Expiratory Flow Limitation During Mechanical Ventilation

Ventilatory response of peripheral chemoreceptors to hypercapnia during exercise above the respiratory compensation point

Peripheral hypercapnic chemosensitivity (PHC) is assessed as the change in ventilation in response to a rapid change in carbon dioxide pressures (Pco2). The increase in chemoresponse from rest to subrespiratory compensation point (RCP) exercise intensities is well-defined but less clear at intensities above the RCP when changes in known ventilatory stimulants occur. Twenty healthy subjects (n = 10 females) completed a maximal exercise test on 1 day, and on a subsequent day, transient hypercapnia was used to test PHC at multiple exercise stages. The transient hypercapnia involved two breaths of 10% CO2 repeated five times during each of the following: sitting at rest on the cycle ergometer, cycling at 40% wmax, cycling at 85% Wmax, at rest on the cycle ergometer immediately following the 85% stage, and cycling at 40% Wmax again following the postexercise rest. The PHC was not different across exercise intensities (0.98 ± 0.37 vs. 0.91 ± 0.39 vs. 0.92 ± 0.42 L·min-1·mmHg-1 for first 40% wmax, 85% wmax and second 40% Wmax, respectively (P = 0.45). There were no differences in PHC between presupra-RCP exercise rest and postsupra-RCP exercise rest (0.52 ± 0.23 vs. 0.53 ± 0.24 L·min-1·mmHg-1, P = 0.8003). Using a repeated-measures correlation to account for within-participant changes, there was a significant relationship between the end-tidal Pco2 and PHC for the 85% intensity (r = 0.5, P < 0.0001) when end-tidal Pco2 was dynamic between the trials. We conclude that the physiological changes (e.g., metabolic milieu and temperature) produced with supra-RCP exercise do not further augment PHC, and that the prestimulus end-tidal Pco2 modulates the PHC.NEW & NOTEWORTHY Exercise at intensities above the respiratory compensation point did not further augment peripheral hypercapnic chemosensitivity (PHC). Moreover, the PHC was not different during a preexercise resting state compared with rest immediately after intense exercise. The lack of differences across both comparisons suggests that exercise itself appears to sensitize the PHC.

Early and late effects of volatile sedation with sevoflurane on respiratory mechanics of critically ill COPD patients

BACKGROUND

The objective was to compare sevoflurane, a volatile sedation agent with potential bronchodilatory properties, with propofol on respiratory mechanics in critically ill patients with COPD exacerbation.

METHODS

Prospective study in an ICU enrolling critically ill intubated patients with severe COPD exacerbation and comparing propofol and sevoflurane after 1:1 randomisation. Respiratory system mechanics (airway resistance, PEEPi, trapped volume, ventilatory ratio and respiratory system compliance), gas exchange, vitals, safety and outcome were measured at inclusion and then until H48. Total airway resistance change from baseline to H48 in both sevoflurane and propofol groups was the main endpoint.

RESULTS

Sixteen patients were enrolled and were sedated for 126 h(61-228) in the propofol group and 207 h(171-216) in the sevoflurane group. At baseline, airway resistance was 21.6cmH2O/l/s(19.8-21.6) in the propofol group and 20.4cmH2O/l/s(18.6-26.4) in the sevoflurane group, (p = 0.73); trapped volume was 260 ml(176-290) in the propofol group and 73 ml(35-126) in the sevoflurane group, p = 0.02. Intrinsic PEEP was 1.5cmH2O(1-3) in both groups after external PEEP optimization. There was neither early (H4) or late (H48) significant difference in airway resistance and respiratory mechanics parameters between the two groups.

CONCLUSIONS

In critically ill patients intubated with COPD exacerbation, there was no significant difference in respiratory mechanics between sevoflurane and propofol from inclusion to H4 and H48.

Expiratory flow limitation during mechanical ventilation: real-time detection and physiological subtypes

BACKGROUND

Tidal expiratory flow limitation (EFLT) complicates the delivery of mechanical ventilation but is only diagnosed by performing specific manoeuvres. Instantaneous analysis of expiratory resistance (Rex) can be an alternative way to detect EFLT without changing ventilatory settings. This study aimed to determine the agreement of EFLT detection by Rex analysis and the PEEP reduction manoeuvre using contingency table and agreement coefficient. The patterns of Rex were explored.

METHODS

Medical patients ≥ 15-year-old receiving mechanical ventilation underwent a PEEP reduction manoeuvre from 5 cmH2O to zero for EFLT detection. Waveforms were recorded and analyzed off-line. The instantaneous Rex was calculated and was plotted against the volume axis, overlapped by the flow-volume loop for inspection. Lung mechanics, characteristics of the patients, and clinical outcomes were collected. The result of the Rex method was validated using a separate independent dataset.

RESULTS

339 patients initially enrolled and underwent a PEEP reduction. The prevalence of EFLT was 16.5%. EFLT patients had higher adjusted hospital mortality than non-EFLT cases. The Rex method showed 20% prevalence of EFLT and the result was 90.3% in agreement with PEEP reduction manoeuvre. In the validation dataset, the Rex method had resulted in 91.4% agreement. Three patterns of Rex were identified: no EFLT, early EFLT, associated with airway disease, and late EFLT, associated with non-airway diseases, including obesity. In early EFLT, external PEEP was less likely to eliminate EFLT.

CONCLUSIONS

The Rex method shows an excellent agreement with the PEEP reduction manoeuvre and allows real-time detection of EFLT. Two subtypes of EFLT are identified by Rex analysis.

TRIAL REGISTRATION

Clinical trial registered with www.thaiclinicaltrials.org (TCTR20190318003). The registration date was on 18 March 2019, and the first subject enrollment was performed on 26 March 2019.

Quantifiable identification of flow-limited ventilator dyssynchrony with the deformed lung ventilator model

BACKGROUND

Ventilator dyssynchrony (VD) can worsen lung injury and is challenging to detect and quantify due to the complex variability in the dyssynchronous breaths. While machine learning (ML) approaches are useful for automating VD detection from the ventilator waveform data, scalable severity quantification and its association with pathogenesis and ventilator mechanics remain challenging.

OBJECTIVE

We develop a systematic framework to quantify pathophysiological features observed in ventilator waveform signals such that they can be used to create feature-based severity stratification of VD breaths.

METHODS

A mathematical model was developed to represent the pressure and volume waveforms of individual breaths in a feature-based parametric form. Model estimates of respiratory effort strength were used to assess the severity of flow-limited (FL)-VD breaths compared to normal breaths. A total of 93,007 breath waveforms from 13 patients were analyzed.

RESULTS

A novel model-defined continuous severity marker was developed and used to estimate breath phenotypes of FL-VD breaths. The phenotypes had a predictive accuracy of over 97% with respect to the previously developed ML-VD identification algorithm. To understand the incidence of FL-VD breaths and their association with the patient state, these phenotypes were further successfully correlated with ventilator-measured parameters and electronic health records.

