Assessment of regional lung recruitment and derecruitment during a PEEP trial based on electrical impedance tomography

Objective To investigate whether electrical impedance tomography (EIT) is capable of monitoring regional lung recruitment and lung collapse during a positive end-expiratory pressure (PEEP) trial. Design Experimental animal study of acute lung injury. Subject Six pigs with saline-lavage-induced acute lung injury. Interventions An incremental and decremental PEEP trial at ten pressure levels was performed. Ventilatory, gas exchange, and hemodynamic parameters were automatically recorded. EIT and computed tomography (CT) scans of the same slice were simultaneously taken at each PEEP level. Measurements and results A significant correlation between EIT and CT analyses of end-expiratory gas volumes (r = 0.98 up to 0.99) and tidal volumes (r = 0.55 up to r = 0.88) could be demonstrated. Changes in global and regional tidal volumes and arterial oxygenation (PaO₂/FiO₂) demonstrated recruitment/derecruitment during the trial, but at different onsets. During the decremental trial, derecruitment first occurred in dependent lung areas. This was indicated by lowered regional tidal volumes measured in this area and by a decrease of PaO₂/FiO₂. At the same time, the global tidal volume still continued to increase, because the increase of ventilation of the non-dependent areas was higher than the loss in the dependent areas. This indicates that opposing regional changes might cancel each other out when combined in a global parameter. Conclusions EIT is suitable for monitoring the dynamic effects of PEEP variations on the regional change of tidal volume. It is superior to global ventilation parameters in assessing the beginning of alveolar recruitment and lung collapse.     

Carbon dioxide rebreathing during non-invasive ventilation delivered by helmet: a bench study

OBJECTIVE: To define how to monitor and limit CO(2) rebreathing during helmet ventilation. DESIGN: Physical model study. SETTING: Laboratory in a university teaching hospital. INTERVENTIONS: We applied pressure-control ventilation to a helmet mounted on a physical model. In series 1 we increased CO(2) production (V'CO(2)) from 100 to 550[Symbol: see text]ml/min and compared mean inhaled CO(2) (iCO(2),mean) with end-inspiratory CO(2) at airway opening (eiCO(2)), end-tidal CO(2) at Y-piece (yCO(2)) and mean CO(2) inside the helmet (hCO(2)). In series 2 we observed, at constant V'CO(2), effects on CO(2) rebreathing of inspiratory pressure, respiratory mechanics, the inflation of cushions inside the helmet and the addition of a flow-by. MEASUREMENTS AND RESULTS: In series 1, iCO(2),mean linearly related to V'CO(2). The best estimate of CO(2) rebreathing was provided by hCO(2): differences between iCO(2),mean and hCO(2), yCO(2) and eiCO(2) were 0.0[Symbol: see text]+/-[Symbol: see text]0.1, 0.4[Symbol: see text]+/-[Symbol: see text]0.2 and -1.3[Symbol: see text]+/-[Symbol: see text]0.5%. In series 2, hCO(2) inversely related to the total ventilation (MVtotal) delivered to the helmet-patient unit. The increase in inspiratory pressure significantly increased MVtotal and lowered hCO(2). The low lung compliance halved the patient:helmet ventilation ratio but led to minor changes in MVtotal and hCO(2). Cushion inflation, although it decreased the helmet's internal volume by 33%, did not affect rebreathing. A 8-l/min flow-by effectively decreased hCO(2). CONCLUSIONS: During helmet ventilation, rebreathing can be assessed by measuring hCO(2) or yCO(2), but not eiCO(2). It is directly related to V'CO(2), inversely related to MVtotal and can be lowered by increasing inspiratory pressure or adding a flow-by.     

Automatic adjustment of pressure support by a computer-driven knowledge-based system during noninvasive ventilation: a feasibility study

Objective

To evaluate the feasibility of using a knowledge-based system designed to automatically titrate pressure support (PS) to maintain the patient in a “respiratory comfort zone” during noninvasive ventilation (NIV) in patients with acute respiratory failure.

