The spontaneous breathing pattern and work of breathing of patients with acute respiratory distress syndrome and acute lung injury

BACKGROUND: The spontaneous breathing pattern and its relationship to compliance, resistance, and work of breathing (WOB) has not been examined in patients with acute respiratory distress syndrome (ARDS) or acute lung injury (ALI). Clinically, the ratio of respiratory frequency to tidal volume (f/VT) during spontaneous breathing may reflect adaptation to altered compliance, resistance, and increased WOB. We examined the relationship between f/VT, WOB, and respiratory system mechanics in patients with ARDS/ALI. METHODS: Data from spontaneous breathing trials were collected from 33 patients (20 with ARDS, 13 with ALI) at various points in their disease course. WOB and respiratory system mechanics were measured with a pulmonary mechanics monitor that incorporates Campbell diagram software. Differences between the patients with ARDS and ALI were assessed with 2-sided unpaired t tests. Multivariate linear regression models were constructed to assess the relationship between f/VT and other pulmonary-related variables. RESULTS: Patients with ARDS had significantly lower compliance than those with ALI (24 +/- 6 mL/cm H2O vs 40 +/- 13 mL/cm H2O, respectively, p < 0.001), but this did not translate into significant differences in either WOB (1.70 +/- 0.59 J/L vs 1.43 +/- 0.90 J/L, respectively, p = 0.30) or f/VT (137 +/- 82 vs 107 +/- 49, respectively, p = 0.23). Multivariate linear regression modeling revealed that peak negative esophageal pressure, central respiratory drive, duration of ARDS/ALI, minute ventilation deficit between mechanical ventilation and spontaneous breathing, and female gender were the strongest predictors of f/VT. CONCLUSION: The characteristic rapid shallow breathing pattern in patients with ARDS/ALI occurs in the context of markedly diminished compliance, elevated respiratory drive, and increased WOB. That f/VT had a strong, inverse relationship to peak negative esophageal pressure also may reflect the influence of muscle weakness.     

Feedback and education improve physician compliance in use of lung-protective mechanical ventilation

Objective Use of lung-protective mechanical ventilation (MV) by applying lower tidal volumes is recommended in patients suffering from acute lung injury (ALI) or acute respiratory distress syndrome (ARDS). Recent data suggest that lung-protective MV may benefit non-ALI/ARDS patients as well. This study analyzed tidal volume settings in three ICUs in The Netherlands to determine the effect of feedback and education concerning use of lung-protective MV.Design and setting Observational study in one academic and two nonacademic closed format ICUs.Patients Intubated mechanically ventilated subjects.Interventions Feedback and education concerning lung-protective MV with special attention to the importance of closely adjusting tidal volumes to predicted body weight (PBW).Results Tidal volumes declined significantly within 6 months after intervention (from 9.8±2.0 at baseline to 8.1±1.7 ml/kg PBW) as the percentage of undesirable ventilation data points, defined as tidal volumes greater than 8 ml/kg PBW (84% vs. 48%). There were no differences between patients meeting the international definition criteria for ALI/ARDS and those not. Only four patients received tidal volumes less than 6 ml/kg PBW. Lower tidal volumes were still used after 12 months. Tidal volumes in patients on mandatory MV and patients breathing on spontaneous modes were similar.Conclusions Feedback and education improve physician compliance in use of lung-protective MV.     

Barriers to providing lung-protective ventilation to patients with acute lung injury

OBJECTIVE: No studies have explored the barriers to implementing lung-protective ventilation in patients with acute lung injury (ALI) and acute respiratory distress syndrome (ARDS). Our objective was to identify barriers to using lung-protective ventilation in patients with ALI/ARDS. DESIGN: Survey with content analysis of open-ended responses. SETTING: Medical center. PARTICIPANTS: Experienced intensive care unit nurses and respiratory therapists network identified through purposive sampling at hospitals from the ARDS Network, a National Institutes of Health-sponsored research consortium. INTERVENTIONS: Survey. RESULTS: Fifty-five surveys representing all ten ARDS Network sites were received. Twenty-seven (49%) of the respondents were intensive care unit nurses, 24 (44%) were respiratory therapists, and four did not indicate their profession. Clinicians had used lung-protective ventilation in a median of 20 (interquartile range, 10-50) patients with ALI/ARDS. Respondents identified physician willingness to relinquish control of ventilator, physician recognition of ALI/ARDS, and physician perceptions of patient contraindications to low tidal volumes as important barriers to initiating lung-protective ventilation. Important barriers to continuing patients on lung-protective ventilation were concerns over patient discomfort and tachypnea and concerns over hypercapnia, acidosis, and hypoxemia. Techniques for overcoming barriers were identified including specific ventilator setup recommendations, clinician education, and tools to assess patient discomfort. CONCLUSIONS: Experienced bedside clinicians perceive important barriers to implementing lung-protective ventilation. Successful strategies to increase use of lung-protective ventilation should target these barriers.     

