Bedside adjustment of proportional assist ventilation to target a predefined range of respiratory effort

Weakness acquired in the cardiac intensive care unit: still the elephant in the room?

Over the past two decades, the cardiac critical care population has shifted to increasingly comorbid and elderly patients often presenting with nonprimary cardiac conditions that exacerbate underlying advanced cardiac disease. Consequently, the modern cardiac intensive care unit (CICU) patient has poor outcome regardless of left ventricular ejection fraction. Importantly, delayed liberation from organ support, independent from premorbid health status and admission severity of illness, has been associated with increased morbidity and mortality up to years post-general critical care. Although a constellation of several acquired morbidities is at play, the most prominent enactor of poor long-term outcome in this population appears to be intensive care unit acquired weakness. Although the specific burden of ICU-acquired morbidities in CICU patients is yet to be clearly defined, it seems unfathomable that patients will not accrue some sort of ICU-related morbidity. There is hence an urgent need to better establish the exact benefit and cost of resource-intensive strategies in both short- and long-term survival of the CICU patient. Consequent and standardized documentation of admission comorbidities, severity of illness indicators, relevant ICU-related complications including weakness, and long-term post-ICU morbidity outcomes can help our understanding of the disease continuum and how to better care for the CICU survivor and their families and caregivers. Given increasing budgetary pressure on healthcare systems worldwide, interventions targeting CICU patients should focus on improving patient-centred long-term outcomes in a cost-effective manner. It will require a holistic and transmural continuity of care model to meet the challenges associated with treating critically ill cardiac patients in the future.

Proportional modes to hasten weaning

PURPOSE OF REVIEW

The purpose of this review is to examine the current state of the evidence, including several recent systematic reviews and meta-analyses, to determine if proportional modes of ventilation have the potential to hasten weaning from mechanical ventilation for adult critically ill patients, compared to pressure support ventilation (PSV), the current standard of care during the recovery and weaning phases of mechanical ventilation.

RECENT FINDINGS

Proportional assist ventilation (PAV) and neurally adjusted ventilatory assist (NAVA) are two commercially available proportional modes that have been studied in randomized controlled trials (RCTs). Although several feasibility studies were not powered to detect differences in clinical outcomes, emerging evidence suggests that both PAV and NAVA may reduce duration of mechanical ventilation, intensive care unit (ICU) length of stay, and hospital mortality compared to PSV, as shown in some small, primarily single-centre studies. Recent meta-analyses suggest that PAV shortens duration of mechanical ventilation and improves weaning success rate, and NAVA may reduce ICU and hospital mortality.

SUMMARY

The current state of the evidence suggests that proportional modes may hasten weaning from mechanical ventilation, but larger, multicentre RCTS are needed to confirm these preliminary findings.

Proportional Assist Ventilation for Minimizing the Duration of Mechanical Ventilation (the PROMIZING study): update to the statistical analysis plan for a randomized controlled trial

BACKGROUND

We previously published the protocol and statistical analysis plan for a randomized controlled trial of Proportional Assist Ventilation for Minimizing the Duration of Mechanical Ventilation: the PROMIZING study in Trials ( https://doi.org/10.1186/s13063-023-07163-w ). This update summarizes changes made to the statistical analysis plan for the trial since the publication of the original protocol and statistical analysis plan.

METHODS/DESIGN

The Proportional Assist Ventilation for Minimizing the Duration of Mechanical Ventilation (PROMIZING) study is a multi-center, open-label, randomized controlled trial designed to determine if ventilation with proportional assist ventilation with load-adjustable gain factors will result in a shorter duration of time spent on mechanical ventilation compared to ventilation with pressure support ventilation for patients with acute respiratory failure. The statistical analysis plan for the trial was incorporated into the original publication of the protocol in Trials ( https://doi.org/10.1186/s13063-023-07163-w ) and was based on version 5.0 of the study protocol and version 1.0 of the statistical analysis plan (SAP), which included plans for both frequentist and Bayesian analyses. We have since updated the SAP to refine the Bayesian analysis plan, update the multistate model diagram, and include plans for a cluster analysis to determine if there is heterogeneity of treatment effect. This update summarizes the changes made and their rationale and provides a refined SAP for the PROMIZING trial with additional background information, in adherence with guidelines for the prospective reporting of SAPs for randomized controlled trials.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT02447692 prospectively registered May 19, 2015.

Challenges in Transitioning from Controlled to Assisted Ventilation in Acute Respiratory Distress Syndrome (ARDS) Management

Acute respiratory distress syndrome (ARDS) presents significant challenges in critical care, primarily due to its inflammatory nature, which leads to impaired gas exchange and respiratory mechanics. While mechanical ventilation (MV) is essential for patient support, the transition from controlled to assisted ventilation is complex and may be associated with intensive care unit-acquired weakness, ventilator-induced diaphragmatic dysfunction and patient self-inflicted lung injury. This paper explores the multifaceted challenges encountered during this transition, with a focus on respiratory effort, sedation management, and monitoring techniques, and investigates innovative approaches to enhance patient outcomes. The key strategies include optimizing sedation protocols, employing advanced monitoring methods like esophageal pressure measurements, and implementing partial neuromuscular blockade to prevent excessive respiratory effort. We also emphasize the importance of personalized treatment plans and the integration of artificial intelligence to facilitate timely transitions. By highlighting early rehabilitation techniques, continuously assessing the respiratory drive, and fostering collaboration among multidisciplinary teams, clinicians can improve the transition from controlled to assisted MV, ultimately enhancing recovery and long-term respiratory health in patients with ARDS.

Transpulmonary pressure monitoring in critically ill patients: pros and cons

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

Fundamental concepts and the latest evidence for esophageal pressure monitoring

Transpulmonary pressure is an essential physiologic concept as it reflects the true pressure across the alveoli, and is a more precise marker for lung stress. To calculate transpulmonary pressure, one needs an estimate of both alveolar pressure and pleural pressure. Airway pressure during conditions of no flow is the most widely accepted surrogate for alveolar pressure, while esophageal pressure remains the most widely measured surrogate marker for pleural pressure. This review will cover important concepts and clinical applications for esophageal manometry, with a particular focus on how to use the information from esophageal manometry to adjust or titrate ventilator support. The most widely used method for measuring esophageal pressure uses an esophageal balloon catheter, although these measurements can be affected by the volume of air in the balloon. Therefore, when using balloon catheters, it is important to calibrate the balloon to ensure the most appropriate volume of air, and we discuss several methods which have been proposed for balloon calibration. In addition, esophageal balloon catheters only estimate the pleural pressure over a certain area within the thoracic cavity, which has resulted in a debate regarding how to interpret these measurements. We discuss both direct and elastance-based methods to estimate transpulmonary pressure, and how they may be applied for clinical practice. Finally, we discuss a number of applications for esophageal manometry and review many of the clinical studies published to date which have used esophageal pressure. These include the use of esophageal pressure to assess lung and chest wall compliance individually which can provide individualized information for patients with acute respiratory failure in terms of setting PEEP, or limiting inspiratory pressure. In addition, esophageal pressure has been used to estimate effort of breathing which has application for ventilator weaning, detection of upper airway obstruction after extubation, and detection of patient and mechanical ventilator asynchrony.