CONCLUSION

This work provides a computational pipeline to identify and quantify the severity of FL-VD breaths and paves the way for a large-scale study of VD causes and effects. This approach has direct application to clinical practice and in meaningful knowledge extraction from the ventilator waveform data.

Effect of expiratory flow limitation on ventilation/perfusion mismatch and perioperative lung function during pneumoperitoneum and Trendelenburg position

BACKGROUND

Laparoscopic surgery and Trendelenburg position may affect the respiratory function and alter the gas exchange. Further the reduction of the lung volumes may contribute to the development of expiratory flow limitation (EFL). The latter is associated with an increased risk of postoperative pulmonary complications. Our aim was to investigate the incidence of EFL and to evaluate its effect on pulmonary function and intraoperative V/Q mismatch.

METHODS

This is a prospective study on patients undergoing elective laparoscopic gynecological surgery. We evaluated respiratory mechanics, V/Q mismatch and presence of EFL after anesthesia induction, during pneumoperitoneum and Trendelenburg position and at the end of surgery. Intraoperative gas exchange and hemodynamic were also recorded. Clinical data were collected until seven days after surgery to evaluate the onset of pulmonary postoperative complications (PPCs).

RESULTS

Among the 66 patients enrolled, 25/66 (38%) exhibited EFL during surgery, of whom 10/66 (15%) after anesthesia induction, and the remaining 15 patients after pneumoperitoneum and Trendelenburg position. Median PEEP able to reverse flow limitation was 7 [7-10] cmH2O after anesthesia induction and 9 [8-15] cmH2O after pneumoperitoneum and Trendelenburg position. Patients with EFL had significantly higher shunt (17 [2-25] vs. 9 [1-19]; P=0.05), low V̇/Q̇ (27 [20-70] vs. 15 [10-22]; P=0.05) and high V̇/Q̇ (10 [7-14] vs. 6 [4-7]; P=0.024). At the end of surgery, only high V/Q was significantly higher in EFL patients. Further, they exhibited higher incidence of postoperative pulmonary complication (48% (12/25) vs. 15% (6/41), P=0.005), hypoxemia and hypercapnia (80% [20/25] vs. 32% [13/41]; P<0.001).

CONCLUSIONS

Expiratory flow limitation is a common phenomenon during gynecological laparoscopic surgery associated with worsen gas exchange, increased V/Q mismatch and altered lung mechanics. Our study showed that patients experiencing EFL during surgery showed a higher risk for PPCs.

Cardiopulmonary interactions in left heart failure

The primary impact of ventilation and ventilatory efforts on left ventricular (LV) function in left ventricular dysfunction relate to how changes in intrathoracic pressure (ITP) alter the pressure gradients for venous return into the chest and LV ejection out of the chest. Spontaneous inspiratory efforts by decreasing ITP increase both of these pressure gradients increasing venous blood flow and impeding LV ejection resulting in increased intrathoracic blood volume. In severe heart failure states when lung compliance is reduced, or airway resistance is increased these negative swings in ITP can be exacerbated leading to LV failure and acute cardiogenic pulmonary edema. By merely reversing these negative swings in ITP by the use of non-invasive continuous positive airway pressure (CPAP), these profoundly detrimental forces can be immediately reversed, and cardiovascular stability can be restored in moments. This forms the clinical rationale for the immediate use of CPAP for the treatment of acute cardiogenic pulmonary edema. Increasing ITP during positive pressure ventilation decreases the pressure gradients for venous return and LV ejection decreasing intrathoracic blood volume. In a hypovolemic patient even with LV dysfunction this can result in hypotension due to inadequate LV preload. Minor increases in ITP as occur using pressure-limited positive-pressure ventilation primarily reverse the increased LV afterload of negative swings in ITP and if fluid overload was already present, minimally alter cardiac output. The effect of changes in lung volume on LV function are related primarily to its effects on right ventricular (RV) function through changes in pulmonary vascular resistance and overdistention (hyperinflation). In acute lung injury with alveolar collapse, positive pressure ventilation may reduce pulmonary vascular resistance if alveolar recruitment predominates. Hyperinflation, however, impedes diastolic filling while simultaneously increasing pulmonary vascular resistance. Thus, increasing lung volume can reduce RV afterload by reversing hypoxic pulmonary vasoconstriction or increase afterload by overdistention. Hyperinflation can also impede RV filling. All of these processes can be readily identified at the bedside using echocardiography.

Effect of the change of mechanical ventilation mode on cerebral oxygen saturation level in neonates

BACKGROUND

This study aimed to apply near-infrared spectroscopy (NIRS) to monitor cerebral oxygen saturation (SrO2) level in neonates before and after the change of mechanical ventilation mode, and thus, the effects of the change of mechanical ventilator mode on SrO2 level in neonates were assessed.

METHODS

This trial was designed as an observational study .A total of 70 neonates who were admitted to the Department of Neonatology of Beijing Luhe Hospital Affiliated to Capital Medical University (Beijing, China) between September 2019 and October 2021 and required respiratory support were included. The variations of SrO2 level before and after the change of mechanical ventilation mode, including changing from Synchronized intermittent mandatory ventilation (SIMV) to noninvasive ventilation (NIV, group 1), and from NIV to oxygen inhalation (group 2), were monitored by Enginmed EGOS-600 A. The changes of SrO2 level at 30 min before and 1 h after the change of ventilation mode were compared between the two groups.

RESULTS

The SrO2 level in the group 1 30 min before, as well as 10 min, 30 min, and 1 h after the change of ventilation mode was 62.54 ± 3.36%, 65.43 ± 3.98%, 64.38 ± 4.23%, and 64.63 ± 3.71%, respectively. The SrO2 level at all the points after the change of ventilation mode increased compared with 30 min before the change (P < 0.05). The SrO2 level in the group 2 at each time point was 62.67 ± 4.69%, 64.61 ± 5.00%, 64.04 ± 4.48%, and 64.55 ± 4.32%, respectively. Compared with 30 min before ventilator weaning, the SrO2 level at all the points after ventilator weaning increased (P < 0.05). Peak inspiratory pressure (PIP) excluding Nasal Continuous Positive Airway Pressure (NCPAP)) in group 1 was lower than that before extubation, and the difference was statistically significant (P = 0) (Table 7).

CONCLUSIONS

SrO2 level showed an increasing trend after the change of ventilation mode, and the increase of SrO2 level at 10 min after the change of ventilation mode was the most prominent. From SIMV to NIV, increased SrO2 levels may be associated with decreased PIP.

Ventilators, Settings, Autotitration Algorithms

The choice of a ventilator model for a single patient is usually based on parameters such as size (portability), presence or absence of battery and ventilatory modes. However, there are many details within each ventilator model about triggering, pressurisation or autotitration algorithms that may go unnoticed, but may be important or may justify some drawbacks that may occur during their use in individual patients. This review is intended to emphasize these differences. Guidance is also provided on the operation of autotitration algorithms, in which the ventilator is able to take decisions based on a measured or estimated parameter. It is important to know how they work and their potential sources of error. Current evidence on their use is also provided.

