Conventional mechanical ventilators rely on pneumatic pressure and flow sensors and controllers to detect breaths. New modes of mechanical ventilation have been developed to better match the assistance delivered by the ventilator to the patient's needs. Among these modes, neurally adjusted ventilatory assist (NAVA) delivers a pressure that is directly proportional to the integral of the electrical activity of the diaphragm recorded continuously through an esophageal probe. In clinical settings, NAVA has been chiefly compared with pressure-support ventilation, one of the most popular modes used during the weaning phase, which delivers a constant pressure from breath to breath. Comparisons with proportional-assist ventilation, which has numerous similarities, are lacking. Because of the constant level of assistance, pressure-support ventilation reduces the natural variability of the breathing pattern and can be associated with asynchrony and/or overinflation. The ability of NAVA to circumvent these limitations has been addressed in clinical studies and is discussed in this report. Although the underlying concept is fascinating, several important questions regarding the clinical applications of NAVA remain unanswered. Among these questions, determining the optimal NAVA settings according to the patient's ventilatory needs and/or acceptable level of work of breathing is a key issue. In this report, based on an investigator-initiated round table, we review the most recent literature on this topic and discuss the theoretical advantages and disadvantages of NAVA compared with other modes, as well as the risks and limitations of NAVA.
Monitoring plays an important role in the current management of patients with acute
respiratory failure but sometimes lacks definition regarding which 'signals' and
'derived variables' should be prioritized as well as specifics related to timing
(continuous versus intermittent) and modality (static versus dynamic). Many new
techniques of respiratory monitoring have been made available for clinical use
recently, but their place is not always well defined. Appropriate use of available
monitoring techniques and correct interpretation of the data provided can help
improve our understanding of the disease processes involved and the effects of
clinical interventions. In this consensus paper, we provide an overview of the
important parameters that can and should be monitored in the critically ill patient
with respiratory failure and discuss how the data provided can impact on clinical
Low tidal volume ventilation, although promoting atelectasis, is a protective strategy against ventilator-induced lung injury. Deep inflation (DI) recruitment maneuvers restore lung volumes, but potentially compromise lung parenchymal and vascular function via repetitive overdistention. Our objective was to examine cardiopulmonary physiological and transcriptional consequences of recruitment maneuvers. C57/BL6 mice challenged with either PBS or LPS via aspiration were placed on mechanical ventilation (5 h) using low tidal volume inflation (TI; 8 μl/g) alone or in combination with intermittent DIs (0.75 ml twice/min). Lung mechanics during TI ventilation significantly deteriorated, as assessed by forced oscillation technique and pressure–volume curves. DI mitigated the TI-induced alterations in lung mechanics, but induced a significant rise in right ventricle systolic pressures and pulmonary vascular resistances, especially in LPS-challenged animals. In addition, DI exacerbated the LPS-induced genome-wide lung inflammatory transcriptome, with prominent dysregulation of a gene cluster involving vascular processes, as well as increases in cytokine concentrations in bronchoalveolar lavage fluid and plasma. Gene ontology analyses of right ventricular tissue expression profiles also identified inflammatory signatures, as well as apoptosis and membrane organization ontologies, as potential elements in the response to acute pressure overload. Our results, although confirming the improvement in lung mechanics offered by DI, highlight a detrimental impact in sustaining inflammatory response and exacerbating lung vascular dysfunction, events contributing to increases in right ventricle afterload. These novel insights should be integrated into the clinical assessment of the risk/benefit of recruitment maneuver strategies.
mechanical ventilation; microarray; pulmonary hypertension; right ventricle; acute lung injury
Inhalation of helium-oxygen (He/O2) mixtures has been explored as a means to lower the work of breathing of patients with obstructive lung disease. Non-invasive ventilation (NIV) with positive pressure support is also used for this purpose. The bench experiments presented herein were conducted in order to compare simulated patient inspiratory effort breathing He/O2 with that breathing medical air, with or without pressure support, across a range of adult, obstructive disease patterns.