Approaches to conventional mechanical ventilation of the patient with acute respiratory distress syndrome

To minimize ventilator-induced lung injury, attention should be directed toward avoidance of alveolar over-distention and cyclical opening and closure of alveoli. The most impressive study of mechanical ventilation to date is the Acute Respiratory Distress Syndrome (ARDS) Network study of higher versus lower tidal volume (V(T)), which reported a reduction in mortality from 39.8% to 31.0% with 6 mL/kg ideal body weight rather than 12 mL/kg ideal body weight (number-needed-to-treat of 12 patients). To achieve optimal lung protection, the lowest plateau pressure and V(T) possible should be selected. What is most important is limitation of V(T) and alveolar distending pressure, regardless of the mode set on the ventilator. Accumulating observational evidence suggests that V(T) should be limited in all mechanically ventilated patients-even those who do not have ALI/ARDS. Evidence does not support the use of pressure controlled inverse-ratio ventilation. Although zero PEEP is probably injurious, an area of considerable controversy is the optimal setting of PEEP. Available evidence does not support the use of higher PEEP, compared to lower PEEP, in unselected patients with acute lung injury (ALI)/ARDS. However, results of a meta-analysis using individual patients from 3 randomized controlled trials suggest that higher PEEP should be used for ARDS, whereas lower PEEP may be more appropriate in patients with ALI. PEEP should be set to maximize alveolar recruitment while avoiding over-distention. Many approaches for setting PEEP have been described, but evidence is lacking that any one approach is superior to any other. In most, if not all, cases of ALI/ARDS, conventional ventilation strategies can be used effectively to provide lung-protective ventilation strategies.     

Automatic selection of breathing pattern using adaptive support ventilation

OBJECTIVE: In a cohort of mechanically ventilated patients to compare the automatic tidal volume (VT)-respiratory rate (RR) combination generated by adaptive support ventilation (ASV) for various lung conditions. DESIGN AND SETTING: Prospective observational cohort study in the 11-bed medicosurgical ICU of a general hospital. PATIENTS: 243 patients receiving 1327 days of invasive ventilation on ASV. MEASUREMENTS: Daily collection of ventilator settings, breathing pattern, arterial blood gases, and underlying clinical respiratory conditions categorized as: normal lungs, ALI/ARDS, COPD, chest wall stiffness, or acute respiratory failure. RESULTS: Overall the respiratory mechanics differed significantly with the underlying conditions. In passive patients ASV delivered different VT-RR combinations based on the underlying condition, providing higher VT and lower RR in COPD than in ALI/ARDS: 9.3ml/kg (8.2-10.8) predicted body weight (PBW) and 13 breaths/min (11-16) vs. 7.6ml/kg (6.7-8.8) PBW and 18 breaths/min (16-22). In patients actively triggering the ventilator the VT-RR combinations did not differ between COPD, ALI/ARDS, and normal lungs. CONCLUSIONS: ASV selects different VT-RR combinations based on respiratory mechanics in passive, mechanically ventilated patients.     

Use of hypothermia to allow low-tidal-volume ventilation in a patient with ARDS

Low-tidal-volume ventilation reduces mortality in patients with ARDS, but there are often challenges in implementing lung-protective ventilation, such as acidosis from hypercapnia. In a patient with severe ARDS we achieved adequate ventilation with a very low tidal volume (4 mL/kg ideal body weight) by inducing mild hypothermia (body temperature 35-36°C).     

