Pleural pressure and optimal positive end-expiratory pressure based on esophageal pressure versus chest wall elastance: incompatible results*

A novel method to quantify breathing effort from respiratory mechanics and esophageal pressure

Breathing effort is important to quantify to understand mechanisms underlying central and obstructive sleep apnea, respiratory-related arousals, and the timing and effectiveness of invasive or noninvasive mechanically assisted ventilation. Current quantitative methods to evaluate breathing effort rely on inspiratory esophageal or epiglottic pressure swings or changes in diaphragm electromyographic (EMG) activity, where units are problematic to interpret and compare between individuals and to measured ventilation. This paper derives a novel method to quantify breathing effort in units directly comparable with measured ventilation by applying respiratory mechanics first principles to convert continuous transpulmonary pressure measurements into "attempted" airflow expected to have arisen without upper airway obstruction. The method was evaluated using data from 11 subjects undergoing overnight polysomnography, including six patients with obesity with severe obstructive sleep apnea (OSA), including one who also had frequent central events, and five healthy-weight controls. Classic respiratory mechanics showed excellent fits of airflow and volume to transpulmonary pressures during wake periods of stable unobstructed breathing (means ± SD, r2 = 0.94 ± 0.03), with significantly higher respiratory system resistance in patients compared with healthy controls (11.2 ± 3.3 vs. 7.1 ± 1.9 cmH2O·L-1·s, P = 0.032). Subsequent estimates of attempted airflow from transpulmonary pressure changes clearly highlighted periods of acute and prolonged upper airway obstruction, including within the first few breaths following sleep onset in patients with OSA. This novel technique provides unique quantitative insights into the complex and dynamically changing interrelationships between breathing effort and achieved airflow during periods of obstructed breathing in sleep.NEW & NOTEWORTHY Ineffective breathing efforts with snoring and obstructive sleep apnea (OSA) are challenging to quantify. Measurements of esophageal or epiglottic pressure swings and diaphragm electromyography are useful, but units are problematic to interpret and compare between individuals and to measured ventilation. This paper derives a novel method that uses esophageal pressure and respiratory mechanics first principles to quantify breathing effort as "attempted" flow and volume in units directly comparable with measured airflow, volume, and ventilation.

Transpulmonary pressure monitoring in critically ill patients: pros and cons

The use of transpulmonary pressure monitoring based on measurement of esophageal pressure has contributed importantly to the personalization of mechanical ventilation based on respiratory pathophysiology in critically ill patients. However, esophageal pressure monitoring is still underused in the clinical practice. This technique allows partitioning of the respiratory mechanics between the lungs and the chest wall, provides information on lung recruitment and risk of barotrauma, and helps titrating mechanical ventilation settings in patients with respiratory failure. In assisted ventilation modes and during non-invasive respiratory support, esophageal pressure monitoring provides important information on the inspiratory effort and work of breathing. Nonetheless, several controversies persist on technical aspects, interpretation and clinical decision-making based on values derived from this monitoring technique. The aim of this review is to summarize the physiological bases of esophageal pressure monitoring, discussing the pros and cons of its clinical applications and different interpretations in critically ill patients undergoing invasive and non-invasive respiratory support.

Setting positive end-expiratory pressure: the use of esophageal pressure measurements

PURPOSE OF REVIEW

To summarize the key concepts, physiological rationale and clinical evidence for titrating positive end-expiratory pressure (PEEP) using transpulmonary pressure ( PL ) derived from esophageal manometry, and describe considerations to facilitate bedside implementation.

RECENT FINDINGS

The goal of an esophageal pressure-based PEEP setting is to have sufficient PL at end-expiration to keep (part of) the lung open at the end of expiration. Although randomized studies (EPVent-1 and EPVent-2) have not yet proven a clinical benefit of this approach, a recent posthoc analysis of EPVent-2 revealed a potential benefit in patients with lower APACHE II score and when PEEP setting resulted in end-expiratory PL values close to 0 ± 2 cmH 2 O instead of higher or more negative values. Technological advances have made esophageal pressure monitoring easier to implement at the bedside, but challenges regarding obtaining reliable measurements should be acknowledged.

SUMMARY

Esophageal pressure monitoring has the potential to individualize the PEEP settings. Future studies are needed to evaluate the clinical benefit of such approach.

Esophageal balloon catheter system identification to improve respiratory effort time features and amplitude determination

Objective. Understanding a patient's respiratory effort and mechanics is essential for the provision of individualized care during mechanical ventilation. However, measurement of transpulmonary pressure (the difference between airway and pleural pressures) is not easily performed in practice. While airway pressures are available on most mechanical ventilators, pleural pressures are measured indirectly by an esophageal balloon catheter. In many cases, esophageal pressure readings take other phenomena into account and are not a reliable measure of pleural pressure.Approach.A system identification approach was applied to provide accurate pleural measures from esophageal pressure readings. First, we used a closed pressurized chamber to stimulate an esophageal balloon and model its dynamics. Second, we created a simplified version of an artificial lung and tried the model with different ventilation configurations. For validation, data from 11 patients (five male and six female) were used to estimate respiratory effort profile and patient mechanics.Main results.After correcting the dynamic response of the balloon catheter, the estimates of resistance and compliance and the corresponding respiratory effort waveform were improved when compared with the adjusted quantities in the test bench. The performance of the estimated model was evaluated using the respiratory pause/occlusion maneuver, demonstrating improved agreement between the airway and esophageal pressure waveforms when using the normalized mean squared error metric. Using the corrected muscle pressure waveform, we detected start and peak times 130 ± 50 ms earlier and a peak amplitude 2.04 ± 1.46 cmH2O higher than the corresponding estimates from esophageal catheter readings.Significance.Compensating the acquired measurements with system identification techniques makes the readings more accurate, possibly better portraying the patient's situation for individualization of ventilation therapy.

Advanced Point-of-care Bedside Monitoring for Acute Respiratory Failure

Advanced respiratory monitoring involves several mini- or noninvasive tools, applicable at bedside, focused on assessing lung aeration and morphology, lung recruitment and overdistention, ventilation-perfusion distribution, inspiratory effort, respiratory drive, respiratory muscle contraction, and patient-ventilator asynchrony, in dealing with acute respiratory failure. Compared to a conventional approach, advanced respiratory monitoring has the potential to provide more insights into the pathologic modifications of lung aeration induced by the underlying disease, follow the response to therapies, and support clinicians in setting up a respiratory support strategy aimed at protecting the lung and respiratory muscles. Thus, in the clinical management of the acute respiratory failure, advanced respiratory monitoring could play a key role when a therapeutic strategy, relying on individualization of the treatments, is adopted.

