Comparison of lung protection strategies using conventional and high-frequency oscillatory ventilation

High frequency oscillatory ventilation suppresses inflammatory response in lung tissue and microdissected alveolar macrophages in surfactant depleted piglets

The impact of high frequency oscillatory ventilation (HFOV) compared with intermittent mandatory ventilation (IMV) on oxygenation and pulmonary inflammatory response was studied in a surfactant depleted piglet model. After establishment of lung injury by bronchoalveolar lavage, piglets either received HFOV (n =5) or IMV (control; n = 5) for eight hours. PaO(2) was higher and mean pulmonary arterial pressure (MPAP) was lower with HFOV (HFOV versus control, mean +/- SEM; endpoint PaO(2): 252 +/- 73 versus 68 +/- 8.4 mm Hg; p < 0.001; MPAP: 22 +/- 2.3 versus 34 +/- 2.5 mm Hg; p < 0.01). mRNA expression of interleukin (IL)-1 beta, IL-6, IL-8, IL-10, TGF-beta 1, Endothelin-1, and adhesion molecules (E-selectin, P-selectin, ICAM-1) in lung tissue was quantified by real time PCR normalized to beta-actin and hypoxanthine-guanine-phosphoribosyl-transferase (HPRT). mRNA expression of all cytokines and adhesion molecules/HPRT was higher in controls (e.g.: HFOV versus control, mean +/- SEM; IL-1 beta/HPRT: 1.6 +/- 0.3 versus 23.1 +/- 8.6 relative units (RU), p < 0.001; IL-8/HPRT: 8.5 +/- 2.0 versus 63.5 +/- 15.2 RU, p < 0.001). IL-8/HPRT gene expression was quantified in microdissected single cells. With HFOV, IL-8 gene expression was highly reduced in alveolar macrophages: 10 +/- 3.4 copies IL-8 mRNA/copy HPRT mRNA versus 356 +/- 142; p < 0.05 (bronchiolar epithelial cells: 33 +/- 16 versus 208 +/- 108; alveolar septum: 2.1 +/- 1.3 versus 26 +/- 11; bronchiolar smooth muscle cells: 1.3 +/- 0.3 versus 2.8 +/- 1.0; vascular smooth muscle cells: 0.7 +/- 0.3 versus 1.1 +/- 0.4). In conclusion, HFOV improved oxygenation, reduced pulmonary arterial pressure and attenuated pulmonary inflammatory response.

[Mechanical ventilation in pediatrics (III). Weaning, complications and other types of ventilation. High-frequency ventilation]

In the era of lung-protective ventilation strategies, high frequency oscillatory ventilation (HFOV) has attracted renewed interest and its use has dramatically increased in neonatal and pediatric intensive care units. HFOV is able to reduce ventilator-induced lung injury by limiting the incidence of volutrauma, atelectrauma, barotrauma and biotrauma. During HFOV, adequate oxygenation and ventilation is achieved by using low tidal volumes and small pressure swings at supraphysiologic frequencies. Unlike other high-frequency ventilation modes, HFOV has an active expiration phase. HFOV constitutes a safe and successful ventilation mode for managing pediatric patients with respiratory insufficiency refractory to optimized conventional mechanical ventilation and provides better results when initiated early. However, the elective use of HFOV requires further studies to identify its benefits over conventional modes of mechanical ventilation and to support its routine use as a first line therapy. In the present article, the Respiratory Working Group of the Spanish Society Pediatric Critical Care reviews the main issues in the pediatric application of HFOV. In addition, a general practical protocol and specific management strategies, as well as the monitoring, patient care and other special features of the use of HFOV in the pediatric setting, are discussed.

High tidal volume upregulates intrapulmonary cytokines in an in vivo mouse model of ventilator-induced lung injury

