Patient-ventilator interaction and sleep in mechanically ventilated patients: Pressure support versus proportional assist ventilation*

OBJECTIVES

To understand the role of patient-ventilator asynchrony in the etiology of sleep disruption and determine whether optimizing patient-ventilator interactions by using proportional assist ventilation improves sleep.

DESIGN

Randomized crossover clinical trial.

SETTING

A tertiary university medical-surgical intensive care unit.

PATIENTS

Thirteen patients during weaning from mechanical ventilation.

INTERVENTIONS

Patients were randomized to receive pressure support ventilation or proportional assist ventilation on the first night and then crossed over to the alternative mode for the second night. Polysomnography and measurement of light, noise, esophageal pressure, airway pressure, and flow were performed from 10 pm to 8 am. Ventilator settings (pressure level during pressure support ventilation and resistive and elastic proportionality factors during proportional assist ventilation) were set to obtain a 50% reduction of the inspiratory work (pressure time product per minute) performed during a spontaneous breathing trial.

MEASUREMENTS AND MAIN RESULTS

Arousals per hour of sleep time during pressure support ventilation were 16 (range 2-74) and 9 (range 1-41) during proportional assist ventilation (p = .02). Overall sleep quality was significantly improved on proportional assist ventilation (p < .05) due to the combined effect of fewer arousals per hour, fewer awakenings per hour (3.5 [0-24] vs. 5.5 [1-24]), and greater rapid eye movement (9% [0-31] vs. 4% [0-23]), and slow wave (3% [0-16] vs. 1% [0-10]) sleep. Tidal volume and minute ventilation were lower on proportional assist ventilation, allowing for a greater increase in Paco2 during the night. Patient-ventilator asynchronies per hour were lower with proportional assist ventilation than with pressure support ventilation (24 +/- 15 vs. 53 +/- 59; p = .02) and correlated with the number of arousals per hour (R = .65, p = .0001).

CONCLUSIONS

Patient ventilator discordance causes sleep disruption. Proportional assist ventilation seems more efficacious than pressure support ventilation in matching ventilatory requirements with ventilator assistance, therefore resulting in fewer patient-ventilator asynchronies and better quality of sleep.

Regional anesthesia in trauma patients

Medical and surgical treatment of the trauma patient has evolved in the last decade. Treatment of pain from multiple fractures or injured organs and surgical anesthesia with regional anesthesia techniques have been used to reduce post-traumatic stress disorder and reduce the adverse effects of general anesthesia. Neuraxial blocks and peripheral nerve block techniques should be practiced by trained emergency and operatory room staff. This article reviews recent publications related to the role of regional anesthesia in trauma patients in the prehospital, emergency, and operatory room settings. It also describes indications, limitations, and practical aspects of regional anesthesia.

Alveolar Recruitment in Pulmonary and Extrapulmonary Acute Respiratory Distress Syndrome

Background

Alveolar recruitment in response to positive end-expiratory pressure (PEEP) may differ between pulmonary and extrapulmonary acute respiratory distress syndrome (ARDS), and alveolar recruitment values may differ when measured by pressure-volume curve compared with static compliance.

Methods

The authors compared PEEP-induced alveolar recruitment in 71 consecutive patients identified in a database. Patients were classified as having pulmonary, extrapulmonary, or mixed/uncertain ARDS. Pressure-volume curves with and without PEEP were available for all patients, and pressure-volume curves with two PEEP levels were available for 44 patients. Static compliance was calculated as tidal volume divided by pressure change for tidal volumes of 400 and 700 ml. Recruited volume was measured at an elastic pressure of 15 cm H2O.

Results

Volume recruited by PEEP (10 +/- 3 cm H2O) was 223 +/- 111 ml in the pulmonary ARDS group (29 patients), 206 +/- 164 ml in the extrapulmonary group (16 patients), and 242 +/- 176 ml in the mixed/uncertain group (26 patients) (P = 0.75). At high PEEP (14 +/- 2 cmH2O, 44 patients), recruited volumes were also similar (P = 0.60). With static compliance, recruitment was markedly underestimated and was dependent on tidal volume (226 +/- 148 ml using pressure-volume curve, 95 +/- 185 ml for a tidal volume of 400 ml, and 23 +/- 169 ml for 700 ml; P &lt; 0.001).

Conclusion

In a large sample of patients, classification of ARDS was uncertain in more than one third of patients, and alveolar recruitment was similar in pulmonary and extrapulmonary ARDS. PEEP levels should not be determined based on cause of ARDS.

Tidal hyperinflation during low tidal volume ventilation in acute respiratory distress syndrome

RATIONALE

Tidal volume and plateau pressure limitation decreases mortality in acute respiratory distress syndrome. Computed tomography demonstrated a small, normally aerated compartment on the top of poorly aerated and nonaerated compartments that may be hyperinflated by tidal inflation.

OBJECTIVES

We hypothesized that despite tidal volume and plateau pressure limitation, patients with a larger nonaerated compartment are exposed to tidal hyperinflation of the normally aerated compartment.

MEASUREMENTS AND MAIN RESULTS

Pulmonary computed tomography at end-expiration and end-inspiration was obtained in 30 patients ventilated with a low tidal volume (6 ml/kg predicted body weight). Cluster analysis identified 20 patients in whom tidal inflation occurred largely in the normally aerated compartment (69.9 +/- 6.9%; "more protected"), and 10 patients in whom tidal inflation occurred largely within the hyperinflated compartments (63.0 +/- 12.7%; "less protected"). The nonaerated compartment was smaller and the normally aerated compartment was larger in the more protected patients than in the less protected patients (p = 0.01). Pulmonary cytokines were lower in the more protected patients than in the less protected patients (p < 0.05). Ventilator-free days were 7 +/- 8 and 1 +/- 2 d in the more protected and less protected patients, respectively (p = 0.01). Plateau pressure ranged between 25 and 26 cm H(2)O in the more protected patients and between 28 and 30 cm H(2)O in the less protected patients (p = 0.006).

CONCLUSIONS

Limiting tidal volume to 6 ml/kg predicted body weight and plateau pressure to 30 cm H(2)O may not be sufficient in patients characterized by a larger nonaerated compartment.

Ischemia and Reperfusion Increases Susceptibility to Ventilator-induced Lung Injury in Rats

OBJECTIVES

Hemorrhagic shock followed by resuscitation (HSR) commonly triggers an inflammatory response that leads to acute respiratory distress syndrome.

HYPOTHESIS

HSR exacerbates mechanical stress-induced lung injury by rendering the lung more susceptible to ventilator-induced lung injury.

METHODS

Rats were subjected to HSR, and were randomized into an HSR + high tidal volume and zero positive end-expiratory pressure (PEEP) or a HSR + low tidal volume with 5 cm H(2)O PEEP. A sham-operated rat + high tidal volume and zero PEEP served as a control.

RESULTS

HSR increased susceptibility to ventilator-induced lung injury as evidenced by an increase in lung elastance and the wet/dry ratio and a reduction in Pa(O(2)) as compared with the other groups. The lung injury observed in the HSR + high tidal volume group was associated with a higher level of interleukin 6 in the lung and blood, increased epithelial cell apoptosis in the kidney and small intestine villi, and a tendency toward high levels of alanine aminotransferase, aspartate aminotransferase, lactate dehydrogenase, and creatinine in plasma.

CONCLUSIONS

HSR priming renders the lung and kidney more susceptible to mechanical ventilation-induced organ injury.

Lung recruitment in patients with the acute respiratory distress syndrome

BACKGROUND

In the acute respiratory distress syndrome (ARDS), positive end-expiratory pressure (PEEP) may decrease ventilator-induced lung injury by keeping lung regions open that otherwise would be collapsed. Since the effects of PEEP probably depend on the recruitability of lung tissue, we conducted a study to examine the relationship between the percentage of potentially recruitable lung, as indicated by computed tomography (CT), and the clinical and physiological effects of PEEP.

METHODS

Sixty-eight patients with acute lung injury or ARDS underwent whole-lung CT during breath-holding sessions at airway pressures of 5, 15, and 45 cm of water. The percentage of potentially recruitable lung was defined as the proportion of lung tissue in which aeration was restored at airway pressures between 5 and 45 cm of water.

