Oxygenation response to a recruitment maneuver during supine and prone positions in an oleic acid-induced lung injury model

Posición prono en respiración espontánea: una lección más del COVID-19

Con la pandemia se implementaron diversas estrategias para evitar la intubación y la ventilación mecánica invasiva. La posición prona tiene claros efectos benéficos en mejorar la oxigenación por diversos mecanismos al tiempo que genera cambios hemodinámicos que pueden optimizar la función del ventrículo derecho.

La evidencia de la posición prona en pacientes con síndrome de dificultad respiratoria aguda en ventilación mecánica invasiva es contundente y obliga a considerarla en las primeras 24 h de pacientes con PaO₂/FiO₂ ≤ 150 mmHg. La posición prona en respiración espontánea puede mejorar la oxigenación en pacientes con falla respiratoria y si se implementa mediante un protocolo que incluya una adecuada selección de pacientes puede evitar la intubación de pacientes en falla respiratoria.

La presente revisión resume los antecedentes históricos, las bases fisiológicas de la posición prona en el paciente despierto, así como la evidencia que evalúa su aplicación en el paciente con COVID-19 al tiempo que resume el protocolo y la experiencia de un centro con esta estrategia como propuesta para estudios multicéntricos.

Inflammatory lung injury in rabbits: effects of high-frequency oscillatory ventilation in the prone position

OBJECTIVE

To compare the effects that prone and supine positioning during high-frequency oscillatory ventilation (HFOV) have on oxygenation and lung inflammation, histological injury, and oxidative stress in a rabbit model of acute lung injury (ALI).

METHODS

Thirty male Norfolk white rabbits were induced to ALI by tracheal saline lavage (30 mL/kg, 38°C). The injury was induced during conventional mechanical ventilation, and ALI was considered confirmed when a PaO2/FiO2 ratio < 100 mmHg was reached. Rabbits were randomly divided into two groups: HFOV in the supine position (SP group, n = 15); and HFOV with prone positioning (PP group, n = 15). For HFOV, the mean airway pressure was initially set at 16 cmH2O. At 30, 60, and 90 min after the start of the HFOV protocol, the mean airway pressure was reduced to 14, 12, and 10 cmH2O, respectively. At 120 min, the animals were returned to or remained in the supine position for an extra 30 min. We evaluated oxygenation indices and histological lung injury scores, as well as TNF-α levels in BAL fluid and lung tissue.

RESULTS

After ALI induction, all of the animals showed significant hypoxemia, decreased respiratory system compliance, decreased oxygenation, and increased mean airway pressure in comparison with the baseline values. There were no statistically significant differences between the two groups, at any of the time points evaluated, in terms of the PaO2 or oxygenation index. However, TNF-α levels in BAL fluid were significantly lower in the PP group than in the SP group, as were histological lung injury scores.

CONCLUSIONS

Prone positioning appears to attenuate inflammatory and histological lung injury during HFOV in rabbits with ALI.

Effects of patient positioning on respiratory mechanics in mechanically ventilated ICU patients

Changes in the body position of patients receiving mechanical ventilation in intensive care unit are frequent. Contrary to healthy humans, little data has explored the physiological impact of position on respiratory mechanics. The objective of present paper is to review the available data on the effect of changing body position on respiratory mechanics in ICU patients receiving mechanical ventilation. Supine position (lying flat) or lateral position do not seem beneficial for critically ill patients in terms of respiratory mechanics. The sitting position (with thorax angulation >30° from the horizontal plane) is associated with improvement of functional residual capacity (FRC), oxygenation and reduction of work of breathing. There is a critical angle of inclination in the seated position above which the increase in abdominal pressure contributes to increase chest wall elastance and offset the increase in FRC. The impact of prone position on respiratory mechanics is complex, but the increase in chest wall elastance is a central mechanism. To sum up, both sitting and prone positions provides beneficial impact on respiratory mechanics of mechanically ventilated patients as compared to supine position.

Effect of setting high APRV guided by expiratory inflection point of pressure-volume curve on oxygen delivery in canine models of severe acute respiratory distress syndrome

In the present study, the effect of setting high airway pressure release ventilation (APRV) pressure guided by an expiratory inflection point of pressure-volume (PV) curve following lung recruitment maneuver (RM) on oxygen delivery (DO₂) in canine models of severe acute respiratory distress syndrome (ARDS) was examined. Canine models of severe ARDS were established by intravenous injection of oleic acid. After injection of sedative muscle relaxants, a PV curve plotted using the super-syringe technique, and the pressure at lower inflection point (LIP) at the inhale branch and the pressure at the point of maximum curvature (PMC) at the exhale branch were measured. The ventilation mode was biphasic positive airway pressure (BiPAP), an inspiration to expiration ratio of 1:2, and P_high 40 cm H₂O, P_low 25 cm H₂O. P_high was decreased to 30 cm H₂O after 90 sec. The dogs were randomized into 3 groups after RM, i.e., Blip group, BiPAP P_low = LIP+2 cm H₂O; Bpmc group, BiPAP P_low = PMC; and Apmc group. In the APRV group, P_high was set as PMC, with an inspiratory duration of 4 sec and expiratory duration of 0.4 sec. PMC was 18±1.4 cm H₂O, and LIP was 11±1.3 cm H₂O. Thirty seconds after RM was stabilized, it was set as 0 h. Hemodynamics, oxygenation and DO₂ were measured at 0, 1, 2 and 4 h after RM in ARDS dogs. The results demonstrated: i) cardiac index (CI) in the 3 groups, where CI was significantly decreased in the Bpmc group at 0, 1, 2 and 4 h after RM compared to prior to RM (P<0.05) as well as in the Blip and Apmc groups (P<0.05). CI in the Blip and Apmc groups was not significantly altered prior to and after RM. ii) Oxygenation at 0, 1, 2 and 4 h in the 3 groups was improved after RM and the oxygenation indices for the 3 groups at 1 and 2 h were not significantly different (P>0.05). However, the oxygenation index in the Blip group at 4 h was significantly lower than those at 0 h for the Apmc and Bpmc groups (P<0.05). Oxygenation for the Apmc group at 4 h was higher than that for the Blip and Bpmc groups (P<0.05). Oxygenation for the Bpmc group was lower than that at 0 h, although the difference was not significant (P>0.05). iii) DO₂ in at 0, 1, 2 and 4 h in the Bpmc group was significantly lower than that in the Blip and Apmc groups, and not significantly improved after RM. DO₂ in the Blip and Apmc groups after RM was improved as compard to that before RM and that in the Bpmc group. However, DO₂ at 4 h in the Blip group was significantly lower than that at 0 h and in the Apmc group (P<0.05). DO₂ at 4 h in the Apmc group was higher than that at 0 h and that in the remaining 2 groups (P<0.05). In conclusion, high APRV pressure guided at PMC of PV curve after RM significantly improved DO₂ in ARDS dogs.

