Volume-related and volume-independent effects of posture on esophageal and transpulmonary pressures in healthy subjects

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.

Distribution of transpulmonary pressure during one-lung ventilation in pigs at different body positions

Background. Global and regional transpulmonary pressure (PL) during one-lung ventilation (OLV) is poorly characterized. We hypothesized that global and regional PL and driving PL (ΔPL) increase during protective low tidal volume OLV compared to two-lung ventilation (TLV), and vary with body position. Methods. In sixteen anesthetized juvenile pigs, intra-pleural pressure sensors were placed in ventral, dorsal, and caudal zones of the left hemithorax by video-assisted thoracoscopy. A right thoracotomy was performed and lipopolysaccharide administered intravenously to mimic the inflammatory response due to thoracic surgery. Animals were ventilated in a volume-controlled mode with a tidal volume (VT) of 6 mL kg-1 during TLV and of 5 mL kg-1 during OLV and a positive end-expiratory pressure (PEEP) of 5 cmH2O. Global and local transpulmonary pressures were calculated. Lung instability was defined as end-expiratory PL<2.9 cmH2O according to previous investigations. Variables were acquired during TLV (TLVsupine), left lung ventilation in supine (OLVsupine), semilateral (OLVsemilateral), lateral (OLVlateral) and prone (OLVprone) positions randomized according to Latin-square sequence. Effects of position were tested using repeated measures ANOVA. Results. End-expiratory PL and ΔPL were higher during OLVsupine than TLVsupine. During OLV, regional end-inspiratory PL and ΔPL did not differ significantly among body positions. Yet, end-expiratory PL was lower in semilateral (ventral: 4.8 ± 2.9 cmH2O; caudal: 3.1 ± 2.6 cmH2O) and lateral (ventral: 1.9 ± 3.3 cmH2O; caudal: 2.7 ± 1.7 cmH2O) compared to supine (ventral: 4.8 ± 2.9 cmH2O; caudal: 3.1 ± 2.6 cmH2O) and prone position (ventral: 1.7 ± 2.5 cmH2O; caudal: 3.3 ± 1.6 cmH2O), mainly in ventral (p ≤ 0.001) and caudal (p = 0.007) regions. Lung instability was detected more often in semilateral (26 out of 48 measurements; p = 0.012) and lateral (29 out of 48 measurements, p < 0.001) as compared to supine position (15 out of 48 measurements), and more often in lateral as compared to prone position (19 out of 48 measurements, p = 0.027). Conclusion. Compared to TLV, OLV increased lung stress. Body position did not affect stress of the ventilated lung during OLV, but lung stability was lowest in semilateral and lateral decubitus position.

The oesophageal balloon for respiratory monitoring in ventilated patients: updated clinical review and practical aspects

There is a well-recognised importance for personalising mechanical ventilation settings to protect the lungs and the diaphragm for each individual patient. Measurement of oesophageal pressure (P oes) as an estimate of pleural pressure allows assessment of partitioned respiratory mechanics and quantification of lung stress, which helps our understanding of the patient's respiratory physiology and could guide individualisation of ventilator settings. Oesophageal manometry also allows breathing effort quantification, which could contribute to improving settings during assisted ventilation and mechanical ventilation weaning. In parallel with technological improvements, P oes monitoring is now available for daily clinical practice. This review provides a fundamental understanding of the relevant physiological concepts that can be assessed using P oes measurements, both during spontaneous breathing and mechanical ventilation. We also present a practical approach for implementing oesophageal manometry at the bedside. While more clinical data are awaited to confirm the benefits of P oes-guided mechanical ventilation and to determine optimal targets under different conditions, we discuss potential practical approaches, including positive end-expiratory pressure setting in controlled ventilation and assessment of inspiratory effort during assisted modes.

Effects of obesity, pneumoperitoneum, and body position on mechanical power of intraoperative ventilation: an observational study

Mechanical power can describe the complex interaction between the respiratory system and the ventilator and may predict lung injury or pulmonary complications, but the power associated with injury of healthy human lungs is unknown. Body habitus and surgical conditions may alter mechanical power but the effects have not been measured. In a secondary analysis of an observational study of obesity and lung mechanics during robotic laparoscopic surgery, we comprehensively quantified the static elastic, dynamic elastic, and resistive energies comprising mechanical power of ventilation. We stratified by body mass index (BMI) and examined power at four surgical stages: level after intubation, with pneumoperitoneum, in Trendelenburg, and level after releasing the pneumoperitoneum. Esophageal manometry was used to estimate transpulmonary pressures. Mechanical power of ventilation and its bioenergetic components increased over BMI categories. Respiratory system and lung power were nearly doubled in subjects with class 3 obesity compared with lean at all stages. Power dissipated into the respiratory system was increased with class 2 or 3 obesity compared with lean. Increased power of ventilation was associated with decreasing transpulmonary pressures. Body habitus is a prime determinant of increased intraoperative mechanical power. Obesity and surgical conditions increase the energies dissipated into the respiratory system during ventilation. The observed elevations in power may be related to tidal recruitment or atelectasis, and point to specific energetic features of mechanical ventilation of patients with obesity that may be controlled with individualized ventilator settings.NEW & NOTEWORTHY Mechanical power describes the complex interaction between a patient's lungs and the ventilator and may be useful in predicting lung injury. However, its behavior in obesity and during dynamic surgical conditions is not understood. We comprehensively quantified ventilation bioenergetics and effects of body habitus and common surgical conditions. These data show body habitus is a prime determinant of intraoperative mechanical power and provide quantitative context for future translation toward a useful perioperative prognostic measurement.

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.

Influence of patient position in thoracoscopic esophagectomy on postoperative pneumonia: a comparative analysis from the National Clinical Database in Japan

BACKGROUND

Two prominent patient positions during thoracoscopic esophagectomy are the left lateral decubitus position (LP) and the prone position (PP). However, whether the patient position during thoracoscopic esophagectomy influences short-term outcomes, especially postoperative pneumonia, remains unclear. We aimed to elucidate the impact of patient position on the occurrence of postoperative pneumonia.

METHODS

We analyzed 9850 patients who underwent oncologic thoracoscopic esophagectomies between 2016 and 2019 from the National Clinical Database. We compared the short-term outcomes between the LP and PP groups, and the primary outcome measure was the incidence of postoperative pneumonia.

RESULTS

This study included 2637 (26.8%) and 7213 (73.2%) patients in the LP and the PP groups, respectively. The baseline characteristics of the two groups were well-balanced. Compared with the LP group, the PP group had a longer operative time and less blood loss. There were no significant differences in the incidences of postoperative pneumonia, recurrent laryngeal nerve palsy, anastomotic leakage, severe complications, and reoperation between the groups. Meanwhile, prolonged ventilation and surgery-related mortality occurred more frequently in the LP than in the PP group (P < 0.001 and 0.046, respectively). After multivariable adjustment, the patient position did not significantly influence the incidence of postoperative pneumonia (odds ratio 0.91, 95% confidence interval 0.80-1.04).

