The Science Journal of the American Association for Respiratory Care

2008 OPEN FORUM Abstracts

VENTILATOR ALARMS IN PC MODE, INVOLVING PEDIATRIC PATIENTS

Cynthia White1, Tanya Scholl1



Introduction: With neonatal and pediatric patients, the PC mode continues to be utilized as one of the primary modes of ventilation. Bedside practitioners may forget that only low pressure and low minute volume alarms can be relied on when using the PC mode of ventilation. The low minute volume alarm must be set tightly in order to improve its sensitivity and clinical usefulness. Setting the low minute alarm value tightly is not always clinically possible or realistic when ventilating these small patients where large ETT leaks are often present.

Bench Study:
A patient scenario in our PICU led us to question why a ventilator's patient safety alarm was not activated in a situation of complete ETT obstruction and no visible chest rise. We set up a bench study with a #4 ETT connected to a test lung to try and answer our questions. A Servoi ventilator was calibrated according to manufacturer's specifications with an infant circuit. The ventilator was placed in PC mode. The ETT was attached to the test lung and then clamped with hemostats to produce a scenario of ETT obstruction. We believe that our clinical situation was reproduced by using this simulated ETT obstruction model. During our simulated obstruction observation, the ventilator continued to cycle normally and normal airway pressure waveforms were displayed on the ventilator's graphics monitor. The ventilator measured an average of 50% of the original tidal volume at the exhalation cassette. The default setting for the ventilator graphics display was set for automatic rescaling. When viewing the scalar graphic waveforms, there was no recognizable change in the morphology of the pressure, flow, or volume waveforms displayed on the ventilators graphics monitor. The ETT was unclamped and the scenario was repeated by obstructing the entire inspiratory limb of the ventilator's circuitry. The inspiratory limb obstruction yielded similar measurement and alarm results as described previously.

Discussion:
In order to activate a ventilator patient safety alarm in a mechanical obstructive scenario, the low minute volume alarm value must be set within tight limits. Some ventilator companies have attempted to address this issue by adding an ETT obstruction alarm into their software. The ETT obstruction feature alarms when there is an acute difference between the percent inspiratory and expiratory tidal volume measurements, thus providing an alarm in the case of ETT obstruction or pinched circuitry.