RESULTS

The percentage of potentially recruitable lung varied widely in the population, accounting for a mean (+/-SD) of 13+/-11 percent of the lung weight, and was highly correlated with the percentage of lung tissue in which aeration was maintained after the application of PEEP (r2=0.72, P<0.001). On average, 24 percent of the lung could not be recruited. Patients with a higher percentage of potentially recruitable lung (greater than the median value of 9 percent) had greater total lung weights (P<0.001), poorer oxygenation (defined as a ratio of partial pressure of arterial oxygen to fraction of inspired oxygen) (P<0.001) and respiratory-system compliance (P=0.002), higher levels of dead space (P=0.002), and higher rates of death (P=0.02) than patients with a lower percentage of potentially recruitable lung. The combined physiological variables predicted, with a sensitivity of 71 percent and a specificity of 59 percent, whether a patient's proportion of potentially recruitable lung was higher or lower than the median.

CONCLUSIONS

In ARDS, the percentage of potentially recruitable lung is extremely variable and is strongly associated with the response to PEEP.

Does dopamine administration in shock influence outcome? Results of the Sepsis Occurrence in Acutely Ill Patients (SOAP) Study

OBJECTIVE

The optimal adrenergic support in shock is controversial. We investigated whether dopamine administration influences the outcome from shock.

DESIGN

Cohort, multiple-center, observational study.

SETTING

One hundred and ninety-eight European intensive care units.

PATIENTS

All adult patients admitted to a participating intensive care unit between May 1 and May 15, 2002.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Patients were followed up until death, until hospital discharge, or for 60 days. Shock was defined as hemodynamic compromise necessitating the administration of vasopressor catecholamines. Of 3,147 patients, 1,058 (33.6%) had shock at any time; 462 (14.7%) had septic shock. The intensive care unit mortality rate for shock was 38.3% and 47.4% for septic shock. Of patients in shock, 375 (35.4%) received dopamine (dopamine group) and 683 (64.6%) never received dopamine. Age, gender, Simplified Acute Physiology Score II, and Sequential Organ Failure Assessment score were comparable between the two groups. The dopamine group had higher intensive care unit (42.9% vs. 35.7%, p=.02) and hospital (49.9% vs. 41.7%, p=.01) mortality rates. A Kaplan-Meier survival curve showed diminished 30 day-survival in the dopamine group (log rank=4.6, p=.032). In a multivariate analysis with intensive care unit outcome as the dependent factor, age, cancer, medical admissions, higher mean Sequential Organ Failure Assessment score, higher mean fluid balance, and dopamine administration were independent risk factors for intensive care unit mortality in patients with shock.

CONCLUSIONS

This observational study suggests that dopamine administration may be associated with increased mortality rates in shock. There is a need for a prospective study comparing dopamine with other catecholamines in the management of circulatory shock.

An evaluation of systemic inflammatory response syndrome signs in the Sepsis Occurrence In Acutely Ill Patients (SOAP) study

OBJECTIVE

To define the frequency and prognostic implications of SIRS criteria in critically ill patients hospitalized in European ICUs.

DESIGN AND SETTING

Cohort, multicentre, observational study of 198 ICUs in 24 European countries.

PATIENTS AND INTERVENTIONS

All 3,147 new adult admissions to participating ICUs between 1 and 15 May 2002 were included. Data were collected prospectively, with common SIRS criteria.

RESULTS

During the ICU stay 93% of patients had at least two SIRS criteria [respiratory rate (82%), heart rate (80%)]. The frequency of having three or four SIRS criteria vs. two was higher in infected than non-infected patients (p < 0.01). In non-infected patients having more than two SIRS criteria was associated with a higher risk of subsequent development of severe sepsis (odds ratio 2.6, p < 0.01) and septic shock (odds ratio 3.7, p < 0.01). Organ system failure and mortality increased as the number of SIRS criteria increased.

CONCLUSIONS

Although common in the ICU, SIRS has prognostic importance in predicting infections, severity of disease, organ failure and outcome.

Sepsis in European intensive care units: results of the SOAP study

OBJECTIVE

To better define the incidence of sepsis and the characteristics of critically ill patients in European intensive care units.

DESIGN

Cohort, multiple-center, observational study.

SETTING

One hundred and ninety-eight intensive care units in 24 European countries.

PATIENTS

All new adult admissions to a participating intensive care unit between May 1 and 15, 2002.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Demographic data, comorbid diseases, and clinical and laboratory data were collected prospectively. Patients were followed up until death, until hospital discharge, or for 60 days. Of 3,147 adult patients, with a median age of 64 yrs, 1,177 (37.4%) had sepsis; 777 (24.7%) of these patients had sepsis on admission. In patients with sepsis, the lung was the most common site of infection (68%), followed by the abdomen (22%). Cultures were positive in 60% of the patients with sepsis. The most common organisms were Staphylococcus aureus (30%, including 14% methicillin-resistant), Pseudomonas species (14%), and Escherichia coli (13%). Pseudomonas species was the only microorganism independently associated with increased mortality rates. Patients with sepsis had more severe organ dysfunction, longer intensive care unit and hospital lengths of stay, and higher mortality rate than patients without sepsis. In patients with sepsis, age, positive fluid balance, septic shock, cancer, and medical admission were the important prognostic variables for intensive care unit mortality. There was considerable variation between countries, with a strong correlation between the frequency of sepsis and the intensive care unit mortality rates in each of these countries.

CONCLUSIONS

This large pan-European study documents the high frequency of sepsis in critically ill patients and shows a close relationship between the proportion of patients with sepsis and the intensive care unit mortality in the various countries. In addition to age, a positive fluid balance was among the strongest prognostic factors for death. Patients with intensive care unit acquired sepsis have a worse outcome despite similar severity scores on intensive care unit admission.

Lack of phosphoinositide 3-kinase-gamma attenuates ventilator-induced lung injury

OBJECTIVE

G protein-coupled receptors may up-regulate the inflammatory response elicited by ventilator-induced lung injury but also regulate cell survival via protein kinase B (Akt) and extracellular signal regulated kinases 1/2 (ERK1/2). The G protein-sensitive phosphoinositide-3-kinase gamma (PI3Kgamma) regulates several cellular functions including inflammation and cell survival. We explored the role of PI3Kgamma on ventilator-induced lung injury.

DESIGN

Prospective, randomized, experimental study.

SETTING

University animal research laboratory.

SUBJECTS

Wild-type (PI3Kgamma), knock-out (PI3Kgamma ), and kinase-dead (PI3Kgamma) mice.

INTERVENTIONS

Three ventilatory strategies (no stretch, low stretch, high stretch) were studied in an isolated, nonperfused model of acute lung injury (lung lavage) in PI3Kgamma, PI3Kgamma, and PI3Kgamma mice.

MEASUREMENTS AND MAIN RESULTS

Reduction in lung compliance, hyaline membrane formation, and epithelial detachment with high stretch were more pronounced in PI3Kgamma than in PI3Kgamma and PI3Kgamma (p < .01). Inflammatory cytokines and IkBalpha phosphorylation with high stretch did not differ among PI3Kgamma, PI3Kgamma, and PI3Kgamma. Apoptotic index (terminal deoxynucleotidyl transferase-mediated biotin-dUTP nick-end labeling) and caspase-3 (immunohistochemistry) with high stretch were larger (p < .01) in PI3Kgamma and PI3Kgamma than in PI3Kgamma. Electron microscopy showed that high stretch caused apoptotic changes in alveolar cells of PI3Kgamma mice whereas PI3Kgamma mice showed necrosis. Phosphorylation of Akt and ERK1/2 with high stretch was more pronounced in PI3Kgamma than in PI3Kgamma and PI3Kgamma (p < .01).

CONCLUSIONS

Silencing PI3Kgamma seems to attenuate functional and morphological consequences of ventilator-induced lung injury independently of inhibitory effects on cytokines release but through the enhancement of pulmonary apoptosis.

High tidal volume and positive fluid balance are associated with worse outcome in acute lung injury

STUDY OBJECTIVES

Recent data have suggested that ventilatory strategy could influence outcomes from acute lung injury (ALI) and ARDS. We tested the hypothesis that infection/sepsis and use of higher tidal volumes than those applied in the ARDS Network (ARDSnet) study (> 7.4 mL/kg of predicted body weight) would worsen outcome in patients with ALI/ARDS.

