Main

High-frequency oscillatory ventilation (HFOV) is used to treat severe neonatal lung disease and has the potential to reduce ventilator-induced lung injury when applied optimally (1,2). HFOV optimally applied aims to recruit the lung and subsequently reduce mean airway pressure (PAW) to an optimal pressure (Popt) that achieves optimal lung recruitment at the lowest distending pressure (open lung strategy) (3,4). However, identification and maintenance of Popt remains challenging, particularly in the newly born, whose lungs are in a highly dynamic state because of fluid reabsorption and establishment of functional residual capacity (5). The lack of appropriate tools for the bedside assessment and continuous monitoring of lung function in infants makes targeting of Popt even more difficult.

Oxygenation, evaluated by monitoring oxygen saturation (SpO2), is most commonly used for targeting Popt during HFOV in clinical practice (6). However, SpO2 is an imperfect guide for PAW titration as it is relatively uniform over a wide range of airway pressures and volumes (7). Beginning with the observations of Suter et al. (8), the notion that lung mechanics can guide pressure settings during mechanical ventilation has been examined carefully. For conventional ventilation, a relationship between end-expiratory lung volume (VL), recruitment, and tidal breath mechanics has been demonstrated repeatedly (9,10). During HFOV, the same theoretical considerations apply (3), but the best means of assessing the mechanics of the oscillated lung remains unclear.

A promising approach to noninvasive bedside assessment of lung mechanics in ventilated newborns is the forced oscillation technique (FOT) (11,12,13,14). FOT measures the mechanical impedance of the respiratory system (ZRS) by evaluating its response to pressure oscillations (15). ZRS is commonly expressed in terms of resistance of the expiratory system (RRS) and reactance of the respiratory system (XRS), which in turn represents the elastic and inertive properties of the system. Recent studies, using broad-band stimulating signals including very-low-frequency components (low frequency forced oscillation technique (LFOT)), showed that the frequency dependence of RRS is a sensitive indicator of mechanical heterogeneities in the lungs and that low-frequency dynamic elastance can be used to assess VL recruitment and distention during ventilation (16,17,18,19,20). Moreover, if proper mathematical interpretative models are applied to LFOT data, it is possible to discriminate between the mechanical properties of airways and lung tissue. This approach identified hysteresivity as the most sensitive parameter to VL recruitment in the preterm lung (21). Unfortunately, clinical application of LFOT for continuous bedside monitoring is limited by its high sensitivity to leaks and, importantly, by the interference of spontaneous breathing on the low-frequency components of the forcing signal. Recently, single-frequency FOT has been applied during conventional mechanical ventilation both in animal models (17,20,22,23) and in ventilated preterm newborns (11). In particular, XRS at 5 Hz (XRS) identified the lowest positive end-expiratory pressure level able to keep the lung fully recruited (22,23), minimizing lung injury (23). Moreover, it has been demonstrated that changes in XRS measured at 5 Hz can also be evaluated during HFOV from the high-amplitude oscillatory waveform delivered by the ventilator allowing, in principle, FOT measurement without suspending the delivery of ventilation to the patient (24).

We hypothesized that monitoring the dynamics of XRS during decremental pressure steps on HFOV, after recruitment, would allow the identification of approaching derecruitment without waiting for physiological variables with long stabilization times, and without bringing the lung to significant derecruitment to identify the lowest PAW able to prevent atelectasis. Based on this hypothesis, we evaluated the temporal change in XRS at each step during a trial of decreasing PAW after volume recruitment. Popt was defined as the lowest PAW at which XRS does not decrease over time within a PAW step. We reasoned that Popt thus defined should maintain recruitment and stability of lung mechanics.

The aims of this study were (i) to characterize the relationships between XRS, measured while delivering HFOV, and pressure, oxygenation, regional VLs, and tidal volume (VT) and (ii) to compare Popt defined by XRS with that defined by oxygenation, VL, or VT, during a trial of decreasing PAW after volume recruitment.