Mechanical ventilation has been demonstrated to exacerbate lung injury, and a sufficiently high tidal volume can induce injury in otherwise healthy lungs. However, it remains controversial whether injurious ventilation per se, without preceding lung injury, can initiate cytokine-mediated pulmonary inflammation. To address this, we developed an in vivo mouse model of acute lung injury produced by high tidal volume (Vt) ventilation. Anesthetized C57BL6 mice were ventilated at high Vt (34.5 +/- 2.9 ml/kg, mean +/- SD) for a duration of 156 +/- 17 min until mean blood pressure fell below 45 mmHg (series 1); high Vt for 120 min (series 2); or low Vt (8.8 +/- 0.5 ml/kg) for 120 or 180 min (series 3). High Vt produced progressive lung injury with a decrease in respiratory system compliance, increase in protein concentration in lung lavage fluid, and lung pathology showing hyaline membrane formation. High-Vt ventilation was associated with increased TNF-alpha in lung lavage fluid at the early stage of injury (series 2) but not the later stage (series 1). In contrast, lavage fluid macrophage inflammatory protein-2 (MIP-2) was increased in all high-Vt animals. Lavage fluid from high-Vt animals contained bioactive TNF-alpha by WEHI bioassay. Low-Vt ventilation induced minimal changes in physiology and pathology with negligible TNF-alpha and MIP-2 proteins and TNF-alpha bioactivity. These results demonstrate that high-Vt ventilation in the absence of underlying injury induces intrapulmonary TNF-alpha and MIP-2 expression in mice. The apparently transient nature of TNF-alpha upregulation may help explain previous controversy regarding the involvement of cytokines in ventilator-induced lung injury.

Positive end-expiratory pressure delays the progression of lung injury during ventilator strategies involving high airway pressure and lung overdistention

OBJECTIVE

Many studies have investigated the protective role of positive end-expiratory pressure (PEEP) on ventilator-induced lung injury. Most assessed lung injury in protocols involving different ventilation strategies applied for the same length of time. This study, however, set out to investigate the protective role of PEEP with respect to the time needed to reach similar levels of lung injury.

DESIGN

Prospective, randomized laboratory animal investigation.

SETTING

The University Laboratory of Ospedale Maggiore, Milano, IRCCS.

SUBJECTS

Anesthetized, paralyzed, and mechanically ventilated Sprague-Dawley rats.

INTERVENTIONS

Three groups of five Sprague-Dawley rats were ventilated using zero end-expiratory pressure ZEEP (PEEP of 0 cm H(2)O) and PEEP of 3 and 6 cm H(2)O and a similar index of lung overdistension (Paw(p)/P(100) congruent with 1.1; where Paw(p) is peak airway pressure and P(100) is the pressure corresponding to total lung capacity). To obtain this, tidal volume was reduced depending on the PEEP. To reach similar levels of lung injury, we measured respiratory system elastance while ventilating the animals and killed them when respiratory system elastance was 150% of baseline. Once target respiratory system elastance was reached, the lung wet-to-dry ratio was obtained.

RESULTS

Rats were ventilated with comparable high airway pressure (Paw(p) of 42.8 +/- 3.1, 43.5 +/- 2.6, and 46.2 +/- 4.4, respectively, for PEEP 0, 3, and 6) obtaining similar overdistension (Paw(p)/P(100) - index of overdistension: 1.17 +/- 0.2, 1.06 +/- 0.1, and 1.19 +/- 0.2). The respiratory system elastance target was reached and wet-to-dry ratio was not different in the three groups, suggesting a similar degree of lung damage. The time taken to achieve the target respiratory system elastance was three times longer with PEEP 3 and 6 (55 +/- 14 mins and 60 +/- 17) as compared with zero end-expiratory pressure (18 +/- 3 mins, p <.001).

CONCLUSION

These findings confirm that PEEP is protective against ventilator-induced lung injury and may enable the clinician to "buy time" in the progression of lung injury.

Ventilation-induced lung injury is associated with an increase in gut permeability

Mechanical ventilation is associated with several harmful effects mainly related to high tidal volumes (Vt). Ventilator-induced lung injury can be responsible for an increased production of inflammatory mediators. We evaluated remote consequences on the gut of lung triggered inflammatory response, neutralizing anti-tumor necrosis factor (TNF) antibody was administered to assess the role of TNF in lung and gut permeability changes. Rats were anesthetized and ventilated for 2 h. A control group (Con: Vt = 10 mL/kg) was compared with a high Vt group (HV: Vt = 30 ml/kg). One microCi of I125-labeled human serum albumin was injected to measure extravascular albumin space. Gut permeability was evaluated by plasma-to-lumen ratio leakage of I125 human serum albumin. Extravascular albumin space increased in the HV group from 446 +/- 50 microL to 2783 +/- 887 microL. Gut index of permeability increased from 5.1 +/- 1.2 to 14.2 +/- 4.9. Anti-TNF antibody prevented both lung and gut increase in permeability. High tidal volume ventilation resulted in an increase in lung edema and gut permeability, antagonism of TNF with neutralizing antibodies abrogated the increase in gut permeability as well as lung edema.

