Assisted spontaneous breathing during early acute lung injury

Lung Protective Ventilation in Brain-Injured Patients: Low Tidal Volumes or Airway Pressure Release Ventilation?

The optimal mode of mechanical ventilation for lung protection is unknown in brain-injured patients as this population is excluded from large studies of lung protective mechanical ventilation. Survey results suggest that low tidal volume (LTV) ventilation is the favored mode likely due to the success of LTV in other patient populations. Airway pressure release ventilation (APRV) is an alternative mode of mechanical ventilation that may offer several benefits over LTV in this patient population. APRV is an inverse-ratio, pressure-controlled mode of mechanical ventilation that utilizes a higher mean airway pressure compared with LTV. This narrative review compares both modes of mechanical ventilation and their consequences in brain-injured patients. Fears that APRV may raise intracranial pressure by virtue of a higher mean airway pressure are not substantiated by the available evidence. Primarily by virtue of spontaneous breathing, APRV often results in improvement in systemic hemodynamics and thereby improvement in cerebral perfusion pressure. Compared with LTV, sedation requirements are lessened by APRV allowing for more accurate neuromonitoring. APRV also uses an open loop system supporting clearance of secretions throughout the respiratory cycle. Additionally, APRV avoids hypercapnic acidosis and oxygen toxicity that may be especially deleterious to the injured brain. Although high-level evidence is lacking that one mode of mechanical ventilation is superior to another in brain-injured patients, several aspects of APRV make it an appealing mode for select brain-injured patients.

Time-varying respiratory system elastance: a physiological model for patients who are spontaneously breathing

BACKGROUND

Respiratory mechanics models can aid in optimising patient-specific mechanical ventilation (MV), but the applications are limited to fully sedated MV patients who have little or no spontaneously breathing efforts. This research presents a time-varying elastance (E(drs)) model that can be used in spontaneously breathing patients to determine their respiratory mechanics.

METHODS

A time-varying respiratory elastance model is developed with a negative elastic component (E(demand)), to describe the driving pressure generated during a patient initiated breathing cycle. Data from 22 patients who are partially mechanically ventilated using Pressure Support (PS) and Neurally Adjusted Ventilatory Assist (NAVA) are used to investigate the physiology relevance of the time-varying elastance model and its clinical potential. E(drs) of every breathing cycle for each patient at different ventilation modes are presented for comparison.

RESULTS

At the start of every breathing cycle initiated by patient, E(drs) is < 0. This negativity is attributed from the E(demand) due to a positive lung volume intake at through negative pressure in the lung compartment. The mapping of E(drs) trajectories was able to give unique information to patients' breathing variability under different ventilation modes. The area under the curve of E(drs) (AUCE(drs)) for most patients is > 25 cmH2Os/l and thus can be used as an acute respiratory distress syndrome (ARDS) severity indicator.

CONCLUSION

The E(drs) model captures unique dynamic respiratory mechanics for spontaneously breathing patients with respiratory failure. The model is fully general and is applicable to both fully controlled and partially assisted MV modes.

Low tidal volume pressure support versus controlled ventilation in early experimental sepsis in pigs

Background

In moderate acute respiratory distress syndrome (ARDS) several studies support the usage of assisted spontaneous breathing modes. Only limited data, however, focus on the application in systemic sepsis and developing lung injury. The present study examines the effects of immediate initiation of pressure support ventilation (PSV) in a model of sepsis-induced ARDS.

Methods

18 anesthetized pigs received a two-staged continuous lipopolysaccharide infusion to induce lung injury. The animals were randomly assigned to PSV or volume controlled (VCV) lung protective ventilation (tidal volume each 6 ml kg^-1, n = 2x9) over six hours. Gas exchange parameters, hemodynamics, systemic inflammation, and ventilation distribution by multiple inert gas elimination and electrical impedance tomography were assessed. The post mortem analysis included histopathological scoring, wet to dry ratio, and alveolar protein content.

