Pressure-targeted, lung-protective ventilatory support in acute lung injury

Comparison of the effects of moderate and severe hypercapnic acidosis on ventilation-induced lung injury

Background

We have proved that hypercapnic acidosis (a PaCO₂ of 80-100 mmHg) protects against ventilator-induced lung injury in rats. However, there remains uncertainty regarding the appropriate target PaCO₂ or if greater CO₂ “doses” (PaCO₂ > 100 mmHg) demonstrate this effect. We wished to determine whether severe acute hypercapnic acidosis can reduce stretch-induced injury, as well as the role of nuclear factor-κB (NF-κB) in the effects of acute hypercapnic acidosis.

Methods

Fifty-four rats were ventilated for 4 hours with a pressure-controlled ventilation mode set at a peak inspiratory pressure (PIP) of 30 cmH₂O. A gas mixture of carbon dioxide with oxygen (FiCO₂ = 4-5%, FiCO₂ = 11-12% or FiCO₂ = 16-17%; FiO₂ = 0.7; balance N₂) was immediately administered to maintain the target PaCO₂ in the NC (a PaCO₂ of 35-45 mmHg), MHA (a PaCO₂ of 80-100 mmHg) and SHA (a PaCO₂ of 130-150 mmHg) groups. Nine normal or non-ventilated rats served as controls. The hemodynamics, gas exchange and inflammatory parameters were measured. The role of NF-κB pathway in hypercapnic acidosis-mediated protection from high-pressure stretch injury was then determined.

Results

In the NC group, high-pressure ventilation resulted in a decrease in PaO₂/FiO₂ from 415.6 (37.1) mmHg to 179.1 (23.5) mmHg (p < 0.001), but improved by MHA (379.9 ± 34.5 mmHg) and SHA (298.6 ± 35.3 mmHg). The lung injury score in the SHA group (7.8 ± 1.6) was lower than the NC group (11.8 ± 2.3, P < 0.05) but was higher than the MHA group (4.4 ± 1.3, P < 0.05). Compared with the NC group, after 4 h of high pressure ventilation, the MHA and SHA groups had decreases in MPO activity of 67% and 33%, respectively, and also declined the levels of TNF-α (58% versus 72%) and MIP-2 (76% versus 60%) in the BALF. Additionally, both hypercapnic acidosis groups reduced stretch–induced NF-κB activation (p < 0.05) and significantly decreased lung ICAM-1 expression (p < 0.05).

Conclusions

Moderate hypercapnic acidosis (PaCO₂ maintained at 80-100 mmHg) has a greater protective effect on high-pressure ventilation-induced inflammatory injury. The potential mechanisms may involve alterations in NF-κB activity.

Pressure and volume limited ventilation for the ventilatory management of patients with acute lung injury: a systematic review and meta-analysis

BACKGROUND

Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are life threatening clinical conditions seen in critically ill patients with diverse underlying illnesses. Lung injury may be perpetuated by ventilation strategies that do not limit lung volumes and airway pressures. We conducted a systematic review and meta-analysis of randomized controlled trials (RCTs) comparing pressure and volume-limited (PVL) ventilation strategies with more traditional mechanical ventilation in adults with ALI and ARDS.

METHODS AND FINDINGS

We searched Medline, EMBASE, HEALTHSTAR and CENTRAL, related articles on PubMed™, conference proceedings and bibliographies of identified articles for randomized trials comparing PVL ventilation with traditional approaches to ventilation in critically ill adults with ALI and ARDS. Two reviewers independently selected trials, assessed trial quality, and abstracted data. We identified ten trials (n = 1,749) meeting study inclusion criteria. Tidal volumes achieved in control groups were at the lower end of the traditional range of 10-15 mL/kg. We found a clinically important but borderline statistically significant reduction in hospital mortality with PVL [relative risk (RR) 0.84; 95% CI 0.70, 1.00; p = 0.05]. This reduction in risk was attenuated (RR 0.90; 95% CI 0.74, 1.09, p = 0.27) in a sensitivity analysis which excluded 2 trials that combined PVL with open-lung strategies and stopped early for benefit. We found no effect of PVL on barotrauma; however, use of paralytic agents increased significantly with PVL (RR 1.37; 95% CI, 1.04, 1.82; p = 0.03).

