Treatment of the patient with acute respiratory failure consists of (1) specific therapy directed toward the underlying disease, (2) respiratory supportive care directed toward the maintenance of adequate gas exchange, and (3) general supportive care. Only the last two aspects are discussed below.
Respiratory support has both nonventilatory and ventilatory aspects.
1. Nonventilatory aspects
The main therapeutic goal in acute hypoxemic respiratory failure is to ensure adequate oxygenation of vital organs. Inspired oxygen concentration should be the lowest value that results in an arterial hemoglobin saturation of 88% or more (PO2 55 mm Hg or more [7.3 kPa or more]). Higher arterial oxygen tensions are of no proven benefit and may be deleterious. Restoration of normoxia may rarely cause hypoventilation in patients with chronic hypercapnia; however, oxygen therapy should not be withheld for fear of causing progressive respiratory acidemia. Hypoxemia in patients with obstructive airway disease is usually easily corrected by administering low-flow oxygen by nasal cannula (1–3 L/min) or Venturi mask (24–40%). Higher concentrations of oxygen are necessary to correct hypoxemia in patients with ARDS, pneumonia, and other parenchymal lung diseases. Humidified, high flow nasal cannulae provide adjustable oxygen delivery and flow-dependent clearance of carbon dioxide from the upper airway resulting in reduced work of breathing and better matching of respiratory demand during respiratory distress. In hypoxemia due to acute respiratory failure, oxygenation with use of humidified, high flow nasal cannulae has been shown to be similar and, in some cases, superior to conventional low flow oxygen supplementation and to noninvasive positive-pressure ventilation.
Ventilatory support consists of maintaining patency of the airway and ensuring adequate alveolar ventilation. Mechanical ventilation may be provided via mask (noninvasive) or through tracheal intubation.
A. NONINVASIVE POSITIVE-PRESSURE VENTILATION
NIPPV delivered via a full-face mask or nasal mask is first-line therapy in COPD patients with hypercapnic respiratory failure who can protect and maintain the patency of their airway, handle their own secretions, and tolerate the mask apparatus. Several studies have demonstrated the effectiveness of this therapy in reducing intubation rates and ICU stays in patients with ventilatory failure. A bilevel positive-pressure ventilation mode is preferred for most patients. Patients with acute lung injury or ARDS or those who suffer from severely impaired oxygenation are less likely to benefit and should be intubated if they require mechanical ventilation.
Indications for tracheal intubation include (1) hypoxemia despite supplemental oxygen; (2) upper airway obstruction; (3) impaired airway protection; (4) inability to clear secretions; (5) respiratory acidosis; (6) progressive general fatigue, tachypnea, use of accessory respiratory muscles, or mental status deterioration; and (7) apnea. Patients in respiratory failure who undergo a trial of NIPPV and do not improve within 30–90 minutes should be intubated. In general, orotracheal intubation is preferred to nasotracheal intubation in urgent or emergency situations because it is easier, faster, and less traumatic. The tip of the endotracheal tube should be positioned 2–4 cm above the carina and be verified by chest radiograph immediately following intubation. Only tracheal tubes with high-volume, low-pressure air-filled cuffs should be used. Cuff inflation pressure should be kept below 20 mm Hg, if possible, to minimize tracheal mucosal injury.
C. MECHANICAL VENTILATION
Indications for mechanical ventilation include (1) apnea, (2) acute hypercapnia that is not quickly reversed by appropriate specific therapy, (3) severe hypoxemia, and (4) progressive patient fatigue despite appropriate treatment.
Several modes of positive-pressure ventilation are available. Controlled mechanical ventilation (CMV; also known as assist-control [A-C]) and synchronized intermittent mandatory ventilation (SIMV) are ventilatory modes in which the ventilator delivers a minimum number of breaths of a specified tidal volume each minute. In both CMV and SIMV, the patient may trigger the ventilator to deliver additional breaths. In CMV, the ventilator responds to breaths initiated by the patient above the set rate by delivering additional full tidal volume breaths. In SIMV, additional breaths are not supported by the ventilator unless the pressure support mode is added. Numerous alternative modes of mechanical ventilation now exist, the most popular being pressure support ventilation (PSV), pressure control ventilation (PCV), and CPAP.