CONCLUSIONS

Although prolonged ventilation and surgery-related mortality occurred more frequently in the LP group than in the PP group, the patient position did not significantly influence the occurrence of postoperative pneumonia.

Pleural and transpulmonary pressures to tailor protective ventilation in children

This review aims to: (1) describe the rationale of pleural (P_PL) and transpulmonary (P_L) pressure measurements in children during mechanical ventilation (MV); (2) discuss its usefulness and limitations as a guide for protective MV; (3) propose future directions for paediatric research. We conducted a scoping review on P_L in critically ill children using PubMed and Embase search engines. We included peer-reviewed studies using oesophageal (P_ES) and P_L measurements in the paediatric intensive care unit (PICU) published until September 2021, and excluded studies in neonates and patients treated with non-invasive ventilation. P_L corresponds to the difference between airway pressure and P_PL Oesophageal manometry allows measurement of P_ES, a good surrogate of P_PL, to estimate P_L directly at the bedside. Lung stress is the P_L, while strain corresponds to the lung deformation induced by the changing volume during insufflation. Lung stress and strain are the main determinants of MV-related injuries with P_L and P_PL being key components. P_L-targeted therapies allow tailoring of MV: (1) Positive end-expiratory pressure (PEEP) titration based on end-expiratory P_L (direct measurement) may be used to avoid lung collapse in the lung surrounding the oesophagus. The clinical benefit of such strategy has not been demonstrated yet. This approach should consider the degree of recruitable lung, and may be limited to patients in which PEEP is set to achieve an end-expiratory P_L value close to zero; (2) Protective ventilation based on end-inspiratory P_L (derived from the ratio of lung and respiratory system elastances), might be used to limit overdistention and volutrauma by targeting lung stress values < 20-25 cmH₂O; (3) P_PL may be set to target a physiological respiratory effort in order to avoid both self-induced lung injury and ventilator-induced diaphragm dysfunction; (4) P_PL or P_L measurements may contribute to a better understanding of cardiopulmonary interactions. The growing cardiorespiratory system makes children theoretically more susceptible to atelectrauma, myotrauma and right ventricle failure. In children with acute respiratory distress, P_PL and P_L measurements may help to characterise how changes in PEEP affect P_PL and potentially haemodynamics. In the PICU, P_PL measurement to estimate respiratory effort is useful during weaning and ventilator liberation. Finally, the use of P_PL tracings may improve the detection of patient ventilator asynchronies, which are frequent in children. Despite these numerous theoritcal benefits in children, P_ES measurement is rarely performed in routine paediatric practice. While the lack of robust clincal data partially explains this observation, important limitations of the existing methods to estimate P_PL in children, such as their invasiveness and technical limitations, associated with the lack of reference values for lung and chest wall elastances may also play a role. P_PL and P_L monitoring have numerous potential clinical applications in the PICU to tailor protective MV, but its usefulness is counterbalanced by technical limitations. Paediatric evidence seems currently too weak to consider oesophageal manometry as a routine respiratory monitoring. The development and validation of a noninvasive estimation of P_L and multimodal respiratory monitoring may be worth to be evaluated in the future.

The calibration of esophageal pressure by proper esophageal balloon filling volume: A clinical study

Background

Esophageal pressure (Pes) can be used as a reliable surrogate for pleural pressure, especially in critically ill patients requiring personalized mechanical ventilation strategies. How to choose the proper esophageal balloon filling volume and then find the optimal value of esophageal pressure remains a challenge. The study aimed to assess the feasibility of catheters for Pes monitoring in mechanically ventilated patients.

Materials and methods

Twelve patients under pressure-controlled mechanical ventilation were included in this study. Raw esophageal pressure was recorded at different balloon filling volumes. Then, the P-V curves were determined. V _WORK was the intermediate linear section on the end-expiratory P-V curve, and V _BEST was the filling volume providing the maximum difference between Pes at end-inspiration and end-expiration. The raw value of Pes was recorded, and the calibrated values of Pes were calculated by calculating the esophageal wall pressure (Pew) and esophageal elastance (Ees).

Results

Twenty-four series of Pes measurements were performed. The mean V _MIN and V _MAX were 2.17 ± 0.49 ml (range, 1.0-3.0 ml) and 6.79 ± 0.83 ml (range, 5.0-9.0 ml), respectively, whereas V _BEST was 4.69 ± 0.16 ml (range, 2.0-8.0 ml). Ees was 1.35 ± 0.51 cm H₂O/ml (range, 0.26-2.38 cm H₂O/ml). The estimated Pew at V _BEST was 3.16 ± 2.19 cm H₂O (range, 0-7.97 cm H₂O). Patients with a body mass index (BMI) ≥ 25 kg/m² had a significantly lower V _MAX (5.88 [5.25-6] vs. 7.25 [7-8] ml, p = 0.006) and a significantly lower V _BEST (3.69 [2.5-4.38] vs. 5.19 [4-6] ml, p = 0.036) than patients with a BMI < 25 kg/m². Patients with positive end-expiratory pressure (PEEP) ≥ 10 cm H₂O had a lower V _MIN and V _BEST than patients with PEEP < 10 cm H₂O, P > 0.05. Patients in the supine position had a higher esophageal pressure than those in the prone position with the same balloon filling volume.

Conclusions

Calibration of esophageal pressure to identify the best filling volume of esophageal balloon catheters is feasible. The esophageal pressure can be influenced by BMI, PEEP, and position. It is necessary to titrate the optimal inflation volume again when the PEEP values or the positions change.

The effect of posture on airflow distribution, airway geometry and air velocity in healthy subjects

BACKGROUND

Gravity, and thus body position, can affect the regional distribution of lung ventilation and blood flow. Therefore, body positioning is a potential tool to improve regional ventilation, thereby possibly enhancing the effect of respiratory physiotherapy interventions. In this proof-of-concept study, functional respiratory imaging (FRI) was used to objectively assess effects of body position on regional airflow distribution in the lungs.

METHODS

Five healthy volunteers were recruited. The participants were asked during FRI first to lie in supine position, afterwards in standardized right lateral position.

RESULTS

In right lateral position there was significantly more regional ventilation also described as Imaging Airflow Distribution in the right lung than in the left lung (P < 0.001). Air velocity was significantly higher in the left lung (P < 0.05). In right lateral position there was significantly more airflow distribution in the right lung than in the left lung (P < 0.001). Significant changes were observed in airway geometry resulting in a decrease in imaged airway volume (P = 0.024) and a higher imaged airway resistance (P = 0.029) in the dependent lung. In general, the effect of right lateral position caused a significant increase in regional ventilation (P < 0.001) in the dependent lung when compared with the supine position.