DESIGN

International cohort, observational study.

SETTING

One hundred ninety-eight European ICUs participating in the Sepsis Occurrence in Acutely Ill Patients study.

PATIENTS OR PARTICIPANTS

All 3,147 adult patients admitted to one of the participating ICUs between May 1, 2002, and May 15, 2002.

INTERVENTIONS

None.

MEASUREMENTS AND RESULTS

Patients were followed up until death, hospital discharge, or for 60 days. Of the 3,147 patients, 393 patients (12.5%) had ALI/ARDS. ICU and hospital mortality was higher in patients with ALI/ARDS than those without ALI/ARDS (38.9% vs 15.6% and 45.5% vs 21.0%, respectively; p < 0.001). A multivariable logistic regression analysis with ICU outcome as the dependent factor showed that the independent risks for mortality were as follows: presence of cancer, use of tidal volumes higher than those used by the ARDSnet study, degree of multiorgan dysfunction, and higher mean fluid balance. Sepsis, septic shock, and oxygenation at the onset of ALI/ARDS were not independently associated with higher mortality rates.

CONCLUSIONS

In addition to comorbidities and organ dysfunction, high tidal volumes and positive fluid balance are associated with a worse outcome from ALI/ARDS.

Is albumin administration in the acutely ill associated with increased mortality? Results of the SOAP study

Introduction

Albumin administration in the critically ill has been the subject of some controversy. We investigated the use of albumin solutions in European intensive care units (ICUs) and its relationship to outcome.

Methods

In a cohort, multicenter, observational study, all patients admitted to one of the participating ICUs between 1 May and 15 May 2002 were followed up until death, hospital discharge, or for 60 days. Patients were classified according to whether or not they received albumin at any time during their ICU stay.

Results

Of 3,147 admitted patients, 354 (11.2%) received albumin and 2,793 (88.8%) did not. Patients who received albumin were more likely to have cancer or liver cirrhosis, to be surgical admissions, and to have sepsis. They had a longer length of ICU stay and a higher mortality rate, but were also more severely ill, as manifested by higher simplified acute physiology score (SAPS) II and sequential organ failure assessment (SOFA) scores than the other patients. A Cox proportional hazard model indicated that albumin administration was significantly associated with decreased 30-day survival. Moreover, in 339 pairs matched according to a propensity score, ICU and hospital mortality rates were higher in the patients who had received albumin than in those who had not (34.8 versus 20.9% and 41.3 versus 27.7%, respectively, both p < 0.001).

Conclusion

Albumin administration was associated with decreased survival in this population of acutely ill patients. Further prospective randomized controlled trials are needed to examine the effects of albumin administration in sub-groups of acutely ill patients.

Mechanical ventilation affects inflammatory mediators in patients undergoing cardiopulmonary bypass for cardiac surgery: a randomized clinical trial

OBJECTIVES

Respiratory support for patients recovering from cardiopulmonary bypass and cardiac surgery uses large tidal volumes and a minimal level of positive end-expiratory pressure. Recent data indicate that these ventilator settings might cause pulmonary and systemic inflammation in patients with acute lung injury. We examined the hypothesis that high tidal volumes and low levels of positive end-expiratory pressure might worsen the inflammatory response associated to cardiopulmonary bypass.

METHODS

Forty patients undergoing elective coronary artery bypass were randomized to be ventilated after cardiopulmonary bypass disconnection with high tidal volume/low positive end-expiratory pressure (10-12 mL/kg and 2-3 cm H2O, respectively) or low tidal volume/high positive end-expiratory pressure (8 mL/kg and 10 cm H2O, respectively). Interleukin 6 and interleukin 8 levels were measured in the bronchoalveolar lavage fluid and plasma. Samples were taken before sternotomy (time 0), immediately after cardiopulmonary bypass separation (time 1), and after 6 hours of mechanical ventilation (time 2).

RESULTS

Interleukin 6 and interleukin 8 levels in the bronchoalveolar lavage fluid and plasma significantly increased at time 1 in both groups but further increased at time 2 only in patients ventilated with high tidal volume/low positive end-expiratory pressure. Interleukin 6 and interleukin 8 levels in the bronchoalveolar lavage fluid and in the plasma at time 2 were higher with high tidal volume/low positive end-expiratory pressure than with low tidal volume/high positive end-expiratory pressure.

CONCLUSION

Mechanical ventilation might be a cofactor able to influence the inflammatory response after cardiac surgery.

Effectiveness of mask and helmet interfaces to deliver noninvasive ventilation in a human model of resistive breathing

The helmet, a transparent latex-free polyvinyl chloride cylinder linked by a metallic ring to a soft collar that seals the helmet around the neck, has been recently proposed as an effective alternative to conventional face mask to deliver pressure support ventilation (PSV) during noninvasive ventilation in patients with acute respiratory failure. We tested the hypothesis that mechanical characteristics of the helmet (large internal volume and high compliance) might impair patient-ventilator interactions compared with standard face mask. Breathing pattern, CO2 clearance, indexes of inspiratory muscle effort and patient-ventilator asynchrony, and dyspnea were measured at different levels of PSV delivered by face mask and helmet in six healthy volunteers before (load-off) and after (load-on) application of a linear resistor. During load-off, no differences in breathing pattern and inspiratory muscle effort were found. During load-on, the use of helmet to deliver pressure support increased inspiratory muscle effort and patient-ventilator asynchrony, worsened CO2 clearance, and increased dyspnea compared with standard face mask. Autocycled breaths accounted for 12 and 25% of the total minute ventilation and for 10 and 23% of the total inspiratory muscle effort during mask and helmet PSV, respectively. We conclude that PSV delivered by helmet interface is less effective in unloading inspiratory muscles compared with PSV delivered by standard face mask. Other ventilatory assist modes should be tested to exploit to the most the potential benefits offered by the helmet.

Proportional assist ventilation

Acute respiratory failure due to derangement of mechanical properties of respiratory system and/or to neuromuscular weakness has been defined as "poor neuroventilatory coupling.'' Proportional assist ventilation (PAV) is an innovative mode of partial ventilatory support designed to "amplify'' patient inspiratory effort and thus to restore a normal "neuroventilatory coupling.'' Potential advantages of PAV in terms of more physiologic patient-ventilator interactions have been evaluated by several studies. We review the literature to elucidate the state of the art of the clinical implementation of PAV.

Independent high-frequency oscillatory ventilation in the management of asymmetric acute lung injury

We present a case of independent lung ventilation in an adult with asymmetric acute lung injury. We applied a conventional protective ventilatory strategy to the more homogeneously infiltrated lung and high-frequency oscillatory ventilation to the almost totally collapsed lung, because a conventional protective strategy exposed this lung to plateau pressure more than 30 cm H2O, whereas high-frequency oscillatory ventilation provided sufficient gas exchange at safer pressure levels. Analysis of a lung computed tomography scan was used to evaluate the efficacy of the ventilatory strategy.

Patient–Ventilator Interaction During the Triggering Phase

Partial patient-controlled mechanical support mode ventilators provide positive pressure assistance whenever a patient's inspiratory effort decreases pressure or flow in the ventilator circuit below the sensitivity set by clinicians; these modes minimize disuse atrophy of the respiratory muscles, can facilitate the weaning process, and usually require lower ventilator pressures. The capability of restoring gas exchange, unloading respiratory muscles, and relieving the patient's dyspnea with partial patient-controlled mechanical support modes depends on matching between the ventilator setting and the patient's ventilatory demand (ie, patient-ventilator interactions).

Acute respiratory distress syndrome: update on the latest developments in basic and clinical research

PURPOSE OF REVIEW

Acute lung injury/acute respiratory distress syndrome is a common, serious condition affecting a heterogeneous population of critically ill patients. Other than low tidal volume ventilation, no specific therapy has improved survival. Understanding the epidemiology, pathogenesis, and lessons to be learned from previous clinical trials is necessary for the development of new therapies and the rational design of studies assessing their efficacy.