Efficacy of prone position in acute respiratory distress syndrome patients: A pathophysiology-based review

Acute respiratory distress syndrome (ARDS) is a syndrome with heterogeneous underlying pathological processes. It represents a common clinical problem in intensive care unit patients and it is characterized by high mortality. The mainstay of treatment for ARDS is lung protective ventilation with low tidal volumes and positive end-expiratory pressure sufficient for alveolar recruitment. Prone positioning is a supplementary strategy available in managing patients with ARDS. It was first described 40 years ago and it proves to be in alignment with two major ARDS pathophysiological lung models; the "sponge lung" - and the "shape matching" -model. Current evidence strongly supports that prone positioning has beneficial effects on gas exchange, respiratory mechanics, lung protection and hemodynamics as it redistributes transpulmonary pressure, stress and strain throughout the lung and unloads the right ventricle. The factors that individually influence the time course of alveolar recruitment and the improvement in oxygenation during prone positioning have not been well characterized. Although patients' response to prone positioning is quite variable and hard to predict, large randomized trials and recent meta-analyses show that prone position in conjunction with a lung-protective strategy, when performed early and in sufficient duration, may improve survival in patients with ARDS. This pathophysiology-based review and recent clinical evidence strongly support the use of prone positioning in the early management of severe ARDS systematically and not as a rescue maneuver or a last-ditch effort.

Prone position in acute respiratory distress syndrome. Rationale, indications, and limits

In the prone position, computed tomography scan densities redistribute from dorsal to ventral as the dorsal region tends to reexpand while the ventral zone tends to collapse. Although gravitational influence is similar in both positions, dorsal recruitment usually prevails over ventral derecruitment, because of the need for the lung and its confining chest wall to conform to the same volume. The final result of proning is that the overall lung inflation is more homogeneous from dorsal to ventral than in the supine position, with more homogeneously distributed stress and strain. As the distribution of perfusion remains nearly constant in both postures, proning usually improves oxygenation. Animal experiments clearly show that prone positioning delays or prevents ventilation-induced lung injury, likely due in large part to more homogeneously distributed stress and strain. Over the last 15 years, five major trials have been conducted to compare the prone and supine positions in acute respiratory distress syndrome, regarding survival advantage. The sequence of trials enrolled patients who were progressively more hypoxemic; exposure to the prone position was extended from 8 to 17 hours/day, and lung-protective ventilation was more rigorously applied. Single-patient and meta-analyses drawing from the four major trials showed significant survival benefit in patients with PaO2/FiO2 lower than 100. The latest PROSEVA (Proning Severe ARDS Patients) trial confirmed these benefits in a formal randomized study. The bulk of data indicates that in severe acute respiratory distress syndrome, carefully performed prone positioning offers an absolute survival advantage of 10-17%, making this intervention highly recommended in this specific population subset.

Gas exchange in the respiratory distress syndromes

This article describes the gas exchange abnormalities occurring in the acute respiratory distress syndrome seen in adults and children and in the respiratory distress syndrome that occurs in neonates. Evidence is presented indicating that the major gas exchange abnormality accounting for the hypoxemia in both conditions is shunt, and that approximately 50% of patients also have lungs regions in which low ventilation-to-perfusion ratios contribute to the venous admixture. The various mechanisms by which hypercarbia may develop and by which positive end-expiratory pressure improves gas exchange are reviewed, as are the effects of vascular tone and airway narrowing. The mechanisms by which surfactant abnormalities occur in the two conditions are described, as are the histological findings that have been associated with shunt and low ventilation-to-perfusion.

Recruitment Maneuver Does not Increase the Risk of Ventilator Induced Lung Injury

BACKGROUND

Mechanical ventilation (MV) may induce lung injury.

AIMS

To assess and evaluate the role of different mechanical ventilation strategies on ventilator-induced lung injury (VILI) in comparison to a strategy which includes recruitment manoeuvre (RM).

STUDY DESIGN

Randomized animal experiment.

METHODS

Thirty male Sprague-Dawley rats were anaesthetised, tracheostomised and divided into 5 groups randomly according to driving pressures; these were mechanically ventilated with following peak alveolar opening (Pao) and positive end-expiratory pressures (PEEP) for 1 hour: Group 15-0: 15 cmH2O Pao and 0 cmH2O PEEP; Group 30-10: 30 cmH2O Pao and 10 cmH2O PEEP; Group 30-5: 30 cmH2O Pao and 5 cmH2O PEEP; Group 30-5&RM: 30 cmH2O Pao and 5 cmH2O PEEP with additional 45 cmH2O CPAP for 30 seconds in every 15 minutes; Group 45-0: 45 cmH2O Pao and 0 cmH2O PEEP Before rats were sacrificed, blood samples were obtained for the evaluation of cytokine and chemokine levels; then, the lungs were subsequently processed for morphologic evaluation.

RESULTS

Oxygenation results were similar in all groups; however, the groups were lined as follows according to the increasing severity of morphometric evaluation parameters: Group 15-0: (0±0.009) < Group 30-10: (0±0.14) < Group 30-5&RM: (1±0.12) < Group 30-5: (1±0.16) < Group 45-0: (2±0.16). Besides, inflammatory responses were the lowest in 30-5&RM group compared to all other groups. TNF-α, IL-1β, IL-6, MCP-1 levels were significantly different between group 30-5&RM and group 15-0 vs. group 45-0 in each group.

CONCLUSION

RM with low PEEP reduces the risk of ventilator-induced lung injury with a lower release of systemic inflammatory mediators in response to mechanical ventilation.

Relationship between regional lung compliance and ventilation homogeneity in the supine and prone position

BACKGROUND

The prone position (PP) improves ventilation homogeneity in acute respiratory distress syndrome. The aim of this study was to investigate whether the alleviation of ventilation inhomogeneity in PP was due to changes in regional lung compliance.

METHODS

Ten lung-lavaged piglets were mechanically ventilated in supine position (SP) and in PP. In each position, positive end-expiratory pressure (PEEP) was reduced from 20 to 6 cmH(2)O in steps of 2 cmH(2)O every 10 min after full lung recruitment. Respiratory mechanics, blood gas, haemodynamic data and whole-lung computed tomography scans were recorded at each PEEP. The compliances of normally aerated (C(normal)) and newly recruited (C(recruited)) lung regions were calculated. Open lung PEEP (OL-PEEP) was defined as the lowest PEEP to maintain full lung recruitment.

RESULTS

At OL-PEEP, PP significantly increased normally aerated lung regions, decreased poorly aerated and hyperinflated lung regions and decreased tidal recruitment and hyperinflation. C(normal) was significantly reduced in PP compared with SP (12.8 ± 4.2 ml/cmH(2)O vs. 20.1 ± 6.2 ml/cmH(2)O, P < 0.001), whereas C(recruited) was increased in PP (13.9 ± 3.9 ml/cmH(2)O vs. 9.4 ± 2.4 ml/cmH(2)O, P < 0.001). C(normal) was correlated with hyperinflated lung regions at end-expiration (rho = 0.67) and end-inspiration (rho = 0.56) at OL-PEEP. C(recruited) was correlated with normally (r(2) = 0.36) and poorly aerated lung regions (rho = -0.58) at OL-PEEP.

CONCLUSION

This surfactant-depleted model shows that the improvement of ventilation homogeneity in PP is related to an increase in C(recruited) and a decrease in C(normal).

