Airway leak size in neonates and autocycling of three flow-triggered ventilators


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Abstract

ObjectivesTo define the spectrum of airway leak in the neonatal population and examine the occurrence rate of autocycling of three flow-triggered ventilators within the defined spectrum of airleak.DesignProspective study of pulmonary function tests of intubated infants and performance of ventilators on a mechanical lung model under simulated clinical conditions.SettingAn intensive care nursery and research laboratory at a university medical center.InterventionsAnalysis of pulmonary function tests of 50 infants from our intensive care nursery, selected at random, to determine size of airleak around the endotracheal tube. The rate of autocycling of ventilators due to airleak of variable size, while connected to a test lung was subsequently studied. Ventilators were set on the assist-control mode with the control rate set at 0 breath/min. Each ventilator was studied at the maximum sensitivity setting, which was 1, 2.5, and 3.3 mL/sec for each ventilator, respectively, and also at decreased sensitivity settings to 10 mL/sec. Airleak size was varied (10% to 45%) by increasing the orifice size within the endotracheal tube adapter/connector sideport and/or the positive end-expiratory pressure level (2 to 8 cm H2 O).Measurements and Main ResultsIn the infants, airleak size was calculated during synchronous ventilator breaths as (inspiratory minus expiratory) tidal volume/expiratory tidal volume times 100% (n equals 25 plus minus 11 breaths/patient). Mean plus minus SD leak size in the infants was 15.6 plus minus 11%. A minimal leak size of 0 to 10% was present in 15 (30%) infants, leak size of 10% to 20% in 24 (48%), leak size of 20% to 30% in seven (14%), and leak size more than 30% in four (8%) infants. The relative tendency of the three ventilators to autocycle is a function of the maximum sensitivity setting, which varies with each ventilator. The ventilator with the maximum sensitivity set at 1 mL/sec autocycled rapidly (more than equals 40 breaths/min) at leak size of more than equals 10%; the ventilator set at 2.5 mL/sec autocycled rapidly at leak size of more than equals 20%; and the ventilator set at 3.3 mL/sec autocycled rapidly at leak size of more than equals 30%. In all ventilators, the rate of autocycling increased with increased leak size, and decreased with decreased sensitivity setting.ConclusionsFlow-triggered ventilators are susceptible to autocycling due to flow compensation to maintain positive end-expiratory pressure levels in the presence of an airway leak. The difference in autocycling is due to the maximum sensitivity setting of each ventilator, and not to intrinsic ventilator flowsensing or other software mechanisms. The 3.3-mL/sec setting was the least prone to autocycling and seems appropriate. The ventilator set at 2.5 mL/sec at the time of this study has been released instead at 4 mL/sec, due to these findings. The ventilator with the maximum setting at 1 mL/sec autocycled readily at leak size of more than equals 10%. Since such a leak size was present in 70% of infants, this setting should be used with caution. Using these guidelines, autocycling of all three ventilators is likely to occur mainly in 8% of infants with leak size of more than 30%. In these cases, lowering the sensitivity setting and/or positive end-expiratory pressure level may decrease autocycling, or may necessitate reintubation with a larger endotracheal tube.(Crit Care Med 1995; 23:1739-1744)

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