Assessment of patient-ventilator breath contribution during neurally adjusted ventilatory assist in patients with acute respiratory failure

Estimation of transpulmonary driving pressure during synchronized mechanical ventilation using a single lower assist maneuver (LAM) in rabbits: a comparison to measurements made with an esophageal balloon

BACKGROUND

Mechanical ventilation is applied to unload the respiratory muscles, but knowledge about transpulmonary driving pressure (ΔPL) is important to minimize lung injury. We propose a method to estimate ΔPL during neurally synchronized assisted ventilation, with a simple intervention of lowering the assist for one breath ("lower assist maneuver", LAM).

METHODS

In 24 rabbits breathing spontaneously with imposed loads, titrations of increasing assist were performed, with two neurally synchronized modes: neurally adjusted ventilatory assist (NAVA) and neurally triggered pressure support (NPS). Two single LAM breaths (not sequentially, but independently) were performed at each level of assist by acutely setting the assist to zero cm H2O (NPS) or NAVA level 0 cm H2O/uV (NAVA) for one breath. NPS and NAVA titrations were followed by titrations in controlled-modes (volume control, VC and pressure control, PC), under neuro-muscular blockade. Breaths from the NAVA/NPS titrations were matched (for flow and volume) to VC or PC. Throughout all runs, we measured diaphragm electrical activity (Edi) and esophageal pressure (PES). We measured ΔPL during the spontaneous modes (PL_PES) and controlled mechanical ventilation (CMV) modes (PL_CMV) with the esophageal balloon. From the LAMs, we derived an estimation of ΔPL ("PL_LAM") using a correction factor (ratio of volume during the LAM and volume during assist) and compared it to measured ΔPL during passive (VC or PC) and spontaneous breathing (NAVA or NPS). A requirement for the LAM was similar Edi to the assisted breath.

RESULTS

All animals successfully underwent titrations and LAMs for NPS/NAVA. One thousand seven-hundred ninety-two (1792) breaths were matched to passive ventilation titrations (matched Vt, r = 0.99). PL_LAM demonstrated strong correlation with PL_CMV (r = 0.83), and PL_PES (r = 0.77). Bland-Altman analysis revealed little difference between the predicted PL_LAM and measured PL_CMV (Bias = 0.49 cm H2O and 1.96SD = 3.09 cm H2O). For PL_PES, the bias was 2.2 cm H2O and 1.96SD was 3.4 cm H2O. Analysis of Edi and PES at peak Edi showed progressively increasing uncoupling with increasing assist.

CONCLUSION

During synchronized mechanical ventilation, a LAM breath allows for estimations of transpulmonary driving pressure, without measuring PES, and follows a mathematical transfer function to describe respiratory muscle unloading during synchronized assist.

Performance of Noninvasive Airway Occlusion Maneuvers to Assess Lung Stress and Diaphragm Effort in Mechanically Ventilated Critically Ill Patients

BACKGROUND

Monitoring and controlling lung stress and diaphragm effort has been hypothesized to limit lung injury and diaphragm injury. The occluded inspiratory airway pressure (Pocc) and the airway occlusion pressure at 100 ms (P0.1) have been used as noninvasive methods to assess lung stress and respiratory muscle effort, but comparative performance of these measures and their correlation to diaphragm effort is unknown. The authors hypothesized that Pocc and P0.1 correlate with diaphragm effort and lung stress and would have strong discriminative performance in identifying extremes of lung stress and diaphragm effort.

METHODS

Change in transdiaphragmatic pressure and transpulmonary pressure was obtained with double-balloon nasogastric catheters in critically ill patients (n = 38). Pocc and P0.1 were measured every 1 to 3 h. Correlations between Pocc and P0.1 with change in transdiaphragmatic pressure and transpulmonary pressure were computed from patients from the first cohort. Accuracy of Pocc and P0.1 to identify patients with extremes of lung stress (change in transpulmonary pressure > 20 cm H2O) and diaphragm effort (change in transdiaphragmatic pressure < 3 cm H2O and >12 cm H2O) in the preceding hour was assessed with area under receiver operating characteristic curves. Cutoffs were validated in patients from the second cohort (n = 13).

