The upper inflection point of the pressure-volume curve. Influence of methodology and of different modes of ventilation

Effect of individualized PEEP titration guided by intratidal compliance profile analysis on regional ventilation assessed by electrical impedance tomography - a randomized controlled trial

Background

The application of positive end-expiratory pressure (PEEP) may reduce dynamic strain during mechanical ventilation. Although numerous approaches for PEEP titration have been proposed, there is no accepted strategy for titrating optimal PEEP. By analyzing intratidal compliance profiles, PEEP may be individually titrated for patients.

Methods

After obtaining informed consent, 60 consecutive patients undergoing general anesthesia were randomly allocated to mechanical ventilation with PEEP 5 cmH₂O (control group) or PEEP individually titrated, guided by an analysis of the intratidal compliance profile (intervention group). The primary endpoint was the frequency of each nonlinear intratidal compliance (C_RS) profile of the respiratory system (horizontal, increasing, decreasing, and mixed). The secondary endpoints measured were respiratory mechanics, hemodynamic variables, and regional ventilation, which was assessed via electrical impedance tomography.

Results

The frequencies of the C_RS profiles were comparable between the groups. Besides PEEP [control: 5.0 (0.0), intervention: 5.8 (1.1) cmH₂O, p < 0.001], the respiratory and hemodynamic variables were comparable between the two groups. The compliance profile analysis showed no significant differences between the two groups. The loss of ventral and dorsal regional ventilation was higher in the control [ventral: 41.0 (16.3)%; dorsal: 25.9 (13.8)%] than in the intervention group [ventral: 29.3 (17.6)%; dorsal: 16.4 (12.7)%; p (ventral) = 0.039, p (dorsal) = 0.028].

Conclusions

Unfavorable compliance profiles indicating tidal derecruitment were found less often than in earlier studies. Individualized PEEP titration resulted in slightly higher PEEP. A slight global increase in aeration associated with this was indicated by regional gain and loss analysis. Differences in dorsal to ventral ventilation distribution were not found.

Trial registration

This clinical trial was registered at the German Register for Clinical Trials (DRKS00008924) on August 10, 2015.

Sizing the lung in dogs: the inspiratory capacity defines the tidal volume

Objective

To evaluate a novel physiological approach for setting the tidal volume in mechanical ventilation according to inspiratory capacity, and to determine if it results in an appropriate mechanical and gas exchange measurements in healthy and critically ill dogs.

Methods

Twenty healthy animals were included in the study to assess the tidal volume expressed as a percentage of inspiratory capacity. For inspiratory capacity measurement, the mechanical ventilator was set as follows: pressure control mode with 35cmH₂O of inspired pressure and zero end-expiratory pressure for 5 seconds. Subsequently, the animals were randomized into four groups and ventilated with a tidal volume corresponding to the different percentages of inspiratory capacity. Subsequently, ten critically ill dogs were studied.

Results

Healthy dogs ventilated with a tidal volume of 17% of the inspiratory capacity showed normal respiratory mechanics and presented expected PaCO₂ values more frequently than the other groups. The respiratory system and transpulmonary driving pressure were significantly higher among the critically ill dogs but below 15 cmH₂O in all cases.

Conclusions

The tidal volume based on the inspiratory capacity of each animal has proven to be a useful and simple tool when setting ventilator parameters. A similar approach should also be evaluated in other species, including human beings, if we consider the potential limitations of tidal volume titration based on the calculated ideal body weight.

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.

Measurement and mathematical modelling of elastic and resistive lung mechanical properties studied at sinusoidal expiratory flow

Elastic pressure/volume (P(el) /V) and elastic pressure/resistance (P(el) /R) diagrams reflect parenchymal and bronchial properties, respectively. The objective was to develop a method for determination and mathematical characterization of P(el) /V and P(el) /R relationships, simultaneously studied at sinusoidal flow-modulated vital capacity expirations in a body plethysmograph. Analysis was carried out by iterative parameter estimation based on a composite mathematical model describing a three-segment P(el) /V curve and a hyperbolic P(el) /R curve. The hypothesis was tested that the sigmoid P(el) /V curve is non-symmetric. Thirty healthy subjects were studied. The hypothesis of a non-symmetric P(el) /V curve was verified. Its upper volume asymptote was nearly equal to total lung capacity (TLC), indicating lung stiffness increasing at high lung volume as the main factor limiting TLC at health. The asymptotic minimal resistance of the hyperbolic P(el) /R relationship reflected lung size. A detailed description of both P(el) /V and P(el) /R relationships was simultaneously derived from sinusoidal flow-modulated vital capacity expirations. The nature of the P(el) /V curve merits the use of a non-symmetric P(el) /V model.

