Comparison between fluctuating PEEP and conventional PEEP in dogs with lung injury induced by blood aspiration

It has been documented that in some patients with acute hypoxic respiratory failure the application of positive end-expiratory pressure (PEEP) may produce no improvement or even a deterioration of pulmonary oxygenation due to an increase in ventilation-perfusion mismatching. Fluctuating PEEP (F-PEEP) is a newly developed PEEP in which end-expiratory pressure (EEP) is periodically changed within a certain range. In a dog model with localized lung injury induced by the aspiration of non-heparinized blood (2 ml.kg body weight-1), F-PEEP in which the EEP was periodically changed from 0.5 to 1.5 kPa at frequencies of 10 min, and conventional PEEP with 3 different fixed EEPs, 0.5, 1.0 and 1.5 kPa (C-PEEP0.5, C-PEEP1.0 and C-PEEP1.5) were each applied for 60 min. F-PEEP produced a periodical change in PaO2 and hemodynamic variables including cardiac output, and in comparison with C-PEEP0.5, C-PEEP1.0 and C-PEEP1.5, a significantly greater improvement of A-aDO2 and dynamic compliance with relatively large cardiac output in the low EEP phase. These results suggest that F-PEEP is a useful mode of artificial ventilation for treating some kinds of acute hypoxic respiratory failure due to increased ventilation-perfusion mismatching.     

Fluctuating PEEP versus conventional PEEP in diffuse and unilateral lung injury induced by oleic acid

Effects of fluctuating positive end-expiratory pressure (F-PEEP), in which end-expiratory pressure (EEP) was periodically changed from 0.5 to 1.5 kPa with a periodic time of 6 min, and conventional PEEP (C-PEEP) with a fixed EEP of 1.0 kPa, were comparatively studied in diffuse (Group I) and unilaterally dominant lung injury (Group II). Although F-PEEP produced cyclic alterations of PaO2 in both groups, PaO2 changed in proportion to EEP in Group I and in reciprocal proportion to EEP in Group II. There was no significant difference between PaO2 and QS/QT during F-PEEP and those during C-PEEP in Group I, whereas in Group II, F-PEEP produced a significantly greater improvement of pulmonary oxygenation at the low EEP phase than C-PEEP. In both groups, the degree of hemodynamic depression was proportional to EEP. These results suggest that F-PEEP should be indicated for acute hypoxic respiratory failure with uneven distribution of lung injury.     

Differential Lung Ventilation with Unilateral PEEP Following Unilateral Hydrochloric Acid Aspiration in the Dog

Differential lung ventilation with positive end expiratory pressure (PEEP) improves pulmonary gas exchange when used in the supportive care of patients with severe unilateral or asymmetrical lung disease. Once the provision of selective PEEP to the two lungs is accomplished, the best method of partitioning the tidal volume between the two lungs is unknown. Twelve mongrel dogs were given a unilateral hydrochloric acid (HCl) aspiration injury. A computer controlled differential lung ventilation system was used to ventilate four dogs with equal volumes to each lung, four dogs with equal driving pressure (end inspiratory pressure-PEEP) to each lung, and four dogs with equal end-tidal CO₂ fraction from each lung. The respiratory rate was feedback controlled to maintain Paco₂ at 4.67 kPa. The dogs were kept supine and ventilated with 30% O₂. Following injury, the PEEP was set at 0 kPa for 1 h. The dogs were then given 1.36 kPa and 2.72 kPa PEEP to the injured lung for 2 h in a cross-over fashion. The assignment of the tidal volume controller, the side of injury, and the PEEP sequence was random. Oxygen tension fell and pulmonary venous admixture increased after giving the HCl injury. In all three groups considered simultaneously, unilateral PEEP improved Pao₂ and venous admixture. The equal tidal volume distribution was the only group to show a significant improvement in Pao₂ at both PEEP increments (0 to 1.36 kPa and 2.72 kPa). There was a significant difference in tidal volume allocation between the three groups with the equal end-tidal and equal pause pressure groups only minimally ventilating the injured lung. With differential lung ventilation and unilateral PEEP, equal partitioning of tidal volume provides the highest Pao₂, compared to the other two methods of partitioning tidal volume.     

