Two methods of setting positive end-expiratory pressure in acute lung injury: an experimental computed tomography volumetric study

Individualized positive end-expiratory pressure following alveolar recruitment manoeuvres in lung-healthy anaesthetized dogs: a randomized clinical trial on early postoperative arterial oxygenation

OBJECTIVE

To assess and compare the effect of intraoperative stepwise alveolar recruitment manoeuvres (ARMs), followed by individualized positive end-expiratory pressure (PEEP), defined as PEEP at maximal respiratory system compliance + 2 cmH₂O (PEEP_maxCrs+2), with that of spontaneous ventilation (SV) and controlled mechanical ventilation (CMV) without ARM or PEEP on early postoperative arterial oxygenation in anaesthetized healthy dogs.

STUDY DESIGN

Prospective, randomized, nonblinded clinical study.

ANIMALS

A total of 32 healthy client-owned dogs undergoing surgery in dorsal recumbency.

METHODS

Dogs were ventilated intraoperatively (inspired oxygen fraction: 0.5) with one of the following strategies: SV, CMV alone, and CMV with PEEP_maxCrs+2 following a single ARM (ARM1) or two ARMs (ARM2, the second ARM at the end of surgery). Arterial blood gas analyses were performed before starting the ventilatory strategy, at the end of surgery, and at 5, 10, 15, 30 and 60 minutes after extubation while breathing room air. Data were analysed using Kruskal-Wallis and Friedman tests (p < 0.050).

RESULTS

At any time point after extubation, PaO₂ was not significantly different between groups. At 5 minutes after extubation, PaO₂ was 95.1 (78.1-104.0), 93.8 (88.3-104.0), 96.9 (86.6-115.0) and 89.1 (87.6-102.0) mmHg in the SV, CMV, ARM1 and ARM2 groups, respectively. PaO₂ decreased at 30 minutes after extubation in the CMV, ARM1 and ARM2 groups (p < 0.050), but it did not decrease after 30 minutes in the SV group. Moderate hypoxaemia (PaO₂, 60-80 mmHg) was observed in one dog in the ARM1 group and two dogs each in the SV and ARM2 groups.

CONCLUSIONS AND CLINICAL RELEVANCE

Intraoperative ARMs, followed by PEEP_maxCrs+2, did not improve early postoperative arterial oxygenation compared with SV or CMV alone in healthy anaesthetized dogs. Therefore, this ventilatory strategy might not be clinically advantageous for improving postoperative arterial oxygenation in healthy dogs undergoing surgery when positioned in dorsal recumbency.

Study of Tidal Volume and Positive End-Expiratory Pressure on Alveolar Recruitment Using Spiro Dynamics in Mechanically Ventilated Patients

Background and Aims

Ventilator setting in the intensive care unit patients is a topic of debate and setting of tidal volume (TV) should be patient-specific based on lung mechanics. In this study, we have evaluated to develop optimal ventilator strategies through continuous and thorough monitoring of respiratory mechanics during ongoing ventilator support to prevent alveolar collapse and alveolar injury in mechanically ventilated patients.

Methods

In our monocentric, randomized, observational study, we had recruited 60 patients and divided them into two groups of 30 each. In Group 1 patients, TV and positive end-expiratory pressure (PEEP) were set according to pressure-volume (P/V) curve obtained by the mechanical ventilator in a conventional manner (control group), and in Group 2, TV and PEEP were set according to P/V curve obtained by the mechanical ventilator using intratracheal catheter. PEEP and TV were set accordingly. TV, PEEP, and PaO₂/FiO₂ (P/F) ratio at days 1, 3, and 7, mortality within 7 days and mortality within 28 days were measured in each group and compared.

Results

We found a significant difference between PEEP and P/F ratio in both groups while intragroup comparison at days 1, 3, and 7. After the intergroup comparison of Group 1 and 2, we observed a significant difference of PEEP and P/F ratio between the groups at day 7 and not on day 1 or 3.

Conclusion

This study concludes that optimal PEEP is more accurate using an intratracheal catheter than the conventional method of deciding ventilator setting. Hence, it is recommended to use intratracheal catheter to obtain more accurate ventilator settings.

Effect of recumbency and body condition score on open-lung positive end-expiratory pressure and respiratory system compliance following a stepwise lung recruitment manoeuvre in healthy dogs during general anaesthesia

The aim was to assess the effects of recumbency and body condition score (BCS) on open-lung positive end-expiratory pressure (OL-PEEP) and quasistatic respiratory system compliance (Crs) following stepwise lung recruitment manoeuvre (RM) in healthy dogs under general anaesthesia. Thirty-four dogs were anaesthetised and mechanically ventilated (tidal volume of 10 mL/kg) without PEEP for 1 min (baseline). A stepwise RM was then performed and the individual OL-PEEP was subsequently applied. The Crs was registered at baseline and every 10-min for 50 min after RM. Dogs were classified into either dorsal or lateral recumbency groups, and as normal (score 4-5/9) or high (≥6/9) BCS groups. The OL-PEEP was higher in lateral than in dorsal recumbency (P = .002), but differences were not observed between normal and high BCS (P = .865). The Crs was increased from baseline at all time points after RM in all groups. The Crs did not differ between dorsally and laterally recumbent dogs at any time point. However, the baseline Crs was significantly lower in dogs with a high BCS than in those with a normal BCS (P < .001); therefore, the absolute change from baseline was considered when comparing Crs after the RM and it was similar in both BCS groups. In conclusion, in anaesthetised healthy dogs the OL-PEEP following RM was lower when dogs were positioned in dorsal than in lateral recumbency. The Crs after RM remained unchanged regardless of the dogs' recumbency. A stepwise RM followed by OL-PEEP could compensate for the potential negative impact of moderately increased BCS on Crs.