RECENT FINDINGS

Acute lung injury/acute respiratory distress syndrome occurs in 6-8% of the general intensive care unit population, with a mortality of 32-45%. A recent epidemiologic study found that multi-organ dysfunction, use of tidal volumes higher than 6 ml/kg, and high mean fluid balance were independent risks for mortality. Although high levels of inflammatory mediators are also markers for acute respiratory distress syndrome development and death, short courses of high-dose steroids are not effective in acute cases. The latest theory of biotrauma proposes cellular mechanisms by which mechanical ventilation incites a local and systemic inflammatory response; protective lung ventilation with low tidal volumes can attenuate this inflammation and injury to distal organs. Endogenous surfactant function is clearly impaired, but no commercially available surfactant preparation has been shown to reduce mortality. Results of trials to determine efficacy of steroids in late cases and optimal fluid management are pending.

SUMMARY

The results of recent clinical trials have raised more questions. Further study of the inflammatory response, surfactant regulation, and the cellular impact of mechanical ventilation should help to develop new therapies, target patients most likely to benefit, and identify appropriate timing of intervention.

Incidence and prognosis of intraabdominal hypertension in a mixed population of critically ill patients: a multiple-center epidemiological study

OBJECTIVE

Intraabdominal hypertension is associated with significant morbidity and mortality in surgical and trauma patients. The aim of this study was to assess, in a mixed population of critically ill patients, whether intraabdominal pressure at admission was an independent predictor for mortality and to evaluate the effects of intraabdominal hypertension on organ functions.

DESIGN

Multiple-center, prospective epidemiologic study.

SETTING

Fourteen intensive care units in six countries.

PATIENTS

A total of 265 consecutive patients admitted for >24 hrs during the 4-wk study period.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Intraabdominal pressure was measured twice daily via the bladder. Data recorded on admission were the patient demographics with Simplified Acute Physiology Score II, Acute Physiology and Chronic Health Evaluation II score, and type of admission; during intensive care stay, Sepsis-Related Organ Failure Assessment score and intraabdominal pressure were measured daily together with fluid balance. Nonsurvivors had a significantly higher mean intraabdominal pressure on admission than survivors: 11.4 +/- 4.8 vs. 9.5 +/- 4.8 mm Hg. Independent predictors for mortality were age (odds ratio, 1.04; 95% confidence interval, 1.01-1.06; p = .003), Acute Physiology and Chronic Health Evaluation II score (odds ratio, 1.1; 95% confidence interval, 1.05-1.15; p < .0001), type of intensive care unit admission (odds ratio, 2.5 medical vs. surgical; 95% confidence interval, 1.24-5.16; p = .01), and the presence of liver dysfunction (odds ratio, 2.5; 95% confidence interval, 1.06-5.8; p = .04). The occurrence of intraabdominal hypertension during the intensive care unit stay was also an independent predictor of mortality (relative risk, 1.85; 95% confidence interval, 1.12-3.06; p = .01). Patients with intraabdominal hypertension at admission had significantly higher Sepsis-Related Organ Failure Assessment scores during the intensive care unit stay than patients without intraabdominal hypertension.

CONCLUSIONS

Intraabdominal hypertension on admission was associated with severe organ dysfunction during the intensive care unit stay. The mean intraabdominal pressure on admission was not an independent risk factor for mortality; however, the occurrence of intraabdominal hypertension during the intensive care unit stay was an independent outcome predictor.

Intrinsic positive end-expiratory pressure in Acute Respiratory Distress Syndrome (ARDS) Network subjects

OBJECTIVE

We tested the hypothesis that subjects randomized to the 6 mL/kg predicted body weight tidal volume study group of the National Institutes of Health Acute Respiratory Distress Syndrome (ARDS) Network study had higher levels of intrinsic positive end-expiratory pressure (PEEP) than subjects randomized to the 12 mL/kg predicted body weight tidal volume study group.

DESIGN

Secondary analysis of a subgroup from a randomized controlled trial.

SETTING

Hospitals located in San Francisco, CA, and Seattle, WA.

PATIENTS

Eighty-four patients enrolled in the ARDS Network tidal volume trial in San Francisco, CA, and Seattle, WA, with records of measurement of intrinsic PEEP.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Intrinsic PEEP was assessed a median of six times over the first 48 hrs of ARDS Network protocol ventilation in study subjects, with no significant difference in number of measurements between subjects randomized to the tidal volume protocol of 6 mL/kg compared with 12 mL/kg. We found that intrinsic PEEP was higher among subjects randomized to the 6 mL/kg protocol, with a median intrinsic PEEP of 1.3 cm H2O (interquartile range, 0-3.1 cm H2O), compared with a median intrinsic PEEP of 0.5 cm H2O (interquartile range, 0-1.5 cm H2O) among subjects randomized to 12 mL/kg (p = .029). There was no difference in total PEEP between the study groups.

CONCLUSIONS

In a subgroup of ARDS Network subjects, intrinsic PEEP was statistically significantly higher in subjects randomized to the 6 mL/kg protocol than those in the 12 mL/kg study group. The amount of intrinsic PEEP was very low in both study groups, and difference of median intrinsic PEEP between the groups was <1 cm H2O. It is unlikely that the difference in intrinsic PEEP between the study groups was clinically important in the ARDS Network study of low tidal volume ventilation.

Continuous positive airway pressure for treatment of postoperative hypoxemia: a randomized controlled trial

CONTEXT

Hypoxemia complicates the recovery of 30% to 50% of patients after abdominal surgery; endotracheal intubation and mechanical ventilation may be required in 8% to 10% of cases, increasing morbidity and mortality and prolonging intensive care unit and hospital stay.

OBJECTIVE

To determine the effectiveness of continuous positive airway pressure compared with standard treatment in preventing the need for intubation and mechanical ventilation in patients who develop acute hypoxemia after elective major abdominal surgery.

DESIGN AND SETTING

Randomized, controlled, unblinded study with concealed allocation conducted between June 2002 and November 2003 at 15 intensive care units of the Piedmont Intensive Care Units Network in Italy.

PATIENTS

Consecutive patients who developed severe hypoxemia after major elective abdominal surgery. The trial was stopped for efficacy after 209 patients had been enrolled.

INTERVENTIONS

Patients were randomly assigned to receive oxygen (n = 104) or oxygen plus continuous positive airway pressure (n = 105).

MAIN OUTCOME MEASURES

The primary end point was incidence of endotracheal intubation; secondary end points were intensive care unit and hospital lengths of stay, incidence of pneumonia, infection and sepsis, and hospital mortality.

RESULTS

Patients who received oxygen plus continuous positive airway pressure had a lower intubation rate (1% vs 10%; P = .005; relative risk [RR], 0.099; 95% confidence interval [CI], 0.01-0.76) and had a lower occurrence rate of pneumonia (2% vs 10%, RR, 0.19; 95% CI, 0.04-0.88; P = .02), infection (3% vs 10%, RR, 0.27; 95% CI, 0.07-0.94; P = .03), and sepsis (2% vs 9%; RR, 0.22; 95% CI, 0.04-0.99; P = .03) than did patients treated with oxygen alone. Patients who received oxygen plus continuous positive airway pressure also spent fewer mean (SD) days in the intensive care unit (1.4 [1.6] vs 2.6 [4.2], P = .09) than patients treated with oxygen alone. The treatments did not affect the mean (SD) days that patients spent in the hospital (15 [13] vs 17 [15], respectively; P = .10). None of those treated with oxygen plus continuous positive airway pressure died in the hospital while 3 deaths occurred among those treated with oxygen alone (P = .12).

CONCLUSION

Continuous positive airway pressure may decrease the incidence of endotracheal intubation and other severe complications in patients who develop hypoxemia after elective major abdominal surgery.

Overview of ventilator-induced lung injury mechanisms

PURPOSE OF REVIEW

Mechanical ventilation is the main supportive therapy for patients with acute respiratory distress syndrome. As with any therapy, mechanical ventilation has side effects and may induce lung injury. This review will focus on stretch-dependent activation of alveolar epithelial and endothelial cells and polymorphonuclear leukocytes, and apoptosis/necrosis balance.

RECENT FINDINGS

The past year has seen important research in the area of mechanotransduction and lung native immunity, suggesting further mechanisms of lung inflammation and injury in ventilator-induced lung injury. Research in the past year has also stressed the importance of inflammatory response by alveolar cells and role of polymorphonuclear leukocytes in stretch-induced lung injury and has suggested a role for apoptosis in the maintenance of the alveolar epithelium.