Effect of Selective PEEP on Pulmonary Blood Flow following Unilateral Hydrochloric Acid Aspiration in the Dog

We investigated the effect of unilateral or bilateral positive end-expiratory pressure (PEEP) on pulmonary perfusion in 12 dogs with a hydrochloric acid aspiration injury of the left lung. The lungs were ventilated separately and PEEP was applied to the left lung at 10 cmH2O (1.0 kPa) in six and at 15 cmH2O (1.5 kPa) in six others. Measurements of the right and left pulmonary arterial blood flows (QR and QL) and venous admixture were made before, during and after PEEP. After this study, 5 and 10 cmH2O (0.5 and 1.0 kPa) PEEP were applied to both lungs in six dogs and measurements were repeated. Following the application of PEEP to the left lung, a significant decrease in QL and increase in QR were observed. However, the application of PEEP to both lungs was followed by significant decreases in both QL and QR. The cardiac output decreased slightly during unilateral PEEP and markedly during bilateral PEEP. The venous admixture decreased significantly during PEEP in all the groups. These findings indicate that selective PEEP causes a transfer of pulmonary blood flow from the injured lung to the normal lung, improving ventilation-perfusion inequality, and improves gas exchange without impeding oxygen delivery.     

Delayed derecruitment after removal of PEEP in patients with acute lung injury

Background: A step decrease in positive end-expiratory airway pressure (PEEP) is not followed by an instantaneous loss of the PEEP-induced increase in end-expiratory lung volume (EELV). Rather, the reduction of EELV is delayed, while adverse PEEP effects on hemodynamics are immediately attenuated upon the drop in airway pressure. Step PEEP increments were applied to the lungs of patients with acute lung injury. It was investigated retrospectively whether enlargement of end-expiratory lung volume and changes in lung mechanics persist 45 min after removal of the PEEP increment.
Methods: In 14 patients with acute lung injury (LIS score 2.7) EELV and volume-dependent dynamic compliance of the respiratory system (C_dyn,rs) were determined 45 min after removal of an additional PEEP increment (0.64 kPa added to baseline PEEP of 1.0 kPa).
Results: Nine patients kept an EELV gain of 13% (SD 7) and showed improved C_dyn,rs. In 5 patients, EELV was reduced (by 9% (SD 6)) and C_dyn,rs unchanged after removal of the PEEP increment compared to baseline.
Conclusion: A subgroup of patients with acute lung injury, the characteristics of which remain to be defined, benefit from prolonged recruitment effects up to 45 min after removal of a PEEP increment, while sequelae of continuously increased airway pressures are minimised.     

Lateral Positioning With Differential Lung Ventilation and Unilateral PEEP Following Unilateral Acid Aspiration in the Dog

Body position can significantly alter the efficiency of gas exchange following unilateral lung injury. We systematically examined three positions during differential lung ventilation with unilateral positive end-expiratory pressure (PEEP) following unilateral hydrochloric acid aspiration in the dog. Twelve mongrel dogs were intubated with a double-lumen endobronchial tube and mechanically ventilated with a microcomputer-controlled pair of ventilators. A tidal volume of 7.5 ml/kg was delivered to each lung. The PaCO2 was maintained at 4.67 kPa. A unilateral injury was induced with an injection of 0.1 N hydrochloric acid (2.5 ml/kg) into one lumen of the endobronchial tube. 0.984 kPa PEEP was applied to the injured lung and the dogs were placed sequentially in one of three positions (supine, lateral decubitus with injured lung non-dependent, and lateral decubitus with injured lung dependent) for 1 h apiece. There was no significant difference between the three positions with regard to PaO2 (F (2, 10) = 1.60, P = 0.25) of venous admixture (F (2, 10) = 0.49, P = 0.63). Our data indicated that position did not alter oxygenation. This was probably due to the use of differential ventilation with unilateral PEEP which eliminated redistribution of ventilation between the two lungs and minimized position-dependent changes in pulmonary blood flow.     

