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136 PULMONARY INSUFFICIENCY AND ACUTE RESPIRATORY DISTRESS SYNDROME (ARDS)

136 PULMONARY INSUFFICIENCY AND ACUTE RESPIRATORY DISTRESS SYNDROME (ARDS)
Harrison’s Manual of Medicine

136

PULMONARY INSUFFICIENCY AND ACUTE RESPIRATORY DISTRESS SYNDROME (ARDS)

Clinical Characteristics and Pathophysiology
Complications
Prognosis
Bibliography

ARDS is a descriptive term applied to many acute, diffuse infiltrative lung lesions of diverse etiologies (Table 136-1) with severe arterial hypoxemia. Acute lung injury (ALI) is a mild form of ARDS.

Table 136-1 Conditions That May Lead to the Acute Respiratory Distress Syndrome

Clinical Characteristics and Pathophysiology
Earliest sign is often tachypnea followed by dyspnea. Arterial blood gas shows reduction of PO2 and PCO2 with widened alveolar-arterial O2 difference. Physical exam and CXR may be normal initially. With progression, pt becomes cyanotic, dyspneic, and increasingly tachypneic. Crackles become audible diffusely, and CXR shows diffuse, bilateral, interstitial and alveolar infiltrates.
ARDS increases lung water without increasing hydrostatic forces. Toxic gases (chlorine, NO2, smoke) and gastric acid aspiration damage the alveolar- capillary membrane directly, whereas sepsis increases alveolar-capillary permeability by producing activation and aggregation of formed blood elements. Though radiologically diffuse, regional lung dysfunction is non-homogeneous, with severe ventilation-perfusion imbalance and actual shunting of blood through collapsed alveoli.

TREATMENT
Early in illness, supplemental O2 may be sufficient to correct hypoxemia, but with progression mechanical ventilatory support is necessary. Goal of therapy is to provide adequate tissue O2 delivery—determined by arterial oxygen saturation (SaO2), hemoglobin (Hb), cardiac output, and blood flow distribution. Reasonable objective is to achieve 90% saturation (PaO2 8 kPa, or 60 mmHg) with the lowest inspired O2 concentration practical to avoid O2 toxicity (FIO2 < 0.6). Hb should be ³100 g/L (³10 g/dL).

Cardiac output is supported as necessary with IV fluids and inotropic agents. Pulmonary artery catheter insertion may be necessary for accurate assessment of ventricular filling pressures, hemodynamics, and O2 transport.

Because airspace disease is nonhomogeneous, large-volume breaths are administered preferentially to normal areas of lung, causing alveolar overdistention and furthering lung injury (“volutrauma”). Avoidance of overdistention is accomplished by using pressure-cycled mechanical ventilation with maximal distending pressure of 35 cmH2O or by using low tidal volumes (5–7 mL/kG ideal body weight) with the ventilator in volume-cycled mode. Positive end-expiratory pressure (PEEP, >12 cmH2O) is used to prevent alveolar collapse.

Pressure or volume limitation frequently results in hypoventilation and hypercarbia. Inadequate oxygenation is corrected with: (1) prone positioning, which improves gas exchange; and (2) prolonged inspiratory times (inverse ratio ventilation), which increases mean lung volumes. Recruitment breaths (sustained large-volume breaths lasting >30 sec) may also markedly improve oxygenation. Techniques such as inhaled nitric oxide or partial liquid ventilation have not been shown to improve outcome.

Complications

1.
LV failure is a common, easily missed complication, particularly in pts receiving mechanical ventilation.

2.
Secondary bacterial infection may be obscured by the diffuse roentgenographic changes.

3.
Bronchial obstruction may be caused by endotracheal or tracheostomy tubes.

4.
Pneumothorax and pneumomediastinum may cause abrupt deterioration in pts receiving mechanical ventilation.
Prognosis
Overall mortality rate is 50% and varies with the intrinsic mortality of the underlying condition, but preliminary studies suggest new ventilator strategies reduce mortality. If ARDS occurs as a result of extrapulmonic sepsis, multiple organ failure may supervene.
Bibliography

For a more detailed discussion, see Moss M, Ingram RH Jr: Acute Respiratory Distress Syndrome, Chap. 265, p. 1523, HPIM-15.

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4 comments on “136 PULMONARY INSUFFICIENCY AND ACUTE RESPIRATORY DISTRESS SYNDROME (ARDS)

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