SUMMARY

The proportion of patients receiving protective ventilatory strategies remains modest. If efforts to minimize the iatrogenic consequences of mechanical ventilation are to succeed, there must be a greater understanding of the signal transduction mechanisms and the development of potential pharmacologic targets to modulate the molecular and cellular effects of lung stretch.

Airway pressure-time curve profile (stress index) detects tidal recruitment/hyperinflation in experimental acute lung injury

OBJECTIVE

To evaluate whether the shape of the airway pressure-time (Paw-t) curve during constant flow inflation corresponds to radiologic evidence of tidal recruitment or tidal hyperinflation in an experimental model of acute lung injury.

DESIGN

Prospective randomized laboratory animal investigation.

SETTING

Department of Clinical Physiology, University of Uppsala, Sweden.

SUBJECTS

Anesthetized, paralyzed, and mechanically ventilated pigs.

INTERVENTIONS

Acute lung injury was induced by lung lavage. During constant inspiratory flow, the Paw-t curve was fitted to a power equation: airway pressure =a x time + c, where coefficient b (stress index) describes the shape of the curve:b = 1, straight curve; b < 1, progressive increase in slope; and b > 1, progressive decrease in slope. Tidal volume (Vt) was 6 mL/kg, and positive end-expiratory pressure was set to obtain a b value between 0.9 and 1.1 before (b = 1) and after (b = 1 after recruiting maneuver) application of a recruiting maneuver. Positive end-expiratory pressure was decreased and Vt increased to obtain 0.9 >b > 0.8 and 0.8 >b > 0.6, whereas positive end-expiratory pressure and Vt were both increased to obtain 1.3 >b > 1.1 and 1.5 >b > 1.3. Experimental conditions sequence was random.

MEASUREMENTS AND MAIN RESULTS

Pulmonary computed tomography was obtained during end-expiratory and end-inspiratory occlusions. Tidal recruitment was quantified as nonaerated (between -100 and +100 Hounsfield units) lung area at end-expiration minus end-inspiration. Tidal hyperinflation was quantified as hyperinflated (between -900 and -1000 Hounsfield units) lung area at end-inspiration minus end-expiration. Computed tomography images showed that tidal recruitment and tidal hyperinflation corresponded to b < 1 and b > 1, respectively. Stress index values and tidal recruitment and tidal hyperinflation values were significantly correlated (R =.917 and R =.911, p <.0001, respectively).

CONCLUSIONS

Shape of the Paw-t curve detects tidal recruitment and tidal hyperinflation.

How respiratory system mechanics may help in minimising ventilator-induced lung injury in ARDS patients

The main supportive therapy in acute respiratory distress syndrome patients is mechanical ventilation. As with any therapy, mechanical ventilation has side-effects, and may induce lung injury (ventilator-induced lung injury (VILI)/ventilator-associated lung injury). The mechanical factors responsible for VILI are thought to be related to tidal recruitment/derecruitment of previously collapsed alveoli and/or pulmonary overdistension. The volume/pressure (V/P) curve of the respiratory system in patients as well as in animal models of acute lung injury (ALI) has a characteristic sigmoid shape, with a lower inflection point (LIP) corresponding to the pressure/end-expiratory volume required to initiate recruitment of collapsed alveoli, and an upper inflection point (UIP) corresponding to the pressure/end inspiratory volume at which alveolar overdistension occurs. "Protective" ventilatory approaches have therefore set out to minimise mechanical injury by using the V/P curve to individualise positive end-expiratory pressure (PEEP) (PEEP above the LIP) and tidal volume (by setting end-inspiratory V/P below the UIP) since a large number of experimental studies correlate P/V curves to histological and biological manifestations of VILI and two randomised trials showed that protective ventilatory strategy individually tailored to the P/V curve minimised pulmonary and systemic inflammation and decreased mortality in patients with ALI. However, despite the fact that several studies have: 1) proposed new techniques to perform pressure/volume curves at the bedside, 2) confirmed that the lower inflection point and upper inflection point correspond to computed tomography scan evidence of atelectasis and overdistension, and 3) demonstrated the ability of the pressure/volume curve to estimate alveolar recruitment with positive end-expiratory pressure, no large studies have assessed whether such measurement can be performed in all intensive care units as a monitoring tool to orient ventilator therapy. Preliminary experimental and clinical studies show that the shape of the dynamic inspiratory pressure/time profile during constant flow inflation (stress index), allows prediction of a ventilatory strategy that minimises the occurrence of ventilator-induced lung injury.

The accuracy of transcranial Doppler to detect vasospasm in patients with aneurysmal subarachnoid hemorrhage

OBJECTIVES

To examine the accuracy of transcranial Doppler to detect cerebral vasospasm in a patient population with aneurysmal subarachnoid hemorrhage.

DESIGN

Prospective blind comparison of transcranial Doppler with cerebral angiography. Diagnostic accuracy of transcranial Doppler was assessed using receiver operating characteristic (ROC) analysis and likelihood ratios. Sensitivity and specificity were calculated using directly measured middle cerebral artery diameter as reference standard.

SETTING

Intensive Care Unit of a large university teaching hospital.

PATIENTS AND PARTICIPANTS

Twenty-two patients with subarachnoid hemorrhage were included. Patients underwent angiography on admission and after 8 days to diagnose vasospasm and were defined as having clinical vasospasm, angiographic vasospasm, or no vasospasm.

MEASUREMENTS AND RESULTS

Sensitivity and specificity were 1.00 and 0.75 for angiographic vasospasm and both equal to 1.00 for clinical vasospasm diagnosis. A transcranial Doppler mean velocity threshold value of 100 cm/s for angiographic vasospasm and 160 cm/s for clinical vasospasm detection were chosen by ROC analysis.

CONCLUSIONS

A Transcranial Doppler mean velocity threshold of 160 cm/s, calculated by the ROC analysis, accurately detects clinical vasospasm. A daily transcranial Doppler examination performed by a trained operator should be routinely used to provide early identification of patients at high risk and to orient therapeutic decisions.

Injurious mechanical ventilation and end-organ epithelial cell apoptosis and organ dysfunction in an experimental model of acute respiratory distress syndrome

CONTEXT

Recent clinical trials have demonstrated a decrease in multiple organ dysfunction syndrome (MODS) and mortality in patients with acute respiratory distress syndrome (ARDS) treated with a protective ventilatory strategy.

OBJECTIVE

To examine the hypothesis that an injurious ventilatory strategy may lead to end-organ epithelial cell apoptosis and organ dysfunction.

DESIGN AND SETTING

In vivo animals: 24 rabbits with acid-aspiration lung injury were ventilated with injurious or noninjurious ventilatory strategies. In vitro: rabbit epithelial cells were exposed to plasma from in vivo rabbit studies. In vivo human: plasma samples from patients included in a previous randomized controlled trial examining a lung protective strategy were analyzed (lung protection group, n = 9 and controls, n = 11).

MAIN OUTCOME MEASURES

In vivo animals: biochemical markers of liver and renal dysfunction; apoptosis in end organs. In vitro: induction of apoptosis in LLC-RK1 renal tubular epithelial cells. In vivo human: correlation of plasma creatinine and soluble Fas ligand.

RESULTS

The injurious ventilatory strategy led to increased rates of epithelial cell apoptosis in the kidney (mean [SE]: injurious, 10.9% [0.88%]; noninjurious, 1.86% [0.17%]; P<.001) and small intestine villi (injurious, 6.7% [0.66%]; noninjurious, 0.97% [0.14%]; P<.001), and led to the elevation of biochemical markers indicating renal dysfunction in vivo. Induction of apoptosis was increased in LLC-RK1 cells incubated with plasma from rabbits ventilated with injurious ventilatory strategy at 4 hours (P =.03) and 8 hours (P =.002). The Fas:Ig, a fusion protein that blocks soluble Fas ligand, attenuated induction of apoptosis in vitro. There was a significant correlation between changes in soluble Fas ligand and changes in creatinine in patients with ARDS (R = 0.64, P =.002).

CONCLUSIONS

Mechanical ventilation can lead to epithelial cell apoptosis in the kidney and small intestine, accompanied by biochemical evidence of organ dysfunction. This may partially explain the high rate of MODS observed in patients with ARDS and the decrease in morbidity and mortality in patients treated with a lung protective strategy.

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.

