Pulmonary dead-space fraction as a risk factor for death in the acute respiratory distress syndrome

Reproducibility of the respiratory dead space measurements in mechanically ventilated children using the CO2SMO monitor

OBJECTIVES

To assess the reproducibility of respiratory dead space measurements in ventilated children.

DESIGN

Prospective study.

SETTING

University pediatric intensive care unit.

PATIENTS

Thirty-two mechanically ventilated children (0.13-15.4 years) who were clinically stable.

METHODS

The single-breath CO(2) test (SBT-CO(2)) was recorded using the CO(2)SMO Plus from the mean of 30 ventilatory cycles during 1 h (at T0, T15, T30, T45, and T60). Airway dead space was determined automatically (Novametrix Medical Systems, USA), and manually by Bohr- Enghoff equations using data obtained by SBT-CO(2). At the end of the study period, arterial blood gas was sampled in order to calculate alveolar and physiologic dead space. Intrasubject reproducibility of measurements was evaluated by the intraclass correlation coefficient. Two-way analysis of variance was used to evaluate the relationships between time and measurements. The two methods for calculating airway dead space were compared by using two-tailed Student's t-test and Bland-Altman analysis.

RESULTS

Airway dead space measurement had a good reproducibility during the 1-h period, whatever the method used (intraclass correlation coefficient: 0.84 to 0.87). No significant difference was observed with time. Airway dead space values from the SBT-CO(2) method were smaller than those from Bohr-Enghoff equations. Physiologic dead space values from the SBT-CO2 method were similar to those from Bohr-Enghoff equations.

CONCLUSION

The measurement of airway dead space by the CO(2)SMO Plus was reproducible over a 1-h period in children requiring mechanical ventilation, provided ventilatory parameters were constant throughout the study. SBT-CO(2) analysis may provide a bedside non-invasive monitoring of volumetric capnography.

[Treatment of acute respiratory distress syndrome in a treatment center. Success is dependent on risk factors]

SUBJECT

Mortality rates remain high for the acute respiratory distress syndrome (ARDS) despite standardised treatment algorithms. Little is known about prognostic factors and exclusion criteria for advanced treatment including extracorporeal membrane oxygenation (ECMO).

METHODS

In an observational study design a cohort of 93 patients with severe ARDS admitted to a referral centre were analysed according to ventilatory and vital parameters.

RESULTS

Overall survival rate was 70% and in patients who received ECMO treatment it was 67%. In patients exhibiting relevant co-morbidity the odds ratio for fatal outcome increased to 4.7 (95% CI: 3.3-24.9), and patients with multiple organ failure had a 7.5-fold increase (95% CI: 2.3-25.2) for risk of death. Survivors demonstrated a more pronounced improvement in oxygenation ( p<0.05) and CO(2) removal ( p<0.05) than non-survivors.

CONCLUSIONS

Advanced treatment of ARDS including ECMO represents a therapeutic option if none of the currently considered contraindications are present. An improvement in gas exchange parameters, but not a defined value per se may be useful as a prognostic factor for favourable outcome.

Plasma protein C levels in patients with acute lung injury: Prognostic significance

Decreased circulating protein C is a marker of a prothrombotic state that has been associated with poor clinical outcomes in sepsis. However, protein C has not been measured in patients with acute lung injury and acute respiratory distress syndrome (ALI/ARDS). In this study, we measured circulating and intra-alveolar concentrations of protein C in 45 patients with ALI/ARDS from septic and nonseptic causes. Plasma protein C levels were lower in ALI/ARDS compared with normal controls. Lower levels of plasma protein C were associated with worse clinical outcomes, including death, fewer ventilator-free days, and more nonpulmonary organ failures, even when only patients without sepsis were analyzed. In summary, the protein C system is markedly disrupted in patients with ALI/ARDS from both septic and nonseptic causes. The protein C system may be a therapeutic target in patients with ALI/ARDS.

