Assessment of patient-ventilator breath contribution during neurally adjusted ventilatory assist in patients with acute respiratory failure

Ling Liu, Songqiao Liu, Jianfeng Xie, Yi Yang, Arthur S Slutsky, Jennifer Beck, Christer Sinderby, Haibo Qiu, Ling Liu, Songqiao Liu, Jianfeng Xie, Yi Yang, Arthur S Slutsky, Jennifer Beck, Christer Sinderby, Haibo Qiu

Abstract

Introduction: We previously showed in animals that the ratio of inspired tidal volume (Vtinsp) to inspiratory peak electrical activity of the diaphragm (EAdipk) can be used to quantify the respective patient and ventilator breath contributions (PVBCs) during neurally adjusted ventilatory assist (NAVA). The PVBC index has not been tested clinically.

Methods: We studied 12 intubated and mechanically ventilated patients with acute respiratory failure and measured EAdipk, airway (Paw) and inspiratory esophageal pressure (Pes) and Vtinsp. We applied 11 different NAVA levels, increasing them every 3 minutes in steps of 0.3 cm H₂O/μV from 0 to 3.0 cmH₂O/μV. At each NAVA level, one breath was non-assisted (NAVA level 0). PVBC indices were calculated by relating Vtinsp/EAdipk of the non-assisted breath to Vtinsp/EAdipk of the assisted breath(s) using one ((N1)PVBC) or the mean value of five preceding assisted breaths ((X5)PVBC). During assisted breaths, inspiratory changes in Pes (∆Pes) and transpulmonary (ΔPtp) pressures were used to calculate the relative contribution of patient to total inspiratory lung-distending pressures (ΔPes/ΔPtp). Matching of respiratory drive indices and squaring of the PVBC was evaluated for their effect on the correlation between PVBC and ΔPes/ΔPtp. Linear regression analysis and Bland-Altman analysis were applied to compare indices.

Results: Using an average of five assisted breaths prior to the non-assisted breath and squaring the PVBC ((X5)PVBC(2)) improved determination coefficients (P <0.05), adjusted the regression slope and intercept between PVBC and ΔPes/ΔPtp toward identity (P <0.05) and reduced bias (P <0.05). Matching EAdipk between non-assisted and assisted breaths within the range of 0.77 to 1.30 improved the relationship between (X5)PVBC(2) and ΔPes/ΔPtp (P <0.05) and abolished the need for EAdi normalization in the PVBC calculation (R(2) = 0.96; bias = 0.16 ± 0.06; precision = 0.33 ± 0.08 (mean and 95% confidence interval)).

Conclusions: This clinical study confirms previous experimental results showing that the PVBC(2) predicts the contribution of the inspiratory muscles versus that of the ventilator during NAVA, when differences in effort (EAdi) between non-assisted and assisted breaths are limited. PVBC could help to quantify and standardize the adjustment of the level of assist, and hence reduce the risks of excessive ventilatory assist in patients.

Trial registration: ClinicalTrials.gov NCT01663480. Registered 9 August 2012.

