Published in:
01-07-2012 | Experimental
Assessment of patient–ventilator breath contribution during neurally adjusted ventilatory assist
Published in: Intensive Care Medicine | Issue 7/2012
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Purpose
During neurally adjusted ventilatory assist (NAVA), it is difficult to quantify the relative contribution of the patient versus the ventilator to the inspiratory tidal volume (Vtinsp). To solve this problem, we developed an index, the “patient–ventilator breath contribution” (PVBC), using the inspiratory deflection of the diaphragmatic electrical activity (∆EAdi) and Vtinsp during assisted and non-assisted breaths. This study evaluated the PVBC index in an experimental setup.
Method
Nine intubated and sedated rabbits were studied during repeated ramp increases of the NAVA level. One breath was non-assisted at each NAVA level. The PVBC index was evaluated during resistive loading and after acute lung injury. PVBC was calculated by relating Vtinsp/∆EAdi of a non-assisted breath to that of the preceding assisted breath. The PVBC was compared to the relative contribution of esophageal pressure (∆Pes) to transpulmonary pressure deflections (∆P
L,dyn).
Results
The relationship between PVBC and ∆Pes/∆P
L,dyn was slightly curvilinear with an intercept different from zero (y = −1x
2 + 1.64x + 0.21) and a determination coefficient (R
2) of 0.95. Squaring the PVBC values resulted in a near perfect linear relationship (y = 1.02x + 0.05) between PVBC2 and ∆Pes/∆P
L,dyn with an R
2 of 0.97.
Conclusion
This study shows that Vtinsp and EAdi can be used to predict the contribution of the inspiratory muscles versus that of the ventilator during NAVA. If clinically applicable, this could serve to quantify and standardize the adjustment of the level of assist, and hence reduce the risks of excessive ventilatory assist. Further studies are required to evaluate if this method is clinically applicable.