CONCLUSIONS

This systematic review suggests that PVL strategies for mechanical ventilation in ALI and ARDS reduce mortality and are associated with increased use of paralytic agents.

Acute exacerbation of idiopathic interstitial pneumonia following lung surgery in 3 of 68 consecutive patients: a retrospective study

BACKGROUND

Acute exacerbation (AE) of idiopathic interstitial pneumonia (IIP) is occasionally observed after lung surgery. However, the risk of lung surgery in patients with IIPs is not yet clearly known.

SUBJECTS AND METHODS

We conducted a retrospective study of consecutive patients who underwent lung surgery for cancer or for the diagnosis of interstitial pneumonia (IP) between 2000 and 2006. Patients who developed AE following the lung operation were assessed.

RESULTS

The data of 68 consecutive patients (males: 56, females: 12) with IP who underwent lung surgery were analyzed. The lung surgery included lobectomy for lung cancer in 48 patients [idiopathic pulmonary fibrosis (IPF) 31, non-IPF 17], and lung biopsy in 20 patients [IPF 8, non-specific interstitial pneumonia (NSIP) 8, unclassified 4]. Three patients with IPF (4.4% in total) developed AE after the operation (2 lobectomy, 1 biopsy). The triggers of AE were considered to be prolonged ventilation at a large tidal volume with oxygen supplementation at a high concentration. At the time of the AE, the extent of parenchymal involvement on the HRCT images was greater on the non-operated side. All three patients died of respiratory failure 12 to 82 days after the onset of AE despite corticosteroid therapy.

CONCLUSION

It is essential to be aware of the risk of AE of IPF following lung operation. Intraoperative respiratory management, such as oxygen supplementation at a high concentration and/or prolonged mechanical ventilation are likely possible etiologic factors.

High frequency oscillatory ventilation in burn patients with the acute respiratory distress syndrome

BACKGROUND

High frequency oscillatory ventilation (HFOV) improves gas exchange while providing lung protective effects during the ventilation of patients with the acute respiratory distress syndrome (ARDS). The purpose of this study was to review our experience with HFOV in adult burn patients with oxygenation failure secondary to ARDS.

METHODS

Retrospective cohort review of all burn patients treated with HFOV at a regional adult burn center.

RESULTS

All values are reported as the mean +/- standard deviation (S.D.). HFOV was used on 28 occasions in 25 patients (age 44 +/- 16 years, %TBSA burns 40 +/- 15, and a 28% incidence of inhalation injury) who had severe oxygenation failure from ARDS (PaO2/FiO2 ratio 98 +/- 26, and oxygenation index (OI) (FiO2 x 100 x mean airway pressure/PaO2) 27 +/- 10) following 4.8 +/- 4.4 days of conventional mechanical ventilation (CMV). After switching from CMV to HFOV, there were significant improvements in the PaO(2)/FiO2 ratio within 1h and in the oxygenation index within 24 h. The duration of HFOV was 6.1 +/- 5.8 days. HFOV was continued during 26 surgeries for 14 patients where a mean of 18 +/- 9% TBSA burns were excised and closed. The only complications related to HFOV were three episodes of severe hypercapnia. In-hospital mortality was 32%.

CONCLUSIONS

HFOV was safe, and was highly effective in correcting oxygenation failure associated with ARDS in burn patients, and can be successfully used as an intra-operative ventilation modality for burn patients.

Ventilator treatment in the Nordic countries. A multicenter survey

BACKGROUND

A 1-day point prevalence study was performed in the Nordic countries to identify ventilator-treatment strategies in the region.

MATERIAL AND METHODS

On 30 May 30 2001 all mechanically ventilated patients in 27 intensive care units (ICUs) were registered via the internet. The results are shown as medians (25th, 75th percentile).

