Spontaneous breathing trial and post-extubation work of breathing in morbidly obese critically ill patients.

BACKGROUND: Predicting whether an obese critically ill patient can be successfully extubated may be specially challenging. Several weaning tests have been described but no physiological study has evaluated the weaning test that would best reflect the post-extubation inspiratory effort. METHODS: This was a physiological randomized crossover study in a medical and surgical single-center Intensive Care Unit, in patients with body mass index (BMI) >35 kg/m2 who were mechanically ventilated for more than 24 h and underwent a weaning test. After randomization, 17 patients were explored using five settings : pressure support ventilation (PSV) 7 and positive end-expiratory pressure (PEEP) 7 cmH2O; PSV 0 and PEEP 7cmH2O; PSV 7 and PEEP 0 cmH2O; PSV 0 and PEEP 0 cmH2O; and a T piece, and after extubation. To further minimize interaction between each setting, a period of baseline ventilation was performed between each step of the study. We hypothesized that the post-extubation work of breathing (WOB) would be similar to the T-tube WOB. RESULTS: Respiratory variables and esophageal and gastric pressure were recorded. Inspiratory muscle effort was calculated as the esophageal and trans-diaphragmatic pressure time products and WOB. Sixteen obese patients (BMI 44 kg/m2 ± 8) were included and successfully extubated. Post-extubation inspiratory effort, calculated by WOB, was 1.56 J/L ± 0.50, not statistically different from the T piece (1.57 J/L ± 0.56) or PSV 0 and PEEP 0 cmH2O (1.58 J/L ± 0.57), whatever the index of inspiratory effort. The three tests that maintained pressure support statistically underestimated post-extubation inspiratory effort (WOB 0.69 J/L ± 0.31, 1.15 J/L ± 0.39 and 1.09 J/L ± 0.49, respectively, p < 0.001). Respiratory mechanics and arterial blood gases did not differ between the five tests and the post-extubation condition. CONCLUSIONS: In obese patients, inspiratory effort measured during weaning tests with either a T-piece or a PSV 0 and PEEP 0 was not different to post-extubation inspiratory effort. In contrast, weaning tests with positive pressure overestimated post-extubation inspiratory effort. TRIAL REGISTRATION: Clinical trial.gov (reference NCT01616901 ), 2012, June 4th.

Comparative evaluation of three interfaces for non-invasive ventilation: a randomized cross-over design physiologic study on healthy volunteers.

INTRODUCTION: Interface choice is crucial for non-invasive ventilation (NIV) success. We compared a new interface, the helmet next (HN), with the facial mask (FM) and the standard helmet (HS) in twelve healthy volunteers. METHODS: In this study, five NIV trials were randomly applied, preceded and followed by a trial of unassisted spontaneous breathing (SB). Baseline settings, for example, 5 cmH2O of both inspiratory pressure support (PS) and positive end-expiratory pressure (PEEP), were applied through FM, HS and HN, while increased settings (PS and PEEP of 8 cmH2O) were only applied through HS and HN. We measured flow, airway, esophageal and gastric pressures, and calculated inspiratory effort indexes and trigger delays. Comfort was assessed with a visual-analog-scale. RESULTS: We found that FM, HS and HN at baseline settings were not significantly different with respect to inspiratory effort indexes and comfort. Inspiratory trigger delay and time of synchrony (TI,synchrony) were significantly improved by FM compared to both helmets, whereas expiratory trigger delay was shorter with FM, as opposed to HS only. HN at increased settings performed better than FM in decreasing inspiratory effort measured by pressure-time product of transdiaphragmatic pressure (PTPdi)/breath (10.7 ± 9.9 versus 17.0 ± 11.0 cmH2Os), and PTPdi/min (128 ± 96 versus 204 ± 81 cmH2Os/min), and PTPdi/L (12.6 ± 9.9 versus 30.2 ± 16.8 cmH2Os/L). TI, synchrony was inferior between HN and HS at increased settings and FM. CONCLUSIONS: HN might hold some advantages with respect to interaction and synchrony between subject and ventilator, but studies on patients are needed to confirm these findings. TRIAL REGISTRATION: ClinicalTrials.gov NCT01610960.

