Respiratory and haemodynamic effects of the prone position at two different levels of PEEP in a canine acute lung injury model

Relationship between regional lung compliance and ventilation homogeneity in the supine and prone position

BACKGROUND

The prone position (PP) improves ventilation homogeneity in acute respiratory distress syndrome. The aim of this study was to investigate whether the alleviation of ventilation inhomogeneity in PP was due to changes in regional lung compliance.

METHODS

Ten lung-lavaged piglets were mechanically ventilated in supine position (SP) and in PP. In each position, positive end-expiratory pressure (PEEP) was reduced from 20 to 6 cmH(2)O in steps of 2 cmH(2)O every 10 min after full lung recruitment. Respiratory mechanics, blood gas, haemodynamic data and whole-lung computed tomography scans were recorded at each PEEP. The compliances of normally aerated (C(normal)) and newly recruited (C(recruited)) lung regions were calculated. Open lung PEEP (OL-PEEP) was defined as the lowest PEEP to maintain full lung recruitment.

RESULTS

At OL-PEEP, PP significantly increased normally aerated lung regions, decreased poorly aerated and hyperinflated lung regions and decreased tidal recruitment and hyperinflation. C(normal) was significantly reduced in PP compared with SP (12.8 ± 4.2 ml/cmH(2)O vs. 20.1 ± 6.2 ml/cmH(2)O, P < 0.001), whereas C(recruited) was increased in PP (13.9 ± 3.9 ml/cmH(2)O vs. 9.4 ± 2.4 ml/cmH(2)O, P < 0.001). C(normal) was correlated with hyperinflated lung regions at end-expiration (rho = 0.67) and end-inspiration (rho = 0.56) at OL-PEEP. C(recruited) was correlated with normally (r(2) = 0.36) and poorly aerated lung regions (rho = -0.58) at OL-PEEP.

CONCLUSION

This surfactant-depleted model shows that the improvement of ventilation homogeneity in PP is related to an increase in C(recruited) and a decrease in C(normal).

[The prone position in acute respiratory distress syndrome: a critical systematic review]

OBJECTIVE

To do a critical systematic review regarding effects of prone positioning (PP) in patients with acute respiratory distress syndrome (ARDS).

METHODS

A systematic review (Highwire, Medline, Cochrane Library from 1976 to 2004), using the keywords: prone position, acute respiratory distress syndrome, allowed us to include the human studies on PP in ARDS patients, independantly of their objectives or their type of protocol. To appreciate the studies validity, we scored the quality evidence of the studies in order to grade our conclusions.

RESULTS AND CONCLUSION

The qualitative analysis of the 58 included studies (1,500 patients returned prone, 4,000 episodes of PP) led to the following main conclusions: 1) the PP improves oxygenation in the majority of ARDS patients (level of evidence I); 2) the PP improves the pulmonary haemodynamics without altering the systemic haemodynamics (level of evidence III); 3) the PP enhances the recruitment maneuvers (level of evidence III); 4) because there are no formal predictive criteria for response to the PP, a "trial of PP" or better two PP trials are necessary to look for the responders; 5) the PP should be performed as early as possible in the course of severe ARDS; 6) the optimal duration of PP is 18 to 23 hours daily, and it should be continued until improvement of arterial oxygenation, or loss of the positive effect of PP on arterial oxygenation or evidently patient's death.

Lung recruitment during mechanical positive pressure ventilation in the PICU: what can be learned from the literature?

A literature review was conducted to assess the evidence for recruitment manoeuvres used in conventional mechanical positive pressure ventilation. A total of 61 studies on recruitment manoeuvres were identified: 13 experimental, 31 ICU, 6 PICU and 12 anaesthesia studies. Recruitment appears to be a continuous process during inspiration and expiration and is determined by peak inspiratory pressure (PIP) and positive end expiratory pressure (PEEP). Single or repeated recruitment manoeuvres may result in a statistically significant increase in oxygenation; however, this is short lasting and clinically irrelevant, especially in late ARDS and pneumonia. Temporary PIP elevation may be effective but only after PEEP loss (for example disconnection and tracheal suctioning). Continuous PEEP elevation and prone positioning can increase P(a)O2 significantly. Adverse haemodynamic or barotrauma effects are reported in various studies. No data exist on the effect of recruitment manoeuvres on mortality, morbidity, length of stay or duration of mechanical ventilation. Although recruitment manoeuvres can improve oxygenation, they can potentially increase lung injury, which eventually determines outcome. Based on the presently available literature, prone position and sufficient PEEP as part of a lung protective ventilation strategy seem to be the safest and most effective recruitment manoeuvres. As paediatric physiology is essentially different from adult, paediatric studies are needed to determine the role of recruitment manoeuvres in the PICU.