High-frequency oscillatory ventilation for acute respiratory distress syndrome in adult patients

INTRODUCTION

High-frequency oscillatory ventilation (HFOV) using an open-lung strategy has been demonstrated to improve oxygenation in neonatal and pediatric respiratory failure, without increasing barotrauma. Animal studies using small (<4 mm) endotracheal tubes have shown reduced histopathologic evidence of lung injury and inflammatory mediator release, suggesting reduced ventilator-induced lung injury.

CLINICAL STUDIES

During the last decade, case reports and observational studies of HFOV in patients failing conventional ventilation strategies have suggested improved oxygenation in adult patients with severe acute respiratory distress syndrome. These reports have also suggested that early (2 days) initiation of HFOV is more likely to result in survival than delayed initiation (>7 days). A recently published randomized, controlled trial in acute respiratory distress syndrome patients (n = 148) comparing HFOV with a pressure-control ventilation strategy (Pao(2)/Fio(2) ratio of <or=200 mm Hg on positive end-expiratory pressure of >10 cm H(2)O) demonstrated early (<16 hrs) improvement in Pao(2)/Fio(2) (p =.008) in the HFOV group but no significant difference in oxygenation index between the two groups during the initial 72 hrs of treatment. Thirty-day mortality was 37% in the HFOV group and 52% in the conventional ventilation group (p =.102). There was no significant difference between treatment groups in the prevalence of barotrauma, hemodynamic instability, or mucus plugging. This study suggests that HFOV is as effective and safe as the conventional strategy to which it was compared.

CLINICAL APPLICATION

For clinical use in adults, a trial of HFOV may be considered when Fio(2) requirements exceed 60% and mean airway pressure is approaching 20 cm H(2)O or higher (or, alternatively, positive end-expiratory pressure of >15 cm H(2)O). It is currently unknown whether initiating HFOV at a lower severity threshold would result in reduced ventilator-associated lung injury or mortality.

FUTURE DIRECTIONS

Future studies should compare different algorithms of applying HFOV to determine the optimal techniques for achieving oxygenation and ventilation, while minimizing ventilator-associated lung injury. The potential role of adjunctive therapies used with HFOV (e.g., prone ventilation, inhaled nitric oxide, aerosolized vasodilators, liquid ventilation) will require further research.

High-frequency oscillatory ventilation in adults with acute respiratory distress syndrome

The last decade has seen increased appreciation of ventilator-induced lung injury. The understanding that the process of mechanical ventilation can itself damage lungs has spurned the search for ventilation strategies that are more lung protective. High-frequency oscillatory ventilation is a mode of high-frequency ventilation that may accomplish all of the current goals of lung protection. Historically, much of the data evaluating high-frequency oscillatory ventilation came from neonatal and pediatric populations. In the past year, a number of provocative and exciting studies have been published that contribute significantly to our understanding of high-frequency oscillatory ventilation, its role in preventing and reducing ventilator-induced lung injury, and its use in the support of adult patients with lung injury. In this article, we discuss the current understanding of high-frequency oscillatory ventilation and highlight the most recent literature addressing its application in adult patients with acute respiratory distress syndrome.

Science review: mechanisms of ventilator-induced injury

Acute respiratory distress syndrome (ARDS) and acute lung injury are among the most frequent reasons for intensive care unit admission, accounting for approximately one-third of admissions. Mortality from ARDS has been estimated as high as 70% in some studies. Until recently, however, no targeted therapy had been found to improve patient outcome, including mortality. With the completion of the National Institutes of Health-sponsored Acute Respiratory Distress Syndrome Network low tidal volume study, clinicians now have convincing evidence that ventilation with tidal volumes lower than those conventionally used in this patient population reduces the relative risk of mortality by 21%. These data confirm the long-held suspicion that the role of mechanical ventilation for acute hypoxemic respiratory failure is more than supportive, in that mechanical ventilation can also actively contribute to lung injury. The mechanisms of the protective effects of low tidal volume ventilation in conjunction with positive end expiratory pressure are incompletely understood and are the focus of ongoing studies. The objective of the present article is to review the potential cellular mechanisms of lung injury attributable to mechanical ventilation in patients with ARDS and acute lung injury.