Roles for early response cytokines during Escherichia coli pneumonia revealed by mice with combined deficiencies of all signaling receptors for TNF and IL-1

During infection, inflammation is essential for host defense, but it can injure tissues and compromise organ function. TNF-alpha and IL-1 (alpha and beta) are early response cytokines that facilitate inflammation. To determine the roles of these cytokines with overlapping functions, we generated mice deficient in all of the three receptors mediating their effects (TNFR1, TNFR2, and IL-1RI). During Escherichia coli pneumonia, receptor deficiency decreased neutrophil recruitment and edema accumulation to half of the levels observed in wild-type mice. Thus these receptors contributed to maximal responses, but substantial inflammation progressed independently of them. Receptor deficiency compromised antibacterial efficacy for some infectious doses. Decreased ventilation during E. coli pneumonia was not affected by receptor deficiency. However, the loss of lung compliance during pneumonia was substantially attenuated by receptor deficiency. Thus during E. coli pneumonia in mice, the lack of signaling from TNF-alpha and IL-1 decreases inflammation and preserves lung compliance.

Decrease in PaCO2 with prone position is predictive of improved outcome in acute respiratory distress syndrome

OBJECTIVE

To determine whether gas exchange improvement in response to the prone position is associated with an improved outcome in acute lung injury (ALI)/acute respiratory distress syndrome (ARDS).

DESIGN

Retrospective analysis of patients in the pronation arm of a controlled randomized trial on prone positioning and patients enrolled in a previous pilot study of the prone position.

SETTING

Twenty-eight Italian and two Swiss intensive care units.

PATIENTS

We studied 225 patients meeting the criteria for ALI or ARDS.

INTERVENTIONS

Patients were in prone position for 10 days for 6 hrs/day if they met ALI/ARDS criteria when assessed each morning. Respiratory variables were recorded before and after 6 hrs of pronation with unchanged ventilatory settings.

MEASUREMENTS AND MAIN RESULTS

We measured arterial blood gas alterations to the first pronation and the 28-day mortality rate. The independent risk factors for death in the general population were the Pao2/Fio2 ratio (odds ratio, 0.992; confidence interval, 0.986-0.998), the minute ventilation/Paco2 ratio (odds ratio, 1.003; confidence interval, 1.000-1.006), and the concentration of plasma creatinine (odds ratio, 1.385; confidence interval, 1.116-1.720). Pao2 responders (defined as the patients who increased their Pao2/Fio2 by > or =20 mm Hg, 150 patients, mean increase of 100.6 +/- 61.6 mm Hg [13.4 +/- 8.2 kPa]) had an outcome similar to the nonresponders (59 patients, mean decrease -6.3 +/- 23.7 mm Hg [-0.8 +/- 3.2 kPa]; mortality rate 44% and 46%, respectively; relative risk, 1.04; confidence interval, 0.74-1.45, p =.65). The Paco2 responders (defined as patients whose Paco2 decreased by > or =1 mm Hg, 94 patients, mean decrease -6.0 +/- 6 mm Hg [-0.8 +/- 0.8 kPa]) had an improved survival when compared with nonresponders (115 patients, mean increase 6 +/- 6 mm Hg [0.8 +/- 0.8 kPa]; mortality rate 35.1% and 52.2%, respectively; relative risk, 1.48; confidence interval, 1.07-2.05, p =.01).

CONCLUSION

ALI/ARDS patients who respond to prone positioning with reduction of their Paco2 show an increased survival at 28 days. Improved efficiency of alveolar ventilation (decreased physiologic deadspace ratio) is an important marker of patients who will survive acute respiratory failure.

Preventive Effect of Tranilast on Oleic Acid-Induced Lung Injury in Guinea Pigs

Acute respiratory distress syndrome or acute lung injury (ARDS)/(ALI) involve the severe lung injury with pulmonary vascular hyper-permeability and hypoxemia induced by inflammatory reactions. Since ARDS/ALI carries high mortality, the development of new drugs against ARDS/ALI is required. We examined the effect of tranilast, an anti-allergic drug, on vascular hyper-permeability in the lungs and airways, and on hypoxemia, in oleic acid (OA)-induced acute lung injury, an animal model of ARDS/ALI. The increase in pulmonary and airway vascular permeability and the decrease in partial oxygen pressure of arterial blood induced by an intravenous injection of OA were drastically ameliorated by the oral administration of tranilast in a dose-dependent manner. This is the first report to prove that tranilast prevents pulmonary and airway vascular permeability and hypoxemia induced by OA. These results suggest that tranilast may be a candidate drug for the treatment of ARDS/ALI.

Current limitations of volumetric capnography in surfactant-depleted small lungs

OBJECTIVE

To investigate the suitability of volumetric capnography for assessing alveolar gas exchange in very small, surfactant-depleted lungs.

DESIGN

Prospective animal trial.

