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Chapter 30. Principles and Goals of Mechanical Ventilation

A 70-year-old man is receiving mechanical ventilation for acute respiratory failure secondary to an exacerbation of underlying COPD. He is sedated and paralyzed on volume assist-control ventilation, at initial settings of: VT = 560 mL (8 mL/kg); f = 20 breaths/min; PEEP = 5 cm H2O; and FIO2 = 0.40. Within 2 minutes, his SaO2% = 94%, BP = 140/100 mm Hg, HR = 90 b/min, and PPEAK and PPLAT are 24 and 21 cm H2O, respectively. One hour later, the patient has become hemodynamically unstable, with PB = 90/70 mm Hg, HR = 126 b/min, SaO2 = 85%, PPEAK = 34 cm H2O, and PPLAT = 32 cm H2O. Results of a portable chest x-ray and arterial blood gas analysis are pending. What should be the next intervention in treating this patient?

a. Increase VT to 700 mL (10 mL/kg) to improve patient's lung expansion.

b. Increase f to 24/min to increase V̇E, given that the patient is sedated and paralyzed.

c. Decrease f to 12/min and then recheck airway pressures in 5 minutes.

d. Perform aggressive endotracheal suctioning and chest percussion.

The most correct answer is c, decrease respiratory frequency and recheck PAW.

This patient demonstrates the classic clinical scenario of increased auto-PEEP. Given his underlying exacerbation of COPD with accompanying alveolar distention and air-trapping, the initial f of 20/min is too rapid to permit complete alveolar emptying and causes further alveolar hyperinflation with hemodynamic compromise. This is reflected in the increased peak pressure commensurate with an increased plateau pressure. Decreasing the ventilator rate will facilitate alveolar emptying by increasing the duration of expiration, thereby reversing the air-trapping and auto-PEEP pathophysiology, with a resultant drop in both peak and plateau pressures. Increasing either VT (answer a) or ventilator rate (answer b) will worsen the auto-PEEP. Although tracheobronchial suctioning would help clear mucus plugging and airway secretions (answer d), it would not alter the underlying auto-PEEP process.

A 34-year-old male is admitted to the ICU for a ruptured appendix. He has never smoked and his past medical history is unremarkable. On day 2 in the unit, his peritonitis worsens and he exhibits respiratory failure necessitating intubation and mechanical ventilation. His attending physician decides to implement a VT = 500 mL and orders that ventilator f be modified only as needed to maintain stable arterial blood gases, and "a little hypercapnia is OK." Which of the following events occurring ...

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