Results

Within six hours both groups developed a mild to moderate ARDS with comparable systemic inflammatory response and without signs of improving gas exchange parameters during PSV. The PSV group showed signs of more homogenous ventilation distribution by electrical impedance tomography, but only slightly less hyperinflated lung compartments by multiple inert gas elimination. Post mortem and histopathological assessment yielded no significant intergroup differences.

Conclusions

In a porcine model of sepsis-induced mild ARDS immediate PSV was not superior to VCV. This contrasts with several experimental studies from non-septic mild to moderate ARDS. The present study therefore assumes that not only severity, but also etiology of lung injury considerably influences the response to early initiation of PSV.

Effects of sigh during pressure control and pressure support ventilation in pulmonary and extrapulmonary mild acute lung injury

INTRODUCTION

Sigh improves oxygenation and lung mechanics during pressure control ventilation (PCV) and pressure support ventilation (PSV) in patients with acute respiratory distress syndrome. However, so far, no study has evaluated the biological impact of sigh during PCV or PSV on the lung and distal organs in experimental pulmonary (p) and extrapulmonary (exp) mild acute lung injury (ALI).

METHODS

In 48 Wistar rats, ALI was induced by Escherichia coli lipopolysaccharide either intratracheally (ALIp) or intraperitoneally (ALIexp). After 24 hours, animals were anesthetized and mechanically ventilated with PCV or PSV with a tidal volume of 6 mL/kg, FiO2 = 0.4, and PEEP = 5 cmH2O for 1 hour. Both ventilator strategies were then randomly assigned to receive periodic sighs (10 sighs/hour, Sigh) or not (non-Sigh, NS). Ventilatory and mechanical parameters, arterial blood gases, lung histology, interleukin (IL)-1β, IL-6, caspase-3, and type III procollagen (PCIII) mRNA expression in lung tissue, and number of apoptotic cells in lung, liver, and kidney specimens were analyzed.

RESULTS

In both ALI etiologies: (1) PCV-Sigh and PSV-Sigh reduced transpulmonary pressure, and (2) PSV-Sigh reduced the respiratory drive compared to PSV-NS. In ALIp: (1) PCV-Sigh and PSV-Sigh decreased alveolar collapse as well as IL-1β, IL-6, caspase-3, and PCIII expressions in lung tissue, (2) PCV-Sigh increased alveolar-capillary membrane and endothelial cell damage, and (3) abnormal myofibril with Z-disk edema was greater in PCV-NS than PSV-NS. In ALIexp: (1) PSV-Sigh reduced alveolar collapse, but led to damage to alveolar-capillary membrane, as well as type II epithelial and endothelial cells, (2) PCV-Sigh and PSV-Sigh increased IL-1β, IL-6, caspase-3, and PCIII expressions, and (3) PCV-Sigh increased the number of apoptotic cells in the lung compared to PCV-NS.

CONCLUSIONS

In these models of mild ALIp and ALIexp, sigh reduced alveolar collapse and transpulmonary pressures during both PCV and PSV; however, improved lung protection only during PSV in ALIp.

Ventilatory strategies in septic patients. Results from a nationwide observational trial

BACKGROUND

Mortality in intensive care unit (ICU) patients is affected by multiple variables. The possible impact of the mode of ventilation has not yet been clarified; therefore, a secondary analysis of the "epidemiology of sepsis in Germany" study was performed. The aims were (1) to describe the ventilation strategies currently applied in clinical practice, (2) to analyze the association of the different modes of ventilation with mortality and (3) to investigate whether the ratio between arterial partial pressure of oxygen and inspired fraction of oxygen (PF ratio) and/or other respiratory variables are associated with mortality in septic patients needing ventilatory support.

METHODS

A total of 454 ICUs in 310 randomly selected hospitals participated in this national prospective observational 1-day point prevalence of sepsis study including 415 patients with severe sepsis or septic shock according to the American College of Chest Physicians/Society of Critical Care Medicine criteria.