Study protocol for a randomized controlled trial of Proportional Assist Ventilation for Minimizing the Duration of Mechanical Ventilation: the PROMIZING study

BACKGROUND

Proportional assist ventilation with load-adjustable gain factors (PAV+) is a mechanical ventilation mode that delivers assistance to breathe in proportion to the patient's effort. The proportional assistance, called the gain, can be adjusted by the clinician to maintain the patient's respiratory effort or workload within a normal range. Short-term and physiological benefits of this mode compared to pressure support ventilation (PSV) include better patient-ventilator synchrony and a more physiological response to changes in ventilatory demand.

METHODS

The objective of this multi-centre randomized controlled trial (RCT) is to determine if, for patients with acute respiratory failure, ventilation with PAV+ will result in a shorter time to successful extubation than with PSV. This multi-centre open-label clinical trial plans to involve approximately 20 sites in several continents. Once eligibility is determined, patients must tolerate a short-term PSV trial and either (1) not meet general weaning criteria or (2) fail a 2-min Zero Continuous Positive Airway Pressure (CPAP) Trial using the rapid shallow breathing index, or (3) fail a spontaneous breathing trial (SBT), in this sequence. Then, participants in this study will be randomized to either PSV or PAV+ in a 1:1 ratio. PAV+ will be set according to a target of muscular pressure. The weaning process will be identical in the two arms. Time to liberation will be the primary outcome; ventilator-free days and other outcomes will be measured.

DISCUSSION

Meta-analyses comparing PAV+ to PSV suggest PAV+ may benefit patients and decrease healthcare costs but no powered study to date has targeted the difficult to wean patient population most likely to benefit from the intervention, or used consistent timing for the implementation of PAV+. Our enrolment strategy, primary outcome measure, and liberation approaches may be useful for studying mechanical ventilation and weaning and can offer important results for patients.

TRIAL REGISTRATION

ClinicalTrials.gov NCT02447692 . Prospectively registered on May 19, 2015.

Diaphragmatic ultrasound in weaning ventilated patients: a reliable predictor?

Timely weaning of mechanical ventilation is clinically difficult, as both early and late discontinuations are associated with adverse events impeding the patient's favorable course in the critical care unit. Many parameters aid in judging whether the patient is going to be safely extubated; however, some remain deficient. We herein describe diaphragmatic ultrasound as a new modality for this purpose, detailing diaphragm excursion and thickness as means of it.

Respiratory drive, inspiratory effort, and work of breathing: review of definitions and non-invasive monitoring tools for intensive care ventilators during pandemic times

Technological advances in mechanical ventilation have been essential to increasing the survival rate in intensive care units. Usually, patients needing mechanical ventilation use controlled ventilation to override the patient’s respiratory muscles and favor lung protection. Weaning from mechanical ventilation implies a transition towards spontaneous breathing, mainly using assisted mechanical ventilation. In this transition, the challenge for clinicians is to avoid under and over assistance and minimize excessive respiratory effort and iatrogenic diaphragmatic and lung damage. Esophageal balloon monitoring allows objective measurements of respiratory muscle activity in real time, but there are still limitations to its routine application in intensive care unit patients using mechanical ventilation. Like the esophageal balloon, respiratory muscle electromyography and diaphragmatic ultrasound are minimally invasive tools requiring specific training that monitor respiratory muscle activity. Particularly during the coronavirus disease pandemic, non invasive tools available on mechanical ventilators to monitor respiratory drive, inspiratory effort, and work of breathing have been extended to individualize mechanical ventilation based on patient’s needs. This review aims to identify the conceptual definitions of respiratory drive, inspiratory effort, and work of breathing and to identify non invasive maneuvers available on intensive care ventilators to measure these parameters. The literature highlights that although respiratory drive, inspiratory effort, and work of breathing are intuitive concepts, even distinguished authors disagree on their definitions.

Proportional assist ventilation relieves clinically significant dyspnea in critically ill ventilated patients

INTRODUCTION

Dyspnea is common and often severe symptom in mechanically ventilated patients. Proportional assist ventilation (PAV) is an assist ventilatory mode that adjusts the level of assistance to the activity of respiratory muscles. We hypothesized that PAV reduce dyspnea compared to pressure support ventilation (PSV).

PATIENTS AND METHODS

Mechanically ventilated patients with clinically significant dyspnea were included. Dyspnea intensity was assessed by the Dyspnea-Visual Analog Scale (D-VAS) and the Intensive Care-Respiratory Distress Observation Scale (IC-RDOS) at inclusion (PSV-Baseline), after personalization of ventilator settings in order to minimize dyspnea (PSV-Personalization), and after switch to PAV. Respiratory drive was assessed by record of electromyographic activity of inspiratory muscles, the proportion of asynchrony was analyzed.

RESULTS

Thirty-four patients were included (73% males, median age of 66 [57-77] years). The D-VAS score was lower with PSV-Personalization (37 mm [20‒55]) and PAV (31 mm [14‒45]) than with PSV-Baseline (62 mm [28‒76]) (p < 0.05). The IC-RDOS score was lower with PAV (4.2 [2.4‒4.7]) and PSV-Personalization (4.4 [2.4‒4.9]) than with PSV-Baseline (4.8 [4.1‒6.5]) (p < 0.05). The electromyographic activity of parasternal intercostal muscles was lower with PAV and PSV-Personalization than with PSV-Baseline. The asynchrony index was lower with PAV (0% [0‒0.55]) than with PSV-Baseline and PSV-Personalization (0.68% [0‒2.28] and 0.60% [0.31‒1.41], respectively) (p < 0.05).

CONCLUSION

In mechanically ventilated patients exhibiting clinically significant dyspnea with PSV, personalization of PSV settings and PAV results in not different decreased dyspnea and activity of muscles to a similar degree, even though PAV was able to reduce asynchrony more effectively.

Flow Index accurately identifies breaths with low or high inspiratory effort during pressure support ventilation

Background

Flow Index, a numerical expression of the shape of the inspiratory flow-time waveform recorded during pressure support ventilation, is associated with patient inspiratory effort. The aim of this study was to assess the accuracy of Flow Index in detecting high or low inspiratory effort during pressure support ventilation and to establish cutoff values for the Flow index to identify these conditions. The secondary aim was to compare the performance of Flow index,of breathing pattern parameters and of airway occlusion pressure (P_0.1) in detecting high or low inspiratory effort during pressure support ventilation.