Design and setting

Prospective crossover interventional study in an intensive care unit of a university hospital.

Patients

Twenty patients.

Interventions

After initial NIV setting and startup in conventional PS by the chest physiotherapist NIV was continued for 45?min with the automated PS activated.

Measurements and results

During automated PS minute-volume was maintained constant while respiratory rate decreased significantly from its pre-NIV value (20?±?3 vs. 25?±?3?bpm). There was a trend towards a progressive lowering of dyspnea. In hypercapnic patients PaCO₂ decreased significantly from 61?±?9 to 51?±?2?mmHg, and pH increased significantly from 7.31?±?0.05 to 7.35?±?0.03. Automated PS was well tolerated. Two system malfunctions occurred prompting physiotherapist intervention.

Conclusions

The results of this feasibility study suggest that the system can be used during NIV in patients with acute respiratory failure. Further studies should now determine whether it can improve patient-ventilator interaction and reduce caregiver workload.

     

Performance of noninvasive ventilation modes on ICU ventilators during pressure support: a bench model study

Objective Noninvasive ventilation (NIV) is often applied with ICU ventilators. However, leaks at the patient-ventilator interface interfere with several key ventilator functions. Many ICU ventilators feature an NIV-specific mode dedicated to preventing these problems. The present bench model study aimed to evaluate the performance of these modes. Design and setting Bench model study in an intensive care research laboratory of a university hospital. Methods Eight ICU ventilators, widely available in Europe and featuring an NIV mode, were connected by an NIV mask to a lung model featuring a plastic head to mimic NIV conditions, driven by an ICU ventilator imitating patient effort. Tests were conducted in the absence and presence of leaks, the latter condition with and without activation of the NIV mode. Trigger delay, trigger-associated inspiratory workload, and pressurization were tested in conditions of normal respiratory mechanics, and cycling was also assessed in obstructive and restrictive conditions. Results On most ventilators leaks led to an increase in trigger delay and workload, a decrease in pressurization, and delayed cycling. On most ventilators the NIV mode partly or totally corrected these problems, but with large variations between machines. Furthermore, on some ventilators the NIV mode worsened the leak-induced dysfunction. Conclusions The results of this bench-model NIV study confirm that leaks interfere with several key functions of ICU ventilators. Overall, NIV modes can correct part or all of this interference, but with wide variations between machines in terms of efficiency. Clinicians should be aware of these differences when applying NIV with an ICU ventilator. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Electrical impedance tomography: the holy grail of ventilation and perfusion monitoring?

This review summarizes the state-of-the-art in electrical impedance tomography (EIT) for ventilation and perfusion imaging. EIT is a relatively new technology used to image regional impedance distributions in a cross-sectional area of the body. After the introduction, a brief overview of the recent history is provided followed by a review of the literature on regional ventilation monitoring using EIT. Several recently presented indices that are useful to extract information from EIT image streams are described. Selected experimental and clinical findings are discussed with respect to future routine applications in intensive care. Finally, past and ongoing research activities aimed at obtaining cardiac output and regional perfusion information from EIT image streams are summarized.     

Electrical impedance tomography: a method for monitoring regional lung aeration and tidal volume distribution?

Objective To demonstrate the monitoring capacity of modern electrical impedance tomography (EIT) as an indicator of regional lung aeration and tidal volume distribution.Design and setting Short-term ventilation experiment in an animal research laboratory.Patients and participants One newborn piglet (body weight: 2 kg).Interventions Surfactant depletion by repeated bronchoalveolar lavage, surfactant administration.Measurements and results EIT scanning was performed at an acquisition rate of 13 images/s during two ventilatory manoeuvres performed before and after surfactant administration. During the scanning periods of 120 s the piglet was ventilated with a tidal volume of 10 ml/kg at positive end-expiratory pressures (PEEP) in the range of 0–30 cmH₂O, increasing and decreasing in 5 cmH₂O steps. Local changes in aeration and ventilation with PEEP were visualised by EIT scans showing the regional shifts in end-expiratory lung volume and distribution of tidal volume, respectively. In selected regions of interest EIT clearly identified the changes in local aeration and tidal volume distribution over time and after surfactant treatment as well as the differences between stepwise inflation and deflation.Conclusions Our data indicate that modern EIT devices provide an assessment of regional lung aeration and tidal volume and allow evaluation of immediate effects of a change in ventilation or other therapeutic intervention. Future use of EIT in a clinical setting is expected to optimise the selection of appropriate ventilation strategies.     