Comparison of sleep quality with mechanical versus spontaneous ventilation during weaning of critically III tracheostomized patients.

BACKGROUND: In mechanically ventilated patients under mechanical ventilation in the ICU, ventilatory mode or settings may influence sleep quality. The aim of this study was to evaluate the direct impact of mechanical ventilation per se on sleep quantity and quality in patients who were able to tolerate separation from mechanical ventilation over prolonged periods. DESIGN AND SETTING: Randomized crossover clinical trial in a medical ICU. PATIENTS: Sixteen conscious patients, free of sedation and tracheostomized because of prolonged weaning from mechanical ventilation, were included in the study when able to tolerate at least 5 hours of spontaneous ventilation. INTERVENTIONS: Patients were randomized to receive either spontaneous ventilation or mechanical ventilation at low levels of pressure support for two crossover periods of 5-hour duration each, from 22:00 to 08:00. Polysomnography was performed throughout the study. MEASUREMENTS AND RESULTS: Total sleep time was higher during mechanical ventilation than during spontaneous ventilation (183 min vs 132 min, p = 0.04). No significant differences between mechanical ventilation and spontaneous ventilation were observed in slow wave sleep time (45 min vs 28 min), rapid eye movement sleep time (11 min vs 3 min), or the fragmentation index (25 vs 23 arousals and awakenings per hr). In four patients, however, our analysis of patient-ventilator interaction suggested that the ventilatory settings were suboptimal and could have been improved to potentially improve sleep. CONCLUSIONS: In difficult-to-wean tracheostomized patients, sleep quality was similar with or without the ventilator. Sleep quantity was higher during mechanical ventilation. Reconnection to the ventilator during the night period may favor sleep efficiency in tracheostomized patients in prolonged weaning.

Sleep in hypercapnic critical care patients under noninvasive ventilation: conventional versus dedicated ventilators.

OBJECTIVE: To compare sleep quality between two types of ventilators commonly used for noninvasive ventilation: conventional ICU ventilators and dedicated noninvasive ventilators; and to evaluate sleep during and between noninvasive ventilation sessions in critically ill patients. DESIGN: Physiological sleep study with a randomized assessment of the ventilator type. SETTING: Medical ICU in a university hospital. PATIENTS: Twenty-four patients admitted for acute hypercapnic respiratory failure requiring noninvasive ventilation. INTERVENTIONS: Patients were randomly assigned to receive noninvasive ventilation with either an ICU ventilators (n = 12) or a dedicated noninvasive ventilators (n = 12), and their sleep and respiratory parameters were recorded by polysomnography from 4 PM to 9 AM on the second, third, or fourth day after noninvasive ventilation initiation. MEASUREMENTS AND MAIN RESULTS: Sleep architecture was similar between ventilator groups, including sleep fragmentation (number of arousals and awakenings/hr), but the dedicated noninvasive ventilators group showed a higher patient-ventilator asynchrony-related fragmentation (28% [17-44] vs. 14% [7.0-22]; p = 0.02), whereas the ICU ventilators group exhibited a higher noise-related fragmentation. Ineffective efforts were more frequent in the dedicated noninvasive ventilators group than in the ICU ventilators group (34 ineffective efforts/hr of sleep [15-125] vs. two [0-13]; p < 0.01), possibly as a result of a higher tidal volume (7.2 mL/kg [6.7-8.8] vs. 5.8 [5.1-6.8]; p = 0.04). More sleep time occurred and sleep quality was better during noninvasive ventilation sessions than during spontaneous breathing periods (p < 0.05) as a result of greater slow wave and rapid eye movement sleep and lower fragmentation. CONCLUSIONS: There were no observed differences in sleep quality corresponding to the type of ventilator used despite slight differences in patient-ventilator asynchrony. Noninvasive ventilation sessions did not prevent patients from sleeping; on the contrary, they seem to aid sleep when compared with unassisted breathing.