Assessment of different computing methods of inspiratory transpulmonary pressure in patients with multiple mechanical problems

While plateau airway pressure alone is an unreliable estimate of lung overdistension inspiratory transpulmonary pressure (PL) is an important parameter to reflect it in patients with ARDS and there is no concensus about which computation method should be used to calculate it. Recent studies suggest that different formulas may lead to different tidal volume and PEEP settings. The aim of this study is to compare 3 different inspiratory PL measurement method; direct measurement (PL_D), elastance derived (PL_E) and release derived (PL_R) methods in patients with multiple mechanical abnormalities. 34 patients were included in this prospective observational study. Measurements were obtained during volume controlled mechanical ventilation in sedated and paralyzed patients. During the study day airway and eosephageal pressures, flow, tidal volume were measured and elastance, inspiratory PL_E, PL_D and PL_R were calculated. Mean age of the patients was 67 ± 15 years and APACHE II score was 27 ± 7. Most frequent diagnosis of the patients were pneumonia (71%), COPD exacerbation(56%), pleural effusion (55%) and heart failure(50%). Mean plateau pressure of the patients was 22 ± 5 cmH₂O and mean respiratory system elastance was 36.7 ± 13 cmH₂O/L. E_L/E_RS% was 0.75 ± 0.35%. Mean expiratory transpulmonary pressure was 0.54 ± 7.7 cmH₂O (min: − 21, max: 12). Mean PL_E (18 ± 9 H₂O) was significantly higher than PL_D (13 ± 9 cmH₂O) and PL_R methods (11 ± 9 cmH₂O). There was a good aggreement and there was no bias between the measurements in Bland–Altman analysis. The estimated bias was similar between the PL_D and PL_E (− 3.12 ± 11 cmH₂O) and PL_E and PL_R (3.9 ± 10.9 cmH₂O) measurements. Our results suggest that standardization of calculation method of inspiratory PL is necessary before using it routinely to estimate alveolar overdistension.

Individualization of PEEP and tidal volume in ARDS patients with electrical impedance tomography: a pilot feasibility study

Background

In mechanically ventilated patients with acute respiratory distress syndrome (ARDS), electrical impedance tomography (EIT) provides information on alveolar cycling and overdistension as well as assessment of recruitability at the bedside. We developed a protocol for individualization of positive end-expiratory pressure (PEEP) and tidal volume (V_T) utilizing EIT-derived information on recruitability, overdistension and alveolar cycling. The aim of this study was to assess whether the EIT-based protocol allows individualization of ventilator settings without causing lung overdistension, and to evaluate its effects on respiratory system compliance, oxygenation and alveolar cycling.

Methods

20 patients with ARDS were included. Initially, patients were ventilated according to the recommendations of the ARDS Network with a V_T of 6 ml per kg predicted body weight and PEEP adjusted according to the lower PEEP/FiO₂ table. Subsequently, ventilator settings were adjusted according to the EIT-based protocol once every 30 min for a duration of 4 h. To assess global overdistension, we determined whether lung stress and strain remained below 27 mbar and 2.0, respectively.

Results

Prospective optimization of mechanical ventilation with EIT led to higher PEEP levels (16.5 [14–18] mbar vs. 10 [8–10] mbar before optimization; p = 0.0001) and similar V_T (5.7 ± 0.92 ml/kg vs. 5.8 ± 0.47 ml/kg before optimization; p = 0.96). Global lung stress remained below 27 mbar in all patients and global strain below 2.0 in 19 out of 20 patients. Compliance remained similar, while oxygenation was significantly improved and alveolar cycling was reduced after EIT-based optimization.

Conclusions

Adjustment of PEEP and V_T using the EIT-based protocol led to individualization of ventilator settings with improved oxygenation and reduced alveolar cycling without promoting global overdistension.

Trial registrationThis study was registered at clinicaltrials.gov (NCT02703012) on March 9, 2016 before including the first patient.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13613-021-00877-7.

Collapsibility of caval vessels and right ventricular afterload: decoupling of stroke volume variation from preload during mechanical ventilation

Collapsibility of caval vessels and stroke volume and pulse pressure variations (SVV, PPV) are used as indicators of volume responsiveness. Their behavior under increasing airway pressures and changing right ventricular afterload is incompletely understood. If the phenomena of SVV and PPV augmentation are manifestations of decreasing preload, they should be accompanied by decreasing transmural right atrial pressures. Eight healthy pigs equipped with ultrasonic flow probes on the pulmonary artery were exposed to positive end-expiratory pressure of 5 and 10 cmH₂O and three volume states (Euvolemia, defined as SVV < 10%, Bleeding, and Retransfusion). SVV and PPV were calculated for the right and PPV for the left side of the circulation at increasing inspiratory airway pressures (15, 20, and 25 cmH₂O). Right ventricular afterload was assessed by surrogate flow profile parameters. Transmural pressures in the right atrium and the inferior and superior caval vessels (IVC and SVC) were determined. Increasing airway pressure led to increases in ultrasonic surrogate parameters of right ventricular afterload, increasing transmural pressures in the right atrium and SVC, and a drop in transmural IVC pressure. SVV and PPV increased with increasing airway pressure, despite the increase in right atrial transmural pressure. Right ventricular stroke volume variation correlated with indicators of right ventricular afterload. This behavior was observed in both PEEP levels and all volume states. Stroke volume variation may reflect changes in right ventricular afterload rather than changes in preload.NEW & NOTEWORTHY Stroke volume variation and pulse pressure variation are used as indicators of preload or volume responsiveness of the heart. Our study shows that these variations are influenced by changes in right ventricular afterload and may therefore reflect right ventricular failure rather than pure volume responsiveness. A zone of collapse detaches the superior vena cava and its diameter variation from the right atrium.

Esophageal Manometry

The estimation of pleural pressure with esophageal manometry has been used for decades, and it has been a fertile area of physiology research in healthy subject as well as during mechanical ventilation in patients with lung injury. However, its scarce adoption in clinical practice takes its roots from the (false) ideas that it requires expertise with years of training, that the values obtained are not reliable due to technical challenges or discrepant methods of calculation, and that measurement of esophageal pressure has not proved to benefit patient outcomes. Despites these criticisms, esophageal manometry could contribute to better monitoring, optimization, and personalization of mechanical ventilation from the acute initial phase to the weaning period. This review aims to provide a comprehensive but comprehensible guide addressing the technical aspects of esophageal catheter use, its application in different clinical situations and conditions, and an update on the state of the art with recent studies on this topic and on remaining questions and ways for improvement.

Individualized PEEP to optimise respiratory mechanics during abdominal surgery: a pilot randomised controlled trial

BACKGROUND

Higher intraoperative driving pressures (ΔP) are associated with increased postoperative pulmonary complications (PPC). We hypothesised that dynamic adjustment of PEEP throughout abdominal surgery reduces ΔP, maintains positive end-expiratory transpulmonary pressures (P_{tp_ee}) and increases respiratory system static compliance (C_rs) with PEEP levels that are variable between and within patients.

METHODS

In a prospective multicentre pilot study, adults at moderate/high risk for PPC undergoing elective abdominal surgery were randomised to one of three ventilation protocols: (1) PEEP≤2 cm H₂O, compared with periodic recruitment manoeuvres followed by individualised PEEP to either optimise respiratory system compliance (PEEP_maxCrs) or maintain positive end-expiratory transpulmonary pressure (PEEP_{Ptp_ee}). The composite primary outcome included intraoperative ΔP, P_{tp_ee}, C_rs, and PEEP values (median (interquartile range) and coefficients of variation [CV_PEEP]).

RESULTS

Thirty-seven patients (48.6% female; age range: 47-73 yr) were assigned to control (PEEP≤2 cm H₂O; n=13), PEEP_maxCrs (n=16), or PEEP_{Ptp_ee} (n=8) groups. The PEEP_{Ptp_ee} intervention could not be delivered in two patients. Subjects assigned to PEEP_maxCrs had lower ΔP (median8 cm H₂O [7-10]), compared with the control group (12 cm H₂O [10-15]; P=0.006). PEEP_maxCrs was also associated with higher P_{tp_ee} (2.0 cm H₂O [-0.7 to 4.5] vs controls: -8.3 cm H₂O [-13.0 to -4.0]; P≤0.001) and higher C_rs (47.7 ml cm H₂O [43.2-68.8] vs controls: 39.0 ml cm H₂O [32.9-43.4]; P=0.009). Individualised PEEP (PEEP_maxCrs and PEEP_{Ptp_ee} combined) varied widely (median: 10 cm H₂O [8-15]; CV_PEEP=0.24 [0.14-0.35]), both between, and within, subjects throughout surgery.

