Table 1.

Factors affecting patient-ventilator synchrony

Factor	Effects
Physiological condition of the patient	Restrictive lung disease: Due to rapid shallow breathing, it may lead to premature cycling Obstructive lung disease: Due to prolonged expiration (expiratory cycle), the inspiratory cycle shortens, resulting in a mismatch between the inspiratory time that the patient demands and this may lead to a delay in cycling
Air leak	Expiratory leak: Leads to auto-triggering Inspiratory leak: Leads to delays in cycling and reduced inspiratory sensitivity. During pressure support, significant leaks result in increases in the inspiratory flow in order to increase airway pressure to attain the set airway pressure. The large increase in inspiratory flow during leak compensation may result in significant side effects, including impaired mask seal and hence causing more leak, or gastric distension Leak leads to loss of PEEP and pressure support, and rebreathing
Level of pressure support	High-pressure support delays expiratory cycle and the ventilator breath will continue into the neural expiration Low-pressure support leads to early expiration and delayed triggering while the respiratory muscles are still contracting. This, in turn, leads to delayed triggering and wasted effort
Interface type	Can affect air leaks and mechanical dead space and CO2 accumulation and PVA Helmet can cause longer trigger delay and a shorter time of synchrony between mechanical support and patient inspiration compared with nasal and oronasal masks
Humidity	Low humidity can increase NAWR and mouth leak, which can lead to unsuccessful acclimatization to NIV in the chronic setting, and to the failure of NIV to improve gas exchange and dyspnea in the acute setting

PVA=Patient-ventilator synchrony, NIV=Noninvasive ventilation, PEEP=Positive end-expiratory pressure, NAWR=Nasal airway resistance