Remifentanil-based sedation to treat noninvasive ventilation failure: a preliminary study

Objective To assess the feasibility and safety of remifentanil-based sedation during noninvasive ventilation (NIV) in patients with NIV failure. Design and setting Prospective clinical investigation in a 16-bed intensive care unit of a university hospital in France. Patients Thirteen patients in NIV failure due to discomfort and/or refusal to continue this ventilatory support: 10 with acute respiratory failure and 3 with acute hypercapnic respiratory failure. Intervention Patients were administered methylene blue and were sedated (Ramsay scale 2–3) by a continuous perfusion of remifentanil during NIV. Cardiorespiratory and ventilatory parameters, blood gas analysis, and adverse events were prospectively recorded. Measurements and results The 13 patients received a total of 125 NIV sessions, totaling 1200 h, of NIV under remifentanil-based sedation (mean remifentanil dose 0.1 ± 0.03 μg/kg per minute). Three patients also required propofol. PaO₂/FIO₂ ratio increased from 134 ± 69 to 187 ± 43 mmHg after 1 h. In patients with acute respiratory failure respiratory rate decreased from 34 ± 12 per minute before remifentanil to 25 ± 4 per minute after 1 h. In the three patients with acute hypercapnic respiratory failure PaCO₂ decreased from 69 ± 7 to 42 ± 5 mmHg. Four patients required endotracheal intubation without aspiration pneumonia. Twelve of the 13 patients left the ICU. Conclusion This pilot study shows that remifentanil-based sedation is safe and effective in the treatment of NIV failure due to low tolerance.     

Control of tracheal cuff pressure: a pilot study using a pneumatic device

OBJECTIVE: To evaluate the efficacy of a simple mechanical device to maintain constant endotracheal cuff pressure (Pcuff) during mechanical ventilation (large encased inflatable cuff connected to the endotracheal cuff and receiving constant pressure from a heavy mass attached to an articulated arm). DESIGN AND SETTING: Single-center, prospective, randomized, crossover, pilot study in a medical intensive care unit. PATIENTS AND PARTICIPANTS: Nine consecutive mechanically ventilated patients (age 62+/-20 years, SAPS II score 39+/-15). INTERVENTIONS: Control day: Pcuff monitored and adjusted with a manometer (Hi-Lo, Tyco Healthcare) according to current recommendations (twice a day and after each intervention on the tracheal tube); initial target Pcuff 22-28 cmH20. Prototype day: test device connected to the endotracheal cuff; same initial target. Continuous Pcuff recording during both days. Control and prototype days in random order. RESULTS: Pcuff values over 50 cmH20 were recorded in six patients during the control day (178+/-159min), never during the prototype day. During the control day, Pcuff was between 30 and 50 cmH20 for 29+/-25% of the time, vs 0.3+/-0.3% during the prototype day (p<0.01). Pcuff was between 15 and 30 cmH20 for 56+/-36% of the time during the control day, vs 95+/-14% during the prototype day p<0.01). During the control day, Pcuff was below 15 cmH20 for 15+/-17% of the time, vs 4.7+/-15% during the prototype day (p<0.05). CONCLUSIONS: The tested device successfully controlled Pcuff with minimal human resource consumption. Prospective studies are required to assess its clinical impact.     

Neurally adjusted ventilatory assist: a ventilation tool or a ventilation toy?