RESULTS

Pocc and P0.1 correlate with change in transpulmonary pressure (R2 = 0.62 and 0.51, respectively) and change in transdiaphragmatic pressure (R2 = 0.53 and 0.22, respectively). Area under receiver operating characteristic curves to detect high lung stress is 0.90 (0.86 to 0.94) for Pocc and 0.88 (0.84 to 0.92) for P0.1. Area under receiver operating characteristic curves to detect low diaphragm effort is 0.97 (0.87 to 1.00) for Pocc and 0.93 (0.81 to 0.99) for P0.1. Area under receiver operating characteristic curves to detect high diaphragm effort is 0.86 (0.81 to 0.91) for Pocc and 0.73 (0.66 to 0.79) for P0.1. Performance was similar in the external dataset.

CONCLUSIONS

Pocc and P0.1 correlate with lung stress and diaphragm effort in the preceding hour. Diagnostic performance of Pocc and P0.1 to detect extremes in these parameters is reasonable to excellent. Pocc is more accurate in detecting high diaphragm effort.

EDITOR’S PERSPECTIVE

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.

Monitoring patient-ventilator breath contribution in the critically ill during neurally adjusted ventilatory assist: reliability and improved algorithms for bedside use

The patient-ventilator breath contribution (PVBC) index estimates the relative contribution of the patient to total tidal volume (Vt_insp) during mechanical ventilation in neurally adjusted ventilator assist mode and has been used to titrate ventilator support. The reliability of this index in ventilated patients is unknown and was investigated in this study. PVBC was calculated by comparing tidal volume (Vt_insp) and diaphragm electrical activity (EAdi) during assisted breaths (Vt_insp/EAdi)_assist and during unassisted breaths (Vt_insp/EAdi)_no-assist. Vt_insp was normalized to peak EAdi (EAdi_peak) using 1) one assisted breath, 2) five consecutive assisted breaths, or 3) five assisted breaths with matching EAdi preceding the unassisted breath (^N1PVBC², ^X5PVBC², and PX5VBCEAdi-matching2 , respectively). In addition, PVBC was calculated by comparing only Vt_insp for breaths with matching EAdi (PVBCβ²). Test-retest reliability of the different PVBC calculation methods was evaluated with the intraclass correlation coefficient (ICC) using five repeated PVBC maneuvers performed with a 1-min interval. In total, 125 PVBC maneuvers were analyzed in 25 patients. ICC [95% confidence interval] values were 0.46 [0.23-0.66], 0.51 [0.33-0.70], and 0.42 [0.14-0.69] for ^N1PVBC², ^X5PVBC², PX5VBCEAdi-matching2 , respectively. Complex waveform analyses showed that insufficient EAdi filtering by the ventilator software affects reliability of PVBC calculation. With our new EAdi-matching techniques reliability improved (PVBCβ² ICC: 0.78 [0.60-0.90]). We conclude that current techniques to calculate PVBC exhibit low reliability and that our newly developed criteria and estimation of PVBC-using Vt_insp of assisted breaths and unassisted breaths with matching EAdi-improves reliability. This may help implementation of PVBC in clinical practice. NEW & NOTEWORTHY The patient-ventilator breath contribution (PVBC) index estimates the relative contribution of the patient to tidal volume generated by the patient and the mechanical ventilator during mechanical ventilation in neurally adjusted ventilator assist mode. It could be used to titrate ventilator support and thus to limit development of diaphragm dysfunction in intensive care unit patients. Currently available methods for bedside assessment of PVBC are unreliable. Our newly developed criteria and estimation of PVBC largely improve reliability and help to quantify patient contribution to total inspiratory effort.

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.