Prospective randomized crossover study of a new closed-loop control system versus pressure support during weaning from mechanical ventilation.

BACKGROUND: Intellivent is a new full closed-loop controlled ventilation that automatically adjusts both ventilation and oxygenation parameters. The authors compared gas exchange and breathing pattern variability of Intellivent and pressure support ventilation (PSV). METHODS: In a prospective, randomized, single-blind design crossover study, 14 patients were ventilated during the weaning phase, with Intellivent or PSV, for two periods of 24 h in a randomized order. Arterial blood gases were obtained after 1, 8, 16, and 24 h with each mode. Ventilatory parameters were recorded continuously in a breath-by-breath basis during the two study periods. The primary endpoint was oxygenation, estimated by the calculation of the difference between the PaO2/FIO2 ratio obtained after 24 h of ventilation and the PaO2/FIO2 ratio obtained at baseline in each mode. The variability in the ventilatory parameters was also evaluated by the coefficient of variation (SD to mean ratio). RESULTS: There were no adverse events or safety issues requiring premature interruption of both modes. The PaO2/FIO2 (mean ± SD) ratio improved significantly from 245 ± 75 at baseline to 294 ± 123 (P = 0.03) after 24 h of Intellivent. The coefficient of variation of inspiratory pressure and positive end-expiratory pressure (median [interquartile range]) were significantly higher with Intellivent, 16 [11-21] and 15 [7-23]%, compared with 6 [5-7] and 7 [5-10]% in PSV. Inspiratory pressure, positive end-expiratory pressure, and FIO2 changes were adjusted significantly more often with Intellivent compared with PSV. CONCLUSIONS: Compared with PSV, Intellivent during a 24-h period improved the PaO2/FIO2 ratio in parallel with more variability in the ventilatory support and more changes in ventilation settings.

Use of maximum end-tidal CO(2) values to improve end-tidal CO(2) monitoring accuracy.

BACKGROUND: The arterial partial pressure of CO(2) (P(aCO(2))) can be grossly estimated by the end-tidal partial pressure of CO(2) (P(ETCO(2))). This principle is used in SmartCare (Dräger, Lübeck, Germany), which is an automated closed-loop system that uses P(ETCO(2)) to estimate alveolar ventilation during mechanical ventilation. OBJECTIVE: To assess whether the maximum P(ETCO(2)) value (instead of the averaged P(ETCO(2)) value) over 2-min or 5-min periods improves P(aCO(2)) estimation, and determine the consequences for the SmartCare system. METHODS: We continuously monitored breath-by-breath P(ETCO(2)) during ventilation with SmartCare in 36 patients mechanically ventilated for various disorders, including 14 patients with COPD. Data were collected simultaneously from SmartCare recordings, every 2 min or 5 min, and through a dedicated software that recorded ventilation data every 10 s. We compared the maximum and averaged P(ETCO(2)) values over 2-min and 5-min periods to the P(aCO(2)) measured from 80 arterial blood samples clinically indicated in 26 patients. We also compared SmartCare's classifications of patient ventilatory status based on averaged P(ETCO(2)) values to what the classifications would have been with the maximum P(ETCO(2)) values. RESULTS: Mean P(aCO(2)) was 44 ± 11 mm Hg. P(aCO(2)) was higher than averaged P(ETCO(2)) by 10 ± 6 mm Hg, and this difference was reduced to 6 ± 6 mm Hg with maximum P(ETCO(2)). The results were similar whether patients had COPD or not. Very few aberrant values (< 0.01%) needed to be discarded. Among the 3,137 classifications made by the SmartCare system, 1.6% were changed by using the maximum P(ETCO(2)) value instead of the averaged P(ETCO(2)) value. CONCLUSIONS: Use of maximum P(ETCO(2)) reduces the difference between P(aCO(2)) and P(ETCO(2)) and improves SmartCare's classification of patient ventilatory status.