Effect of ventilator-induced lung injury on the development of reperfusion injury in a rat lung transplant model

OBJECTIVE

Although mechanical ventilation can potentially worsen preexisting lung injury, its importance in the setting of lung transplantation has not been explored. This study was undertaken to examine the effect of 2 ventilatory strategies on the development of ischemia-reperfusion injury after lung transplantation.

METHODS

In a rat lung transplant model animals were randomized into 2 groups defined by the ventilatory strategy during the early reperfusion period. In conventional mechanical ventilation the transplanted lung was ventilated with a tidal volume equal to 50% of the inspiratory capacity of the left lung and a low positive end-expiratory pressure. In minimal mechanical stress ventilation the transplanted lung was ventilated with a tidal volume equal to 20% of the inspiratory capacity of the left lung, and positive end-expiratory pressure was adjusted according to the shape of the pressure-time curve to minimize pulmonary stress.

RESULTS

After 3 hours of reperfusion, oxygenation from the transplanted lung was significantly higher with minimal mechanical stress ventilation than with conventional ventilation. In addition, elastance, cytokine levels, and morphologic signs of injury were significantly lower in the group with minimal mechanical stress ventilation.

CONCLUSIONS

This study demonstrates that the mode of mechanical ventilation used in the early phase of reperfusion of the transplanted lung can influence ischemia-reperfusion injury, and a protective ventilatory strategy on the basis of minimizing pulmonary mechanical stress can lead to improved lung function after lung transplantation.

Conventional mechanical ventilation is associated with bronchoalveolar lavage-induced activation of polymorphonuclear leukocytes: a possible mechanism to explain the systemic consequences of ventilator-induced lung injury in patients with ARDS

BACKGROUND

Protective ventilatory strategies have resulted in a decreased mortality rate in acute respiratory distress syndrome, but the underlying mechanisms remain unclear. The authors hypothesized that (1) mechanical ventilation modulates activation of polymorphonuclear leukocytes (PMNs), (2) the consequent release of proteinases is correlated with a systemic inflammatory response and with multiple organ dysfunction, and (3) these deleterious effects can be minimized by a protective ventilatory strategy.

METHODS

Human PMNs were incubated with bronchoalveolar lavage fluid obtained from patients at entry or 36 h after randomization to ventilation with either a conventional (control) or a lung-protective strategy. PMN oxidant production and surface expression of adhesion molecules and granule markers, including CD18, CD63, and L-selectin, were measured by flow cytometry. Extracellular elastase activity was quantified using a fluorescent substrate.

RESULTS

Bronchoalveolar lavage obtained from both groups of patients at entry showed similar effects on PMN oxidant production and expression of surface markers. At 36 h, exposure of PMNs to bronchoalveolar lavage fluid from the control group resulted in increased PMN activation as manifested by a significant increase in oxidant production, CD18, and CD63 surface expression, and shedding of L-selectin. By contrast, these variables were unchanged at 36 h in the lung-protective group. There was a significant correlation between the changes of the variables and changes in interleukin-6 level and the number of failing organs.

CONCLUSIONS

Polymorphonuclear leukocytes can be activated by mechanical ventilation, and the consequent release of elastase was correlated with the degree of systemic inflammatory response and multiple organ failure. This result may possibly explain the decreased mortality in acute respiratory distress syndrome patients treated with a lung-protective strategy.

ARDSNet lower tidal volume ventilatory strategy may generate intrinsic positive end-expiratory pressure in patients with acute respiratory distress syndrome

The ARDSNet trial revealed that the use of a smaller tidal volume (VT) reduced mortality by 22%. However, three earlier studies that lowered VT did not find a decrease in mortality. We tested the hypothesis that the increased respiratory rate used in the ARDSNet lower VT strategy might have led to intrinsic positive end-expiratory pressure (PEEP(i)), raising total PEEP (PEEP(total)). Ten patients with acute respiratory distress syndrome (ARDS) were ventilated using the ARDSNet lower VT protocol. Respiratory rate was then reduced (10-15 breaths/minute) to obtain a VT of 12 ml/kg (ARDSNet traditional VT). PEEP on the ventilator (PEEP(nominal): 10.1 +/- 0.7 cm H2O), FIO2 (0.7 +/- 0.1), and minute ventilation (VE: 12.4 +/- 1.7 L/minute) were set using the ARDSNet protocol and maintained constant during the two ventilatory strategies. Values of airway pressure at end-expiration of a regular breath (PEEP(external)) and 3-5 seconds after the onset of an end-expiratory occlusion (PEEP(total)) were measured. PEEP(i) was calculated by subtracting PEEP(external) from PEEP(total). PEEP(total) and PEEP(i) were, respectively, 16.3 +/- 2.9 and 5.8 +/- 3.0 cm H2O during the lower VT strategy and 11.7 +/- 0.9 and 1.4 +/- 1.0 cm H2O during the traditional VT strategy (p < 0.01). The reduced mortality observed with the ARDSNet strategy may have been due to the protective effect of a higher PEEP(total).

Effects of recruiting maneuvers in patients with acute respiratory distress syndrome ventilated with protective ventilatory strategy

BACKGROUND

A lung-protective ventilatory strategy with low tidal volume (VT) has been proposed for use in acute respiratory distress syndrome (ARDS). Alveolar derecruitment may occur during the use of a lung-protective ventilatory strategy and may be prevented by recruiting maneuvers. This study examined the hypothesis that the effectiveness of a recruiting maneuver to improve oxygenation in patients with ARDS would be influenced by the elastic properties of the lung and chest wall.

METHODS

Twenty-two patients with ARDS were studied during use of the ARDSNet lung-protective ventilatory strategy: VT was set at 6 ml/kg predicted body weight and positive end-expiratory pressure (PEEP) and inspiratory oxygen fraction (Fio2) were set to obtain an arterial oxygen saturation of 90-95% and/or an arterial oxygen partial pressure (Pao2) of 60- 80 mmHg (baseline). Measurements of Pao2/Fio2, static volume-pressure curve, recruited volume (vertical shift of the volume-pressure curve), and chest wall and lung elastance (EstW and EstL: esophageal pressure) were obtained on zero end-expiratory pressure, at baseline, and at 2 and 20 min after application of a recruiting maneuver (40 cm H2O of continuous positive airway pressure for 40 s). Cardiac output (transesophageal Doppler) and mean arterial pressure were measured immediately before, during, and immediately after the recruiting maneuver. Patients were classified a priori as responders and nonresponders on the basis of the occurrence or nonoccurrence of a 50% increase in Pao2/Fio2 after the recruiting maneuver.

RESULTS

Recruiting maneuvers increased Pao2/Fio2 by 20 +/- 3% in nonresponders (n = 11) and by 175 +/- 23% (n = 11; mean +/- standard deviation) in responders. On zero end-expiratory pressure, EstL (28.4 +/- 2.2 vs. 24.2 +/- 2.9 cm H2O/l) and EstW (10.4 +/- 1.8 vs. 5.6 +/- 0.8 cm H2O/l) were higher in nonresponders than in responders (P < 0.01). Nonresponders had been ventilated for a longer period of time than responders (7 +/- 1 vs. 1 +/- 0.3 days; P < 0.001). Cardiac output and mean arterial pressure decreased by 31 +/- 2 and 19 +/- 3% in nonresponders and by 2 +/- 1 and 2 +/- 1% in responders (P < 0.01).

CONCLUSIONS

Application of recruiting maneuvers improves oxygenation only in patients with early ARDS who do not have impairment of chest wall mechanics and with a large potential for recruitment, as indicated by low values of EstL.

Proportional assist ventilation

Partial ventilatory support techniques are intended for patients who are unable to maintain a normal alveolar ventilation, despite normal central control for respiration. Proportional assist ventilation (PAV) is a novel mode of partial ventilatory support in which the ventilator generates an instantaneous inspiratory pressure in proportion to the instantaneous effort of the patient. In theory, PAV should normalize the neuro-ventilatory coupling by making the ventilator an extension of patient's respiratory muscles, while leaving to the patient the entire control of all aspects of breathing. PAV, however, shares a common problem with the conventional partial ventilatory support modes. In mechanically ventilated patients, the respiratory system impedance may change over time. These changes may impair the good matching between ventilator output and patient's ventilatory demand and lead to patient-ventilator asynchrony. To take full advantage of PAV, the authors believe that PAV should continuously and automatically adapt to the respiratory system passive mechanics, assessed by continuous noninvasive measurement of total elastance and resistance.