Mechanical ventilation has, since its introduction into clinical practice, undergone a major evolution from controlled ventilation to various modes of assisted ventilation. Neurally adjusted ventilatory assist (NAVA) is the newest development. The implementation of NAVA requires the introduction of a catheter to measure the electrical activity of the diaphragm (EA(di)). NAVA relies, opposite to conventional assisted ventilation modes, on the EA(di) to trigger the ventilator breath and to adjust the ventilatory assist to the neural drive. The amplitude of the ventilator assist is determined by the instantaneous EA(di) and the NAVA level set by the clinician. The NAVA level amplifies the EA(di) signal and determines instantaneous ventilator assist on a breath-to-breath basis. Experimental and clinical data suggest superior patient-ventilator synchrony with NAVA. Patient-ventilator asynchrony is present in 25% of mechanically ventilated patients in the intensive care unit and may contribute to patient discomfort, sleep fragmentation, higher use of sedation, development of delirium, ventilator-induced lung injury, prolonged mechanical ventilation, and ultimately mortality. With NAVA, the reliance on the EA(di) signal, together with an intact ventilatory drive and intact breathing reflexes, allows integration of the ventilator in the neuro-ventilatory coupling on a higher level than conventional ventilation modes. The simple monitoring of the EA(di) signal alone may provide the clinician with important information to guide ventilator management, especially during the weaning process. Although, until now, little evidence proves the superiority of NAVA on clinically relevant end points, it seems evident that patient populations (eg, COPD and small children) with major patient-ventilator asynchrony may benefit from this new ventilatory tool.     

High-frequency oscillatory ventilation reduces lung inflammation: a large-animal 24-h model of respiratory distress

Objective High-frequency oscillatory ventilation (HFOV) may reduce ventilator-induced lung injury in experimental neonatal respiratory distress. However, these data permit no conclusions for large animals or adult patients with acute respiratory distress syndrome (ARDS), because in neonates higher frequencies and lower amplitudes can be used, resulting in lower tidal volumes (VT) and airway pressures. The aim of this study was to compare gas exchange, lung histopathology and inflammatory cytokine expression during lung-protective pressure-controlled ventilation (PCV) and HFOV in a long-term large-animal model of ARDS. Design Prospective, randomized, controlled pilot study. Setting University animal laboratory. Subjects Sixteen female pigs (55.3 ± 3.9 kg). Interventions After induction of ARDS by repeated lavage, the animals were randomly assigned to PCV (VT = 6 ml/kg) and HFOV (6 Hz). After lung injury, a standardised lung recruitment was performed in both groups, and ventilation was continued for 24 h. Measurements and results: After lung recruitment sustained improvements in the oxygenation index were observed in both groups. The mean airway pressure (mPaw) was significantly lower in the HFOV group during the experiment ( p < 0.01). Histologically, lung inflammation was significantly ameliorated in the HFOV group ( p < 0.05). The messenger RNA expression of IL-1-β in lung tissue was significantly lower in the HFOV-treated animals ( p < 0.01). Conclusions These data suggest that HFOV compared with conventional lung-protective ventilation can reduce lung inflammation in a large-animal 24-h model of ARDS. Furthermore, it was shown that lung recruitment leads to sustained improvements in gas exchange with a significantly lower mPaw when HFOV is used. This article is discussed in the editorial available at: .     

C-reactive protein concentration as a predictor of in-hospital mortality after ICU discharge: a prospective cohort study