RESULTS

One hundred and eight patients were included (69% male) with new simplified acute physiology score (SAPS) 48 (37,57) and 4.5 d (2,11) of ventilator treatment. The most frequent indication for ventilator treatment was acute respiratory failure (73%). Airway management was by endotracheal tube (64%), tracheostomy (32%) and facial mask (4%). Pressure regulated ventilator modes were used in 86% of the patients and spontaneous triggering was allowed in 75%. The tidal volume was 7 ml/kg (6,9), peak inspiratory pressure 22 cmH2O (18,26) and positive end-expiratory pressure (PEEP) 6 cmH2O (6,9). FiO2 was 40% (35,50), SaO2 97% (95-98), PaO2 11 kPa (10,13), PaCO2 5.4 kPa (4.7,6.3), pH 7.43 (7.38,7.47) and BE 2.0 mmol/l (- 0.5,5). The PaO2/FiO2 ratio was 220 mmHg (166,283). The peak inspiratory pressure (r=0.37), mean airway pressure (r=0.36), PEEP (r=0.33), tidal volume (r=0.22) and SAPS score (r=0.19) were identified as independent variables in relation to the PaO2/FiO2 ratio.

CONCLUSION

The vast majority of patients were ventilated with pressure-regulated modes. Tidal volume was well below what has been considered conventional in recent large trials. Correlations between the parameters of gas exchange, respiratory mechanics, ventilator settings and physiological status of the patients was poor. It appears that blood gas values are the main tool used to steer ventilator treatment. These results may help to design future interventional studies of ventilator treatment.

New therapies for adults with acute lung injury. High-frequency oscillatory ventilation

High-frequency oscillatory ventilation seems theoretically ideal for the treatment of patients with ARDS, allowing adequate oxygenation and ventilation to be maintained without causing further damage to the already injured lung. High-frequency oscillating ventilation also seems a sound strategy for improving oxygenation in patients who are no longer responding to conventional mechanical ventilation. Currently, HFOV should be used in the adult ICU as one of many ancillary therapies available for the treatment of extremely ill, hypoxemic patients with ARDS. Future research may define the role of HFOV as a more routine strategy for preventing VALI in this patient population.

Reverse-thrust ventilation in hypercapnic patients with acute respiratory distress syndrome. Acute physiological effects

Techniques of tracheal gas insufflation (TGI) have been shown to enhance CO(2) clearance efficiency in mechanically ventilated patients with acute respiratory distress syndrome (ARDS). Clinical studies have explored the effects of such techniques only at moderate intratracheal gas flow rates, with TGI superimposed to mechanical ventilation in a continuous fashion, or synchronized to the expiratory phase of the duty cycle. We examined the effects of intratracheal pulmonary ventilation (ITPV), delivering the entire tidal volume (VT) in the proximity of the tracheal carina, with all the gas flow supplied continuously through a reverse-thrust catheter (RTC). A potential limitation in the application of TGI is dynamic hyperinflation. Therefore, in a subgroup of patients, we also evaluated the effects of ITPV on end-expiratory lung volume (EELV) by respiratory inductive plethysmography (RIP). Eleven patients with ARDS under volume-cycled mechanical ventilation were subsequently switched to ITPV at the same baseline respiratory rate, I:E ratio, and VT. At the same minute volume, Pa(CO(2)) decreased from 70 +/- 12.3 to 59 +/- 9.5 mm Hg, with a percent reduction of 15 +/- 4% (range from 10 to 20%). The CO(2) decrease was greater in patients with higher baseline Pa(CO(2)) levels (DeltaPa(CO(2)) = 0.29 x Pa(CO(2)) - 9.48, r = 0.95). During transition from mechanical ventilation to ITPV, tracheal positive end-expiratory pressure (PEEP(tr)) decreased with a correspondent decrease in EELV. Both were restored by increasing the PEEP at the ventilator by 3.6 +/- 2.0 cm H(2)O. These data suggest that in patients with ARDS ITPV effectively reduces dead space ventilation and the employment of the RTC may limit or avoid dynamic hyperinflation.

Long-term follow-up of survivors of acute lung injury: lack of effect of a ventilation strategy to prevent barotrauma

OBJECTIVES

To determine the effect of a ventilation strategy to prevent barotrauma on long-term outcome in survivors of acute lung injury.

DESIGN

Prospective blinded cohort analysis.

SETTING

Three university-affiliated medical-surgical intensive care units.

PATIENTS

A total of 28 survivors of acute lung injury, 1-2 yrs after diagnosis, from a multicenter prospective randomized controlled trial comparing pressure (peak inflation pressure < or =30 cm H2O) and volume (tidal volume < or =8 mL/kg) limited ventilation to a conventional (peak inflation pressure < or =50 cm H2O, tidal volume 10-15 mL/kg) ventilation strategy.