Temporal relationship between endothelin-1 concentrations and cerebral vasospasm in patients with aneurysmal subarachnoid hemorrhage

BACKGROUND AND PURPOSE

Endothelin 1 (ET-1) is a potent vasoconstrictor that may play a role in cerebral vasospasm following subarachnoid hemorrhage (SAH). However, data regarding its pathogenic role in the development of vasospasm are controversial. We planned a prospective, observational clinical study to investigate the temporal relationship between increased ET-1 production and cerebral vasospasm or other neurological sequelae after SAH.

METHODS

ET-1 levels in cerebrospinal fluid (CSF) were measured in 20 SAH patients from admission (within 24 hours from the bleeding) until day 7. Patients received a daily transcranial Doppler study and a neurological evaluation. On day 7, angiography was performed to verify the degree and extent of vasospasm. Patients were then classified as having (1) clinical vasospasm, (2) angiographic vasospasm, (3) no vasospasm, or (4) poor neurological condition without significant vasospasm (low Glasgow Coma Scale score [GCS]).

RESULTS

On admission, ET-1 levels were increased in the low-GCS group compared with the other groups (P=0.04). On day 4 ET-1 levels were not significantly different among groups, whereas on day 7 ET-1 levels were significantly increased in both the clinical vasospasm and low-GCS groups compared with the angiographic vasospasm and no vasospasm groups (P<0.005). Moreover, when the low-GCS group was excluded, there was a significant relationship between vasospasm grade and CSF ET-1 levels (R(2)=0.73).

CONCLUSIONS

CSF ET-1 levels were markedly elevated in patients with clinical manifestations of vasospasm (day 7) and with a poor neurological condition not related to vasospasm. However, ET-1 levels were low in clinical vasospasm patients before clinical symptoms were evident (day 4) and remained low in angiographic vasospasm patients throughout the study period. Thus, our data suggest that CSF ET-1 levels are increased in conditions of severe neuronal damage regardless whether this was due to vasospasm or to the primary hemorrhagic event. In addition, CSF ET-1 levels paralleled the neurological deterioration but were not predictive of vasospasm.

[Traditional artificial ventilation. General principles]

Mechanical ventilation is generally applied on patients affected by acute respiratory failure (ARF) on the basis of hemogasanalysis, hemodynamics and X-ray features. The modification of ventilatory pattern is based on problem the modification of data concerning mechanical variation. In this study the most important mechanical measures and their consequences on ventilatory strategy are discussed.

Continuous positive airway pressure facilitates spontaneous breathing in weaning chronic obstructive pulmonary disease patients by improving breathing pattern and gas exchange

OBJECTIVE

To elucidate the effects of continuous positive airway pressure (CPAP) on breathing pattern, gas exchange and the ability to sustain spontaneous breathing (SB) in chronic obstructive pulmonary disease (COPD) patients with dynamic hyperinflation.

DESIGN

Prospective study with two randomised trials of SB without and with CPAP in each patient.

SETTING

Medical intensive care units (ICUs) in two university hospitals.

PATIENTS

Nine dynamically hyperinflated, intubated COPD patients recuperating from acute exacerbation.

INTERVENTIONS

One SB trial with CPAP (5-7.5 cmH2O), one without (control) in each patient.

MEASUREMENTS

airway opening pressure, gas flow and thus breathing pattern, oxygen uptake, carbon dioxide excretion, arterial blood gases, dyspnoea and respiratory drive (P100).

RESULTS

With CPAP, intrinsic positive end-expiratory pressure (PEEPi) fell from 11.4 to 6.3 cm H2O (p < 0.05). Eight patients sustained SB with CPAP for the maximum time planned (30 min), one failed after 18 min. In contrast, only four patients successfully completed the control trial, the others failing after 5-18 min (p < 0.05). Dyspnoea-gauged on a visual analogue scale by five patients--was less severe or occurred later with CPAP. Breathing with CPAP tended to be slower (18.9 vs 22.2 min(-1), p < 0.05) and deeper (tidal volume 370 vs 323 ml). At the end of the control run, PaCO2 was higher (60 vs 55 mmHg, p < 0.05) and still rising while being stable at the end of the CPAP trial.

CONCLUSION

CPAP helps severely ill COPD patients sustain SB. Apparently it does so by promoting slower, deeper breathing and thus facilitating carbon dioxide elimination.

Pressure-time curve predicts minimally injurious ventilatory strategy in an isolated rat lung model

BACKGROUND

We tested the hypothesis that the pressure-time (P-t) curve during constant flow ventilation can be used to set a noninjurious ventilatory strategy.

METHODS

In an isolated, nonperfused, lavaged model of acute lung injury, tidal volume and positive end-expiratory pressure were set to obtain: (1) a straight P-t curve (constant compliance, minimal stress); (2) a downward concavity in the P-t curve (increasing compliance, low volume stress); and (3) an upward concavity in the P-t curve (decreasing compliance, high volume stress). The P-t curve was fitted to: P = a. tb +c, where b describes the shape of the curve, b = 1 describes a straight P-t curve, b < 1 describes a downward concavity, and b > 1 describes an upward concavity. After 3 h, lungs were analyzed for histologic evidence of pulmonary damage and lavage concentration of inflammatory mediators. Ventilator-induced lung injury occurred when injury score and cytokine concentrations in the ventilated lungs were higher than those in 10 isolated lavaged rats kept statically inflated for 3 h with an airway pressure of 4 cm H2O.

RESULTS

The threshold value for coefficient b that discriminated best between lungs with and without histologic and inflammatory evidence of ventilator-induced lung injury (receiver-operating characteristic curve) ranged between 0.90-1.10. For such threshold values, the sensitivity of coefficient b to identify noninjurious ventilatory strategy was 1.00. A significant relation (P < 0.001) between values of coefficient b and injury score, interleukin-6, and macrophage inflammatory protein-2 was found.

CONCLUSIONS

The predictive power of coefficient b to predict noninjurious ventilatory strategy in a model of acute lung injury is high.

Mechanical ventilation: lessons from the ARDSNet trial

The acute respiratory distress syndrome (ARDS) is an inflammatory disease of the lungs characterized clinically by bilateral pulmonary infiltrates, decreased pulmonary compliance and hypoxemia. Although supportive care for ARDS seems to have improved over the past few decades, few studies have shown that any treatment can decrease mortality for this deadly syndrome. In the 4 May 2000 issue of New England Journal of Medicine, the results of an NIH-sponsored trial were presented; they demonstrated that the use of a ventilatory strategy that minimizes ventilator-induced lung injury leads to a 22% decrease in mortality. The implications of this study with respect to clinical practice, further ARDS studies and clinical research in the critical care setting are discussed.

Compensation for increase in respiratory workload during mechanical ventilation. Pressure-support versus proportional-assist ventilation

Variation in respiratory impedance may occur in mechanically ventilated patients. During pressure-targeted ventilatory support, this may lead to patient-ventilator asynchrony. We assessed the hypothesis that during pressure-support ventilation (PSV), preservation of minute ventilation (V E) consequent to added mechanical loads would result in an increase in respiratory rate (RR) due to the large reduction in tidal volume (VT). WITH proportional-assist ventilation (PAV), preservation of V E would occur through the preservation of VT, with a smaller effect on RR. We anticipated that this compensatory strategy would result in greater patient comfort and a reduce work of breathing. An increase in respiratory impedance was obtained by chest and abdominal binding in 10 patients during weaning from mechanical ventilation. V E remained constant in both ventilatory modes after chest and abdominal compression. During PSV, this maintenance of VE was obtained through a 58 +/- 3% increase in RR that compensated for a 29 +/- 2% reduction in VT. The magnitudes of the reduction in VT (10 +/- 3%) and of the increase in RR (14 +/- 2%) were smaller (p < 0. 001) during PAV. During both PSV and PAV, chest and abdominal compression caused increases in both the pressure-time product (PTP) of the diaphragm per minute (142.9 +/- 26.9 cm H(2)O. s/min, PSV, and 117.6 +/- 16.4 cm H(2)O. s/min, PAV) and per liter (13.4 +/- 2.5 cm H(2)O. s/L, PSV, and 9.6 +/- 0.7 cm H(2)O. s/L, PAV). These increments were greater (p < 0.001) during PSV than during PAV. The capability of keeping VT and V E constant through increases in inspiratory effort after increases in mechanical loads is relatively preserved only during PAV. The ventilatory response to an added respiratory load during PSV required greater muscle effort than during PAV.