CONCLUSIONS

Changing body position leads to significant changes in regional lung ventilation, objectively assessed by FRI The volume based on the imaging parameters in the dependent lung is smaller in the lateral position than in the supine position. In right lateral decubitus position, airflow distribution is greater in dependent lung compared to the nondependent lung.

TRIAL REGISTRATION

The trial has been submitted to www.

CLINICALTRIALS

gov with identification number NCT01893697 on 07/02/2013.

Modeling the influence of gravity and the mechanical properties of elastin and collagen fibers on alveolar and lung pressure-volume curves

The relationship between pressure (P) and volume (V) in the human lung has been extensively studied. However, the combined effects of gravity and the mechanical properties of elastin and collagen on alveolar and lung P–V curves during breathing are not well understood. Here, we extended a previously established thick-walled spherical model of a single alveolus with wavy collagen fibers during positive pressure inflation. First, we updated the model for negative pressure-driven inflation that allowed incorporation of a gravity-induced pleural pressure gradient to predict how the static alveolar P–V relations vary spatially throughout an upright human lung. Second, by introducing dynamic surface tension and collagen viscoelasticity, we computed the hysteresis loop of the lung P–V curve. The model was tested by comparing its predicted regional ventilation to literature data, which offered insight into the effects of microgravity on ventilation. The model has also produced novel testable predictions for future experiments about the variation of mechanical stresses in the septal walls and the contribution of collagen and elastin fibers to the P–V curve and throughout the lung. The model may help us better understand how mechanical stresses arising from breathing and pleural pressure variations affect regional cellular mechanotransduction in the lung.

Partition of respiratory mechanics in patients with acute respiratory distress syndrome and association with outcome: a multicentre clinical study

PURPOSE

In acute respiratory distress syndrome (ARDS), physiological parameters associated with outcome may help defining targets for mechanical ventilation. This study aimed to address whether transpulmonary pressures (P_L), including transpulmonary driving pressure (DP_L), elastance-derived plateau P_L, and directly-measured end-expiratory P_L, are better associated with 60-day outcome than airway driving pressure (DP_aw). We also tested the combination of oxygenation and stretch index [PaO₂/(FiO₂*DP_aw)].

METHODS

Prospective, observational, multicentre registry of ARDS patients. Respiratory mechanics were measured early after intubation at 6 kg/ml tidal volume. We compared the predictive power of the parameters for mortality at day-60 through receiver operating characteristic (ROC) and assessed their association with 60-day mortality through unadjusted and adjusted Cox regressions. Finally, each parameter was dichotomized, and Kaplan-Meier survival curves were compared.

RESULTS

385 patients were enrolled 2 [1-4] days from intubation (esophageal pressure and arterial blood gases in 302 and 318 patients). As continuous variables, DP_aw, DP_L, and oxygenation stretch index were associated with 60-day mortality after adjustment for age and Sequential Organ Failure Assessment, whereas elastance-derived plateau P_L was not. DP_aw and DP_L performed equally in ROC analysis (P = 0.0835). DP_aw had the best-fit Cox regression model. When dichotomizing the variables, DP_aw ≥ 15, DP_L ≥ 12, plateau P_L ≥ 24, and oxygenation stretch index < 10 exhibited lower 60-day survival probability. Directly measured end-expiratory P_L ≥ 0 was associated with better outcome in obese patients.

CONCLUSION

DP_L was equivalent predictor of outcome than DP_aw. Our study supports the soundness of limiting lung and airway driving pressure and maintaining positive end-expiratory P_L in obese patients.

Oesophageal pressure as a surrogate of pleural pressure in mechanically ventilated patients

Background

Oesophageal pressure (P_oes) is used to approximate pleural pressure (P_pl) and therefore to estimate transpulmonary pressure (P_L). We aimed to compare oesophageal and regional pleural pressures and to calculate transpulmonary pressures in a prospective physiological study on lung transplant recipients during their stay in the intensive care unit of a tertiary university hospital.

Methods

Lung transplant recipients receiving invasive mechanical ventilation and monitored by oesophageal manometry and dependent and nondependent pleural catheters were investigated during the post-operative period. We performed simultaneous short-time measurements and recordings of oesophageal manometry and pleural pressures. Expiratory and inspiratory P_L were computed by subtracting regional P_pl or P_oes from airway pressure; inspiratory P_L was also calculated with the elastance ratio method.

Results

16 patients were included. Among them, 14 were analysed. Oesophageal pressures correlated with dependent and nondependent pleural pressures during expiration (R²=0.71, p=0.005 and R²=0.77, p=0.001, respectively) and during inspiration (R²=0.66 for both, p=0.01 and p=0.014, respectively). P_L values calculated using P_oes were close to those obtained from the dependent pleural catheter but higher than those obtained from the nondependent pleural catheter both during expiration and inspiration.

Conclusions

In ventilated lung transplant recipients, oesophageal manometry is well correlated with pleural pressure. The absolute value of P_oes is higher than P_pl of nondependent lung regions and could therefore underestimate the highest level of lung stress in those at high risk of overinflation.

During controlled ventilation without respiratory muscle activity, absolute oesophageal pressure is higher than the pleural pressure of the nondependent lung regions and could therefore underestimate the highest level of lung stress in that lunghttps://bit.ly/3a95CUh

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.

High Pleural Pressure Prevents Alveolar Overdistension and Hemodynamic Collapse in ARDS with Class III Obesity

Rationale: Obesity is characterized by elevated pleural pressure (Ppl) and worsening atelectasis during mechanical ventilation in patients with acute respiratory distress syndrome (ARDS).

Objectives: To determine the effects of a lung recruitment maneuver (LRM) in the presence of elevated Ppl on hemodynamics, left and right ventricular pressure, and pulmonary vascular resistance. We hypothesized that elevated Ppl protects the cardiovascular system against high airway pressure and prevents lung overdistension.

Methods: First, an interventional crossover trial in adult subjects with ARDS and a body mass index ≥ 35 kg/m² (n = 21) was performed to explore the hemodynamic consequences of the LRM. Second, cardiovascular function was studied during low and high positive end-expiratory pressure (PEEP) in a model of swine with ARDS and high Ppl (n = 9) versus healthy swine with normal Ppl (n = 6).