CONCLUSIONS

This pilot study suggests that individualised PEEP management strategies applied during abdominal surgery reduce driving pressure, maintain positive P_{tp_ee} and increase static compliance. The wide range of PEEP observed suggests that an individualised approach is required to optimise respiratory mechanics during abdominal surgery.

CLINICAL TRIAL REGISTRATION

NCT02671721.

Transpulmonary driving pressure during mechanical ventilation-validation of a non-invasive measurement method

BACKGROUND

Transpulmonary driving pressure plays an important role in today's understanding of ventilator induced lung injury. We have previously validated a novel non-invasive method based on stepwise increments of PEEP to assess transpulmonary driving pressure in anaesthetised patients with healthy lungs. The aim of this study was to validate the method in patients who were mechanically ventilated for different diagnoses requiring intensive care.

METHODS

We measured transpulmonary pressure (Ptp) and calculated transpulmonary driving pressure (ΔPtp) in 31 patients undergoing mechanical ventilation in an intensive care unit. Parallel triplicate measurements were performed with the PEEP step method (PtpPSM) and the conventional oesophageal balloon method (Ptpconv). Their agreement was compared using the intraclass correlation coefficient (ICC) and the Bland Altman plot.

RESULT

The coefficient of variation for the repeated measurements was 4,3 for ΔPtpPSM and 9,2 for ΔPtpconv. The ICC of 0,864 and the Bland Altman plot indicate good agreement between the two methods.

CONCLUSION

The non-invasive method can be applied in mechanically ventilated patients to measure transpulmonary driving pressure with good repeatability and accuracy comparable to the traditional oesophageal balloon method.

Esophageal Manometry and Regional Transpulmonary Pressure in Lung Injury

RATIONALE

Esophageal manometry is the clinically available method to estimate pleural pressure, thus enabling calculation of transpulmonary pressure (Pl). However, many concerns make it uncertain in which lung region esophageal manometry reflects local Pl.

OBJECTIVES

To determine the accuracy of esophageal pressure (Pes) and in which regions esophageal manometry reflects pleural pressure (Ppl) and Pl; to assess whether lung stress in nondependent regions can be estimated at end-inspiration from Pl.

METHODS

In lung-injured pigs (n = 6) and human cadavers (n = 3), Pes was measured across a range of positive end-expiratory pressure, together with directly measured Ppl in nondependent and dependent pleural regions. All measurements were obtained with minimal nonstressed volumes in the pleural sensors and esophageal balloons. Expiratory and inspiratory Pl was calculated by subtracting local Ppl or Pes from airway pressure; inspiratory Pl was also estimated by subtracting Ppl (calculated from chest wall and respiratory system elastance) from the airway plateau pressure.

MEASUREMENTS AND MAIN RESULTS

In pigs and human cadavers, expiratory and inspiratory Pl using Pes closely reflected values in dependent to middle lung (adjacent to the esophagus). Inspiratory Pl estimated from elastance ratio reflected the directly measured nondependent values.

CONCLUSIONS

These data support the use of esophageal manometry in acute respiratory distress syndrome. Assuming correct calibration, expiratory Pl derived from Pes reflects Pl in dependent to middle lung, where atelectasis usually predominates; inspiratory Pl estimated from elastance ratio may indicate the highest level of lung stress in nondependent "baby" lung, where it is vulnerable to ventilator-induced lung injury.

PEEP titration in moderate to severe ARDS: plateau versus transpulmonary pressure

BACKGROUND

Although lung protection with low tidal volume and limited plateau pressure (P_plat) improves survival in acute respiratory distress syndrome patients (ARDS), the best way to set positive end-expiratory pressure (PEEP) is still debated.

METHODS

This study aimed to compare two strategies using individual PEEP based on a maximum P_plat (28-30 cmH₂O, the Express group) or on keeping end-expiratory transpulmonary pressure positive (0-5 cmH₂O, P_Lexpi group). We estimated alveolar recruitment (Vrec), end-expiratory lung volume and alveolar distension based on elastance-related end-inspiratory transpulmonary pressure (P_L,EL).

RESULTS

Nineteen patients with moderate to severe ARDS (PaO₂/FiO₂ < 150 mmHg) were included with a baseline PEEP of 7.0 ± 1.8 cmH₂O and a PaO₂/FiO₂ of 91.2 ± 31.2 mmHg. PEEP and oxygenation increased significantly from baseline with both protocols; PEEP Express group was 14.2 ± 3.6 cmH₂O versus 16.7 ± 5.9 cmH₂O in P_Lexpi group. No patient had the same PEEP with the two protocols. Vrec was higher with the latter protocol (299 [0 to 875] vs. 222 [47 to 483] ml, p = 0.049) and correlated with improved oxygenation (R² = 0.45, p = 0.002). Two and seven patients in the Express and P_L,expi groups, respectively, had P_L,EL > 25 cmH₂O.

CONCLUSIONS

There is a great heterogeneity of P_Lexpi when P_plat is used to titrate PEEP but with limited risk of over-distension. A PEEP titration for a moderate positive level of P_Lexpi might slightly improve alveolar recruitment and oxygenation but increases the risk of over-distension in one-third of patients.

Driving Pressure and Transpulmonary Pressure: How Do We Guide Safe Mechanical Ventilation?

The physiological concept, pathophysiological implications and clinical relevance and application of driving pressure and transpulmonary pressure to prevent ventilator-induced lung injury are discussed.

Detailed measurements of oesophageal pressure during mechanical ventilation with an advanced high-resolution manometry catheter

Background

Oesophageal pressure (PES) is used for calculation of lung and chest wall mechanics and transpulmonary pressure during mechanical ventilation. Measurements performed with a balloon catheter are suggested as a basis for setting the ventilator; however, measurements are affected by several factors. High-resolution manometry (HRM) simultaneously measures pressures at every centimetre in the whole oesophagus and thereby provides extended information about oesophageal pressure. The aim of the present study was to evaluate the factors affecting oesophageal pressure using HRM.

Methods

Oesophageal pressure was measured using a high-resolution manometry catheter in 20 mechanically ventilated patients (15 in the ICU and 5 in the OR). Different PEEP levels and different sizes of tidal volume were applied while pressures were measured continuously. In 10 patients, oesophageal pressure was also measured using a conventional balloon catheter for comparison. A retrospective analysis of oesophageal pressure measured with HRM in supine and sitting positions in 17 awake spontaneously breathing patients is also included.

Results

HRM showed large variations in end-expiratory PES (PESEE) and tidal changes in PES (ΔPES) along the oesophagus. Mean intra-individual difference between the minimum and maximum end-expiratory oesophageal pressure (PESEE at baseline PEEP) and tidal variations in oesophageal pressure (ΔPES at tidal volume 6 ml/kg) recorded by HRM in the different sections of the oesophagus was 23.7 (7.9) cmH₂O and 7.6 (3.9) cmH₂O respectively. Oesophageal pressures were affected by tidal volume, level of PEEP, part of the oesophagus included and patient positioning. HRM identified simultaneous increases and decreases in PES within a majority of individual patients. Compared to sitting position, supine position increased PESEE (mean difference 12.3 cmH₂O), pressure variation within individual patients and cardiac artefacts. The pressure measured with a balloon catheter did not correspond to the average pressure measured with HRM within the same part of the oesophagus.