Noninvasive positive pressure in acute exacerbations of chronic obstructive pulmonary disease

PURPOSE OF REVIEW

Noninvasive positive pressure ventilation (NIV) is standard of care for patients with acute exacerbations of chronic obstructive pulmonary disease (AECOPD). We review the most current evidence and highlight areas of uncertainty and ongoing research. We highlight key concepts for the clinician caring for patients with AECOPD which require NIV.

RECENT FINDINGS

Implementation of NIV in AECOPD is not uniform in spite of the evidence and guidelines. Initiation of NIV should be done early and following protocols. Low-intensity NIV remains the standard of care, although research and guidelines are evaluating higher intensity NIV. Scores to predict NIV failure continue to be refined to allow early identification and interventions. Several areas of uncertainty remain, among them are interventions to improve tolerance, length of support and titration and nutritional support during NIV.

SUMMARY

The use of NIV in AECOPD is the standard of care as it has demonstrated benefits in several patient-centered outcomes. Current developments and research is related to the implementation and adjustment of NIV.

An identifiable model of lung mechanics to diagnose and monitor COPD

BACKGROUND

Current methods to diagnose and monitor COPD employ spirometry as the gold standard to identify lung function reduction with reduced forced expiratory volume (FEV₁)/vital capacity (VC) ratio. Current methods utilise linear assumptions regarding airway resistance, where nonlinear resistance modelling may provide rapid insight into patient specific condition and disease progression. This study examines model-based expiratory resistance in healthy lungs and those with progressively more severe COPD.

METHODS

Healthy and COPD pressure (P)[cmH₂O] and flow (Q)[L/s] data is obtained from the literature, and 5 intermediate levels of COPD and responses are created to simulate COPD progression and assess model-based metric resolution. Linear and nonlinear single compartment models are used to identify changes in inspiratory (R_1,insp) and linear (R_1,exp)/nonlinear (R₂Φ) expiratory resistance with disease severity and over the course of expiration.

RESULTS

R_1,insp increases from 2.1 to 7.3 cmH₂O/L/s, R_1,exp increases from 2.4 to 10.0 cmH₂O/L/s with COPD severity. Nonlinear R₂Φ increases (mean R₂Φ: 2.5 cmH₂O/L/s (healthy) to 24.4 cmH₂O/L/s (COPD)), with increasing end-expiratory nonlinearity as COPD severity increases.

CONCLUSION

Expiratory resistance is increasingly highly nonlinear with COPD severity. These results show a simple, nonlinear model can capture fundamental COPD dynamics and progression from regular breathing data, and such an approach may be useful for patient-specific diagnosis and monitoring.

Expiratory Muscles of Respiration and Weaning Failure: What do We Know So Far?

How to cite this article: Majeed NA, Nasa P. Expiratory Muscles of Respiration and Weaning Failure: What do We Know So Far? Indian J Crit Care Med 2023;27(1):1-3.

Monitoring Lung Injury Severity and Ventilation Intensity during Mechanical Ventilation

Acute respiratory distress syndrome (ARDS) is a severe form of respiratory failure burden by high hospital mortality. No specific pharmacologic treatment is currently available and its ventilatory management is a key strategy to allow reparative and regenerative lung tissue processes. Unfortunately, a poor management of mechanical ventilation can induce ventilation induced lung injury (VILI) caused by physical and biological forces which are at play. Different parameters have been described over the years to assess lung injury severity and facilitate optimization of mechanical ventilation. Indices of lung injury severity include variables related to gas exchange abnormalities, ventilatory setting and respiratory mechanics, ventilation intensity, and the presence of lung hyperinflation versus derecruitment. Recently, specific indexes have been proposed to quantify the stress and the strain released over time using more comprehensive algorithms of calculation such as the mechanical power, and the interaction between driving pressure (DP) and respiratory rate (RR) in the novel DP multiplied by four plus RR [(4 × DP) + RR] index. These new parameters introduce the concept of ventilation intensity as contributing factor of VILI. Ventilation intensity should be taken into account to optimize protective mechanical ventilation strategies, with the aim to reduce intensity to the lowest level required to maintain gas exchange to reduce the potential for VILI. This is further gaining relevance in the current era of phenotyping and enrichment strategies in ARDS.

Stepwise Ventilator Waveform Assessment to Diagnose Pulmonary Pathophysiology

Clinicians can use mechanical waveform analysis as a diagnostic tool to identify pulmonary pathophysiology. This review offers an approach to develop a hypothesis of a patient’s lung pathophysiology.

Overnight variation in tidal expiratory flow limitation in COPD patients and its correction: an observational study

Background

Tidal expiratory flow limitation (EFL_T) is common among COPD patients. Whether EFL_T changes during sleep and can be abolished during home ventilation is not known.

Methods

COPD patients considered for noninvasive ventilation used a ventilator which measured within-breath reactance change at 5 Hz (∆Xrs) and adjusted EPAP settings to abolish EFL_T. Participants flow limited (∆Xrs > 2.8) when supine underwent polysomnography (PSG) and were offered home ventilation for 2 weeks. The EPAP pressure that abolished EFL_T was measured and compared to that during supine wakefulness. Ventilator adherence and subjective patient perceptions were obtained after home use.

Results

Of 26 patients with supine EFL_T, 15 completed overnight PSG and 10 the home study. In single night and 2-week home studies, EFL_T within and between participants was highly variable. This was unrelated to sleep stage or body position with only 14.6% of sleep time spent within 1 cmH₂O of the awake screening pressure. Over 2 weeks, mean EPAP was almost half the mean maximum EPAP (11.7 vs 6.4 cmH₂O respectively). Group mean ∆Xrs was ≤ 2.8 for 77.3% of their home use with a mean time to abolish new EFL_T of 5.91 min. Adherence to the ventilator varied between 71 and 100% in prior NIV users and 36–100% for naïve users with most users rating therapy as comfortable.

Conclusions

Tidal expiratory flow limitation varies significant during sleep in COPD patients. This can be controlled by auto-titrating the amount of EPAP delivered. This approach appears to be practical and well tolerated by patients.

Trial registration: The trial was retrospectively registered at CT.gov NCT04725500.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12931-021-01913-7.

Three bedside techniques to quantify dynamic pulmonary hyperinflation in mechanically ventilated patients with chronic obstructive pulmonary disease

BACKGROUND

Dynamic pulmonary hyperinflation may develop in patients with chronic obstructive pulmonary disease (COPD) due to dynamic airway collapse and/or increased airway resistance, increasing the risk of volutrauma and hemodynamic compromise. The reference standard to quantify dynamic pulmonary hyperinflation is the measurement of the volume at end-inspiration (Vei). As this is cumbersome, the aim of this study was to evaluate if methods that are easier to perform at the bedside can accurately reflect Vei.

