The Science Journal of the American Association for Respiratory Care

2000 OPEN FORUM Abstracts

ADAPTIVE SUPPORT VENTILATION REDUCED PEAK PRESSURES, IMPROVED BLOOD GASES, AND MINIMIZED AIR TRAPPING IN A CHILD WITH STATUS ASTHMATICUS

Melissa K. Brown RRT, Susan E. Duthie M.D., Department of Critical Care, Children's Hospital and Health Center, San Diego, California

Introduction: Adaptive Support Ventilation (ASV, Hamilton GALILEO, Hamilton Medical, Reno, NV.) is a closed-loop ventilator mode. It uses respiratory mechanics measurements from a variable orifice flowmeter positioned at the patient's airway to provide feedback to the ventilator on the patient's status. The Least Square Fit method allows the simultaneous measurement of Airway Pressure, Airway flow, Tidal Volume, Compliance, Resistance, and Auto PEEP. The target tidal volume and rate is calculated using Otis' equation for the least work of breathing. Inspiratory pressure and machine rate are then adjusted by the controller to meet the targets with the secondary priority of maintaining three expiratory time constants to minimize air trapping. ASV employs a lung protective strategy to minimize barotrauma. The operator sets the minimum minute volume, body weight, PEEP, and FiO2. Case Summary: The patient is a 13 year old, 50 kg male, who is a known asthmatic, with a one day history of cough and URI. He presented to an outlying Emergency Department in respiratory distress and was intubated when he failed to improve with therapy. He was transported to CHSD PICU. Upon admission he was placed on a Hamilton GALILEO ventilator with the settings P-SIMV, 40/5, rate 12, Inspiratory time (Ti) of 1.2 seconds, I:E ratio of 1:3.2, 70/30 heliox. Arterial blood gases on these settings were pH 7.13, PCO2 68, BE -6.9, PO2 75. He was treated with IV Terbutaline, Atrovent, IV Magnesium, heliox, and steroids. After 24 hours the patient's blood gases were pH 7.40, PCO2 43, BE +1.7, PO2 90 with an expiratory minute volume (Ve) of 4.3 l/min. The patient's ventilator mode was changed to ASV with the settings of 50kg body weight, 95% minute volume. The ventilator chose reduced pressures of 24/5, rate of 10-12, Ti=2.0 seconds, Vt =400cc, Ve of 4.6 l/min and I:E ratio varied between 1:1.5 to 1:2 to maintain zero air trapping. Follow up ABG was pH 7.44, PCO2 26, BE-6.7, PO2 94. The % minute volume was weaned to 85%. The patient was weaned to 70% minute volume ventilation over three days. After a brief trial of Pressure Support of 5, he was successfully extubated to a heliox 75/25 mask. Discussion: The ASV algorithm has as its first priority a lung protective strategy to minimize peak pressures and barotrauma. Its second priority is to ensure adequate time for complete emptying of the lung. This pediatric case report shows the use of ASV in an asthmatic. It also shows the ability of the controller to tailor the settings to patient disease with subsequent improvement in gas exchange. There was a reduction in peak pressures and no air trapping or Auto PEEP was observed. There are already studies documenting ASV's ability to choose settings that provide adequate gas exchange. There is one study showing ASV's ability to adapt to changing lung mechanics. More documentation is needed on how the ventilator controller will respond to difficult to ventilate respiratory patients and its effect on their outcome.

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