Effect of a lung recruitment maneuver by high-frequency oscillatory ventilation in experimental acute lung injury on organ blood flow in pigs

Computed Tomography Assessment of Tidal Lung Overinflation in Domestic Cats Undergoing Pressure-Controlled Mechanical Ventilation During General Anesthesia

Objective

This study aimed to evaluate lung overinflation at different airway inspiratory pressure levels using computed tomography in cats undergoing general anesthesia.

Study Design

Prospective laboratory study.

Animals

A group of 17 healthy male cats, aged 1.9–4.5 years and weighing 3.5 ± 0.5 kg.

Methods

Seventeen adult male cats were ventilated in pressure-controlled mode with airway pressure stepwise increased from 5 to 15 cmH₂O in 2 cmH₂O steps every 5 min and then stepwise decreased. The respiratory rate was set at 15 movements per min and end-expiratory pressure at zero (ZEEP). After 5 min in each inspiratory pressure step, a 4 s inspiratory pause was performed to obtain a thoracic juxta-diaphragmatic single slice helical CT image and to collect respiratory mechanics data and an arterial blood sample. Lung parenchyma aeration was defined as overinflated, normally-aerated, poorly-aerated, and non-aerated according to the CT attenuation number (−1,000 to −900 HU, −900 to −500 HU, −500 to −100 HU, and −100 to +100 HU, respectively).

Result

At 5 cmH₂O airway pressure, tidal volume was 6.7± 2.2 ml kg⁻¹, 2.1% (0.3–6.3%) of the pulmonary parenchyma was overinflated and 84.9% (77.6%−87.6%) was normally inflated. Increases in airway pressure were associated with progressive distention of the lung parenchyma. At 15 cmH₂O airway pressure, tidal volume increased to 31.5± 9.9 ml kg⁻¹ (p < 0.001), overinflated pulmonary parenchyma increased to 28.4% (21.2–30.6%) (p < 0.001), while normally inflated parenchyma decreased 57.9% (53.4–62.8%) (p < 0.001). Tidal volume and overinflated lung fraction returned to baseline when airway pressure was decreased. A progressive decrease was observed in arterial carbon dioxide partial pressure (PaCO₂) and end-tidal carbon dioxide (ETCO₂) when the airway pressures were increased above 9 cmH₂O (p < 0.001). The increase in airway pressure promoted an elevation in pH (p < 0.001).

Conclusions and Clinical Relevance

Ventilation with 5 and 7 cmH₂O of airway pressure prevents overinflation in healthy cats with highly compliant chest walls, despite presenting acidemia by respiratory acidosis. This fact can be controlled by increasing or decreasing respiratory rate and inspiratory time.

High-frequency oscillatory ventilation: still a role?

PURPOSE OF REVIEW

In light of emerging data from clinical trials, the place of high-frequency oscillatory ventilation (HFOV) in the management of acute respiratory distress syndrome (ARDS) is uncertain. This review provides an overview of these new clinical data and also explores new areas of investigation for HFOV in adults.

RECENT FINDINGS

While prior meta-analyses suggested benefit for HFOV, updated systematic reviews published this year, which include two large recent clinical trials, now show no statistically significant impact of HFOV on mortality in adults with ARDS. It is possible that HFOV would be safer and more effective with a more individualized approach to setting mean airway pressure (mPaw). Possible techniques to achieve this include titrating mPaw in response to oxygenation or hemodynamic changes after HFOV initiation, by measuring respiratory system impedance, or by following echocardiographic changes.

SUMMARY

Although not first-line, HFOV remains a tool in the armamentarium of the intensivist managing the patient with severe ARDS and refractory hypoxemia. A refinement in the approach to delivering HFOV is warranted, with more attention paid to its adverse hemodynamic consequences.

Is there still a role for high-frequency oscillatory ventilation in neonates, children and adults?

Critically ill patients with respiratory pathology often require mechanical ventilation and while low tidal volume ventilation has become the mainstay of treatment, achieving adequate gas exchange may not be attainable with conventional ventilator modalities. In attempt to achieve gas exchange goals and also mitigate lung injury, high frequency ventilation is often implemented which couples low tidal volumes with sustained mean airway pressure. This manuscript presents the physiology of high-frequency oscillatory ventilation, reviews the currently available data on its use and provides strategies and approaches for this mode of ventilation.

High-frequency oscillatory ventilation and short-term outcome in neonates and infants undergoing cardiac surgery: a propensity score analysis

INTRODUCTION

Experience with high-frequency oscillatory ventilation (HFOV) after congenital cardiac surgery is limited despite evidence about reduction in pulmonary vascular resistance after the Fontan procedure. HFOV is recommended in adults and children with acute respiratory distress syndrome. The aim of the present study was to assess associations between commencement of HFOV on the day of surgery and length of mechanical ventilation, length of Intensive Care Unit (ICU) stay and mortality in neonates and infants with respiratory distress following cardiac surgery.