Methods

Data from 24 subjects was included in the analysis, accounting for a total of 702 breaths. Breaths with high inspiratory effort were defined by a pressure developed by inspiratory muscles (P_musc) greater than 10 cmH₂O while breaths with low inspiratory effort were defined by a P_musc lower than 5 cmH₂O. The areas under the receiver operating characteristic curves of Flow Index and respiratory rate, tidal volume,respiratory rate over tidal volume and P_0.1 were analyzed and compared to identify breaths with low or high inspiratory effort.

Results

P_musc, P_0.1, Pressure Time Product and Flow Index differed between breaths with high, low and intermediate inspiratory effort, while RR, RR/V_T and V_T/kg of IBW did not differ in a statistically significant way. A Flow index higher than 4.5 identified breaths with high inspiratory effort [AUC 0.89 (CI 95% 0.85–0.93)], a Flow Index lower than 2.6 identified breaths with low inspiratory effort [AUC 0.80 (CI 95% 0.76–0.83)].

Conclusions

Flow Index is accurate in detecting high and low spontaneous inspiratory effort during pressure support ventilation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13054-021-03855-4.

Refractory ineffective triggering during pressure support ventilation: effect of proportional assist ventilation with load-adjustable gain factors

Background

Ineffective triggering is frequent during pressure support ventilation (PSV) and may persist despite ventilator adjustment, leading to refractory asynchrony. We aimed to assess the effect of proportional assist ventilation with load-adjustable gain factors (PAV+) on the occurrence of refractory ineffective triggering.

Design

Observational assessment followed by prospective cross-over physiological study.

Setting

Academic medical ICU.

Patients

Ineffective triggering was detected during PSV by a twice-daily inspection of the ventilator’s screen. The impact of pressure support level (PSL) adjustments on the occurrence of asynchrony was recorded. Patients experiencing refractory ineffective triggering, defined as persisting asynchrony at the lowest tolerated PSL, were included in the physiological study.

Interventions

Physiological study: Flow, airway, and esophageal pressures were continuously recorded during 10 min under PSV with the lowest tolerated PSL, and then under PAV+ with the gain adjusted to target a muscle pressure between 5 and 10 cmH₂O.

Measurements

Primary endpoint was the comparison of asynchrony index between PSV and PAV+ after PSL and gain adjustments.

Results

Among 36 patients identified having ineffective triggering under PSV, 21 (58%) exhibited refractory ineffective triggering. The lowest tolerated PSL was higher in patients with refractory asynchrony as compared to patients with non-refractory ineffective triggering. Twelve out of the 21 patients with refractory ineffective triggering were included in the physiological study. The median lowest tolerated PSL was 17 cmH₂O [12–18] with a PEEP of 7 cmH₂O [5–8] and FiO₂ of 40% [39–42]. The median gain during PAV+ was 73% [65–80]. The asynchrony index was significantly lower during PAV+ than PSV (2.7% [1.0–5.4] vs. 22.7% [10.3–40.1], p < 0.001) and consistently decreased in every patient with PAV+. Esophageal pressure–time product (PTPes) did not significantly differ between the two modes (107 cmH₂O/s/min [79–131] under PSV vs. 149 cmH₂O/s/min [129–170] under PAV+, p = 0.092), but the proportion of PTPes lost in ineffective triggering was significantly lower with PAV+ (2 cmH₂O/s/min [1–6] vs. 8 cmH₂O/s/min [3–30], p = 0.012).

Conclusions

Among patients with ineffective triggering under PSV, PSL adjustment failed to eliminate asynchrony in 58% of them (21 of 36 patients). In these patients with refractory ineffective triggering, switching from PSV to PAV+ significantly reduced or even suppressed the incidence of asynchrony.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13613-021-00935-0.

Patient-Self Inflicted Lung Injury: A Practical Review

Patients with severe lung injury usually have a high respiratory drive, resulting in intense inspiratory effort that may even worsen lung damage by several mechanisms gathered under the name “patient-self inflicted lung injury” (P-SILI). Even though no clinical study has yet demonstrated that a ventilatory strategy to limit the risk of P-SILI can improve the outcome, the concept of P-SILI relies on sound physiological reasoning, an accumulation of clinical observations and some consistent experimental data. In this review, we detail the main pathophysiological mechanisms by which the patient’s respiratory effort could become deleterious: excessive transpulmonary pressure resulting in over-distension; inhomogeneous distribution of transpulmonary pressure variations across the lung leading to cyclic opening/closing of nondependent regions and pendelluft phenomenon; increase in the transvascular pressure favoring the aggravation of pulmonary edema. We also describe potentially harmful patient-ventilator interactions. Finally, we discuss in a practical way how to detect in the clinical setting situations at risk for P-SILI and to what extent this recognition can help personalize the treatment strategy.

Respiratory Support Adjustments and Monitoring of Mechanically Ventilated Patients Performing Early Mobilization: A Scoping Review

This scoping review is aimed to summarize current knowledge on respiratory support adjustments and monitoring of metabolic and respiratory variables in mechanically ventilated adult patients performing early mobilization.

DATA SOURCES

Eight electronic databases were searched from inception to February 2021, using a predefined search strategy.

STUDY SELECTION

Two blinded reviewers performed document selection by title, abstract, and full text according to the following criteria: mechanically ventilated adult patients performing any mobilization intervention, respiratory support adjustments, and/or monitoring of metabolic/respiratory real-time variables.

DATA EXTRACTION

Four physiotherapists extracted relevant information using a prespecified template.

DATA SYNTHESIS

From 1,208 references screened, 35 documents were selected for analysis, where 20 (57%) were published between 2016 and 2020. Respiratory support settings (ventilatory modes or respiratory variables) were reported in 21 documents (60%). Reported modes were assisted (n = 11) and assist-control (n = 9). Adjustment of variables and modes were identified in only seven documents (20%). The most frequent respiratory variable was the Fio2, and only four studies modified the level of ventilatory support. Mechanical ventilator brand/model used was not specified in 26 documents (74%). Monitoring of respiratory, metabolic, and both variables were reported in 22 documents (63%), four documents (11%) and 10 documents (29%), respectively. These variables were reported to assess the physiologic response (n = 21) or safety (n = 13). Monitored variables were mostly respiratory rate (n = 26), pulse oximetry (n = 22), and oxygen consumption (n = 9). Remarkably, no study assessed the work of breathing or effort during mobilization.