Measurements and Main Results: Subjects with ARDS and obesity (body mass index = 57 ± 12 kg/m²) after LRM required an increase in PEEP of 8 (95% confidence interval [95% CI], 7–10) cm H₂O above traditional ARDS Network settings to improve lung function, oxygenation and V./Q. matching, without impairment of hemodynamics or right heart function. ARDS swine with high Ppl demonstrated unchanged transmural left ventricular pressure and systemic blood pressure after the LRM protocol. Pulmonary arterial hypertension decreased (8 [95% CI, 13–4] mm Hg), as did vascular resistance (1.5 [95% CI, 2.2–0.9] Wood units) and transmural right ventricular pressure (10 [95% CI, 15–6] mm Hg) during exhalation. LRM and PEEP decreased pulmonary vascular resistance and normalized the V./Q. ratio.

Conclusions: High airway pressure is required to recruit lung atelectasis in patients with ARDS and class III obesity but causes minimal overdistension. In addition, patients with ARDS and class III obesity hemodynamically tolerate LRM with high airway pressure.

Clinical trial registered with www.clinicaltrials.gov (NCT 02503241).

Body Habitus and Dynamic Surgical Conditions Independently Impair Pulmonary Mechanics during Robotic-assisted Laparoscopic Surgery

Background

Body habitus, pneumoperitoneum, and Trendelenburg positioning may each independently impair lung mechanics during robotic laparoscopic surgery. This study hypothesized that increasing body mass index is associated with more mechanical strain and alveolar collapse, and these impairments are exacerbated by pneumoperitoneum and Trendelenburg positioning.

Methods

This cross-sectional study measured respiratory flow, airway pressures, and esophageal pressures in 91 subjects with body mass index ranging from 18.3 to 60.6 kg/m2. Pulmonary mechanics were quantified at four stages: (1) supine and level after intubation, (2) with pneumoperitoneum, (3) in Trendelenburg docked with the surgical robot, and (4) level without pneumoperitoneum. Subjects were stratified into five body mass index categories (less than 25, 25 to 29.9, 30 to 34.9, 35 to 39.9, and 40 or higher), and respiratory mechanics were compared over surgical stages using generalized estimating equations. The optimal positive end-expiratory pressure settings needed to achieve positive end-expiratory transpulmonary pressures were calculated.

Results

At baseline, transpulmonary driving pressures increased in each body mass index category (1.9 ± 0.5 cm H2O; mean difference ± SD; P &lt; 0.006), and subjects with a body mass index of 40 or higher had decreased mean end-expiratory transpulmonary pressures compared with those with body mass index of less than 25 (–7.5 ± 6.3 vs. –1.3 ± 3.4 cm H2O; P &lt; 0.001). Pneumoperitoneum and Trendelenburg each further elevated transpulmonary driving pressures (2.8 ± 0.7 and 4.7 ± 1.0 cm H2O, respectively; P &lt; 0.001) and depressed end-expiratory transpulmonary pressures (–3.4 ± 1.3 and –4.5 ± 1.5 cm H2O, respectively; P &lt; 0.001) compared with baseline. Optimal positive end-expiratory pressure was greater than set positive end-expiratory pressure in 79% of subjects at baseline, 88% with pneumoperitoneum, 95% in Trendelenburg, and ranged from 0 to 36.6 cm H2O depending on body mass index and surgical stage.

Conclusions

Increasing body mass index induces significant alterations in lung mechanics during robotic laparoscopic surgery, but there is a wide range in the degree of impairment. Positive end-expiratory pressure settings may need individualization based on body mass index and surgical conditions.

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What We Already Know about This Topic

What This Article Tells Us That Is New

Body Position Alters Mechanical Power and Respiratory Mechanics During Thoracic Surgery

BACKGROUND

During thoracic surgery, patients are usually positioned in lateral decubitus and only the dependent lung ventilated. The ventilated lung is thus exposed to the weight of the contralateral hemithorax and restriction of the dependent chest wall. We hypothesized that mechanical power would increase during one-lung ventilation in the lateral position.

METHODS

We performed a prospective, observational, single-center study from December 2016 to May 2017. Thirty consecutive patients undergoing general anesthesia with mechanical ventilation (mean age, 68 ± 11 years; body mass index, 25 ± 5 kg·m) for thoracic surgery were enrolled. Total and partitioned mechanical power, lung and chest wall elastance, and esophageal pressure were compared in supine and lateral position with double- and one-lung ventilation and with closed and open chest both before and after surgery. Mixed factorial ANOVA for repeated measurements was performed, with both step and the period before or after surgery as 2 within-subject factors, and left or right body position during surgery as a fixed, between-subject factor. Appropriate interaction terms were included.

RESULTS

The mechanical power was higher in lateral one-lung ventilation compared to both supine and lateral position double-lung ventilation (11.1 ± 3.0 vs 8.2 ± 2.7 vs 8.7 ± 2.6; mean difference, 2.9 J·minute [95% CI, 1.4-4.4 J·minute] and 2.4 J·minute [95% CI, 0.9-3.9 J·minute]; P < .001 and P = .002, respectively). Lung elastance was higher during lateral position one-lung ventilation compared to both lateral and supine double-lung ventilation (24.3 ± 8.7 vs 9.5 ± 3.8 vs 10.0 ± 3.8; mean difference, 14.7 cm H2O·L [95% CI, 11.2-18.2 cm H2O·L] and 14.2 cm H2O·L [95% CI, 10.8-17.7 cm H2O·L], respectively) and was higher compared to predicted values (20.1 ± 7.5 cm H2O·L). Chest wall elastance increased in lateral position double-lung ventilation compared to supine (11.1 ± 3.8 vs 6.6 ± 3.4; mean difference, 4.5 cm H2O·L [95% CI, 2.6-6.3 cm H2O·L]) and was lower in lateral position one-lung ventilation with open chest than with a closed chest (3.5 ± 1.9 vs 7.1 ± 2.8; mean difference, 3.6 cm H2O·L [95% CI, 2.4-4.8 cm H2O·L]). The end-expiratory esophageal pressure decreased moving from supine position to lateral position one-lung ventilation while increased with the opening of the chest wall.

CONCLUSIONS

Mechanical power and lung elastance are increased in the lateral position with one-lung ventilation. Esophageal pressure monitoring may be used to follow these changes.

Emerging concepts in ventilation-induced lung injury

Ventilation-induced lung injury results from mechanical stress and strain that occur during tidal ventilation in the susceptible lung. Classical descriptions of ventilation-induced lung injury have focused on harm from positive pressure ventilation. However, injurious forces also can be generated by patient effort and patient–ventilator interactions. While the role of global mechanics has long been recognized, regional mechanical heterogeneity within the lungs also appears to be an important factor propagating clinically significant lung injury. The resulting clinical phenotype includes worsening lung injury and a systemic inflammatory response that drives extrapulmonary organ failures. Bedside recognition of ventilation-induced lung injury requires a high degree of clinical acuity given its indistinct presentation and lack of definitive diagnostics. Yet the clinical importance of ventilation-induced lung injury is clear. Preventing such biophysical injury remains the most effective management strategy to decrease morbidity and mortality in patients with acute respiratory distress syndrome and likely benefits others at risk.