Positive end-expiratory pressure-induced functional recruitment in patients with acute respiratory distress syndrome.

OBJECTIVE: In acute respiratory distress syndrome, alveolar recruitment improves gas exchange only if perfusion of the recruited alveolar units is adequate. To evaluate functional recruitment induced by positive end-expiratory pressure, we assessed pulmonary conductance for gas exchange based on lung diffusion for carbon monoxide and its components, including pulmonary capillary blood volume. DESIGN: Prospective, randomized, crossover study. SETTING: Medical intensive care unit of a university hospital. PATIENTS: Sixteen patients with lung injury/acute respiratory distress syndrome as well as eight control patients under invasive ventilation and eight healthy volunteers. INTERVENTIONS: Mechanical ventilation with two levels of positive end-expiratory pressure (5 and 15 cm H2O). MEASUREMENTS AND MAIN RESULTS: Lung diffusion for carbon monoxide and lung volumes, arterial blood gas analysis, and pressure-volume curves. In patients with acute respiratory distress syndrome, high positive end-expiratory pressure induced a 23% mean lung diffusion for carbon monoxide increase (4.4 +/- 1.7 mm Hg . min vs. 3.6 +/- 1.4 mL . mm Hg . min). In control patients and in healthy volunteers, lung diffusion for carbon monoxide values were (median [interquartile range]) 5.5 [3.8-8.0] mm Hg . min and 19.6 [15.1-20.6] mL . mm Hg . min, respectively. Among patients with acute respiratory distress syndrome, eight showed a >20% lung diffusion for carbon monoxide increase (responders) when increasing positive end-expiratory pressure. In the other eight, lung diffusion for carbon monoxide decreased or showed a <5% increase (nonresponders) with high positive end-expiratory pressure. Compared with nonresponders, responders at low positive end-expiratory pressure had smaller lungs with higher capillary blood volume-to-lung-volume ratio, higher values of the lower inflection point, and significantly greater increases in pulmonary capillary blood volume with high positive end-expiratory pressure. High positive end-expiratory pressure increased PaO2/Fio2 only in the responders. CONCLUSIONS: The functional response to positive end-expiratory pressure in patients with acute lung injury/acute respiratory distress syndrome seems better when the lungs are smaller and with a higher capillary blood-volume-to-lung-volume ratio. Lung diffusion for carbon monoxide measurement supplies additional information about functional lung recruitment, which is not synonymous with mechanical recruitment.

Poor sleep quality is associated with late noninvasive ventilation failure in patients with acute hypercapnic respiratory failure.

OBJECTIVE: To determine whether sleep quality helps to predict noninvasive ventilation outcome in patients with acute hypercapnic respiratory failure. Despite an initial clinical improvement, nearly one fourth of patients may fail noninvasive ventilation after several days. Because late intubation is associated with a poor prognosis, it may be useful to identify factors that may predict or explain late noninvasive ventilation failure. PATIENTS: We prospectively studied 27 hypercapnic patients in a medical intensive care unit who required noninvasive ventilation for >48 hrs. INTERVENTIONS: A 17-hr sleep polysomnography (3 PM-8 AM) was recorded 2 days to 4 days after noninvasive ventilation initiation. Late noninvasive ventilation failure was defined as death, endotracheal intubation, or persistent need for noninvasive ventilation on day 6. MEASUREMENTS AND MAIN RESULTS: An abnormal electroencephalographic pattern that eluded analysis by standard sleep-scoring criteria was noted in seven (50%) of the 14 patients with late noninvasive ventilation failure compared with one (8%) of the 13 patients successfully treated with noninvasive ventilation (p = .03). No clinical or laboratory variables explained the electroencephalographic differences. Patients failing noninvasive ventilation had poorer sleep quality with greater circadian sleep-cycle disruption and less nocturnal rapid eye movement sleep (6 mins [range, 0-12] vs. 26 mins [range, 6-49], p = .03), compared with patients successfully treated with noninvasive ventilation. Noninvasive ventilation failure was associated with delirium during the intensive care unit stay (64% vs. 0%). CONCLUSIONS: Late noninvasive ventilation failure in elderly patients with acute hypercapnic respiratory failure was associated with early sleep disturbances including an abnormal electroencephalographic pattern, disruption of the circadian sleep cycle, and decreased rapid eye movement sleep.

