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

The ventilator of the future: key principles and unmet needs

Persistent shortcomings of invasive positive pressure ventilation make it less than an ideal intervention. Over the course of more than seven decades, clinical experience and scientific investigation have helped define its range of hazards and limitations. Apart from compromised airway clearance and lower airway contamination imposed by endotracheal intubation, the primary hazards inherent to positive pressure ventilation may be considered in three broad categories: hemodynamic impairment, potential for ventilation-induced lung injury, and impairment of the respiratory muscle pump. To optimize care delivery, it is crucial for monitoring and machine outputs to integrate information with the potential to impact the underlying requirements of the patient and/or responses of the cardiopulmonary system to ventilatory interventions. Trending analysis, timely interventions, and closer communication with the caregiver would limit adverse clinical trajectories. Judging from the rapid progress of recent years, we are encouraged to think that insights from physiologic research and emerging technological capability may eventually address important aspects of current deficiencies.

Practical Aspects of Esophageal Pressure Monitoring in Patients with Acute Respiratory Distress Syndrome

Esophageal pressure (P_es) monitoring is a minimally invasive advanced respiratory monitoring method with the potential to guide ventilation support management. P_es monitoring enables the separation of lung and chest wall mechanics and estimation of transpulmonary pressure, which is recognized as an important risk factor for lung injury during both spontaneous breathing and mechanical ventilation. Appropriate balloon positioning, calibration, and measurement techniques are important to avoid inaccurate results. Both the approach of using absolute expiratory Pes values and the approach based on tidal Pes difference have shown promising results for ventilation adjustments, with the potential to decrease the risk of ventilator-induced lung injury.

Respiratory Fluctuation of Peak Systolic Velocities in the Carotid Doppler Waveforms of Patients With Obstructive Airway Disease: Evidence of Pulsus Paradoxus With Experimental Validation

OBJECTIVES

To characterize fluctuations in peak systolic velocities (PSVs) in Doppler waveforms of the carotid artery in patients with and without obstructive airway disease and in volunteers subjected to incremental levels of airway resistance in an experimental model.

METHODS

The PSV variation in common carotid waveforms was measured in 100 patients who had had a carotid ultrasound examination and no respiratory or carotid disease. This was compared to that of patients who had this study during an admission for acute exacerbation of chronic obstructive pulmonary disease (COPD). The PSV variation was correlated with pulmonary function testing. In addition, 14 healthy volunteers were asked to breathe through 5 resistors. Simultaneous recordings were made of Doppler waveforms in the common carotid artery, cardiac activity, and respiration. Peak systolic velocity changes from inspiration to expiration were calculated.

RESULTS

Of the 100 patients without respiratory disease, the magnitude of the PSV variation averaged 6.3 cm/s. Of the 33 patients with COPD, the PSV variation averaged 16.5 cm/s. Nineteen of the 33 patients with COPD had concurrent pulmonary function testing; there was a statistically significant correlation between the PSV variation and forced vital capacity and forced expiratory volume indices. For the volunteers, mean velocity changes were 7.1, 6.6, 8.3, 15.1, and 16.1 cm/s for 0.00-, 2.15-, 3.27-, 3.58-, and 5.77-cm H₂ O/L/s levels of breathing resistance, respectively. There was a statistically significant relationship between an increasing airway load and the decline in PSV during inspiration (P = .02).

CONCLUSIONS

The PSV variation is greater in patients with increased airway resistance. Similar changes are evident in volunteers breathing into resistors. These findings likely reflect pulsus paradoxus.

Volume Capnography in the Intensive Care Unit: Potential Clinical Applications

Volume capnography provides a noninvasive, continuous display of the fractional concentration or partial pressure of carbon dioxide (Pco₂) versus exhaled volume. Derived measurements and calculations are influenced by changes in both ventilation and perfusion and are therefore useful for assessing both respiratory and cardiovascular function. This article provides an evidence-based review of several potential uses of volume capnography in the intensive care unit: 1) monitoring the effectiveness of ventilation by using end-tidal Pco₂ as a surrogate for arterial Pco₂, 2) assessing volume responsiveness, 3) measuring cardiac output, 4) determining prognosis in patients with the acute respiratory distress syndrome, 5) optimizing alveolar recruitment, and 6) excluding pulmonary embolism. Studies performed during the past few decades have clearly shown that volume capnography can provide important prognostic information in patients with acute respiratory distress syndrome and that end-tidal Pco₂ should not be used to estimate or even to monitor the direction of change in the arterial Pco₂ in mechanically ventilated intensive care unit patients. Unfortunately, few conclusions can be made from studies evaluating other potential applications. Of these, the most promising are the noninvasive measurement of cardiac output and optimization of alveolar recruitment in patients with acute respiratory distress syndrome and in mechanically ventilated, morbidly obese patients.