METHODS

A logistic regression model was used to develop a propensity score, which accounted for the probability of being switched from conventional mechanical ventilation (CMV) to HFOV on the day of surgery. It included baseline characteristics, type of procedure and postoperative variables, and was used to match each patient with HFOV with a control patient, in whom CMV was used exclusively. Length of mechanical ventilation, ICU stay and mortality rates were compared in the matched set.

RESULTS

Overall, 3,549 neonates and infants underwent cardiac surgery from January 2001 through June 2010, 120 patients were switched to HFOV and matched with 120 controls. After adjustment for the delay to sternal closure, duration of renal replacement therapy, occurrence of pulmonary hypertension and year of surgery, the probability of successful weaning over time and the probability of ICU delivery over time were significantly higher in patients with HFOV, adjusted hazard ratios and 95% confidence intervals: 1.63, 1.17 to 2.26 (P = 0.004). and 1.65, 95% confidence intervals: 1.20 to 2.28 (P = 0.002) respectively. No association was found with mortality.

CONCLUSIONS

When commenced on the day of surgery in neonates and infants with respiratory distress following cardiac surgery, HFOV was associated with shorter lengths of mechanical ventilation and ICU stay than CMV.

Lung recruitment--a guide for clinicians

Recruitment manoeuvres play an important role in minimising ventilator associated lung injury (VALI) particularly when lung protective ventilation strategies are employed and as such clinicians should consider their application. This paper provides evidence-based recommendations for clinical practice with regard to alveolar recruitment. It includes recommendations for timing of recruitment, strategies of recruitment and methods of measuring the efficacy of recruitment manoeuvres and contributes to knowledge about the risks associated with recruitment manoeuvres. There are a range of methods for recruiting alveoli, most notably by manipulating positive end expiratory pressure (PEEP) and peak inspiratory pressure (PIP) with consensus as to the most effective not yet determined. A number of studies have demonstrated that improvement in oxygenation is rarely sustained following a recruitment manoeuvre and it is questionable whether improved oxygenation should be the clinician's goal. Transient haemodynamic compromise has been noted in a number of studies with a few studies reporting persistent, harmful sequelae to recruitment manoeuvres. No studies have been located that assess the impact of recruitment manoeuvres on length of ventilation, length of stay, morbidity or mortality. Recruitment manoeuvres restore end expiratory lung volume by overcoming threshold opening pressures and are most effective when applied after circuit disconnection and airway suction. Whether this ultimately improves outcomes in adult or paediatric populations is unknown.

Year in review 2006: Critical Care--Respirology

The present article summarises and places in context original research articles from the respirology section published in Critical Care in 2006. Twenty papers were identified and were grouped by topic into those addressing acute lung injury and ventilator-induced lung injury, those examining high-frequency oscillation, those studying pulmonary physiology and mechanics, those assessing tracheostomy, and those exploring other topics.

Bench-to-bedside review: high-frequency oscillatory ventilation in adults with acute respiratory distress syndrome

Mechanical ventilation is the cornerstone of therapy for patients with acute respiratory distress syndrome (ARDS). Paradoxically, mechanical ventilation can exacerbate lung damage – a phenomenon known as ventilator-induced lung injury. While new ventilation strategies have reduced the mortality rate in patients with ARDS, this mortality rate still remains high. High-frequency oscillatory ventilation (HFOV) is an unconventional form of ventilation that may improve oxygenation in patients with ARDS, while limiting further lung injury associated with high ventilatory pressures and volumes delivered during conventional ventilation. HFOV has been used for almost two decades in the neonatal population, but there is more limited experience with HFOV in the adult population. In adults, the majority of the published literature is in the form of small observational studies in which HFOV was used as 'rescue' therapy for patients with very severe ARDS who were failing conventional ventilation. Two prospective randomized controlled trials, however, while showing no mortality benefit, have suggested that HFOV, compared with conventional ventilation, is a safe and effective ventilation strategy for adults with ARDS. Several studies suggest that HFOV may improve outcomes if used early in the course of ARDS, or if used in certain populations. This review will summarize the evidence supporting the use of HFOV in adults with ARDS.

Ventilatory management of ARDS: high frequency oscillation and lung recruitment!

Many aspects of ventilatory management in patients with ARDS are still controversial and one of the major controversies is should HFO or CMV ideally be used to manage this patients. As shown by David et al. when the two approaches to ventilatory support are applied using similar principles the physiologic outcomes appear to be similar. With both approaches the use of lung recruitment maneuvers early in ARDS (1 to 3 day) after hemodynamic stabilization in patients without baratrauma is promising. The key to managing ARDS regardless of mode is to use an open lung protective ventilatory strategy. It is not the mode that makes the difference, it is the approach used to apply the mode!