METHODS

Vei was assessed in COPD patients under controlled protective mechanical ventilation (7 ± mL/kg) on zero end-expiratory pressure, using three techniques in a fixed order: (1) reference standard (Vei_reference): passive exhalation to atmosphere from end-inspiration in a calibrated glass burette; (2) ventilator maneuver (Vei_maneuver): measuring the expired volume during a passive exhalation of 45s using the ventilator flow sensor; (3) formula (Vei_formula): (Vt × P_plateau)/(P_plateau - PEEP_i), with Vt tidal volume, P_plateau is plateau pressure after an end-inspiratory occlusion, and PEEP_i is intrinsic positive end-expiratory pressure after an end-expiratory occlusion. A convenience sample of 17 patients was recruited.

RESULTS

Vei_reference was 1030 ± 380 mL and had no significant correlation with P_plateau (r² = 0.06; P = 0.3710) or PEEP_i (r² = 0.11; P = 0.2156), and was inversely related with P_drive (calculated as P_plateau -PEEP_i) (r² = 0.49; P = 0.0024). A low bias but rather wide limits of agreement and fairly good correlations were found when comparing Vei_maneuver and Vei_formula to Vei_reference. Vei remained stable during the study period (low bias 15 mL with high agreement (95% limits of agreement from - 100 to 130 mL) and high correlation (r² = 0.98; P < 0.0001) between both measurements of Vei_reference).

CONCLUSIONS

In patients with COPD, airway pressures are not a valid representation of Vei. The three techniques to quantify Vei show low bias, but wide limits of agreement.

Associations Between Expiratory Flow Limitation and Postoperative Pulmonary Complications in Patients Undergoing Cardiac Surgery

OBJECTIVES

To determine whether driving pressure and expiratory flow limitation are associated with the development of postoperative pulmonary complications (PPCs) in cardiac surgery patients.

DESIGN

Prospective cohort study.

SETTING

University Hospital San Raffaele, Milan, Italy.

PARTICIPANTS

Patients undergoing elective cardiac surgery.

MEASUREMENTS AND MAIN RESULTS

The primary endpoint was the occurrence of a predefined composite of PPCs. The authors determined the association among PPCs and intraoperative ventilation parameters, mechanical power and energy load, and occurrence of expiratory flow limitation (EFL) assessed with the positive end-expiratory pressure test. Two hundred patients were enrolled, of whom 78 (39%) developed one or more PPCs. Patients with PPCs, compared with those without PPCs, had similar driving pressure (mean difference [MD] -0.1 [95% confidence interval (CI), -1.0 to 0.7] cmH₂O, p = 0.561), mechanical power (MD 0.5 [95% CI, -0.3 to 1.1] J/m, p = 0.364), and total energy load (MD 95 [95% CI, -78 to 263] J, p = 0.293), but they had a higher incidence of EFL (51% v 38%, p = 0.005). Only EFL was associated independently with the development of PPCs (odds ratio 2.46 [95% CI, 1.28-4.80], p = 0.007).

CONCLUSIONS

PPCs occurred frequently in this patient population undergoing cardiac surgery. PPCs were associated independently with the presence of EFL but not with driving pressure, total energy load, or mechanical power.

Management of Intraoperative Mechanical Ventilation to Prevent Postoperative Complications after General Anesthesia: A Narrative Review

Mechanical ventilation (MV) is still necessary in many surgical procedures; nonetheless, intraoperative MV is not free from harmful effects. Protective ventilation strategies, which include the combination of low tidal volume and adequate positive end expiratory pressure (PEEP) levels, are usually adopted to minimize the ventilation-induced lung injury and to avoid post-operative pulmonary complications (PPCs). Even so, volutrauma and atelectrauma may co-exist at different levels of tidal volume and PEEP, and therefore, the physiological response to the MV settings should be monitored in each patient. A personalized perioperative approach is gaining relevance in the field of intraoperative MV; in particular, many efforts have been made to individualize PEEP, giving more emphasis on physiological and functional status to the whole body. In this review, we summarized the latest findings about the optimization of PEEP and intraoperative MV in different surgical settings. Starting from a physiological point of view, we described how to approach the individualized MV and monitor the effects of MV on lung function.

Diagnostic Insights from Plethysmographic Alveolar Pressure Assessed during Spontaneous Breathing in COPD Patients

Since its introduction in the clinical practice, body plethysmography has assisted pneumologists in the diagnosis of respiratory diseases and patients’ follow-up, by providing easy assessment of absolute lung volumes and airway resistance. In the last decade, emerging evidence suggested that estimation of alveolar pressure by electronically-compensated plethysmographs may contain information concerning the mechanics of the respiratory system which goes beyond those provided by the simple value of airway resistance or conductance. Indeed, the systematic study of expiratory alveolar pressure-flow loops produced during spontaneous breathing at rest has shown that the marked expansion of expiratory loops in chronic obstructive pulmonary disease patients mainly reflects the presence of tidal expiratory flow-limitation. The presence of this phenomenon can be accurately predicted on the basis of loop-derived parameters. Finally, we present results suggesting that plethysmographic alveolar pressure may be used to estimate non-invasively intrinsic positive end-expiratory pressure (PEEPi) in spontaneously breathing patients, a task which previously could be only accomplished by introducing a balloon-tipped catheter in the esophagus.

Non-interventional monitoring of expiratory flow limitation during experimental mechanical ventilation

Background

Expiratory flow limitation (EFL) is common among patients in the intensive care unit under mechanical ventilation (MV) and may have significant clinical consequences. In the present study, we examine the possibility of non-interventional detection of EFL during experimental MV.

Methods

Eight artificially ventilated New Zealand rabbits were included in the experiments. EFL was induced during MV by application of negative expiratory pressure (−5, −8 and −10 hPa) and detected by the negative expiratory pressure technique. Airway pressure (P_aw) and gas flow (V′) were digitally recorded and processed off-line for the evaluation of respiratory mechanics. The method is based on the computation and monitoring of instantaneous respiratory resistance R_rs(t). The resistive pressure (P_aw,res(t)) is calculated by subtracting from P_aw its elastic component and the end-expiratory pressure, as assessed by linear regression. Then, R_rs(t) is computed as the instant ratio P_aw,res(t)/V′(t).

Results

Two completely different patterns of expiratory R_rs(t) separate the cases with EFL from those without EFL. Small and random fluctuations are noticed when EFL is absent, whereas the onset of EFL is accompanied by an abrupt and continuous rise in R_rs(t), towards the end of expiration. Thus, EFL is not only detected but may also be quantified from the volume still to be expired at the time EFL occurs.

Conclusion

The proposed technique is a simple, accurate and non-interventional tool for EFL monitoring during MV.

Respiratory system resistance in expiratory flow limitation during mechanical ventilationhttps://bit.ly/34hU6Bv

How to ventilate obstructive and asthmatic patients

Exacerbations are part of the natural history of chronic obstructive pulmonary disease and asthma. Severe exacerbations can cause acute respiratory failure, which may ultimately require mechanical ventilation. This review summarizes practical ventilator strategies for the management of patients with obstructive airway disease. Such strategies include non-invasive mechanical ventilation to prevent intubation, invasive mechanical ventilation, from the time of intubation to weaning, and strategies intended to prevent post-extubation acute respiratory failure. The role of tracheostomy, the long-term prognosis, and potential future adjunctive strategies are also discussed. Finally, the physiological background that underlies these strategies is detailed.