Effect of Deep Sedation on Mechanical Power in Moderate to Severe Acute Respiratory Distress Syndrome: A Prospective Self-Control Study

Mechanical power (MP) is a parameter for assessing ventilator-induced lung injury (VILI) in patients with acute respiratory distress syndrome (ARDS). Deep sedation inhibits the respiratory center and reduces the excessive spontaneous breathing in ARDS patients, thereby reducing transpulmonary pressure (Ptp) and lung injury. However, the effect of sedation on MP in ARDS patients is not yet clear. Therefore, the purpose of this study was to investigate the effect of deep sedation on MP in ARDS patients. Patients with moderate to severe ARDS who required mechanical ventilation were considered. Different degrees of sedation were performed on patients in three stages after 24 hours of mechanical ventilation. The three stages are as follows: stage 1 (H+3): 0 to 3 hours of sedation; patients' Ramsay score was 2-3 to obtain mild sedation; stage 2 (H+6): 4 to 6 hours of sedation; the sedation depth was adjusted to 5-6 points; and stage 3 (H+9): 7 to 9 hours of sedation; the sedation depth was adjusted to 2-3 points. Under deep sedation (H+6), MP, respiratory rate (RR), and Ptp were significantly lower than the ones in the patients under mild sedation (H+3) (all P < 0.01) although PaO₂/FiO₂ (P/F) and static lung compliance (Cst) were significantly higher (both P < 0.01). However, no significant difference in the above parameters was observed between H+3 and H+9. Correlation analysis showed that ΔMP was significantly and positively correlated with ΔRR and ΔPtp (both P < 0.001), while no correlation was observed neither between ΔMP and ΔCst nor between ΔMP and ΔP/F. The 28-day Kaplan-Meier survival curve showed the occurrence of 19 deaths, and the overall survival rate was 63.46%. The survival rate was 53.12% in the high-MP (HMP) group and 80.95 in the low-MP (LMP) group (P < 0.05). In conclusion, deep sedation significantly reduced MP in patients with moderate to severe ARDS, thereby reducing the occurrence of VILI. In addition, MP monitoring in deep sedation predicted the 28-day survival of patients with moderate to severe ARDS.

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.

Extracorporeal Membrane Oxygenation Can Successfully Support Patients With Severe Acute Respiratory Distress Syndrome in Lieu of Mechanical Ventilation

OBJECTIVES

Extracorporeal membrane oxygenation is increasingly used in the management of severe acute respiratory distress syndrome. With extracorporeal membrane oxygenation, select patients with acute respiratory distress syndrome can be managed without mechanical ventilation, sedation, or neuromuscular blockade. Published experience with this approach, specifically with attention to a patient's respiratory drive following cannulation, is limited.

DESIGN

We describe our experience with three consecutive patients with severe acute respiratory distress syndrome supported with right jugular-femoral configuration of venovenous extracorporeal membrane oxygenation without therapeutic anticoagulation as an alternative to lung-protective mechanical ventilation. Outcomes are reported including daily respiratory rate, vital capacities, and follow-up pulmonary function testing.

RESULTS

Following cannulation, patients were extubated within 24 hours. During extracorporeal membrane oxygenation support, all patients were able to maintain a normal respiratory rate and experienced steady improvements in vital capacities. Patients received oral nutrition and ambulated daily. At follow-up, no patients required supplemental oxygen.

CONCLUSIONS

Our results suggest that venovenous extracorporeal membrane oxygenation can provide a safe and effective alternative to lung-protective mechanical ventilation in carefully selected patients. This approach facilitates participation in physical therapy and avoids complications associated with mechanical ventilation.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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

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

Respiratory mechanics to understand ARDS and guide mechanical ventilation

OBJECTIVE

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

SIGNIFICANCE

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

Measurements Obtained From Esophageal Balloon Catheters Are Affected by the Esophageal Balloon Filling Volume in Children With ARDS

BACKGROUND

Esophageal balloon inflation volume may affect the accuracy of transpulmo-nary pressure estimates in adults, but the effect is unknown in pediatrics. Using a combination bench and human study, we sought to determine a range of optimal filling volumes for esophageal balloon catheters and to derive a technique to inflate catheters to yield the most accurate estimates of pleural pressure.

METHODS

In the laboratory study, we evaluated 4 pediatric and adult esophageal balloon catheters, a liquid-filled catheter, and a micro-tip catheter, both with and without a model esophagus. We compared the measured esophageal pressure for each type of catheter within a pressurized chamber. Esophageal balloon catheters were also tested by manipulating the esophageal balloon inflation volume, and we attempted to derive a filling-volume technique that would assure accuracy. We then tested the feasibility of this technique in 5 mechanically ventilated pediatric subjects with ARDS.

RESULTS

In the laboratory study, smaller inflation volumes underestimated the chamber pressure at higher chamber pressures, and larger inflation volumes overestimated the chamber pressure at lower chamber pressures. Using an optimal filling-volume technique resulted in a mean total error that ranged from -0.53 to -0.10 cm H₂O. The optimal filling-volume values for the pediatric catheters were 0.2-0.6 mL, and 0.4-0.8 mL for the adult catheters. When correctly positioned and calibrated, the micro-tip transducer and liquid-filled catheters were within ± 1 cm H₂O of chamber pressure for all ranges of pressure. In the clinical study, high variability in measured esophageal pressure and subsequent transpulmonary pressure during exhalation and during inhalation was observed within the manufacturer's recommended esophageal balloon inflation ranges.

CONCLUSIONS

Manufacturer-recommended esophageal balloon inflation ranges do not assure accuracy. Individual titration of esophageal balloon volume may improve accuracy. Better esophageal catheters are needed to provide reliable esophageal pressure measurements in children.

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

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

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

Background

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

Methods

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

Results

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

Conclusions

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