Optimal peep in ARDS. Changing concepts and current controversies

Saved by the PEEP: Resolution of Complete Unilateral Lung Collapse Secondary to Mucus Plugging With Ventilator Technique

Bronchoscopy can be used to resolve respiratory failure caused by tenacious mucus plugs. However, emergent bronchoscopy to resolve mucus plugging is not always available in small rural hospitals around the country. We present a case in which increasing the positive end-expiratory pressure settings on the ventilator resulted in immediate improvement in patient oxygenation and imaging findings during a respiratory emergency caused by mucus plugging.

The acute effectiveness and safety of the constant-flow, pressure-volume curve to improve hypoxemia in acute lung injury

OBJECTIVE

To investigate the effectiveness of the constant-flow, pressure-volume curve (PVC) to prescribe positive end-expiratory pressure (PEEP) in acute lung injury (ALI) and risk of cardiopulmonary deterioration during the PVC process.

DESIGN

A retrospective, cohort study.

SETTING

A surgical intensive care unit (ICU) of a tertiary, university hospital.

PATIENTS

Fifty consecutive ventilated patients diagnosed with ALI undergoing the PVC maneuver from 1999 to 2003.

INTERVENTIONS

Titration of PEEP based on the lower inflection point of the constant-flow, pressure-volume curve.

MEASUREMENTS AND MAIN RESULTS

Patients were divided into 2 groups based on PVC-guided PEEP changes of <3 cm H2O (PVC-NC or "no change") or ≥3 cm H2O (PVC-CHG or "change") from the initial empiric prescription. There was a greater increase in partial pressure of arterial oxygen (PaO2)/fractional concentration of inspired oxygen (FiO2) in the PVC-CHG group, with a mean change of 80 ± 50 (95% confidence interval [CI] 61, 98) versus 42 ± 54 (95% CI 17, 67) in the PVC-NC group. Eighty-two percent of patients (41/50) showed an increase in ratio of partial pressure of arterial oxygen to fraction of inspired oxygen (PaO2/FiO2) by 20% within 6 to 24 hours after the PVC test-greater in the PVC-CHG group (OR 1.44, 95% CI 1.02, 2.01). Thirteen percent (4/30) within the PVC-CHG group and none within the PVC-NC group (0/20) required a 25% increase in vasoactive infusion rates (P = .089) in relation to the procedure. Univariate logistic regression showed that PVC-CHG was significantly associated with a 20% change in PaO2/FiO2 (OR 7.54, 95% CI 1.37, 41.41). Multivariate logistic modeling showed that PVC-guided PEEP changes of ≥3 cm H2O, age ≤65 years, and pre-PVC FiO2 ≥ .85 were significantly associated with a 20% increase in PaO2/FiO2 (receiver operator area under the curve = .86).

CONCLUSIONS

In the setting of acute lung injury, use of the constant-flow, pressure-volume curve to prescribe PEEP appears associated with improvement in oxygenation with limited risk of acute, process-related, cardiopulmonary deterioration.

Cardiac output estimation using pulmonary mechanics in mechanically ventilated patients

The application of positive end expiratory pressure (PEEP) in mechanically ventilated (MV) patients with acute respiratory distress syndrome (ARDS) decreases cardiac output (CO). Accurate measurement of CO is highly invasive and is not ideal for all MV critically ill patients. However, the link between the PEEP used in MV, and CO provides an opportunity to assess CO via MV therapy and other existing measurements, creating a CO measure without further invasiveness.

This paper examines combining models of diffusion resistance and lung mechanics, to help predict CO changes due to PEEP. The CO estimator uses an initial measurement of pulmonary shunt, and estimations of shunt changes due to PEEP to predict CO at different levels of PEEP. Inputs to the cardiac model are the PV loops from the ventilator, as well as the oxygen saturation values using known respiratory inspired oxygen content. The outputs are estimates of pulmonary shunt and CO changes due to changes in applied PEEP. Data from two published studies are used to assess and initially validate this model.

The model shows the effect on oxygenation due to decreased CO and decreased shunt, resulting from increased PEEP. It concludes that there is a trade off on oxygenation parameters. More clinically importantly, the model also examines how the rate of CO drop with increased PEEP can be used as a method to determine optimal PEEP, which may be used to optimise MV therapy with respect to the gas exchange achieved, as well as accounting for the impact on the cardiovascular system and its management.

Positive end-expiratory pressure improves survival in a rodent model of cardiopulmonary resuscitation using high-dose epinephrine

BACKGROUND

Multiple interventions have been tested in models of cardiopulmonary resuscitation (CPR) to optimize drug use, chest compressions, and ventilation. None has studied the effects of positive end-expiratory pressure (PEEP) on outcome. We hypothesized that because PEEP can reverse pulmonary atelectasis, lower pulmonary vascular resistance, and potentially improve cardiac output, its use during CPR would increase survival.

METHODS

Anesthetized Sprague-Dawley rats were exposed to 1 min of asphyxial cardiac arrest. Resuscitation was standardized and consisted of chest compressions, oxygen (Fio(2) 1.0), and IV epinephrine 30 microg/kg (Series 1) and 10 microg/kg (Series 2). Left ventricular function was assessed by echocardiography (Series 1), and animals were randomized to receive either 5 cm H(2)O PEEP or zero PEEP at commencement of CPR and throughout resuscitation. Survival was defined as the presence of a spontaneous circulation 60 or 120 min (Series 2) after initial resuscitation.