Selective PEEP in Acute Bilateral Lung Disease. Effect on Patients in the Lateral Posture

Seven patients with acute respiratory failure due to diffuse and fairly uniform lung disease were studied during mechanical ventilation in the lateral decubital position with: (a) zero end-expiratory pressure (ZEEP) through a double-lumen oro-bronchial tube to permit a recording of the ventilation to each lung; (b) bilateral positive end-expiratory pressure (PEEP) of 1.2 kPa, with maintenance of ventilation distribution between lungs as observed during ZEEP; (c) selective PEEP of 1.2 kPa, applied to the dependent lung only, with ventilation as during ZEEP; and (d) conventional PEEP of 1.2 kPa applied to both lungs through a single-lumen tube, with free distribution of ventilation between the lungs. During ZEEP, 69% of ventilation was distributed to the non-dependent and 31% to the dependent lung; cardiac output was 6.51 X min-1, venous admixture (QS/QT) 40% and arterial oxygen tension (PaO2) 8.3 kPa. With bilateral PEEP, functional residual capacity (FRC) increased by 0.331, cardiac output was reduced to 5.11 X min-1 and venous admixture to 32%. PaO2 increased to 10.1 kPa. With selective PEEP the dependent lung FRC increased by 0.211 and the FRC of the non-dependent lung decreased by 0.081. Cardiac output increased to 6.11 X min-1, which was no longer significantly different from that during ZEEP. Venous admixture remained at the same level as with bilateral PEEP.(ABSTRACT TRUNCATED AT 250 WORDS)     

Alveolar recruitment strategy and PEEP improve oxygenation,dynamic compliance of respiratory system and end‐expiratory lung volume in pediatric patients undergoing cardiac surgery for congenital heart disease

Objective: Optimizing alveolar recruitment by alveolar recruitment strategy (ARS) and maintaining lung volume with adequate positive end‐expiratory pressure (PEEP) allow preventing ventilator‐induced lung injury (VILI). Knowing that PEEP has its most beneficial effects when dynamic compliance of respiratory system (Crs) is maximized, we hypothesize that the use of 8 cm H₂O PEEP with ARS results in an increase in Crs and end‐expiratory lung volume (EELV) compared to 8 cm H₂O PEEP without ARS and to zero PEEP in pediatric patients undergoing cardiac surgery for congenital heart disease. Methods: Twenty consecutive children were studied. Three different ventilation strategies were applied to each patient in the following order: 0 cm H₂O PEEP, 8 cm H₂O PEEP without an ARS, and 8 cm H₂O PEEP with a standardized ARS. At the end of each ventilation strategy, Crs, EELV, and arterial blood gases were measured. Results: EELV, Crs, and P_aO₂/FiO₂ ratio changed significantly (P < 0.001) with the application of 8 cm H2O + ARS. Mean P_aCO₂– PETCO₂ difference between 0 PEEP and 8 cm H2O PEEP + ARS was also significant (P < 0.05). Conclusion: An alveolar recruitment strategy with relative high PEEP significantly improves Crs, oxygenation, P_aCO₂– PETCO₂ difference, and EELV in pediatric patients undergoing cardiac surgery for congenital heart disease.     

Hemodynamic changes during laparoscopy with positive end–expiratory pressure ventilation

Hemodynamic measurements were performed in 10 healthy women undergoing elective laparoscopy for the investigation of infertility. A standardized anesthetic technique which included the application of positive end-expiratory pressure (PEEP), 0.49 kPa (3.7 mmHg) was utilized. The following variables were studied: cardiac output, stroke volume and left ventricular ejection time (determined non-invasively with impedance cardiography), heart rate, blood pressure, total peripheral vascular resistance and end-tidal carbon dioxide (ET-CO2). The combination of 25 degrees head-down tilt and PEEP ventilation during laparoscopy was associated with a pressure response that restored arterial pressures to essentially pre-anesthetic levels. Net cardiac effects were small. With this regime low pressure 0.7-1.1 kPa (5-8 mmHg) intra-abdominal insufflation with CO2 was associated with only minor cardiovascular changes. There were no indications that 0.49 kPa PEEP during laparoscopy produced adverse cardiovascular effects. The application of PEEP reduced (P less than 0.001) ET-CO2. There was no net increase in ET-CO2 after CO2-insufflation compared to the measurement after induction of anesthesia. This is in contrast to earlier studies without PEEP where a significant net increase in ET-CO2 was reported after CO2-insufflation.     

