The Science Journal of the American Association for Respiratory Care

2006 OPEN FORUM Abstracts

An Evaluation of Respiratory Rate Change Response Times of a Conventional & Closed Loop Mode of Ventilation in a Simulated Spontaneous Breathing Model.  

David A. Grooms, BS, RRT, Elizabeth Ryland, RRT.  Sentara Leigh Hospital , Norfolk , VA

Background:  Manipulation of the ventilator respiratory rate (RR) control is a common practice of bedside clinicians. The goal of manipulating RR is to provide patients with acceptable minute volumes to ensure adequate alveolar ventilation, and minimize patient work of breathing (WOB).  In the event a patient becomes tachypneic, or demonstrates an increased WOB, practioners will commonly increase the ventilator RR in an attempt to "capture" or take over the patients breathing pattern. The purpose of this study was to assess ventilator RR change response times in a simulated spontaneous breathing model of a conventional Pressure Control Mode (PC/AC) and a closed loop mode of ventilation, Adaptive Tidal Volume Support (AVTS). 

Methods: A simulated spontaneous breathing model was utilized using a dual chamber, Double TTL test lung (Michigan Instruments).  One test lung (driving model) was powered by a Hamilton Galileo Gold  ventilator (Hamilton Medical Inc.) using minimum settings to generate spontaneous breathing of a second test lung (spontaneous model) connected to another Hamilton Galileo ventilator. The Pressure Control (PC) mode was used for the driving ventilator, however settings did differ to simulate different patient scenarios/WOB/P0.1, etc. There were two stages, consisting of two arms per stage. Stage I driving vent settings were PC/AC mode, PC of 5cmH2O (set to create low to moderate WOB, P0.1 of -2.8), RR 12bpm, I-time 1.3 seconds, PEEP 5.  Stage II driving vent settings were PC/AC mode, PC of 25cmH2O (set to create increased WOB, P0.1 -8.2), RR 30 bpm, I-time 0.6 seconds, PEEP 5cmH2O.  Each stage consisted of a direct observation of the first machine initiated breath that was delivered to the spontaneous model when the RR control was set below, equal to, and above the simulated spontaneous rate. Time to mandatory breath delivery without disruption of spontaneous breathing cycle was observed and determined by stop watch, and by measurement of graphic waveform analysis.  Increases to RR control settings were consistently activated during the beginning of inspiration.  This was discovered to optimize breath delivery in the shortest time frame due to cycle time criteria.  Following data collection on each RR increase, the RR control setting was reset below the spontaneous rate for 2 minutes to allow equilibration between the two models.  If there was no delivery of a machine initiated breath, the observation was limited to less than 3 minutes (180 seconds).    

Results:  Stage I:  RR control settings of 10-20 bpm demonstrated response times of >3min to 2.9 seconds in PC/AC (Arm I) and >3min to 36 seconds in AVTS (Arm II).  Stage II: RR control settings of 28-36bpm demonstrated response times of >3min to 1.54 seconds in PC/AC (Arm I) and >3min to 20 seconds in AVTS mode (Arm II).

Conclusion:  Conventional PC/AC produced faster response times of mandatory breath type delivery when the RR control setting was adjusted above the spontaneous breathing rate. There was a significant delay in mandatory breath type delivery in AVTS.  The response time consistently improved as the mandatory RR control setting was increased in both modes. An unexpected and significant finding of this study demonstrated that the initial delivery of mandatory breaths is dependent on which phase of the time cycle the change is activated.  For example, when RR changes above the spontaneous RR were activated in the beginning phase of inspiration, it resulted in an increased mandatory breath response time, in contrast to being initiated in the mid expiration phase.  When activated in mid expiration, the mandatory breath was delayed by the anticipated spontaneous breath.

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