CONCLUSIONS

Little information on respiratory support adjustments during mobilization of mechanically ventilated patients was identified. Monitoring of metabolic and respiratory variables is also scant. More studies on the effects of adjustments of the level/mode of ventilatory support on exercise performance and respiratory muscle activity monitoring for safe and efficient implementation of early mobilization in mechanically ventilated patients are needed.

Lung- and Diaphragm-Protective Ventilation

Mechanical ventilation can cause acute diaphragm atrophy and injury, and this is associated with poor clinical outcomes. Although the importance and impact of lung-protective ventilation is widely appreciated and well established, the concept of diaphragm-protective ventilation has recently emerged as a potential complementary therapeutic strategy. This Perspective, developed from discussions at a meeting of international experts convened by PLUG (the Pleural Pressure Working Group) of the European Society of Intensive Care Medicine, outlines a conceptual framework for an integrated lung- and diaphragm-protective approach to mechanical ventilation on the basis of growing evidence about mechanisms of injury. We propose targets for diaphragm protection based on respiratory effort and patient-ventilator synchrony. The potential for conflict between diaphragm protection and lung protection under certain conditions is discussed; we emphasize that when conflicts arise, lung protection must be prioritized over diaphragm protection. Monitoring respiratory effort is essential to concomitantly protect both the diaphragm and the lung during mechanical ventilation. To implement lung- and diaphragm-protective ventilation, new approaches to monitoring, to setting the ventilator, and to titrating sedation will be required. Adjunctive interventions, including extracorporeal life support techniques, phrenic nerve stimulation, and clinical decision-support systems, may also play an important role in selected patients in the future. Evaluating the clinical impact of this new paradigm will be challenging, owing to the complexity of the intervention. The concept of lung- and diaphragm-protective ventilation presents a new opportunity to potentially improve clinical outcomes for critically ill patients.

Proportional modes of ventilation: technology to assist physiology

Proportional modes of ventilation assist the patient by adapting to his/her effort, which contrasts with all other modes. The two proportional modes are referred to as neurally adjusted ventilatory assist (NAVA) and proportional assist ventilation with load-adjustable gain factors (PAV+): they deliver inspiratory assist in proportion to the patient’s effort, and hence directly respond to changes in ventilatory needs. Due to their working principles, NAVA and PAV+ have the ability to provide self-adjusted lung and diaphragm-protective ventilation. As these proportional modes differ from ‘classical’ modes such as pressure support ventilation (PSV), setting the inspiratory assist level is often puzzling for clinicians at the bedside as it is not based on usual parameters such as tidal volumes and PaCO₂ targets. This paper provides an in-depth overview of the working principles of NAVA and PAV+ and the physiological differences with PSV. Understanding these differences is fundamental for applying any assisted mode at the bedside. We review different methods for setting inspiratory assist during NAVA and PAV+ , and (future) indices for monitoring of patient effort. Last, differences with automated modes are mentioned.

Detection of strong inspiratory efforts from the analysis of central venous pressure swings: a preliminary clinical study

BACKGROUND

Swings of central venous pressure (ΔCVP) may reflect those of pleural and esophageal (ΔPES) pressure and, therefore, the strength of inspiration. Strong inspiratory efforts can produce some harm. Herein we preliminarily assessed the diagnostic accuracy of ΔCVP for strong inspiratory efforts in critically-ill subjects breathing spontaneously.

METHODS

We measured ΔCVP and ΔPES in 48 critically-ill subjects breathing spontaneously with zero end-expiratory pressure (ZEEP) or 10 cmH<inf>2</inf>O of continuous positive airway pressure (CPAP). The overall diagnostic accuracy of ΔCVP for strong inspiratory efforts (arbitrarily defined as ΔPES >8 mmHg) was described as the area under the receiver operating characteristic (ROC) curve, with 0.50 indicating random guess. The agreement between ΔCVP and ΔPES was assessed with the Bland-Altman analysis.

RESULTS

ΔCVP recognized strong inspiratory efforts with an area under the ROC curve of 0.95 (95% confidence intervals, 0.85-0.99) with ZEEP and 0.89 (0.76-0.96) with CPAP, both significantly larger than 0.50 (P<0.001). With the best cut-off value around 8 mmHg, the diagnostic accuracy of ΔCVP was 0.92 (0.80-0.98) with ZEEP and 0.94 (0.83-0.99) with CPAP. With ZEEP, the median difference between ΔCVP and ΔPES (bias) was -0.2 mmHg, and the 95% limits of agreement (LoA) were -3.9 and +5.5 mmHg. With CPAP, bias was -0.1 mmHg, and 95%-LoA were -5.8 and +4.5 mmHg. In both cases, ΔCVP correlated with ΔPES (r<inf>s</inf> 0.81 and 0.67; P<0.001 for both).

CONCLUSIONS

In critically-ill subjects breathing spontaneously, ΔCVP recognized strong inspiratory efforts with acceptable accuracy. Even so, it sometimes largely differed from ∆PES.

Effects of assist parameter on the performance of proportional assist ventilation in a lung model of chronic obstructive pulmonary disease

BACKGROUND

How the assist parameters affect synchronization and inspiratory workload in proportional assist ventilation (PAV) remains unknown.

PURPOSE

This bench study aimed to optimize the PAV parameters by evaluating their effects on patient-ventilator synchrony and work of breathing (WOB) in a chronic obstructive pulmonary disease (COPD) model during noninvasive ventilation, compared with the pressure support ventilation (PSV) mode.

METHODS

The Respironics V60 ventilator was connected to an ASL5000 lung simulator, which simulates lung mechanics in COPD (compliance, 50mL/cmH₂O; expiratory resistance, 20 cmH₂O/L/s; respiratory rate, 15 breaths/min; inspiratory time, 1.6 s). PAV was applied with different assistance levels, including flow assist (FA, 40-90% respiratory resistance) and volume assist (VA, 50-90% elastance). PSV was assessed using the same model. Measurements were obtained at a leak flow rate of 25-28 L/min. Performance characteristics, simulator-ventilator synchrony, and WOB were assessed.

RESULTS

Runaway was prone to occur, and severe premature cycling was observed with VA75+FA level>65%. Compared with PSV, lower tidal volume (≤400mL) was observed during PAV with VA75+FA40-50 and FA50+VA40-80; similar and improved cycling synchrony was observed for FA50+VA80 and FA50+VA90 (cycling delay: -117.60±6.13 and -61.50±8.03 vs. -101.20±7.32ms). The reduced triggering workload was observed for VA75+FA60-65 and FA50+VA80-90. Total and patient WOB was improved with all tested assist level combinations, except for FA50+VA90.

