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FOCUS POINTS
1.Type II pneumocytes develop at 24 to 26 weeks of gestation and begin producing surfactant. Eight to ten percent of the number of adult alveoli are present at birth
Infants and children have reduced functional residual capacity (FRC) and higher oxygen consumption (6 to 8 ml/kg/min) rendering them susceptible to faster oxygen desaturation.
Etiologies for increased ventilation to perfusion (V/Q) mismatch during thoracic surgery include lateral positioning, general anesthesia and blunting of hypoxic pulmonary vasoconstriction, mechanical ventilation and surgical manipulation or single-lung ventilation.
One-lung ventilation (OLV) can be achieved utilizing single lumen endotracheal tubes inserted into the main stem bronchus, endobronchial blockers, Univent tubes or double lumen tubes dependent on the size of the patient.
Management of hypoxemia during OLV includes 100% oxygen, continuous positive airway pressure (CPAP) to the nondependent lung, positive end-expiratory pressure (PEEP) to the dependent lung, double lung ventilation and in extremis occlusion of the pulmonary artery to the operative lung.
Thoracic and mediastinal masses are lesions that may cause airway compromise prior to or during a procedure under anesthesia. Thorough preoperative evaluation that includes review of echocardiogram, available imaging, and symptoms allow for a safer anesthetic. Spontaneous ventilation with min sedation may be the anesthetic of choice.
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DEVELOPMENT OF THE RESPIRATORY SYSTEM
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The morphological development of the lung begins several weeks after conception and continues well into the first decade of life. Four stages of lung development and maturation were described initially by Dubreuil,1 which was then updated in 1988 by Thurlbeck.2 The first stage, called the embryonic stage, spans from 2–3 to 7–8 weeks of gestation and begins as an outgrowth of endoderm, surrounded by mesenchymal tissue, from the ventral foregut. This outgrowth extends into the pleuroperitoneal cavity and eventually forms lung buds.3 A concurrent event is the development of the tracheoesophageal ridge. Incomplete separation of this ridge may lead to formation of a tracheoesophageal fistula.4 The next phase, the pseudoglandular stage, extends from 7 to 16 weeks postconception and is punctuated by the formation of the conducting airways and pulmonary vasculature. Therefore, a fetus cannot survive if born at this stage of development.5 The diaphragm forms during this time frame and consequently, any disruption may lead to herniation of abdominal organs into the thorax and hypoplasia of the lung, that is, congenital diaphragmatic hernia.6 The canalicular phase, extending from 17 to 28 weeks postconception, is characterized by the development of acini and adjacent vascular growth. During this stage, bronchi and bronchioles increase in luminal diameter.7 Type II pneumocytes develop by 24 to 26 weeks’ gestation and potentially as early as 20 weeks.8,9 These cells produce surfactant, which acts to reduce surface tension and stabilize airspaces.10 Capillary networks surrounding the acini proliferate and allow for sufficient respiratory exchange by 26 to 28 weeks.11 Premature infants can survive if born at 24 weeks; however, exogenous surfactant ...