Conclusions

The intra-individual variability in PESEE and ΔPES is substantial, and as a result, the balloon on the conventional catheter is affected by many different pressures along its length. Oesophageal pressures are not only affected by lung and chest wall mechanics but are a complex product of many factors, which is not obvious during conventional measurements. For correct calculations of transpulmonary pressure, factors influencing oesophageal pressures need to be known. HRM, which is available at many hospitals, can be used to increase the knowledge concerning these factors.

Trial registration

ClinicalTrials.gov, NCT02901158

Electronic supplementary material

The online version of this article (10.1186/s13054-019-2484-8) contains supplementary material, which is available to authorized users.

Esophageal pressure monitoring: why, when and how?

PURPOSE OF REVIEW

Esophageal manometry has shown its usefulness to estimate transpulmonary pressure, that is lung stress, and the intensity of spontaneous effort in patients with acute respiratory distress syndrome. However, clinical uptake of esophageal manometry in ICU is still low. Thus, the purpose of review is to describe technical tips to adequately measure esophageal pressure at the bedside, and then update the most important clinical applications of esophageal manometry in ICU.

RECENT FINDINGS

Each esophageal balloon has its own nonstressed volume and it should be calibrated properly to measure pleural pressure accurately: transpulmonary pressure calculated on absolute esophageal pressure reflects values in the lung regions adjacent to the esophageal balloon (i.e. dependent to middle lung). Inspiratory transpulmonary pressure calculated from airway plateau pressure and the chest wall to respiratory system elastance ratio reasonably reflects lung stress in the nondependent 'baby' lung, at highest risk of hyperinflation. Also esophageal pressure can be used to detect and minimize patient self-inflicted lung injury.

SUMMARY

Esophageal manometry is not a complicated technique. There is a large potential to improve clinical outcome in patients with acute respiratory distress syndrome, acting as an early detector of risk of lung injury from mechanical ventilation and vigorous spontaneous effort.

Should we titrate peep based on end-expiratory transpulmonary pressure?-yes

Ventilator management of patients with acute respiratory distress syndrome (ARDS) has been characterized by implementation of basic physiology principles by minimizing harmful distending pressures and preventing lung derecruitment. Such strategies have led to significant improvements in outcomes. Positive end expiratory pressure (PEEP) is an important part of a lung protective strategy but there is no standardized method to set PEEP level. With widely varying types of lung injury, body habitus and pulmonary mechanics, the use of esophageal manometry has become important for personalization and optimization of mechanical ventilation in patients with ARDS. Esophageal manometry estimates pleural pressures, and can be used to differentiate the chest wall and lung (transpulmonary) contributions to the total respiratory system mechanics. Elevated pleural pressures may result in negative transpulmonary pressures at end expiration, leading to lung collapse. Measuring the esophageal pressures and adjusting PEEP to make transpulmonary pressures positive can decrease atelectasis, derecruitment of lung, and cyclical opening and closing of airways and alveoli, thus optimizing lung mechanics and oxygenation. Although there is some spatial and positional artifact, esophageal pressures in numerous animal and human studies in healthy, obese and critically ill patients appear to be a good estimate for the "effective" pleural pressure. Multiple studies have illustrated the benefit of using esophageal pressures to titrate PEEP in patients with obesity and with ARDS. Esophageal pressure monitoring provides a window into the unique physiology of a patient and helps improve clinical decision making at the bedside.

Technical aspects of bedside respiratory monitoring of transpulmonary pressure

Transpulmonary pressure, that is the difference between airway pressure (Paw) and pleural pressure, is considered one of the most important parameters to know in order to set a safe mechanical ventilation in acute respiratory distress syndrome (ARDS) patients but also in critically ill obese patients, in abdominal pathologies or in pathologies affecting the chest wall itself. Transpulmonary pressure should rely on the assessment of intrathoracic pleural pressure. Esophageal pressure (Pes) is considered the best surrogate of pleural pressure in critically ill patients, but concerns about its reliability exist. The aim of this article is to describe the technique of Pes measurement in mechanically ventilated patients: the catheter insertion, the proper balloon placement and filling, the validation test and specific procedures to remove the main artifacts will be discussed.

Interpretation of the transpulmonary pressure in the critically ill patient

Mechanical ventilation is a life-saving procedure, which takes over the function of the respiratory muscles while buying time for healing to take place. However, it can also promote or worsen lung injury, so that careful monitoring of respiratory mechanics is suggested to titrate the level of support and avoid injurious pressures and volumes to develop. Standard monitoring includes flow, volume and airway pressure (Paw). However, Paw represents the pressure acting on the respiratory system as a whole, and does not allow to differentiate the part of pressure that is spent di distend the chest wall. Moreover, if spontaneous breathing efforts are allowed, the Paw is the sum of that applied by the ventilator and that generated by the patient. As a consequence, monitoring of Paw has significant shortcomings. Assessment of esophageal pressure (Pes), as a surrogate for pleural pressure (Ppl), may allow the clinicians to discriminate between the elastic behaviour of the lung and the chest wall, and to calculate the degree of spontaneous respiratory effort. In the present review, the characteristics and limitations of airway and transpulmonary pressure monitoring will be presented; we will highlight the different assumptions underlying the various methods for measuring transpulmonary pressure (i.e., the elastance-derived and the release-derived method, and the direct measurement), as well as the potential application of transpulmonary pressure assessment during both controlled and spontaneous/assisted mechanical ventilation in critically ill patients.

Can we estimate transpulmonary pressure without an esophageal balloon?-yes

A protective ventilation strategy is based on separation of lung and chest wall mechanics and determination of transpulmonary pressure. So far, this has required esophageal pressure measurement, which is cumbersome, rarely used clinically and associated with lack of consensus on the interpretation of measurements. We have developed an alternative method based on a positive end expiratory pressure (PEEP) step procedure where the PEEP-induced change in end-expiratory lung volume is determined by the ventilator pneumotachograph. In pigs, lung healthy patients and acute lung injury (ALI) patients, it has been verified that the determinants of the change in end-expiratory lung volume following a PEEP change are the size of the PEEP step and the elastic properties of the lung, ∆PEEP × Clung. As a consequence, lung compliance can be calculated as the change in end-expiratory lung volume divided by the change in PEEP and esophageal pressure measurements are not needed. When lung compliance is determined in this way, transpulmonary driving pressure can be calculated on a breath-by-breath basis. As the end-expiratory transpulmonary pressure increases as much as PEEP is increased, it is also possible to determine the end-inspiratory transpulmonary pressure at any PEEP level. Thus, the most crucial factors of ventilator induced lung injury can be determined by a simple PEEP step procedure. The measurement procedure can be repeated with short intervals, which makes it possible to follow the course of the lung disease closely. By the PEEP step procedure we may also obtain information (decision support) on the mechanical consequences of changes in PEEP and tidal volume performed to improve oxygenation and/or carbon dioxide removal.

