Chest
Volume 96, Issue 6, December 1989, Pages 1360-1363
Journal home page for Chest

Clinical Investigations in Critical Care
Continuous In-line Nebulizers Complicate Pressure Support Ventilation

https://doi.org/10.1378/chest.96.6.1360Get rights and content

Patients ventilated in the pressure support mode must generate a negative airway pressure before the ventilator will deliver a breath. Inserting a continuous-flow nebulizer between the patient and the sensor in the ventilator makes it more difficult for the patient to generate this negative pressure. We observed two mechanically ventilated patients who were unable to initiate ventilator breaths in the pressure support mode while bronchodilators were being administered through a continuous-flow nebulizer. In neither case did ventilator alarms sound. Using a lung model, we found that when the nebulizer flow rate exceeded the mean inspiratory flow rate of the test lung, the negative pressure necessary to trigger the pressure support ventilator could not be generated. Critical care providers need to be aware of this potential complication, since it may lead to serious underventilation of their patients.

Section snippets

Case 1

A 73-year-old woman with a history of congestive heart failure and COPD (FEV1, 1.0 L) presented in respiratory failure. Her trachea was intubated and lungs mechanically ventilated with a Siemens 900C ventilator in the SIMV mode at a rate of 14 breaths/min and a tidal volume of 700 ml. By the third hospital day, the SIMV rate had been reduced to 6 breaths/min, but her spontaneous respiratory rate was 35 breaths/min and she appeared dyspneic. The ventilator was changed to the pressure support

Methods

To document the effect of a continuous, in-line nebulizer on pressure support ventilation, a lung model similar to that of Katz and co-workers2 was devised. A test lung (Bio-Tek, Inc) was driven by a separate bellows that was inflated by a mechanical ventilator (Hamilton Medical) (Fig 1). A sine wave inspiratory flow pattern was produced to mimic a patient breathing with an I:E ratio of 1:1 at varying respiratory rates and tidal volumes. The test lung compliance was set at 0.035 L/cm H2O. A

Results

The calculated mean inspiratory flow rates generated by the test lung at different respiratory rates and tidal volumes are shown in Figure 2. These flows are recorded in L/min so that they may be correlated with the nebulizer flow rates. The shaded area represents inspiratory flow rates at which the test lung was unable to trigger the ventilator with the flow through the nebulizer set at 6 L/min. The numbers in parentheses represent the measured peak inspiratory flow rates at the appropriate

Discussion

The liter flow rate of the nebulizer is a major factor that contributes to the occurrence of this problem. Six L/min is commonly used to power continuous nebulizers, but as many as 10 L/min may be used on occasion. The data in Figure 1 show that high nebulizer flow rates will make it more difficult for patients to trigger the ventilator. In our experiments the ventilator was not triggered whenever the nebulizer flow rate exceeded the mean inspiratory flow rate generated by the test lung. The

ACKNOWLEDGMENTS

The writers wish to thank Drs. Richard K. Albert, Thomas R. Martin, Bruce H. Culver, and David J. Pierson for their critical review of this manuscript.

References (5)

There are more references available in the full text version of this article.

Cited by (0)

Manuscript received January 16; revision accepted May 30.

View full text