A 67-year-old woman with a history of end-stage chronic obstructive pulmonary disease (COPD) is admitted to the hospital because of severe respiratory distress. She requires intubation in the emergency department because of respiratory acidosis. The patient's mechanical ventilator setting is in assist control (AC) mode, with the respiratory rate at 16 breaths per minute, the tidal volume at 500 mL, and the fraction of inspired oxygen (FIO2) at 80%. Her vital signs are temperature, 100.4°F (38°C); heart rate, 104 beats per minute (bpm); pulse oximetry reading, 90%; and blood pressure, 130 mm Hg systolic, 80 mm Hg diastolic.
Two hours later, the nurse pages you because the patient's blood pressure had decreased to 75 mm Hg systolic, 50 mm Hg diastolic. Physical examination reveals diffuse and bilateral wheezing with distinctive heart sounds. A frontal chest radiograph (A) is obtained.
What should be performed immediately to reverse the hypotension?
a. Starting a dopamine infusion
b. Placing a chest tube
c. Decreasing the ventilator rate
d. Increasing the tidal volume
Chest radiograph A reveals hyperinflated lung fields with the endotracheal (ET) tube properly positioned.
Hypotension secondary to severe respiratory distress: The correct answer is C, decreasing the ventilator rate. The cause of this patient^s hypotension is the auto–positive end-expiratory pressure (auto-PEEP) secondary to the hyperinflated lungs induced by a rapid ventilatory rate. Venous return is decreased because of elevated intrathoracic pressures, which cause a decrease in cardiac output and blood pressure. Auto-PEEP, intrinsic positive end-expiratory pressure (PEEP), occult PEEP, and positive end-expiratory alveolar pressure are terms that have been used to describe the failure of alveolar pressure to return to zero (atmospheric pressure) at the end of exhalation.
Auto-PEEP frequently occurs in mechanically ventilated patients, especially those with asthma or COPD. It is the consequence of the ventilator delivering a positive-pressure breath before the patient has had time to completely exhale the previous breath.
When a patient is intubated, a number of factors can contribute to the development of auto-PEEP: the ventilator rate, the end-inspiratory pause, and the inflation volume. Careful monitoring and repeated evaluation of high-risk patients are helpful to identify the problem.
The expiratory flow waveform should be monitored, and if the flow does not return to baseline before the delivery of the next breath, auto-PEEP is present. If auto-PEEP is not recognized, inappropriate clinical decisions (in terms of diagnostics and interventions) may be made.
A 2-minute trial of cessation of mechanical ventilation or decreasing the ventilator rate should be performed. The diagnosis is confirmed if the trial results in rapid improvement in hemodynamics.
In general, auto-PEEP can be diminished by different means: decreasing the ventilatory frequency, decreasing the tidal volume, increasing the expiratory duration, sedating the patient, performing frequent suctioning, and using bronchodilators.
Starting a dopamine infusion in this patient would not treat the cause of the hypotension, and placing a chest tube is indicated only in the case of a pneumothorax, which is one of the complications of auto-PEEP.
During auto-PEEP, a portion of the tidal volume is trapped in the alveoli. Therefore, the answer is to decrease rather than increase the ventilator tidal volume.
In this case, the flow and pressure waveforms revealed an auto-PEEP of 12 cm H2O. The respiratory rate was decreased to 10 breaths per minute, and another chest radiograph (B) was obtained shortly afterward. The patient^s blood pressure increased and stabilized at 130 mm Hg systolic/ 70 mm Hg diastolic.