Sleep quality in mechanically ventilated patients: comparison of three ventilatory modes.

OBJECTIVES: To compare the influence of three ventilatory modes on sleep. DESIGN: Prospective, comparative, crossover study. SETTING: Medical intensive care unit in a university hospital. PATIENTS: Fifteen conscious, nonsedated, mechanically ventilated patients. INTERVENTIONS: Patients were successively ventilated with assist-control ventilation, clinically adjusted pressure support ventilation (cPSV), and automatically adjusted pressure support ventilation (aPSV). Sleep polysomnography was performed during three consecutive 6-hr periods, one with each mode in random order. Airway pressure and thorax and abdomen plethysmography were used to diagnose central apneas and ineffective efforts. MEASUREMENTS AND MAIN RESULTS: The main abnormalities were a low percentage of rapid eye movement (REM) sleep counting, for a median (25th-75th percentiles) of 10% (3.5-12.5) of total sleep, and a highly fragmented sleep with 29 arousals and awakenings per hour of sleep. REM sleep duration was similar in the three ventilatory modes, 7% in assist-control, 4% in aPSV, and 1% during cPSV (p = .54), as well as in the fragmentation index, 31 arousals and awakenings per hour in assist-control, 32 in aPSV, and 34 during cPSV (p = .62). Ineffective efforts occurred similarly with the three modes (seven per hour of sleep in assist-control, 16 in aPSV, and 12 during cPSV) or central apneas during PSV (five in aPSV, seven during cPSV). Minute ventilation was similar with the three modes. CONCLUSIONS: In conscious, mechanically ventilated patients, sleep architecture was highly abnormal, with a short REM stage and a high degree of fragmentation. The ventilatory mode did not influence sleep pattern, arousals, awakenings, and ineffective efforts.

Reduction of patient-ventilator asynchrony by reducing tidal volume during pressure-support ventilation.

OBJECTIVE: To identify ventilatory setting adjustments that improve patient-ventilator synchrony during pressure-support ventilation in ventilator-dependent patients by reducing ineffective triggering events without decreasing tolerance. DESIGN AND SETTING: Prospective physiological study in a 13-bed medical intensive care unit in a university hospital. PATIENTS AND PARTICIPANTS: Twelve intubated patients with more than 10% of ineffective breaths while receiving pressure-support ventilation. INTERVENTIONS: Flow, airway-pressure, esophageal-pressure, and gastric-pressure signals were used to measure patient inspiratory effort. To decrease ineffective triggering the following ventilator setting adjustments were randomly adjusted: pressure support reduction, insufflation time reduction, and change in end-expiratory pressure. MEASUREMENTS AND RESULTS: Reducing pressure support from 20.0 cm H(2)O (IQR 19.5-20) to 13.0 (12.0-14.0) reduced tidal volume [10.2ml /kg predicted body weight (7.2-11.5) to 5.9 (4.9-6.7)] and minimized ineffective triggering events [45% of respiratory efforts (36-52) to 0% (0-7)], completely abolishing ineffective triggering in two-thirds of patients. The ventilator respiratory rate increased due to unmasked wasted efforts, with no changes in patient respiratory rate [26.5 breaths/min (23.1-31.9) vs. 29.4 (24.6-34.5)], patient effort, or arterial PCO(2). Shortening the insufflation time reduced ineffective triggering events and patient effort, while applying positive end-expiratory pressure had no influence on asynchrony. CONCLUSIONS: Markedly reducing pressure support or inspiratory duration to reach a tidal volume of about 6 ml/kg predicted body weight eliminated ineffective triggering in two-thirds of patients with weaning difficulties and a high percentage of ineffective efforts without inducing excessive work of breathing or modifying patient respiratory rate.