OBJECTIVE: The objective was to assess the ability of potential clinical predictors and inflammatory markers within 24 h of intensive care unit (ICU) discharge to predict subsequent in-hospital mortality. DESIGN AND SETTING: A prospective cohort study of 603 consecutive patients who survived their first ICU admission, between 1 June and 31 December 2005, in a 22-bed multidisciplinary ICU of a university hospital. MEASUREMENTS AND RESULTS: A total of 26 in-hospital deaths after ICU discharge (4.3%) were identified. C-reactive protein (CRP) concentrations at ICU discharge were associated with subsequent in-hospital mortality in the univariate analysis (mean CRP concentrations of non-survivors=174 vs. survivors=85.6 mg/l, p=0.001). CRP concentrations remained significantly associated with post-ICU mortality (a 10-mg/l increment in CRP concentrations increased the odds ratio [OR] of death: 1.09, 95% confidence interval [CI]: 1.03-1.16); after adjusting for age, the Acute Physiology and Chronic Health Evaluation (APACHE) II predicted mortality, and the Delta Sequential Organ Failure Assessment (Delta SOFA) score. The area under the receiver operating characteristic curve of this multivariate model to discriminate between survivors and non-survivors after ICU discharge was 0.85 (95% CI: 0.73-0.96). The destination and timing of ICU discharge, and the Discharge SOFA score, white cell counts and fibrinogen concentrations at ICU discharge were not significantly associated with in-hospital mortality after ICU discharge. CONCLUSIONS: A high CRP concentration at ICU discharge was an independent predictor of in-hospital mortality after ICU discharge in our ICU.     

Comparison of lung injury after normal or small volume optimized resuscitation in a model of hemorrhagic shock

Objective To compare lung injury induced by a hemorrhagic shock resuscitated with normal saline or with small volumes of a hypertonic/hyperoncotic solution. Design and setting Randomized, controlled, laboratory study in an animal research laboratory. Subjects Nineteen pigs (43 ± 4 kg). Interventions After anesthesia and mechanical ventilation animals were bled to induce a 2-h deep shock and resuscitated for 2 h using normal saline (NS, 2 ml/kg per minute, n = 7) or the association of 7.2% NaCl with 6% hydroxyethylstarch 200/0.5 (HSHES, 4 ml/kg in 10 min followed by 0.2 ml/kg per minute, n = 7) to reach cardiac index and mixed venous oxygen saturation goals. Lungs were removed 6 h after the initiation of hemorrhage. Five animals were used as controls without hemorrhage. Measurements and results Resuscitation goals were achieved using 90 ± 17 ml/kg NS or 6.8 ± 1.9 ml/kg HSHES. Lung injury was noted in both hemorrhage groups but was not influenced by the type of resuscitation. Extravascular lung water was measured at 9.6 ± 1.8 ml/kg in the NS group, 9.2 ± 1.6 ml/kg in the HSHES, group and 6.4 ± 1 m/kg in the control group. The degree of histological alveolar membrane focal thickening and interstitial neutrophil infiltration were significantly more pronounced in the hemorrhage groups with no difference between the two types of fluid loading. Finally, pulmonary levels of IL-8 were higher after hemorrhage regardless of the type of resuscitation. Conclusions When included in an optimized and goal directed resuscitation, the use of normal saline or a small volume of hypertonic/hyperoncotic solution does not result in a different early hemorrhage-induced lung injury.     

Regional distribution of blood flow during proximal aortic cross‐clamping: An experimental study using coloured microspheres

Objective. To investigate the effect of thoracic aortic cross‐clamping on blood perfusion of the brain, spinal cord, heart, muscular tissue and visceral organs. Material and methods. Nine pigs underwent 30?min cross‐clamping of the descending thoracic aorta. Multiple coloured microspheres (15.0?µm±0.1) were infused into the left ventricle before and during aortic cross‐clamping (XC) and after declamping (DC). Tissue samples were analysed by spectrophotometry. Results. Blood perfusion of the middle and lower segments of the spinal cord was significantly reduced during aortic XC. Perfusion of the brain was not significantly altered by aortic XC, while perfusion of myocardium increased 3‐fold. During XC, perfusion of the deltoid muscle and diaphragm increased 5‐fold and 13‐fold, respectively, while a decrease was found in the gluteus muscle. Renal blood flow was significantly reduced during XC. Finally, XC induced a significant decrease of perfusion in the bowel, spleen, liver and pancreas. Conclusion. During XC of the thoracic aorta, the perfusion of the muscular tissue was significantly increased proximal to the level of XC. The circulation of the brain was unchanged, probably because of autoregulatory mechanisms. Blood perfusion of the myocardium increased 3‐fold during XC.