Effect of inspired oxygen fraction on alveolar derecruitment in acute respiratory distress syndrome

[Positive end-expiratory pressure : adjustment in acute lung injury]

Treatment of patients suffering from acute lung injury is a challenge for the treating physician. In recent years ventilation of patients with acute hypoxic lung injury has changed fundamentally. Besides the use of low tidal volumes, the most beneficial setting of positive end-expiratory pressure (PEEP) has been in the focus of researchers. The findings allow adaption of treatment to milder forms of acute lung injury and severe forms. Additionally computed tomography techniques to assess the pulmonary situation and recruitment potential as well as bed-side techniques to adjust PEEP on the ward have been modified and improved. This review gives an outline of recent developments in PEEP adjustment for patients suffering from acute hypoxic and hypercapnic lung injury and explains the fundamental pathophysiology necessary as a basis for correct treatment.

Low Tidal Volume Ventilation in Patients without Acute Respiratory Distress Syndrome: A Paradigm Shift in Mechanical Ventilation

Protective ventilation with low tidal volume has been shown to reduce morbidity and mortality in patients suffering from acute lung injury (ALI) and acute respiratory distress syndrome (ARDS). Low tidal volume ventilation is associated with particular clinical challenges and is therefore often underutilized as a therapeutic option in clinical practice. Despite some potential difficulties, data have been published examining the application of protective ventilation in patients without lung injury. We will briefly review the physiologic rationale for low tidal volume ventilation and explore the current evidence for protective ventilation in patients without lung injury. In addition, we will explore some of the potential reasons for its underuse and provide strategies to overcome some of the associated clinical challenges.

Hyperoxia may be beneficial

The current practice of mechanical ventilation comprises the use of the least inspiratory O2 fraction associated with an arterial O2 tension of 55 to 80 mm Hg or an arterial hemoglobin O2 saturation of 88% to 95%. Early goal-directed therapy for septic shock, however, attempts to balance O2 delivery and demand by optimizing cardiac function and hemoglobin concentration, without making use of hyperoxia. Clearly, it has been well-established for more than a century that long-term exposure to pure O2 results in pulmonary and, under hyperbaric conditions, central nervous O2 toxicity. Nevertheless, several arguments support the use of ventilation with 100% O2 as a supportive measure during the first 12 to 24 hrs of septic shock. In contrast to patients without lung disease undergoing anesthesia, ventilation with 100% O2 does not worsen intrapulmonary shunt under conditions of hyperinflammation, particularly when low tidal volume-high positive end-expiratory pressure ventilation is used. In healthy volunteers and experimental animals, exposure to hyperoxia may cause pulmonary inflammation, enhanced oxidative stress, and tissue apoptosis. This, however, requires long-term exposure or injurious tidal volumes. In contrast, within the timeframe of a perioperative administration, direct O2 toxicity only plays a negligible role. Pure O2 ventilation induces peripheral vasoconstriction and thus may counteract shock-induced hypotension and reduce vasopressor requirements. Furthermore, in experimental animals, a redistribution of cardiac output toward the kidney and the hepato-splanchnic organs was observed. Hyperoxia not only reverses the anesthesia-related impairment of the host defense but also is an antibiotic. In fact, perioperative hyperoxia significantly reduced wound infections, and this effect was directly related to the tissue O2 tension. Therefore, we advocate mechanical ventilation with 100% O2 during the first 12 to 24 hrs of septic shock. However, controlled clinical trials are mandatory to test the safety and efficacy of this approach.

Clinicians' response to hyperoxia in ventilated patients in a Dutch ICU depends on the level of FiO2

PURPOSE

Hyperoxia may induce pulmonary injury and may increase oxidative stress. In this retrospective database study we aimed to evaluate the response to hyperoxia by intensivists in a Dutch academic intensive care unit.

METHODS

All arterial blood gas (ABG) data from mechanically ventilated patients from 2005 until 2009 were extracted from an electronic storage database of a mixed 32-bed intensive care unit in a university hospital in Amsterdam. Mechanical ventilation settings at the time of the ABG tests were retrieved.