Evaluation of stroke volume variation and pulse pressure variation as predictors of fluid responsiveness in patients undergoing protective one-lung ventilation

In order to investigate whether the hemodynamic indices, including stroke volume variation (SVV) and pulse pressure variation (PPV) could predict fluid responsiveness in patients undergoing protective one-lung ventilation. 60 patients scheduled for a combined thoracoscopic and laparoscopic esophagectomy were enrolled and randomized into two groups. The patients in the protective group (Group P) were ventilated with a tidal volume of 6 mL/kg, an inspired oxygen fraction (FiO2) of 80%, and a positive end expiratory pressure (PEEP) of 5 cm H2O. Patients in the conventional group (Group C) were ventilated with a tidal volume of 8 mL/kg and a FiO2 of 100%. Dynamic variables were collected before and after fluid loading (7 mL/kg hydroxyethyl starch 6%, 0.4 mL/kg/min). Patients whose stroke volume index (SVI) increased by more than 15% were defined as responders. Data collected from 45 patients were finally analyzed. Twelve of 24 patients in Group P and 10 of 21 patients in Group C were responders. SVV and PPV significantly changed after the fluid loading. The receive operating characteristic (ROC) analysis showed that the thresholds for SVV and PPV to discriminate responders were 8.5% for each, with a sensitivity of 66.7% (SVV) and 75% (PPV) and a specificity of 50% (SVV) and 83.3% (PPV) in Group P. However, the thresholds for SVV and PPV were 8.5% and 7.5% with a sensitivity of 80% (SVV) and 90% (PPV) and a specificity of 70% (SVV) and 80% (PPV) in Group C. We found SVV and PPV could predict fluid responsiveness in protective one-lung ventilation, but the accuracy and ability of SVV and PPV were weak compared with the role they played in a conventional ventilation strategy.

Effects of different tidal volume ventilation on paraquat-induced acute lung injury in piglets

Background

The aim of this study was to explore the effects of different tidal volume (V_T) ventilation on paraquat-induced acute lung injury or acute respiratory distress syndrome (ALI/ARDS) in piglets.

Material/Methods

We developed ALI/ARDS models in piglets by intraperitoneal injection of paraquat (PQ). The piglets were randomly divided into three groups: small V_T group (V_T=6 ml/kg, n=6), middle V_T group (V_T=10 ml/kg, n=6), and large V_T group (V_T=15 ml/kg, n=6), with the positive end-expiratory pressure (PEEP) set as 10 cmH₂O. The hemodynamics were monitored by pulse-indicated continuous cardiac output (PiCCO) and the airway pressure changes and blood gas analysis indexes were recorded at different time points. The pathological changes were observed by lung puncture.

Results

The piglets showed ALI/ARDS in 4.5±0.8 hours after PQ intraperitoneal injection. PH, PaO₂ and oxygenation indexes in the three groups all decreased after modeling success compared with baseline, and PaCO₂ increased significantly. PH in the small VT group decreased most obviously after ventilation for 6 hours. PaO₂ and oxygenation indexes in the small VT group showed the most obvious increase after ventilation for 2 hours and were much higher than the other two groups after ventilation for 6 hours. PaCO₂ increased gradually after mechanical ventilation and the small VT group showed most obvious increase. The ELWI increased obviously after ventilation for 2 hours and then the small VT group clearly decreased. PIP and plateau pressure (Pplat) in the small VT group decreased gradually and in the middle and large VT group they increased after ventilation. The lung histopathology showed that the large VT group had the most severe damage and the small VT group had only minimal damage.

Conclusions

Small tidal volume ventilation combined with PEEP could alleviate the acute lung injury induced by paraquat and improve oxygenation.

Positive end-expiratory pressure

PURPOSE OF REVIEW

In the last 2 years, several reports have dealt with recruitment/positive end-expiratory pressure (PEEP) selection. Most of them confirm previous results and few add new information.

RECENT FINDINGS

It has been definitely confirmed that opening pressures are different throughout the acute respiratory distress syndrome lung parenchyma, ranging from 5-10 up to 30-40 cmH2O. The highest opening pressures are required to open the most dependent lung regions. It has been found that in 2 s, most of the recruitable lung regions may be open when a proper pressure is applied. The best way to assess recruitment is computed tomography scanning, whereas lung mechanics are a reasonable bedside surrogate. Impedance tomography has been increasingly tested, whereas gas exchange is the less reliable indicator of recruitment. A large outcome study showed that higher PEEP might provide survival benefit in a subgroup of more severe patients as compared with lower PEEP. To set PEEP in each individual patient, the use of the expiratory limb of the pressure-volume curve has been suggested. Setting PEEP according to transpulmonary pressure has a robust physiological background, although it requires confirmatory study.

SUMMARY

Indiscriminate application of recruitment maneuver in unselected acute respiratory distress syndrome population does not provide benefits. However, in the most severe patients, recruitment maneuver has to be considered and higher PEEP applied. To individualize PEEP, the expiratory phase has to be considered, and the esophageal pressure measurement to compute the transpulmonary pressure should be progressively introduced in clinical practice.