A 46-year-old man is hospitalized with severe acute pancreatitis. Because of progressive respiratory difficulty, he is intubated and placed on mechanical ventilation. His weight is 70 kg (152 lb), blood pressure is 110/70 mm Hg, and heart rate is 90/min. His respiratory rate is 18/min. Chest x-ray shows patchy opacities bilaterally, consistent with pulmonary edema. His current ventilator settings are: assist control mode, respiratory rate of 18/min, tidal volume of 450 mL, FiO2 of 40%, and positive end-expiratory pressure (PEEP) of 5 cm H2O. His blood gas readings are:
pH 7.51 pCO2 22 mmHg pO2 69 mmHg
Which of the following is the best next step in managing this patient?
This patient with pancreatitis has respiratory failure likely due to acute respiratory distress syndrome. The assist control mode of mechanical ventilation delivers a predetermined tidal volume with every breath. Inspiratory cycles can be initiated by the patient, but if the patient fails to breathe at a set minimum rate, then the ventilator will deliver the tidal volume on its own.
In general, tidal volumes should be about 6 mL/kg of ideal body weight; thus, the tidal volume being administered to the patient above is appropriate. However, the arterial blood gas reveals an increased pH (> 7.4) and decreased pCO2, consistent with respiratory alkalosis secondary to hyperventilation. Thus, decreasing the respiratory rate would be the most appropriate next step in his management.
(Choice A) Bronchodilator therapy is useful in patients with obstructive lung disease to help conduct inspired air from the trachea to the alveoli. This patient is not hypoxic and does not appear to be having difficulty with ventilation.
(Choice B) The FiO2 setting of 40% is appropriately low to avoid oxygen toxicity in this patient, and his PaO2 is appropriate.
(Choice D) Increasing the tidal volume would increase ventilation and worsen the patient’s respiratory alkalosis.
(Choice E) Incentive spirometry is utilized to prevent atelectasis in bed-bound patients, particularly following surgical procedures. It has no role in the treatment of intubated, mechanically ventilated patients.
(Choice F) This patient's blood pressure is within normal range. Some patients with severe acute pancreatitis do need support with positive inotropic agents, but they are not indicated in this particular patient at this time.
(Choice G) Chest physiotherapy is utilized in patients with pneumonia or atelectasis to loosen and promote expectoration of secretions. Patients with bronchiectasis require long-term chest physiotherapy.
(Choice H) A tracheostomy may be indicated if prolonged intubation is foreseen or if the patient has difficulty maintaining airway patency and/or managing airway secretions following weaning from mechanical ventilation, but this is not a pressing issue in this patient at this time.
Educational objective:
Ventilation is defined as the product of the respiratory rate and the tidal volume. Respiratory alkalosis results from hyperventilation. In mechanically ventilated patients with respiratory alkalosis in the setting of an appropriate tidal volume, the respiratory rate should be lowered.