Patient breathing was simulated using a dual-chamber mechanical test lung, with the breathing compartment connected to an ICU ventilator operated in NIV mode with medical air or He/O2 (78/22 or 65/35%). Parabolic or linear resistances were inserted at the inlet to the breathing chamber. Breathing chamber compliance was also varied. The inspiratory effort was assessed for the different gas mixtures, for three breathing patterns, with zero pressure support (simulating unassisted spontaneous breathing), and with varying levels of pressure support.
Inspiratory effort increased with increasing resistance and decreasing compliance. At a fixed resistance and compliance, inspiratory effort increased with increasing minute ventilation, and decreased with increasing pressure support. For parabolic resistors, inspiratory effort was lower for He/O2 mixtures than for air, whereas little difference was measured for nominally linear resistance. Relatively small differences in inspiratory effort were measured between the two He/O2 mixtures. Used in combination, reductions in inspiratory effort provided by He/O2 and pressure support were additive.
The reduction in inspiratory effort afforded by breathing He/O2 is strongly dependent on the severity and type of airway obstruction. Varying helium concentration between 78% and 65% has small impact on inspiratory effort, while combining He/O2 with pressure support provides an additive reduction in inspiratory effort. In addition, breathing He/O2 alone may provide an alternative to pressure support in circumstances where NIV is not available or poorly tolerated.
Helium; Oxygen; Heliox; Inspiratory effort; Work of breathing; Airway resistance; Lung compliance; Non-invasive ventilation; Pressure support
The specific burden imposed on Intensive Care Units (ICUs) during the A/H1N1 influenza 2009 pandemic has been poorly explored. An on-line screening registry allowed a daily report of ICU beds occupancy rate by flu infected patients (Flu-OR) admitted in French ICUs.
We conducted a prospective inception cohort study with results of an on-line screening registry designed for daily assessment of ICU burden.
Among the 108 centers participating to the French H1N1 research network on mechanical ventilation (REVA) - French Society of Intensive Care (SRLF) registry, 69 ICUs belonging to seven large geographical areas voluntarily participated in a website screening-registry. The aim was to daily assess the ICU beds occupancy rate by influenza-infected and non-infected patients for at least three weeks. Three hundred ninety-one critically ill infected patients were enrolled in the cohort, representing a subset of 35% of the whole French 2009 pandemic cohort; 73% were mechanically ventilated, 13% required extra corporal membrane oxygenation (ECMO) and 22% died. The global Flu-OR in these ICUs was only 7.6%, but it exceeded a predefined 15% critical threshold in 32 ICUs for a total of 103 weeks. Flu-ORs were significantly higher in University than in non-University hospitals. The peak ICU burden was poorly predicted by observations obtained at the level of large geographical areas.
The peak Flu-OR during the pandemic significantly exceeded a 15% critical threshold in almost half of the ICUs, with an uneven distribution with time, geographical areas and between University and non-University hospitals. An on-line assessment of Flu-OR via a simple dedicated registry may contribute to better match resources and needs.
Non-invasive ventilation is largely used to treat acute and chronic respiratory failure. This ventilation encounters a non negligible rate of failure related to the used interface/mask, but the reasons for this failure remain unclear. In order to shed light into this issue and to better understand the effects of the geometrical design of interfaces, we aimed to quantify flow, pressure and gas composition in terms of CO2 and O2 at the passage through different types of interface (oronasal mask, integral mask and helmet). Especially, we postulated that due to specific gas flow passing throughout interface, the effective dead space added by the interface is not always related to the whole gas volume included in the interface.
Numerical simulations, using computational fluid dynamics, were used to describe pressure, flow and gas composition during ventilation with the different interfaces.
Between the different interfaces the effective dead spaces differed only modestly (110 to 370ml) while their internal volumes were markedly different (110 to 10000ml). It was limited to half the tidal volume for the most voluminous interface, while it was close to the interface gas volume for the less voluminous interfaces. Pressure variations induced by the flow ventilation throughout the interface were negligible.