Bedside respiratory physiology to detect risk of lung injury in acute respiratory distress syndrome

PURPOSE OF REVIEW

The most effective strategies for treating the patient with acute respiratory distress syndrome center on minimizing ventilation-induced lung injury (VILI). Yet, current standard-of-care does little to modify mechanical ventilation to patient-specific risk. This review focuses on evaluation of bedside respiratory mechanics, which when interpreted in patient-specific context, affords opportunity to individualize lung-protective ventilation in patients with acute respiratory distress syndrome.

RECENT FINDINGS

Four biophysical mechanisms of VILI are widely accepted: volutrauma, barotrauma, atelectrauma, and stress concentration. Resulting biotrauma, that is, local and systemic inflammation and endothelial activation, may be thought of as the final common pathway that propagates VILI-mediated multiorgan failure. Conventional, widely utilized techniques to assess VILI risk rely on airway pressure, flow, and volume changes, and remain essential tools for determining overdistension of aerated lung regions, particularly when interpreted cognizant of their limitations. Emerging bedside tools identify regional differences in mechanics, but further study is required to identify how they might best be incorporated into clinical practice.

SUMMARY

Quantifying patient-specific risk of VILI requires understanding each patient's pulmonary mechanics in context of biological predisposition. Tailoring support at bedside according to these factors affords the greatest opportunity to date for mitigating VILI and alleviating associated morbidity.

Global and Regional Respiratory Mechanics During Robotic-Assisted Laparoscopic Surgery: A Randomized Study

BACKGROUND

Pneumoperitoneum and nonphysiological positioning required for robotic surgery increase cardiopulmonary risk because of the use of larger airway pressures (Paws) to maintain tidal volume (VT). However, the quantitative partitioning of respiratory mechanics and transpulmonary pressure (PL) during robotic surgery is not well described. We tested the following hypothesis: (1) the components of driving pressure (transpulmonary and chest wall components) increase in a parallel fashion at robotic surgical stages (Trendelenburg and robot docking); and (2) deep, when compared to routine (moderate), neuromuscular blockade modifies those changes in PLs as well as in regional respiratory mechanics.

METHODS

We studied 35 American Society of Anesthesiologists (ASA) I-II patients undergoing elective robotic surgery. Airway and esophageal balloon pressures and respiratory flows were measured to calculate respiratory mechanics. Regional lung aeration and ventilation was assessed with electrical impedance tomography and level of neuromuscular blockade with acceleromyography. During robotic surgical stages, 2 crossover randomized groups (conditions) of neuromuscular relaxation were studied: Moderate (1 twitch in the train-of-four stimulation) and Deep (1-2 twitches in the posttetanic count).

RESULTS

Pneumoperitoneum was associated with increases in driving pressure, tidal changes in PL, and esophageal pressure (Pes). Steep Trendelenburg position during robot docking was associated with further worsening of the respiratory mechanics. The fraction of driving pressures that partitioned to the lungs decreased from baseline (63% ± 15%) to Trendelenburg position (49% ± 14%, P < .001), due to a larger increase in chest wall elastance (Ecw; 12.7 ± 7.6 cm H2O·L) than in lung elastance (EL; 4.3 ± 5.0 cm H2O·L, P < .001). Consequently, from baseline to Trendelenburg, the component of Paw affecting the chest wall increased by 6.6 ± 3.1 cm H2O, while PLs increased by only 3.4 ± 3.1 cm H2O (P < .001). PL and driving pressures were larger at surgery end than at baseline and were accompanied by dorsal aeration loss. Deep neuromuscular blockade did not change respiratory mechanics, regional aeration and ventilation, and hemodynamics.

CONCLUSIONS

In robotic surgery with pneumoperitoneum, changes in ventilatory driving pressures during Trendelenburg and robot docking are distributed less to the lungs than to the chest wall as compared to routine mechanical ventilation for supine patients. This effect of robotic surgery derives from substantially larger increases in Ecw than ELs and reduces the risk of excessive PLs. Deep neuromuscular blockade does not meaningfully change global or regional lung mechanics.

Assessment of Work of Breathing in Patients with Acute Exacerbations of Chronic Obstructive Pulmonary Disease

The assessment of the work of breathing (WOB) of patients with acute exacerbations of chronic obstructive pulmonary disease (COPD) is difficult, particularly when the patient first presents with acute hypercapnia and respiratory acidosis. Acute exacerbations of COPD patients are in significant respiratory distress and noninvasive measurements of WOB are easier for the patient to tolerate. Given the interest in using alternative therapies to noninvasive ventilation, such as high flow nasal oxygen therapy or extracorporeal carbon dioxide removal, understanding the physiological changes are key and this includes assessment of WOB. This narrative review considers the role of three different methods of assessing WOB in patients with acute exacerbations of COPD. Esophageal pressure is a very well validated measure of WOB, however the ability of patients with acute exacerbations of COPD to tolerate esophageal tubes is poor. Noninvasive alternative measurements include parasternal electromyography (EMG) and electrical impedance tomography (EIT). EMG is easily applied and is a well validated measure of neural drive but is more likely to be degraded by the electrical environment in intensive care or high dependency. EIT is less well validated as a tool for WOB in COPD but extremely well tolerated by patients. Each of the different methods assess WOB in a different way and have different advantages and disadvantages. For research into therapies treating acute exacerbations of COPD, combinations of EIT, EMG and esophageal pressure are likely to be better than only one of these.

Effects of Prone Positioning on Transpulmonary Pressures and End-expiratory Volumes in Patients without Lung Disease

BACKGROUND

The effects of prone positioning on esophageal pressures have not been investigated in mechanically ventilated patients. Our objective was to characterize effects of prone positioning on esophageal pressures, transpulmonary pressure, and lung volume, thereby assessing the potential utility of esophageal pressure measurements in setting positive end-expiratory pressure (PEEP) in prone patients.

METHODS

We studied 16 patients undergoing spine surgery during general anesthesia and neuromuscular blockade. We measured airway pressure, esophageal pressures, airflow, and volume, and calculated the expiratory reserve volume and the elastances of the lung and chest wall in supine and prone positions.

RESULTS

Esophageal pressures at end expiration with 0 cm H2O PEEP decreased from supine to prone by 5.64 cm H2O (95% CI, 3.37 to 7.90; P < 0.0001). Expiratory reserve volume measured at relaxation volume increased from supine to prone by 0.15 l (interquartile range, 0.25, 0.10; P = 0.003). Chest wall elastance increased from supine to prone by 7.32 (95% CI, 4.77 to 9.87) cm H2O/l at PEEP 0 (P < 0.0001) and 6.66 cm H2O/l (95% CI, 3.91 to 9.41) at PEEP 7 (P = 0.0002). Median driving pressure, the change in airway pressure from end expiration to end-inspiratory plateau, increased in the prone position at PEEP 0 (3.70 cm H2O; 95% CI, 1.74 to 5.66; P = 0.001) and PEEP 7 (3.90 cm H2O; 95% CI, 2.72 to 5.09; P < 0.0001).

