A 76-year-old man is brought to the hospital from a nursing home due to confusion, high-grade fever, and decreased oral intake. Three days ago, he underwent cystoscopy with removal of a bladder polyp. The patient has a history of benign prostate hyperplasia and recurrent urinary tract infections. Other medical conditions include type 2 diabetes mellitus, hypertension, osteoarthritis, and gout. On admission, the patient has respiratory failure requiring endotracheal intubation. Mechanical ventilation is initiated and adjusted. He is resuscitated with intravenous normal saline and treated with broad-spectrum intravenous antibiotics. Current ventilator settings are as follows: tidal volume, 370 mL (6 mL/kg of ideal body weight); respiratory rate, 22/min; positive end-expiratory pressure, 5 cm H2O; and FiO2, 70%. Inspiratory plateau pressure is normal (<30 cm H2O). Chest x-ray reveals extensive bilateral alveolar opacities. The results of arterial blood gas analysis are as follows:
| pH | 7.44 |
| PaCO2 | 30 mm Hg |
| PaO2 | 53 mm Hg |
| HCO3- | 21 mEq/L |
Which of the following is the best next step in management of this patient?
Show Explanatory Sources
This patient with respiratory distress, hypoxemia (PaO2 <60 mm Hg), and bilateral opacities likely has acute respiratory distress syndrome (ARDS) due to sepsis of urinary origin. ARDS can develop due to direct (eg, aspiration, pneumonia) or indirect (eg, trauma, sepsis, pancreatitis) lung injury. In ARDS, severe hypoxemia is due mainly to an intrapulmonary shunt effect caused by neutrophilic lung inflammation and flooding of fibrinous proteinaceous fluid into alveoli.
Positive end-expiratory pressure (PEEP) improves oxygenation by recruiting flooded alveoli and propping them open throughout the respiratory cycle; elevated PEEP levels (eg, ≥10 cm H2O) are frequently required in ARDS. Increasing PEEP must be carefully done because it can increase the risk of barotrauma; however, increasing PEEP has several beneficial effects:
Additional alveoli are recruited, making more of the lung available for gas exchange (reduced shunting).
As more alveolar units are recruited, lung compliance may improve because the pressure delivered by the ventilator is distributed throughout a larger lung volume (ie, if the lung is underrecruited, increasing PEEP may improve compliance).
Mean airway pressure is increased, providing a larger pressure gradient for oxygen transfer (diffusion) across the alveolar-capillary membrane (ie, ↑ PAO2).
Oxygenation can also be improved by increasing the FiO2; patients are often provided high (≥60% or 0.6) FiO2 immediately after intubation. However, FiO2 is usually weaned to <60% as quickly as possible to avoid potential oxygen toxicity (eg, this patient may have been provided 100% FiO2 and subsequently quickly decreased to 70% FiO2) (Choice A). Increasing PEEP addresses the underlying pathophysiology of ARDS. In patients requiring improved oxygenation, PEEP should be increased as tolerated to allow for a reduction in FiO2 (Choice E).
(Choices C and D) This patient's ventilation is adequate, as confirmed by mild hypocapnia. Increasing respiratory rate or tidal volume (VT) would worsen respiratory alkalosis without improving oxygenation. In addition, increasing VT may cause alveolar overdistension and increase the risk of volutrauma. This patient's VT (~6 mL/kg) is consistent with lung-protective ventilation commonly used in ARDS.
Educational objective:
Oxygenation in mechanically ventilated patients is determined by the FiO2 and positive end-expiratory pressure (PEEP). Increasing PEEP recruits collapsed alveoli in acute respiratory distress syndrome (ARDS), reducing intrapulmonary shunting and allowing reduction of FiO2. Optimal PEEP settings may improve mortality rates in patients with severe ARDS.