CHAPTER 27: Pulmonology

PULMONARY DISEASES AND SYNDROMES

ACUTE RESPIRATORY DISTRESS SYNDROME

An acute, diffuse, inflammatory lung injury that leads to increased pulmonary vascular permeability, increased lung weight, diffuse alveolar damage, and hypoxemia.

Berlin definition of acute respiratory distress syndrome (ARDS)

EPIDEMIOLOGY

• Accounts for approximately 5% of hospitalized and mechanically ventilated pediatric patients

• Incidence in children varies greatly, but in the United States ranges between 2.9 and 9.5 cases/100,000 children per year

• Mortality depends on age, etiology, pre-morbid conditions, and severity of oxygenation deficit:

  • ✓ Mortality rate in children ranges between 22 and 35%

ETIOLOGY

• Most common etiology: Sepsis

• Others include systemic inflammatory response syndrome (SIRS), pneumonia, aspiration, smoke inhalation, trauma, drowning, pancreatitis, and massive blood transfusions

• Risk of developing ARDS is higher in adults with a history of alcoholism and obesity

PATHOPHYSIOLOGY

• ARDS follows a predictable progression of histologic and clinical stages:

  • ✓ Exudative stage: Release of proinflammatory cytokines and influx of neutrophils lead to epithelial and endothelial injury, noncardiogenic pulmonary edema, impaired gas exchange, surfactant deficiency/deactivation, and diffuse alveolar damage

  • ✓ Proliferative stage: Proliferation of type II alveolar cells, squamous metaplasia, interstitial infiltration by myofibroblasts, and early deposition of collagen

  • ✓ Fibrotic stage: Obliteration of normal lung architecture, diffuse fibrosis, collagen deposition

CLINICAL MANIFESTATIONS

• The initial presentation can include cyanosis, dyspnea, tachypnea, and diffuse crackles

DIAGNOSTICS

• Arterial blood gas: Profound hypoxemia, usually refractory to supplemental oxygen administration; elevated alveolar-arterial oxygen gradient. Calculate PaO₂/FiO₂ to determine severity of ARDS. Initially the PCO₂ is low because of hyperventilation, but later on there is CO₂ retention

• Indicators of end-organ damage and multiorgan failure: Follow liver enzyme levels, coagulation studies, cardiac enzymes

• Chest radiograph: Diffuse bilateral alveolar infiltrates

• Chest CT scan: Bilateral patchy airspace opacities in dependent areas of lung

• Expected changes in lung function and mechanics: Decreased total lung capacity, decreased functional residual capacity (FRC), large intrapulmonary shunt fraction (Q_S/Q_T), decreased pulmonary compliance

MANAGEMENT

Management involves supportive ventilation and correction of underlying causes while treating comorbidities and limiting complications.

• Ventilation: Goal is to achieve adequate (not necessarily normal) alveolar gas exchange while limiting complications by using low tidal volume to avoid alveolar overdistention, low FiO₂ to reduce oxidant injury, and high positive end-expiratory pressure (PEEP) to limit barotrauma and maximize lung recruitment:

  • ✓ Minimize tidal volume: Goal 4–8 mL/kg

  • ✓ Permissive hypercapnia: Smaller tidal volumes result in hypoventilation, therefore an elevation of PaCO₂

  • ✓ PEEP: 5–15cm H₂O to prevent or reverse atelectasis

  • ✓ Limit inflating (plateau) pressure (e.g, after an inspiratory hold) to less than 30cm H₂O if possible

  • ✓ Goal SpO₂ 88–95%; attempt to use “nontoxic” FiO₂ of ...