CONCLUSIONS

End-expiratory esophageal pressure decreases, and end-expiratory transpulmonary pressure and expiratory reserve volume increase, when patients are moved from supine to prone position. Mean respiratory system driving pressure increases in the prone position due to increased chest wall elastance. The increase in end-expiratory transpulmonary pressure and expiratory reserve volume may be one mechanism for the observed clinical benefit with prone positioning.

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.

Regulation of Cell Behavior by Hydrostatic Pressure

Hydrostatic pressure (HP) regulates diverse cell behaviors including differentiation, migration, apoptosis, and proliferation. Abnormal HP is associated with pathologies including glaucoma and hypertensive fibrotic remodeling. In this review, recent advances in quantifying and predicting how cells respond to HP across several tissue systems are presented, including tissues of the brain, eye, vasculature and bladder, as well as articular cartilage. Finally, some promising directions on the study of cell behaviors regulated by HP are proposed.

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.

Effect of transpulmonary pressure-guided positive end-expiratory pressure titration on lung injury in pigs with acute respiratory distress syndrome

To investigate the effect of positive end-expiratory pressure (PEEP) guided by transpulmonary pressure or with maximum oxygenation-directed PEEP on lung injury in a porcine model of acute respiratory distress syndrome (ARDS). The porcine model of ARDS was induced in 12 standard pigs by intratracheal infusion with normal saline. The pigs were then randomly divided into two groups who were ventilated with the lung-protective strategy of low tidal volume (VT) (6 ml/kg), using different methods to titrate PEEP level: transpulmonary pressure (TP group; n = 6) or maximum oxygenation (MO group; n = 6). Gas exchange, pulmonary mechanics, and hemodynamics were determined and pulmonary inflammatory response indices were measured after 4 h of ventilation. The titrated PEEP level in the TP group (6.12 ± 0.89 cmH₂O) was significantly lower than that in the MO group (11.33 ± 2.07 cmH2O) (P < 0.05). The PaO₂/FiO₂ (P/F) after PEEP titration both improved in the TP and MO groups as compared with that at T0 (when the criteria for ARDS were obtained). The P/F in the TP group did not differ significantly from that in the MO group during the 4 h of ventilation (P > 0.05). Respiratory system compliance and lung compliance were significantly improved in the TP group compared to the MO group (P < 0.05). The VD/VT in the TP group was significantly lower than that in the MO group after 4 h of ventilation (P < 0.05). Central venous pressure increased and the cardiac index decreased significantly in the MO group as compared with the TP group (P < 0.05), whereas oxygen delivery did not differ significantly between the groups (P > 0.05). The pulmonary vascular permeability index and the extravascular lung water index in the TP group were significantly lower than those in the MO group (P < 0.05). The TP group had a lower lung wet to dry weight ratio, lung injury score, and MPO, TNF-, and IL-8 concentrations than the MO group (P < 0.05). In summary, in a pig model of ARDS, ventilation with low VT and transpulmonary pressure-guided PEEP adjustment was associated with improved compliance, reduced dead space ventilation, increased cardiac output, and relieved lung injury, as compared to maximum oxygenation-guide PEEP adjustment.

Effect of Titrating Positive End-Expiratory Pressure (PEEP) With an Esophageal Pressure-Guided Strategy vs an Empirical High PEEP-Fio2 Strategy on Death and Days Free From Mechanical Ventilation Among Patients With Acute Respiratory Distress Syndrome: A Randomized Clinical Trial

IMPORTANCE

Adjusting positive end-expiratory pressure (PEEP) to offset pleural pressure might attenuate lung injury and improve patient outcomes in acute respiratory distress syndrome (ARDS).

OBJECTIVE

To determine whether PEEP titration guided by esophageal pressure (PES), an estimate of pleural pressure, was more effective than empirical high PEEP-fraction of inspired oxygen (Fio2) in moderate to severe ARDS.

DESIGN, SETTING, AND PARTICIPANTS

Phase 2 randomized clinical trial conducted at 14 hospitals in North America. Two hundred mechanically ventilated patients aged 16 years and older with moderate to severe ARDS (Pao2:Fio2 ≤200 mm Hg) were enrolled between October 31, 2012, and September 14, 2017; long-term follow-up was completed July 30, 2018.

INTERVENTIONS

Participants were randomized to PES-guided PEEP (n = 102) or empirical high PEEP-Fio2 (n = 98). All participants received low tidal volumes.

MAIN OUTCOMES AND MEASURES

The primary outcome was a ranked composite score incorporating death and days free from mechanical ventilation among survivors through day 28. Prespecified secondary outcomes included 28-day mortality, days free from mechanical ventilation among survivors, and need for rescue therapy.

RESULTS

Two hundred patients were enrolled (mean [SD] age, 56 [16] years; 46% female) and completed 28-day follow-up. The primary composite end point was not significantly different between treatment groups (probability of more favorable outcome with PES-guided PEEP: 49.6% [95% CI, 41.7% to 57.5%]; P = .92). At 28 days, 33 of 102 patients (32.4%) assigned to PES-guided PEEP and 30 of 98 patients (30.6%) assigned to empirical PEEP-Fio2 died (risk difference, 1.7% [95% CI, -11.1% to 14.6%]; P = .88). Days free from mechanical ventilation among survivors was not significantly different (median [interquartile range]: 22 [15-24] vs 21 [16.5-24] days; median difference, 0 [95% CI, -1 to 2] days; P = .85). Patients assigned to PES-guided PEEP were significantly less likely to receive rescue therapy (4/102 [3.9%] vs 12/98 [12.2%]; risk difference, -8.3% [95% CI, -15.8% to -0.8%]; P = .04). None of the 7 other prespecified secondary clinical end points were significantly different. Adverse events included gross barotrauma, which occurred in 6 patients with PES-guided PEEP and 5 patients with empirical PEEP-Fio2.

CONCLUSIONS AND RELEVANCE

Among patients with moderate to severe ARDS, PES-guided PEEP, compared with empirical high PEEP-Fio2, resulted in no significant difference in death and days free from mechanical ventilation. These findings do not support PES-guided PEEP titration in ARDS.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier NCT01681225.

