Not all types of asynchrony are created equal

Dear Editor,We read with interest the articleentitled, ‘‘Asynchrony, neural drive,ventilator variability and COMFORT:NAVA versus pressure support inpediatric patients’’ [1]. The authorsfound that the use of NeurallyAdjusted Ventilatory Assist (NAVA)improved patient-ventilator syn-chrony and patient comfort comparedto pressure support whether deter-mined by the treating physician oroptimized by means of protocolizedadjustments in trigger sensitivity,inspiratory rise time and terminationcriteria, airway pressure and appliedPEEP.First, we appreciate the citation ofour previous work entitled ‘‘Neurallytriggered breaths reduce trigger delayand improve ventilator response timesin ventilated infants with bronchioli-tis’’ [2]. However, the commentary onour work is not accurate. We com-pared NAVA to volume supportrather than pressure support, and itwas in human infants with bronchi-olitis, not in a mechanical lungmodel. This difference is importantbecause few studies have been pub-lished examining the use of NAVA inpediatric patients.Second, in their study, the authorslimited their documentation ofasynchrony to autotriggered, double-triggered and non-triggered breaths(types of trigger asynchrony) as wellas an asynchrony index. In a morerecent study that used the samemethodology as our infant study, wealso found that the use of NAVAsignificantly reduced the overallincidence of asynchrony, particularlytrigger asynchrony in a recoveringlung injury animal model. However,we also found that the incidence offlow asynchrony, short cycles andmore common with neurally triggeredbreaths than with pneumatically trig-gered breaths [3]. Flow asynchronyoccurs if ventilator flow does notequal patient flow. This type ofasynchrony can be detrimental topatients because it can lead to auto-PEEP and other types of asynchrony.Short cycles may be viewed graphi-cally as double triggering and aredefined as inspiratory time less thanhalf the mean inspiratory time. Theseasynchronous breaths can result fromimproperly set termination criteriasuch that the ventilator inspiratorytime ends before the patient’s inspi-ratory effort is complete. The loss ofventilator support during inspirationcan result in increased work ofbreathing. Expiratory asynchrony, inparticular shortened expiratory time,can also promote air trapping andauto-PEEP [4]. We believe that thesetypes of asynchrony should not beoverlooked, as all types of asyn-chrony can lead to prolonged durationof ventilation [5].