CONCLUSIONS

PAV reduces WOB but can induce asynchrony if improper settings are set, but the most optimal settings still need more clinical observations.

Monitoring Patient Respiratory Effort During Mechanical Ventilation: Lung and Diaphragm-Protective Ventilation

This article is one of ten reviews selected from the Annual Update in Intensive Care and Emergency Medicine 2020. Other selected articles can be found online at https://www.biomedcentral.com/collections/annualupdate2020. Further information about the Annual Update in Intensive Care and Emergency Medicine is available from http://www.springer.com/series/8901.

Respiratory drive in the acute respiratory distress syndrome: pathophysiology, monitoring, and therapeutic interventions

Neural respiratory drive, i.e., the activity of respiratory centres controlling breathing, is an overlooked physiologic variable which affects the pathophysiology and the clinical outcome of acute respiratory distress syndrome (ARDS). Spontaneous breathing may offer multiple physiologic benefits in these patients, including decreased need for sedation, preserved diaphragm activity and improved cardiovascular function. However, excessive effort to breathe due to high respiratory drive may lead to patient self-inflicted lung injury (P-SILI), even in the absence of mechanical ventilation. In the present review, we focus on the physiological and clinical implications of control of respiratory drive in ARDS patients. We summarize the main determinants of neural respiratory drive and the mechanisms involved in its potentiation, in health and ARDS. We also describe potential and pitfalls of the available bedside methods for drive assessment and explore classical and more “futuristic” interventions to control drive in ARDS patients.

Comparison of Inspiratory Effort, Workload and Cycling Synchronization Between Non-Invasive Proportional-Assist Ventilation and Pressure-Support Ventilation Using Different Models of Respiratory Mechanics

Background

This study assessed lung models for the influence of respiratory mechanics and inspiratory effort on breathing pattern and simulator-ventilator cycling synchronization in non-invasive ventilation.

Material/Methods

A Respironics V60 ventilator was connected to an active lung simulator modeling mildly restrictive, severely restrictive, obstructive and mixed obstructive/restrictive profiles. Pressure-support ventilation (PSV) and proportional-assist ventilation (PAV) were set to obtain similar tidal volume (V_T). PAV was applied at flow assist (FA) 40–90% of resistance (Rrs) and volume assist (VA) 40–90% of elastance (Ers). Measurements were performed with system air leak of 25–28 L/minute. Ventilator performance and simulator-ventilator asynchrony were evaluated.

Results

At comparable V_T, PAV had slightly lower peak inspiratory flow and higher driving pressure compared with PSV. Premature cycling occurred in the obstructive, severely restrictive and mildly restrictive models. During PAV, time for airway pressure to achieve 90% of maximum during inspiration (T90) in the severely restrictive model was shorter than those of the obstructive and mixed obstructive/restrictive models and close to that measured in the PSV mode. Increasing FA level reduced inspiratory trigger workload (PTP₃₀₀) in obstructive and mixed obstructive/restrictive models. Increasing FA level decreased inspiratory time (T_I) and tended to aggravate premature cycling, whereas increasing VA level attenuated this effect.

Conclusions

PAV with an appropriate combination of FA and VA decreases work of breathing during the inspiratory phase and improves simulator-ventilator cycling synchrony. FA has greater impact than VA in the adaptation to inspiratory effort demand. High VA level might help improve cycling synchrony.

A novel non-invasive method to detect excessively high respiratory effort and dynamic transpulmonary driving pressure during mechanical ventilation

Background

Excessive respiratory muscle effort during mechanical ventilation may cause patient self-inflicted lung injury and load-induced diaphragm myotrauma, but there are no non-invasive methods to reliably detect elevated transpulmonary driving pressure and elevated respiratory muscle effort during assisted ventilation. We hypothesized that the swing in airway pressure generated by respiratory muscle effort under assisted ventilation when the airway is briefly occluded (ΔP_occ) could be used as a highly feasible non-invasive technique to screen for these conditions.

Methods

Respiratory muscle pressure (P_mus), dynamic transpulmonary driving pressure (ΔP_L,dyn, the difference between peak and end-expiratory transpulmonary pressure), and ΔP_occ were measured daily in mechanically ventilated patients in two ICUs in Toronto, Canada. A conversion factor to predict ΔP_L,dyn and P_mus from ΔP_occ was derived and validated using cross-validation. External validity was assessed in an independent cohort (Nanjing, China).

Results

Fifty-two daily recordings were collected in 16 patients. In this sample, P_mus and ΔP_L were frequently excessively high: P_mus exceeded 10 cm H₂O on 84% of study days and ΔP_L,dyn exceeded 15 cm H₂O on 53% of study days. ΔP_occ measurements accurately detected P_mus > 10 cm H₂O (AUROC 0.92, 95% CI 0.83–0.97) and ΔP_L,dyn > 15 cm H₂O (AUROC 0.93, 95% CI 0.86–0.99). In the external validation cohort (n = 12), estimating P_mus and ΔP_L,dyn from ΔP_occ measurements detected excessively high P_mus and ΔP_L,dyn with similar accuracy (AUROC ≥ 0.94).

Conclusions

Measuring ΔP_occ enables accurate non-invasive detection of elevated respiratory muscle pressure and transpulmonary driving pressure. Excessive respiratory effort and transpulmonary driving pressure may be frequent in spontaneously breathing ventilated patients.

Standardized Unloading of Respiratory Muscles during Neurally Adjusted Ventilatory Assist: A Randomized Crossover Pilot Study

WHAT WE ALREADY KNOW ABOUT THIS TOPIC

WHAT THIS ARTICLE TELLS US THAT IS NEW: BACKGROUND:: Currently, there is no standardized method to set the support level in neurally adjusted ventilatory assist (NAVA). The primary aim was to explore the feasibility of titrating NAVA to specific diaphragm unloading targets, based on the neuroventilatory efficiency (NVE) index. The secondary outcome was to investigate the effect of reduced diaphragm unloading on distribution of lung ventilation.

METHODS

This is a randomized crossover study between pressure support and NAVA at different diaphragm unloading at a single neurointensive care unit. Ten adult patients who had started weaning from mechanical ventilation completed the study. Two unloading targets were used: 40 and 60%. The NVE index was used to guide the titration of the assist in NAVA. Electrical impedance tomography data, blood-gas samples, and ventilatory parameters were collected.

RESULTS

The median unloading was 43% (interquartile range 32, 60) for 40% unloading target and 60% (interquartile range 47, 69) for 60% unloading target. NAVA with 40% unloading led to more dorsal ventilation (center of ventilation at 55% [51, 56]) compared with pressure support (52% [49, 56]; P = 0.019). No differences were found in oxygenation, CO2, and respiratory parameters. The electrical activity of the diaphragm was higher during NAVA with 40% unloading than in pressure support.

