Intratidal compliance-volume curve as an alternative basis to adjust positive end-expiratory pressure: a study in isolated perfused rabbit lungs

Pathophysiology and Clinical Meaning of Ventilation-Perfusion Mismatch in the Acute Respiratory Distress Syndrome

Acute respiratory distress syndrome (ARDS) remains an important clinical challenge with a mortality rate of 35-45%. It is being increasingly demonstrated that the improvement of outcomes requires a tailored, individualized approach to therapy, guided by a detailed understanding of each patient's pathophysiology. In patients with ARDS, disturbances in the physiological matching of alveolar ventilation (V) and pulmonary perfusion (Q) (V/Q mismatch) are a hallmark derangement. The perfusion of collapsed or consolidated lung units gives rise to intrapulmonary shunting and arterial hypoxemia, whereas the ventilation of non-perfused lung zones increases physiological dead-space, which potentially necessitates increased ventilation to avoid hypercapnia. Beyond its impact on gas exchange, V/Q mismatch is a predictor of adverse outcomes in patients with ARDS; more recently, its role in ventilation-induced lung injury and worsening lung edema has been described. Innovations in bedside imaging technologies such as electrical impedance tomography readily allow clinicians to determine the regional distributions of V and Q, as well as the adequacy of their matching, providing new insights into the phenotyping, prognostication, and clinical management of patients with ARDS. The purpose of this review is to discuss the pathophysiology, identification, consequences, and treatment of V/Q mismatch in the setting of ARDS, employing experimental data from clinical and preclinical studies as support.

Marathoners' Breathing Pattern Protects Against Lung Injury by Mechanical Ventilation: An Ex Vivo Study Using Rabbit Lungs

Background

Breathing during a marathon is often empirically conducted in a so-called "2:2 breathing rhythm," which is based on a four-phase cycle, consisting of the 1st and 2nd inspiratory and the 1st and 2nd expiratory phases. We developed a prototype ventilator that can perform intermittent positive pressure ventilation, mimicking the breathing cycle of the 2:2 breathing rhythm. This mode of ventilation was named the marathoners' breathing rhythm ventilation (MBV). We hypothesized that MBV may have a lung protective effect.

Methods

We examined the effects of the MBV on the pulmonary pre-edema model in isolated perfused rabbit lungs. The pulmonary pre-edema state was induced using bloodless perfusate with low colloid osmotic pressure. The 14 isolated rabbit lung preparations were randomly divided into the conventional mechanical ventilation (CMV) group and MBV group, (both had an inspiratory/expiratory ratio of 1/1). In the CMV group, seven rabbit lungs were ventilated using the Harvard Ventilator 683 with a tidal volume (TV) of 8 mL/kg, a respiratory rate (RR) of 30 cycles/min, and a positive end-expiratory pressure (PEEP) of 2 cmH₂O for 60 min. In the MBV group, seven rabbit lungs were ventilated using the prototype ventilator with a TV of 6 mL/kg, an RR of 30 cycles/min, and a PEEP of 4 cmH₂O (first step) and 2 cmH₂O (second step) for 60 min. The time allocation of the MBV for one cycle was 0.3 s for each of the 1st and 2nd inspiratory and expiratory phases with 0.2 s of intermittent resting between each phase.

Results

Peak airway pressure and lung wet-to-dry ratio after 60 min of ventilation were lower in the MBV group than in the CMV group.

Conclusion

MBV was considered to have a lung-protective effect compared to CMV.

Monitoring of intratidal lung mechanics: a Graphical User Interface for a model-based decision support system for PEEP-titration in mechanical ventilation

In mechanical ventilation, a careful setting of the ventilation parameters in accordance with the current individual state of the lung is crucial to minimize ventilator induced lung injury. Positive end-expiratory pressure (PEEP) has to be set to prevent collapse of the alveoli, however at the same time overdistension should be avoided. Classic approaches of analyzing static respiratory system mechanics fail in particular if lung injury already prevails. A new approach of analyzing dynamic respiratory system mechanics to set PEEP uses the intratidal, volume-dependent compliance which is believed to stay relatively constant during one breath only if neither atelectasis nor overdistension occurs. To test the success of this dynamic approach systematically at bedside or in an animal study, automation of the computing steps is necessary. A decision support system for optimizing PEEP in form of a Graphical User Interface (GUI) was targeted. Respiratory system mechanics were analyzed using the gliding SLICE method. The resulting shapes of the intratidal compliance-volume curve were classified into one of six categories, each associated with a PEEP-suggestion. The GUI should include a graphical representation of the results as well as a quality check to judge the reliability of the suggestion. The implementation of a user-friendly GUI was successfully realized. The agreement between modelled and measured pressure data [expressed as root-mean-square (RMS)] tested during the implementation phase with real respiratory data from two patient studies was below 0.2 mbar for data taken in volume controlled mode and below 0.4 mbar for data taken in pressure controlled mode except for two cases with RMS < 0.6 mbar. Visual inspections showed, that good and medium quality data could be reliably identified. The new GUI allows visualization of intratidal compliance-volume curves on a breath-by-breath basis. The automatic categorisation of curve shape into one of six shape-categories provides the rational decision-making model for PEEP-titration.