Study of the role of epidermal growth factor on lung fluid transport in rabbits with acute lung injury caused by endotoxin

The aim of this study was to investigate the effect of epidermal growth factor (EGF) on the lung fluid transport of rabbits with acute lung injury caused by endotoxin and evaluate its therapeutic action. A total of 24 rabbits were randomly divided into control, simple acute lung injury (ALI) and EGF only treatment groups. ALI rabbit models were constructed by the administration of endotoxin (lipopolysaccharide, LPS) and subsequent treatment with EGF. Arterial partial pressure of oxygen, lung pathomorphological changes and wet/dry weight (W/D) of the left lobe of lung tissue were observed at various time points. Results showed that following treatment with EGF, the breathing status of the rabbits continued to improve. An increase was noted in PaO₂ at 12 h after EGF treatment and 24 h later PaO₂ had significantly increased. A marked decrease was observed in the value of W/D and the exudation was reduced. The extrinsic EGF decreased the exudation of pulmonary capillaries and improved lung water transport. Our findings verified that epidermal growth factor had repaired the effect of ALI through continuous 48-h observation. Therefore, the present study demonstrated the therapeutic action of EGF.

Improvement in ARDS experimental model installation: low mortality rate and maintenance of hemodynamic stability

INTRODUCTION

Many experimental models using lung lavage have been developed for the study of acute respiratory distress syndrome (ARDS). The original technique has been modified by many authors, resulting in difficulties with reproducibility. There is insufficient detail on the lung injury models used, including hemodynamic stability during animal preparation and drawbacks encountered such as mortality. The authors studied the effects of the pulmonary recruitment and the use of fixed tidal volume (Vt) or fixed inspiratory pressure in the experimental ARDS model installation.

METHODS

Adult rabbits were submitted to repeated lung lavages with 30 ml/kg warm saline until the ARDS definition (PaO₂/FiO₂ ≤ 100) was reached. The animals were divided into three groups, according to the technique used for mechanical ventilation: 1) fixed Vt of 10 ml/kg; 2) fixed inspiratory pressure (IP) with a tidal volume of 10 ml/kg prior to the first lung lavage; and 3) fixed Vt of 10 ml/kg with pulmonary recruitment before the first lavage.

RESULTS

The use of alveolar recruitment maneuvers, and the use of a fixed Vt or IP between the lung lavages did not change the number of lung lavages necessary to obtain the experimental model of ARDS or the hemodynamic stability of the animals during the procedure. A trend was observed toward an increased mortality rate with the recruitment maneuver and with the use of a fixed IP.

DISCUSSION

There were no differences between the three study groups, with no disadvantage in method of lung recruitment, either fixed tidal volume or fixed inspiratory pressure, regarding the number of lung lavages necessary to obtain the ARDS animal model. Furthermore, the three different procedures resulted in good hemodynamic stability of the animals, and low mortality rate.

Prone position and recruitment manoeuvre: the combined effect improves oxygenation

Introduction

Among the various methods for improving oxygenation while decreasing the risk of ventilation-induced lung injury in patients with acute respiratory distress syndrome (ARDS), a ventilation strategy combining prone position (PP) and recruitment manoeuvres (RMs) can be practiced. We studied the effects on oxygenation of both RM and PP applied in early ARDS patients.

Methods

We conducted a prospective study. Sixteen consecutive patients with early ARDS fulfilling our criteria (ratio of arterial oxygen partial pressure to fraction of inspired oxygen (PaO₂/FiO₂) 98.3 ± 28 mmHg; positive end expiratory pressure, 10.7 ± 2.8 cmH₂O) were analysed. Each patient was ventilated in both the supine position (SP) and the PP (six hours in each position). A 45 cmH₂O extended sigh in pressure control mode was performed at the beginning of SP (RM1), one hour after turning to the PP (RM2) and at the end of the six-hour PP period (RM3).

Results

The mean arterial oxygen partial pressure (PaO₂) changes after RM1, RM2 and RM3 were 9.6%, 15% and 19%, respectively. The PaO₂ improvement after a single RM was significant after RM3 only (P < 0.05). Improvements in PaO₂ level and PaO₂/FiO₂ ratio were transient in SP but durable during PP. PaO₂/FiO₂ ratio peaked at 218 mmHg after RM3. PaO₂/FiO₂ changes were significant only after RM3 and in the pulmonary ARDS group (P = 0.008). This global strategy had a benefit with regard to oxygenation: PaO₂/FiO₂ ratio increased from 98.3 mmHg to 165.6 mmHg 13 hours later at the end of the study (P < 0.05). Plateau airway pressures decreased after each RM and over the entire PP period and significantly after RM3 (P = 0.02). Some reversible side effects such as significant blood arterial pressure variations were found when extended sighs were performed.

Conclusions

In our study, interventions such as a 45 cmH₂O extended sigh during PP resulted in marked oxygenation improvement. Combined RM and PP led to the highest increase in PaO₂/FiO₂ ratio without major clinical side effects.

New and conventional strategies for lung recruitment in acute respiratory distress syndrome

This article is one of ten reviews selected from the Yearbook of Intensive Care and Emergency Medicine 2010 (Springer Verlag) and co-published as a series in Critical Care. Other articles in the series can be found online at http://ccforum.com/series/yearbook. Further information about the Yearbook of Intensive Care and Emergency Medicine is available from http://www.springer.com/series/2855.

Prolonged recruitment manoeuvre improves lung function with less ultrastructural damage in experimental mild acute lung injury

The effects of prolonged recruitment manoeuvre (PRM) were compared with sustained inflation (SI) in paraquat-induced mild acute lung injury (ALI) in rats. Twenty-four hours after ALI induction, rats were anesthetized and mechanically ventilated with VT=6 ml/kg and positive end-expiratory pressure (PEEP)=5 cmH(2)O for 1h. SI was performed with an instantaneous pressure increase of 40 cmH(2)O that was sustained for 40s, while PRM was done by a step-wise increase in positive inspiratory pressure (PIP) of 15-20-25 cmH(2)O above a PEEP of 15 cm H(2)O (maximal PIP=40 cmH(2)O), with interposed periods of PIP=10 cmH(2)O above a PEEP=15 cmH(2)O. Lung static elastance and the amount of alveolar collapse were more reduced with PRM than SI, yielding improved oxygenation. Additionally, tumour necrosis factor-alpha, interleukin-6, interferon-gamma, and type III procollagen mRNA expressions in lung tissue and lung epithelial cell apoptosis decreased more in PRM. In conclusion, PRM improved lung function, with less damage to alveolar epithelium, resulting in reduced pulmonary injury.

Effect of recruitment and body positioning on lung volume and oxygenation in acute lung injury model

The mechanism of oxygenation improvement after recruitment manoeuvres or prone positioning in acute lung injury or acute respiratory distress syndrome is still unclear. We tried to determine the mechanism responsible for the effects of recruitment manoeuvres or prone positioning on lung aeration using a whole lung computed tomography scan in an oleic acid induced acute lung injury canine model. Twelve adult mongrel dogs were allocated into either the supine group (n=6) or the prone group (n=6). After the establishment of acute lung injury, three recruitment manoeuvres were performed at one-hour intervals. Haemodynamic and ventilatory variables, arterial blood gas analyses and CT scans of the whole lung were obtained 90 minutes after oleic acid injection and five minutes before and after each recruitment manoeuvre. Recruitment manoeuvres in the supine position improved oxygenation (P=0.025) that correlated with increase of the poorly- and well-aerated dorsal (dependent) lung volume (r=0.436, P=0.016). Prone positioning increased oxygenation (P=0.004) that also correlated with increase of the poorly- and well-aerated dorsal (nondependent) lung volume (r=0.787, P<0.001). However, the recruitment manoeuvre in the prone position had no effect on oxygenation despite an increase in ventral (dependent) lung volume. The increase in PaO2 after recruitment manoeuvres in the supine position or after prone positioning is related to the increase of the poorly- and well-aerated dorsal lung.