Development and Evaluation of a Flow-Dividing Unit for Differential Ventilation and Selective PEEP

Differential ventilation with selective positive end-expiratory pressure (PEEP) was studied in a two-compartment lung model, using one ventilator and a flow-dividing unit consisting of inspiratory flow resistors and an inspiratory threshold valve. The compliance of each lung compartment was varied between 0.15 and 0.23 1 X kPa-1 and the resistance was varied from 0 to 3.5 kPa X 1(-1) X s. The minute volume was 12 1 and the respiratory frequency 12/min, with an inspiratory:expiratory ratio of 1:2. An even distribution of ventilation to the two lung compartments was obtained with the inspiratory flow resistors or the threshold valve under all conditions studied. However, a stepwise increase in the inspiratory resistance of one lung compartment from 1.0 to 2.5 or from 2.5 to 3.5 kPa X 1(-1) X s required readjustment of the inspiratory flow resistor to achieve an even distribution of ventilation, whereas the inspiratory threshold valve needed no readjustment. Large differences in the inspiratory impedance of the two lung compartments caused asynchronous gas delivery when the ventilation distribution was adjusted by means of the flow resistors. Use of the threshold valve resulted in synchronous gas delivery. The flow-dividing unit consists of non-active elements and can thus be connected to any ventilator.     

Effects of PEEP on extra vascular lung water and central blood volume in the dog

Twenty-four mongrel dogs were anaesthetized and ventilated mechanically in the supine position. Extravascular lung water (EVLW) and central blood volume (CBV) were measured with a double indicator (dye/cold) dilution technique. Both indicators were detected intravascularly in the aortic root with a fibreoptic thermistor catheter. Seven dogs ventilated with a positive end-expiratory pressure (PEEP) of 1.0 kPa (10 cmH2O) for a short period of time (less than 20 min) displayed no significant change in EVLW as measured with the indicator dilution technique (= EVLWi), while reductions were seen in both CBV (15%, P less than 0.01) and cardiac output (CO-thermodilution technique) (10%, P less than 0.05). Another seven dogs ventilated with a PEEP of 1.0 kPa for 8 h showed a gradual increase in EVLWi. After 8 h, a mean increase of 34% (P less than 0.01) was recorded, and the increase was also verified by post-mortem gravimetric determination of EVLW (= EVLWg), displaying an increase of 61% (P less than 0.01). In five dogs ventilated with zero end-expiratory pressure (ZEEP) for 8 h, no changes in EVLWi, CO, and CBV were observed, and EVLWg was mean 4.39 g/kg body weight (BW). Five additional dogs were sacrificed after 15 min of anaesthesia without catheterization and EVLWg was found to be 4.24 g/kg BW. It is concluded that EVLWi does not change measurably during ZEEP or short periods of PEEP. However, long periods (8 h) of PEEP result in elevated EVLWi values. Gravimetry supports these conclusions.(ABSTRACT TRUNCATED AT 250 WORDS)     

Both lung recruitment maneuver and PEEP are needed to increase oxygenation and lung volume after cardiac surgery

BACKGROUND: Patients ventilated after cardiac surgery commonly have impaired oxygenation, mainly due to lung collapse. We have previously found that PaO2 and end-expiratory lung volume (EELV) were increased by a lung recruitment maneuver (LRM) followed by positive end-expiratory pressure (PEEP). The aim of this study was to evaluate whether only PEEP or only a LRM could give similar effects. METHODS: Thirty circulatory stable patients (aged 55-79 years) mechanically ventilated after cardiac surgery were randomized to receive LRM (four 10-s insufflations to an airway pressure of 45 cmH2O) and zero end-expiratory pressure (LRM-group), PEEP 12 cmH2O (PEEP-group) or LRM in combination with PEEP 12 cmH2O (LRM + PEEP-group). The set end-expiratory pressure was kept for 75 min. Before, during and after the intervention, EELV (SF6 washout technique) and blood gases were measured. RESULTS: Initial EELV and PaO2 were similar in all groups. In the LRM-group, PaO2 and EELV increased transiently (P < 0.0001), but returned at 5 min to the initial values. In the PEEP-group, PaO2 did not change but EELV increased to 155 +/- 27% of the initial value (P < 0.0001). In the LRM+PEEP-group, PaO2 and EELV increased to 212 +/- 66% and 178 +/- 31% of the initial values (P < 0.0001), respectively, and were maintained during PEEP application. CONCLUSION: In patients ventilated after cardiac surgery: (1) PEEP increased lung volume but not PaO2, (2) a lung recruitment maneuver without subsequent PEEP had no sustained effect, and (3) both a lung recruitment maneuver and PEEP were needed to increase and maintain the increased lung volume and PaO2.     