Effective dead space is not related to the internal gas volume included in the interface, suggesting that this internal volume should not be considered as a limiting factor for their efficacy during non-invasive ventilation. Patient’s comfort and synchrony have also to be taken into account.
Computer Simulation; Equipment Design; Face; anatomy & histology; Humans; Positive-Pressure Respiration; instrumentation; Respiratory Dead Space; interfaces/masks; non-invasive ventilation; dead space
Noninvasive ventilation (NIV) is frequently used for the management of acute respiratory failure (ARF) in very old patients (≥ 80 years), often in the context of a do-not-intubate order (DNI). We aimed to determine its efficacy and long-term outcome.
Prospective cohort of all patients admitted to the medical ICU of a tertiary hospital during a 2-year period and managed using NIV. Characteristics of patients, context of NIV, and treatment intensity were compared for very old and younger patients. Six-month survival and functional status were assessed in very old patients.
During the study period, 1,019 patients needed ventilatory support and 376 (37%) received NIV. Among them, 163 (16%) very old patients received ventilatory support with 60% of them managed using NIV compared with 32% of younger patients (p < 0.0001). Very old patients received NIV more frequently with DNI than in younger patients (40% vs. 8%). Such cases were associated with high mortality for both very old and younger patients. Hospital mortality was higher in very old than in younger patients but did not differ when NIV was used for cardiogenic pulmonary edema or acute-on-chronic respiratory failure (20% vs. 15%) and in postextubation (15% vs. 17%) out of a context of DNI. Six-month mortality was 51% in very old patients, 67% for DNI patients, and 77% in case of NIV failure and endotracheal intubation. Of the 30 hospital survivors, 22 lived at home and 13 remained independent for activities of daily living.
Very old patients managed using NIV have an overall satisfactory 6-month survival and functional status, except for endotracheal intubation after NIV failure.
End-expiratory lung volume (EELV) is decreased in acute respiratory distress
syndrome (ARDS), and bedside EELV measurement may help to set positive
end-expiratory pressure (PEEP). Nitrogen washout/washin for EELV measurement is
available at the bedside, but assessments of accuracy and precision in real-life
conditions are scant. Our purpose was to (a) assess EELV measurement precision in
ARDS patients at two PEEP levels (three pairs of measurements), and (b) compare
the changes (Δ) induced by PEEP for total EELV with the PEEP-induced changes
in lung volume above functional residual capacity measured with passive spirometry
(ΔPEEP-volume). The minimal predicted increase in lung volume was calculated
from compliance at low PEEP and ΔPEEP to ensure the validity of lung-volume
Thirty-four patients with ARDS were prospectively included in five
university-hospital intensive care units. ΔEELV and ΔPEEP volumes were
compared between 6 and 15 cm H2O of PEEP.
After exclusion of three patients, variability of the nitrogen technique was less
than 4%, and the largest difference between measurements was 81 ± 64 ml.
ΔEELV and ΔPEEP-volume were only weakly correlated (r2
= 0.47); 95% confidence interval limits, -414 to 608 ml). In four
patients with the highest PEEP (≥ 16 cm H2O), ΔEELV was
lower than the minimal predicted increase in lung volume, suggesting flawed
measurements, possibly due to leaks. Excluding those from the analysis markedly
strengthened the correlation between ΔEELV and ΔPEEP volume (r2
In most patients, the EELV technique has good reproducibility and accuracy, even
at high PEEP. At high pressures, its accuracy may be limited in case of leaks. The
minimal predicted increase in lung volume may help to check for accuracy.
To evaluate the effects of acute hypercapnia induced by positive end-expiratory pressure (PEEP) variations at constant plateau pressure (Pplat) in patients with severe acute respiratory distress syndrome (ARDS) on right (R) and left ventricular (LV) function.