SETTINGS

Animal laboratory in a university setting.

SUBJECTS

Twenty-one ventilated newborn piglets (age <12 hrs; median weight, 890 g; range, 560-1435 g).

INTERVENTIONS

Bronchoalveolar lavage with instillation of 30 mL/kg normal saline. Ventilatory, circulatory, and lung mechanic variables were measured before and 0, 30, and 60 mins after bronchoalveolar lavage.

MEASUREMENTS AND MAIN RESULTS

The alveolar deadspace fraction calculated by the Bohr and the Bohr/Enghoff equations increased three-fold (p<.001) after bronchoalveolar lavage in capnograms with distinct alveolar plateau, whereas in capnograms without alveolar plateau no statistical significant difference was seen. The main problem of capnography in small and especially stiff lungs was the high number of discarded records exclusively caused by a missing alveolar plateau. Rates of discarded records of capnography were 9.5% before lavage and increased (p<.01) to 52.4%, 47.6%,42.8% after bronchoalveolar lavage (0, 30, and 60 mins). With decreasing exhalation time, the number of discarded records increased significantly. No plateau was seen in >75% of recorded files with exhalation times <200 msecs. The effect of bronchoalveolar lavage on all variables measured was quite different, with the highest impact on required ventilatory settings, calculated oxygenation variables, and compliance. The effect of bronchoalveolar lavage on arterio-alveolar CO2 difference, CO2 production, and alveolar deadspace was much lower and statistically significant only in capnograms with alveolar plateau.

CONCLUSIONS

Volumetric capnography is a useful tool to detect impaired alveolar gas exchange in surfactant-depleted small lungs. However, the method failed if there was no alveolar plateau in the volumetric capnogram especially in stiff lungs with short exhalation times.

Estimating alveolar dead space from the arterial to end-tidal CO(2) gradient: a modeling analysis

Using an original, validated, high-fidelity model of pulmonary physiology, we compared the arterial to end-tidal CO(2) gradient divided by the arterial CO(2) tension (Pa-E'CO(2)/PaCO(2)) with alveolar dead space expressed as a fraction of alveolar tidal volume, calculated in the conventional manner using Fowler's technique and the Bohr equation: (VDalv/VTalv)(Bohr-Fowler). We examined the variability of Pa-E'CO(2)/PaCO(2) and of (VDalv/VTalv)(Bohr-Fowler) in the presence of three ventilation-perfusion defects while varying CO(2) production (Vdot;CO(2)), venous admixture, and anatomical dead space fraction (VDanat). Pa-E'CO(2)/PaCO(2) was approximately 59.5% of (VDalv/VTalv)(Bohr-Fowler). During constant alveolar configuration, the factors examined (Vdot;CO(2), pulmonary shunt fraction, and VDanat) each caused variation in (VDalv/VTalv)(Bohr-Fowler) and in Pa-E'CO(2)/PaCO(2). Induced variation was slightly larger for Pa-E'CO(2)/PaCO(2) during changes in VDanat, but was similar during variation of venous admixture and Vdot;CO(2). Pa-E'CO(2)/PaCO(2) may be a useful serial measurement in the critically ill patient because all the necessary data are easily obtained and calculation is significantly simpler than for (VDalv/VTalv)(Bohr-Fowler).

IMPLICATIONS

Using an original, validated, high-fidelity model of pulmonary physiology, we have demonstrated that the arterial to end-tidal carbon dioxide pressure gradient may be used to robustly and accurately quantify alveolar dead space. After clinical validation, its use could replace that of conventionally calculated alveolar dead space fraction, particularly in the critically ill.

Validation of an Original Mathematical Model of CO2 Elimination and Dead Space Ventilation

We present an original, mathematical model of ventilation and gas-exchange. Our aim was to validate it using data from previous clinical investigations, allowing our use of it in future investigations. The first previous investigation used a low-dead space, double-lumen, tracheal tube (DLT). We matched the model's PaCO(2) and airway pressures (P(AW)) to the patient mean during use of the DLT and a single-lumen tube (SLT). The model's resulting PaCO(2), PECO(2) and P(AW) were compared with the patients' as tidal volume (VT) changed with constant minute volume. The second investigation examined dead space during anesthesia. The model's VT, respiratory rate, CO(2) production, temperature, and alveolar and anatomical dead spaces were matched to each mechanically ventilated subject. Bias and precision in predictions of PaCO(2) and PECO(2) were calculated. The model's bias in prediction of dead space reduction by the DLT was 6.9%. Bias in prediction of P(AW) was 0.1% (peak) and -5.13% (mean), of PaCO(2) was 1.2% (DLT) and 1.5% (SLT) and of PECO(2) was 1.7% (DLT) and 1.3% (SLT). Prediction of PaCO(2) and PECO(2) in the second investigation (as 95% confidence interval of bias): PaCO(2) -2.6% to 0.8% and PECO(2) -4.9% to 1.2%. This validation allows future application of our model in appropriate theoretical investigations.