CONCLUSIONS

In this pilot study, NAVA could be titrated to different diaphragm unloading levels based on the NVE index. Less unloading was associated with greater diaphragm activity and improved ventilation of the dependent lung regions.

A Pilot Bench Study of Decision Support for Proportional Assist Ventilation

The purpose was to develop a bench setup for testing a decision support system (DSS) for proportional assist ventilation (PAV). The test setup was based on a patient simulator connected to a mechanical ventilator with the DSS measurement sensors connected to the respiratory circuit. A test case was developed with parameters of lung mechanics reflecting a patient with mild acute respiratory distress syndrome. Five experiments were performed starting at different levels of percentage support (%Supp) and continuing until the DSS advised to remain at current settings. Final advice ranged from %Supp of 50-70%, indicating some dependence of baseline level, but with resulting patient effort estimates indicating that this may not be clinically important. Further studies are required of test cases reflecting different patient types and in patients.

Proportional modes versus pressure support ventilation: a systematic review and meta-analysis

Background

Proportional modes (proportional assist ventilation, PAV, and neurally adjusted ventilatory assist, NAVA) could improve patient–ventilator interaction and consequently may be efficient as a weaning mode. The purpose of this systematic review is to examine whether proportional modes improved patient–ventilator interaction and whether they had an impact on the weaning success and length of mechanical ventilation, in comparison with PSV.

Methods

We searched PubMed, EMBASE, and the Cochrane Central Register of Controlled Trials from inception through May 13, 2018. We included both parallel-group and crossover randomized studies that examined the efficacy of proportional modes in comparison with PSV in mechanically ventilated adults. The primary outcomes were (1) asynchrony index (AI), (2) weaning failure, and (3) duration of mechanical ventilation.

Results

We included 15 studies (four evaluated PAV, ten evaluated NAVA, and one evaluated both modes). Although the use of proportional modes was not associated with a reduction in AI (WMD − 1.43; 95% CI − 3.11 to 0.25; p = 0.096; PAV—one study, and NAVA—seven studies), the use of proportional modes was associated with a reduction in patients with AI > 10% (RR 0.15; 95% CI 0.04–0.58; p = 0.006; PAV—two studies, and NAVA—five studies), compared with PSV. There was a significant heterogeneity among studies for AI, especially with NAVA. Compared with PSV, use of proportional modes was associated with a reduction in weaning failure (RR 0.44; 95% CI 0.26–0.75; p = 0.003; PAV—three studies) and duration of mechanical ventilation (WMD − 1.78 days; 95% CI − 3.24 to − 0.32; p = 0.017; PAV—three studies, and NAVA—two studies). Reduced duration of mechanical ventilation was found with PAV but not with NAVA.

Conclusion

The use of proportional modes was associated with a reduction in the incidence with AI > 10%, weaning failure and duration of mechanical ventilation, compared with PSV. However, reduced weaning failure and duration of mechanical ventilation were found with only PAV. Due to a significant heterogeneity among studies and an insufficient number of studies, further investigation seems warranted to better understand the impact of proportional modes.

Clinical trial registration PROSPERO registration number, CRD42017059791. Registered 20 March 2017

Electronic supplementary material

The online version of this article (10.1186/s13613-018-0470-y) contains supplementary material, which is available to authorized users.

Assessing breathing effort in mechanical ventilation: physiology and clinical implications

Recent studies have shown both beneficial and detrimental effects of patient breathing effort in mechanical ventilation. Quantification of breathing effort may allow the clinician to titrate ventilator support to physiological levels of respiratory muscle activity. In this review we will describe the physiological background and methodological issues of the most frequently used methods to quantify breathing effort, including esophageal pressure measurement, the work of breathing, the pressure-time-product, electromyography and ultrasound. We will also discuss the level of breathing effort that may be considered optimal during mechanical ventilation at different stages of critical illness.

Oxygen Therapeutics and Mechanical Ventilation Advances

Advances in intensive care unit (ICU) therapeutics are plentiful and rooted in technological enhancements as well as recognition of patient care priorities. A plethora of new devices and modes are available for use to enhance patient safety and support liberation from mechanical ventilation while preserving oxygenation and carbon dioxide clearance. Increased penetrance of closed loop systems is one means to reduce care variation in appropriate populations. The intelligent design of the ICU space needs to integrate the footprint of that device and the data streaming from it into a coherent whole that supports patient, family, and caregivers.

Critical illness-associated diaphragm weakness

Diaphragm weakness is highly prevalent in critically ill patients. It may exist prior to ICU admission and may precipitate the need for mechanical ventilation but it also frequently develops during the ICU stay. Several risk factors for diaphragm weakness have been identified; among them sepsis and mechanical ventilation play central roles. We employ the term critical illness-associated diaphragm weakness to refer to the collective effects of all mechanisms of diaphragm injury and weakness occurring in critically ill patients. Critical illness-associated diaphragm weakness is consistently associated with poor outcomes including increased ICU mortality, difficult weaning, and prolonged duration of mechanical ventilation. Bedside techniques for assessing the respiratory muscles promise to improve detection of diaphragm weakness and enable preventive or curative strategies. Inspiratory muscle training and pharmacological interventions may improve respiratory muscle function but data on clinical outcomes remain limited.

Mechanical Ventilation: State of the Art

Mechanical ventilation is the most used short-term life support technique worldwide and is applied daily for a diverse spectrum of indications, from scheduled surgical procedures to acute organ failure. This state-of-the-art review provides an update on the basic physiology of respiratory mechanics, the working principles, and the main ventilatory settings, as well as the potential complications of mechanical ventilation. Specific ventilatory approaches in particular situations such as acute respiratory distress syndrome and chronic obstructive pulmonary disease are detailed along with protective ventilation in patients with normal lungs. We also highlight recent data on patient-ventilator dyssynchrony, humidified high-flow oxygen through nasal cannula, extracorporeal life support, and the weaning phase. Finally, we discuss the future of mechanical ventilation, addressing avenues for improvement.

A Pilot Randomized Trial Comparing Weaning From Mechanical Ventilation on Pressure Support Versus Proportional Assist Ventilation

OBJECTIVES

Despite protocols incorporating spontaneous breathing trials, 31% of ICU patients experience difficult or prolonged weaning from mechanical ventilation. Nonfatiguing modes such as pressure support ventilation are recommended. Proportional assist ventilation provides assistance in proportion to patient effort, which may optimize weaning. However, it is not known how proportional assist ventilation performs relative to pressure support ventilation over a prolonged period in the complex ICU setting. The purpose of this study was to compare the physiologic and clinical performance (failure rate), safety, and feasibility of protocols using daily spontaneous breathing trial plus pressure support ventilation versus proportional assist ventilation until ventilation discontinuation.

