoperative ventilation modes, and anesthetic effects on pulmonary blood flow. Acute anesthetic effects are largely reversed at the end of surgery; however, certain surgeries have postoperative implications for lung function. For instance, after upper abdominal and thoracic surgery, lung capacities are reduced by approximately 40% for the first few days following surgery, and measurable decrement in respiratory mechanics persists for up to 2 weeks. Other causes of [V with dot above]/[Q with dot above] mismatch include partial airway obstruction (postextubation stridor, laryngospasm or bronchospasm); inadequate tidal volumes and cough (from recurarization or residual sedation); and atelectasis (from sustained reduction of peak air flows and total lung volume during and after anesthesia).