IMPLICATIONS

We present an original, mathematical model of ventilation and gas exchange. We validate it against previously published clinical data to allow its use in future theoretical investigations where data may be unavailable from patients.

Chemokines Accumulate in the Lungs of Rats with Severe Pulmonary Embolism Induced by Polystyrene Microspheres

Pulmonary thromboembolism (PEm) is a serious and life threatening disease and the most common cause of acute pulmonary vascular occlusion. Even following successful treatment of PEm, many patients experience long-term disability due to diminished heart and lung function. Considerable damage to the lungs presumably occurs due to reperfusion injury following anti-occlusive treatments for PEm and the resulting chronic inflammatory state in the lung vasculature. We have used a rat model of irreversible PEm to ask whether pulmonary vascular occlusion in the absence of reperfusion is itself sufficient to induce an inflammatory response in lungs. By adjusting the severity of the vascular occlusion, we were able to generate hypertensive and nonhypertensive PEm, and then examine lung tissue for expression of CXC and C-C chemokine genes and bronchoalveolar lavage (BAL) fluid for the presence of chemokine proteins. Hypertensive and nonhypertensive PEm resulted in increased expression of both CXC and C-C chemokines genes in lung tissues. Hypertensive PEm was also associated with a 50-100-fold increase in protein content in lung BAL fluid, which included the CXC chemokines cytokine-induced neutrophil chemoattractant and macrophage-inflammatory protein 2. The presence of chemokines in BALs was reflected by a potent neutrophil chemotactic activity in in vitro chemotaxis assays. Abs to cytokine-induced neutrophil chemoattractant blocked the in vitro neutrophil chemotactic activity of BAL by 44%. Our results indicate that the ischemia and hypertension associated with PEm are sufficient to induce expression of proinflammatory mediators such as chemokines, and establish a proinflammatory environment in the ischemic lung even before reperfusion.

Acute lung injury and the acute respiratory distress syndrome

Although ALI/ARDS mortality rates have improved over the last several decades, they remain high, particularly in the geriatric patient population. Although considerable progress has been made in understanding the pathogenesis of the disease, a large number of promising treatments have proven unsuccessful. One exception has been in the area of ventilator management, where a strategy of protective ventilation with low tidal volumes has demonstrated a significant mortality benefit. Basic research continues to help advance our understanding of this complex syndrome and identify interesting new directions of investigation. The results of several large, randomized trials of new ventilatory and pharmacologic strategies currently underway may help identify successful methods of treating this important disease.

Protein C and thrombomodulin in human acute lung injury

Decreased circulating protein C and increased circulating thrombomodulin are markers of the prothrombotic, antifibrinolytic state associated with poor outcomes in sepsis but have not been measured in patients with ALI (acute lung injury)/ARDS (acute respiratory distress syndrome). We measured circulating and intra-alveolar protein C and thrombomodulin in 45 patients with ALI/ARDS from septic and nonseptic causes and correlated the levels with clinical outcomes. Plasma protein C levels were lower in ALI/ARDS compared with normal. Lower levels of protein C were associated with worse clinical outcomes, including death, fewer ventilator-free days, and more nonpulmonary organ failures, even when only patients without sepsis were analyzed. Levels of thrombomodulin in pulmonary edema fluid from ALI/ARDS patients were >10-fold higher than normal plasma and 2-fold higher than ALI/ARDS plasma. Higher edema fluid thrombomodulin levels were associated with worse clinical outcomes. The higher levels in edema fluid compared with plasma suggest local release of soluble thrombomodulin in the lung, possibly from a lung epithelial source. To determine whether lung epithelial cells can release thrombomodulin, A549 cells and primary isolates of human alveolar type II cells were exposed to H2O2 or inflammatory cytokines. Both epithelial cell types released thrombomodulin into the media. In summary, the protein C system is markedly disrupted in patients with ALI/ARDS from both septic and nonseptic causes. The protein C system may be a potential therapeutic target in patients with ALI/ARDS.