Posição prona

A posição prona é uma manobra utilizada para combater a hipoxemia nos pacientes com síndrome do desconforto respiratório agudo. Apesar de hoje ser considerada um modo eficaz de melhorar a oxigenação, os mecanismos fisiológicos que levam à melhora da função respiratória ainda não estão completamente esclarecidos. O objetivo principal desta revisão é discutir os aspectos fisiológicos e clínicos da posição prona na síndrome do desconforto respiratório agudo.

Prone positioning improves oxygenation without adverse hemodynamic effects during partial liquid ventilation in a canine model of acute lung injury

BACKGROUND

Partial liquid ventilation (PLV) and prone positioning can improve the arterial oxygenation (PaO2) in acute lung injury (ALI). We evaluated the effect of prolonged prone positioning during partial liquid ventilation (PLV) in a canine model of acute lung injury.

METHODS

Six mongrel dogs (weighing 17.4 +/- 0.7 kg each) were anesthetized, intubated and mechanically ventilated. After 1 hour of baseline stabilization, the dogs' lungs were instilled with 40 mL/kg perfluorocarbon (PFC). PLV was first performed in the supine position for 1 hour (S1), then in the prone position for 3 hours with hourly measurements (P1, P2, P3), and finally, PLV was performed with the animal turned back to the supine position for 1 hour (S2).

RESULTS

After instillation of the PFC, the PaO2 significantly increased from 992 +/- 32.6 mmHg at baseline to 198.1 +/- 59.2 mmHg at S1 (p = 0.001). When the dogs were turned to the prone position, the PaO2 further increased to 288.3 +/- 80.9 mmHg at P1 (p = 0.008 vs. S1): this increase was maintained for 3 hours, but the PaO2 decreased to 129.4 +/- 62.5 mmHg at S2 (p < 0.001 vs. P3). Similar changes were seen in the shunt fraction. There were no significant differences for the systemic hemodynamic parameters between the prone and supine positions.

CONCLUSION

Prolonged prone positioning during PLV in an animal model of ALI appears to improve oxygenation without any hemodynamic compromise.

Prone position and positive end-expiratory pressure in acute respiratory distress syndrome

OBJECTIVE

To determine whether positive end-expiratory pressure (PEEP) and prone position present a synergistic effect on oxygenation and if the effect of PEEP is related to computed tomography scan lung characteristic.

DESIGN

Prospective randomized study.

SETTING

French medical intensive care unit.

PATIENTS

Twenty-five patients with acute respiratory distress syndrome.

INTERVENTIONS

After a computed tomography scan was obtained, measurements were performed in all patients at four different PEEP levels (0, 5, 10, and 15 cm H2O) applied in random order in both supine and prone positions.

MEASUREMENTS AND MAIN RESULTS

Analysis of variance showed that PEEP (p <.001) and prone position (p <.001) improved oxygenation, whereas the type of infiltrates did not influence oxygenation. PEEP and prone position presented an additive effect on oxygenation. Patients presenting diffuse infiltrates exhibited an increase of Pao2/Fio2 related to PEEP whatever the position, whereas patients presenting localized infiltrates did not have improved oxygenation status when PEEP was increased in both positions. Prone position (p <.001) and PEEP (p <.001) reduced the true pulmonary shunt. Analysis of variance showed that prone position (p <.001) and PEEP (p <.001) reduced the true pulmonary shunt. The decrease of the shunt related to PEEP was more pronounced in patients presenting diffuse infiltrates. A lower inflection point was identified in 22 patients (88%) in both supine and prone positions. There was no difference in mean lower inflection point value between the supine and the prone positions (8.8 +/- 2.7 cm H2O vs. 8.4 +/- 3.4 cm H2O, respectively).

CONCLUSIONS

PEEP and prone positioning present additive effects. The prone position, not PEEP, improves oxygenation in patients with acute respiratory distress syndrome with localized infiltrates.