MEASUREMENTS AND MAIN RESULTS

Physicians blinded as to treatment group evaluated 20 of 28 survivors (treatment group, 7; control group, 13). Exercise tolerance in the 6-minute walk test was comparable to patients with chronic respiratory disease and equivalent between groups (treatment group, 373+/-171 m vs. control group, 375+/-129 m; p = .84). Pulmonary function testing showed reduced diffusing capacity (treatment group, 64+/-29% predicted vs. control group, 74+/-14% predicted; p = .68) and normal volumes, flows, and blood gases. Two domains of disease-specific Health Related Quality of Life assessed by the Chronic Respiratory Questionnaire were worse for patients in the treatment group compared with the control group (Emotional Function 3.8+/-1.4 vs. 5.1+/-0.08; p = .05, Mastery 4.7+/-1.7 vs. 6.2+/-0.8; p = .03). There were no between-group differences in the scores of the Spitzer Quality of Life Index (a generic Health Related Quality of Life instrument), although they were reduced (7.5+/-1.9) and comparable to patients with chronic disease.

CONCLUSIONS

We found that 1-2 yrs after the onset of their illness, survivors of acute lung injury have reductions in quality of life and exercise tolerance which are similar to patients with chronic diseases. We were unable to show that a limited ventilation strategy improves either long-term pulmonary function or quality of life in survivors of acute lung injury.

Activation of human macrophages by mechanical ventilation in vitro

Positive-pressure mechanical ventilation supports gas exchange in patients with respiratory failure but is also responsible for significant lung injury. In this study, we have developed an in vitro model in which isolated lung cells can be submitted to a prolonged cyclic pressure-stretching strain resembling that of conventional mechanical ventilation. In this model, cells cultured on a Silastic membrane were elongated up to 7% of their initial diameter, corresponding to a 12% increase in cell surface. The lung macrophage was identified as the main cellular source for critical inflammatory mediators such as tumor necrosis factor-alpha, the chemokines interleukin (IL)-8 and -6, and matrix metalloproteinase-9 in this model system of mechanical ventilation. These mediators were measured in supernatants from ventilated alveolar macrophages, monocyte-derived macrophages, and promonocytic THP-1 cells. Nuclear factor-kappaB was found to be activated in ventilated macrophages. Synergistic proinflammatory effects of mechanical stress and molecules such as bacterial endotoxin were observed, suggesting that mechanical ventilation might be particularly deleterious in preinjured or infected lungs. Dexamethasone prevented IL-8 and tumor necrosis factor-alpha secretion in ventilated macrophages. Mechanical ventilation induced low levels of IL-8 secretion by alveolar type II-like cells. Other lung cell types such as endothelial cells, bronchial cells, and fibroblasts failed to produce IL-8 in response to a prolonged cyclic pressure-stretching load. This model is of particular value for exploring physical stress-induced signaling pathways, as well as for testing the effects of novel ventilatory strategies or adjunctive substances aimed at modulating cell activation induced by mechanical ventilation.

Effects of recruitment of collapsed lung units on the elastic pressure-volume relationship in anaesthetised healthy adults

BACKGROUND

The elastic pressure-volume (Pel-V) curve of the respiratory system can be used as a guide for improved ventilator management. The understanding of curves recorded for sick patients can be improved with better knowledge of the Pel-V relationship observed in healthy humans. Dynamic Pel-V curves were determined over an extended volume range in 15 anaesthetised and muscle-relaxed healthy humans. The influence of a recruitment manoeuvre was studied.

METHODS

Dynamic Pel-V curves were determined during a single prolonged insufflation before and after the recruitment manoeuvre. A mathematical three-segment model of the curve including a linear intermediate segment, delineated by the lower (LIP) and upper (UIP) inflection points, was used for characterisation of the recorded curves.

RESULTS

The model gave an adequate description of the recorded Pel-V curves. Before the recruitment manoeuvre, compliance increased until the LIP was reached at 20 cm H2O (1.9 L). Then followed a long linear segment. After the recruitment manoeuvre, compliance increased during insufflation until a LIP was reached at 13 cm H2O (1.2 L). Above the LIP followed a shorter linear segment (compliance = 140 mL/cm H2O) and then an upper segment with decreasing compliance.

