Tidal Volume Predictive of NIV Failure
Monitored tidal volumes persistently greater than 9.5 ml/kg predicted body weight (PBW) during NIV suggest the need for intubation and are predictive of NIV failure. 1
Critical Threshold for NIV Failure
The 9.5 ml/kg PBW threshold has high diagnostic accuracy: In patients with moderate-to-severe hypoxemia (PaO2/FiO2 up to 200 mm Hg), expired tidal volume above 9.5 ml/kg PBW predicted NIV failure with 82% sensitivity and 87% specificity. 2
Patients who failed NIV had significantly higher mean expired tidal volumes compared to those who succeeded (10.6 ml/kg PBW vs 8.5 ml/kg PBW, p = 0.001), and this was independently associated with NIV failure in multivariate analysis. 2
Physiological Rationale
High tidal volumes during NIV indicate excessive transpulmonary pressure swings, which increase the risk of patient self-inflicted lung injury (P-SILI), particularly in patients with high respiratory drive. 1
Large tidal volumes during NIV can be significantly higher than during invasive ventilation, especially when substantial inspiratory pressure is delivered and exacerbated by the high inspiratory demand in acute hypoxic respiratory failure. 1
The total pressure dissipated to inflate the lungs can be excessive during NIV, and such large transpulmonary pressures with resulting large tidal volumes may exacerbate lung injury if prolonged over time. 1
Clinical Monitoring Algorithm
When initiating NIV, monitor the following parameters closely:
Measure expired tidal volume at each NIV session and calculate the average over time. 2
If tidal volumes persistently exceed 9.5 ml/kg PBW, this signals impending NIV failure and the need for intubation should be strongly considered. 1, 2
A rapid shallow breathing index (RSBI) greater than 105 breaths/min/L may also be associated with need for intubation in patients receiving NIV. 1
Target Tidal Volume During NIV
The ideal target during NIV is to maintain expired tidal volume between 6-8 ml/kg PBW, though this is difficult to achieve in the majority of patients with de novo acute hypoxemic respiratory failure. 2
A low expired tidal volume is almost impossible to achieve in most patients receiving NIV for acute hypoxemic respiratory failure, and inability to maintain low tidal volumes is independently associated with NIV failure. 2
Critical Pitfalls to Avoid
Delayed intubation is associated with increased mortality in patients with acute respiratory failure, so failure to recognize persistently high tidal volumes (>9.5 ml/kg PBW) and delay intubation can have devastating consequences. 1
Deterioration can occur abruptly during NIV, so patients should be monitored closely with frequent reassessment. 1
If there is no substantial improvement in gas exchange and respiratory rate within a few hours of NIV initiation, invasive mechanical ventilation should be started without delay. 1
The ability to use lung protective ventilator strategies (maintaining low tidal volume of 6 ml/kg PBW) may be more difficult with NIV than with invasive ventilation. 1