A comprehensive review of pediatric endotracheal suctioning: Effects, indications, and clinical practice

OBJECTIVE

To provide a comprehensive, evidence-based review of pediatric endotracheal suctioning: effects, indications, and clinical practice.

METHODS

PubMed, Cumulative Index of Nursing and Allied Health Literature, and PEDro (Physiotherapy Evidence Database) electronic databases were searched for English language articles, published between 1962 and June 2007. Owing to the paucity of objective pediatric data, all reports dealing with this topic were examined, including adult and neonatal studies.

RESULTS

One hundred eighteen references were included in the final review. Despite the widespread use of endotracheal suctioning, very little high-level evidence dealing with pediatric endotracheal suctioning exists. Studies of mechanically ventilated neonatal, pediatric, and adult patients have shown that suctioning causes a range of potentially serious complications. Current practice guidelines are not based on evidence from controlled clinical trials. There is no clear evidence that endotracheal suctioning improves respiratory mechanics, with most studies pointing to the detrimental effect it has on lung mechanics. Suctioning should be performed when obstructive secretions are present rather than routinely. There is no clear evidence for the superiority of closed- or open-system suctioning, nor is there clear evidence for appropriate vacuum pressures and suction catheter size. Sterility does not seem to be necessary when suctioning. Preoxygenation has short-term benefits, but the longer-term impact is unknown. Routine saline instillation before suctioning should not be performed. Recruitment maneuvers performed after suctioning have not been shown to be useful as standard practice.

CONCLUSIONS

Endotracheal suctioning is a procedure used regularly in the pediatric intensive care unit. Despite this, good evidence supporting its practice is limited. Further, controlled clinical studies are needed to develop evidence-based protocols for endotracheal suctioning of infants and children, and to examine the impact of different suctioning techniques on the duration of ventilatory support, incidence of nosocomial infection, and length of pediatric intensive care unit and hospital stay.

How best to recruit the injured lung?

Sustained re-opening of collapsed lung tissue (recruitment) requires the application of airway pressures that exceed those of the tidal cycle. The post-maneuver PEEP as well as the duration of high pressure application are also key factors in its success, with their accompanying potential for hemodynamic compromise. Although a wide variety of recruiting maneuvers have been described, the technique that strikes the best balance between efficacy and risk may well vary among patients with differing right heart loading status and lung properties.

Recruitment maneuver in pulmonary and extrapulmonary experimental acute lung injury

OBJECTIVE

The aim of this study is to test the hypothesis that recruitment maneuvers (RMs) might act differently in models of pulmonary (p) and extrapulmonary (exp) acute lung injury (ALI) with similar transpulmonary pressure changes.

DESIGN

Prospective, randomized, controlled experimental study.

SETTING

University research laboratory.

SUBJECTS

Wistar rats were randomly divided into four groups. In control groups, sterile saline solution was intratracheally (0.1 mL, Cp) or intraperitoneally (1 mL, Cexp) injected, whereas ALI animals received Escherichia coli lipopolysaccharide intratracheally (100 microg, ALIp) or intraperitoneally (1 mg, ALIexp). After 24 hrs, animals were mechanically ventilated (tidal volume, 6 mL/kg; positive end-expiratory pressure, 5 cm H2O) and three RMs (pressure inflations to 40 cm H2O for 40 secs, 1 min apart) applied.

MEASUREMENTS AND MAIN RESULTS

PaO2, lung resistive and viscoelastic pressures, static elastance, lung histology (light and electron microscopy), and type III procollagen messenger RNA expression in pulmonary tissue were measured before RMs and at the end of 1 hr of mechanical ventilation. Mechanical variables, gas exchange, and the fraction of area of alveolar collapse were similar in both ALI groups. After RMs, lung resistive and viscoelastic pressures and static elastance decreased more in ALIexp (255%, 180%, and 118%, respectively) than in ALIp (103%, 59%, and 89%, respectively). The amount of atelectasis decreased more in ALIexp than in ALIp (from 58% to 19% and from 59% to 33%, respectively). RMs augmented type III procollagen messenger RNA expression only in the ALIp group (19%), associated with worsening in alveolar epithelium injury but no capillary endothelium lesion, whereas the ALIexp group showed a minor detachment of the alveolar capillary membrane.

CONCLUSIONS

Given the same transpulmonary pressures, RMs are more effective at opening collapsed alveoli in ALIexp than in ALIp, thus improving lung mechanics and oxygenation with limited damage to alveolar epithelium.

[Hemodynamic variability caused by pressure-volume plotting and alveolar recruitment maneuvers in patients with adult respiratory distress syndrome]

OBJECTIVES

The plotting of pressure-volume curves and the performance of alveolar recruitment maneuvers are common practices in the care of patients with adult respiratory distress syndrome (ARDS), even though potentially harmful hemodynamic effects are associated with sustaining a high intrathoracic pressure. Our aim was to analyze hemodynamic and ventilatory changes related to these 2 maneuvers and to assess the short-term effectiveness of recruitment.

PATIENTS AND METHODS

The patients had ARDS and were being monitored with a catheter connected to a PiCCO system. All measurements were taken in sinus rhythm and with adequate vascular filling. Values recorded during plotting of the quasistatic pressure-volume curve and the recruitment maneuver (sustained airway pressure of 40 cm H2O) were the cardiac index, mean arterial pressure, heart rate, systolic volume index, and oxygen saturation (SpO2). Blood gas measurements were recorded before the maneuvers and 15 minutes afterwards.

RESULTS

All parameters decreased significantly in the 14 patients studied. The mean (SD) maximum decreases, from which all patients recovered within 2 minutes, were as follows: cardiac index, 26% (16%); mean arterial pressure, 6% (6%); heart rate, 4% (5%), systolic volume index, 21% (15%); and SpO2, 3% (3%). Significant increases in PaO2 (7% [6%]) and the ratio of PaO2 to the fraction of inspired oxygen were recorded after the recruitment maneuver (P=.016 and P=.014, respectively), but the changes were not clinically significant.

CONCLUSIONS

The hemodynamic disturbances associated with the alveolar recruitment maneuver based on sustaining a high end-expiratory pressure and the minor improvement in oxygenation achieved as a result suggest that the routine use of that maneuver in ARDS patients is of questionable value.

A recruitment manoeuvre performed after endotracheal suction does not increase dynamic compliance in ventilated paediatric patients: a randomised controlled trial

QUESTION

Does a recruitment manoeuvre after suctioning have any immediate or short-term effect on ventilation and gas exchange in mechanically-ventilated paediatric patients?

DESIGN

Randomised controlled trial with concealed allocation, assessor blinding, and intention-to-treat analysis.

PARTICIPANTS

Forty-eight paediatric patients with heterogeneous lung pathology. Fourteen patients were subsequently excluded from analysis due to large leaks around the endotracheal tube.

INTERVENTION

The experimental group received a single standardised suctioning procedure followed five minutes later by a standardised recruitment manoeuvre. The control group received only the single suctioning procedure.

OUTCOME MEASURES

Measurements of ventilation (dynamic lung compliance, expiratory airway resistance, mechanical and spontaneous expired tidal volume, respiratory rate) and gas exchange (transcutaneous oxygen saturation) were recorded, on three occasions before and on two occasions after the recruitment manoeuvre, using a respiratory profile monitor.