CONCLUSION

Pel-V curves recorded before and after the recruitment manoeuvre show that large lung compartments close during anaesthesia and that high pressures are needed to achieve recruitment even in the normal lung. Accordingly, the LIP does not define the end of recruitment during insufflation.

Volume recruitment and oxygenation in pulmonary edema: a comparison between HFOV and CMV

PURPOSE

In acute lung injury, edema floods alveoli decreasing mean lung volume (MLV) and increasing pulmonary venous admixture (Ova/Qt). We reasoned that a ventilatory strategy that uses large tidal volumes (VT) might recruit volume differently than a strategy that uses very small VT (high-frequency oscillatory ventilation, HFOV) which may require an inflation maneuver to total lung capacity (TLC) for full recruitment.

MATERIALS AND METHODS

We studied six dogs with pulmonary edema induced by oleic acid injury and compared HFOV with conventional mechanical ventilation (CMV). Increasing mean airway opening pressure (Pao) from 6 to 14 cm H2O raised MLV from 932+/-162 to 1,550+/-210 mL and from 872+/-145 to 1,242+/-192 mL during CMV and HFOV, respectively, whereas Qva/Qt decreased from 24.1+/-8.5 to 9.3+/-4.3% and from 42.2+/-6.8 to 30.4+/-9.3%. We repeated our measurements at a Pao of 14 cm H2O after an inflation maneuver to TLC.

RESULTS

Intlation to TLC recruited additional lung volume and decreased Qva/Qt further only during HFOV. After an inflation to TLC, we observed a rapid isobaric volume loss from the deflation limb of the pressure-volume curve during both CMV and HFOV.

CONCLUSIONS

We conclude that after oleic acid injury in dogs pressure-volume hysteresis has two components: a recruitable portion associated with gas exchange improvement and a nonrecruitable portion. At the level of PEEP used in this study (8.5 cm H2O), full lung recruitment during HFOV required inflation to TLC, whereas during CMV it was accomplished by the relatively large VT.

Critical care management of the patient with acute respiratory distress syndrome (ARDS). Part 2--A review of modes and strategies for ventilating the patient with poorly compliant lungs

Mechanical ventilation strategies for patients suffering from acute respiratory distress syndrome (ARDS) have traditionally relied on volume cycling. Due to the poor lung compliance characteristic of ARDS, these patients may be exposed to very high inspiratory pressures to achieve sufficient tidal volumes for adequate gas exchange. This greatly increases the risk of ventilator-induced lung injury associated with alveolar over-distention. The literature review explores the rationales behind alternative ventilation modes and strategies introduced to reduce the risk of ventilator-induced lung injury for the patient with ARDS.

Tracing best PEEP by applying PEEP as a RAMP

OBJECTIVE

The aim of this study was to show the feasibility of a slow, continuously increasing level of positive end-expiratory pressure (PEEP) (ramp manoeuvre) in selecting best PEEP and to evaluate whether best PEEP, as defined by maximal oxygen transport, coincides with best systemic arterial oxygenation or best compliance.

DESIGN

In 11 anaesthetized piglets, PEEP was increased between 0 cmH2O (zero end-expiratory pressure; ZEEP) and 15 cmH2O (PEEP15) with a constant rate of 0.67 cmH2O x min(-1). This ramp manoeuvre was performed both under normal conditions and after induction of an experimental lung oedema. During the ramp manoeuvre, haemodynamic and pulmonary variables were monitored almost continuously.

RESULTS

During the rise in PEEP, cardiac output declined in a non-linear way. In the series with normal conditions, best PEEP was always found at ZEEP. In the series with experimental lung oedema, best PEEP, as defined by maximum oxygen transport, was found at PEEP1-6, as defined by maximal compliance, at PEEP7.5 and by maximal arterial oxygen tension (PaO2) at PEEP10-14.

CONCLUSIONS

Best PEEP according to oxygen transport is lower than best PEEP according to compliance and PaO2; the use of PEEP as a ramp might prevent unnecessarily high levels of PEEP.