Airway pressure release and biphasic intermittent positive airway pressure ventilation: are they ready for prime time?

Airway pressure release ventilation and biphasic positive airway pressure ventilation are being used increasingly as alternative strategies to conventional assist control ventilation for patients with acute respiratory distress syndrome (ARDS) and acute lung injury. By permitting spontaneous breathing throughout the ventilatory cycle, these modes offer several advantages over conventional strategies to improve the pathophysiology in these patients, including gas exchange, cardiovascular function, and reducing or eliminating the need for heavy sedation and paralysis. Whether these surrogate outcomes will translate into better patient outcomes remains to be determined. The purpose of this review is to summarize the rationale behind the use of these ventilatory strategies in ARDS, the clinical experience with the use of these modes, and their future applications in trauma patients.

Clinical review: respiratory mechanics in spontaneous and assisted ventilation

Pulmonary disease changes the physiology of the lungs, which manifests as changes in respiratory mechanics. Therefore, measurement of respiratory mechanics allows a clinician to monitor closely the course of pulmonary disease. Here we review the principles of respiratory mechanics and their clinical applications. These principles include compliance, elastance, resistance, impedance, flow, and work of breathing. We discuss these principles in normal conditions and in disease states. As the severity of pulmonary disease increases, mechanical ventilation can become necessary. We discuss the use of pressure–volume curves in assisting with poorly compliant lungs while on mechanical ventilation. In addition, we discuss physiologic parameters that assist with ventilator weaning as the disease process abates.

Pressure–Volume Curve Does Not Predict Steady-State Lung Volume in Canine Lavage Lung Injury

To better understand strategies for recruiting and maintaining lung volume in acute lung injury, we examined relationships between steady-state lung volume and cumulative cyclic recruitment/derecruitment volume history and the quasi-static pressure-volume curve, in an animal saline lavage lung injury model. Small-volume tidal pressure-volume loops performed after inflation from functional residual capacity demonstrated incremental, cyclic recruitment only if the peak pressure achieved exceeded the pressure at which the compliance increased (Pflex) on the pressure-volume curve, whereas loops performed after deflation from total lung capacity remained close to the envelope deflation curve. Recruitment continued to occur up to and beyond a peak inspiratory airway pressure of 40 cm H(2)O, as demonstrated by both the tidal loops and by computed tomography-derived lung volume data. Tidal-specific compliance was relatively constant across positive end-expiratory pressure levels after inflation from functional residual capacity, but peaked at moderate positive end-expiratory pressure after deflation from total lung capacity, further demonstrating the effects of volume history and providing experimental validation of the recruitment models of Hickling (AJRCCM 2001;163:69-78). These results support the interpretation of Pflex as pressure threshold for recruitment, but otherwise do not suggest a role for the pressure-volume curve in predicting steady-state lung volume.

Pressure-volume relationships in acute lung injury: methodological and clinical implications

BACKGROUND

Pressure-volume relationships (PV curves) are the only available method for bedside monitoring of respiratory mechanics. Alveolar recruitment modifies the results obtained from the PV curves. We hypothesized that method-related differences may influence PV-curve guided ventilatory management.

METHODS

Twelve acute lung injury (ALI) patients [PaO2/FiO2 13.0 +/- 1.5 kPa (97.6 +/- 11.3 mmHg), bilateral pulmonary infiltrates] were studied. Two PV curves [one at variable, and another at constant level of positive end-expiratory pressure (PEEP)] were obtained from each patient using constant inspiratory flow and end-inspiratory and -expiratory occlusions. Upper and lower inflection points (UIP, LIP) were estimated. Recruitment due to PEEP and during inflation was assessed by respiratory inductive plethysmography (RIP).

RESULTS

(1) Pressure-volume curves at constant PEEP tended to provide higher LIP values compared with curves at variable PEEP (mean difference +/- SEM 5.1 +/- 1.9 cmH2O); and (2) recruitment occurred throughout the PV curve with no relationship with LIP or UIP.

CONCLUSION

Pressure-volume curves obtained using variable PEEP translate a different physiological reality and seem to be clinically more relevant than curves constructed at constant PEEP. If curves constructed at constant PEEP are used to set the ventilator, unnecessarily high PEEP levels may be used. Respiratory inductive plethysmography technology may be used for monitoring of recruitment at the bedside.

Derecruitment of the lung induced by stepwise lowering of positive end-expiratory pressure in patients with adult respiratory distress syndrome

BACKGROUND AND OBJECTIVE

It has recently been suggested that recruitment proceeds far above the lower inflection point of the elastic pressure-volume (Pel/V) curve of the respiratory system. Accordingly, the value of the lower inflection point as a guide to set the positive end-expiratory pressure (PEEP) has been challenged. Our aim was to evaluate the derecruitment induced by stepwise decreasing PEEP levels.