RESULTS

There was no difference between the experimental and the control group in dynamic compliance, expired airway resistance, or oxygen saturation either immediately after the recruitment manoeuvre, or after 25 minutes. The experimental group decreased mechanical expired tidal volume by 0.3 ml/kg (95% CI 0.1 to 0.6), increased spontaneous expired tidal volume by 0.3 ml/kg (95% CI 0.0 to 0.6), and increased total respiratory rate by 3 bpm (95% CI 1 to 4) immediately after the recruitment manoeuvre compared with the control group, but these differences disappeared after 25 minutes.

CONCLUSION

There is insufficient evidence to support performing recruitment manoeuvres after suctioning infants and children.

[The prone position in acute respiratory distress syndrome: a critical systematic review]

OBJECTIVE

To do a critical systematic review regarding effects of prone positioning (PP) in patients with acute respiratory distress syndrome (ARDS).

METHODS

A systematic review (Highwire, Medline, Cochrane Library from 1976 to 2004), using the keywords: prone position, acute respiratory distress syndrome, allowed us to include the human studies on PP in ARDS patients, independantly of their objectives or their type of protocol. To appreciate the studies validity, we scored the quality evidence of the studies in order to grade our conclusions.

RESULTS AND CONCLUSION

The qualitative analysis of the 58 included studies (1,500 patients returned prone, 4,000 episodes of PP) led to the following main conclusions: 1) the PP improves oxygenation in the majority of ARDS patients (level of evidence I); 2) the PP improves the pulmonary haemodynamics without altering the systemic haemodynamics (level of evidence III); 3) the PP enhances the recruitment maneuvers (level of evidence III); 4) because there are no formal predictive criteria for response to the PP, a "trial of PP" or better two PP trials are necessary to look for the responders; 5) the PP should be performed as early as possible in the course of severe ARDS; 6) the optimal duration of PP is 18 to 23 hours daily, and it should be continued until improvement of arterial oxygenation, or loss of the positive effect of PP on arterial oxygenation or evidently patient's death.

Therapeutic aspects of fat embolism syndrome

Signs and symptoms of clinical fat embolism syndrome (FES) usually begin within 24-48 hours after trauma. The classic triad involves pulmonary changes, cerebral dysfunction, and petechial rash. Clinical diagnosis is key because laboratory and radiographic diagnosis is not specific and can be inconsistent. The duration of FES is difficult to predict because it is often subclinical or may be overshadowed by other illnesses or injuries. Medical care is prophylactic or supportive, including early fixation and general ICU management to ensure adequate oxygenation and ventilation, hemodynamic stability, prophylaxis of deep venous thrombosis, stress-related gastrointestinal bleeding, and nutrition. Studies support early fracture fixation as a method to reduce recurrent fat embolism and FES. The main therapeutic interventions once FES has been clinically diagnosed are directed towards support of pulmonary and neurological manifestations and management of acute lung injury (ALI) and acute respiratory distress syndrome (ARDS).

Prone position augments recruitment and prevents alveolar overinflation in acute lung injury

RATIONALE

Mechanical ventilation in the prone position may be an effective means of recruiting nonaerated alveolar units and minimizing ventilation-induced lung injury.

OBJECTIVES

To evaluate and quantify regional lung volume alterations when patients with lobar or diffuse acute lung injury (ALI) were turned prone after a recruitment maneuver.

METHODS

In 21 patients with ALI, a recruitment maneuver was applied in the supine position followed by a multislice spiral computed tomography (CT) scan; then, patients were turned prone and a second CT scan was performed.

MAIN RESULTS

Both the recruitment maneuver and prone position resulted in improved oxygenation in patients with lobar ALI. Prone position also resulted in increased respiratory system compliance and decreased Pa(CO(2)) in lobar ALI. In lobar ALI, the proportion of overinflated and nonaerated areas declined, whereas the proportion of well-aerated areas increased in the prone position. The decrease in overinflated areas was observed mainly in the ventral areas. The dorsal regions showed a decrease in nonaerated areas and an increase in well-aerated areas. Recruitment maneuver and prone position improved oxygenation but had no effect either on Pa(CO(2)) or on the respiratory system compliance of patients with diffuse ALI. These patients responded to prone position with a decrease in nonaerated areas.

CONCLUSIONS

Prone position recruited the edematous lung further than recruitment maneuvers and reversed overinflation, resulting in a more homogeneous distribution of aeration. The effects of the prone position were more pronounced in patients with lobar ALI.

Adjuncts to Mechanical Ventilation in ARDS

Since its first description, acute respiratory distress syndrome has been characterized by abnormal physiologic and gas exchange properties of the lungs. Many adjunctive therapies have been developed to reduce the stresses of mechanical ventilation on already damaged lungs. We examined the mechanism of action and the latest clinical trial information of several adjunctive therapies including prone positioning, nitric oxide, extracorporeal membrane oxygenation, arterial venous carbon dioxide removal, and liquid ventilation. While all of these therapies have demonstrated short-term improvements in arterial blood gases and in the limitation of lung injury, none have shown an evidence-based survival benefit.

Acute respiratory distress syndrome in pregnancy

OBJECTIVE

To summarize the pathophysiology and treatment of acute lung injury and acute respiratory distress syndrome (ARDS) during pregnancy.

DATA SOURCE

Review of select articles from MEDLINE, including published abstracts, case reports, observational studies, controlled trials, review articles, and institutional experience.

DATA SUMMARY

ARDS occurs in pregnancy and may have unique causes. Despite extensive clinical research to improve the management of ARDS, mortality remains high, and few strategies have shown a mortality benefit. Furthermore, in most published studies, pregnancy is an exclusionary criterion, and thus, few treatments have been adequately evaluated in obstetric populations. The treatment of ARDS in pregnancy is extrapolated from studies performed in the general ARDS patient population, with consideration given to the normal physiologic changes of pregnancy. In general, the best support of the fetus is support of the mother. From the age of viability (24-26 wks at most institutions) until full term, decisions regarding delivery should be made based primarily on the standard obstetric indications.

CONCLUSIONS

Little evidence exists regarding the management of ARDS specifically in pregnancy, and thus, treatment approaches must be drawn from studies performed in a general patient population. A multidisciplinary approach involving maternal-fetal medicine, neonatology, anesthesiology, and intensivist clinicians is essential to optimizing maternal and fetal outcomes.

Alveolar recruitment in acute lung injury

Alveolar recruitment is one of the primary goals of respiratory care for acute lung injury. It is aimed at improving pulmonary gas exchange and, even more important, at protecting the lungs from ventilator-induced trauma. This review addresses the concept of alveolar recruitment for lung protection in acute lung injury. It provides reasons for why atelectasis and atelectrauma should be avoided; it analyses current and future approaches on how to achieve and preserve alveolar recruitment; and it discusses the possibilities of detecting alveolar recruitment and derecruitment. The latter is of particular clinical relevance because interventions aimed at lung recruitment are often undertaken without simultaneous verification of their effectiveness.

Ventilator strategies for posttraumatic acute respiratory distress syndrome: airway pressure release ventilation and the role of spontaneous breathing in critically ill patients

PURPOSE OF REVIEW

Patients who experience severe trauma are at increased risk for the development of acute lung injury and acute respiratory distress syndrome. The management strategies used to treat respiratory failure in this patient population should be comprehensive. Current trends in the management of acute lung injury and acute respiratory distress syndrome consist of maintaining acceptable gas exchange while limiting ventilator-associated lung injury.

