Chapter 38

A 12-year-old boy with a history of Duchenne muscular dystrophy presents for scoliosis surgery. He is wheelchair-bound and uses biphasic positive airway pressure (BiPAP) at night. An echocardiogram performed 3 months ago showed an ejection fraction of 45%. Pulmonary function tests showed a vital capacity 45% of the predicted value.

PREOPERATIVE CONSIDERATIONS

Duchenne muscular dystrophy (DMD), or pseudohypertrophic muscular dystrophy, is the most common muscular dystrophy seen in children. DMD is an X-linked recessive disease associated with a lack of dystrophin. The absence of dystrophin results in instability and increased permeability of the sarcolemma and elevated intracellular calcium. If the sarcolemma is exposed to volatile agents or succinylcholine, this instability and permeability may worsen, leading to a compensatory hypermetabolic response, including hyperkalemia, hyperthermia, tachycardia, and rhabdomyolysis. Beginning in early childhood, DMD is characterized by progressive deterioration of skeletal, cardiac, and smooth muscles. Proximal muscles are affected first, whereas distal muscles, especially calf muscles, may appear hypertrophied. The Gower sign, in which patients use their arms in order to stand up, reflects the severe proximal weakness. Patients frequently are wheelchair-bound by early adolescence. The major causes of morbidity and mortality are respiratory and cardiac failure.

Preoperative evaluation should include pulmonary function testing, if possible. Vital capacity (VC) less than 50% predicted may be associated with the need for postoperative mechanical ventilation, and a VC less than 30% predicted may lead to serious postoperative complications, even with mechanical ventilation. The preoperative oxygen saturation on room air should be noted. Patients with DMD are at increased risk for pneumonia due to their diminished ability to cough and clear secretions. They may be at increased risk of aspiration due to gastrointestinal hypomotility in conjunction with weak laryngeal reflexes. Preoperative electrocardiogram changes may include tall R waves in V1, deep Q waves in the limb leads, decreased PR intervals, and sinus tachycardia. Mitral regurgitation or mitral valve prolapse may be present as a result of papillary muscle dysfunction. Dilated cardiomyopathy is common in older adolescent patients and is a major cause of mortality.

ANESTHETIC MANAGEMENT

• Flush the anesthesia machine preoperatively (usually 10 L/min for 20 minutes; consult manufacturer) and change the soda lime (“trigger-free technique”).

• Avoid succinylcholine and volatile agents. Trigger-free anesthetic (eg, total intravenous anesthesia or regional) is preferred.

• Use an arterial line for measurement of arterial blood gases, electrolytes, and hematocrit.

• Monitor central venous pressure to assess intravascular volume and monitor for congestive heart failure (CHF).

• Prepare to treat acute rhabdomyolysis or hyperkalemia with sodium bicarbonate, insulin, calcium chloride, and mannitol.

• Decompress the stomach with a nasogastric tube.

• Transfuse as necessary to maximize oxygen delivery.

• Monitor urine output closely to assess volume status.

• Use cautious narcotic administration for pain.

• There must be careful attention to positioning the patient, to prevent nerve injuries and to minimize respiratory compromise.

POSTOPERATIVE CONSIDERATIONS

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