Prospective observational study in two academic intensive care units enrolling 11 adults with severe ARDS (PaO2/FiO2<150 mm Hg at PEEP >5 cm H2O). We compared three ventilatory strategies, each used for 1 hour, with Pplat at 22 [20–25] cm H2O: low PEEP (5.4 cm H2O) or high PEEP (11.0 cm H2O) with compensation of the tidal volume reduction by either a high respiratory rate (high PEEP/high rate) or instrumental dead space decrease (high PEEP/low rate). We assessed RV function (transesophageal echocardiography), alveolar dead space (expired CO2), and alveolar recruitment (pressure-volume curves).
Compared to low PEEP, PaO2/FiO2 ratio and alveolar recruitment were increased with high PEEP. Alveolar dead space remained unchanged. Both high-PEEP strategies induced higher PaCO2 levels (71 [60–94] and 75 [53–84], vs. 52 [43–68] mm Hg) and lower pH values (7.17 [7.12–7.23] and 7.20 [7.16–7.25] vs. 7.30 [7.24–7.35]), as well as a significant decrease in cardiac index, RV dilatation and LV deformation. The decrease in stroke index tended to be negatively correlated to the increase in alveolar recruitment with high PEEP.
Acidosis and hypercapnia induced by tidal volume reduction and increase in PEEP at constant Pplat was associated with impaired RV function and hemodynamics despite a positive effect on oxygenation and alveolar recruitment.
Acidosis, Respiratory; etiology; Acute Disease; Aged; Analysis of Variance; Echocardiography, Transesophageal; Female; Humans; Hypercapnia; diagnosis; etiology; Intensive Care; methods; Linear Models; Male; Middle Aged; Positive-Pressure Respiration; adverse effects; methods; Prospective Studies; Respiratory Dead Space; Respiratory Distress Syndrome, Adult; complications; metabolism; physiopathology; therapy; Respiratory Rate; Severity of Illness Index; Statistics, Nonparametric; Tidal Volume; Ventricular Dysfunction, Right; diagnosis; etiology; lung injury; acidosis; right heart; alveolar recruitment; alveolar dead space
Several factors, including diuretic use and sepsis, interfere with the fractional excretion of sodium, which is used to distinguish transient from persistent acute kidney injury (AKI). These factors do not affect the fractional excretion of urea (FeUrea). However, there are conflicting data on the diagnostic accuracy of FeUrea.
We conducted an observational, prospective, multicenter study at three ICUs in university hospitals. Unselected patients, except those with obstructive AKI, were admitted to the participating ICUs during a six-month period. Transient AKI was defined as AKI caused by renal hypoperfusion and reversal within three days. The results are reported as medians (interquartile ranges).
A total of 203 patients were included. According to our definitions, 67 had no AKI, 54 had transient AKI and 82 had persistent AKI. FeUrea was 39% (28 to 40) in the no-AKI group, 41% (29 to 54) in the transient AKI group and 32% (22 to 51) in the persistent AKI group (P = 0.12). FeUrea was of little help in distinguishing transient AKI from persistent AKI, with the area under the receiver operating characteristic curve being 0.59 (95% confidence interval, 0.49 to 0.70; P = 0.06). Sensitivity was 63% and specificity was 54% with a cutoff of 35%. In the subgroup of patients receiving diuretics, the results were similar.
FeUrea may be of little help in distinguishing transient AKI from persistent AKI in critically ill patients, including those receiving diuretic therapy. Additional studies are needed to evaluate alternative markers or strategies to differentiate transient from persistent AKI.
acute kidney failure; ICU; fractional excretion of sodium; acute tubular necrosis; diuretics; sensitivity and specificity
In patients with acute lung injury (ALI) and/or acute respiratory distress syndrome (ARDS), recent randomised controlled trials (RCTs) showed a consistent trend of mortality reduction with prone ventilation. We updated a meta-analysis on this topic.