Regional distribution of transpulmonary pressure

The pressure across the lung, so-called transpulmonary pressure (P_L), represents the main force acting toward to provide lung movement. During mechanical ventilation, P_L is provided by respiratory system pressurization, using specific ventilator setting settled by the operator, such as: tidal volume (V_T), positive end-expiratory pressure (PEEP), respiratory rate (RR), and inspiratory airway flow. Once P_L is developed throughout the lungs, its distribution is heterogeneous, being explained by the elastic properties of the lungs and pleural pressure gradient. There are different methods of P_L calculation, each one with importance and some limitations. Among the most known, it can be quoted: (I) direct measurement of P_L; (II) elastance derived method at end-inspiration of P_L; (III) transpulmonary driving pressure. Recent studies using pleural sensors in large animal models as also in human cadaver have added new and important information about P_L heterogeneous distribution across the lungs. Due to this heterogeneous distribution, lung damage could happen in specific areas of the lung. In addition, it is widely accepted that high P_L can cause lung damage, however the way it is delivered, whether it's compressible or tensile, may also further damage despite the values of P_L achieved. According to heterogeneous distribution of P_L across the lungs, the interstitium and lymphatic vessels may also interplay to disseminate lung inflammation toward peripheral organs through thoracic lymph tracts. Thus, it is conceivable that juxta-diaphragmatic area associated strong efforts leading to high values of P_L may be a source of dissemination of inflammatory cells, large molecules, and plasma contents able to perpetuate inflammation in distal organs.

Value of measuring esophageal pressure to evaluate heart-lung interactions-applications for invasive hemodynamic monitoring

Evaluation of intrathoracic pressure is the cornerstone of the understanding of heart-lung interactions, but is not easily feasible at the bedside. Esophageal pressure (P_es) has been shown to be a good surrogate for intrathoracic pressure and can be more easily measured using a small esophageal catheter, but is not routinely employed. It can provide crucial information for the study of heart-lung interactions in both controlled and spontaneous ventilation. This review presents the physiological basis, the technical aspects and the value in clinical practice of the measurement of P_es.

Fifty Years of Research in ARDS. Vt Selection in Acute Respiratory Distress Syndrome

Mechanical ventilation (MV) is critical in the management of many patients with acute respiratory distress syndrome (ARDS). However, MV can also cause ventilator-induced lung injury (VILI). The selection of an appropriate Vt is an essential part of a lung-protective MV strategy. Since the publication of a large randomized clinical trial demonstrating the benefit of lower Vts, the use of Vts of 6 ml/kg predicted body weight (based on sex and height) has been recommended in clinical practice guidelines. However, the predicted body weight approach is imperfect in patients with ARDS because the amount of aerated lung varies considerably due to differences in inflammation, consolidation, flooding, and atelectasis. Better approaches to setting Vt may include limits on end-inspiratory transpulmonary pressure, lung strain, and driving pressure. The limits of lowering Vt have not yet been established, and some patients may benefit from Vts that are lower than those in current use. However, lowering Vts may result in respiratory acidosis. Tactics to reduce respiratory acidosis include reductions in ventilation circuit dead space, increases in respiratory rate, higher positive end-expiratory pressures in patients who recruit lung in response to positive end-expiratory pressure, recruitment maneuvers, and prone positioning. Mechanical adjuncts such as extracorporeal carbon dioxide removal may be useful to normalize pH and carbon dioxide levels, but further studies will be necessary to demonstrate benefit with this technology.

Respiratory mechanics to understand ARDS and guide mechanical ventilation

OBJECTIVE

As precision medicine is becoming a standard of care in selecting tailored rather than average treatments, physiological measurements might represent the first step in applying personalized therapy in the intensive care unit (ICU). A systematic assessment of respiratory mechanics in patients with the acute respiratory distress syndrome (ARDS) could represent a step in this direction, for two main reasons. Approach and Main results: On the one hand, respiratory mechanics are a powerful physiological method to understand the severity of this syndrome in each single patient. Decreased respiratory system compliance, for example, is associated with low end expiratory lung volume and more severe lung injury. On the other hand, respiratory mechanics might guide protective mechanical ventilation settings. Improved gravitationally dependent regional lung compliance could support the selection of positive end-expiratory pressure and maximize alveolar recruitment. Moreover, the association between driving airway pressure and mortality in ARDS patients potentially underlines the importance of sizing tidal volume on respiratory system compliance rather than on predicted body weight.

SIGNIFICANCE

The present review article aims to describe the main alterations of respiratory mechanics in ARDS as a potent bedside tool to understand severity and guide mechanical ventilation settings, thus representing a readily available clinical resource for ICU physicians.

Esophageal pressure: research or clinical tool?

Esophageal manometry has traditionally been utilized for respiratory physiology research, but clinicians have recently found numerous applications within the intensive care unit. Esophageal pressure (P_Es) is a surrogate for pleural pressures (P_Pl), and the difference between airway pressure (P_AO) and P_Es provides a good estimate for the pressure across the lung also known as the transpulmonary pressure (P_L). Differentiating the effects of mechanical ventilation and spontaneous breathing on the respiratory system, chest wall, and across the lung allows for improved personalization in clinical decision making. Measuring P_L in acute respiratory distress syndrome (ARDS) may help set positive end expiratory pressure (PEEP) to prevent derecruitment and atelectrauma, while assuring peak pressures do not cause over distension during tidal breathing and recruitment maneuvers. Monitoring P_Es allows improved insight into patient-ventilator interactions and may help in decisions to adjust sedation and paralytics to correct dyssynchrony. Intrinsic PEEP (auto-PEEP) may be monitored using esophageal manometry, which may also improve patient comfort and synchrony with the ventilator. Finally, during weaning, P_Es may be used to better predict weaning success and allow for rapid intervention during failure. Improved consistency in definition and terminology and further outcomes research is needed to encourage more widespread adoption; however, with clear clinical benefit and increased ease of use, it appears time to reintroduce basic physiology into personalized ventilator management in the intensive care unit.

Transpulmonary Pressure: The Importance of Precise Definitions and Limiting Assumptions

Recent studies applying the principles of respiratory mechanics to respiratory disease have used inconsistent and mutually exclusive definitions of the term "transpulmonary pressure." By the traditional definition, transpulmonary pressure is the pressure across the whole lung, including the intrapulmonary airways, (i.e., the pressure difference between the opening to the pulmonary airway and the pleural surface). However, more recently transpulmonary pressure has also been defined as the pressure across only the lung tissue (i.e., the pressure difference between the alveolar space and the pleural surface), traditionally known as the "elastic recoil pressure of the lung." Multiple definitions of the same term, and failure to recognize their underlying assumptions, have led to different interpretations of lung physiology and conclusions about appropriate therapy for patients. It is our view that many current controversies in the physiological interpretation of disease are caused by the lack of consistency in the definitions of these common physiological terms. In this article, we discuss the historical uses of these terms and recent misconceptions that may have resulted when these terms were confused. These misconceptions include assertions that normal pleural pressure must be negative (subatmospheric) and that a pressure in the pleural space may not be substantially positive when a subject is relaxed with an open airway. We urge specificity and uniformity when using physiological terms to define the physical state of the lungs, the chest wall, and the integrated respiratory system.

Is there a role for clowns in paediatric intensive care units?

