Should mechanical ventilation be guided by esophageal pressure measurements?

The pleural gradient does not reflect the superimposed pressure in patients with class III obesity

BACKGROUND

The superimposed pressure is the primary determinant of the pleural pressure gradient. Obesity is associated with elevated end-expiratory esophageal pressure, regardless of lung disease severity, and the superimposed pressure might not be the only determinant of the pleural pressure gradient. The study aims to measure partitioned respiratory mechanics and superimposed pressure in a cohort of patients admitted to the ICU with and without class III obesity (BMI ≥ 40 kg/m2), and to quantify the amount of thoracic adipose tissue and muscle through advanced imaging techniques.

METHODS

This is a single-center observational study including ICU-admitted patients with acute respiratory failure who underwent a chest computed tomography scan within three days before/after esophageal manometry. The superimposed pressure was calculated from lung density and height of the largest axial lung slice. Automated deep-learning pipelines segmented lung parenchyma and quantified thoracic adipose tissue and skeletal muscle.

RESULTS

N = 18 participants (50% female, age 60 [30-66] years), with 9 having BMI < 30 and 9  ≥ 40 kg/m2. Groups showed no significant differences in age, sex, clinical severity scores, or mortality. Patients with BMI ≥ 40 exhibited higher esophageal pressure (15.8 ± 2.6 vs. 8.3 ± 4.9 cmH2O, p = 0.001), higher pleural pressure gradient (11.1 ± 4.5 vs. 6.3 ± 4.9 cmH2O, p = 0.04), while superimposed pressure did not differ (6.8 ± 1.1 vs. 6.5 ± 1.5 cmH2O, p = 0.59). Subcutaneous and intrathoracic adipose tissue were significantly higher in subjects with BMI ≥ 40 and correlated positively with esophageal pressure and pleural pressure gradient (p < 0.05). Muscle areas did not differ between groups.

CONCLUSIONS

In patients with class III obesity, the superimposed pressure does not approximate the pleural pressure gradient, which is higher than in patients with lower BMI. The quantity and distribution of subcutaneous and intrathoracic adiposity also contribute to increased pleural pressure gradients in individuals with BMI ≥ 40. This study introduces a novel physiological concept that provides a solid rationale for tailoring mechanical ventilation in patients with high BMI, where specific guidelines recommendations are lacking.

Association of different positive end-expiratory pressure selection strategies with all-cause mortality in adult patients with acute respiratory distress syndrome

BACKGROUND

The acute respiratory distress syndrome (ARDS) has high morbidity and mortality. Positive end-expiratory pressure (PEEP) is commonly used in patients with ARDS but the best method to select the optimal PEEP level and reduce all-cause mortality is unclear. The primary objective of this network meta-analysis is to summarize the available evidence and to compare the effect of different PEEP selection strategies on all-cause mortality in adult patients with ARDS.

METHODS

We will search MEDLINE, Cochrane Central Register of Controlled Trials, PubMed, EMBASE, and LILACS from inception onwards for randomized controlled trials assessing the effect of PEEP selection strategies in adult patients with moderate to severe ARDS. We will exclude studies that did not use a lung-protective ventilation approach as part of the comparator or intervention strategy. The primary outcome will be all-cause mortality (at the longest available follow-up and up to 90 days). Secondary outcomes will include barotrauma, ventilator-free days, intensive care unit and hospital length of stay, and changes in oxygenation. Two reviewers will independently screen all citations, full-text articles, and extract study-data. We will assess the risk of bias for each of the outcomes using version 2 of the Cochrane risk of bias tool for randomized controlled trials. If feasible, Bayesian network meta-analyses will be conducted to obtain pooled estimates of all potential head-to-head comparisons. We will report pairwise and network meta-analysis treatment effect estimates as risk ratios and risk differences, together with the associated 95% credible intervals. We will assess certainty in effect estimates using GRADE methodology.

DISCUSSION

The present study will inform clinical decision-making for adult patients with ARDS and will improve our understanding of the limitations of the available literature assessing PEEP selection strategies. Finally, this information may also inform the design of future randomized trials, including the selection of interventions, comparators, and predictive enrichment strategies.

TRIAL REGISTRATION

PROSPERO 2020 CRD42020193302 .

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.

Reliability of transpulmonary pressure-time curve profile to identify tidal recruitment/hyperinflation in experimental unilateral pleural effusion

The stress index (SI) is a parameter that characterizes the shape of the airway pressure-time profile (P/t). It indicates the slope progression of the curve, reflecting both lung and chest wall properties. The presence of pleural effusion alters the mechanical properties of the respiratory system decreasing transpulmonary pressure (Ptp). We investigated whether the SI computed using Ptp tracing would provide reliable insight into tidal recruitment/overdistention during the tidal cycle in the presence of unilateral effusion. Unilateral pleural effusion was simulated in anesthetized, mechanically ventilated pigs. Respiratory system mechanics and thoracic computed tomography (CT) were studied to assess P/t curve shape and changes in global lung aeration. SI derived from airway pressure (Paw) was compared with that calculated by Ptp under the same conditions. These results were themselves compared with quantitative CT analysis as a gold standard for tidal recruitment/hyperinflation. Despite marked changes in tidal recruitment, mean values of SI computed either from Paw or Ptp were remarkably insensitive to variations of PEEP or condition. After the instillation of effusion, SI indicates a preponderant over-distension effect, not detected by CT. After the increment in PEEP level, the extent of CT-determined tidal recruitment suggest a huge recruitment effect of PEEP as reflected by lung compliance. Both SI in this case were unaffected. We showed that the ability of SI to predict tidal recruitment and overdistension was significantly reduced in a model of altered chest wall-lung relationship, even if the parameter was computed from the Ptp curve profile.