RCTs that compared ventilation of adult patients with ALI/ARDS in prone versus supine position were included in this study-level meta-analysis. Analysis was made by a random-effects model. The effect size on intensive care unit (ICU) mortality was computed in the overall included studies and in two subgroups of studies: those that included all ALI or hypoxemic patients, and those that restricted inclusion to only ARDS patients. A relationship between studies' effect size and daily prone duration was sought with meta-regression. We also computed the effects of prone positioning on major adverse airway complications.
Seven RCTs (including 1,675 adult patients, of whom 862 were ventilated in the prone position) were included. The four most recent trials included only ARDS patients, and also applied the longest proning durations and used lung-protective ventilation. The effects of prone positioning differed according to the type of study. Overall, prone ventilation did not reduce ICU mortality (odds ratio = 0.91, 95% confidence interval = 0.75 to 1.2; P = 0.39), but it significantly reduced the ICU mortality in the four recent studies that enrolled only patients with ARDS (odds ratio = 0.71; 95% confidence interval = 0.5 to 0.99; P = 0.048; number needed to treat = 11). Meta-regression on all studies disclosed only a trend to explain effect variation by prone duration (P = 0.06). Prone positioning was not associated with a statistical increase in major airway complications.
Long duration of ventilation in prone position significantly reduces ICU mortality when only ARDS patients are considered.
A high respiratory rate associated with the use of small tidal volumes, recommended for acute lung injury (ALI), shortens time for gas diffusion in the alveoli. This may decrease CO2 elimination. We hypothesized that a post-inspiratory pause could enhance CO2 elimination and reduce PaCO2 by reducing dead space in ALI. In 15 mechanically ventilated patients with ALI and hypercapnia, a 20% post-inspiratory pause (Tp20) was applied during a period of 30 min between two ventilation periods without post-inspiratory pause (Tp0). Other parameters were kept unchanged. The single breath test for CO2 was recorded every 5 minutes to measure tidal CO2 elimination (VtCO2), airway dead space (VDaw) and slope of the alveolar plateau. PaO2, PaCO2, physiological and alveolar dead space (VDphys, VDalv) were determined at the end of each 30 minute period. The post-inspiratory pause, 0.7±0.2 s, induced on average less than 0.5 cm H2O of intrinsic PEEP. During Tp20, VtCO2 increased immediately by 28±10% (14±5 ml per breath compared to 11±4 for Tp0) and then decreased without reaching the initial value within 30 minutes. The addition of a post-inspiratory pause decreased significantly VDaw by 14% and VDphys by 11% with no change in VDalv. During Tp20, the slope of alveolar plateau initially fell to 65±10 % of baseline value and continued to decrease. Tp20 induced a 10±3% decrease in PaCO2 at 30 minutes (from 55±10 to 49±9 mmHg, p<0.001) with no significant variation in PaO2. Post-inspiratory pause has a significant influence on CO2 elimination when small tidal volumes are used during mechanical ventilation for ALI.
Adult; Aged; Aged, 80 and over; Blood Gas Analysis; Carbon Dioxide; blood; metabolism; Female; Humans; Lung Diseases; metabolism; physiopathology; Male; Middle Aged; Pneumonia; metabolism; physiopathology; Positive-Pressure Respiration; Respiration, Artificial; Respiratory Dead Space; physiology; Respiratory Distress Syndrome, Adult; metabolism; physiopathology; Respiratory Mechanics; physiology; Tidal Volume; physiology; Gas Exchange; Dead Space; Mechanical Ventilation; ARDS
To compare 13 commercially available, new-generation, intensive-care-unit (ICU) ventilators regarding trigger function, pressurization capacity during pressure-support ventilation (PSV), accuracy of pressure measurements and expiratory resistance.
Design and Setting
Bench study at a research laboratory in a university hospital.
Four turbine-based ventilators and nine conventional servo-valve compressed-gas ventilators were tested using a two-compartment lung model.