Evidence-Based Management of the Critically Ill Adult With SARS-CoV-2 Infection

Human infection by the novel viral pathogen SARS-CoV-2 results in a clinical syndrome termed Coronavirus Disease 2019 (COVID-19). Although the majority of COVID-19 cases are self-limiting, a substantial minority of patients develop disease severe enough to require intensive care. Features of critical illness associated with COVID-19 include hypoxemic respiratory failure, acute respiratory distress syndrome (ARDS), shock, and multiple organ dysfunction syndrome (MODS). In most (but not all) respects critically ill patients with COVID-19 resemble critically ill patients with ARDS due to other causes and are optimally managed with standard, evidence-based critical care protocols. However, there is naturally an intense interest in developing specific therapies for severe COVID-19. Here we synthesize the rapidly expanding literature around the pathophysiology, clinical presentation, and management of COVID-19 with a focus on those points most relevant for intensivists tasked with caring for these patients. We specifically highlight evidence-based approaches that we believe should guide the identification, triage, respiratory support, and general ICU care of critically ill patients infected with SARS-CoV-2. In addition, in light of the pressing need and growing enthusiasm for targeted COVID-19 therapies, we review the biological basis, plausibility, and clinical evidence underlying these novel treatment approaches.

The Crashing Obese Patient

The obesity pandemic now affects hundreds of millions of people worldwide. As obesity rates continue to increase, emergency physicians are called on with increasing frequency to resuscitate obese patients. This article discusses important anatomic, physiologic, and practical challenges imposed by obesity on resuscitative care. Impacts on hemodynamic monitoring, airway and ventilator management, and pharmacologic therapy are discussed. Finally, several important clinical scenarios (trauma, cardiac arrest, and sepsis), in which alterations to standard treatments may benefit obese patients, are highlighted.

Expiratory Resistances Prevent Expiratory Diaphragm Contraction, Flow Limitation, and Lung Collapse

Rationale: Tidal expiratory flow limitation (tidal-EFL) is not completely avoidable by applying positive end-expiratory pressure and may cause respiratory and hemodynamic complications in ventilated patients with lungs prone to collapse. During spontaneous breathing, expiratory diaphragmatic contraction counteracts tidal-EFL. We hypothesized that during both spontaneous breathing and controlled mechanical ventilation, external expiratory resistances reduce tidal-EFL.Objectives: To assess whether external expiratory resistances 1) affect expiratory diaphragmatic contraction during spontaneous breathing, 2) reduce expiratory flow and make lung compartments more homogeneous with more similar expiratory time constants, and 3) reduce tidal atelectasis, preventing hyperinflation.Methods: Three positive end-expiratory pressure levels and four external expiratory resistances were tested in 10 pigs after lung lavage. We analyzed expiratory diaphragmatic electric activity and respiratory mechanics. On the basis of computed tomography scans, four lung compartments-not inflated (atelectasis), poorly inflated, normally inflated, and hyperinflated-were defined.Measurements and Main Results: Consequently to additional external expiratory resistances, and mainly in lungs prone to collapse (at low positive end-expiratory pressure), 1) the expiratory transdiaphragmatic pressure decreased during spontaneous breathing by >10%, 2) expiratory flow was reduced and the expiratory time constants became more homogeneous, and 3) the amount of atelectasis at end-expiration decreased from 24% to 16% during spontaneous breathing and from 32% to 18% during controlled mechanical ventilation, without increasing hyperinflation.Conclusions: The expiratory modulation induced by external expiratory resistances preserves the positive effects of the expiratory brake while minimizing expiratory diaphragmatic contraction. External expiratory resistances optimize lung mechanics and limit tidal-EFL and tidal atelectasis, without increasing hyperinflation.

Go with the flow-clinical importance of flow curves during mechanical ventilation: A narrative review

Most clinicians pay attention to tidal volume and airway pressures and their curves during mechanical ventilation. On the other hand, inspiratory–expiratory flow curves also provide a plethora of information, but much less attention is paid to them. Flow curves chronologically show the velocity and direction of inspiration and expiration and are influenced by the respiratory mechanics, the patient’s effort, and the mode of ventilation and its settings. When the ventilator setting does not synchronize with the patient’s respiratory pattern, the patient can easily have worsening breathing effort, patient–ventilator asynchrony, which can lead to prolonged ventilator support or lung injury. The information provided by the flow curves during mechanical ventilation, such as respiratory mechanics, the patient’s effort, and patient–ventilator interactions, are very helpful when adjusting the ventilator setting. If clinicians can monitor and assess the flow curves information appropriately, it can be a useful diagnostic and therapeutic tool at the bedside. There may be association between inspiratory effort and flow, and this may further guide us, especially in the weaning process and when patients are not synchronizing with the ventilator. In this review, we try to gather information about “flow” that is scattered around in the literature and textbooks in one place. We will summarize the different flow waveforms utilized in commonly used ventilator modes with their advantages and disadvantages, information gained by the flow curves (i.e., flow-time, flow-volume, and flow-pressure), how to detect and manage asynchronies, and some ideas for future uses. Flow waveforms shapes and patterns are very beneficial for the management of patients undergoing mechanical ventilatory support. Attention to those waveforms can potentially improve patient outcomes. Clinicians should be familiar with this information and how to act upon them.

Intraabdominal Pressure Targeted Positive End-expiratory Pressure during Laparoscopic Surgery: An Open-label, Nonrandomized, Crossover, Clinical Trial

BACKGROUND

Pneumoperitoneum for laparoscopic surgery is associated with a rise of driving pressure. The authors aimed to assess the effects of positive end-expiratory pressure (PEEP) on driving pressure at varying intraabdominal pressure levels. It was hypothesized that PEEP attenuates pneumoperitoneum-related rises in driving pressure.

METHODS

Open-label, nonrandomized, crossover, clinical trial in patients undergoing laparoscopic cholecystectomy. "Targeted PEEP" (2 cm H2O above intraabdominal pressure) was compared with "standard PEEP" (5 cm H2O), with respect to the transpulmonary and respiratory system driving pressure at three predefined intraabdominal pressure levels, and each patient was ventilated with two levels of PEEP at the three intraabdominal pressure levels in the same sequence. The primary outcome was the difference in transpulmonary driving pressure between targeted PEEP and standard PEEP at the three levels of intraabdominal pressure.