High-frequency oscillatory ventilation for acute respiratory distress syndrome in adults: a randomized, controlled trial

Observational studies of high-frequency oscillatory ventilation in adults with the acute respiratory distress syndrome have demonstrated improvements in oxygenation. We designed a multicenter, randomized, controlled trial comparing the safety and effectiveness of high-frequency oscillatory ventilation with conventional ventilation in adults with acute respiratory distress syndrome; 148 adults with acute respiratory distress syndrome (Pa(O2)/fraction of inspired oxygen <or= 200 mm Hg on 10 or more cm H2O positive end-expiratory pressure) were randomized to high-frequency oscillatory ventilation (n = 75) or conventional ventilation (n = 73). Applied mean airway pressure was significantly higher in the high-frequency oscillation group compared with the conventional ventilation group throughout the first 72 hours (p = 0.0001). The high-frequency oscillation group showed early (less than 16 hours) improvement in Pa(O2)/fraction of inspired oxygen compared with the conventional ventilation group (p = 0.008); however, this difference did not persist beyond 24 hours. Oxygenation index decreased similarly over the first 72 hours in both groups. Thirty-day mortality was 37% in the high-frequency oscillation group and was 52% in the conventional ventilation group (p = 0.102). The percentage of patients alive without mechanical ventilation at Day 30 was 36% and 31% in the high-frequency oscillation and conventional ventilation groups, respectively (p = 0.686). There were no significant differences in hemodynamic variables, oxygenation failure, ventilation failure, barotraumas, or mucus plugging between treatment groups. We conclude that high-frequency oscillation is a safe and effective mode of ventilation for the treatment of acute respiratory distress syndrome in adults.

Neonatal respiratory failure

The classic entity of neonatal distress syndrome, as a lung disease expressing predominant surfactant deficiency, is currently changing to a more complex disease of the developing lung as a result of the number of extremely immature preterm infants. Prenatal factors, such as the fetal inflammatory response syndrome influence short- and long-term outcome in these premature infants presenting with respiratory distress syndrome at birth. Therefore, various previously dismissed treatment strategies, such as surfactant prophylaxis or newer anti-inflammatory approaches have to be reinvestigated in this emerging population. Despite the resurrection of a new picture of bronchopulmonary dysplasia, lung injury induced by mechanical ventilation remains a major issue in neonatal intensive care. With the advance in understanding of mechanical ventilation, it is becoming evident, that improvement in outcome can not be achieved by restoring normal lung physiology in the diseased lung using sophisticated ventilators and ventilation modes. A more disease specific ventilator strategy that will target as early as possible homogenous lung opening while at the same time avoiding overdistention of the lung, has the potential to affect outcome. The possible antiinflammatory properties of surfactant-proteins, nitric oxide and corticosteroids, despite some drawbacks, may show to have a synergistic effect. However, this needs further exploration.

Reduced tidal volumes and lung protective ventilatory strategies: where do we go from here?

Three major determinants of lung injury associated with mechanical ventilation have been clearly identified: high pressure/high volume, the shear forces caused by intratidal collapse and decollapse leading to barotrauma/volotrauma/biotrauma. The lung protective strategy aims to reduce the impact of all three determinants. A groundbreaking study showed that reduced tidal volume is less dangerous than high tidal volume, but the researchers did not apply "full" lung protective strategy and did not take into account the shear forces. "Full" protective lung strategy was tested in only one study and in a limited number of patients. Several physiologic studies strongly suggest the advantages of the lung protective strategy.

Pro/con clinical debate: is high-frequency oscillatory ventilation useful in the management of adult patients with respiratory failure?

In neonatal and pediatric intensive care units, high-frequency oscillatory ventilation (HFOV) has become an increasingly common therapy. This may not have been the case if researchers had not persisted in investigating the therapy after early disappointing clinical trials. Devices capable of providing this therapy to adults have become commercially available relatively recently. However, there are many questions that need to be answered regarding HFOV in adults: Is HFOV in adults superior to conventional mechanical ventilation? Who is the ideal candidate for HFOV? When should it be applied? What is the best technique with which to apply it? When should a patient on HFOV be converted back to conventional ventilation? What is the safety and efficacy of the device? As outlined in the following debate, there are several compelling arguments for and against the use of HFOV at this point in adults.