RESULTS

The results of 126,778 ABG tests from 5,498 mechanically ventilated patients were retrieved including corresponding ventilator settings. In 28,222 (22%) of the ABG tests the arterial oxygen tension (PaO(2)) was >16 kPa (120 mmHg). In only 25% of the tests with PaO(2) >16 kPa (120 mmHg) was the fraction of inspired oxygen (FiO(2)) decreased. Hyperoxia was accepted without adjustment in ventilator settings if FiO(2) was 0.4 or lower.

CONCLUSION

Hyperoxia is frequently seen but in most cases does not lead to adjustment of ventilator settings if FiO(2) <0.41. Implementation of guidelines concerning oxygen therapy should be improved and further research is needed concerning the effects of frequently encountered hyperoxia.

Ventilatory strategies for hypoxemia during cardiac surgery: survey validation for anesthesiologists in Brazil

BACKGROUND AND OBJECTIVES

Perioperative hypoxemia is common in cardiac surgeries, and atelectasis is the main cause. Besides, we can mention extracorporeal circulation (ECC), dissection of internal thoracic arteries, and previous clinical status of the patient among others as its causes. The present study elaborated an anonymous questionnaire to observe ventilatory strategies for hypoxemia in cardiac surgeries adopted by five thousand anesthesiologists all over the country.

METHODS

Questionnaires were sent via e-mail for five thousand anesthesiologists in Brazil.

RESULTS

Out of the questionnaires sent, 81 valid responses were received. Among the answers, 65 (80%) anesthesiologists use volume-controlled ventilation (VCV), while 16 (20%) prefer pressure-controlled ventilation (PCV). The tidal volume (Vt) used is lower than 10 mL.kg(-1), for 46 (61%) versus 20 (30%) who adopt a Vt greater than 10 mL.kg(-1). Forty-seven (58%) use PEEP and 15 (21%) use FiO(2) above 60%. In the case of intraoperative hypoxemia, 20.9% increase or introduce PEEP, 70.3% increase the FiO(2), 19.7% use alveolar recruitment maneuvers, 13.5% increase the tidal volume, and 20.9% check for the presence of failures in the anesthesia equipment. Responses were sent from 15 states.

CONCLUSIONS

The conducts described in the questionnaires are compatible with those of the international literature. Adjusting the questionnaires format and the way to approach anesthesiologists, new studies could be undertaken.

Prospective evaluation of a decision support system for setting inspired oxygen in intensive care patients

PURPOSE

The aim of the study was to prospectively evaluate a decision support system for its ability to provide appropriate suggestions of inspired oxygen fraction in intensive care patients comparing with levels used by clinicians in attendance.

MATERIALS AND METHODS

Thirteen mechanically ventilated patients were studied in an intensive care unit where up to 4 experiments were performed during 2 consecutive days. Inspired oxygen fraction was selected in each experiment by both the decision support system and attending clinicians, and each selection was evaluated by measuring arterial oxygen saturation.

RESULTS

Median (interquartile range [range]) changes in inspired oxygen fraction from baseline level by attending clinicians and the decision support system were 0.00 (-0.05 to 0.00 [-0.10 to 0.05]) and -0.03 (-0.07 to 0.01 [-0.16 to 0.12]), respectively. Clinician ranges of inspired oxygen fraction and arterial oxygen saturation were 0.25 to 0.70 and 0.92 to 0.99, respectively. Decision support system ranges of inspired oxygen fraction and arterial oxygen saturation were 0.26 to 0.54 and 0.94 to 0.99, respectively.

CONCLUSIONS

The decision support system selects appropriate levels of inspired oxygen fraction in intensive care patients and could be used for automatic frequent assessment of patients, freeing the focus of clinicians to concentrate on more challenging therapy.

Acute lung injury and outcomes after thoracic surgery

PURPOSE OF REVIEW

The present review evaluates the evidence available in the literature tracking perioperative mortality and morbidity as well as the pathogenesis and management of acute lung injury (ALI) in patients undergoing thoracotomy.

RECENT FINDINGS

Over the last decade, despite increasing age and comorbid conditions, the operative mortality has remained unchanged for patients undergoing lung resection, whereas procedure-related complications have declined. Better clinical outcomes are achieved in high-volume hospitals and when procedures are performed by a thoracic surgeon. Postthoracotomy ALI has become the leading cause of operative death, its incidence has remained stable (2-5%) and earlier diagnosis can be made by assessing the extravascular lung water volume with the single-indicator dilution technique. The pathogenesis of ALI implicates a multiple-hit sequence of various triggering factors (e.g. oxidative stress and surgical-induced inflammation) in addition to injurious ventilatory settings and genetic predisposition.