Effects of acute hypoxia and lipopolysaccharide on nitric oxide synthase-2 expression in acute lung injury

The potential role of nitric oxide synthase-2 (NOS2) in acute lung injury (ALI) has gained increasing attention. This study evaluates the effects of hypoxia, an important feature of ALI, on NOS2 expression in a rat model of ALI caused by exposure to hypoxia and LPS. Exposure to hypoxia alone had no effect on the expression of NOS2 in rat lungs. LPS treatment resulted in a significant increase in NOS2 in the lungs, which was further enhanced by concomitant exposure to hypoxia. Immunohistochemical analysis and in situ hybridization showed no changes in the expression of NOS2 in lung resident cells under any conditions. The increase in NOS2 levels is mainly due to the influx of NOS2-expressing inflammatory cells. By morphologic analysis, these inflammatory cells were identified as neutrophils, lymphocytes, and monocytes. In vitro experiments of lung epithelial and endothelial cell lines showed no detectable expression of NOS2 with any of the treatments. In a macrophage cell line, LPS-induced NOS2 expression was not affected by the concomitant exposure to hypoxia. In conclusion, LPS increases NOS2 expression in rat lungs through the recruitment of NOS2-producing leukocytes. Simultaneous exposure to LPS and hypoxia results in a greater influx of inflammatory cells that further enhances NOS2 expression.

Dead space ventilation in critically ill children with lung injury

STUDY OBJECTIVE

In children with acute lung injury, there is an increase in minute ventilation (E) and inefficient gas exchange due to a high level of physiologic dead space ventilation (VD/VT). Mechanical ventilation with positive end-expiratory pressure, when used in critically ill patients to correct hypoxemia, may contribute to increased VD/VT. The purpose of this study was to measure metabolic parameters and VD/VT in critically ill children.

DESIGN

A cross-sectional study.

SETTING

Pediatric ICU of a university hospital.

PATIENTS

A total of 45 mechanically intubated children (mean age, 5.5 years).

INTERVENTIONS

Indirect calorimetry was used to measure metabolic parameters. VD/VT parameters were calculated using the modified Bohr-Enghoff equation. ARDS was defined based on criteria by The American-European Consensus Conference.

MEASUREMENTS AND RESULTS

The group mean (+/- SD) ventilatory equivalent for oxygen (VeqO(2)) and ventilatory equivalent for carbon dioxide (VeqCO(2)) were 2.9 +/- 1 and 3.3 +/- 1 L per 100 mL, respectively. The group mean VD/VT was 0.48 +/- 0.2. When compared to non-ARDS patients (33 patients), the patients with ARDS (12 patients) had a significantly higher VeqO(2) (3.3 +/- 1 vs 2.8 +/- 1 L per 100 mL, respectively; p < 0.05), a significantly higher VeqCO(2) (3.7 +/- 1 L/100 vs 3.1 +/- 1 L per 100 mL, respectively; p < 0.05), and a significantly higher VD/VT (0.62 +/- 0.14 vs 0.43 +/- 0.15, respectively; p < 0.0005).

CONCLUSIONS

Critically ill children with ARDS have increased VD/VT. Increased VD/VT was the main cause of the excess of E demand in these patients. Increased metabolic demands, as shown by the VeqO(2), VeqCO(2), and ventilatory support, are the major determinants of E requirements in children with ARDS.

Noncardiogenic pulmonary edema

Pulmonary edema is differentiated into two categories--cardiogenic and noncardiogenic. Noncardiogenic pulmonary edema is due to changes in permeability of the pulmonary capillary membrane as a result of either a direct or an indirect pathologic process. It is a spectrum of illness ranging from the less severe form of ALI to the severe ARDS. The mainstay of treatment is mechanical ventilation with maximization of ventilation and oxygenation through the judicious use of PEEP. Newer ventilation techniques, such as high-frequency oscillatory ventilation and partial fluid ventilation, are promising but are in the early stages of clinical testing. Mortality rates remain high despite increasing intensive care unit care.

Chronic alcohol abuse, acute respiratory distress syndrome, and multiple organ dysfunction

OBJECTIVE

To review the effects of chronic alcohol abuse on the incidence, severity, and pathogenesis of acute respiratory distress syndrome (ARDS) and multiple organ dysfunction.