Results

Animal Characteristics

Nine lambs (six male) with a mean (SD) birth weight of 3.5 (0.9) kg and cord pH immediately prior to birth of 7.38 (0.03) were studied. All animals completed the protocol without complications, including no evidence of pneumothorax. Table 1 reports ventilator settings, arterial blood gas, and hemodynamic variables at relevant protocol steps. After attaining Pmax, gas exchange improved markedly, whereas mean arterial pressure and heart rate decreased. As PAW was reduced, blood pressure and heart rate were restored. Changing oscillatory frequency to 5 Hz for the duration of FOT measurements did not modify VL.

Table 1 Ventilatory and physiological indices at selected protocol steps

Relationship Between XRS and Other Variables

Figure 1 shows the relationships of oxygenation, XRS, VL, and VT with PAW during mapping of the deflation limb in a representative animal. As PAW was reduced from Pmax to 12 cmH2O, the lung displayed a predominantly elastic behavior: XRS increased (became less negative, indicating an increase in respiratory system compliance), suggesting a reduction in tissue distension. For pressure values lower than 12 cmH2O, there was no subsequent sustained benefit in XRS, and a noticeable reduction between t0min and t2min occurred with subsequent steps, suggesting that the applied pressure was no longer able to maintain recruitment. Therefore, for the subject shown, Popt defined by XRS was 12 cmH2O. This was also the upper corner pressure (Popt value) of the PAWVL relationship. In contrast, oxygenation was unchanged between Pmax and a PAW of 10 cmH2O and then decreased rapidly as PAW was reduced further, resulting in an oxygenation-defined Popt of 10 cmH2O. Finally, the PAWVT relationship was dome shaped with maximum VT (Popt) occurring at 14 cmH2O.

Figure 1
figure 1

Relationship between pressure and relevant variables in a representative animal. Relationship between mean airway pressure (PAW) and (a) oxygenation (SpO2/FiO2), (b) lung volume (VL), (c) reactance of the respiratory system (XRS), and (d) tidal volume (VT) during the deflation series in a representative animal. Change in VL and XRS values immediately after (t0min; open circles) and 2 min after (t2min; closed circles) each PAW decrement are also shown.

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Combining data from all animals, Pmax occurred on average (SD) at 28.2 (4.5) cmH2O ( Figure 2 ). Attaining Pmax resulted in a marked improvement in oxygenation, lung mechanics, VL, and VT compared with Pstart ( Table 2 ). Thereafter, on the deflation limb, Pcl occurred at 10.7 (5.9) cmH2O.

Figure 2
figure 2

Mean relationship between pressure and relevant variables. Relationship between mean airway pressure (PAW) at key steps in the mapping of the pressure–volume relationship and (a) SpO2/FiO2, (b) lung volume (VL), (c) reactance of the respiratory system (XRS), and (d) tidal volume (VT). In each figure, data for initial pressure (Pstart), maximum pressure (Pmax), and five decremental PAW series before closing pressure (Pcl) are shown. VL is expressed as a percentage of the VL at Pmax. The XRS and VL data at t0min (solid lines, closed circles) and t2min (open circles, dotted lines) of each PAW step are shown. Optimal pressure (Popt) defined by oxygenation occurred at Pcl+2 cmH2O, whereas XRS, VL, and VT identified a Popt at Pcl+6 cmH2O, and the point of sign change in the t0min to t2min relationship for each parameter. All data are expressed as mean and SEM. SpO2, oxygen saturation.

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Table 2 Changes in oxygenation, reactance, VL, and VT attaining maximal recruitment

Overall, oxygenation remained stable until Pcl+2 cmH2O. At 2 min, the average PAWXRS relationship presented a maximum at Pcl+6 cmH2O. At pressures below Pcl+6 cmH2O, XRS decreased between t0min and t2min, indicating that VL recruitment could not be maintained. The XRS-defined Popt of Pcl+6 cmH2O also coincided with the upper corner pressure of the PAWVL relationship and with the maximum of the PAWVT relationship, indicating a common Popt value. In addition, below Pcl+6 cmH2O, both VL and VT were unstable between t0min and t2min, suggesting that this was the PAW value in which derecruitment became significant within the lung. The regional VL data supported this. Below Pcl+6 cmH2O, the dorsal hemithorax demonstrated a greater change in VL than the ventral hemithorax, suggesting a nonuniform gravity-dependent pattern of derecruitment ( Figure 3 ).