SUMMARY

Knowledge of the perioperative risk factors of major complications and understanding of the mechanisms of postthoracotomy ALI enable anesthesiologists to implement 'protective' lung strategies including the use of low tidal volume (VT) with recruitment maneuvers, a goal-directed fluid approach and prophylactic treatment with inhaled beta2-adrenergic agonists.

Impact of mild hypoxemia on renal function and renal resistive index during mechanical ventilation

RATIONALE

Short-term hypoxemia affects diuresis and natriuresis in healthy individuals. No data are available on the impact of the mild hypoxemia levels usually tolerated in critically ill patients receiving mechanical ventilation.

OBJECTIVES

To assess the renal effects of mild hypoxemia during mechanical ventilation for acute lung injury (ALI).

METHODS

Prospective, physiological study in 12 mechanically ventilated patients with ALI. Patients were studied at baseline with an arterial saturation (SaO(2)) of 96% [94-98] then a comparison was performed between SaO(2) values of 88-90% (mild hypoxemia) and 98-99% (high oxygenation).

MAIN RESULTS

FiO(2) was set at 0.25 [0.23-0.32] and 0.7 [0.63-0.8], respectively, to obtain SaO(2) of 89 [89-90] and 99% [98-99]. Hemodynamic or respiratory parameters were not significantly affected by FiO(2) levels. Compared with high oxygenation level, mild hypoxemia using low FiO(2) was associated with increase in diuresis (median [interquartile range], 67 [55-105] vs. 55 [45-60] ml/h; P = 0.003) and in doppler-based renal resistive index (RI) (0.78 [0.66-0.85] vs. 0.72 [0.60-0.78]; P = 0.003). The 2-h calculated creatinine clearance also increased (63 [46-103] vs. 35 [30-85] ml/min; P = 0.005) without change in urinary creatinine (P = 0.13). No significant change in natriuresis was observed. Half of the patients were under norepinephrine infusion and the renal response did not differ according to the presence of vasopressors.

CONCLUSION

In patients with ALI, mild hypoxemia related to short-term low FiO(2) induce increases in diuresis and in renal RI. This latter point suggests intra-renal mechanisms that need to be further investigated.

FIO2 and acute respiratory distress syndrome definition during lung protective ventilation

OBJECTIVE

PaO2/FIO2 ratio (P/F) is the marker of hypoxemia used in the American-European Consensus Conference on lung injury. A high FIO2 level has been reported to variably alter PaO2/FIO2. We investigated the effect of high FIO2 levels on the course of P/F in lung protective mechanically ventilated patients with acute respiratory distress syndrome.

DESIGN

Prospective, controlled, interventional study.

SETTING

University teaching French medical intensive care unit.

PATIENTS

Twenty-four patients with acute respiratory distress syndrome having P/F between 100 and 200 mm Hg at FIO2 0.5 received low-volume controlled ventilation (V(T) = 6 mL/kg predicted body weight) with a positive end-expiratory pressure at 2 cm H2O above the lower inflection point if present, or 10 cm H2O.

INTERVENTION

The following FIO2 levels were applied randomly for 20 mins: 0.5, 0.6, 0.7, 0.8, 0.9, and 1.

MEASUREMENTS AND RESULTS

Increasing FIO2 above 0.7 was associated with a significant increase in P/F (p < 0.001). The mean P/F change between FIO2 0.5 and 1 (Delta P/F) was 47% +/- 35%. Sixteen patients (67%) had a P/F >200 at FIO2 1 whereas P/F was <200 at FIO2 0.5. Venous admixture (Q(VA)/Q(T)) decreased linearly for each FIO2 step (p < 0.001). The Q(VA)/Q(T) change between FIO2 0.5 and 1 was strongly correlated with Delta P/F (r = 0.84). Delta P/F was higher in patients with true shunt <30% (64% [54-93]) than in those with shunt >30% (20% [10-36]; p = 0.003).