Figures

Figure 1
Figure 1
Description of reference points used for calculation of respiratory variables. Reference points are indicated for calculating peak airway pressure (Pawpk), change in esophageal pressure (Pes) from onset of electrical activity of the diaphragm (EAdi) to peak (nadir) (ΔPes), change in transpulmonary pressure (Ptp) from onset of EAdi to peak (ΔPtp), peak EAdi (EAdipk) and inspiratory tidal volume (Vtinsp). Neural inspiratory period (Ti) and total breath time (Ttot) are indicated at bottom. Breathing frequency (Fb) was calculated as 60/Ttot.
Figure 2
Figure 2
Example of neurally adjusted ventilatory assist level titration in one patient. (A) Effect of 11 increases of the neurally adjusted ventilatory assist (NAVA) level in steps of 0.3 cmH2O/μV (x-axis) on peak airway pressure (Pawpk), peak inspiratory electrical activity of the diaphragm (EAdipk), tidal volume (Vtinsp) during assisted (closed circles) and non-assisted (open circles) breaths and esophageal pressure peak inspiratory change from onset of EAdi (ΔPes). (B) and (C) Respective raw signals for Paw, EAdi, volume and Pes for five assisted and one unassisted breath (NAVA zero breath is indicated by yellow vertical bars) at a low NAVA level (B, red tracings), which relates to the red dot in (A), and at high NAVA level (C, green tracings), which relates to the green dot in (A).
Figure 3
Figure 3
Impact of breath averaging and breath matching on coefficient of determination between patient-ventilator breath contribution indices and ratio of inspiratory changes in esophageal pressure and transpulmonary pressure.Left: Coefficient of determination (R2) between patient-ventilator breath contribution (PVBC) and ratio of inspiratory changes in esophageal pressure and transpulmonary pressure (ΔPes/ΔPtp) (y-axis) is plotted against different matching criteria of increasingly strict inclusion levels (x-axis). Data are presented for PVBC indices when PVBC is calculated with one assisted breath (N1PVBC, open symbols) or with five assisted breaths averaged (X5PVBC, closed symbols). The determination coefficient was found to improve for X5PVBC at matching levels for ratios of inspiratory peak electrical activity of the diaphragm (EAdipk) with versus without assist (EAdipk,no-assist/EAdipk,assist) ranging from 0.77 to 1.30 ($P <0.05, orange closed symbols) and to become significantly higher than N1PVBC (*P <0.05). Increasing the matching of the neural inspiratory time (Ti) between assisted and non-assisted breaths (Tino-assist/Tiassist, blue symbols) did not result in any improvement in R2. Values are presented as mean with 95% confidence interval. Right: Same as left graph, but for PVBC2. The R2-value between PVBC2 and ΔPes/ΔPtp (y-axis) is plotted when PVBC2 is calculated with one assisted breath (N1PVBC2, open symbols) or with five assisted breaths averaged (X5PVBC2, closed symbols) and when different matching criteria are used of increasingly strict inclusion levels (x-axis). The determination coefficient was found to improve for X5PVBC2 at matching levels for EAdipk,no-assist/EAdipk,assist of 0.77 to 1.30 ($P <0.05, *P <0.05, orange closed symbols) and to become significantly higher than N1PVBC2 (*P <0.05). Increasing the matching of the Ti between assisted and non-assisted breaths (Tino-assist/Tiassist, blue symbols) did not result in any improvement in R2. Values are presented as mean with 95% confidence interval.
Figure 4
Figure 4
Distribution of ratios between peak inspiratory electrical activity of the diaphragm without assist to peak inspiratory electrical activity of the diaphragm with assist. Histograms showing distribution of all electrical activity of the diaphragm (EAdi) matching index values (EAdipk,no-assist/EAdipk,assist) when calculated using a single assisted breath (left) or using the average of five breaths preceding the non-assisted breath (right). Of all the breaths, 78% fell between 0.77 and 1.30.
Figure 5
Figure 5
Group mean values of measured variables during neurally adjusted ventilatory assist level titration.Leftmost panel: Mean and 95% confidence interval values (y-axis) for peak inspiratory change in esophageal pressure from onset of electrical activity of the diaphragm (ΔPes, open blue circles), applied positive end-expiratory pressure (PEEP, open green squares), peak airway pressure (Pawpk, green solid squares), peak inspiratory change in transpulmonary pressure from onset of electrical activity of the diaphragm (ΔPtp, blue solid circles) above PEEP, as the neurally adjusted ventilatory assist (NAVA) level was increased from 0 to 3.0 cmH2O/μV (x-axis). Next three panels to the right: Mean and 95% confidence interval values plotted for peak electrical activity of the diaphragm (EAdipk), inspiratory tidal volume (Vtinsp) for assisted (closed symbols) and non-assisted breaths (open symbols) and breathing frequency (Fb) during increasing NAVA levels.
Figure 6
Figure 6
Relationship between patient-ventilator breath contribution indices and ratio of inspiratory changes in esophageal and transpulmonary pressure indices with and without electrical activity of the diaphragm normalization. (A) Group mean values and 95% confidence interval for indices of patient-ventilator breath contribution calculated from single non-assisted breath compared with five averaged electrical activity of the diaphragm (EADi) values from the five preceding breaths (X5PVBC), X5PVBC2 and ratio of inspiratory changes in esophageal and transpulmonary pressure (ΔPes/ΔPtp) with increasing neurally adjusted ventilatory assist (NAVA) levels. (B) and (C)X5PVBC and X5PVBC2 plotted against ΔPes/ΔPtp, respectively. Note how X5PVBC2 corrected the regression slope against ΔPes/ΔPtp. (D) Group mean values and 95% confidence intervals for X5(PVBCβ), X5(PVBCβ)2 and ΔPes/ΔPtp indices with increasing NAVA levels. (E) and (F)X5(PVBCβ) and X5(PVBCβ)2 plotted against ΔPes/ΔPtp, respectively. Note how X5PVBCβ2 corrected the regression slope against ΔPes/ΔPtp. All values were calculated with the five-breath averaging technique and EAdi matching criteria (EAdipk,no-assist/EAdipk,assist) of 0.77 to 1.30.

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Source: PubMed

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