RESULTS

Thirty patients were included and analyzed. Targeted PEEP was 10, 14, and 17 cm H2O at intraabdominal pressure of 8, 12, and 15 mmHg, respectively. Compared to standard PEEP, targeted PEEP resulted in lower median transpulmonary driving pressure at intraabdominal pressure of 8 mmHg (7 [5 to 8] vs. 9 [7 to 11] cm H2O; P = 0.010; difference 2 [95% CI 0.5 to 4 cm H2O]); 12 mmHg (7 [4 to 9] vs.10 [7 to 12] cm H2O; P = 0.002; difference 3 [1 to 5] cm H2O); and 15 mmHg (7 [6 to 9] vs.12 [8 to 15] cm H2O; P < 0.001; difference 4 [2 to 6] cm H2O). The effects of targeted PEEP compared to standard PEEP on respiratory system driving pressure were comparable to the effects on transpulmonary driving pressure, though respiratory system driving pressure was higher than transpulmonary driving pressure at all intraabdominal pressure levels.

CONCLUSIONS

Transpulmonary driving pressure rises with an increase in intraabdominal pressure, an effect that can be counterbalanced by targeted PEEP. Future studies have to elucidate which combination of PEEP and intraabdominal pressure is best in term of clinical outcomes.

Titration of extra-PEEP against intrinsic-PEEP in severe asthma by electrical impedance tomography: A case report and literature review

RATIONALE

The use of extra-positive end-expiratory pressure (PEEP) at a level of 80% intrinsic-PEEP (iPEEP) to improve ventilation in severe asthma patients with control ventilation remains controversial. Electrical impedance tomography (EIT) may provide regional information for determining the optimal extra-PEEP to overcome gas trapping and distribution. Moreover, the experience of using EIT to determine extra-PEEP in severe asthma patients with controlled ventilation is limited.

PATIENTS CONCERNS

A severe asthma patient had 12-cmH2O iPEEP using the end-expiratory airway occlusion method at Zero positive end-expiratory pressures (ZEEP). How to titrate the extra-PEEP to against iPEEP at bedside?

DIAGNOSES AND INTERVENTIONS

An incremental PEEP titration was performed in the severe asthma patient with mechanical ventilation. An occult pendelluft phenomenon of the ventral and dorsal regions was found during the early and late expiration periods when the extra-PEEP was set to <6 cmH2O. If the extra-PEEP was elevated from 4 to 6 cmH2O, a decrease in the end-expiratory lung impedance (EELI) and a disappearance of the pendelluft phenomenon were observed during the PEEP titration. Moreover, there was broad disagreement as to the "best" extra-PEEP settings according to the various EIT parameters. The regional ventilation delay had the lowest extra-PEEP value (10 cmH2O), whereas the value was 12 cmH2O for the lung collapse/overdistension index and 14 cmH2O for global inhomogeneity.

OUTCOMES

The extra-PEEP was set at 6 cmH2O, and the severe whistling sound was improved. The patient's condition further became better under the integrated therapy.

LESSONS

A broad literature review shows that this was the 3rd case of using EIT to titrate an extra-PEEP to against PEEPi. Importantly, the visualization of occult pendelluft and possible air release during incremental PEEP titration was documented for the first time during incremental PEEP titration in patients with severe asthma. Examining the presence of the occult pendelluft phenomenon and changes in the EELI by EIT might be an alternative means for determining an individual's extra-PEEP.

New physiological insights in ventilation during cardiopulmonary resuscitation

PURPOSE OF REVIEW

In the setting of cardiopulmonary resuscitation (CPR), classical physiological concept about ventilation become challenging. Ventilation may exert detrimental hemodynamic effects that must be balanced with its expected benefits. The risks of hyperventilation have been thoroughly addressed, even questioning the need for ventilation, emphasizing the need to prioritize chest compression quality. However, ventilation is mandatory for adequate gas exchange as soon as CPR is prolonged. Factors affecting the capability of chest compressions to produce alveolar ventilation are poorly understood. In this review, we discuss the conventional interpretation of interactions between ventilation and circulation, from the perspective of novel physiological observations.

RECENT FINDINGS

Many patients with cardiac arrest exhibit 'intrathoracic airway closure.' This phenomenon is associated with lung volume reduction, impedes chest compressions to generate ventilation and overall limits the delivered ventilation. This phenomenon can be reversed by the application of small levels of positive end-expiratory pressure. Also, a novel interpretation of the capnogram can rate the magnitude of this phenomenon, contributing to clarify the physiological meaning of exhaled CO2 and may help assess the real amount of delivered ventilation.

SUMMARY

Recent advances in the understanding of ventilatory physiology during CPR shows that capnogram analysis not only provides information on the quality of resuscitation but also on the amount of ventilation produced by chest compressions and on the total amount of ventilation.

Airway Closure during Surgical Pneumoperitoneum in Obese Patients

BACKGROUND

Airway closure causes lack of communication between proximal airways and alveoli, making tidal inflation start only after a critical airway opening pressure is overcome. The authors conducted a matched cohort study to report the existence of this phenomenon among obese patients undergoing general anesthesia.

METHODS

Within the procedures of a clinical trial during gynecological surgery, obese patients underwent respiratory/lung mechanics and lung volume assessment both before and after pneumoperitoneum, in the supine and Trendelenburg positions, respectively. Among patients included in this study, those exhibiting airway closure were compared to a control group of subjects enrolled in the same trial and matched in 1:1 ratio according to body mass index.

RESULTS

Eleven of 50 patients (22%) showed airway closure after intubation, with a median (interquartile range) airway opening pressure of 9 cm H2O (6 to 12). With pneumoperitoneum, airway opening pressure increased up to 21 cm H2O (19 to 28) and end-expiratory lung volume remained unchanged (1,294 ml [1,154 to 1,363] vs. 1,160 ml [1,118 to 1,256], P = 0.155), because end-expiratory alveolar pressure increased consistently with airway opening pressure and counterbalanced pneumoperitoneum-induced increases in end-expiratory esophageal pressure (16 cm H2O [15 to 19] vs. 27 cm H2O [23 to 30], P = 0.005). Conversely, matched control subjects experienced a statistically significant greater reduction in end-expiratory lung volume due to pneumoperitoneum (1,113 ml [1,040 to 1,577] vs. 1,000 ml [821 to 1,061], P = 0.006). With airway closure, static/dynamic mechanics failed to measure actual lung/respiratory mechanics. When patients with airway closure underwent pressure-controlled ventilation, no tidal volume was inflated until inspiratory pressure overcame airway opening pressure.

CONCLUSIONS

In obese patients, complete airway closure is frequent during anesthesia and is worsened by Trendelenburg pneumoperitoneum, which increases airway opening pressure and alveolar pressure: besides preventing alveolar derecruitment, this yields misinterpretation of respiratory mechanics and generates a pressure threshold to inflate the lung that can reach high values, spreading concerns on the safety of pressure-controlled modes in this setting.

Expiratory flow limitation in intensive care: prevalence and risk factors

BACKGROUND

Expiratory flow limitation (EFL) is characterised by a markedly reduced expiratory flow insensitive to the expiratory driving pressure. The presence of EFL can influence the respiratory and cardiovascular function and damage the small airways; its occurrence has been demonstrated in different diseases, such as COPD, asthma, obesity, cardiac failure, ARDS, and cystic fibrosis. Our aim was to evaluate the prevalence of EFL in patients requiring mechanical ventilation for acute respiratory failure and to determine the main clinical characteristics, the risk factors and clinical outcome associated with the presence of EFL.

