BP = blood pressure; PO = per os.

B. Inadequate Respiratory Drive

1. During early recovery from anesthesia, residual effects of IV and inhalational anesthetics blunt the ventilatory responses to both hypercarbia and hypoxemia. Sedatives augment depression from opioids or anesthetics and reduce the conscious desire to ventilate.

2. Hypoventilation and hypercarbia may evolve insidiously during transfer and admission to the PACU. Although the effects of intraoperative medications are usually waning, the peak depressant effect of IV opioid given just before transfer occurs in the PACU.

3. Patients may communicate lucidly and even complain of pain while experiencing significant opioid-induced hypoventilation. A balance must be struck between an acceptable level of postoperative ventilatory depression and a tolerable level of pain or agitation.

4. Patients with abnormal CO₂/pH responses from morbid obesity, chronic airway obstruction, or sleep apnea are more sensitive to respiratory depressants.

5. The abrupt diminution of a noxious stimulus (tracheal extubation, placement of a postoperative block) may promote hypoventilation or airway obstruction by altering the balance between arousal from discomfort and depression from medication.

C. Increased airway resistance increases the work of breathing and CO₂ production.

1. In postoperative patients, increased upper airway resistance is caused by obstruction in the pharynx, larynx (laryngospasm, laryngeal edema), or large airways (extrinsic compression from hematoma).

2. Weakness from residual neuromuscular relaxation may contribute but is seldom the primary cause of airway compromise.

3. Laryngospasm can usually be overcome by providing gentle positive pressure (10–20 mm Hg continuous) in the oropharynx by mask with 100% oxygen. Prolonged laryngospasm is relieved with a small dose of succinylcholine (0.1 mg/kg) or deepening sedation with propofol.

D. Decreased compliance accentuates the work of breathing. Obesity affects pulmonary compliance, especially when adipose tissue compresses the thoracic cage or increases intra-abdominal pressure in the supine or lateral positions. Allowing patients to recover in a semi-sitting (semi-Fowler’s) position reduces the work of breathing.

E. Neuromuscular and Skeletal Problems

1. Postoperative airway obstruction and hypoventilation are accentuated by incomplete reversal of neuromuscular relaxation. Residual paralysis compromises airway patency, the ability to overcome airway resistance, airway protection, and the ability to clear secretions.

2. PACU staff should be aware of patients who have received nondepolarizing muscle relaxants but have not received reversal agents because these patients often exhibit low levels of residual paralysis and in the presence of severe kyphosis or scoliosis may cause postoperative ventilatory insufficiency.

3. Simple tests help assess the patient’s mechanical ability to ventilate. The ability to sustain head elevation in a supine position, a forced vital capacity of 10 to 12 mL/kg, an inspiratory pressure that is lower than –25 cm H₂O, and tactile train-of-four assessment imply that the strength of ventilatory muscles is adequate to sustain ventilation and to take a large enough breath to cough. However, none of these clinical end points reliably predicts the recovery of airway protective reflexes, and failure on these tests does not necessarily indicate the need for assisted ventilation.

F. Inadequate Postoperative Oxygenation

1. Systemic arterial partial pressure of oxygen (PaO₂) is the best indicator of pulmonary oxygen transfer from alveolar gas to pulmonary capillary blood.

2. Arterial hemoglobin saturation monitored by pulse oximetry yields less information on alveolar–arterial gradients and is not helpful in assessing the impact of hemoglobin dissociation curve shifts or carboxyhemoglobin.

3. In postoperative patients, the acceptable lower limit for PaO₂ varies with individual patient characteristics. Maintaining PaO₂ between 80 and 100 mm Hg (saturation, 93%–97%) ensures adequate oxygen availability. Little benefit is derived from elevating PaO₂ above 110 mm Hg because hemoglobin is saturated, and the amount of additional oxygen dissolved in plasma is negligible.

4. During mechanical ventilation, a PaO₂ above 80 mm Hg with 0.4 fraction of inspired oxygen (FIO₂) and 5 cm H₂O positive end-expiratory pressure (PEEP), continuous positive airway pressure (CPAP), or a spontaneous breathing trial usually predicts sustained adequate oxygenation after tracheal extubation.

G. Obstructive sleep apnea (OSA) is a syndrome in which patients exhibit a period of partial or complete obstruction of the upper airway (Table 54-4).

1. It is estimated that 2% of women and 4% of men show overt symptoms of OSA.

2. The perioperative management of a patient with OSA must start preoperatively with a well-planned anesthetic, taking into account the type, location, and recovery of surgery.

a. Postoperative management concerns include analgesia, oxygenation, patient positioning, and monitoring.

b. Regional anesthesia with minimal sedation (rather than increased use of opioids) is best for recovery.

c. Patients who use CPAP or noninvasive positive-pressure ventilation should continue to use these therapies.

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