Effect of PEEP and inhaled nitric oxide on pulmonary gas exchange during gaseous and partial liquid ventilation with small volumes of perfluorocarbon

BACKGROUND: Partial liquid ventilation, positive end-expiratory pressure (PEEP) and inhaled nitric oxide (NO) can improve ventilation/perfusion mismatch in acute lung injury (ALI). The aim of the present study was to compare gas exchange and hemodynamics in experimental ALI during gaseous and partial liquid ventilation at two different levels of PEEP, with and without the inhalation of nitric oxide. METHODS: Seven pigs (24+/-2 kg BW) were surfactant-depleted by repeated lung lavage with saline. Gas exchange and hemodynamic parameters were assessed in all animals during gaseous and subsequent partial liquid ventilation at two levels of PEEP (5 and 15 cmH2O) and intermittent inhalation of 10 ppm NO. RESULTS: Arterial oxygenation increased significantly with a simultaneous decrease in cardiac output when PEEP 15 cmH2O was applied during gaseous and partial liquid ventilation. All other hemodynamic parameters revealed no relevant changes. Inhalation of NO and instillation of perfluorocarbon had no additive effects on pulmonary gas exchange when compared to PEEP 15 cmH2O alone. CONCLUSION: In experimental lung injury, improvements in gas exchange are most distinct during mechanical ventilation with PEEP 15 cmH2O without significantly impairing hemodynamics. Partial liquid ventilation and inhaled NO did not cause an additive increase of PaO2.     

Relaxation of tracheomotor tone by PEEP: influence of hypocapnia and acidemia

The effect of PEEP on airway smooth muscle tone is difficult to assess using standard lung resistance and compliance techniques. In this study we isolated the extrathoracic trachea from lower airway pressure by performing a low cervical tracheostomy in dogs. We then measured the pressure (PTE) within a water-filled cuff of an endotracheal tube which was placed in the isolated extra-thoracic tracheal segment above the tracheostomy as a measure of tracheomotor tone. Sudden application of 10 cm H2O PEEP in normocapneic animals (PaCO2 = 5.6 +/- 0.2 kPa) caused an immediate dilation in this extrathoracic tracheal segment (PTE decreased from 5.7 +/- 0.3 kPa to 2.4 +/- 0.4 kPa). This decrease in tracheomotor tone was transient, returning to control level by four minutes in spite of 10 cm H2O PEEP maintained on the lower airway. With zero end-expiratory pressure the respiratory rate was increased from 9 to 19 breaths per minute and PaCO2 reduced to 3.9 +/- 0.2 kPa. No detectable tracheomotor dilation was observed after the application of 10 cm H2O PEEP to the lower airway. When exogenous CO2 was added to the inspired gas mixture at a respiratory rate of 19/min, the PaCO2 increased from 4.2 +/- 0.2 kPa to 6.7 +/- 0.4 kPa and a tracheomotor dilation in response to PEEP was again detectable. Finally, a 0.1 N infusion of HCl was infused into hypocapneic animals (PaCO2 = 3.7 +/- 0.3 kPa; pH = 7.47 +/- 0.02). After 30-70 minutes, pH decreased to 7.26 +/- 0.02 and PaCO2 remained 3.5 +/- 0.3 kPa.(ABSTRACT TRUNCATED AT 250 WORDS)     

Differential Ventilation and Selective PEEP During Anaesthesia in the Lateral Decubitus Posture

