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 within the next 4 hours would alert the ICU staff that the patient's suspected acute lung injury has abruptly and significantly worsened?
a. The patient's PPEAK decreases by 6 cm H2O pressure.
b. His shunt fraction calculated at bedside decreases from 45% to 28%
c. His calculated plasma [HCO3−] increases from 17 mM to 20 mM.
d. The patient's SaO2 decreases from 92% to 89% by pulse oximetry.
e. His PPEAK and PPLAT both increase by 8 cm H2O pressure.
The most correct answer is e.
PPEAK and PPLAT increase in parallel as would be expected in a patient with an acute onset or worsening of a restrictive parenchymal process, in this instance caused by edema. Either a decrease in PPEAK (answer a) or in shunt fraction (answer b) over a 4-hour period would be interpreted as welcome signs that the patient's acute lung injury has stabilized and that MV was appropriate. Likewise, a modest increase in [HCO3–] (answer c) would be considered an acceptable side effect of MV, particularly in a patient likely to have metabolic acidosis that consumes body bicarbonate stores. Although his SaO2 = 89% is approaching a critical value (answer d), a 3% change would not be the most troubling development in such a patient.
A 44-year-old man weighing 90 kg is in the intensive care unit for the past 4 days due to severe head and abdominal trauma from a motor vehicle accident. He has been on IV lipid emulsion throughout this period. Worsening neurological deficits now necessitate intubation and mechanical ventilation (MV). Current ventilator settings are delivering 400 mL room air/breath at 14 breaths/min. Blood gas analyses 10 minutes ago reveal a pHa = 7.31, PaCO2 = 56 mm Hg, and PaO2 = 60 mm Hg; pulse oximetry shows SaO2 = 72%. Which of the following is the most appropriate next step in managing this patient?
a. Switch his IV feeding solution to 5% dextrose.
b. Add 15 cm H2O of positive end-expiratory pressure.
c. Convert him from room air to an FIO2 = 0.50.
d. Double his breathing frequency on MV.
e. Increase his VT on MV by 150 mL/breath.
The most correct answer is e, increase the patient's VT by 150 mL/breath.
The current ventilator setting is delivering < 5 mL/kg and is inducing respiratory acidosis with an excessively high VD/VT ratio, which will not be corrected simply by increasing ventilator f (answer d). By Dalton's law, his low PaO2 is caused by alveolar hypoventilation and supplemental oxygen is probably unnecessary (answer c). Likewise, changing the feeding solution to 5% dextrose will only increase the patient's PaCO2 because of the increased respiratory quotient that occurs with carbohydrate catabolism (answer a). Consideration of using PEEP is premature at this point (answer b).