RECENT FINDINGS

Currently, two distinct forms of ventilator-associated lung injury are recognized to produce alveolar stress failure and have been termed low-volume lung injury (intratidal alveolar recruitment and derecruitment) and high-volume lung injury (alveolar stretch and overdistension). Pathologically, alveolar stress failure from low- and high-volume ventilation can produce lung injury in animal models and is termed ventilator-induced lung injury. The management goal in acute lung injury and acute respiratory distress syndrome challenges clinicians to achieve the optimal balance that both limits the forms of alveolar stress failure and maintains effective gas exchange. The integration of new ventilator modes that include the augmentation of spontaneous breathing during mechanical ventilation may be beneficial and may improve the ability to attain these goals.

SUMMARY

Airway pressure release ventilation is a mode of mechanical ventilation that maintains lung volume to limit intra tidal recruitment /derecruitment and improves gas exchange while limiting over distension. Clinical and experimental data demonstrate improvements in arterial oxygenation, ventilation-perfusion matching (less shunt and dead space ventilation), cardiac output, oxygen delivery, and lower airway pressures during airway pressure release ventilation. Mechanical ventilation with airway pressure release ventilation permits spontaneous breathing throughout the entire respiratory cycle, improves patient comfort, reduces the use of sedation, and may reduce ventilator days.

Posição prona

A posição prona é uma manobra utilizada para combater a hipoxemia nos pacientes com síndrome do desconforto respiratório agudo. Apesar de hoje ser considerada um modo eficaz de melhorar a oxigenação, os mecanismos fisiológicos que levam à melhora da função respiratória ainda não estão completamente esclarecidos. O objetivo principal desta revisão é discutir os aspectos fisiológicos e clínicos da posição prona na síndrome do desconforto respiratório agudo.

Bench-to-bedside review: adjuncts to mechanical ventilation in patients with acute lung injury

Mechanical ventilation is indispensable for the survival of patients with acute lung injury and acute respiratory distress syndrome. However, excessive tidal volumes and inadequate lung recruitment may contribute to mortality by causing ventilator-induced lung injury. This bench-to-bedside review presents the scientific rationale for using adjuncts to mechanical ventilation aimed at optimizing lung recruitment and preventing the deleterious consequences of reduced tidal volume. To enhance CO₂ elimination when tidal volume is reduced, the following are possible: first, ventilator respiratory frequency can be increased without necessarily generating intrinsic positive end-expiratory pressure; second, instrumental dead space can be reduced by replacing the heat and moisture exchanger with a conventional humidifier; and third, expiratory washout can be used for replacing the CO₂-laden gas present at end expiration in the instrumental dead space by a fresh gas (this method is still experimental). For optimizing lung recruitment and preventing lung derecruitment there are the following possibilities: first, recruitment manoeuvres may be performed in the most hypoxaemic patients before implementing the preset positive end-expiratory pressure or after episodes of accidental lung derecruitment; second, the patient can be turned to the prone position; third, closed-circuit endotracheal suctioning is to be preferred to open endotracheal suctioning.

Pressure-volume curve variations after a recruitment manoeuvre in acute lung injury/ARDS patients: implications for the understanding of the inflection points of the curve

BACKGROUND AND OBJECTIVE

Although the pressure-volume (P-V) curve has been proposed in the management of mechanically ventilated patients, its interpretation remains unclear. Our aim has been to study the variations of the P-V curve after a recruitment manoeuvre (RM). Our hypothesis was that the lower inflection point (LIP) represents the presence of compressive atelectases, so it should not change after lung recruitment, while the upper inflection point (UIP) reflects reabsorptive atelectases, and an effective recruitment should result in changes at this level.

METHODS

Two P-V curves (quasi-static method) separated by an RM (40 cmH2O, two consecutive manoeuvres) were plotted in 35 postoperative patients with criteria of acute lung injury/acute respiratory distress syndrome (ARDS). LIP, UIP and expiratory inflection point (EIP) were defined as the first point where the curve consistently starts to separate from the line.

RESULTS

One to six measurements were obtained per patient (73 procedures). Neither the lower nor the EIPs varied significantly after the RM (P = 0.11 and 0.35, respectively). An UIP was observed in 18 curves (25%) before the RM and disappeared on nine occasions after the recruitment. Similar results were obtained when first measurements only were analysed, and when the cause (pulmonary vs. extrapulmonary), severity of lung injury or duration of mechanical ventilation at first measurement were studied.

CONCLUSIONS

An RM does not modify the LIP significantly, but induces the disappearance of the UIP in 50% of the cases in which this point is found.

High-frequency oscillatory ventilation and adjunctive therapies: Inhaled nitric oxide and prone positioning

OBJECTIVE

To review the use of high-frequency oscillatory ventilation (HFOV) with adjunctive therapies (inhaled nitric oxide [iNO] and prone positioning [PP]) in adult patients with acute respiratory distress syndrome (ARDS).

DATA SOURCES

Published studies evaluating the use of iNO, PP, and HFOV in adult patients with ARDS.

DATA SUMMARY

Despite ongoing preclinical and clinical research, the therapeutic armamentarium for ARDS remains limited. Although a pressure- and volume-limited strategy aimed at mitigating ventilator-associated lung injury has demonstrated mortality benefit, patients with severe ARDS may still develop life-threatening hypoxemia. As a result, various salvage therapies aimed at improving oxygenation, including HFOV, iNO, and PP alone or in combination, have been evaluated in patients with refractory ARDS. Although the few preclinical and clinical trials of combination therapy to date have shown promising improvements in oxygenation and other physiological variables, with few adverse clinical events, the impact on survival awaits the performance of large randomized trials.

CONCLUSIONS

There is limited clinical data to recommend the widespread use of combination therapy in patients with ARDS. In the subset of patients with life-threatening hypoxemia from refractory ARDS, combination therapy is safe and may be considered for salvage therapy. More rigorous randomized, controlled trials are needed to help delineate the therapeutic role of combination therapy in adults with ARDS.

Differential effects of sustained inflation recruitment maneuvers on alveolar epithelial and lung endothelial injury

OBJECTIVE

The role of recruitment maneuvers in mechanical ventilation for patients with the acute respiratory distress syndrome and acute lung injury remains uncertain in part due to a lack of data on the effects of specific recruitment maneuvers on lung injury severity. The primary objective of this study was to determine the effect of one type of recruitment maneuver--sustained inflation--on alveolar epithelial and lung endothelial injury in experimental acute lung injury.

DESIGN

Randomized experimental study.

SETTING

Academic research laboratory.

SUBJECTS

Forty-nine Sprague-Dawley rats.

INTERVENTIONS

Lung injury was induced in anesthetized, ventilated rats by instillation of acid (pH 1.5) into the airspaces. Rats were ventilated with a tidal volume of 6 mL/kg and a positive end-expiratory pressure of 5 cm H(2)O with or without a sustained inflation recruitment maneuver repeated every 30 mins. Each recruitment maneuver consisted of two 30-sec inflations to total lung capacity (30 cm H(2)O) 1 min apart.