DATA SOURCES

A summary of published medical literature from MEDLINE search files and other reviews published concerning chronic alcohol abuse and critical illness.

DATA SUMMARY

A history of chronic alcohol abuse is associated with an increased incidence and severity of ARDS in critically ill patients. In two separate epidemiologic studies, involving 571 intensive care patients, chronic alcohol abuse was a significant comorbid variable that increased the incidence of ARDS by nearly three-fold and was associated with more severe nonpulmonary organ dysfunction. In addition, nearly 50% of all ARDS patients had a significant history of chronic alcohol abuse, making the association between chronic alcohol abuse and ARDS a common scenario in the intensive care unit. By using animal models of chronic ethanol ingestion, researchers have identified alcohol-mediated alterations in epithelial and endothelial cell function, surfactant synthesis and secretion, alveolar-capillary barrier function, and lung matrix content and composition. More importantly, similar changes have been reported in humans with a history of chronic alcohol abuse. Individuals with a history of chronic alcohol abuse have decreased concentrations of glutathione in the epithelial lining fluid of the lung, which do not significantly increase after 1 wk of abstinence from alcohol. The total protein concentration in the epithelial lining fluid also is increased in these individuals with a history of chronic alcohol abuse compared with healthy controls, suggesting alterations in alveolar-capillary barrier function.

CONCLUSIONS

Chronic alcohol abuse is associated with an increased incidence of ARDS and the severity of multiple organ dysfunction. This research has implications in understanding the diagnosis of, and prognosis for, critically ill patients who are at risk of developing ARDS. It also may lead to the development of novel therapies for those patients at greatest risk of acute lung injury as a consequence of chronic alcohol abuse.

Future research directions in acute lung injury: summary of a National Heart, Lung, and Blood Institute working group

Acute lung injury (ALI) and its more severe form, the acute respiratory distress syndrome (ARDS), are syndromes of acute respiratory failure that result from acute pulmonary edema and inflammation. The development of ALI/ARDS is associated with several clinical disorders including direct pulmonary injury from pneumonia and aspiration as well as indirect pulmonary injury from trauma, sepsis, and other disorders such as acute pancreatitis and drug overdose. Although mortality from ALI/ARDS has decreased in the last decade, it remains high. Despite two major advances in treatment, low VT ventilation for ALI/ARDS and activated protein C for severe sepsis (the leading cause of ALI/ARDS), additional research is needed to develop specific treatments and improve understanding of the pathogenesis of these syndromes. The NHLBI convened a working group to develop specific recommendations for future ALI/ARDS research. Improved understanding of disease heterogeneity through use of evolving biologic, genomic, and genetic approaches should provide major new insights into pathogenesis of ALI. Cellular and molecular methods combined with animal and clinical studies should lead to further progress in the detection and treatment of this complex disease.

Epidemiology and outcome of acute respiratory failure in intensive care unit patients

OBJECTIVES

To summarize the prevalence of various forms of acute respiratory failure in acutely ill patients and review the major factors involved in the outcome of these patients.

DATA SOURCES AND SELECTION

MEDLINE search for published studies reporting the prevalence or outcome for patients with acute respiratory failure and cited reference studies and abstracts from a recent international meeting in the intensive care medicine field.

DATA SYNTHESIS AND EXTRACTION

From the selected articles, information was obtained regarding the prevalence of acute respiratory failure, including acute respiratory distress syndrome and acute lung injury as defined by the North American-European Consensus Conference, the outcome, and the factors influencing mortality rates in this population of patients.

CONCLUSIONS

The prevalence of acute respiratory failure varies according to the definition used and the population studied. Nonsurvivors of acute respiratory distress syndrome die predominantly of respiratory failure in <20% of cases. The relatively high mortality rates of acute lung injury/acute respiratory distress syndrome are primarily related to the underlying disease, the severity of the acute illness, and the degree of organ dysfunction.

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.

Lung recruitment maneuvers in acute respiratory distress syndrome and facilitating resolution

OBJECTIVES

To summarize the possible ways that acute respiratory distress syndrome (ARDS) lungs can be recruited and to present the experimental and clinical results of these maneuvers, along with the possible effects on patient outcome.

DATA SOURCES

Selected published medical literature from 1972 to 2002 and personal observations.