The effects of pressure control versus volume control assisted ventilation on patient work of breathing in acute lung injury and acute respiratory distress syndrome

BACKGROUND: Patient work of breathing (WOB) during assisted ventilation is reduced when inspiratory flow (V(I)) from the ventilator exceeds patient flow demand. Patients in acute respiratory failure often have unstable breathing patterns and their requirements for V(I) may change from breath to breath. Volume control ventilation (VCV) traditionally incorporates a pre-set ventilator V(I) that remains constant even under conditions of changing patient flow demand. In contrast, pressure control ventilation (PCV) incorporates a variable decelerating flow wave form with a high ventilator V(I) as inspiration commences. We compared the effects of flow patterns on assisted WOB during VCV and PCV. METHODS: WOB was measured with a BICORE CP-100 monitor (incorporating a Campbell Diagram) in a prospective, randomized cross-over study of 18 mechanically ventilated adult patients with acute lung injury (ALI) or acute respiratory distress syndrome (ARDS). Tidal volume, inspiratory time, and mean ventilator V(I) were constant in each mode. RESULTS: At comparable levels of respiratory drive and minute ventilation, patient WOB was significantly lower with PCV than with VCV (0.59 +/- 0.42 J/L vs 0.70 +/- 0.58 J/L, respectively, p < 0.05). Ventilator peak V(I) was significantly higher with PCV than with VCV (103.2 +/- 22.8 L/min vs 43.8 L/min, respectively, p < 0.01). CONCLUSIONS: In the setting of ALI and ARDS, PCV significantly reduced patient WOB relative to VCV. The decrease in patient WOB was attributed to the higher ventilator peak V(I) of PCV.     

高呼气末正压通气结合小潮气量对急性肺损伤/急性呼吸窘迫综合征患者预后影响的荟萃分析

目的 系统评价高呼气末正压(PEEP)与低PEEP机械通气对急性肺损伤/急性呼吸窘迫综合征(ALI/ARDS)患者预后的影响.方法 通过检索美国<医学索引>、荷兰<医学文摘>、Cochrane临床试验数据库、中国生物医学文献数据库(CBM)和中国期刊网全文数据库(CNKI)等文献数据库,全面收集全世界范围内高PEEP与低PEEP治疗ALI/ARDS患者的随机对照试验(RCT),提取文献中的相关资料和评估方法学质量,而后采用Cochrane协作网RevMan 5.0软件对资料进行荟萃分析(Meta分析).结果 最终纳入6个RCT共2484例ALI/ARDS患者.A亚组的3个RCT中试验组采用了高PEEP(相对于对照组),对照组采用了低PEEP(相对于试验组),两组均采用了小潮气量(6 ml/kg)通气;B亚组的3个RCT中试验组采用了高PEEP加小潮气量,对照组采用了低PEEP加传统潮气量通气.合并结果显示,B亚组中高PEEP加小潮气量通气策略可以降低患者的28 d病死率[Peto比值比(OR)=0.40,95%可信区间(95%CI)0.22～0.72,P=0.003]和气压伤发生率(OR=0.20,95%CI 0.05～0.82,P=0.02);A亚组中,两组患者的28 d病死率(OR=0.86,95%CI 0.72～1.02,P=0.08)和气压伤发生率(OR=1.19,95%CI 0.89～1.58,P=0.25)差异无统计学意义.结论 高PEEP加小潮气量通气可以改善ALI/ARDS患者的28 d病死率和气压伤发生率,单独高PEEP的作用需要进一步评价.

Abstract：

Objective To compare the effects of high and low positive end-expiratory pressure (PEEP) levels on prognosis of patients with acute lung injury/acute respiratory distress syndrome (ALI/ARDS). Methods The data in PubMed, EMbase, Cochrane Library, CBM and CNKI were retrieved. All randomized controlled trials (RCTs) of treatment of ALI/ARDS with PEEP with high or low level were included. Study selection and assessment, data collection and analyses were undertaken by two independent reviewers. Meta-analyses were done using Cochrane Collaboration's RevMan 5.0 software.Results Six RCTs, involving a total of 2 484 patients of ALI/ARDS were included in the review. According to ventilation strategy, all trials were divided into subgroup A (high PEEP+low tidal volume of 6 ml/kg vs.low PEEP+low tidal volume) and subgroup B (high PEEP+low tidal volume vs. low PEEP+traditional tidal volume). In subgroup B, there were three RCTs, and high PEEP was found to be associated with a lower 28-day mortality [odds ratio (OR)=0. 40, 95% confidence interval (95%CI) 0.22 -0.72, P=0.003]and a lower barotraumas (OR = 0.20,95%CI 0.05 - 0.82, P = 0.02) in patients with ALI/ARDS. In subgroup A, there were three RCTs, and it was found that the differences in 28-day mortality (OR=0.86,95%CI 0.72 - 1.02, P = 0.08) and barotraumas (OR = 1.19, 95%CI 0.89 - 1.58, P= 0.25) were not significant. Conclusion As compared with conventional ventilation, high PEEP and low tidal volume ventilation are associated with improved survival and a lower rate of barotrauma in patients with ALI/ARDS.It is necessary to further confirm the role of high PEEP only in the ventilation strategy in patients with ALI/ARDS.     