Estimation of the diaphragm neuromuscular efficiency index in mechanically ventilated critically ill patients

Background

Diaphragm dysfunction develops frequently in ventilated intensive care unit (ICU) patients. Both disuse atrophy (ventilator over-assist) and high respiratory muscle effort (ventilator under-assist) seem to be involved. A strong rationale exists to monitor diaphragm effort and titrate support to maintain respiratory muscle activity within physiological limits. Diaphragm electromyography is used to quantify breathing effort and has been correlated with transdiaphragmatic pressure and esophageal pressure. The neuromuscular efficiency index (NME) can be used to estimate inspiratory effort, however its repeatability has not been investigated yet. Our goal is to evaluate NME repeatability during an end-expiratory occlusion (NMEoccl) and its use to estimate the pressure generated by the inspiratory muscles (Pmus).

Methods

This is a prospective cohort study, performed in a medical-surgical ICU. A total of 31 adult patients were included, all ventilated in neurally adjusted ventilator assist (NAVA) mode with an electrical activity of the diaphragm (EAdi) catheter in situ. At four time points within 72 h five repeated end-expiratory occlusion maneuvers were performed. NMEoccl was calculated by delta airway pressure (ΔPaw)/ΔEAdi and was used to estimate Pmus. The repeatability coefficient (RC) was calculated to investigate the NMEoccl variability.

Results

A total number of 459 maneuvers were obtained. At time T = 0 mean NMEoccl was 1.22 ± 0.86 cmH₂O/μV with a RC of 82.6%. This implies that when NMEoccl is 1.22 cmH₂O/μV, it is expected with a probability of 95% that the subsequent measured NMEoccl will be between 2.22 and 0.22 cmH2O/μV. Additional EAdi waveform analysis to correct for non-physiological appearing waveforms, did not improve NMEoccl variability. Selecting three out of five occlusions with the lowest variability reduced the RC to 29.8%.

Conclusions

Repeated measurements of NMEoccl exhibit high variability, limiting the ability of a single NMEoccl maneuver to estimate neuromuscular efficiency and therefore the pressure generated by the inspiratory muscles based on EAdi.

Electronic supplementary material

The online version of this article (10.1186/s13054-018-2172-0) contains supplementary material, which is available to authorized users.

Effects of neurally adjusted ventilatory assist on air distribution and dead space in patients with acute exacerbation of chronic obstructive pulmonary disease

Background

Neurally adjusted ventilatory assist (NAVA) could improve patient-ventilator interaction; its effects on ventilation distribution and dead space are still unknown. The aim of this study was to evaluate the effects of varying levels of assist during NAVA and pressure support ventilation (PSV) on ventilation distribution and dead space in patients with acute exacerbation of chronic obstructive pulmonary disease (AECOPD).

Methods

Fifteen mechanically ventilated patients with AECOPD were included in the study. The initial PSV levels were set to 10 cmH₂O for 10 min. Thereafter, the ventilator mode was changed to NAVA for another 10 min with the same electrical activity of the diaphragm as during PSV. Furthermore, the ventilation mode was switched between PSV and NAVA every 10 min in the following order: PSV 5 cmH₂O; NAVA 50%; PSV 15 cmH₂O; and NAVA 150% (relative to the initial NAVA support level). Ventilation distribution in the lung was evaluated in percentages in regions of interest (ROI) of four anteroposterior segments of equal height (ROI1 to ROI4 represents ventral, mid-ventral, mid-dorsal, and dorsal, respectively). Blood gases, ventilation distribution (electrical impedance tomography), diaphragm activity (B-mode ultrasonography), and dead space fraction (PeCO₂ and PaCO₂) were measured.

Results

The trigger and cycle delays were lower during NAVA than during PSV. The work of trigger was significantly lower during NAVA compared to PSV. The diaphragm activities based on ultrasonography were higher during NAVA compared to the same support level during PSV. The ventilation distribution in ROI4 increased significantly (P < 0.05) during NAVA compared to PSV (except for a support level of 50%). Similar results were found in ROI3 + 4. NAVA reduced dead space fraction compared to the corresponding support level of PSV.

Conclusions

NAVA was superior to PSV in AECOPD for increasing ventilation distribution in ROI4 and reducing dead space.