DATA SUMMARY

In the experimental setting, repeated derecruitments accentuate lung injury during mechanical ventilation, whereas open lung concept strategies can attenuate lung injury. In the clinical setting, recruitment maneuvers that use a continuous positive airway pressure of 40 cm H(2)O for 40 secs improve oxygenation in patients with early ARDS who do not have an impairment in the chest wall. High intermittent positive end-expiratory pressure (PEEP), intermittent sighs, or high-pressure controlled ventilation improves short-term oxygenation in ARDS patients. Both conventional and electrical impedance thoracic tomography studies indicate that high airway pressures increase the lung volume and recruitment percentage of lung tissue in ARDS patients. To sustain the recruited ARDS lungs, it is important to maintain adequate PEEP levels. High PEEP/low tidal volume ventilation was seen to reduce inflammatory mediators in both bronchoalveolar lavage and plasma, compared with low PEEP/high tidal volume ventilation, after 36 hrs of mechanical ventilation in ARDS patients. Recruitment maneuvers that used continuous positive airway pressure levels of 35-40 cm H(2)O for 40 secs, with PEEP set at 2 cm H(2)O above the Pflex (the lowest inflection point on the pressure-volume curve), and tidal volume <6 mL/kg were associated with a 28-day intensive care unit survival rate of 62%. This contrasted with a survival rate of only 29% with conventional ventilation (defined as the lowest PEEP for acceptable oxygenation without hemodynamic impairment with a tidal volume of 12 mL/kg), without recruitment maneuvers (number needed to treat = 3; p <.001).

CONCLUSIONS

High airway pressures can open collapsed ARDS lungs and partially open edematous ARDS lungs. High PEEP levels and low tidal volume ventilation decrease bronchoalveolar and plasma inflammatory mediators and improve survival compared with low PEEP/high tidal volume ventilation. In the near future, thoracic computed tomography associated with high-performance monitoring of regional ventilation (electrical impedance tomography) may be used at the bedside to determine the optimal mechanical ventilation of ARDS patients.

Monitorización de la función respiratoria en el niño con ventilación mecánica (II): complianza, resistencia, hiperinsuflación dinámica, espacio muerto y trabajo respiratorio

Several parameters can be used to study respiratory mechanics in children on mechanical ventilation. Compliance is a measure of the distensibility of the respiratory system. In mechanical ventilation two measures of compliance can be used. Static compliance (pulmonary distensibility) can be measured in volume modes by the application of an inspiratory pause with the respiratory system at rest (sedated patients without inspiratory effort). Dynamic compliance does not require an inspiratory pause and the respiratory system need not be at rest. Compliance can be calculated numerically or expressed graphically in the volume-pressure curve. Many respirators can calculate inspiratory and expiratory airway resistance (including endotracheal tube resistance). Several measures can be used to detect dynamic hyperinflation (intrinsic PEEP, auto-PEEP, trapped air volume) after application of an expiratory pause. The latest respirators can perform these measurements almost automatically. Work of breathing and respiratory effort can also be analyzed by measuring several parameters (pressure-time product, imposed work of breathing, P 0.1, maximum inspiratory pressure). However, these measures have not yet been standardized in children.

Effect of position, nitric oxide, and almitrine on lung perfusion in a porcine model of acute lung injury

In a porcine model of oleic acid-induced lung injury, the effects of inhaled nitric oxide (iNO) and intravenous almitrine bismesylate (ivALM), which enhances the hypoxic pulmonary vasoconstriction on the distribution of regional pulmonary blood flow (PBF), were assessed. After injection of 0.12 ml/kg oleic acid, 20 anesthetized and mechanically ventilated piglets [weight of 25 +/- 2.6 (SD) kg] were randomly divided into four groups: supine position, prone position, and 10 ppm iNO for 40 min followed by 4 microg x kg(-1) x min(-1) ivALM for 40 min in supine position and in prone position. PBF was measured with positron emission tomography and H(2)15O. The redistribution of PBF was studied on a pixel-by-pixel basis. Positron emission tomography scans were performed before and then 120, 160, and 200 min after injury. With prone position alone, although PBF remained prevalent in the dorsal regions it was significantly redistributed toward the ventral regions (P < 0.001). A ventral redistribution of PBF was also obtained with iNO regardless of the position (P = 0.043). Adjunction of ivALM had no further effect on PBF redistribution. PP and iNO have an additive effect on ventral redistribution of PBF.