Figure 3
figure 3

Dynamic changes in regional lung volume (VL). Difference in regional end-expiratory volume recorded at t0min and t2min for the 2-min period at each mean airway pressure (PAW) step for the ventral (open circles) and dorsal (closed circle) hemithoraces during the last five PAW steps prior to closing pressure (Pcl) (deflation series). Y axis: absolute percent difference in normalized regional volume. Data expressed as mean and SEM. *P < 0.05 between volumes in the dorsal and ventral region (two-way ANOVA for repeated measurements with Tukey post hoc test).

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Popt Defined by Oxygenation, XRS, VL, and VT

In all subjects, oxygenation defined a significantly lower Popt than reactance, VL, and VT. Mean (95% confidence interval) Popt defined by oxygenation was 4.4 (1.9, 7.0) cmH2O lower than the Popt defined by XRS, 4.5 (2.5, 6.5) cmH2O lower than VL-defined Popt, and 3.6 (2.1, 5.1) cmH2O lower than VT-defined Popt. Oxygenation was not different between the Popt for oxygenation and XRS. Popt for VL and VT resulted in a higher SpO2/FiO2 (fraction of inspired oxygen) than Popt for oxygenation. XRS and VT at each of the Popt points did not differ significantly. VL at the Popt for oxygenation was lower than at the Popt for XRS, VL, and VT ( Table 3 ).

Table 3 Ventilatory and physiological indices at the optimal pressure defined by oxygenation, reactance, end-expiratory volume, and oscillatory VT

Discussion

In this preterm lamb model, we used FOT during HFOV to characterize the relationship between PAW and XRS measured at 5 Hz. The use of oscillatory mechanics to characterize lung recruitment and to target the optimal PAW was compared with oxygenation, relative VL, and VT. Within the recruited lung, optimal PAW according to XRS, as defined by its stability while at a given PAW, was closely aligned with the upper corner pressure of the PAWVL relationship, uniform recruitment, and maximal VT. These results suggest that FOT may have use in improving the application of HFOV.

A unique feature of the present study is that the identification of the optimal pressure has been based on the stability of XRS over time while at a given PAW. The rationale behind this definition is that the optimal PAW should be the lowest pressure able to maintain VL after achieving full lung recruitment. Additionally, with our approach to FOT, reactance can be continuously monitored without interrupting ventilation, and thus, it is possible to track its changes over time, allowing a definition of Popt that is independent from the time constants of the lung. In this way, it may be possible to identify the lowest PAW that prevents derecruitment before oxygenation deteriorates.

It is well established that lung mechanics can be used to determine an optimal pressure during mechanical ventilation. However, assessing lung mechanics during HFOV is problematic, and clinically usable tools are lacking. Attempts to estimate the mechanical properties of the respiratory system from its response to the pressure oscillations delivered by the ventilator have been made in animal models (25) and, more recently, in infants using VT as an indirect indicator of lung compliance, either via electrical impedance tomography (EIT) (26) or respiratory inductive plethysmography and pneumotachography at the airway opening (27). The similarity between the relationship of XRS and VT with pressure is not surprising, because at a given pressure amplitude, the maximum VT that could be delivered is predominantly influenced by compliance. However, since the oscillatory periods are quite small during HFOV compared with the time constants of the lung, the actual VT is influenced not only by compliance but also by resistance and the oscillatory frequency. Thus, the differences in VT that we observed at 5 Hz would have been less obvious at higher oscillatory frequencies (28). Moreover, VT does not increase linearly with compliance but asymptotically, which means that above a given compliance threshold VT becomes largely independent from compliance, particularly at high oscillatory frequencies (29). Therefore, although the shape of the VT and XRS curves during PAW maneuvers are similar, XRS appears a more robust indicator of lung elastance.