CONCLUSION

The P/F ratio increased significantly with a FIO2 >0.7. P/F variation, induced by a switch from FIO2 0.5 to 1, was responsible for two thirds of patients changing from the acute respiratory distress syndrome to the acute lung injury stage of the American-European Consensus Conference definition. FIO2 should be carefully defined for the screening of lung-injured patients.

Short-term administration of a high oxygen concentration is not injurious in an ex-vivo rabbit model of ventilator-induced lung injury

BACKGROUND

Mechanical ventilation and administration of a high oxygen concentration are simultaneously used in the management of respiratory failure. We conducted this study to evaluate the effect of a high inspired oxygen concentration on ventilator-induced lung injury.

METHODS

Forty sets of isolated/perfused rabbit lungs were randomized for 60 min of pressure-control ventilation at a plateau inspiratory pressure of 25 or 15 cm H(2)O and positive end-expiratory pressure of 3 cm H(2)O while receiving 100% or 21% O(2). The temperature, pH, and partial pressure of CO(2) in the perfusate were maintained the same in all groups (n = 10 for each group). The outcome measures used to assess lung injury included: the change in weight gain and ultrafiltration coefficient, the frequency of vascular failure, the histological lesions and the concentration of tumor necrosis factor-alpha and malondialdehyde in the bronchoalveolar lavage fluid.

RESULTS

The two groups ventilated at the higher inspiratory pressure/tidal volume experienced greater weight gain and increases in the ultrafiltration coefficient, more frequently suffered vascular failure, and presented higher composite scores of histological damage than the two groups ventilated at the lower inspiratory pressure/tidal volume. Hyperoxia was not found to further increase any of the monitored markers of lung injury. No difference was noticed among the four experimental groups in the alveolar lavage fluid levels of tumor necrosis factor-alpha or malondialdehyde.

CONCLUSIONS

These findings suggest that short-term administration of a high oxygen concentration is not a major determinant of ventilator-induced lung injury in this experimental model.

Impact of supplemental oxygen in mechanically ventilated adult and infant mice

The aim of the present study was to determine the short-term effects of hyperoxia on respiratory mechanics in mechanically ventilated infant and adult mice. Eight and two week old BALB/c mice were exposed to inspired oxygen fractions [Formula: see text] of 0.21, 0.3, 0.6, and 1.0, respectively, during 120 min of mechanical ventilation. Respiratory system mechanics and inflammatory responses were measured. Using the low-frequency forced oscillation technique no differences were found in airway resistance between different [Formula: see text] groups when corrected for changes in gas viscosity. Coefficients of lung tissue damping and elastance were not different between groups and showed similar changes over time in both age groups. Inflammatory responses did not differ between groups at either age. Hyperoxia had no impact on respiratory mechanics during mechanical ventilation with low tidal volume and positive end-expiratory pressure. Hence, supplemental oxygen can safely be applied during short-term mechanical ventilation strategies in infant and adult mice.

Appropriate Ventilatory Settings for Thoracic Surgery: Intraoperative and Postoperative

Mechanical ventilation of patients undergoing thoracic surgery is often challenging. These patients frequently have significant underlying comorbidities, including cardiopulmonary disease, and often must undergo 1-lung ventilation. Perioperative respiratory complications are common and are multifactorial in etiology. Increasing evidence suggests that mechanical ventilation is associated with, and may even cause, lung damage in both sick and healthy patients. Gas exchange to provide acceptable end-organ oxygenation remains a primary goal but so too is minimization of risks for acute lung injury. Every ventilator strategy is associated with potential beneficial and adverse side effects. Understanding the impact of various ventilation strategies allows clinicians to provide optimal care for patients.

The influence of venous admixture on alveolar dead space and carbon dioxide exchange in acute respiratory distress syndrome: computer modelling

INTRODUCTION

Alveolar dead space reflects phenomena that render arterial partial pressure of carbon dioxide higher than that of mixed alveolar gas, disturbing carbon dioxide exchange. Right-to-left shunt fraction (Qs/Qt) leads to an alveolar dead space fraction (VdAS/VtA; where VtA is alveolar tidal volume). In acute respiratory distress syndrome, ancillary physiological disturbances may include low cardiac output, high metabolic rate, anaemia and acid-base instability. The purpose of the present study was to analyze the extent to which shunt contributes to alveolar dead space and perturbs carbon dioxide exchange in ancillary physiological disturbances.