Hospital clowning is a programme in healthcare facilities involving visits from specially trained actors. In the paediatric intensive care unit (PICU), clowning may appear inappropriate and less intuitive. The patient could appear too ill and/or sedated, the environment too crowded or chaotic and the parents too stressed. Relying on our experience with professionally trained clowns both in France and Canada, the purpose of this article is to offer a model for hospital clowning and to suggest standards of practice for the implementation of clowning in PICUs. In this work, we provide a framework for the implementation of clown care in the PICU, to overcome the challenges related to the complex technical environment, the patient's critical illness and the high parental stress levels. Regardless of the specifics of the PICU, our experience suggests that professional clown activity is feasible, safe and can offer multiple benefits to the child, his/her parents and to hospital personnel. Due to the specific challenges in the PICU, clowns must be educated and prepared to work in this highly specialised environment. We stress that prior to clowning in a PICU, professional performers must be highly trained, experienced, abide by a code of ethics and be fully accepted by the treating healthcare team.

Transpulmonary pressure: importance and limits

Transpulmonary pressure (P_L) is computed as the difference between airway pressure and pleural pressure and separates the pressure delivered to the lung from the one acting on chest wall and abdomen. Pleural pressure is measured as esophageal pressure (P_ES) through dedicated catheters provided with esophageal balloons. We discuss the role of P_L in assessing the effects of mechanical ventilation in patients with acute respiratory distress syndrome (ARDS). In the supine position, directly measured P_L represents the pressure acting on the alveoli and airways. Because there is a pressure gradient in the pleural space from the non-dependent to the dependent zones, the pressure in the esophagus probably represents the pressure at a mid-level between sternal and vertebral regions. For this reason, it has been proposed to set the end-expiratory pressure in order to get a positive value of P_L. This improves oxygenation and compliance. P_L can also be estimated from airway pressure plateau and the ratio of lung to respiratory elastance (elastance-derived method). Some data suggest that this latter calculation may better estimate P_L in the nondependent lung zones, at risk for hyperinflation. Elastance-derived P_L at end-inspiration (P_Lend-insp) may be a good surrogate of end-inspiratory lung stress for the "baby lung", at least in non-obese patients. Limiting end-inspiratory P_L to 20-25 cmH₂O appears physiologically sound to mitigate ventilator-induced lung injury (VILI). Last, lung driving pressure (∆P_L) reflects the tidal distending pressure. Changes in P_L may also be assessed during assisted breathing to take into account the additive effects of spontaneous breathing and mechanical breaths on lung distension. In summary, despite limitations, assessment of P_L allows a deeper understanding of the risk of VILI and may potentially help tailor ventilator settings.

Fifty Years of Research in ARDS. Setting Positive End-Expiratory Pressure in Acute Respiratory Distress Syndrome

Positive end-expiratory pressure (PEEP) has been used during mechanical ventilation since the first description of acute respiratory distress syndrome (ARDS). In the subsequent decades, many different strategies for optimally titrating PEEP have been proposed. Higher PEEP can improve arterial oxygenation, reduce tidal lung stress and strain, and promote more homogenous ventilation by preventing alveolar collapse at end expiration. However, PEEP may also cause circulatory depression and contribute to ventilator-induced lung injury through alveolar overdistention. The overall effect of PEEP is primarily related to the balance between the number of alveoli that are recruited to participate in ventilation and the amount of lung that is overdistended when PEEP is applied. Techniques to assess lung recruitment from PEEP may help to direct safer and more effective PEEP titration. Some PEEP titration strategies attempt to weigh beneficial effects on arterial oxygenation and on prevention of cyclic alveolar collapse with the harmful potential of overdistention. One method for PEEP titration is a PEEP/FiO2 table that prioritizes support for arterial oxygenation. Other methods set PEEP based on mechanical parameters, such as the plateau pressure, respiratory system compliance, or transpulmonary pressure. No single method of PEEP titration has been shown to improve clinical outcomes compared with other approaches of setting PEEP. Future trials should focus on identifying individuals who respond to higher PEEP with recruitment and on clinically important outcomes (e.g., mortality).

The promises and problems of transpulmonary pressure measurements in acute respiratory distress syndrome

PURPOSE OF REVIEW

The optimal strategy for assessing and preventing ventilator-induced lung injury in the acute respiratory distress syndrome (ARDS) is controversial. Recent investigative efforts have focused on personalizing ventilator settings to individual respiratory mechanics. This review examines the strengths and weaknesses of using transpulmonary pressure measurements to guide ventilator management in ARDS.

RECENT FINDINGS

Recent clinical studies suggest that adjusting ventilator settings based on transpulmonary pressure measurements is feasible, may improve oxygenation, and reduce ventilator-induced lung injury.

SUMMARY

The measurement of transpulmonary pressure relies upon esophageal manometry, which requires the acceptance of several assumptions and potential errors. Notably, this includes the ability of localized esophageal pressures to represent global pleural pressure. Recent investigations demonstrated improved oxygenation in ARDS patients when positive end-expiratory pressure was adjusted to target specific end-inspiratory or end-expiratory transpulmonary pressures. However, there are different methods for estimating transpulmonary pressure and different goals for positive end-expiratory pressure titration among recent studies. More research is needed to refine techniques for the estimation and utilization of transpulmonary pressure to guide ventilator settings in ARDS patients.

Transpulmonary and pleural pressure in a respiratory system model with an elastic recoiling lung and an expanding chest wall

Background

We have shown in acute lung injury patients that lung elastance can be determined by a positive end-expiratory pressure (PEEP) step procedure and proposed that this is explained by the spring-out force of the rib cage off-loading the chest wall from the lung at end-expiration. The aim of this study was to investigate the effect of the expanding chest wall on pleural pressure during PEEP inflation by building a model with an elastic recoiling lung and an expanding chest wall complex.

Methods

Test lungs with a compliance of 19, 38, or 57 ml/cmH₂O were placed in a box connected to a plastic container, 3/4 filled with water, connected to a water sack of 10 l, representing the abdomen. The space above the water surface and in the lung box constituted the pleural space. The contra-directional forces of the recoiling lung and the expanding chest wall were obtained by evacuating the pleural space to a negative pressure of 5 cmH₂O. Chest wall elastance was increased by strapping the plastic container. Pressure was measured in the airway and pleura. Changes in end-expiratory lung volume (ΔEELV), during PEEP steps of 4, 8, and 12 cmH₂O, were determined in the isolated lung, where airway equals transpulmonary pressure and in the complete model as the cumulative inspiratory-expiratory tidal volume difference. Transpulmonary pressure was calculated as airway minus pleural pressure.

Results

Lung pressure/volume curves of an isolated lung coincided with lung P/V curves in the complete model irrespective of chest wall stiffness. ΔEELV was equal to the size of the PEEP step divided by lung elastance (EL), ΔEELV = ΔPEEP/EL. The end-expiratory “pleural” pressure did not increase after PEEP inflation, and consequently, transpulmonary pressure increased as much as PEEP was increased.

Conclusions

The rib cage spring-out force causes off-loading of the chest wall from the lung and maintains a negative end-expiratory “pleural” pressure after PEEP inflation. The behavior of the respiratory system model confirms that lung elastance can be determined by a simple PEEP step without using esophageal pressure measurements.