The potential of differential ventilation (DV) with selective positive end-expiratory pressure (PEEP) has been tested versus conventional ventilation with and without general PEEP. Gas exchange and central haemodynamics were studied in 15 subjects with no clinical or radiological signs of pulmonary disease. The rationale of the method was to ensure ventilation of the well-perfused dependent lung and to counteract airway closure within that lung. The subjects were intubated with a double-lumen catheter prior to scheduled abdominal surgery. During general anaesthesia in the lateral posture, they were given DV. The mean inspired oxygen fraction was 0.32. Fifty per cent ("even" tidal volume (VT) distribution) or 70% ("inverted" VT distribution) of the inspired volume was administered to the dependent lung. Two synchronized ventilators were used. In eight subjects DV was also combined with PEEP applied solely to the dependent lung (selective PEEP). The major findings were that DV with even VT distribution reduced venous admixture by 26% ( P <0.05) and the alveolo-arterial oxygen tension gradient (P(A-a)o2) by 30% ( P <0.05) in comparison with conventional ventilation in the lateral position. The addition of selective PEEP further reduced the P(A-a)o2 by 13%. P(A-a)o2 was consequently 43% lower than during conventional ventilation without PEEP in the lateral posture ( P <0.01). Selective PEEP also had less impact on cardiac output than general PEEP (P<0.05). It is concluded that DV with even distribution of VT and selective PEEP can reduce the P(A-a)o2 in anaesthetized lung-healthy subjects in the lateral position.     

Effect of continuous positive airway pressure (CPAP) in patients with chronic obstructive pulmonary chsease (COPD) depending on intrinsic PEEP levels

The application of continuous positive airway pressure (CPAP) is known to reduce inspiratory work of breathing in intubated patients with chronic obstructive pulmonary disease (COPD). This effect is caused by a decrease in elastic work related to a reduction in intrinsic PEEP. The aim of this study was to relate the decrease in inspiratory work due to CPAP to the intrinsic PEEP levels obtained during spontaneous breathing without positive pressure. Ten intubated patients with COPD who had been ventilated for acute respiratory failure were studied. Intrinsic PEEP was determined during tracheal occlusions performed at end-expiration when the patient was breathing without positive airway pressure. Inspiratory work was computed during breathing through a circuit with a CPAP of 0.5 kPa and the same circuit without positive pressure. Intrinsic PEEP-levels ranged from 0.26 to 1.31 kPa. Compared to spontaneous breathing without positive pressure, CPAP reduced the total inspiratory work per liter of ventilation (Wltot) from 1.42±0.48 to 1.24±0.50 J·1^-1 (means±SD P <0.01). This decrease was found to be related to the intrinsic PEEP-levels: the largest reductions were found in the patients with an intrinsic PEEP-level close to the CPAP-level applied. In intubated patients with COPD, the decrease in Wltot due to a CPAP of 0.5 kPa was found to be related to the intrinsic PEEP-levels present when no positive airway pressure was applied. The intrinsic PEEP measured during tracheal occlusions could be used to estimate the effect of CPAP in these patients.     

Effect of elevated PEEP on dynamic variables of fluid responsiveness in a pediatric animal model

Background: Previous studies have demonstrated that stroke volume variation (SVV), pulse pressure variation (PPV) and global end‐diastolic volume (GEDV) can be used to predict the response to fluid administration. Currently, little information is available whether application of different levels of positive end‐expiratory pressure (PEEP), especially in infants and neonates, affects their ability to predict fluid responsiveness. The aim of our study was to assess the effect of increasing PEEP levels on the predictive value of SVV, PPV and GEDV with respect to fluid responsiveness. Methods: Stroke volume variation and PPV were monitored continously in 22 anesthetized piglets during changing PEEP levels (5 and 10 cmH₂O) both before and after fluid loading (FL). GEDV was measured by transpulmonary thermodilution; cardiac output and stroke volume (SV) were measured by pulmonary artery thermodilution. A positive response to FL was defined as ≥15% increase in SV. Results: Fluid loading induced significant changes in all hemodynamic variables except of heart rate and systemic vascular resistance. At PEEP 5 cmH₂O, SVV, PPV and GEDV significantly correlated with volume induced percentage change in SV, whereas at PEEP 10 cmH₂O, this correlation was abolished for PPV. As assessed by receiver operating characteristic curve analysis, SVV and GEDV, independent of PEEP level applied, were the best predictors of a positive response to FL [area under the curve: SVV = 0.88; GEDV = 0.80]. Conclusions: In this pediatric animal model, SVV and GEDV were sensitive and specific predictors of fluid responsiveness during increasing PEEP levels.     