METHODS

A comprehensive model of pulmonary gas exchange was based upon known equations and iterative mathematics.

RESULTS

The alveolar dead space fraction caused by shunt increased nonlinearly with Qs/Qt and, under 'basal conditions', reached 0.21 at a Qs/Qt of 0.6. At a Qs/Qt of 0.4, reduction in cardiac output from 5 l/minute to 3 l/minute increased VdAS/VtA from 0.11 to 0.16. Metabolic acidosis further augmented the effects of shunt on VdAS/VtA, particularly with hyperventilation. A Qs/Qt of 0.5 may increase arterial carbon dioxide tension by about 15% to 30% if ventilation is not increased.

CONCLUSION

In acute respiratory distress syndrome, perturbation of carbon dioxide exchange caused by shunt is enhanced by ancillary disturbances such as low cardiac output, anaemia, metabolic acidosis and hyperventilation. Maintained homeostasis mitigates the effects of shunt.

Effects of ventilation with 100% oxygen during early hyperdynamic porcine fecal peritonitis

OBJECTIVE

Early goal-directed therapy aims at balancing tissue oxygen delivery and demand. Hyperoxia (i.e., pure oxygen breathing) has not been studied in this context, since sepsis increases oxygen radical production, which is believed to be directly related to the oxygen tension. On the other hand, oxygen breathing improved survival in various shock models. Therefore, we hypothesized that hyperoxia may be beneficial during early septic shock.

DESIGN

Laboratory animal experiments.

SETTING

Animal research laboratory at university medical school.

SUBJECTS

Twenty domestic pigs of either gender.

INTERVENTIONS

After induction of fecal peritonitis, anesthetized and instrumented pigs were ventilated with either 100% oxygen or supplemental oxygen as needed to maintain arterial hemoglobin oxygen saturation > or = 90%. Normotensive and hyperdynamic hemodynamics were achieved using hydroxyethyl starch and norepinephrine infusion.

MEASUREMENTS AND MAIN RESULTS

Before and at 12, 18, and 24 hrs of peritonitis, we measured lung compliance; systemic, pulmonary, and hepatosplanchnic hemodynamics; gas exchange; acid-base status; blood isoprostanes; nitrates; DNA strand breaks; and organ function. Gluconeogenesis and glucose oxidation were calculated from blood isotope and expiratory 13CO2 enrichments during continuous intravenous 1,2,3,4,5,6-(13)C6-glucose. Apoptosis in lung and liver was assessed postmortem (TUNEL staining). Hyperoxia did not affect lung mechanics or gas exchange but redistributed cardiac output to the hepatosplanchnic region, attenuated regional venous metabolic acidosis, increased glucose oxidation, improved renal function, and markedly reduced the apoptotic death rate in liver and lung.

CONCLUSIONS

During early hyperdynamic porcine septic shock, 100% oxygen improved organ function and attenuated tissue apoptosis without affecting lung function and oxidative or nitrosative stress. Therefore, it might be considered as an additional measure in the first phase of early goal-directed therapy.

How to monitor lung recruitment in patients with acute lung injury

PURPOSE OF REVIEW

Bedside assessment of lung recruitment is critical for setting mechanical ventilation during acute respiratory distress syndrome. We review recent findings on this topic and attempt to provide a clinical approach to estimating lung recruitment.

RECENT FINDINGS

Because of intrinsic limitations in considering single parameters of gas exchange as tools to estimate lung recruitment, investigators have combined different respiratory variables, including respiratory mechanics, to enhance the likelihood of predicting lung recruitment. Confusions on interpreting the physiologic rationale of gas-exchange variations as associated with lung recruitment are still widespread. Techniques of lung imaging, in particular computed-tomography scanning, are still the most applied for reference measurement. Dynamic computed-tomography scanning may allow continuous monitoring of the effects of mechanical ventilation on lung parenchyma. Among the new techniques proposed, electric impedance and positron emission tomography are the most promising. Despite progress, computed-tomography scanning still represents the best technique to measure lung recruitment in clinical practice.

SUMMARY

Two approaches should be considered to estimate lung recruitment: the use of computed-tomography scanning and indices combining different respiratory variables. Future studies, especially on lung-perfusion distribution, are warranted to improve our knowledge of the pathophysiology of lung recruitment.