Response characteristics of esophageal balloon catheters handmade using latex and nonlatex materials

The measurement of esophageal pressure allows for the calculation of several important and clinically useful parameters of respiratory mechanics. Esophageal pressure is often measured with balloon-tipped catheters. These catheters may be handmade from natural latex condoms and polyethylene tubing. Given the potential of natural latex to cause allergic reaction, it is important to determine whether esophageal catheter balloons can be fabricated, by hand, using nonlatex condoms as construction materials. To determine the static and dynamic response characteristics of esophageal balloon catheters handmade from latex and nonlatex materials, six esophageal catheter balloons were constructed from each of the following condom materials: natural latex, synthetic polyisoprene, and polyurethane (18 total). Static compliance and working volume range of each balloon catheter was obtained from their pressure-volume characteristics in water. The dynamic response of balloon catheters were measured via a pressure “step” test, from which a third-order underdamped transfer function was modeled. The dynamic ranges of balloon catheters were characterized by the frequencies corresponding to ±5% amplitude- and phase-distortion (f_A5% and f_φ5%). Balloon catheters handmade from polyurethane condoms displayed the smallest working volume range and lowest static balloon compliance. Despite this lower compliance, f_A5% and f_φ5% were remarkably similar between all balloon materials. Our findings suggest that polyisoprene condoms are an ideal nonlatex construction material to use when fabricating esophageal catheter balloons by hand.

Transpulmonary pressure and lung elastance can be estimated by a PEEP-step manoeuvre

BACKGROUND

Transpulmonary pressure is a key factor for protective ventilation. This requires measurements of oesophageal pressure that is rarely used clinically. A simple method may be found, if it could be shown that tidal and positive end-expiratory pressure (PEEP) inflation of the lungs with the same volume increases transpulmonary pressure equally. The aim of the present study was to compare tidal and PEEP inflation of the respiratory system.

METHODS

A total of 12 patients with acute respiratory failure were subjected to PEEP trials of 0-4-8-12-16 cmH2O. Changes in end-expiratory lung volume (ΔEELV) following a PEEP step were determined from cumulative differences in inspiratory-expiratory tidal volumes. Oesophageal pressure was measured with a balloon catheter.

RESULTS

Following a PEEP increase from 0 to 16 cmH2O end-expiratory oesophageal pressure did not increase (0.5 ± 4.0 cmH2O). Average increase in EELV following a PEEP step of 4 cmH2O was 230 ± 132 ml. The increase in EELV was related to the change in PEEP divided by lung elastance (El) derived from oesophageal pressure as ΔPEEP/El. There was a good correlation between transpulmonary pressure by oesophageal pressure and transpulmonary pressure based on El determined as ΔPEEP/ΔEELV, r(2)  = 0.80, y = 0.96x, mean bias -0.4 ± 3.0 cmH2 O with limits of agreement from 5.4 to -6.2 cmH2O (2 standard deviations).

CONCLUSION

PEEP inflation of the respiratory system is extremely slow, and allows the chest wall complex, especially the abdomen, to yield and adapt to intrusion of the diaphragm. As a consequence a change in transpulmonary pressure is equal to the change in PEEP and transpulmonary pressure can be determined without oesophageal pressure measurements.

Advances in ventilator-associated lung injury: prevention is the target

Mechanical ventilation (MV) is the main supportive treatment in respiratory failure due to different etiologies. However, MV might aggravate ventilator-associated lung injury (VALI). Four main mechanisms leading to VALI are: 1) increased stress and strain, induced by high tidal volume (VT); 2) increased shear stress, i.e. opening and closing, of previously atelectatic alveolar units; 3) distribution of perfusion and 4) biotrauma. In severe acute respiratory distress syndrome patients, low VT, higher levels of positive end expiratory pressure, long duration prone position and neuromuscular blockade within the first 48 hours are associated to a better outcome. VALI can also occur by using high VT in previously non injured lungs. We believe that prevention is the target to minimize injurious effects of MV. This review aims to describe pathophysiology of VALI, the possible prevention and treatment as well as monitoring MV to minimize VALI.

Lung elastance and transpulmonary pressure can be determined without using oesophageal pressure measurements

INTRODUCTION

The aim of the present study was to demonstrate that lung elastance and transpulmonary pressure can be determined without using oesophageal pressure measurements.

METHODS

Studies were performed on 13 anesthetized and sacrificed ex vivo pigs. Tracheal and oesophageal pressures were measured and changes in end-expiratory lung volume (ΔEELV) determined by spirometry as the cumulative inspiratory-expiratory tidal volume difference. Studies were performed with different end-expiratory pressure steps [change in end-expiratory airway pressure (ΔPEEP)], body positions and with abdominal load.

RESULTS

A PEEP increase results in a multi-breath build-up of end-expiratory lung volume. End-expiratory oesophageal pressure did not increase further after the first expiration, constituting half of the change in ΔEELV following a PEEP increase, even though end-expiratory volume continued to increase. This resulted in a successive left shift of the chest wall pressure-volume curve. Even at a PEEP of 12 cmH(2) O did the end-expiratory oesophageal (pleural) pressure remain negative.

CONCLUSIONS

A PEEP increase resulted in a less than expected increase in end-expiratory oesophageal pressure, indicating that the chest wall and abdomen gradually can accommodate changes in lung volume. The rib cage end-expiratory spring-out force stretches the diaphragm and prevents the lung from being compressed by abdominal pressure. The increase in transpulmonary pressure following a PEEP increase was closely related to the increase in PEEP, indicating that lung compliance can be calculated from the ratio of the change in end-expiratory lung volume and the change in PEEP, ΔEELV/ΔPEEP.