Acute effects of tidal volume strategy on hemodynamics, fluid balance, and sedation in acute lung injury

OBJECTIVE: To examine the effects of mechanical ventilation with a tidal volume of 6 mL/kg compared with 12 mL/kg predicted body weight on hemodynamics, vasopressor use, fluid balance, diuretics, sedation, and neuromuscular blockade within 48 hrs in patients with acute lung injury and acute respiratory distress syndrome. DESIGN: Retrospective analysis of a previously conducted randomized, clinical trial. SETTING: Two adult intensive care units at a tertiary university medical center and a large county hospital. PATIENTS: One hundred eleven patients who were enrolled in the National Institutes of Health ARDS Network trial at the University of California, San Francisco. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: Compared with 12 mL/kg predicted body weight, treatment with a tidal volume of 6 mL/kg predicted body weight had no adverse effects on hemodynamics. There were also no differences in the need for supportive therapies, including vasopressors, intravenous fluids, or diuretics. In addition, there were no differences in body weight, urine output, and fluid balance. Finally, there was no difference in the need for sedation or neuromuscular blockade between the two tidal volume protocols. CONCLUSIONS: When compared with ventilation with 12 mL/kg predicted body weight, patients treated with the lung-protective 6 mL/kg predicted body weight tidal volume protocol had no difference in their supportive care requirements. Therefore, concerns regarding potential adverse effects of this protocol should not preclude its use in patients with acute lung injury or the acute respiratory distress syndrome.     

Protective effects of low tidal volume ventilation in a rabbit model of Pseudomonas aeruginosa-induced acute lung injury

OBJECTIVE: To determine whether low "stretch" mechanical ventilation protects animals from clinical sepsis after direct acute lung injury with Pseudomonas aeruginosa as compared with high "stretch" ventilation. DESIGN: Prospective study. SETTING: Experimental animal laboratory. SUBJECTS: Twenty-seven anesthetized and paralyzed rabbits. INTERVENTIONS: P. aeruginosa (109 colony forming units) was instilled into the right lungs of rabbits that were then ventilated at a tidal volume of either 15 mL/kg (n = 11) or 6 mL/kg (n = 7) for 8 hrs. Control animals were ventilated at a tidal volume of either 15 mL/kg (n = 4) or 6 mL/kg (n = 5) for 8 hrs, but an instillate without bacteria was used. A positive end-expiratory pressure of 3-5 cm H2O was used for all experiments. Radiolabeled albumin was used as a marker of alveolar epithelial permeability. MEASUREMENTS AND MAIN RESULTS: Hemodynamics, arterial blood gas determination, alveolar permeability, wet-to-dry ratios on lungs, and time course of bacteremia were determined. When final values were compared with the values at the beginning of the experiment, there were significant decreases in mean arterial pressure (from 104 +/- 15 to 57 +/- 20 mm Hg), pH (from 7.46 +/- 0.04 to 7.24 +/- 15), Pao2 (from 528 +/- 35 to 129 +/- 104 torr [70.4 +/- 4.7 to 17.2 +/- 13.9 kPa]), and temperature (from 38.2 +/- 1 to 36.2 +/- 1.2 degrees C) in the high tidal volume group, whereas no significant differences were found in the low tidal volume group. Decreased alveolar permeability was shown in the low tidal volume group, as was decreased extravascular lung water in the uninstilled lung in the low tidal volume group (12.7 +/- 2.5 vs. 4.3 +/- 0.45 g H2O/g dry lung). No noteworthy difference was noted in the time course of bacteremia, although there was a trend toward earlier bacteremia in the high tidal volume group. CONCLUSIONS: In our animal model of P. aeruginosa-induced acute lung injury, low tidal volume ventilation was correlated with improved oxygenation, hemodynamic status, and acid-base status as well as decreased alveolar permeability and contralateral extravascular lung water.     