Critical care management of the patient with acute respiratory distress syndrome (ARDS). Part 1: Pathophysiology and implications for mechanical ventilation

Acute respiratory distress syndrome (ADRS) is a severe, life-threatening consequence of certain pulmonary and systemic insults. It is thought to result from a dramatic change in the permeability of the alveolar-capillary membrane, allowing the movement of fluid and proteins into alveolar air spaces. These changes are followed by inactivation of surfactant, bringing about a significant alteration in lung compliance. It is common for the devastating changes to lung function in ARDS to necessitate the patient being supported by mechanical ventilation. However, the poor compliance of the ARDS-affected lung can greatly increase the risk of ventilator induced lung injury. This has led to a concern that traditional ventilation strategies may in fact be perpetuating the very conditions they attempt to compensate for.

Tracheal gas insufflation improves ventilatory efficiency during metacholine-induced bronchospasm

INTRODUCTION

Barotrauma and cardiovascular insufficiency are frequently encountered problems in patients with acute bronchospastic disease who require mechanical ventilation. Permissive hypercapnia is a recognized strategy for minimizing these adverse effects; however, it has potential risks. Tracheal gas insufflation (TGI) has been shown to increase carbon dioxide elimination efficiency and thus could permit mechanical ventilation at lower peak airway pressures without inducing hypercapnia. However, caution exists as to the impact of TGI on lung volumes, given that expiratory flow limitation is a hallmark of bronchospastic disease.

PURPOSE

To examine these issues, we studied ventilatory and hemodynamic effects of continuous TGI as an adjunct to mechanical ventilation before and after methacholine-induced bronchospasm.

MATERIALS AND METHODS

Ten anesthetized, paralyzed dogs were ventilated on volume-controlled mechanical ventilation during administration of continuous TGI (0, 2, 6, and 10 L/min) while total inspired minute ventilation (ventilator-derived minute ventilation plus TGI) was kept constant. In an additional step, with TGI flow of 10 L/min, total inspired minute ventilation was decreased by 30%.

RESULTS

PaCO2 decreased (44 +/- 7 mm Hg at zero flow to 34 +/- 7 mm Hg at 6 L/min and 31 +/- 6 mm Hg at 10 L/min, respectively, P < .05), as did the dead space to tidal volume ratio at TGI of 6 and 10 L/min compared with zero flow. There were no significant changes in end-expiratory transpulmonary pressure, mean arterial pressure, or cardiac output. During the highest TGI flow (10 L/min), with a 30% reduction of total inspired minute ventilation, both PaCO2 and peak airway pressure remained less than during zero flow conditions.

CONCLUSION

We conclude that TGI increases carbon dioxide elimination efficiency during constant and decreased minute ventilation conditions without any evidence of hyperinflation or hemodynamic instability during methacholine-induced bronchospasm.

The intensive care management, mortality and prognostic indicators in severe community-acquired pneumococcal pneumonia

OBJECTIVE

To determine mortality and factors that might predict outcome in severe community-acquired pneumococcal pneumonia treated by a standard protocol.

DESIGN

Prospective, non-concurrent study.

SETTING

Respiratory intensive care unit (ICU) in a teaching hospital.

PATIENTS

63 patients who were diagnosed by positive blood culture or Gram stain and culture of sputum or tracheal aspirate were included.

MEASUREMENTS AND RESULTS

Clinical features, severity scores including Acute Physiology and Chronic Health Evaluation (APACHE) II, organ failure and lung injury scores, and the clinical course in the ICU were documented; 79% of patients required mechanical ventilation. Bacteraemia was present in 34 patients (54%); there were no distinguishing clinical features between bacteraemic and non-bacteraemic cases. The overall mortality was 21%, with only 5 deaths (15% mortality) in the bacteraemic group. Shock and a very low serum albumin (< 26 g/l) were the only clinical features that differentiated survivors from non-survivors; lung injury, APACHE II and multiple organ failure scores were all predictive of outcome. The positive predictive value and specificity in predicting death in individuals for the modified British Thoracic Society rule 1 were 26 and 64%; APACHE II > 2057 and 88%; > 2 organ failure 64 and 92%; and lung injury > 233 and 73%, respectively.

CONCLUSIONS

These results suggest that even in bacteraemic cases mortality should be below 25% with intensive care management and that conventional scoring systems, while predictive of group mortality, are unreliable in individuals.