METHODS

Seven consecutive sedated and paralysed patients with acute respiratory distress syndrome were studied. Multiple Pel/V curves of the respiratory system were recorded at PEEP levels progressively decreasing in steps of 3.75 cmH2O from + 15 to zero according to the principles of the low flow inflation method.

RESULTS

Multiple Pel/V curves shifted towards lower volumes at decreasing PEEP. Dynamic compliance was higher for Pel/V curves recorded from lower PEEP levels. A high correlation (r = 0.99) was found between dynamic compliance and PEEP. The lower inflection point was on average 9.2 cmH2O. However, the transition between the lower segment and the linear part of the Pel/V curve was in general smooth to the eye. The upper inflection point was on average 23.8 cmH2O. A high correlation (r = 0.98) between the upper inflection point and PEEP was found.

CONCLUSIONS

The lower inflection point is a poor indicator of alveolar closure. The evaluation of derecruitment induced by a stepwise reduction in PEEP seems to be more useful than individual titration of PEEP and tidal volume in patients with adult respiratory distress syndrome.

Temporal change, reproducibility, and interobserver variability in pressure-volume curves in adults with acute lung injury and acute respiratory distress syndrome

OBJECTIVES

To assess the reproducibility of the static pressure-volume curve of the respiratory system by using a "mini-syringe" technique; to assess the temporal change in upper (UIP) and lower inflection points (LIP) measured from pressure-volume curves of the respiratory system; to assess the inter- and intraobserver variability in detection of the UIP and LIP in patients with acute lung injury (ALI)/acute respiratory distress syndrome (ARDS); and to compare the syringe and multiple occlusion techniques for determining LIP and UIP.

DESIGN

Prospective observational study.

SETTING

Academic medical-surgical critical care unit.

PATIENTS

Consecutive patients with ALI or ARDS.

INTERVENTIONS

Static inspiratory pressure-volume curves of the respiratory system were determined twice on day 1 of diagnosis of ALI/ARDS and then once daily for up to 6 days by using the syringe technique. Pressure-volume curves were determined from zero positive end-expiratory pressure. At each time point, three separate measurements of the pressure-volume curve were made to determine reproducibility. A 100-mL graduated syringe was used to inflate patients' lungs with 50- to 100-mL increments up to an airway pressure of 45 cm H2O or a total volume of 2 L; each volume step was maintained for 2-3 secs until a plateau airway pressure was recorded. On day 1, the static pressure-volume curve also was determined by using the multiple occlusion technique. In a random and blinded sequence, the pressure-volume curves were examined visually by three critical care physicians on three different occasions, to determine the intra- and interobserver variability in visual detection of the LIP and UIP. Observers were given objective instructions to visually identify LIP and UIP.

MEASUREMENTS AND MAIN RESULTS

Eleven patients were enrolled, with a total of 134 pressure-volume curves generated. LIP and UIP could be detected in 90-94% and 61-68% of curves, respectively. When the three successive pressure-volume curves were compared, both the LIP and UIP were within 3 cm H2O in >65% of curves. The index of reliability (intraclass correlation coefficient) in LIP and UIP was 0.92 and 0.89 for interobserver variability and 0.90 and 0.88 for intraobserver variability. Daily variability was as high as 7 cm H2O for LIP and 5 cm H2O for UIP. When pressure-volume curves obtained by using the multiple occlusion and syringe techniques were compared, LIP was within 2 cm H2O, and UIP was within 4 cm H2O with the two techniques.

CONCLUSIONS

The static pressure-volume curve of the respiratory system is reasonably reproducible, thus avoiding the need for multiple measurements at a single time. We found excellent interobserver and intraobserver correlation in manual identification of the LIP and UIP. Both LIP and UIP show appreciable daily variability in patients with ALI/ARDS. The multiple occlusion and syringe techniques generate similar values for LIP and UIP.

New aspects of ventilation in acute lung injury

Recent recognition that artificial ventilation may cause damage to the acutely injured lung has caused renewed interest in ventilation techniques that minimise this potential harm. Many ventilation techniques have proved beneficial in small trials of very specific patient groups, but most have subsequently failed to translate into improved patient outcome in larger trials. An exception to this is 'protective ventilation' using reduced tidal volumes (to lower airway pressure) and increased PEEP (to reduce pulmonary collapse). Results of trials of protective ventilation have been encouraging, and the technique should now be adopted more widely. High frequency ventilation, inverse ratio ventilation, prone positioning and inhaled nitric oxide are all techniques that may be considered when, in spite of optimal artificial ventilation, the patient's gas exchange remains dangerously poor. Under these circumstances, the choice of technique is dependent on their availability, local expertise and individual patient needs.

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.