METHODS

Patients admitted to the intensive care unit (ICU) with an expected length of mechanical ventilation of 72 h were enrolled in this prospective, observational study. Patients were evaluated, within 24 h from ICU admission and for at least 72 h, in terms of respiratory mechanics, presence of EFL through the PEEP test, daily fluid balance and followed for outcome measurements.

RESULTS

Among the 121 patients enrolled, 37 (31%) exhibited EFL upon admission. Flow-limited patients had higher BMI, history of pulmonary or heart disease, worse respiratory dyspnoea score, higher intrinsic positive end-expiratory pressure, flow and additional resistance. Over the course of the initial 72 h of mechanical ventilation, additional 21 patients (17%) developed EFL. New onset EFL was associated with a more positive cumulative fluid balance at day 3 (103.3 ml/kg) compared to that of patients without EFL (65.8 ml/kg). Flow-limited patients had longer duration of mechanical ventilation, longer ICU length of stay and higher in-ICU mortality.

CONCLUSIONS

EFL is common among ICU patients and correlates with adverse outcomes. The major determinant for developing EFL in patients during the first 3 days of their ICU stay is a positive fluid balance. Further studies are needed to assess if a restrictive fluid therapy might be associated with a lower incidence of EFL.

Regional air trapping in acute exacerbation of obstructive lung diseases measured with electrical impedance tomography: a feasibility study

BACKGROUND

Since bronchial abnormalities often exhibit spatial non-uniformity which may be not correctly assessed by conventional global lung function measures, regional information may help to characterize the disease progress. We hypothesized that regional air trapping during mechanical ventilation could be characterized by regional end-expiratory flow (EEF) derived from electrical impedance tomography (EIT).

METHODS

Twenty-five patients suffering from chronic obstructive pulmonary disease (COPD grade 3 or 4) or severe asthma with acute exacerbation were examined prospectively. Patients were ventilated under assist-control mode. EIT measurements were conducted before and one hour after inhaled combined corticosteroid and long-acting β2 agonist, on two consecutive days. Regional EEF was calculated as derivative of relative impedance for every image pixel in the lung regions. The results were normalized to global flow values measured by the ventilator.

RESULTS

Regional and global EEF were highly correlated (P<0.00001) and regional effects of medication and disease progression were visible in the regional EEF maps. The sums of regional EEF in lung regions were 3.8 [2.0, 5.1] and 3.6 [1.9, 4.5] L/min in COPD patients before and after medication (median [lower, upper quartiles]; P=0.37). The corresponding values in asthma patients were 3.0 [2.5, 4.2] and 2.2 [1.7, 3.2] L/min (P<0.05). Histograms of regional EEF showed high spatial heterogeneity of EEF before medication. After one day of treatment, the histograms exhibited less heterogeneous and a decrease in EEF level.

CONCLUSIONS

Regional EEF characterizes air trapping and intrinsic PEEP, which could provide diagnostic information for monitoring the disease progress during treatment.

Dynamic hyperinflation and intrinsic positive end-expiratory pressure in ARDS patients

Background

In ARDS patients, changes in respiratory mechanical properties and ventilatory settings can cause incomplete lung deflation at end-expiration. Both can promote dynamic hyperinflation and intrinsic positive end-expiratory pressure (PEEP). The aim of this study was to investigate, in a large population of ARDS patients, the presence of intrinsic PEEP, possible associated factors (patients’ characteristics and ventilator settings), and the effects of two different external PEEP levels on the intrinsic PEEP.

Methods

We made a secondary analysis of published data. Patients were ventilated with a tidal volume of 6–8 mL/kg of predicted body weight, sedated, and paralyzed. After a recruitment maneuver, a PEEP trial was run at 5 and 15 cmH₂O, and partitioned mechanics measurements were collected after 20 min of stabilization. Lung computed tomography scans were taken at 5 and 45 cmH₂O. Patients were classified into two groups according to whether or not they had intrinsic PEEP at the end of an expiratory pause.

Results

We enrolled 217 sedated, paralyzed patients: 87 (40%) had intrinsic PEEP with a median of 1.1 [1.0–2.3] cmH₂O at 5 cmH₂O of PEEP. The intrinsic PEEP significantly decreased with higher PEEP (1.1 [1.0–2.3] vs 0.6 [0.0–1.0] cmH₂O; p < 0.001). The applied tidal volume was significantly lower (480 [430–540] vs 520 [445–600] mL at 5 cmH₂O of PEEP; 480 [430–540] vs 510 [430–590] mL at 15 cmH₂O) in patients with intrinsic PEEP, while the respiratory rate was significantly higher (18 [15–20] vs 15 [13–19] bpm at 5 cmH₂O of PEEP; 18 [15–20] vs 15 [13–19] bpm at 15 cmH₂O). At both PEEP levels, the total airway resistance and compliance of the respiratory system were not different in patients with and without intrinsic PEEP. The total lung gas volume and lung recruitability were also not different between patients with and without intrinsic PEEP (respectively 961 [701–1535] vs 973 [659–1433] mL and 15 [0–32] % vs 22 [0–36] %).

Conclusions

In sedated, paralyzed ARDS patients without a known obstructive disease, the amount of intrinsic PEEP during lung-protective ventilation is negligible and does not influence respiratory mechanical properties.

Physiological effects of high-flow oxygen in tracheostomized patients

Background

High-flow oxygen therapy via nasal cannula (HFOT_NASAL) increases airway pressure, ameliorates oxygenation and reduces work of breathing. High-flow oxygen can be delivered through tracheostomy (HFOT_TRACHEAL), but its physiological effects have not been systematically described. We conducted a cross-over study to elucidate the effects of increasing flow rates of HFOT_TRACHEAL on gas exchange, respiratory rate and endotracheal pressure and to compare lower airway pressure produced by HFOT_NASAL and HFOT_TRACHEAL.

Methods

Twenty-six tracheostomized patients underwent standard oxygen therapy through a conventional heat and moisture exchanger, and then HFOT_TRACHEAL through a heated humidifier, with gas flow set at 10, 30 and 50 L/min. Each step lasted 30 min; gas flow sequence during HFOT_TRACHEAL was randomized. In five patients, measurements were repeated during HFOT_TRACHEAL before tracheostomy decannulation and immediately after during HFOT_NASAL. In each step, arterial blood gases, respiratory rate, and tracheal pressure were measured.

Results

During HFOT_TRACHEAL, PaO₂/FiO₂ ratio and tracheal expiratory pressure slightly increased proportionally to gas flow. The mean [95% confidence interval] expiratory pressure raise induced by 10-L/min increase in flow was 0.2 [0.1–0.2] cmH₂O (ρ = 0.77, p < 0.001). Compared to standard oxygen, HFOT_TRACHEAL limited the negative inspiratory swing in tracheal pressure; at 50 L/min, but not with other settings, HFOT_TRACHEAL increased mean tracheal expiratory pressure by (mean difference [95% CI]) 0.4 [0.3–0.6] cmH₂O, peak tracheal expiratory pressure by 0.4 [0.2–0.6] cmH₂O, improved PaO₂/FiO₂ ratio by 40 [8–71] mmHg, and reduced respiratory rate by 1.9 [0.3–3.6] breaths/min without PaCO₂ changes. As compared to HFOT_TRACHEAL, HFOT_NASAL produced higher tracheal mean and peak expiratory pressure (at 50 L/min, mean difference [95% CI]: 3 [1–5] cmH₂O and 4 [1–7] cmH₂O, respectively).