RESULTS

Of the 415 patients, 331 required ventilatory support. Pressure controlled ventilation (PCV) was the most frequently used ventilatory mode (70.6 %) followed by assisted ventilation (AV 21.7 %) and volume controlled ventilation (VCV 7.7 %). Hospital mortality did not differ significantly among patients ventilated with PCV (57 %), VCV (71 %) or AV (51 %, p=0.23). A PF ratio equal or less than 300 mmHg was found in 83.2 % of invasively ventilated patients (n=316). In AV patients there was a clear trend to a higher PF ratio (204±70 mmHg) than in controlled ventilated patients (PCV 179±74 mmHg, VCV 175±75 mmHg, p=0.0551). Multiple regression analysis identified the tidal volume to pressure ratio (tidal volume divided by peak inspiratory airway pressure, odds ratio OR=0.94, 95 % confidence interval 95% CI=0.89-0.99), acute renal failure (OR=2.15, 95% CI=1.01-4.55) and acute physiology and chronic health evaluation (APACHE) II score (OR=1.09, 95% CI=1.03-1.15) but not the PF ratio (univariate analysis OR=0.998, 95 % CI=0.995-1.001) as independent risk factors for in-hospital mortality.

CONCLUSIONS

This representative survey revealed that severe sepsis or septic shock was frequently associated with acute lung injury. Different ventilatory modes did not affect mortality. The tidal volume to inspiratory pressure ratio but not the PF ratio was independently associated with mortality.

Effects of controlled and pressure support mechanical ventilation on rat diaphragm muscle

PURPOSE

The objective of this study was to analyze the effects of Pressure Controlled Ventilation mode (PCV-C) and PSV mode in diaphragm muscle of rats.

METHODS

Wistar rats (n=18) were randomly assigned to the control group or to receive 6 hours of PCV and PSV. After this period, animals were euthanized and their diaphragms were excised, frozen in liquid nitrogen and stored in at -80º C for further histomorphometric analysis.

RESULTS

Results showed a 15% decrease in cross-sectional area of muscle fibers on the PCV-C group when compared to the control group (p<0.001) and by 10% when compared to the PSV group (p<0.05). Minor diameter was decreased in PCV-C group by 9% when compared with the control group (p<0.001) and by 6% when compared to the PSV group (p<0.05). When myonuclear area was analyzed, a 16% decrease was observed in the PCV-C group when compared to the PSV group (p<0.05). No significant difference between the groups was observed in myonuclear perimeter (p>0.05).

CONCLUSION

Short-term controlled mechanical ventilation seems to lead to muscular atrophy in diaphragm fibers. The PSV mode may attenuate the effects of VIDD.

Clinical Course of ICU Patients With Severe Pandemic 2009 Influenza A (H1N1) Pneumonia

Received June 10, 2010, and in revised form July 26, 2010. Accepted for publication August 12, 2010. Background: A number of different modalities have been employed in addition to conventional ventilation to improve oxygenation in patients with severe 2009 pandemic influenza A (H1N1) pneumonia. Outcomes with ventilatory and rescue therapies for H1N1 influenza-related acute respiratory distress syndrome (ARDS) have been varied. 1 – 6 A single intensive care unit (ICU) experience with management of laboratory-confirmed 2009 pandemic influenza A (H1N1) ARDS with a combination of proning and airway pressure release ventilation (APRV) is described. Methods: A retrospective review of medical records of ICU patients seen at Utah Valley Regional Medical Center during the first and second waves of the H1N1 influenza pandemic was done. Results: Fourteen ICU patients were managed with invasive ventilation for 2009 pandemic influenza A (H1N1)-related ARDS. Hypoxemia refractory to conventional ventilation was noted in 11 of 14 patients despite application of APRV. Following proning in patients on APRV, improvement of hypoxemia and hemodynamic status was achieved. Only 2 of 11 patients on APRV and proning required continuous dialysis. Mortality in intubated patients receiving a combination of proning and APRV was 27.3% (3/11) with 2 of these dying during the first wave of the H1N1 influenza pandemic. In all, 3 of 11 patients on proning and APRV underwent tracheostomy, with 2 of these undergoing tube thoracostomy. ARDSnet fluid-conservative protocol was safely tolerated in 8 of 11 of the intubated patients following initiation of proning and APRV. Conclusions: Proning in combination with APRV provides improvement of hypoxemia with limitation of end-organ dysfunction and thereby facilitates recovery from severe 2009 pandemic influenza A (H1N1).