Effect of adrenoreceptors on endotoxin-induced cytokines and lipid peroxidation in lung explants

Lung tissue may be an important source of systemic inflammation associated with sepsis and the acute respiratory distress syndrome (ARDS). An ex vivo model of freshly explanted lung tissue in culture was developed to evaluate the ability of lipopolysaccharide (LPS) to directly stimulate lung tissues under conditions where indirect mechanisms such as recruitment of blood-derived inflammatory cells could not be implicated. Under control conditions, lung explants produced a high level of macrophage inflammatory protein-2 (MIP-2). Eight hours after LPS challenge, there were marked increases in the production of tumor necrosis factor-alpha (TNF-alpha) from 0.18 +/- 0.04 to 4.13 +/- 0.23 ng/ml/g tissue (p < 0.05), MIP-2 from 60.0 +/- 7.4 to 165.6 +/- 10.3 ng/ml/g tissue (p < 0.05), and tissue lipid peroxidation (malonaldehyde from 27.6 +/- 2.5 to 48.4 +/- 17.5 microM/g tissue; and 4-hydroxyalkenal from 34.0 +/- 3.0 to 59.7 +/- 18.8 microM/g tissue, both p < 0.05) from lung explants. Treatment with the beta-adrenoreceptor agonist isoproterenol (1 ng/ml) attenuated LPS-induced release of TNF-alpha and lipid peroxidation in association with an increase in intracellular cAMP levels. The adenylate cyclase activator, forskolin, also inhibited LPS-induced changes in TNF-alpha and lipid peroxidation. In conclusion, increasing intracellular levels of cAMP through beta-adrenoreceptor activation can attenuate the acute inflammatory response induced in the lung by LPS. LPS did not significantly impair the beta-adrenoreceptor reactivity in lung explants. Lung explants allow for the quantitative assessment of pulmonary inflammatory responses independent of influences from the circulation, and thus may be a useful ex vivo model to investigate cellular and molecular mechanisms of lung injury.

Time-course of impairment of respiratory mechanics after cardiac surgery and cardiopulmonary bypass

OBJECTIVE

Cardiopulmonary bypass (CPB) is associated with abnormalities of lung function characterized by an increase in static elastance of the respiratory system. We examined the following: a) the effects of CPB on the total inspiratory volume-pressure (V-P) relationship of the respiratory system; b) the relative contribution of the chest wall and lung to the impairment of respiratory system mechanics; and c) the time-course of such impairment.

DESIGN

Prospective, interventional study.

SETTING

Surgical and medical intensive care units in a teaching hospital.

PATIENTS

Eight adult patients scheduled for elective open heart surgery to correct valvular dysfunction.

INTERVENTIONS

V-P curves (interrupter technique) of the respiratory system were partitioned between the chest wall and lung by measurements of esophageal pressure. Measurements were obtained before sternotomy (control), immediately after, 4 hrs after, and 7 hrs after separation from CPB.

MEASUREMENTS AND MAIN RESULTS

Control V-P relationships of the respiratory system and lung showed lower inflection points (Pflex) at pressure values of 5.9+/-2.3 and 4.3+/-2.5 cm H2O, respectively. Immediately after and 4 hrs after separation from CPB, both curves had sigmoid shapes because of lower Pflex and formation of upper inflection (UIP) points. The pressures corresponding to the Pflex increased significantly (p < .001) by 56%+/-3% and 78%+/-4%, whereas the UIP corresponded to a pressure value of 42.34+/-8.5 and 35.6+/-7.8 cm H2O in the respiratory system and lung, respectively. A linear V-P relationship of the chest wall was observed during the control condition and after separation from CPB. Four hours later, no further increases in the pressure values corresponding to Pflex were observed on the inspiratory V-P curves of the respiratory system and lung, whereas the UIP occurred at a pressure of 35.6+/-9.1 and 29.7+/-8.4 cm H2O, respectively. A UIP was present on the V-P curve of the chest wall at a volume of 0.77+/-0.02 L. Seven hours after separation from CPB, the inspiratory V-P curves of the respiratory system, chest wall, and lung returned to normal.

CONCLUSIONS

Sternotomy and CPB produced immediate changes in lung mechanics. Chest wall mechanics were affected only 4 hrs after sternotomy. Seven hours after disconnection from CPB, all mechanics had returned to normal.

Effect of mechanical ventilation on inflammatory mediators in patients with acute respiratory distress syndrome: a randomized controlled trial

CONTEXT

Studies have shown that an inflammatory response may be elicited by mechanical ventilation used for recruitment or derecruitment of collapsed lung units or to overdistend alveolar regions, and that a lung-protective strategy may reduce this response.

OBJECTIVE

To test the hypothesis that mechanical ventilation induces a pulmonary and systemic cytokine response that can be minimized by limiting recruitment or derecruitment and overdistention.

DESIGN AND SETTING

Randomized controlled trial in the intensive care units of 2 European hospitals from November 1995 to February 1998, with a 28-day follow-up.

PATIENTS

Forty-four patients (mean [SD] age, 50 [18] years) with acute respiratory distress syndrome were enrolled, 7 of whom were withdrawn due to adverse events.

INTERVENTIONS

After admission, volume-pressure curves were measured and bronchoalveolar lavage and blood samples were obtained. Patients were randomized to either the control group (n = 19): tidal volume to obtain normal values of arterial carbon dioxide tension (35-40 mm Hg) and positive end-expiratory pressure (PEEP) producing the greatest improvement in arterial oxygen saturation without worsening hemodynamics; or the lung-protective strategy group (n = 18): tidal volume and PEEP based on the volume-pressure curve. Measurements were repeated 24 to 30 and 36 to 40 hours after randomization.

MAIN OUTCOME MEASURES

Pulmonary and systemic concentrations of inflammatory mediators approximately 36 hours after randomization.

RESULTS

Physiological characteristics and cytokine concentrations were similar in both groups at randomization. There were significant differences (mean [SD]) between the control and lung-protective strategy groups in tidal volume (11.1 [1.3] vs 7.6 [1.1] mL/kg), end-inspiratory plateau pressures (31.0 [4.5] vs 24.6 [2.4] cm H2O), and PEEP (6.5 [1.7] vs 14.8 [2.7] cm H2O) (P<.001). Patients in the control group had an increase in bronchoalveolar lavage concentrations of interleukin (IL) 1beta, IL-6, and IL-1 receptor agonist and in both bronchoalveolar lavage and plasma concentrations of tumor necrosis factor (TNF) alpha, IL-6, and TNF-alpha, receptors over 36 hours (P<.05 for all). Patients in the lung-protective strategy group had a reduction in bronchoalveolar lavage concentrations of polymorphonuclear cells, TNF-alpha, IL-1beta, soluble TNF-alpha receptor 55, and IL-8, and in plasma and bronchoalveolar lavage concentrations of IL-6, soluble TNF-alpha receptor 75, and IL-1 receptor antagonist (P<.05). The concentration of the inflammatory mediators 36 hours after randomization was significantly lower in the lung-protective strategy group than in the control group (P<.05).

CONCLUSIONS

Mechanical ventilation can induce a cytokine response that may be attenuated by a strategy to minimize overdistention and recruitment/derecruitment of the lung. Whether these physiological improvements are associated with improvements in clinical end points should be determined in future studies.

Ventilatory support in the acute respiratory distress syndrome

Ventilatory support in the acute respiratory distress syndrome (ARDS) has undergone considerable transformation in the 1990s. Current approaches include lung protective techniques which, while attempting to recruit and maintain lung volume, limit the shear stresses associated with ventilation by avoiding both alveolar overdistension and cyclical end-expiratory collapse. In addition, gas exchange targets have been liberalized and ventilatory conduct is much more tailored to individual pulmonary mechanics. Assessment of the inspiratory volume-pressure (V-P) curve provides information which can direct ventilator settings. Recent information from clinical trials has provided new insights into appropriate ventilatory modification and set the foundation for future clinical investigations.