Conclusions

As compared to standard oxygen, 50 L/min of HFOT_TRACHEAL are needed to improve oxygenation, reduce respiratory rate and provide small degree of positive airway expiratory pressure, which, however, is significantly lower than the one produced by HFOT_NASAL.

Electrical impedance tomography in perioperative medicine: careful respiratory monitoring for tailored interventions

BACKGROUND

Electrical impedance tomography (EIT) is a non-invasive radiation-free monitoring technique that provides images based on tissue electrical conductivity of the chest. Several investigations applied EIT in the context of perioperative medicine, which is not confined to the intraoperative period but begins with the preoperative assessment and extends to postoperative follow-up.

MAIN BODY

EIT could provide careful respiratory monitoring in the preoperative assessment to improve preparation for surgery, during anaesthesia to guide optimal ventilation strategies and to monitor the hemodynamic status and in the postoperative period for early detection of respiratory complications. Moreover, EIT could further enhance care of patients undergoing perioperative diagnostic procedures. This narrative review summarizes the latest evidence on the application of this technique to the surgical patient, focusing also on possible future perspectives.

CONCLUSIONS

EIT is a promising technique for the perioperative assessment of surgical patients, providing tailored adaptive respiratory and haemodynamic monitoring. Further studies are needed to address the current technological limitations, confirm the findings and evaluate which patients can benefit more from this technology.

Expiratory muscle dysfunction in critically ill patients: towards improved understanding

INTRODUCTION

This narrative review summarizes current knowledge on the physiology and pathophysiology of expiratory muscle function in ICU patients, as shared by academic professionals from multidisciplinary, multinational backgrounds, who include clinicians, clinical physiologists and basic physiologists.

RESULTS

The expiratory muscles, which include the abdominal wall muscles and some of the rib cage muscles, are an important component of the respiratory muscle pump and are recruited in the presence of high respiratory load or low inspiratory muscle capacity. Recruitment of the expiratory muscles may have beneficial effects, including reduction in end-expiratory lung volume, reduction in transpulmonary pressure and increased inspiratory muscle capacity. However, severe weakness of the expiratory muscles may develop in ICU patients and is associated with worse outcomes, including difficult ventilator weaning and impaired airway clearance. Several techniques are available to assess expiratory muscle function in the critically ill patient, including gastric pressure and ultrasound.

CONCLUSION

The expiratory muscles are the "neglected component" of the respiratory muscle pump. Expiratory muscles are frequently recruited in critically ill ventilated patients, but a fundamental understanding of expiratory muscle function is still lacking in these patients.

Décubitus ventral et kinésithérapie respiratoire : y a-t-il une indication ? Description d’un cas clinique

Contexte : Le décubitus ventral (DV) est appliqué dans un objectif de recrutement alvéolaire, dans le cadre de syndrome de détresse respiratoire aiguë (SDRA). Le DV mobilise parfois des sécrétions bronchiques, interrogeant l’intérêt d’une kinésithérapie de désencombrement.

Matériel et méthode : Une femme de 43 ans, myopathe de Steinert, est hospitalisée pour une insuffisance hépatique aiguë. À j3, elle présente une pneumopathie d’inhalation, suivie d’un SDRA. Le positionnement en DV est réalisé, mobilisant des sécrétions bronchiques. Une séance de kinésithérapie respiratoire est alors appliquée.

Résultats : Après la mise en DVet la séance de kinésithérapie, la quantité de sécrétions recueillies est de 2,4 g. Le rapport entre la pression partielle en oxygène et la fraction inspirée en oxygène (PaO2/FiO2) s’améliore, passant de 64 à 180 au bout de 11 heures de DV. La pression motrice et la pression de plateau sont restées inférieures aux valeurs délétères au cours de la séance de kinésithérapie, celle-ci n’ayant pas généré d’hypoxie pendant ou après la séance.

Discussion : Le positionnement en DV libère les parties postérieures des poumons, permettant une amélioration du rapport PaO2/FiO2. La clairance mucociliaire a été améliorée, mais il n’est pas possible de discriminer les effets du DVou de la kinésithérapie. Dans la littérature, la kinésithérapie respiratoire n’a pas montré son efficacité pour ces patients, même si aucun effet délétère n’a été observé à travers ce cas clinique. Les risques de dé-recrutement alvéolaire restent importants.

Conclusion : Il est difficile de recommander en pratique courante la kinésithérapie respiratoire de désencombrement en DV. Des études ultérieures sont nécessaires, dans un objectif de recherche centré plutôt sur le recrutement alvéolaire que sur le désencombrement, chez ces patients fragiles.

Biomedical engineer's guide to the clinical aspects of intensive care mechanical ventilation

Background

Mechanical ventilation is an essential therapy to support critically ill respiratory failure patients. Current standards of care consist of generalised approaches, such as the use of positive end expiratory pressure to inspired oxygen fraction (PEEP–FiO₂) tables, which fail to account for the inter- and intra-patient variability between and within patients. The benefits of higher or lower tidal volume, PEEP, and other settings are highly debated and no consensus has been reached. Moreover, clinicians implicitly account for patient-specific factors such as disease condition and progression as they manually titrate ventilator settings. Hence, care is highly variable and potentially often non-optimal. These conditions create a situation that could benefit greatly from an engineered approach. The overall goal is a review of ventilation that is accessible to both clinicians and engineers, to bridge the divide between the two fields and enable collaboration to improve patient care and outcomes. This review does not take the form of a typical systematic review. Instead, it defines the standard terminology and introduces key clinical and biomedical measurements before introducing the key clinical studies and their influence in clinical practice which in turn flows into the needs and requirements around how biomedical engineering research can play a role in improving care. Given the significant clinical research to date and its impact on this complex area of care, this review thus provides a tutorial introduction around the review of the state of the art relevant to a biomedical engineering perspective.

Discussion

This review presents the significant clinical aspects and variables of ventilation management, the potential risks associated with suboptimal ventilation management, and a review of the major recent attempts to improve ventilation in the context of these variables. The unique aspect of this review is a focus on these key elements relevant to engineering new approaches. In particular, the need for ventilation strategies which consider, and directly account for, the significant differences in patient condition, disease etiology, and progression within patients is demonstrated with the subsequent requirement for optimal ventilation strategies to titrate for patient- and time-specific conditions.

Conclusion

Engineered, protective lung strategies that can directly account for and manage inter- and intra-patient variability thus offer great potential to improve both individual care, as well as cohort clinical outcomes.