XRS is not yet available in the clinical setting, but measurements can be readily obtained from the response of the respiratory system to the delivered oscillatory waveform using existing monitors. This would allow real-time XRS measurement to be used for titration of PAW to achieve an optimal VL during HFOV and to identify individualized optimal ventilatory strategies.

Oxygenation is the most commonly used indicator of the volumetric response to PAW (4,6,7,30), but its sensitivity to detect subtle regional volume changes is poor, it is insensitive to tissue distension and unable to define a narrow optimal point of ventilation (7,26,31). Our study suggested that oxygenation requires significant heterogeneous derecruitment before appreciable change was observed. A lag in the oxygen response to volume loss from the previously recruited lung has been described in newly born term lambs (32) and pediatric lung disease piglet models receiving HFOV (24). In contrast, changes of XRS over time at a given PAW were very sensitive even to partial (i.e., dorsal) VL recruitment and derecruitment. In particular, the average difference between Popt based on XRS and Popt based on oxygenation was about 4 cmH2O but highly variable (range: 0–10 cmH2O). Therefore, the PAW associated with optimal VL and mechanics cannot be simply extrapolated from Popt based on oxygenation.

It is possible that the time at each PAW step was too short to demonstrate a steady state in the physiological measurements. A change in XRS over time indicates instability of lung mechanics at that PAW and, as evident by the EIT data, increasing heterogeneity. It is possible that if we had waited for longer, all variables (including SpO2) would have stabilized to a value indicating VL derecruitment at a higher PAW. These results highlight a potential for XRS to identify that the lung is approaching derecruitment in real time at the bedside and without the need to reach suboptimal saturation levels. In the present study, we used the point of maximal curvature on the deflation limb of the pressure-VL curve to define Popt according to VL. The point of maximal curvature identifies the PAW at which there is an increase in volume loss. EIT is unable to delineate VL loss due to a reduction in tissue distension or derecruitment. This highlights the potential benefit of a measure of lung mechanics compared with a measure of VL in identifying a precise point of optimal PAW.

FOT was assessed at 5 Hz because the use of XRS at 5 Hz has been validated for the assessment of VL recruitment (17), lung tissue distension, and PAW titration (19). However, infants usually receive HFOV at higher frequencies. Therefore, performing FOT measurements required changing the prevailing frequency and also the amplitude to maintain suitable VTs. In the present study, EIT measurements confirmed that end-expiratory VL did not change during the measurement, but changes in intrapulmonary pressure cannot be excluded. Mathematical modeling suggests that higher oscillatory frequencies would provide a similar PAWXRS relationship but with lower sensitivity to changes occurring in the lung periphery (33). In the present study, reactance evaluated at 10 Hz identified the same Popt as XRS. In the future, higher oscillatory frequencies could be adopted after having confirmed the validity of the methodology in vivo.

This study has some limitations not already mentioned. SpO2, XRS, VL, and VT may be influenced by other factors than the volume state of the lung. In the present study, cuffed endotracheal tubes were used and spontaneous ventilation was suppressed to optimize the quality of the recordings. Whether similar results can be replicated in the clinical setting warrants investigation. Exogenous surfactant and antenatal corticosteroids were not used in our model. Both are known to influence lung mechanics. We contend that the relationships seen are unlikely to differ but the exact values may differ. The limitations of EIT have been well described previously (34), in particular, EIT is not commercially available for neonatal use and the different chest shape of the newborn lamb compared with the human infant may influence the regional EIT data (34). The use of SpO2/FiO2 to report the oxygenation response to PAW changes was a pragmatic decision. This variable has been used in clinical “open lung” studies in newborns (6). However, the point of optimal partial pressure of arterial oxygen may not be equal to the point of optimal SpO2/FiO2. This may also explain why other studies found a closer correlation between the optimal pressure defined by oxygenation and by lung mechanics (22,35). Lung injury analysis was not performed in our study and it is thus unclear whether targeting an optimal point defined by XRS, VL changes or VT is more or less lung protective than targeting oxygenation.