PEEP is useful in improving oxygenation in patients with diffuse parenchymal lung disease, such as ARDS. It should be used cautiously in patients with localized parenchymal disease, emphysema, hyperinflation, or very high airway pressure requirements during mechanical ventilation.
D. COMPLICATIONS OF MECHANICAL VENTILATION
Potential complications of mechanical ventilation are numerous. Migration of the tip of the endotracheal tube into a main bronchus can cause atelectasis of the contralateral lung and overdistention of the intubated lung. Barotrauma refers to rupture and loss of integrity of the alveolar space secondary to high transmural pressures applied during positive-pressure ventilation. Barotrauma is manifested by subcutaneous emphysema, pneumomediastinum, subpleural air cysts, pneumothorax, or systemic gas embolism (eFigure 9–33). Volutrauma is sometimes used to refer to subtle parenchymal injury due to overdistention of alveoli from excessive tidal volumes without alveolar rupture, mediated through inflammatory rather than physical mechanisms. The principal strategy to avoid volutrauma is the use of low tidal volume ventilation.
Barotrauma in ARDS. A: Chest radiograph demonstrates diffuse lung consolidation secondary to ARDS. Parenchymal stippling is present, with lucent perivascular halos secondary to pulmonary interstitial emphysema. B: On chest radiograph 4 days later, pneumomediastinum is now identified with extensive subcutaneous emphysema. C: I n another patient with ARDS, subpleural cysts (arrow) and parenchymal stippling due to pulmonary interstitial emphysema are present. (Reproduced, with permission, from Bongard FS, Sue DY [editors]. Current Critical Care Diagnosis & Treatment. Originally published by Appleton & Lange. Copyright © 1994 by The McGraw-Hill Companies, Inc.)
Acute respiratory alkalosis caused by overventilation is common. Hypotension induced by elevated intrathoracic pressure that results in decreased return of systemic venous blood to the heart may occur in patients treated with PEEP, particularly those with intravascular volume depletion, and in patients with severe airflow obstruction at high respiratory rates that promote “breath stacking” (dynamic hyperinflation). Ventilator-associated pneumonia is another serious complication of mechanical ventilation.
B. General Supportive Care
Maintenance of adequate nutrition is vital; parenteral nutrition should be used only when conventional enteral feeding methods are not possible. Overfeeding, especially with carbohydrate-rich formulas, should be avoided, because it can increase CO2 production and may potentially worsen or induce hypercapnia in patients with limited ventilatory reserve. However, failure to provide adequate nutrition is more common. Hypokalemia and hypophosphatemia may worsen hypoventilation due to respiratory muscle weakness. Sedative-hypnotics and opioid analgesics should be titrated carefully to avoid oversedation, leading to prolongation of intubation. Temporary paralysis with a nondepolarizing neuromuscular blocking agent is used to facilitate mechanical ventilation and to lower oxygen consumption. Prolonged muscle weakness due to an acute myopathy is a potential complication of these agents. Myopathy is more common in patients with kidney injury and in those given concomitant corticosteroids.
Psychological and emotional support of the patient and family, skin care to avoid pressure injuries (previously called pressure ulcers), and meticulous avoidance of health care–associated infection and complications of tracheal tubes are vital aspects of comprehensive care for patients with acute respiratory failure.
Attention must also be paid to preventing complications associated with serious illness. Stress gastritis and ulcers may be avoided by administering sucralfate (1 g orally twice a day), histamine H2-receptor antagonists, or PPIs. There is some concern that the latter two agents, which raise gastric pH, may permit increased growth of gram-negative bacteria in the stomach, predisposing to pharyngeal colonization and ultimately HCAP; many clinicians therefore prefer sucralfate. The risk of DVT and PE may be reduced by subcutaneous administration of heparin (5000 units every 12 hours), the use of LMWH (see Table 14–14), or placement of sequential compression devices on the lower extremities.