Diaphragmatic dysfunction in patients with ICU-acquired weakness and its impact on extubation failure.

PURPOSE: Diaphragm function is rarely studied in intensive care patients with unit-acquired weakness (ICUAW) in whom weaning from mechanical ventilation is challenging. The aim of the present study was to evaluate the diaphragm function and the outcome using a multimodal approach in ICUAW patients. METHODS: Patients were eligible if they were diagnosed for ICUAW [Medical Research Council (MRC) Score <48], mechanically ventilated for at least 48 h and were undergoing a spontaneous breathing trial. Diaphragm function was assessed using magnetic stimulation of the phrenic nerves (change in endotracheal tube pressure), maximal inspiratory pressure and ultrasonographically (thickening fraction). Diaphragmatic dysfunction was defined by a change in endotracheal tube pressure below 11 cmH2O. The endpoints were to describe the correlation between diaphragm function and ICUAW and its impact on extubation. RESULTS: Among 185 consecutive patients ventilated for more than 48 h, 40 (22 %) with a MRC score of 31 [20-36] were included. Diaphragm dysfunction was observed with ICUAW in 32 patients (80 %). Change in endotracheal tube pressure and MRC score were not correlated. Maximal inspiratory pressure was correlated with change in endotracheal tube pressure after magnetic stimulation of the phrenic nerves (r = 0.43; p = 0.005) and MRC score (r = 0.34; p = 0.02). Thickening fraction was less than 20 % in 70 % of the patients and was statistically correlated with change in endotracheal tube pressure (r = 0.4; p = 0.02) but not with MRC score. Half of the patients could be extubated without needing reintubation within 72 h. CONCLUSION: Diaphragm dysfunction is frequent in patients with ICU-acquired weakness (80 %) but poorly correlated with the ICU-acquired weakness MRC score. Half of the patients with ICU-acquired weakness were successfully extubated. Half of the patients who failed the weaning process died during the ICU stay.

A new classification for sleep analysis in critically ill patients.

BACKGROUND: Patients in intensive care units (ICUs) experience severe sleep alterations and conventional sleep scoring rules are difficult to use in these patients. In a previous study, we showed that abnormal sleep EEG and wake EEG patterns could predict the outcome of noninvasive ventilation in a group of patients treated for acute respiratory failure. Our aims were to assess the prevalence of these abnormal sleep/wake EEG patterns in a larger group and search for objective parameters to help their identification. METHODS: We reviewed sleep studies previously performed with full polysomnography during 17-h in conscious nonsedated ICU patients receiving invasive ventilation during weaning or noninvasive ventilation for acute respiratory failure. RESULTS: We included 57 patients. Sleep scoring using conventional rules was not feasible in 16 (28%) patients due to the absence of stage-2 markers. Wake EEG in these 16 patients, although recognizable, showed abnormal features, including decreased reactivity to eye opening and slower peak EEG frequency compared to patients with normal sleep-wake EEGs. CONCLUSION: In almost one third of awake mechanically ventilated ICU patients, sleep cannot be classified with standard criteria. Two new states, atypical sleep and pathologic wakefulness, need to be added. We suggest rules for scoring these states. The origin and links with outcomes of these abnormal EEG patterns deserve investigation.

Delivery of helium­oxygen mixture during spontaneous breathing: evaluation of three high-concentration face masks.