Optimizing positive end-expiratory pressure by oscillatory mechanics minimizes tidal recruitment and distension: an experimental study in a lavage model of lung injury

Introduction

It is well established that during mechanical ventilation of patients with acute respiratory distress syndrome cyclic recruitment/derecruitment and overdistension are potentially injurious for lung tissues. We evaluated whether the forced oscillation technique (FOT) could be used to guide the ventilator settings in order to minimize cyclic lung recruitment/derecruitment and cyclic mechanical stress in an experimental model of acute lung injury.

Methods

We studied six pigs in which lung injury was induced by bronchoalveolar lavage. The animals were ventilated with a tidal volume of 6 ml/kg. Forced oscillations at 5 Hz were superimposed on the ventilation waveform. Pressure and flow were measured at the tip and at the inlet of the endotracheal tube respectively. Respiratory system reactance (Xrs) was computed from the pressure and flow signals and expressed in terms of oscillatory elastance (E_X5). Positive end-expiratory pressure (PEEP) was increased from 0 to 24 cm H₂O in steps of 4 cm H₂O and subsequently decreased from 24 to 0 in steps of 2 cm H₂O. At each PEEP step CT scans and E_X5 were assessed at end-expiration and end-inspiration.

Results

During deflation the relationship between both end-expiratory and end-inspiratory E_X5 and PEEP was a U-shaped curve with minimum values at PEEP = 13.4 ± 1.0 cm H₂O (mean ± SD) and 13.0 ± 1.0 cm H₂O respectively. E_X5 was always higher at end-inspiration than at end-expiration, the difference between the average curves being minimal at 12 cm H₂O. At this PEEP level, CT did not show any substantial sign of intra-tidal recruitment/derecruitment or expiratory lung collapse.

Conclusions

Using FOT it was possible to measure E_X5 both at end-expiration and at end-inspiration. The optimal PEEP strategy based on end-expiratory E_X5 minimized intra-tidal recruitment/derecruitment as assessed by CT, and the concurrent attenuation of intra-tidal variations of E_X5 suggests that it may also minimize tidal mechanical stress.

Adaptive SLICE method: an enhanced method to determine nonlinear dynamic respiratory system mechanics

The objective of this paper is to introduce and evaluate the adaptive SLICE method (ASM) for continuous determination of intratidal nonlinear dynamic compliance and resistance. The tidal volume is subdivided into a series of volume intervals called slices. For each slice, one compliance and one resistance are calculated by applying a least-squares-fit method. The volume window (width) covered by each slice is determined based on the confidence interval of the parameter estimation. The method was compared to the original SLICE method and evaluated using simulation and animal data. The ASM was also challenged with separate analysis of dynamic compliance during inspiration. If the signal-to-noise ratio (SNR) in the respiratory data decreased from +∞ to 10 dB, the relative errors of compliance increased from 0.1% to 22% for the ASM and from 0.2% to 227% for the SLICE method. Fewer differences were found in resistance. When the SNR was larger than 40 dB, the ASM delivered over 40 parameter estimates (42.2 ± 1.3). When analyzing the compliance during inspiration separately, the estimates calculated with the ASM were more stable. The adaptive determination of slice bounds results in consistent and reliable parameter values. Online analysis of nonlinear respiratory mechanics will profit from such an adaptive selection of interval size.

Dynamic respiratory mechanics in acute lung injury/acute respiratory distress syndrome: research or clinical tool?

PURPOSE OF REVIEW

Classic static measurements of lung mechanics have been used mainly for research purposes, but have not gained widespread clinical acceptance. Instead, dynamic measurements have been used, but interpretation of results has been hampered by lack of clear definitions. The review provides an overview of possible definitions and a description of methods for evaluating lung mechanics in acute lung injury/acute respiratory distress syndrome patients.

RECENT FINDINGS

Compliance measured using static techniques is significantly higher compared to measurements during ongoing ventilation. This indicates that lung mechanic properties depend on flow velocity during inflation and the time allowed for equilibration of viscoelastic forces. Thus, methods for evaluating lung mechanics should be clearly defined in terms of whether they are classically static, i.e. excluding resistance to flow and equilibration of viscoelastic forces, or truly dynamic, i.e. including flow resistance and unequilibrated viscoelastic forces. New techniques have emerged which make it possible to monitor lung mechanics during ongoing, therapeutic ventilation, 'functional lung mechanics', where the impact of flow resistance on tube and airway resistance has been eliminated, providing alveolar pressure/volume curves.