Trial registration

Clinicaltrials.gov, NCT02289573. Registered on 12 November 2014.

Novel insights in ICU-acquired respiratory muscle dysfunction: implications for clinical care

This article is one of ten reviews selected from the Annual Update in Intensive Care and Emergency medicine 2017. Other selected articles can be found online at http://ccforum.com/series/annualupdate2017. Further information about the Annual Update in Intensive Care and Emergency Medicine is available from http://www.springer.com/series/8901.

The differential diagnosis for failure to wean from mechanical ventilation

PURPOSE OF REVIEW

In this review, we discuss the causes for a failed weaning trial and specific diagnostic tests that could be conducted to identify the cause for weaning failure. We briefly highlight treatment strategies that may enhance the chance of weaning success.

RECENT FINDINGS

Impaired respiratory mechanics, respiratory muscle dysfunction, cardiac dysfunction, cognitive dysfunction, and metabolic disorders are recognized causes for weaning failure. In addition, iatrogenic factors may be at play. Most studies have focused on respiratory muscle dysfunction and cardiac dysfunction. Recent studies demonstrate that both ultrasound and electromyography are valuable tools to evaluate respiratory muscle function in ventilated patients. Sophisticated ultrasound techniques and biomarkers such as B-type natriuretic peptide, are valuable tools to identify cardiac dysfunction as a cause for weaning failure. Once a cause for weaning failure has been identified specific treatment should be instituted. Concerning treatment, both strength training and endurance training should be considered for patients with respiratory muscle weakness. Inotropes and vasodilators should be considered in case of heart failure.

SUMMARY

Understanding the complex pathophysiology of weaning failure in combination with a systematic diagnostic approach allows identification of the primary cause of weaning failure. This will help the clinician to choose a specific treatment strategy and therefore may fasten liberation from mechanical ventilation.

Endotoxemia accelerates diaphragm dysfunction in ventilated rabbits

BACKGROUND

Ventilators may induce diaphragm dysfunction, and most of the septic population who are admitted to the intensive care unit require mechanical ventilation. However, there is no evidence that sepsis accelerates the onset of ventilator-induced diaphragm dysfunction or affects the microcirculation. Our study investigated whether lipopolysaccharide (LPS)-induced endotoxemia accelerated diaphragm dysfunction in ventilated rabbits by evaluating microcirculation, lipid accumulation, and diaphragm contractility.

METHODS

After anesthesia and tracheostomy, 25 invasively monitored and mechanically ventilated New Zealand white rabbits were randomized to control (n = 5), controlled mechanical ventilation (CMV) (n = 5), pressure support ventilation (PSV; n = 5), CMV or PSV with LPS-induced endotoxemia (CMV-LPS and PSV-LPS, respectively; n = 5 for each). Rabbits were anesthetized and ventilated for 24 h, except the control rabbits (30 min). Diaphragmatic contractility was evaluated using neuromechanical and neuroventilatory efficiency. We evaluated the following at the end of the protocol: (1) diaphragm microcirculation; (2) lipid accumulation; and (3) diaphragm muscular fibers structure.

RESULTS

Diaphragm contractility, microcirculation, lipid accumulation, and fiber structures were severely compromised in endotoxemic animals after 24 h compared to nonendotoxemic rabbits. Moreover, a slight but significant increase in lipid accumulation was observed in CMV and PSV groups compared with controls (P < 0.05).

CONCLUSIONS

Endotoxemia accelerates the diaphragm dysfunction process in ventilated rabbits, affects the microcirculation, and results in diaphragmatic lipid accumulation and contractility impairment.

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.

Strategies to optimize respiratory muscle function in ICU patients

Respiratory muscle dysfunction may develop rapidly in critically ill ventilated patients and is associated with increased morbidity, length of intensive care unit stay, costs, and mortality. This review briefly discusses the pathophysiology of respiratory muscle dysfunction in intensive care unit patients and then focuses on strategies that prevent the development of muscle weakness or, if weakness has developed, how respiratory muscle function may be improved. We propose a simple strategy for how these can be implemented in clinical care.