Prediction of post-extubation work of breathing

OBJECTIVE: To evaluate which mode of preextubation ventilatory support most closely approximates the work of breathing performed by spontaneously breathing patients after extubation. DESIGN: Prospective observational design. SETTING: Medical, surgical, and coronary intensive care units in a university hospital. PATIENTS: A total of 22 intubated subjects were recruited when weaned and ready for extubation. INTERVENTIONS: Subjects were ventilated with continuous positive airway pressure at 5 cm H2O, spontaneous ventilation through an endotracheal tube (T piece), and pressure support ventilation at 5 cm H2O in randomized order for 15 mins each. At the end of each interval, we measured pulmonary mechanics including work of breathing reported as work per liter of ventilation, respiratory rate, tidal volume, negative change in esophageal pressure, pressure time product, and the airway occlusion pressure 100 msec after the onset of inspiratory flow, by using a microprocessor-based monitor. Subsequently, subjects were extubated, and measurements of pulmonary mechanics were repeated 15 and 60 mins after extubation. MEASUREMENTS AND MAIN RESULTS: There were no statistical differences between work per liter of ventilation measured during continuous positive airway pressure, T piece, or pressure support ventilation (1.17+/-0.67 joule/L, 1.11+/-0.57 joule/L, and 0.97+/-0.57 joule/L, respectively). However, work per liter of ventilation during all three preextubation modes was significantly lower than work measured 15 and 60 mins after extubation (p < .05). Tidal volume during pressure support ventilation and continuous positive airway pressure (0.46+/-0.11 L and 0.44+/-0.11 L, respectively) were significantly greater than tidal volume during both T-piece breathing and spontaneous breathing 15 mins after extubation (p < .05). Negative change in esophageal pressure, the airway occlusion pressure 100 msec after the onset of inspiratory flow, and pressure time product were significantly higher after extubation than during any of the three preextubation modes (p < .05). CONCLUSIONS: Work per liter of ventilation, negative change in esophageal pressure, the airway occlusion pressure 100 msec after the onset of inspiratory flow, and pressure time product all significantly increase postextubation. Tidal volume during continuous positive airway pressure or pressure support ventilation overestimates postextubation tidal volume.     

Association between tidal volume size,duration of ventilation,and sedation needs in patients without acute respiratory distress syndrome: an individual patient data meta-analysis

Purpose

Mechanical ventilation with lower tidal volumes (≤6 ml/kg of predicted body weight, PBW) could benefit patients without acute respiratory distress syndrome (ARDS). However, tidal volume reduction could be associated with increased patient discomfort and sedation needs, and consequent longer duration of ventilation. The aim of this individual patient data meta-analysis was to assess the associations between tidal volume size, duration of mechanical ventilation, and sedation needs in patients without ARDS.

Methods

Studies comparing ventilation with different tidal volume sizes in patients without ARDS were screened for inclusion. Corresponding authors were asked to provide individual participant data. Patients were assigned to three groups based on tidal volume size (≤6 ml/kg PBW, 6–10 ml/kg PBW, or ≥10 ml/kg PBW). Ventilator-free days, alive at day 28, and dose and duration of sedation (propofol and midazolam), analgesia (fentanyl and morphine), and neuromuscular blockade (NMB) were compared.

Results

Seven investigations (2,184 patients) were included in the analysis. The number of patients breathing without assistance by day 28 was higher in the group ventilated with tidal volume ≤6 ml/kg PBW compared to those ventilated with tidal volume ≥10 ml/kg PBW (93.1 vs. 88.6 %; p = 0.027, respectively). Only two investigations (187 patients) could be included in the meta-analysis of sedation needs. There were neither differences in the percentage of study days that patients received sedatives, opioids, or NMBA nor in the total dose of benzodiazepines, propofol, opioids, and NMBA.

Conclusions

This meta-analysis suggests that use of lower tidal volumes in patients without ARDS at the onset of mechanical ventilation could be associated with shorter duration of ventilation. Use of lower tidal volumes seems not to affect sedation or analgesia needs, but this must be confirmed in a robust, well-powered randomized controlled trial.     