DESIGN

Single-center, unblinded pilot randomized controlled trial.

SETTING

Medical-surgical ICU of a tertiary-care hospital.

PATIENTS

Adult patients intubated greater than 36 hours were randomized if they met eligibility criteria for partial ventilatory support, tolerated pressure support ventilation greater than or equal to 30 minutes, and either failed or did not meet criteria for a spontaneous breathing trial.

INTERVENTIONS

Patients were randomized to the pressure support ventilation or proportional assist ventilation protocol (PAV+, Puritan Bennett 840; Covidien, Boulder, CO). Both protocols used progressive decreases in level of assistance as tolerated, coupled with daily assessment for spontaneous breathing trials.

MEASUREMENTS AND MAIN RESULTS

Of 54 patients randomized, outcome data are available for 50 patients; 27 were randomized to receive proportional assist ventilation and 23 to receive pressure support ventilation. There were no adverse events linked to the study interventions, and protocol violations were infrequent. Recruitment was slower than projected (1.3 patients per month). The median (interquartile range) time from randomization to successful extubation was 3.9 days (2.8-8.4 d) on proportional assist ventilation versus 4.9 days (2.9-26.3 d) on pressure support ventilation (p = 0.39). Time to live ICU discharge was 7.3 days (5.2-11.4 d) on proportional assist ventilation versus 12.4 days (7.5-30.8 d) on pressure support ventilation (p = 0.03).

CONCLUSION

This pilot study demonstrates the utility, safety, and feasibility of the weaning protocols and provides important information to guide the design of a future randomized controlled trial comparing weaning from mechanical ventilation on pressure support ventilation versus proportional assist ventilation.

Limited predictability of maximal muscular pressure using the difference between peak airway pressure and positive end-expiratory pressure during proportional assist ventilation (PAV)

Background

If the proportional assist ventilation (PAV) level is known, muscular effort can be estimated from the difference between peak airway pressure and positive end-expiratory pressure (PEEP) (ΔP) during PAV. We conjectured that deducing muscle pressure from ΔP may be an interesting method to set PAV, and tested this hypothesis using the oesophageal pressure time product calculation.

Methods

Eleven mechanically ventilated patients with oesophageal pressure monitoring under PAV were enrolled. Patients were randomly assigned to seven assist levels (20–80%, PAV20 means 20% PAV gain) for 15 min. Maximal muscular pressure calculated from oesophageal pressure (P_{mus, oes}) and from ΔP (P_{mus, aw}) and inspiratory pressure time product derived from oesophageal pressure (PTP_oes) and from ΔP (PTP_aw) were determined from the last minute of each level. P_{mus, oes} and PTP_oes with consideration of PEEPi were expressed as P_{mus, oes, PEEPi} and PTP_{oes, PEEPi}, respectively. Pressure time product was expressed as per minute (PTP_oes, PTP_{oes, PEEPi}, PTP_aw) and per breath (PTP_{oes, br}, PTP_{oes, PEEPi, br}, PTP_{aw, br}).

Results

PAV significantly reduced the breathing effort of patients with increasing PAV gain (PTP_oes 214.3 ± 80.0 at PAV20 vs. 83.7 ± 49.3 cmH₂O•s/min at PAV80, PTP_{oes, PEEPi} 277.3 ± 96.4 at PAV20 vs. 121.4 ± 71.6 cmH₂O•s/min at PAV80, p < 0.0001). P_{mus, aw} overestimates P_{mus, oes} for low-gain PAV and underestimates P_{mus, oes} for moderate-gain to high-gain PAV. An optimal P_{mus, aw} could be achieved in 91% of cases with PAV60. When the PAV gain was adjusted to P_{mus, aw} of 5–10 cmH₂O, there was a 93% probability of PTP_oes <224 cmH₂O•s/min and 88% probability of PTP_{oes, PEEPi} < 255 cmH₂O•s/min.

Conclusion

Deducing maximal muscular pressure from ΔP during PAV has limited accuracy. The extrapolated pressure time product from ΔP is usually less than the pressure time product calculated from oesophageal pressure tracing. However, when the PAV gain was adjusted to P_{mus, aw} of 5–10 cmH₂O, there was a 90% probability of PTP_oes and PTP_{oes, PEEPi} within acceptable ranges. This information should be considered when applying ΔP to set PAV under various gains.

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Comparison Between Neurally Adjusted Ventilatory Assist and Pressure Support Ventilation Levels in Terms of Respiratory Effort

OBJECTIVES

To understand the potential equivalence between neurally adjusted ventilatory assist and pressure support ventilation levels in terms of respiratory muscle unloading. To compare the respiratory pattern, variability, synchronization, and neuromuscular coupling within comparable ranges of assistance.

DESIGN

Prospective single-center physiologic study.

SETTING

A 13-bed university medical ICU.

PATIENTS

Eleven patients recovering from respiratory failure.

INTERVENTIONS

The following levels of assistance were consecutively applied in a random order: neurally adjusted ventilatory assist levels: 0.5, 1, 1.5, 2, 2.5, 3, 4, 5, and 7 cm H2O/μvolt; pressure support levels: 7, 10, 15, 20, and 25 cm H2O.

MEASUREMENTS AND MAIN RESULTS

Flow, airway pressure, esophageal pressures, and peak electrical activity of the diaphragm were continuously recorded. Breathing effort was calculated. To express the percentage of assist assumed by the ventilator, the total pressure including muscular and ventilator pressure was calculated. The median percentage of assist ranged from 33% (24-47%) to 82% (72-90%) between pressure support 7 and 25 cm H2O. Similar levels of unloading were observed for neurally adjusted ventilatory assist levels from 0.5 cm H2O/μvolt (46% [40-51%]) to 2.5 cm H2O/μvolt (80% [74-84%]). Tidal variability was higher during neurally adjusted ventilatory assist and ineffective efforts appeared only in pressure support. In neurally adjusted ventilatory assist, double triggering occurred sometimes when electrical activity of the diaphragm signal depicted a biphasic aspect, and an abnormal oscillatory pattern was frequently observed from 4 cm H2O/μvolt. For both modes, the relationship between peak electrical activity of the diaphragm and muscle pressure depicted a curvilinear profile.

CONCLUSIONS

In patients recovering from acute respiratory failure, levels of neurally adjusted ventilatory assist between 0.5 and 2.5 cm H2O/μvolt are comparable to pressure support levels ranging from 7 to 25 cm H2O in terms of respiratory muscle unloading. Neurally adjusted ventilatory assist provides better patient-ventilator interactions but can be sometimes excessively sensitive to electrical activity of the diaphragm in terms of triggering.