SUMMARY

Functional lung mechanics obtained during ongoing ventilator treatment have the potential to provide information for optimizing ventilator management in critically ill patients.

Using pressure-volume curves to set proper PEEP in acute lung injury

The evolution of respiratory care on patients with acute respiratory distress syndrome (ARDS) has been focused on preventing the deleterious effects of mechanical ventilation, termed ventilator-induced lung injury (VILI). Currently, reduced tidal volume is the standard of ventilatory care for patients with ARDS. The current focus, however, has shifted to the proper setting of positive end-expiratory pressure (PEEP). The whole lung pressure-volume (P/V) curve has been used to individualize setting proper PEEP in patients with ARDS, although the physiologic interpretation of the curve remains under debate. The purpose of this review is to present the pros and cons of using P/V curves to set PEEP in patients with ARDS. A systematic analysis of recent and relevant literature was conducted. It has been hypothesized that proper PEEP can be determined by identifying P/V curve inflection points. Acquiring a dynamic curve presents the key to the curve's bedside application. The lower inflection point of the inflation limb has been shown to be the point of massive alveolar recruitment and therefore an option for setting PEEP. However, it is becoming widely accepted that the upper inflection point (UIP) of the deflation limb of the P/V curve represents the point of optimal PEEP. New methods used to identify optimal PEEP, including tomography and active compliance measurements, are currently being investigated. In conclusion, we believe that the most promising method for determining proper PEEP settings is use of the UIP of the deflation limb. However, tomography and dynamic compliance may offer superior bedside availability.

Dynamic versus static respiratory mechanics in acute lung injury and acute respiratory distress syndrome

OBJECTIVES

It is not clear whether the mechanical properties of the respiratory system assessed under the dynamic condition of mechanical ventilation are equivalent to those assessed under static conditions. We hypothesized that the analyses of dynamic and static respiratory mechanics provide different information in acute respiratory failure.

DESIGN

Prospective multiple-center study.

SETTING

Intensive care units of eight German university hospitals.

PATIENTS

A total of 28 patients with acute lung injury and acute respiratory distress syndrome.

INTERVENTIONS

None.

MEASUREMENTS

Dynamic respiratory mechanics were determined during ongoing mechanical ventilation with an incremental positive end-expiratory pressure (PEEP) protocol with PEEP steps of 2 cm H2O every ten breaths. Static respiratory mechanics were determined using a low-flow inflation.

MAIN RESULTS

The dynamic compliance was lower than the static compliance. The difference between dynamic and static compliance was dependent on alveolar pressure. At an alveolar pressure of 25 cm H2O, dynamic compliance was 29.8 (17.1) mL/cm H2O and static compliance was 59.6 (39.8) mL/cm H2O (median [interquartile range], p < .05). End-inspiratory volumes during the incremental PEEP trial coincided with the static pressure-volume curve, whereas end-expiratory volumes significantly exceeded the static pressure-volume curve. The differences could be attributed to PEEP-related recruitment, accounting for 40.8% (10.3%) of the total volume gain of 1964 (1449) mL during the incremental PEEP trial. Recruited volume per PEEP step increased from 6.4 (46) mL at zero end-expiratory pressure to 145 (91) mL at a PEEP of 20 cm H2O (p < .001). Dynamic compliance decreased at low alveolar pressure while recruitment simultaneously increased. Static mechanics did not allow this differentiation. The decrease in static compliance occurred at higher alveolar pressures compared with the dynamic analysis.

CONCLUSIONS

Exploiting dynamic respiratory mechanics during incremental PEEP, both compliance and recruitment can be assessed simultaneously. Based on these findings, application of dynamic respiratory mechanics as a diagnostic tool in ventilated patients should be more appropriate than using static pressure-volume curves.

Alveolar recruitment in acute lung injury

Alveolar recruitment is one of the primary goals of respiratory care for acute lung injury. It is aimed at improving pulmonary gas exchange and, even more important, at protecting the lungs from ventilator-induced trauma. This review addresses the concept of alveolar recruitment for lung protection in acute lung injury. It provides reasons for why atelectasis and atelectrauma should be avoided; it analyses current and future approaches on how to achieve and preserve alveolar recruitment; and it discusses the possibilities of detecting alveolar recruitment and derecruitment. The latter is of particular clinical relevance because interventions aimed at lung recruitment are often undertaken without simultaneous verification of their effectiveness.