Comparison of prone positioning and high-frequency oscillatory ventilation in patients with acute respiratory distress syndrome

OBJECTIVE: Both prone position and high-frequency oscillatory ventilation (HFOV) have the potential to facilitate lung recruitment, and their combined use could thus be synergetic on gas exchange. Keeping the lung open could also potentially be lung protective. The aim of this study was to compare physiologic and proinflammatory effects of HFOV, prone positioning, or their combination in severe acute respiratory distress syndrome (ARDS). DESIGN:: Prospective, comparative randomized study. SETTING: A medical intensive care unit. PATIENTS: Thirty-nine ARDS patients with a Pao2/Fio2 ratio <150 mm Hg at positive end-expiratory pressure > or =5 cm H2O. INTERVENTIONS: After 12 hrs on conventional lung-protective mechanical ventilation (tidal volume 6 mL/kg of ideal body weight, plateau pressure not exceeding the upper inflection point, and a maximum of 35 cm H2O; supine-CV), 39 patients were randomized to receive one of the following 12-hr periods: conventional lung-protective mechanical ventilation in prone position (prone-CV), HFOV in supine position (supine-HFOV), or HFOV in prone position (prone-HFOV). MEASUREMENTS AND MAIN RESULTS: Prone-CV (from 138 +/- 58 mm Hg to 217 +/- 110 mm Hg, p < .0001) and prone-HFOV (from 126 +/- 40 mm Hg to 227 +/- 64 mm Hg, p < 0.0001) improved the Pao2/Fio2 ratio whereas supine-HFOV did not alter the Pao2/Fio2 ratio (from 134 +/- 57 mm Hg to 138 +/- 48 mm Hg). The oxygenation index ({mean airway pressure x Fio2 x 100}/Pao2) decreased in the prone-CV and prone-HFOV groups and was lower than in the supine-HFOV group. Interleukin-8 increased significantly in the bronchoalveolar lavage fluid (BALF) in supine-HFOV and prone-HFOV groups compared with prone-CV and supine-CV. Neutrophil counts were higher in the supine-HFOV group than in the prone-CV group. CONCLUSIONS: Although HFOV in the supine position does not improve oxygenation or lung inflammation, the prone position increases oxygenation and reduces lung inflammation in ARDS patients. Prone-HFOV produced similar improvement in oxygenation like prone-CV but was associated with higher BALF indexes of inflammation. In contrast, supine-HFOV did not improve gas exchange and was associated with enhanced lung inflammation.     

Biological markers of lung injury before and after the institution of positive pressure ventilation in patients with acute lung injury

Background  

Several biological markers of lung injury are predictors of morbidity and mortality in patients with acute lung injury (ALI). The low tidal volume lung-protective ventilation strategy is associated with a significant decrease in plasma biomarker levels compared to the high tidal volume ventilation strategy. The primary objective of this study was to test whether the institution of lung-protective positive pressure ventilation in spontaneously ventilating patients with ALI exacerbates pre-existing lung injury by using measurements of biomarkers of lung injury before and after intubation.     

Evaluation of auto-regulated inspiratory support during rebreathing and acute lung injury in pigs

BACKGROUND: Auto-regulated inspiratory support mode (ARIS) is an original closed-loop pressure-support system that regulates the slope ("A") and the initial level ("B") of the applied inspiratory pressure, in order to achieve an optimal minute ventilation under constrained respiratory frequency, tidal volume, and maximum inspiratory airway pressure. The servo-controlled design results in a more or less decreasing applied pressure. OBJECTIVE: The aim of this study was to evaluate the ARIS behavior, compared with pressure-support ventilation at a constant applied pressure. METHODS: ARIS and pressure-support ventilation were randomly applied to 2 pig models of increasing ventilatory demand induced by a rebreathing test (n = 6), and of altered lung compliance induced by bronchoalveolar lavage (n = 6). The breathing pattern, work of breathing, and blood gas values were compared. ARIS automatically increased the mean inspiratory airway pressure in both groups. This increase was obtained in the rebreathing group by increasing "B" (35 +/- 3.5 cm H2O vs 42.8 +/- 2.5 cm H2O) and in the lung-injury group by decreasing the absolute value of "A" (25 +/- 5.5 cm H2O/s vs 14.7 +/- 8.6 cm H2O/s). RESULTS: There were significant differences (p < 0.05) between ARIS and pressure-support ventilation. In the rebreathing group, tidal volume was 692 +/- 63 mL versus 606 +/- 96 mL, work of breathing was 1.17 +/- 0.45 J/L versus 1.44 +/- 0.27 J/L, and P(aCO2) was 54 +/- 9 mm Hg versus 63 +/- 7 mm Hg. In the lung-injury group, respiratory frequency was 25 +/- 4 breaths/min versus 42 +/- 10 breaths/min, tidal volume was 477 +/- 67 mL versus 300 +/- 63 mL, work of breathing was 0.54 +/- 0.3 J/L versus 0.99 +/- 0.45 J/L, and P(aCO2) was 36 +/- 8 mm Hg versus 53 +/- 15 mm Hg. CONCLUSIONS: The ARIS servo control operates correctly, maintaining efficient ventilation facing an increase in respiratory demand or a decrease in respiratory system compliance.     