Peep titration based on the open lung approach during one lung ventilation in thoracic surgery: a physiological study

BACKGROUND

During thoracic surgery in lateral decubitus, one lung ventilation (OLV) may impair respiratory mechanics and gas exchange. We tested a strategy based on an open lung approach (OLA) consisting in lung recruitment immediately followed by a decremental positive-end expiratory pressure (PEEP) titration to the best respiratory system compliance (C_RS) and separately quantified the elastic properties of the lung and the chest wall. Our hypothesis was that this approach would improve gas exchange. Further, we were interested in documenting the impact of the OLA on partitioned respiratory system mechanics.

METHODS

In thirteen patients undergoing upper left lobectomy we studied lung and chest wall mechanics, transpulmonary pressure (P_L), respiratory system and transpulmonary driving pressure (ΔP_RS and ΔP_L), gas exchange and hemodynamics at two time-points (a) during OLV at zero end-expiratory pressure (OLV_pre-OLA) and (b) after the application of the open-lung strategy (OLV_post-OLA).

RESULTS

The external PEEP selected through the OLA was 6 ± 0.8 cmH₂O. As compared to OLV_pre-OLA, the PaO₂/FiO₂ ratio went from 205 ± 73 to 313 ± 86 (p = .05) and C_L increased from 56 ± 18 ml/cmH₂O to 71 ± 12 ml/cmH₂O (p = .0013), without changes in C_CW. Both ΔP_RS and ΔP_L decreased from 9.2 ± 0.4 cmH₂O to 6.8 ± 0.6 cmH₂O and from 8.1 ± 0.5 cmH₂O to 5.7 ± 0.5 cmH₂O, (p = .001 and p = .015 vs OLV_pre-OLA), respectively. Hemodynamic parameters remained stable throughout the study period.

CONCLUSIONS

In our patients, the OLA strategy performed during OLV improved oxygenation and increased C_L and had no clinically significant hemodynamic effects. Although our study was not specifically designed to study ΔP_RS and ΔP_L, we observed a parallel reduction of both after the OLA.

TRIAL REGISTRATION

TRN: ClinicalTrials.gov , NCT03435523 , retrospectively registered, Feb 14 2018.

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.

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.

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.

Obesity and Weaning from Mechanical Ventilation-An Exploratory Study

Introduction

Obesity is associated with increased risk of hypercapnic respiratory failure, prolonged duration on mechanical ventilation, and extended weaning periods.

Objective

Pilot study to determine whether morbidly obese adult tracheotomized subjects (body mass index [BMI] ⩾ 40) can be more efficiently weaned from the ventilator by optimizing their positive end-expiratory pressure (PEEP) using either an esophageal balloon or the best achieved static effective compliance.

Methods

We randomly assigned 25 morbidly obese adult tracheotomized subjects (median [interquartile range] BMI 53.4 [26.4]; range 40.4-113.8) to 1 of 2 methods of setting PEEP; using either titration guided by esophageal balloon to overcome negative transpulmonary pressure (Ptp) (goal Ptp 0-5 cmH₂O) (ESO group) or titration to maximize static effective lung compliance (Cstat group). Our outcomes of interest were number of subjects weaned by day 30 and time to wean.

Results

At day 30, there was no significant difference in percentage of subjects weaned. 8/13 subjects (62%) in the ESO Group were weaned vs. 9/12(75%) in the Cstat Group (P = 0.67). Among the 17 subjects who weaned, median time to ventilator liberation was significantly shorter in the ESO group: 3.5 days vs Cstat group 14 days (P = .01). Optimal PEEP in the ESO and Cstat groups was similar (ESO mean ± SD = 26.5 ± 5.7 cmH₂O and Cstat 24.2 ± 7 cmH₂O (P = .38).

Conclusions

Optimization of PEEP using esophageal balloon to achieve positive transpulmonary pressure did not change the proportion of patients weaned. Among patients who weaned, use of the esophageal balloon resulted in faster liberation from mechanical ventilation. There were no adverse consequences of the high PEEP (mean 25.4; range 13-37 cmH₂O) used in our study. The study was approved by the Institutional Review Board at our institution (UMCIRB#10-0343) and registered with clinicaltrials.gov (NCT02323009).

Intraoperative Ventilation of Morbidly Obese Patients Guided by Transpulmonary Pressure

BACKGROUND

Bariatric surgery has proven a successful approach in the treatment of morbid obesity and its concomitant diseases such as diabetes mellitus and arterial hypertension. Aiming for optimal management of this challenging patient cohort, tailored concepts directly guided by individual patient physiology may outperform standardized care. Implying esophageal pressure measurement and electrical impedance tomography-increasingly applied monitoring approaches to individually adjust mechanical ventilation in challenging circumstances like acute respiratory distress syndrome (ARDS) and intraabdominal hypertension-we compared our institutions standard ventilator regimen with an individually adjusted positive end expiratory pressure (PEEP) level aiming for a positive transpulmonary pressure (P _L) throughout the respiratory cycle.

METHODS

After obtaining written informed consent, 37 patients scheduled for elective bariatric surgery were studied during mechanical ventilation in reverse Trendelenburg position. Before and after installation of capnoperitoneum, PEEP levels were gradually raised from a standard value of 10 cm H₂O until a P _L of 0 +/- 1 cm H₂O was reached. Changes in ventilation were monitored by electrical impedance tomography (EIT) and arterial blood gases (ABGs) were obtained at the end of surgery and 5 and 60 min after extubation, respectively.

RESULTS

To achieve the goal of a transpulmonary pressure (P _L) of 0 cm H₂O at end expiration, PEEP levels of 16.7 cm H₂O (95% KI 15.6-18.1) before and 23.8 cm H₂O (95% KI 19.6-40.4) during capnoperitoneum were necessary. EIT measurements confirmed an optimal PEEP level between 10 and 15 cm H₂O before and 20 and 25 cm H₂O during capnoperitoneum, respectively. Intra- and postoperative oxygenation did not change significantly.

CONCLUSION

Patients during laparoscopic bariatric surgery require high levels of PEEP to maintain a positive transpulmonary pressure throughout the respiratory cycle. EIT monitoring allows for non-invasive monitoring of increasing PEEP demand during capnoperitoneum. Individually adjusted PEEP levels did not result in improved postoperative oxygenation.

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.

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).

Use of the injection test to indicate the oesophageal balloon position in patients without spontaneous breathing: a clinical feasibility study

Objective

To investigate the clinical feasibility of the injection test for balloon placement during oesophageal pressure measurement in patients without spontaneous breathing.

Methods

The injection test was performed in 12 mechanically ventilated patients under deep sedation and paralysis. During withdrawal of the balloon from the stomach and air injection into the gastric lumen of the catheter, the presence of the injection test wave in the balloon pressure tracing indicated that the whole balloon was positioned above the lower oesophageal sphincter (LES). The positive pressure occlusion test was performed at different balloon positions.