Effect of alveolar recruitment maneuver in early acute respiratory distress syndrome according to antiderecruitment strategy, etiological category of diffuse lung injury, and body position of the patient

OBJECTIVE

To assess how the level of positive end-expiratory pressure (PEEP) (antiderecruitment strategy), etiological category of diffuse lung injury, and body position of the patient modify the effect of the alveolar recruitment maneuver (ARM) in acute respiratory distress syndrome (ARDS).

DESIGN

Prospective clinical trial.

SETTING

Medical intensive care unit at a tertiary hospital.

PATIENTS

Forty-seven patients with early ARDS, including 19 patients from our preliminary study.

INTERVENTION

From baseline ventilation at a tidal volume of 8 mL/kg and PEEP of 10 cm H2O, the ARM (a stepwise increase in the level of PEEP up to 30 cm H2O with a concomitant decrease in the magnitude of tidal volume down to 2 mL/kg) was given with (ARM + PEEP, n = 20) or without (ARM only, n = 19) subsequent increase of PEEP to 15 cm H2O. In eight other patients, PEEP was increased to 15 cm H2O without a preceding ARM (PEEP only).

MEASUREMENTS AND RESULTS

In all three groups, Pao2 was increased by the respective intervention (all p<.05). In the ARM-only group, Pao2 at 15 mins after intervention was lower than Pao2 immediate after intervention (p =.046). In the ARM + PEEP group, no such decrease in Pao2 was observed, and Pao2 at 15, 30, 45, and 60 mins after intervention was higher than in the ARM-only group (all p<.05). Compared with the PEEP-only group, Pao2 of the ARM + PEEP group was higher immediately after intervention and at the later time points (all p <.05). Compared with patients with ARDS associated with direct lung injury (pulmonary ARDS), patients with ARDS associated with indirect lung injury (extrapulmonary ARDS) showed a greater increase in Pao2 (27 +/- 21% vs. 130 +/- 112%; p=.002) and a greater decrease in radiologic scores (1.0 +/- 2.4 vs. 3.4 +/- 1.5; p=.005) after the ARM. The increase in Pao2 induced by the ARM was greater for patients in the supine position than for patients in the prone position (61 +/- 82% vs. 21 +/- 14%; p=.028). Consequently, Pao immediately after the ARM was similar in the two groups of patients in different positions.

CONCLUSIONS

After the ARM, a sufficient level of PEEP is required as an antiderecruitment strategy. Pulmonary ARDS and extrapulmonary ARDS may be different pathophysiologic entities. An effective ARM may obviate the need for the prone position in ARDS at least in terms of oxygenation.

Strategies to optimize alveolar recruitment

Low tidal volume (4-8 mL/kg) during mechanical ventilation in adult respiratory distress syndrome is the standard of care. However, there are questions regarding the approach to setting positive end-expiratory pressure and the use of recruitment maneuvers in patients with adult respiratory distress syndrome. Animal data and preliminary patient data suggest that lung recruitment maneuvers can markedly improve arterial oxygen tension (PaO2). Prone positioning has also become established a method of recruiting lung and improving PaO2 in those with adult respiratory distress syndrome. The data suggest that recruitment maneuvers in the prone position are most effective in improving PaO2 and that the positive end-expiratory pressure level required to sustain the improved PaO2 is less in the prone position than in the supine position.

Oxygenation response to a recruitment maneuver during supine and prone positions in an oleic acid-induced lung injury model

Prone position and recruitment maneuvers (RM) are proposed as adjuncts to mechanical ventilation to open up the lung and keep it open. We studied the oxygenation response to a RM (composed of a 30-s sustained inflation at 60 cm H(2)O airway pressure) performed in prone and supine positions in dogs after oleic acid- induced lung injury using an inspired O(2) fraction of 0.60. In one group (n = 6) first supine then prone positions were examined after a RM at 8 cm H(2)O and 15 cm H(2)O of positive end-expiratory pressure (PEEP). In the second group (n = 6) the sequence of positions was reversed. Prone positioning after supine position always improved oxygenation, whereas the decrement in Pa(O(2)) was relatively small when dogs were returned to the supine position. Oxygenation improved in both groups after a RM, and the improvement was sustained (after 15 min) in the prone position at 8 cm H(2)O of PEEP, but 15 cm H(2)O of PEEP was required in supine position. Our results suggest that a RM improves oxygenation more effectively with a decreased PEEP requirement for the preservation of the oxygenation response in prone compared with supine position.