Three levels of effort were simulated. Each ventilator was evaluated at four PSV levels (5, 10, 15, and 20 cm H2O), with and without positive end-expiratory pressure (5 cm H2O, Trigger function was assessed as the time from effort onset to detectable pressurization. Pressurization capacity was evaluated using the airway pressure-time product computed as the net area under the pressure-time curve over the first 0.3 s after inspiratory effort onset. Expiratory resistance was evaluated by measuring trapped volume in controlled ventilation. Significant differences were found across the ventilators, with a range of triggering-delay from 42 ms to 88 ms for all conditions averaged (P<.001). Under difficult conditions, the triggering delay was longer than 100 ms and the pressurization was poor with five ventilators at PSV5 and three at PSV10, suggesting an inability to unload patient’s effort. On average, turbine-based ventilators performed better than conventional ventilators, which showed no improvement compared to a 2000 bench comparison.
Technical performances of trigger function, pressurization capacity and expiratory resistance vary considerably across new-generation ICU ventilators. ICU ventilators seem to have reached a technical ceiling in recent years, and some ventilators still perform inadequately.
Equipment Design; Hospitals, University; Humans; Intensive Care Units; Models, Biological; Respiration, Artificial; instrumentation; standards; Work of Breathing; physiology; Mechanical ventilation; Pressure-support ventilation; Work of breathing; Inspiratory trigger; Bench study
Fluid resuscitation is widely used, and many patients are therefore exposed to plasma volume expanders. Among these, colloids, particularly hydroxyethyl starches, have been shown in recent experiments and clinical studies to induce acute kidney injury. The mechanisms of colloid-induced acute kidney injury remain incompletely elucidated. The risks associated with colloid osmotic pressure elevation in vivo and the high incidence of osmotic nephrosis lesions in experimental models and clinical studies indicate that hydroxyethyl starches can no longer be considered safe.
Background and Aims:
To understand the practice patterns of noninvasive ventilation (NIV) use by Indian physicians.
Subjects and Methods:
Around three thousand physicians from all over India were mailed a questionnaire that could capture the practice patterns of NIV use.
Completed responses were received from 648 physicians (21.6%). Majority (n = 469, 72.4%, age 40 ± 9 years, M:F 409:60) use NIV in their clinical practice. NIV was most exclusively being used in the ICU setting (68.4%) and the commonest indication for its use was chronic obstructive pulmonary disease (COPD) (71.4%). A significant number did not report use of a conventional ventilator for NIV support (62%). Oronasal mask was the overwhelming favorite among the sampled physicians (68.2%). In most of the cases, the treating physician initiated NIV (60.8%) and a baseline blood gas analysis was performed in only 71.1% of the cases (315/443). Nasal bridge pressure sores was the commonest complication (64.2%).
NIV is being widely used in clinical practice in India for various indications. COPD is the most common indication for its deployment. There seems to be a marked variability in the patterns relating to actual deployment of NIV, including the site of initiation, protocols for initiation followed, and monitoring of patients.
Noninvasive ventilation; questionnaire-based study; survey
The pressure–volume (PV) curve is a physiological tool proposed for diagnostic or monitoring purposes during mechanical ventilation of acute respiratory distress syndrome. The reduction in compliance measured by the PV curve and the different inflection points on the curve are considered interesting markers of the severity of and the levels of opening and closing pressures. Tracing a curve, however, may in itself influence the degree of opening or distension of the lung, and interpretation of the curve has to take this effect into account. In some individuals tracing the curve may even have moderate hemodynamic effects. Fortunately, on average, most of these effects are transient or negligible and do not invalidate the PV curve measurement.
There are few issues in critical care medicine that have a less clearly defined standard of care than the intravenous fluid choice for resuscitation. Natural colloids (such as albumin) became popular during the Second World War when there was a need to develop a portable, easily stored, blood substitute. Early successes led to widespread use and a multibillion dollar industry. It is not surprising given the large demand, high costs and potential adverse effects of natural colloids that synthetic colloids have emerged. In the present article, two groups of clinical investigators remind us of the controversies surrounding the use of synthetic colloids.
fluid resuscitation; hydroxyethylstarches; intensive care unit; sepsis