MEASUREMENTS AND MAIN RESULTS

The use of recruitment maneuvers significantly improved oxygenation, compliance, end-expiratory lung volume, functional residual capacity, and deadspace fraction. Recruitment maneuvers reduced extravascular lung water and lung endothelial injury as measured by protein permeability (217 +/- 28 vs. 314 +/- 70 extravascular plasma equivalents [microL], p < .05). However, recruitment maneuvers did not prevent alveolar epithelial injury. Epithelial permeability and bronchoalveolar lavage RTI40 levels, a marker of type I cell injury, were similar with or without recruitment maneuvers. Recruitment maneuvers decreased epithelial fluid transport, a functional marker of epithelial injury. Recruitment maneuvers did not reduce markers of airspace inflammation.

CONCLUSIONS

Sustained inflation recruitment maneuvers improve respiratory mechanics and oxygenation and may protect the lung endothelium but do not reduce alveolar epithelial injury. Because of the differential effects of recruitment maneuvers on the lung endothelium and alveolar epithelium, the net effect in clinical acute lung injury may not be beneficial. Additional clinical studies will be needed to assess the net impact of recruitment maneuvers in patients with acute lung injury.

Recruitment maneuvers during prone positioning in patients with acute respiratory distress syndrome

OBJECTIVE

To evaluate the interaction of recruitment maneuvers and prone positioning on gas exchange and venous admixture in patients with early extrapulmonary acute respiratory distress syndrome ventilated with high levels of positive end-expiratory pressure. We hypothesized that a sustained inflation performed after 6 hrs of prone positioning would induce sustained improvement in oxygenation (Pao2/Fio2) and venous admixture.

DESIGN

Prospective, interventional study.

SETTING

Tertiary care, postoperative intensive care unit.

PATIENTS

Fifteen patients with early extrapulmonary acute respiratory distress syndrome.

INTERVENTIONS

After 6 hrs of prone positioning, a sustained inflation was performed with 50 cm H2O maintained for 30 secs. Data were recorded in supine position, after 6 hrs of prone positioning, at 3, 30, and 180 mins following the sustained inflation.

MEASUREMENTS AND MAIN RESULTS

A response to prone positioning was observed in nine of 15 patients leading to an improvement of Pao2/Fio2 (147 +/- 37 torr vs. 225 +/- 77 torr, p = .005) and venous admixture (35.4 +/- 8.3% vs. 28.9 +/- 9.8%, p = .001). Six patients did not respond to prone positioning. Following the sustained inflation, the responders to prone positioning showed a further increase of Pao2/Fio2 and decrease of venous admixture at 3 mins (Pao2/Fio2, 225 +/- 77 torr vs. 368 +/- 90 torr, p = .018; venous admixture, 28.9 +/- 9.8% vs. 18.9 +/- 6.7%, p = .05). In all six nonresponders to prone positioning, an improvement of Pao2/Fio2 and venous admixture occurred at 3 mins following the sustained inflation (128 +/- 18 torr vs. 277 +/- 59 torr, p = .03; venous admixture, 34.2 +/- 6.0% vs. 23.8 +/- 6.3%, p = .05). The beneficial effects of the sustained inflation remained significantly elevated over 3 hrs in responders and nonresponders to prone positioning.

CONCLUSION

In patients with early extrapulmonary acute respiratory distress syndrome, a sustained inflation performed after 6 hrs of prone positioning induced further and sustained improvement of oxygenation and venous admixture in both responders and nonresponders to prone positioning.

Intercomparison of recruitment maneuver efficacy in three models of acute lung injury*

OBJECTIVE

To compare the relative efficacy of three forms of recruitment maneuvers in diverse models of acute lung injury characterized by differing pathoanatomy.

DESIGN

We compared three recruiting maneuver (RM) techniques at three levels of post-RM positive end-expiratory pressure in three distinct porcine models of acute lung injury: oleic acid injury; injury induced purely by the mechanical stress of high-tidal airway pressures; and pneumococcal pneumonia.

SETTING

Laboratory in a clinical research facility.

SUBJECTS

Twenty-eight anesthetized mixed-breed pigs (23.8 +/- 2.6 kg).

INTERVENTIONS

The RM techniques tested were sustained inflation, extended sigh or incremental positive end-expiratory pressure, and pressure-controlled ventilation.

PRIMARY MEASUREMENTS

Oxygenation and end-expiratory lung volume.

MAIN RESULTS

The post-RM positive end-expiratory pressure level was the major determinant of post-maneuver PaO2, independent of the RM technique. The pressure-controlled ventilation RM caused a lasting increase of PaO2 in the ventilator-induced lung injury model, but in oleic acid injury and pneumococcal pneumonia, there were no sustained oxygenation differences for any RM technique (sustained inflation, incremental positive end-expiratory pressure, or pressure-controlled ventilation) that differed from raising positive end-expiratory pressure without RM.

CONCLUSIONS

Recruitment by pressure-controlled ventilation is equivalent or superior to sustained inflation, with the same peak pressure in all tested models of acute lung injury, despite its lower mean airway pressure and reduced risk for hemodynamic compromise. Although RM may improve PaO2 in certain injury settings when traditional tidal volumes are used, sustained improvement depends on the post-RM positive end-expiratory pressure value.

Methods to prevent ventilator-associated lung injury: a summary

Mechanical ventilation can cause ventilator-associated lung injury (VALI). This may manifest itself in various forms such as pneumothorax or, at the most extreme level, multi-system organ failure. The exact mechanisms by which the injury occurs are not known but appear to involve the conversion of mechanical stimulation of alveolar membranes into intracellular signalling, with subsequent upregulation of inflammatory mediators that produce the damage. This has been termed biotrauma. Furthermore, disruption of alveolar-capillary membranes may allow the release of these mediators into the systemic circulation that underpins the systemic inflammatory response syndrome. Various protective ventilatory strategies may be employed in order to reduce the lung damage and shall be discussed in this paper.

Transient hemodynamic effects of recruitment maneuvers in three experimental models of acute lung injury*

OBJECTIVE

Elevated lung volumes and increased pleural pressures associated with recruitment maneuvers (RM) may adversely affect pulmonary vascular resistance and cardiac filling or performance. We investigated the hemodynamic consequences of three RM techniques after inducing acute lung injury.

DESIGN

Prospective, randomized, controlled experimental study.

SETTING

Hospital research laboratory.

SUBJECTS

Thirteen anesthetized, mechanically ventilated pigs.

INTERVENTIONS

We induced three types of acute lung injury: oleic acid injury (n = 4); ventilator-induced lung injury (n = 4); and pneumonia (n = 5). All three models were designed to initiate a similar severity of oxygenation impairment. RM methods tested were sustained inflation, incremental positive end-expiratory pressure (PEEP) with a limited peak pressure, and pressure-controlled ventilation with increased PEEP and a fixed driving pressure. From a baseline PEEP of 8 cm H2O, all interventions were tested using post-RM PEEP levels of 8, 12, and 16 cm H2O. Cardiac output by thermodilution and systemic and pulmonary artery pressures were measured frequently during the RM and for 15 mins after its completion.

MEASUREMENTS AND MAIN RESULTS

During the RM, cardiac output decreased to a greater extent in the pneumonia model (0.49 of baseline cardiac output) than in the oleic acid injury (0.67 of baseline) or ventilator-induced lung injury (0.79 of baseline) models. Cardiac output recovered to the baseline value by 5 mins post-RM in oleic acid injury and ventilator-induced lung injury models. However, cardiac output remained decreased 15 mins post-RM in the pneumonia model. There were no differences in hemodynamic parameters among RM methods in oleic acid injury and ventilator-induced lung injury models. In the pneumonia model, however, cardiac output decreased to a greater extent during the RM with sustained inflation (to 0.33 of baseline cardiac output) compared with pressure-controlled ventilation (to 0.68 of baseline).