Conclusion

XRS can be used to map lung mechanics during PAW maneuvers on HFOV. In our preterm lamb model, XRS defined an optimal PAW significantly greater than oxygenation and similar to expiratory VL and oscillatory VT, making this FOT approach a potential bedside tool for optimizing PAW with an open lung approach. The potential for XRS stability to guide the clinical application of HFOV, and improve lung protection, warrants further investigation.

Methods

The study was performed at the Murdoch Childrens Research Institute, Melbourne, Australia and approved by the institution’s animal ethics committee. Animals were cared for in accordance with the guidelines of the National Health and Medical Research Council of Australia.

Animal Preparation

Preterm lambs at 131–132 d gestation were delivered via cesarean section from anaesthetized and sedated ewes. After delivery of the fetal head, the carotid and external jugular vessels were catheterized. Lamb was intubated with a 4.0 mm cuffed endotracheal tube (ETT). Lung liquid was drained passively for 10 s and the ETT clamped thereafter. No surfactant was administrated. The fetal chest was exteriorized and dried prior to applying 16 custom-made needle electrodes equidistant around the chest at a level 1 cm above the xiphisternum. The electrodes were connected to a GoeMF II EIT system (CareFusion, Hoechberg, Germany) and electrode placement, conductance, and signal stability were confirmed. The lamb was weighed at delivery, placed supine, and a 10-s reference EIT recording was performed (SciEIT, Carefusion, Hoechberg, Germany). The ETT was unclamped and HFOV commenced immediately (Sensormedics 3100B, Carefusion, Yorba Linda, CA). The total time between delivery and initiation of HFOV was below 90 s.

Measurements and Monitoring

SpO2, systemic blood pressure, heart rate, and body temperature were continuously monitored from birth (HP48S, Hewllett Packard, Andover, MA). The equipment for FOT consisted of pressure and flow sensors placed at the proximal end of the ETT. Airway opening pressure (PAO) was measured using a pressure transducer (30 Inch-D-4V, All Sensors, Morgan Hill, CA) and flow with a custom-made mesh-type heated pneumotachograph coupled with a differential pressure transducer (1 Inch-D-4V, All Sensors). The frequency response of the pressure and flow sensors was evaluated on a bench model of immature lung and found to be flat in the range of frequencies used in this study. PAO and flow signals were digitized (DAQCARD 6036-E, National Instruments, Austin, TX) at a sampling rate of 600 Hz and recorded to a laptop computer using custom-built programs for data acquisition developed using LabVIEW software (National Instruments). Changes in global and regional electrical impedance, which are related to VL, were measured by EIT sampling at 44 Hz using the methodology we described previously (36). All parameters were measured continuously throughout mapping of the pressure–volume relationship described as follows.

Mapping of the Pressure–Volume Relationship

HFOV was commenced at a PAW of 14 cmH2O, amplitude 45 cmH2O, frequency 10 Hz, inspiratory time 33%, and FiO2 0.4. After a 10-min equilibration period at these baseline settings, PAW was increased by 4 cmH2O every 2 min until SpO2 no longer improved, or a maximum PAW (Pmax) of 34 cmH2O was obtained (PAW values above 34 cmH2O were associated with significant hemodynamic compromise in a pilot group). Mapping of the deflation limb then followed, initially with stepwise reduction in PAW in 4 cmH2O decrements at 2-min intervals, and then 2 cmH2O decrements below a PAW of 18 cmH2O, until a closing pressure (Pcl) was identified. Pcl was defined as the PAW at which an increase in FiO2 by at least 0.1 was required to maintain SpO2 between 88 and 94%. Immediately after (t0min) and 2 min after (t2min) each PAW decrement, oscillatory frequency was reduced to 5 Hz and the amplitude to 20 cmH2O (to keep the flow peaks within the working range of the flow sensor and to limit the increase in VT as frequency was reduced) for ~10 s for the calculation of XRS. XRS was assessed at 5 Hz to optimize the sensitivity of the measurement to changes in peripheral mechanics (33); the duration of the measurements was kept short to prevent changes in VL or gas exchange associated with the change in oscillatory frequency. Arterial blood gas analysis was performed at the initial PAW, Pmax, and Pcl. Amplitude was adjusted to achieve a partial pressure of arterial carbon dioxide of 45–55 mmHg. The amplitude was kept constant during the decremental PAW series, except during XRS measurement, to avoid the introduction of confounding variables. Throughout the experiment, FiO2 was adjusted to maintain SpO2 in the target range (88–94%). Lambs were euthanized humanely by intravenous pentabarbitone overdose at study completion.