Reduced activation of immunomodulatory transcription factors during positive end-expiratory pressure adjustment based on volume-dependent compliance in isolated perfused rabbit lungs

BACKGROUND

Repeated alveolar collapse and cyclic alveolar overdistension with associated activation of inflammatory signalling cascades contribute to ventilator-induced lung injury (VILI). The appropriate positive end-expiratory pressure (PEEP) which prevents or ameliorates VILI is unknown. In the isolated perfused lung, repeated adjustments of PEEP based on the continuously analysed intratidal compliance-volume curve have previously been shown to result in full end-expiratory alveolar recruitment and low risk of cyclic alveolar overdistension. Accordingly, we tested the hypothesis that such ventilatory management reduces intrapulmonary activation of the immunomodulatory transcription factors nuclear factor kappaB (NF-kappaB), activator protein 1 (AP-1) and cAMP-responsive element binding protein (CREB) which induce the expression of various chemokines and cytokines.

METHODS

Isolated perfused rabbit lungs were randomly allocated to one of three groups: zero end-expiratory pressure (ZEEP) to induce repeated alveolar collapse (n=6), high PEEP to induce cyclic alveolar overdistension (n=6) and repeated PEEP adjustments based on intratidal compliance-volume curve analysis by the slice method to minimize repeated alveolar collapse and overdistension (n=9). All lungs were ventilated with a tidal volume of 6 ml kg(-1) bodyweight for 120 min. Thereafter, activation of transcription factors NF-kappaB, AP-1 and CREB in lung tissue was analysed by electrophoretic mobility shift assay.

RESULTS

High PEEP was associated with the highest activation of NF-kappaB and AP-1 and repeated PEEP adjustments with the lowest activation when compared with the other two study groups (P<0.001). In contrast, activation of CREB did not differ between groups. Activated NF-kappaB and AP-1 protein complexes consisted mainly of the transactivators p50/p65 and c-Fos/Jun, respectively.

CONCLUSIONS

In isolated perfused rabbit lungs, repeated adjustments of PEEP based on the continuously analysed intratidal compliance-volume curve were associated with less activation of early steps of inflammatory signalling cascades than ventilation with ZEEP or high PEEP.

Respiratory monitoring tools in the intensive care unit

PURPOSE OF REVIEW

Respiratory monitoring tools remain important in the overall assessment of a patient in the ICU. A working knowledge of the various tools is helpful in applying their use to patient care.

RECENT FINDINGS

The past year has seen tremendous research in this domain of patient care, fueled by rapid advances in modern technology. Still, the ICU clinician continues to rely on tried and true established tools while awaiting proof of clinical efficacy of newer modalities.

SUMMARY

This review discusses traditional methods of respiratory system evaluation and their current controversies. Attention is also given to newer modalities, including those that are investigational or currently limited to bench application, that show promise of future application in ICU clinical practice.

Different strategies to keep the lung open: a study in isolated perfused rabbit lungs

OBJECTIVE

Atelectatic alveoli can be recruited or kept open either by sustained inflation maneuvers or by positive end-expiratory pressure (PEEP). Little is known about potential interactions between both approaches. Especially, it is not known whether the recruiting effect of sustained inflation maneuvers is maintained in combination with a low PEEP, as suggested recently. In an attempt to answer this question, we combined sustained inflation maneuvers with either high or low PEEP. Both approaches were compared with a strategy likely to result in alveolar atelectasis and with another ensuring adequate alveolar recruitment by adjustment of PEEP alone.

DESIGN

Randomized block design.

SETTING

Laboratory.

SUBJECTS

Isolated perfused rabbit lungs (n = 28).

INTERVENTIONS

The lungs were ventilated with a tidal volume of 8 mL/kg. After stabilization, the lungs were randomized to one of four ventilatory strategies, which then were followed for 120 mins: a) PEEP 1 cm H2O (PEEP1, negative control); b) PEEP 1 cm H2O and 30 sec-sustained inflations (20 cm H2O) every 30 mins (SI-1); c) PEEP 3 cm H2O combined with sustained inflations (SI-3); and d) PEEP repeatedly adjusted following a previously established strategy ensuring full alveolar recruitment (DYN, positive control).

MEASUREMENTS AND MAIN RESULTS

Distribution of ventilation and perfusion (Va/Q distribution) was analyzed by the multiple inert gas elimination technique. Volume-dependent compliance within the tidal volume was determined by using the slice method. Shunt and Va/Q mismatch significantly differed between SI-1 and SI-3, indicating full alveolar recruitment only in the latter. Data of SI-1 did not differ substantially from those of PEEP1, and data obtained in SI-3 were similar to those of DYN.

CONCLUSIONS

First, enduring alveolar recruitment by sustained inflation maneuvers is only possible when the alveoli are stabilized thereafter by sufficient PEEP. Second, a ventilation strategy that uses repeated sustained inflations on a comparably high PEEP may not be superior to adequate adjustment of PEEP alone.