CONCLUSIONS

We conclude that RM transiently but profoundly depressed cardiac output in three models of acute lung injury. The results imply that a lung recruiting maneuver should be used with caution, especially when using sustained inflation in the setting of pneumonia.

Prone positioning improves oxygenation without adverse hemodynamic effects during partial liquid ventilation in a canine model of acute lung injury

BACKGROUND

Partial liquid ventilation (PLV) and prone positioning can improve the arterial oxygenation (PaO2) in acute lung injury (ALI). We evaluated the effect of prolonged prone positioning during partial liquid ventilation (PLV) in a canine model of acute lung injury.

METHODS

Six mongrel dogs (weighing 17.4 +/- 0.7 kg each) were anesthetized, intubated and mechanically ventilated. After 1 hour of baseline stabilization, the dogs' lungs were instilled with 40 mL/kg perfluorocarbon (PFC). PLV was first performed in the supine position for 1 hour (S1), then in the prone position for 3 hours with hourly measurements (P1, P2, P3), and finally, PLV was performed with the animal turned back to the supine position for 1 hour (S2).

RESULTS

After instillation of the PFC, the PaO2 significantly increased from 992 +/- 32.6 mmHg at baseline to 198.1 +/- 59.2 mmHg at S1 (p = 0.001). When the dogs were turned to the prone position, the PaO2 further increased to 288.3 +/- 80.9 mmHg at P1 (p = 0.008 vs. S1): this increase was maintained for 3 hours, but the PaO2 decreased to 129.4 +/- 62.5 mmHg at S2 (p < 0.001 vs. P3). Similar changes were seen in the shunt fraction. There were no significant differences for the systemic hemodynamic parameters between the prone and supine positions.

CONCLUSION

Prolonged prone positioning during PLV in an animal model of ALI appears to improve oxygenation without any hemodynamic compromise.

Novel approaches in conventional mechanical ventilation for paediatric acute lung injury

Acute lung injury remains a major cause of morbidity and mortality in paediatric intensive care units. Research over the past decade has altered our understanding of the pathophysiology of acute lung injury and the effects of mechanical ventilation on the lung. As a result, approaches to conventional mechanical ventilation of the injured lung are now largely centred around preservation of adequate gas exchange while protecting the lung from further ventilator-induced lung injury. Current techniques for accomplishing these goals include adjusting the ventilator based on the measurement and interpretation of pressure-volume curves, limitation of inspiratory tidal volumes, use of elevated levels of positive end-expiratory pressure, recruiting manoeuvres and prone positioning. The currently available data regarding the efficacy and appropriate use of these techniques are reviewed.

Acute respiratory distress syndrome

Several combination therapies have been described throughout this article: in all likelihood, it is combination therapy that will allow improved survival of ARDS patients. As medicine moves into the future, clinical trials evaluating the efficacy of therapies for ARDS will be performed. In human critical care medicine, a large forward step was taken when ALI and ARDS were clearly defined. Unfortunately. as good as the definition is, ALI and ARDS occur secondary to many different underlying pathologic processes,perhaps obscuring the benefits of certain therapies for ARDS based on the underlying condition, for example, trauma versus sepsis. Selection of patients entering any ARDS trial is crucial: not only must those patients meet the strict definition of ARDS but the underlying disease process should be clearly identified. Identification of patients suffering from different disease processes before the onset of ARDS will allow for stratification of outcomes according to the intervention and the underlying pathology--comparing apples to apples and not to oranges. We are in a unique position in veterinary medicine. Although frequently financially limited by our clients, we have the opportunity to achieve several goals. First, we need to clearly define what constitutes ALI and ARDS in veterinary medicine. Do we want to rely on the human definitions? Probably not; however, as a group, we need to determine what we will accept as definitions. For example, we may not be able perform right heart catheterizations on all our patients to meet the wedge pressure requirement of human beings of less than 18 mm Hg. Do we agree that a PAOP of less than 18 mmHg is appropriate for animals, and is it appropriate for all animals? Will we accept another measure, for example, pulmonary artery diameter increases with echocardiographic evidence of acceptable left heart function? What is acceptable left heart function? As veterinarians, what do we consider to be hypoxemia? Is it the same in all species that we work with? What do we define as acute onset? Most human ARDS cases occur while patients are in hospital being treated for other problems, whereas many of our patients present already in respiratory distress. If we are unable to ventilate patients for economic or practical reasons, what do we use as the equivalent of the Pao2/Flo, ratio'? Reliance on the pathologist is not reasonable, because many disease processes can look similar to ARDS under the microscope. If anything, ALI and ARDS are clinical diagnoses. It is time for veterinarians to reach a consensus on the definition for ALI and ARDS in our patients. Only when we have a consensus of definition can rational prospective clinical trials of therapies be designed.

Clinical review: the implications of experimental and clinical studies of recruitment maneuvers in acute lung injury

Mechanical ventilation can cause and perpetuate lung injury if alveolar overdistension, cyclic collapse, and reopening of alveolar units occur. The use of low tidal volume and limited airway pressure has improved survival in patients with acute lung injury or acute respiratory distress syndrome. The use of recruitment maneuvers has been proposed as an adjunct to mechanical ventilation to re-expand collapsed lung tissue. Many investigators have studied the benefits of recruitment maneuvers in healthy anesthetized patients and in patients ventilated with low positive end-expiratory pressure. However, it is unclear whether recruitment maneuvers are useful when patients with acute lung injury or acute respiratory distress syndrome are ventilated with high positive end-expiratory pressure, and in the presence of lung fibrosis or a stiff chest wall. Moreover, it is unclear whether the use of high airway pressures during recruitment maneuvers can cause bacterial translocation. This article reviews the intrinsic mechanisms of mechanical stress, the controversy regarding clinical use of recruitment maneuvers, and the interactions between lung infection and application of high intrathoracic pressures.

Positive end-expiratory pressure after a recruitment maneuver prevents both alveolar collapse and recruitment/derecruitment

We tested the hypothesis that collapsed alveoli opened by a recruitment maneuver would be unstable or recollapse without adequate positive end-expiratory pressure (PEEP) after recruitment. Surfactant deactivation was induced in pigs by Tween instillation. An in vivo microscope was placed on a lung area with significant atelectasis and the following parameters measured: (1) the number of alveoli per field and (2) alveolar stability (i.e., the change in alveolar size from peak inspiration to end expiration). We previously demonstrated that unstable alveoli cause lung injury. A recruitment maneuver (peak pressure = 45 cm H2O, PEEP = 35 cm H2O for 1 minute) was applied and alveolar number and stability were measured. Pigs were then separated into two groups with standard ventilation plus (1) 5 PEEP or (2) 10 PEEP and alveolar number and stability were again measured. The recruitment maneuver opened a significant number of alveoli, which were stable during the recruitment maneuver. Although both 5 PEEP and 10 PEEP after recruitment demonstrated improved oxygenation, alveoli ventilated with 10 PEEP were stable, whereas alveoli ventilated with 5 PEEP showed significant instability. This suggests recruitment followed by inadequate PEEP permits unstable alveoli and may result in ventilator-induced lung injury despite improved oxygenation.