Hypercapnia in late-phase ALI/ARDS: providing spontaneous breathing using pumpless extracorporeal lung assist

Objective  The fibroproliferative phase of late ALI/ARDS as described by Hudson and Hough (Clin Chest Med 27:671–677, 2006) is associated with pronounced reductions in pulmonary compliance and an accompanying hypercapnia complicating low tidal volume mechanical ventilation. We report the effects of extracorporeal CO₂ removal by means of a novel pumpless extracorporeal lung assist (p-ECLA) on tidal volumes, airway pressures, breathing patterns and sedation management in pneumonia patients during late-phase ARDS. Design  Retrospective analysis. Setting  Fourteen-bed university hospital ICU. Patients  Ten consecutive late-phase ALI/ARDS patients with low pulmonary compliance, and severe hypercapnia. Intervention  Gas exchange, tidal volumes, airway pressures, breathing patterns and sedation requirements before (baseline) and after (2–4 days) initiation of treatment with p-ECLA were analysed. Patients were ventilated in a pressure-controlled mode with PEEP adjusted to pre-defined oxygenation goals. Measurements and main results  Median reduction in pCO₂ was 50% following institution of p-ECLA. Extracorporeal CO₂ removal enabled significant reduction in tidal volumes (to below 4 ml/kg predicted body weight) and inspiratory plateau pressures [30 (28.5/32.3) cmH₂O, median 25, 75% percentiles]. Normalization of pCO₂ levels permitted significant reduction in the dosages of analgesics and sedatives. The proportion of assisted spontaneous breathing increased within 24 h of instituting p-ECLA. Conclusion  Elimination of CO₂ by p-ECLA therapy allowed reduction of ventilator-induced shear stress through ventilation with tidal volumes below 4 ml/kg predicted body weight in pneumonia patients with severely impaired pulmonary compliance during late-phase ARDS. p-ECLA treatment supported control of breathing pattern while sedation requirements were reduced and facilitated the implementation of assisted spontaneous breathing.     

Tidal volume increases do not affect alveolar mechanics in normal lung but cause alveolar overdistension and exacerbate alveolar instability after surfactant deactivation

OBJECTIVE: We utilized microscopy to measure the impact of increasing tidal volume on individual alveolar mechanics (i.e., the dynamic change in alveolar size during tidal ventilation) in the living porcine lung. DESIGN: In three anesthetized, mechanically ventilated pigs, we observed normal alveoli (n = 27) and alveoli after surfactant deactivation by Tween 20 lavage (n = 26) at three different tidal volumes (6, 12, and 15 mL/kg). Alveolar area was measured at peak inspiration (I) and at end expiration (E) by image analysis and I minus E was calculated as an index of alveolar stability (I-Edelta). MEASUREMENTS AND MAIN RESULTS: In normal alveoli, increasing tidal volume did not change alveolar area at I (6 mL/kg = 9726 +/- 848 microm; 15 mL/kg = 9,637 +/- 884 microm ), E (6 mL/kg = 9747 +/- 800 microm; 15 mL/kg = 9742 +/- 853 microm ), or I-Edelta (6 mL/kg = -21 +/- 240 microm; 15 mL/kg = -105 +/- 229 microm ). In contrast, with surfactant deactivation, increasing tidal volume significantly increased alveolar area at I (6 mL/kg = 11,413 +/- 1032 microm; 15 mL/kg = 13,917 +/- 1214 microm ), at E (6 mL/kg = 10,462 +/- 906 microm; 15 mL/kg = 12,000 +/- 1066 microm ), and I-Edelta (6 mL/kg = 825 +/- 276 microm; 15 mL/kg = 1917 +/- 363 microm ). Moreover, alveolar instability (increased I-Edelta) was significantly increased at all tidal volumes with altered surface tension when compared with normal alveoli. CONCLUSIONS: We conclude that high tidal volume ventilation does not alter alveolar mechanics in the normal lung; however, in the surfactant-deactivated lung, it causes alveolar overdistension and exacerbates alveolar instability.     