Effects of PEEP levels following repeated recruitment maneuvers on ventilator-induced lung injury

Background: Different levels of positive end-expiratory pressure (PEEP) with and without a recruitment maneuver (RM) may have a significant impact on ventilator-induced lung injury but this issue has not been well addressed.
Methods: Anesthetized rats received hydrochloric acid (HCl, pH 1.5) aspiration, followed by mechanical ventilation with a tidal volume of 6 ml/kg. The animals were randomized into four groups of 10 each: (1) high PEEP at 6 cm H₂O with an RM by applying peak airway pressure at 30 cm H₂O for 10 s every 15 min; (2) low PEEP at 2 cm H₂O with RM; (3) high PEEP alone; and (4) low PEEP alone.
Results: The mean arterial pressure and the amounts of fluid infused were similar in the four groups. Application of the higher PEEP improved oxygenation compared with the lower PEEP groups ( P <0.05). The lung compliance was better reserved, and the systemic cytokine responses and lung wet to dry ratio were lower in the high PEEP than in the low PEEP group for a given RM ( P <0.05).
Conclusions: The use of a combination of periodic RM and the higher PEEP had an additive effect in improving oxygenation and pulmonary mechanics and attenuation of inflammation.     

Pulmonary effects of body position, PEEP, and surfactant depletion in dogs

The influence of position (sphinx, lateral, supine), surfactant depletion, and different positive end-expiratory pressure (PEEP) on functional residual capacity (FRC), series dead space (VdS) and compliance of the respiratory system (Crs) were evaluated in five dogs. Ventilation homogeneity as measured by an index (multiple breath alveolar mixing efficiency), oxygenation, and cardiovascular hemodynamics were additionally examined. The dogs were anesthetized with halothane, paralyzed, and mechanically ventilated. FRC and VdS were found to be notably large in dogs, 45 +/- 8 ml/kg and 6 +/- 1 ml/kg, respectively. FRC and ventilation homogeneity were improved in the sphinx position (prone position with upright head). Surfactant depletion by lung lavage with 37 degrees C saline caused an immediate and stable decrease in FRC, Crs, and oxygenation (P less than 0.05, respectively) for about 5 h without marked effects on the circulatory system. FRC and VdS increased with increasing PEEP. At the highest PEEP, 10 cmH2O (1 kPa), Crs decreased (P less than 0.05) and ventilation became more uneven, indicating alveolar overdistension.     

Evaluation of a recruitment maneuver with positive inspiratory pressure and high PEEP in patients with severe ARDS

BACKGROUND: To evaluate the effect of a recruitment maneuver (RM) with constant positive inspiratory pressure and high positive end-expiratory pressure (PEEP) on oxygenation and static compliance (Cs) in patients with severe acute respiratory distress syndrome (ARDS). METHODS: Eight patients with ARDS ventilated with lung-protective strategy and an arterial partial pressure of oxygen to inspired oxygen fraction ratio (PaO2/FIO2) < or =100 mmHg regardless of PEEP were prospectively studied. The RM was performed in pressure-controlled ventilation at FIO2 of 1.0 until PaO2 reached 250 mmHg or a maximal plateau pressure/PEEP of 60/45 cmH2O was achieved. The RM was performed with stepwise increases of 5 cmH2O of PEEP every 2 min and thereafter with stepwise decreases of 2 cmH2O of PEEP every 2 min until a drop in PaO2 >10% below the recruitment PEEP level. Data was collected before (preRM), during and after 30 min (posRM). RESULTS: The PaO2/FIO2 increased from 83 +/- 22 mmHg preRM to 118 +/- 32 mmHg posRM (P = 0.001). The Cs increased from 28 +/- 10 ml cmH2O(-1) preRM to 35 +/- 12 ml cmH2O(-1) posRM (P = 0.025). The PEEP was 12 +/- 3 cmH2O preRM and was set at 15 +/- 4 cmH2O posRM (P = 0.025). The PEEP of recruitment was 36 +/- 9 cmH2O and the collapsing PEEP was 13 +/- 4 cmH2O. The PaO2 of recruitment was 225 +/- 105 mmHg, with five patients reaching a PaO2 > or = 250 mmHg. The FIO2 decreased from 0.76 +/- 0.16 preRM to 0.63 +/- 0.15 posRM (P = 0.001). No major complications were detected. CONCLUSION: Recruitment maneuver was safe and useful to improve oxygenation and Cs in patients with severe ARDS ventilated with lung-protective strategy.