Effect of PEEP, blood volume, and inspiratory hold maneuvers on venous return

According to Guyton's model of circulation, mean systemic filling pressure (MSFP), right atrial pressure (RAP), and resistance to venous return (RVR) determine venous return. MSFP has been estimated from inspiratory hold-induced changes in RAP and blood flow. We studied the effect of positive end-expiratory pressure (PEEP) and blood volume on venous return and MSFP in pigs. MSFP was measured by balloon occlusion of the right atrium (MSFPRAO), and the MSFP obtained via extrapolation of pressure-flow relationships with airway occlusion (MSFPinsp_hold) was extrapolated from RAP/pulmonary artery flow (QPA) relationships during inspiratory holds at PEEP 5 and 10 cmH2O, after bleeding, and in hypervolemia. MSFPRAO increased with PEEP [PEEP 5, 12.9 (SD 2.5) mmHg; PEEP 10, 14.0 (SD 2.6) mmHg, P = 0.002] without change in QPA [2.75 (SD 0.43) vs. 2.56 (SD 0.45) l/min, P = 0.094]. MSFPRAO decreased after bleeding and increased in hypervolemia [10.8 (SD 2.2) and 16.4 (SD 3.0) mmHg, respectively, P < 0.001], with parallel changes in QPA. Neither PEEP nor volume state altered RVR ( P = 0.489). MSFPinsp_hold overestimated MSFPRAO [16.5 (SD 5.8) vs. 13.6 (SD 3.2) mmHg, P = 0.001; mean difference 3.0 (SD 5.1) mmHg]. Inspiratory holds shifted the RAP/QPA relationship rightward in euvolemia because inferior vena cava flow (QIVC) recovered early after an inspiratory hold nadir. The QIVC nadir was lowest after bleeding [36% (SD 24%) of preinspiratory hold at 15 cmH2O inspiratory pressure], and the QIVC recovery was most complete at the lowest inspiratory pressures independent of volume state [range from 80% (SD 7%) after bleeding to 103% (SD 8%) at PEEP 10 cmH2O of QIVC before inspiratory hold]. The QIVC recovery thus defends venous return, possibly via hepatosplanchnic vascular waterfall.

Effect of high-frequency oscillatory ventilation on esophageal and transpulmonary pressures in moderate-to-severe acute respiratory distress syndrome

Background

High-frequency oscillatory ventilation (HFOV) has not been shown to be beneficial in the management of moderate-to-severe acute respiratory distress syndrome (ARDS). There is uncertainty about the actual pressure applied into the lung during HFOV. We therefore performed a study to compare the transpulmonary pressure (P_L) during conventional mechanical ventilation (CMV) and different levels of mean airway pressure (mPaw) during HFOV.

Methods

This is a prospective randomized crossover study in a university teaching hospital. An esophageal balloon catheter was used to measure esophageal pressures (Pes) at end inspiration and end expiration and to calculate P_L. Measurements were taken during ventilation with CMV (CMVpre) after which patients were switched to HFOV with three 1-h different levels of mPaw set at +5, +10 and +15 cm H₂O above the mean airway pressure measured during CMV. Patients were thereafter switched back to CMV (CMVpost).

Results

Ten patients with moderate-to-severe ARDS were included. We demonstrated a linear increase in Pes and P_L with the increase in mPaw during HFOV. Contrary to CMV, P_L was always positive during HFOV whatever the level of mPaw applied but not associated with improvement in oxygenation. We found significant correlations between mPaw and Pes.

Conclusion

HFOV with high level of mPaw increases transpulmonary pressures without improvement in oxygenation.

Severe hypoxemia: which strategy to choose

BACKGROUND

Acute respiratory distress syndrome (ARDS) is characterized by a noncardiogenic pulmonary edema with bilateral chest X-ray opacities and reduction in lung compliance, and the hallmark of the syndrome is hypoxemia refractory to oxygen therapy. Severe hypoxemia (PaO2/FiO2 < 100 mmHg), which defines severe ARDS, can be found in 20-30 % of the patients and is associated with the highest mortality rate. Although the standard supportive treatment remains mechanical ventilation (noninvasive and invasive), possible adjuvant therapies can be considered. We performed an up-to-date clinical review of the possible available strategies for ARDS patients with severe hypoxemia.

MAIN RESULTS

In summary, in moderate-to-severe ARDS or in the presence of other organ failure, noninvasive ventilatory support presents a high risk of failure: in those cases the risk/benefit of delayed mechanical ventilation should be evaluated carefully. Tailoring mechanical ventilation to the individual patient is fundamental to reduce the risk of ventilation-induced lung injury (VILI): it is mandatory to apply a low tidal volume, while the optimal level of positive end-expiratory pressure should be selected after a stratification of the severity of the disease, also taking into account lung recruitability; monitoring transpulmonary pressure or airway driving pressure can help to avoid lung overstress. Targeting oxygenation of 88-92 % and tolerating a moderate level of hypercapnia are a safe choice. Neuromuscular blocking agents (NMBAs) are useful to maintain patient-ventilation synchrony in the first hours; prone positioning improves oxygenation in most cases and promotes a more homogeneous distribution of ventilation, reducing the risk of VILI; both treatments, also in combination, are associated with an improvement in outcome if applied in the acute phase in the most severe cases. The use of extracorporeal membrane oxygenation (ECMO) in severe ARDS is increasing worldwide, but because of a lack of randomized trials is still considered a rescue therapy.

CONCLUSION

Severe ARDS patients should receive a holistic framework of respiratory and hemodynamic support aimed to ensure adequate gas exchange while minimizing the risk of VILI, by promoting lung recruitment and setting protective mechanical ventilation. In the most severe cases, NMBAs, prone positioning, and ECMO should be considered.

Pulse Pressure Variation Adjusted by Respiratory Changes in Pleural Pressure, Rather Than by Tidal Volume, Reliably Predicts Fluid Responsiveness in Patients With Acute Respiratory Distress Syndrome

OBJECTIVES

1) To evaluate the ability of pulse pressure variation adjusted by respiratory changes in pleural pressure to predict fluid responsiveness compared with pulse pressure variation alone. 2) To identify factors explaining the poor performance of pulse pressure variation in acute respiratory distress syndrome.

DESIGN

Prospective study.

SETTING

Forty-bed university hospital general ICU.

PATIENTS

Ninety-six mechanically ventilated acute respiratory distress syndrome patients requiring fluid challenge.

INTERVENTIONS

Fluid challenge, 500 mL saline over 20 minutes.

MEASUREMENTS AND MAIN RESULTS

Before fluid challenge, esophageal pressure was measured at the end-inspiratory and end-expiratory occlusions. Change in pleural pressure was calculated as the difference between esophageal pressure measured at end-inspiratory and end-expiratory occlusions. Hemodynamic measurements were obtained before and after the fluid challenge. Patients were ventilated with tidal volume 7.0 ± 0.8 mL/kg predicted body weight. The fluids increased cardiac output by greater than 15% in 52 patients (responders). Adjusting pulse pressure variation for changes in pleural pressure (area under the receiver operating characteristic curve, 0.94 [0.88-0.98]) and the ratio of chest wall elastance to total respiratory system elastance (area under the receiver operating characteristic curve, 0.93 [0.88-0.98]) predicted fluid responsiveness better than pulse pressure variation (area under the receiver operating characteristic curve, 0.78 [0.69-0.86]; all p < 0.01). The gray zone approach identified a range of pulse pressure variation/changes in pleural pressure values (1.94-2.1) in 3.1% of patients for whom fluid responsiveness could not be predicted reliably. On logistic regression analysis, two independent factors affected the correct classification of fluid responsiveness at a 12% pulse pressure variation cutoff: tidal volume (adjusted odds ratio 1.57/50 mL; 95% CI, 1.05-2.34; p = 0.027) and chest wall elastance/respiratory system elastance (adjusted odds ratio, 2.035/0.1 unit; 95% CI, 1.36-3.06; p = 0.001). In patients with chest wall elastance/respiratory system elastance above the median (0.28), pulse pressure variation area under the receiver operating characteristic curve was 0.94 (95% CI, 0.84-0.99) compared with 0.76 (95% CI, 0.61-0.87) otherwise (p = 0.02).