Data Analysis

SpO2 at t2min was used as the measure of oxygenation at each pressure step and the SpO2/FiO2 ratio (37) was calculated to compare oxygenation at different protocol steps. XRS was determined from the PAO and flow signals during the oscillatory cycles at t0min and t2min by spectral analysis using the cross-spectrum method (38). The difference between XRS at t0min and t2min was used to evaluate the dynamics of recruitment, with a decrease in XRS from t0min to t2min taken to indicate derecruitment (17). Alterations in VL between t0min and t2min were categorized similarly.

Relative VL was determined as the mean value of the low-pass filtered (frequency < 0.1 Hz) impedance changes measured by EIT. To better compare VL and XRS, VL was averaged over the 10 s during each 5-Hz period, providing a single data point at t0min and at t2min for each PAW . The EIT signal at each PAW was referenced to the value immediately prior to unclamping the ETT (baseline). The cross-sectional EIT images of the lungs were divided into ventral (nongravity dependent) and dorsal (dependent) hemithoraces. To allow for intersubject comparison, the volume in each region of interest (global, dorsal, and ventral) was normalized to the volume in that region at the Pmax for each subject (100%) and baseline (0%) (7,23). VT (VT) was computed by integrating the flow signal. The oscillatory volumes during the 10 s at 5 Hz were averaged for comparison with XRS and VL.

Definitions of Optimal PAW

Four different definitions of Popt were evaluated: (i) oxygenation-based Popt, defined as Pcl+2 cmH2O (4,6); (ii) XRS-based Popt, defined as the lowest PAW that prevented a decrease in XRS between t0min and t2min; (iii) VL-based Popt, defined as the upper corner pressure, using t2min data, on the deflation limb of the VLPAW curve (the deflation limb of the VLPAW curve was fitted according to the model described by Venegas et al. (39): VL = a + b/(1+e–(Pc)/d). a, b, c, and d are the fitting parameters and all have a physiological correlate: a is the lower asymptote, b is the total change in VL, c corresponds to the PAW at the point of highest compliance, and d, in units of pressure, represents the distance from c of the high compliance portion of the curve. The upper corner pressure, where the function rapidly changes slope, was computed as c+2d and corresponds to the intersection between the tangent to the pressure–volume curve at the point of maximal compliance (P = c) and the upper asymptote) (39); and (iv) VT-based Popt, defined as the lowest PAW that maximized VT at t2min.

Statistical Analysis

Data were tested for normality using the Kolmogorov–Smirnov test. Differences in PAW and resultant oxygenation, XRS, VL, and VT at key points of the pressure–volume relationship were evaluated using t-tests or one-way ANOVA for repeated measurements as appropriate. Significance of differences between the dorsal and the ventral VL was assessed by two-way ANOVA for repeated measurements (using PAW and region as factors). The Tukey post-test was applied to all ANOVA analyses. P < 0.05 was considered significant. Statistical analysis was performed using SigmaStat 3.1 (Systat Software, Chicago, IL).

Statement of Financial Support

D.G.T. is supported by a National Health and Medical Research Council Clinical Research Fellowship (Australia, grant ID 491286). D.G.T., E.J.P., and R.B. are supported by the Victorian Government Operational Infrastructure Support Program (Victoria, Australia). M.N. and M.L.V. are supported by a grant from Fondazione MBBM (Monza, Italy).

Disclosure

Politecnico di Milano University, the institution of E.Z. and R.L.D., owns a patent on the use of forced oscillation technique for the detection of lung volume recruitment/derecruitment. The other authors have no competing interests to declare.