The risk for TV to be greater than 6 ml/kg IBW was significantly increased with spontaneous breaths assisted by PSV modes (for PSV OR 19.36; [12.37–30.65]) and significantly reduced in APRV (OR 0.44; [0.26–0.72]) and PSV with guaranteed volume mode.
By comparing the ventilator and Edi timings, PVA was identified, including wasted efforts (clear effort observed on Edi with no ventilator assist), auto-triggered breath (ventilator assist delivered in the absence of Edi increase), double triggering (two rapidly successive assists following a single effort), and inspiratory trigger and cycling-off errors.
The cycling-off dys-synchronies are reported as absolute values as cycling-off dys-synchronies usually have a wide distribution of values with positive or negative values [ 17, 29]; (c) ineffective effort: neural breath not assisted by the ventilator; (d) autotriggered breath: initiation of mechanical assist by the ventilator without an inflection of EAdi.
Comparison of Vtinsp corrected for neural inspiratory drive for a non-assisted breath (Vtinsp/EAdipk)no-assist to that during an assisted breath (that is, (Vtinsp/EAdipk)assist) resulted in a PVBC index that closely reflected the ratio between patient (ΔPes) and total (patient + ventilator = ΔPtp) mechanical inspiratory effort.
Consequently, if the breath without assist failed to meet the inclusion criteria for EAdipk or Ti matching, we would lose the PVBC calculation for that NAVA level.
For this purpose, we calculated PVBC simply as the ratio between Vtinsp for non-assisted to assisted breaths (Vtinsp,no-assist/Vtinsp,assist), subsequently referred to as PVBCβ (using same annotations as the PVBC).
Using EAdi to match non-assisted and assisted breaths eliminated the need to correct for changes in neural respiratory drive and allowed computation of PVBC based on inspiratory volume alone.
Matching EAdipk between non-assisted and assisted breaths within the range of 0.77 to 1.30 improved the relationship between X5PVBC and ΔPes/ΔPtp (P <0.05) and abolished the need for EAdi normalization in the PVBC calculation (R = 0.96; bias = 0.16 ± 0.06; precision = 0.33 ± 0.08 (mean and 95% confidence interval)).
PVBC indices were calculated by relating Vtinsp/EAdipk of the non-assisted breath to Vtinsp/EAdipk of the assisted breath(s) using one (N1PVBC) or the mean value of five preceding assisted breaths (X5PVBC).
As mentioned in the Introduction above, to calculate the PVBC-related indices, we assumed that either respiratory drive should be fairly similar for the unassisted and the assisted breaths or breaths could be corrected by using respiratory drive to "normalize" Vtinsp [ 12].
