assisted / supported mechanical ventilation is designed to interact with patient muscle activity and ' share ' the work of breathing . if properly done , assisted /
, the ventilatory 's flow and pressure delivery must synchronize with patient effort during all three phases of breath delivery : breath initiation , flow delivery , and breath termination ( cycling ) .
dys - synchronous interactions can overload ventilatory muscles ( ' imposed ' loads ) , compromise alveolar ventilation , overdistend alveolar units , disrupt sleep patterns , and cause patient discomfort , prompting additional sedation .
carlucci and colleagues   have recently studied patient - ventilator interactions in 69 acutely ill patients receiving non - invasive ventilation ( niv ) for a variety of reasons .
they grouped these patients according to obstructive or restrictive pathophysiologies and carefully measured respiratory system mechanics ( including esophageal pressure - a surrogate for pleural pressure ) .
the patients were then initiated on niv according to a standard protocol and were assessed for triggering dys - synchronies defined as missed triggers , delayed triggers , or double triggers .
importantly , they found that these dys - synchronies were common ( 58% of patients experienced them with missed triggers being the most frequent ) .
trigger dys - synchronies were associated with niv intolerance and their prevalence was similar among the different patho - physiologic patterns .
this study is important as it underscores that patient - ventilator trigger dys - synchrony is common   and can be a major barrier to niv effectiveness .
this study also underscores the fact that the causes of trigger dys - synchrony are complex and affect all types of patients requiring assisted / supported mechanical ventilation [ 3 - 5 ] . like invasive mechanical ventilation , niv trigger dys - synchrony can stem from insensitive / poorly responsive breath triggering mechanisms . however , other factors also involved likely include ventilatory muscle loading patterns and their effects on the ventilatory controller , ventilatory muscle function , lung volumes at end - expiration ( that is , intrinsic positive end - expiratory pressure ( peep ) ) , gas exchange effectiveness , and cortical inputs to the patient 's ventilatory control center such as pain [ 3 - 5 ] . in addition , the triggering process with niv can be further hampered by the mask system with its inherent leaks and claustrophobia exaggerating dyspnea and neural drive .
importantly , this study did not address issues of flow synchrony or cycle synchrony , two other forms of patient ventilator interactions likely to impact niv tolerance but not well studied in the current literature .
flow synchrony is defined as the ability of the ventilator to provide flow in accordance with patient demand as manifest by the ventilatory muscle contraction pattern .
if flow is synchronous with that contraction pattern , the inspiratory muscle pressure time profile conceptually should resemble a near normal pattern .
instead it means providing flow to ' re - shape ' the inspiratory muscle 's pressure - time or pressure - volume profile to a more physiologic shape . in general
, the variable flow features of pressure targeted breaths tend to flow synchronize easier with patient effort than clinician set flow - volume targeted breaths .
an assisted / supported mechanical breath termination shorter than the neural inspiratory time ( machine ti < neural ti ) can lead to muscle activity beyond the machine 's flow delivery phase , which can lead to high muscle loading , excessive tidal volumes and/or triggering of a second breath .
in contrast , when mechanical breath cycling terminates after the inspiratory effort has ended ( machine ti > neural ti ) , dyspnea and expiratory muscle recruitment may occur in an effort to terminate the breath .
importantly , it is worth noting that dys - synchronous interactions often result in anxiety and dyspnea , which can stimulate overall ventilatory drive .
thus , improving synchrony in one area ( for example , triggering ) can help facilitate achieving synchrony in other areas ( for example , flow demand ) . in summary ,
patient - ventilator interactions are complex and are important to recognize and manage during both invasive and non - invasive mechanical ventilation .
carlucci and colleagues have demonstrated how common the trigger dys - synchronies are during niv and that understanding them requires assessment not only of respiratory system mechanics , but also the complex interactions of neural drive , gas exchange , muscle function , and muscle loading .
much research is needed in this area as dys - synchronies of all types can lead to overloaded muscles and dyspneic patients requiring sedation - two factors that clearly can lead to prolonged mechanical ventilation days and poor outcomes .
 patient - ventilator trigger dys - synchronies are common with the use of assisted forms of mechanical ventilatory support , including non - invasive mechanical ventilatory support ( niv ) . 
 future system designs need to address this in order to improve the effectiveness of niv . 
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