RESULTS

There were no baseline differences between the groups. In Series 1, administration of 5 cm H(2)O PEEP (Fio(2) 1.0 and 0.21) was associated with improved survival compared with zero PEEP (7/9 and 6/6 vs 0/9, P < 0.01 and <0.001, respectively). Application of 5 cm H(2)O PEEP (Fio(2) 1.0) increased left ventricular end-diastolic area, systemic oxygenation, and functional residual capacity. Use of PEEP during CPR did not adversely affect left ventricular systolic function or arterial blood pressure. The outcome differences were not due to increased oxygenation because the rank order of survival was 5 cm H(2)O PEEP (Fio(2) 1.0) approximately 5 cm H(2)O PEEP (Fio(2) 0.21) > zero PEEP (Fio(2) 1.0), whereas the rank order of Pao(2) was 5 cm H(2)O PEEP (Fio(2) 1.0) > 5 cm H(2)O PEEP (Fio(2) 0.21) approximately zero PEEP (Fio(2) 1.0). In an additional series in which epinephrine 10 microg/kg was used (Series 2), the survival was 100% with no beneficial effects of PEEP.

CONCLUSION

In asphyxial cardiac arrest in a small rodent model, continuous application of PEEP (5 cm H(2)O) during and after CPR had beneficial effects on survival that were independent of oxygenation and without adverse cardiovascular effects.

[High PEEP vs. conventional PEEP in the acute respiratory distress syndrome: a systematic review and meta-analysis]

OBJECTIVE

To perform a systematic review and meta-analysis of the literature to evaluate the effects of high PEEP versus conventional PEEP on mortality and on the risk of barotrauma in patients with the acute respiratory distress syndrome (ARDS).

SOURCE OF DATA

Computer search of Medline, Embase, CINAHL, CANCERLIT, Pascal-Biomed, ACP Journal Club, Cochrane library (CDSR, DARE, CCTR), ISI Proceedings, Current Contents, and Web of Science, as well as manual search of selected references.

SELECTION OF STUDIES

Controlled random clinical trials published after NAECC (1994) that evaluated the effect of two levels of PEEP and that reported the mortality and incidence of barotrauma in the series.

DATA EXTRACTION

By two investigators working independently, with discrepancies resolved by group consensus. Contingency tables were elaborated and the RRs with corresponding confidence intervals were obtained for each study.

RESULTS

Four articles were selected for the meta-analysis of mortality and three for the meta-analysis of barotrauma. No effects of PEEP level on mortality were found (RR 0.73, 95% CI: 0.49 to 1.10) or on the incidence of barotrauma (RR 0.50, 95% CI: 0.14 to 1.73). However, an analysis of the studies in which PEEP was individualized in function of Pflex showed a significant decrease in mortality (RR 0.59, 95% CI: 0.43 to 0.82) (p=0.001)

CONCLUSIONS

The use of high or conventional PEEP in function of oxygenation does not affect mortality or the incidence of barotrauma in patients with ARDS. However, there might be a decrease in mortality associated to high PEEP individualized in function of the pulmonary mechanics of each patient.

Relationship between dynamic respiratory mechanics and disease heterogeneity in sheep lavage injury*

OBJECTIVE

Acute respiratory distress syndrome and acute lung injury are characterized by heterogeneous flooding/collapse of lung tissue. An emerging concept for managing these diseases is to set mechanical ventilation so as to minimize the impact of disease heterogeneity on lung mechanical stress and ventilation distribution. The goal of this study was to determine whether changes in lung mechanical heterogeneity with increasing positive end-expiratory pressure in an animal model of acute lung injury could be detected from the frequency responses of resistance and elastance.

DESIGN

Prospective, experimental study.

SETTING

Research laboratory at a veterinary hospital.

SUBJECTS

Female sheep weighing 48 +/- 2 kg.

INTERVENTIONS

In five saline-lavaged sheep, we acquired whole-lung computed tomography scans, oxygenation, static elastance, and dynamic respiratory resistance and elastance at end-expiratory pressure levels of 7.5-20 cm H2O.

MEASUREMENTS AND MAIN RESULTS

As end-expiratory pressure increased, computed tomography-determined alveolar recruitment significantly increased but was accompanied by significant alveolar overdistension at 20 cm H2O. An optimal range of end-expiratory pressures (15-17.5 cm H2O) was identified where alveolar recruitment was significantly increased without significant overdistension. This range corresponded to the end-expiratory pressure levels that maximized oxygenation, minimized peak-to-peak ventilation pressures, and minimized indexes reflective of the mechanical heterogeneity (e.g., frequency dependence of respiratory resistance and low-frequency elastance). Static elastance did not demonstrate any significant pressure dependence or reveal an optimal end-expiratory pressure level.

CONCLUSIONS

We conclude that dynamic mechanics are more sensitive than static mechanics in the assessment of the functional trade-off of recruitment relative to overdistension in a sheep model of lung injury. We anticipate that monitoring of dynamic respiratory resistance and elastance ventilator settings can be used to optimize ventilator management in acute lung injury.

Monitorización de la mecánica ventilatoria

Monitoring the mechanical properties (both static and dynamic) of the respiratory tract is essential for the diagnosis and prognosis of the disease causing any condition and to take the pertinent therapeutic decisions. These measurements can maintain a lung function in optimum conditions during mechanical ventilation and choose the best time to disconnect the patient. Until recently, monitoring the respiration of patients undergoing mechanical ventilation required sophisticated and complicated apparatuses or difficult techniques. Instrumentalization of the respiratory as a monitor has evolved in such a way that many respirators can graphically reproduce in real time the changes that occur in some variables of the respiratory cycle. Using these monitoring techniques improves knowledge of lung behavior of each patient with ARDS, thus facilitating its management. It may also improve decision-making in each individual patient, improving their prognosis.