PURPOSE: To evaluate the efficacy of delivering a mixture of helium and oxygen gas (He­O2) in spontaneous ventilation. Three high oxygen flow reservoir masks were tested: the Heliox21, specifically designed for helium; the Hi-Ox80 mask, with an inspiratory and an expiratory valve; and a standard high-concentration face mask. METHODS: This prospective randomized crossover study was performed in six healthy volunteers in a laboratory setting. Volunteers breathed a mixture of 78% He/22% O2 through each of the masks under two different breathing conditions (rest and hyperventilation: minute ventilation of 14.9 ± 6.1 and 26.7 ± 8.7 L min(−1), respectively) and four different He­O2 flow rates (7, 10, 12, and 15 L min(−1)). RESULTS: A nasopharyngeal catheter was used to estimate He pharyngeal concentration (Fp [He]) in the airways in order to determine the percentage of contamination with room air (% air cont) at end-expiration. Under all testing conditions, the Hi-Ox80 mask presented a significantly lower % air cont. During resting breathing pattern, a Fp [He] higher than 50% was achieved in 54% of the tests performed with the Hi-Ox80 mask compared to 29% for the Heliox21 mask and only 17% for the standard mask. At hyperventilation, a Fp [He] higher than 50% was achieved in 17% of the tests performed with the Hi-Ox mask compared to 4% for the other two masks. CONCLUSION: He­O2 administration via the usual high-concentration reservoir masks results in significant dilution by room air. The Hi-Ox80 mask minimized room air contamination and much more frequently achieved a pharyngeal He concentration higher than 50%.

Bench test evaluation of volume delivered by modern ICU ventilators during volume-controlled ventilation.

PURPOSE: During volume-controlled ventilation, part of the volume delivered is compressed into the circuit. To correct for this phenomenon, modern ventilators use compensation algorithms. Humidity and temperature also influence the delivered volume. METHODS: In a bench study at a research laboratory in a university hospital, we compared nine ICU ventilators equipped with compensation algorithms, one with a proximal pneumotachograph and one without compensation. Each ventilator was evaluated under normal, obstructive, and restrictive conditions of respiratory mechanics. For each condition, three tidal volumes (V (T)) were set (300, 500, and 800 ml), with and without an inspiratory pause. The insufflated volume and the volume delivered at the Y-piece were measured independently, without a humidification device, under ambient temperature and pressure and dry gas conditions. We computed the actually delivered V (T) to the lung under body temperature and pressure and saturated water vapour conditions (BTPS). RESULTS: For target V (T) values of 300, 500, and 800 ml, actually delivered V (T) under BTPS conditions ranged from 261 to 396 ml (-13 to +32%), from 437 to 622 ml (-13 to +24%), and from 681 to 953 ml (-15 to +19%), respectively (p < 0.01). Respiratory system mechanics and application of an inspiratory pause significantly affected actually delivered V (T). Assuming a set V (T) of 6 ml/kg of predicted body weight, a difference of 1-2 ml/kg with actually delivered V (T) would be commonly observed. CONCLUSION: The difference between preset V (T) and actually delivered V (T) is clinically meaningful and differs across modern ICU ventilators.

A bench study of intensive-care-unit ventilators: new versus old and turbine-based versus compressed gas-based ventilators.

OBJECTIVE: To compare 13 commercially available, new-generation, intensive-care-unit (ICU) ventilators in terms of 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. METHODS: 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. RESULTS: Significant differences were found across the ventilators, with a range of triggering delays from 42 to 88 ms for all conditions averaged (P < 0.001). Under difficult conditions, the triggering delay was longer than 100 ms and the pressurization was poor for 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 bench comparison in 2000. CONCLUSION: Technical performance of trigger function, pressurization capacity, and expiratory resistance differs considerably across new-generation ICU ventilators. ICU ventilators seem to have reached a technical ceiling in recent years, and some ventilators still perform inadequately.