Clinical challenges in mechanical ventilation

Mechanical ventilation supports gas exchange and alleviates the work of breathing when the respiratory muscles are overwhelmed by an acute pulmonary or systemic insult. Although mechanical ventilation is not generally considered a treatment for acute respiratory failure per se, ventilator management warrants close attention because inappropriate ventilation can result in injury to the lungs or respiratory muscles and worsen morbidity and mortality. Key clinical challenges include averting intubation in patients with respiratory failure with non-invasive techniques for respiratory support; delivering lung-protective ventilation to prevent ventilator-induced lung injury; maintaining adequate gas exchange in severely hypoxaemic patients; avoiding the development of ventilator-induced diaphragm dysfunction; and diagnosing and treating the many pathophysiological mechanisms that impair liberation from mechanical ventilation. Personalisation of mechanical ventilation based on individual physiological characteristics and responses to therapy can further improve outcomes.

Accuracy of delivered airway pressure and work of breathing estimation during proportional assist ventilation: a bench study

Background

Proportional assist ventilation+ (PAV+) delivers airway pressure (P_aw) in proportion to patient effort (P_mus) by using the equation of motion of the respiratory system. PAV+ calculates automatically respiratory mechanics (elastance and resistance); the work of breathing (WOB) is estimated by the ventilator. The accuracy of P_mus estimation and hence accuracy of the delivered P_aw and WOB calculation have not been assessed. This study aimed at assessing the accuracy of delivered P_aw and calculated WOB by PAV+ and examining the factors influencing this accuracy.

Methods

Using an active lung model with different respiratory mechanics, we compared (1) the actual delivered P_aw by the ventilator to the theoretical P_aw as defined by the equation of motion and (2) the WOB value displayed by the ventilator to the WOB measured from a Campbell diagram.

Results

Irrespective of respiratory mechanics and gain, the ventilator provided a P_aw approximately 25 % lower than expected. This underassistance was greatest at the beginning of the inspiration. Intrinsic PEEP (PEEPi), associated with an increase in trigger delay, was a major factor affecting PAV+ accuracy. The absolute value of total WOB displayed by the ventilator was underestimated, but the changes in WOB were accurately detected by the ventilator.

Conclusion

The assistance provided by PAV+ well follows P_mus but with a constant underassistance. This is associated with an underestimation by the ventilator of the WOB. PEEPi can be a major factor contributing to PAV+ inaccuracy. Clinical recommendations should include using a high trigger sensitivity and a careful PEEP titration.

Electronic supplementary material

The online version of this article (doi:10.1186/s13613-016-0131-y) contains supplementary material, which is available to authorized users.

Assessing effort and work of breathing

PURPOSE OF REVIEW

Spontaneous breathing has been shown to induce both positive and negative effects on the function and on injury of lungs and diaphragm during critical illness; thus, monitoring of the breathing effort generated by the patient might be valuable for a better understanding of the mechanisms of disease and to set properly ventilation. The purpose of this review is to summarize the recent findings on the different techniques available to measure the patient's breathing effort, mainly during spontaneous assisted ventilation.

RECENT FINDINGS

Although esophageal pressure measurement remains the solid reference technique to quantitate the breathing effort, other tools have been developed and tested. These include the diaphragmatic electromyogram, whose voltage is linearly related to the pressure generated by the diaphragm, ultrasound, which relies on the measurement of diaphragmatic displacement or thickening, and other approaches, which derive breathing effort solely from the airway flow and pressure tracings.

SUMMARY

The development of measurement techniques and their introduction in clinical practice will allow us to understand the role of spontaneous breathing effort in the pathophysiology of lung injury and weaning failure, and how to adjust the breathing workload in an individual patient.

The application of esophageal pressure measurement in patients with respiratory failure

This report summarizes current physiological and technical knowledge on esophageal pressure (Pes) measurements in patients receiving mechanical ventilation. The respiratory changes in Pes are representative of changes in pleural pressure. The difference between airway pressure (Paw) and Pes is a valid estimate of transpulmonary pressure. Pes helps determine what fraction of Paw is applied to overcome lung and chest wall elastance. Pes is usually measured via a catheter with an air-filled thin-walled latex balloon inserted nasally or orally. To validate Pes measurement, a dynamic occlusion test measures the ratio of change in Pes to change in Paw during inspiratory efforts against a closed airway. A ratio close to unity indicates that the system provides a valid measurement. Provided transpulmonary pressure is the lung-distending pressure, and that chest wall elastance may vary among individuals, a physiologically based ventilator strategy should take the transpulmonary pressure into account. For monitoring purposes, clinicians rely mostly on Paw and flow waveforms. However, these measurements may mask profound patient-ventilator asynchrony and do not allow respiratory muscle effort assessment. Pes also permits the measurement of transmural vascular pressures during both passive and active breathing. Pes measurements have enhanced our understanding of the pathophysiology of acute lung injury, patient-ventilator interaction, and weaning failure. The use of Pes for positive end-expiratory pressure titration may help improve oxygenation and compliance. Pes measurements make it feasible to individualize the level of muscle effort during mechanical ventilation and weaning. The time is now right to apply the knowledge obtained with Pes to improve the management of critically ill and ventilator-dependent patients.

[Pressure support ventilation and proportional assist ventilation during weaning from mechanical ventilation]

OBJECTIVES

To compare tolerance, duration of mechanical ventilation (MV) and clinical outcomes during weaning from MV in patients subjected to either pressure support ventilation (PSV) or proportional assist ventilation (PAV).

DESIGN

A prospective, observational study was carried out.

SETTING

Intensive Care Unit.

PATIENTS

A total of 40 consecutive subjects were allocated to either the PSV or the PAV group until each group contained 20 patients. Patients were included in the study when they met the criteria to begin weaning and the attending physician decided to initiate the weaning process. The physician selected the modality and set the ventilatory parameters.

INTERVENTIONS

None.

VARIABLES OF INTEREST

Demographic data, respiratory mechanics, ventilatory parameters, duration of MV, and clinical outcomes (reintubation, tracheostomy, mortality).

RESULTS

Baseline characteristics were similar in both groups. No significant differences were observed between the PSV and PAV groups in terms of the total duration of MV (10 [5-18] vs. 9 [7-19] days; P=.85), reintubation (5 [31%] vs. 3 [19%]; P=.69), or mortality (4 [20%] vs. 5 [25%] deaths; P=1). Eight patients (40%) in the PSV group and 6 patients (30%) in the PAV group (P=.74) required a return to volume assist-control ventilation due to clinical deterioration.

CONCLUSIONS

Tolerance, duration of MV and clinical outcomes during weaning from mechanical ventilation were similar in PSV and PAV.