CONCLUSIONS

In acute respiratory distress syndrome patients, pulse pressure variation adjusted by changes in pleural pressure is a reliable fluid responsiveness predictor despite the low tidal volume (< 8 mL/kg). The poor predictive ability of pulse pressure variation in acute respiratory distress syndrome patients is more related to low chest wall elastance/respiratory system elastance ratios than to a low tidal volume.

In vivo calibration of esophageal pressure in the mechanically ventilated patient makes measurements reliable

Background

Esophageal pressure (Pes) can provide information to guide mechanical ventilation in acute respiratory failure. However, both relative changes and absolute values of Pes can be affected by inappropriate filling of the esophageal balloon and by the elastance of the esophagus wall. We evaluated the feasibility and effectiveness of a calibration procedure consisting in optimization of balloon filling and subtraction of the pressure generated by the esophagus wall (Pew).

Methods

An esophageal balloon was progressively filled in 36 patients under controlled mechanical ventilation. V_BEST was the filling volume associated with the largest tidal increase of Pes. Esophageal wall elastance was quantified and Pew was computed at each filling volume. Different filling strategies were compared by performing a validation occlusion test.

Results

Fifty series of measurements were performed. V_BEST was 3.5 ± 1.9 ml (range 0.5–6.0). Esophagus elastance was 1.1 ± 0.5 cmH₂O/ml (0.3–3.1). Both Pew and the result of the occlusion test differed among filling strategies. At filling volumes of 0.5, V_BEST and 4.0 ml respectively, Pew was 0.0 ± 0.1, 2.0 ± 1.9, and 3.0 ± 1.7 cmH₂O (p < 0.0001), whereas the occlusion test was satisfactory in 22 %, 98 %, and 88 % of cases (p < 0.0001).

Conclusions

Under mechanical ventilation, an increase of balloon filling above the conventionally recommended low volumes warrants complete transmission of Pes swings, but is associated with significant elevation of baseline. A simple calibration procedure allows finding the filling volume associated with the best transmission of tidal Pes change and subtracting the associated baseline artifact, thus making measurement of absolute values of Pes reliable.

Electronic supplementary material

The online version of this article (doi:10.1186/s13054-016-1278-5) contains supplementary material, which is available to authorized users.

The occlusion tests and end-expiratory esophageal pressure: measurements and comparison in controlled and assisted ventilation

Background

Esophageal pressure is used as a reliable surrogate of the pleural pressure. It is conventionally measured by an esophageal balloon placed in the lower part of the esophagus. To validate the correct position of the balloon, a positive pressure occlusion test by compressing the thorax during an end-expiratory pause or a Baydur test obtained by occluding the airway during an inspiratory effort is used. An acceptable catheter position is defined when the ratio between the changes in esophageal and airway pressure (∆Pes/∆Paw) is close to unity. Sedation and paralysis could affect the accuracy of esophageal pressure measurements. The aim of this study was to evaluate, in mechanically ventilated patients, the effects of paralysis, two different esophageal balloon positions and two PEEP levels on the ∆Pes/∆Paw ratio measured by the positive pressure occlusion and the Baydur tests and on the end-expiratory esophageal pressure and respiratory mechanics (lung and chest wall).

Methods

Twenty-one intubated and mechanically ventilated patients (mean age 64.8 ± 14.0 years, body mass index 24.2 ± 4.3 kg/m², PaO₂/FiO₂ 319.4 ± 117.3 mmHg) were enrolled. In step 1, patients were sedated and paralyzed during volume-controlled ventilation, and in step 2, they were only sedated during pressure support ventilation. In each step, two esophageal balloon positions (middle and low, between 25–30 cm and 40–45 cm from the mouth) and two levels of PEEP (0 and 10 cmH₂O) were applied. The ∆Pes/∆Paw ratio and end-expiratory esophageal pressure were evaluated.

Results

The ∆Pes/∆Paw ratio was slightly higher (+0.11) with positive occlusion test compared with Baydur’s test. The level of PEEP and the esophageal balloon position did not affect this ratio. The ∆Pes and ∆Paw were significantly related to a correlation coefficient of r = 0.984 during the Baydur test and r = 0.909 in the positive occlusion test. End-expiratory esophageal pressure was significantly higher in sedated and paralyzed patients compared with sedated patients (+2.47 cmH₂O) and when esophageal balloon was positioned in the low position (+2.26 cmH₂O). The esophageal balloon position slightly influenced the lung elastance, while the PEEP reduced the chest wall elastance without affecting the lung and total respiratory system elastance.

Conclusions

Paralysis and balloon position did not clinically affect the measurement of the ∆Pes/∆Paw ratio, while they significantly increased the end-expiratory esophageal pressure.

The effects of airway pressure release ventilation on respiratory mechanics in extrapulmonary lung injury

BACKGROUND

Lung injury is often studied without consideration for pathologic changes in the chest wall. In order to reduce the incidence of lung injury using preemptive mechanical ventilation, it is important to recognize the influence of altered chest wall mechanics on disease pathogenesis. In this study, we hypothesize that airway pressure release ventilation (APRV) may be able to reduce the chest wall elastance associated with an extrapulmonary lung injury model as compared with low tidal volume (LVt) ventilation.

METHODS

Female Yorkshire pigs were anesthetized and instrumented. Fecal peritonitis was established, and the superior mesenteric artery was clamped for 30 min to induce an ischemia/reperfusion injury. Immediately following injury, pigs were randomized into (1) LVt (n = 3), positive end-expiratory pressure (PEEP) 5 cmH2O, V t 6 cc kg(-1), FiO2 21 %, and guided by the ARDSnet protocol or (2) APRV (n = 3), P High 16-22 cmH2O, P Low 0 cmH2O, T High 4.5 s, T Low set to terminate the peak expiratory flow at 75 %, and FiO2 21 %. Pigs were monitored continuously for 48 h. Lung samples and bronchoalveolar lavage fluid were collected at necropsy.

RESULTS

LVt resulted in mild acute respiratory distress syndrome (ARDS) (PaO2/FiO2 = 226.2 ± 17.1 mmHg) whereas APRV prevented ARDS (PaO2/FiO2 = 465.7 ± 66.5 mmHg; p < 0.05). LVt had a reduced surfactant protein A concentration and increased histologic injury as compared with APRV. The plateau pressure in APRV (34.3 ± 0.9 cmH2O) was significantly greater than LVt (22.2 ± 2.0 cmH2O; p < 0.05) yet transpulmonary pressure between groups was similar (p > 0.05). This was because the pleural pressure was significantly lower in LVt (7.6 ± 0.5 cmH2O) as compared with APRV (17.4 ± 3.5 cmH2O; p < 0.05). Finally, the elastance of the lung, chest wall, and respiratory system were all significantly greater in LVt as compared with APRV (all p < 0.05).

CONCLUSIONS

APRV preserved surfactant and lung architecture and maintenance of oxygenation. Despite the greater plateau pressure and tidal volumes in the APRV group, the transpulmonary pressure was similar to that of LVt. Thus, the majority of the plateau pressure in the APRV group was distributed as pleural pressure in this extrapulmonary lung injury model. APRV maintained a normal lung elastance and an open, homogeneously ventilated lung without increasing lung stress.