Mechanical ventilation in acute lung injury and ARDS. Tidal volume reduction

Traditional mechanical ventilation practices used generous tidal volumes in patients with acute lung injury and acute respiratory distress syndrome (ALI/ARDS). This approach may have caused overdistention of aerated lung units, thus exacerbating lung injury in some patients. Several recent clinical trials of traditional versus lower tidal volume strategies in ALI/ARDS yielded disparate results. In the largest study, the lower tidal volume approach was associated with lower mortality and more ventilator-free days. This article reviews the rationale for tidal volume reduction in ALI/ARDS and the differences between the studies. Several different interpretations of the recent clinical trial results are addressed.     

Respective effects of end-expiratory and end-inspiratory pressures on alveolar recruitment in acute lung injury

OBJECTIVE: A low tidal volume can induce alveolar derecruitment in patients with acute lung injury. This study was undertaken to evaluate whether this resulted mainly from the decrease in tidal volume or from the reduction in end-inspiratory plateau pressure and whether there is any benefit in raising the level of positive end-expiratory pressure (PEEP) while plateau pressure is kept constant. DESIGN: Prospective crossover study. SETTING: Medical intensive care unit of a university teaching hospital. PATIENTS: Fifteen adult patients ventilated for acute lung injury (PaO2/FiO2, 158 +/- 34 mm Hg; lung injury score, 2.7 +/- 0.6). INTERVENTIONS: Three combinations were tested: PEEP at the lower inflection point with 6 mL/kg tidal volume, PEEP at the lower inflection point with 10 mL/kg tidal volume, and high PEEP with tidal volume at 6 mL/kg, keeping the plateau pressure similar to the preceding condition. MEASUREMENTS AND MAIN RESULTS: Pressure-volume curves at zero PEEP and at set PEEP were recorded, and recruitment was calculated as the volume difference between both curves for pressures ranging from 15 to 30 cm H2O. Arterial blood gases were measured for all patients. For a similar PEEP at the lower inflection point (10 +/- 3 cm H2O), tidal volume reduction (10 to 6 mL/kg) led to a significant derecruitment. A low tidal volume (6 mL/kg) with high PEEP (14 +/- 3 cm H2O), however, induced a significantly greater recruitment and a higher Pao than the two other strategies. CONCLUSION: At a given plateau pressure (i.e., similar end-inspiratory distension), lowering tidal volume and increasing PEEP increase recruitment and PaO2.     

Effects of recruitment maneuvers in patients with acute lung injury and acute respiratory distress syndrome ventilated with high positive end-expiratory pressure

Objective: Positive end-expiratory pressure (PEEP) and recruitment maneuvers (RMs) may partially reverse atelectasis and reduce ventilation-associated lung injury. The purposes of this study were to assess a) magnitude and duration of RM effects on arterial oxygenation and on requirements for oxygenation support (Fio2/PEEP) in patients with acute lung injury and acute respiratory distress syndrome (ALI/ARDS) receiving ventilation with low tidal volumes and high levels of PEEP; and b) frequency of adverse respiratory and circulatory events attributable to RMs. Design: Prospective, randomized, crossover study. Setting: Thirty-four intensive care units at 19 hospitals. Patients: Seventy-two patients with early ALI/ARDS. Baseline PEEP and Fio2 were 13.8 +/- 3.0 cm H2O and 0.39 +/- 0.10, respectively (mean +/- sd). Interventions: We conducted RMs by applying continuous positive airway pressure of 35-40 cm H2O for 30 secs. We conducted sham RMs on alternate days. We monitored oxyhemoglobin saturation by pulse oximetry (SpO2), Fio2/PEEP, blood pressure, and heart rate for 8 hrs after RMs and sham RMs. We examined chest radiographs for barotrauma. Measurements and Main Results: Responses to RMs were variable. Greatest increments from baseline SpO2 within 10 mins after RMs were larger than after sham RMs (1.7 +/- 0.2 vs. 0.6 +/- 0.3 %, mean +/- SEM, p < .01). Systolic blood pressure decreased more +/- 1.1 mm Hg, p < .01). Changes in Fio2/PEEP requirements were not significantly different at any time after RMs vs. sham RMs. Barotrauma was apparent on first radiographs after one RM and one sham RM.Conclusions: In ALI/ARDS patients receiving mechanical ventilation with low tidal volumes and high PEEP, short-term effects of RMs as conducted in this study are variable. Beneficial effects on gas exchange in responders appear to be of brief duration. More information is needed to determine the role of recruitment maneuvers in the management of ALI/ARDS.