Results

In each patient, the injection test wave appeared at a distinct balloon depth, with a mean ± standard deviation of 41.9 ± 3.3 cm and range from 37 cm to 47 cm. The optimal ratio of changes in the balloon and airway pressure (0.8–1.2) during the positive pressure occlusion test was obtained when the balloon was located 5 cm and 10 cm above the LES in nine (75%) and three (25%) patients, respectively.

Conclusions

The injection test is feasible for identification of the whole balloon position above the LES during passive ventilation. The middle third of the oesophagus might be the optimal balloon position.

Compression of the lungs by the heart in supine, side-lying, semi-prone positions

[Purpose] Clarification of the differences in the compression volume of the lungs by the heart (CVLH) between postures may facilitate the selection of optimal postures in respiratory care. Determining CVLH in the supine, semi-prone (Sim’s position), and side-lying positions was the aim of this study. [Subjects and Methods] Eight healthy volunteers (six males, two females; mean age, 29.0 ± 9.2 years) were enrolled in the study. Measurements were performed in the supine, right and left semi-prone, and right and left side-lying positions. semi-prone position was inclined 45° ventrally from the side-lying position. A 1.5-T system with a fast advanced spin-echo sequence in the coronal plane was used for magnetic resonance imaging. [Results] CVLH and heart compression ratio were significantly lower in the semi-prone position on both sides than the other positions. The heart was displaced ventrally when semi-prone and a larger area of the heart leaned on the ventral chest wall, localizing compression to part of the ventral region of the dependent lung. [Conclusion] The region of lungs compressed by the heart is reduced in the semi-prone position due to ventral displacement of the heart. These results suggest that maintaining expansion of the dependent lung is easier in the semi-prone position.

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.

Esophageal and transpulmonary pressure in the clinical setting: meaning, usefulness and perspectives

PURPOSE

Esophageal pressure (Pes) is a minimally invasive advanced respiratory monitoring method with the potential to guide management of ventilation support and enhance specific diagnoses in acute respiratory failure patients. To date, the use of Pes in the clinical setting is limited, and it is often seen as a research tool only.

METHODS

This is a review of the relevant technical, physiological and clinical details that support the clinical utility of Pes.

RESULTS

After appropriately positioning of the esophageal balloon, Pes monitoring allows titration of controlled and assisted mechanical ventilation to achieve personalized protective settings and the desired level of patient effort from the acute phase through to weaning. Moreover, Pes monitoring permits accurate measurement of transmural vascular pressure and intrinsic positive end-expiratory pressure and facilitates detection of patient-ventilator asynchrony, thereby supporting specific diagnoses and interventions. Finally, some Pes-derived measures may also be obtained by monitoring electrical activity of the diaphragm.

CONCLUSIONS

Pes monitoring provides unique bedside measures for a better understanding of the pathophysiology of acute respiratory failure patients. Including Pes monitoring in the intensivist's clinical armamentarium may enhance treatment to improve clinical outcomes.

Pneumoperitoneum deteriorates intratidal respiratory system mechanics: an observational study in lung-healthy patients

BACKGROUND

Pneumoperitoneum during laparoscopic surgery leads to atelectasis and impairment of oxygenation. Positive end-expiratory pressure (PEEP) is supposed to counteract atelectasis. We hypothesized that the derecruiting effects of pneumoperitoneum would deteriorate the intratidal compliance profile in patients undergoing laparoscopic surgery.

METHODS

In 30 adult patients scheduled for surgery with pneumoperitoneum, respiratory variables were measured during mechanical ventilation. We calculated the dynamic compliance of the respiratory system (C _RS) and the intratidal volume-dependent C _RS curve using the gliding-SLICE method. The C _RS curve was then classified in terms of indicating intratidal recruitment/derecruitment (increasing profile) and overdistension (decreasing profile). During the surgical interventions, the PEEP level was maintained nearly constant at 7 cm H₂O. Data are expressed as mean [confidence interval].

RESULTS

Baseline C _RS was 60 [54-67] mL cm H₂O^-1. Application of pneumoperitoneum decreased C _RS to 40 [37-43] mL cm H₂O^-1 which partially recovered to 54 [50-59] mL cm H₂O^-1 (P < 0.001) after removal but remained below the value measured before pneumoperitoneum (P < 0.001). Baseline compliance profiles indicated intratidal recruitment/derecruitment in 48 % patients. After induction of pneumoperitoneum, intratidal recruitment/derecruitment was indicated in 93 % patients (P < 0.01), and after removal intratidal recruitment/derecruitment was indicated in 59 % patients. Compliance profiles showing overdistension were not observed.

CONCLUSIONS

Analyses of the intratidal compliance profiles reveal that pneumoperitoneum during laparoscopic surgery causes intratidal recruitment/derecruitment which partly persists after its removal. The analysis of the intratidal volume-dependent C _RS profiles could be used to guide intraoperative PEEP adjustments during elevated intraabdominal pressure.

How much esophageal pressure-guided end-expiratory transpulmonary pressure is sufficient to maintain lung recruitment in lavage-induced lung injury?

BACKGROUND

Because of limitations of the esophageal balloon technique, the value of using esophageal pressure (Pes)-guided end-expiratory transpulmonary pressure (PL-exp) to maintain lung recruitment in adult respiratory distress syndrome is controversial. This study aimed to investigate whether tailoring PL-exp to greater than 0 was enough to maintain lung recruitment.

METHODS

Ten pigs with severe lavage-induced lung injury were mechanically ventilated in a decremental positive end-expiratory pressure (PEEP) trial that was reduced from 20 to 6 cm H2O after full-lung recruitment. Respiratory mechanics, blood gases, hemodynamic data, and whole-lung computed tomography scans were recorded at each PEEP level. Open-lung PEEP (OL-PEEP) was determined by computed tomography, while Pes-guided PEEP (Pes-PEEP) was to maintain PL-exp greater than 0.

RESULTS

OL-PEEP was higher than Pes-PEEP, which induced a higher PL-exp at OL-PEEP than at Pes-PEEP (4.6 [1.6] cm H2O vs. 1.2 [0.6] cm H2O, p < 0.001). Compared with OL-PEEP, the nonaerated lung region was significantly increased at Pes-PEEP. Superimposed pressure (SP) of the lung tissue between the esophageal plane and the dorsal level was higher at Pes-PEEP than at OL-PEEP, whereas PL-exp at the dorsal level was lower at Pes-PEEP than at OL-PEEP (-1.5 [0.7] cm H2O vs. 2.5 [1.5] cm H2O, p < 0.001). The SP correlated with PL-exp at the dorsal level and the nonaerated lung region.

CONCLUSION

In this surfactant-depleted model, maintaining PL-exp just greater than 0 using Pes was unable to maintain lung recruitment; this was partly caused by a lack of compensation for the increased SP between the esophageal plane and the dorsal level.