Performance of bedside transpulmonary thermodilution monitoring for goal-directed hemodynamic management after subarachnoid hemorrhage

BACKGROUND AND PURPOSE

Early goal-directed hemodynamic therapy is of particular importance for adequate cerebral circulation of patients with vasospasm after subarachnoid hemorrhage but is often precluded by the invasiveness of established cardiac output determination using a pulmonary artery catheter. This study was undertaken to validate the usefulness of less invasive goal-directed hemodynamic monitoring by transpulmonary thermodilution technique in patients after subarachnoid hemorrhage.

METHODS

One hundred sixteen patients with subarachnoid hemorrhage who underwent surgical clipping within 24 hours of ictus were investigated. Validation of transpulmonary thermodilution-derived intermittent/continuous cardiac output and cardiac preload (global end diastolic volume) were compared with pulmonary artery catheter-derived reference cardiac output and pulmonary capillary wedge pressure or central venous pressure in 16 patients diagnosed with vasospasm. In a subsequent trial of 100 consecutive cases, clinical results between the new and standard management paradigms were compared.

RESULTS

Transpulmonary thermodilution-derived intermittent cardiac output and transpulmonary thermodilution-derived continuous cardiac output showed close agreement to catheter-derived reference cardiac output with high correlation (r=0.85 and 0.77) and low percentage error (13.5% and 18.0%). Fluid responsiveness to defined volume loading was predicted better with global end diastolic volume than with pulmonary capillary wedge pressure and central venous pressure for larger receiver operating characteristic curve area. Patients receiving early goal-directed management by transpulmonary thermodilution experienced reduced frequencies of vasospasm and cardiopulmonary complications compared with those managed with standard therapy (P<0.05), whereas their functional outcomes at 3 months were not different (P=0.06).

CONCLUSIONS

Goal-directed hemodynamic management guided by transpulmonary thermodilution appears to have a therapeutic advantage for optimizing the prognosis of patients with subarachnoid hemorrhage with vasospasm over conventional methods.

Pressure support ventilation attenuates ventilator-induced protein modifications in the diaphragm

Introduction

Controlled mechanical ventilation (CMV) induces profound modifications of diaphragm protein metabolism, including muscle atrophy and severe ventilator-induced diaphragmatic dysfunction. Diaphragmatic modifications could be decreased by spontaneous breathing. We hypothesized that mechanical ventilation in pressure support ventilation (PSV), which preserves diaphragm muscle activity, would limit diaphragmatic protein catabolism.

Methods

Forty-two adult Sprague-Dawley rats were included in this prospective randomized animal study. After intraperitoneal anesthesia, animals were randomly assigned to the control group or to receive 6 or 18 hours of CMV or PSV. After sacrifice and incubation with ¹⁴C-phenylalanine, in vitro proteolysis and protein synthesis were measured on the costal region of the diaphragm. We also measured myofibrillar protein carbonyl levels and the activity of 20S proteasome and tripeptidylpeptidase II.

Results

Compared with control animals, diaphragmatic protein catabolism was significantly increased after 18 hours of CMV (33%, P = 0.0001) but not after 6 hours. CMV also decreased protein synthesis by 50% (P = 0.0012) after 6 hours and by 65% (P < 0.0001) after 18 hours of mechanical ventilation. Both 20S proteasome activity levels were increased by CMV. Compared with CMV, 6 and 18 hours of PSV showed no significant increase in proteolysis. PSV did not significantly increase protein synthesis versus controls. Both CMV and PSV increased protein carbonyl levels after 18 hours of mechanical ventilation from +63% (P < 0.001) and +82% (P < 0.0005), respectively.

Conclusions

PSV is efficient at reducing mechanical ventilation-induced proteolysis and inhibition of protein synthesis without modifications in the level of oxidative injury compared with continuous mechanical ventilation. PSV could be an interesting alternative to limit ventilator-induced diaphragmatic dysfunction.