Beneficial hemodynamic effects of prone positioning in patients with acute respiratory distress syndrome. (Fortenberry)

Jozwiak M, Teboul JL, Anguel N, Persichini R, Silva S, Chemla D, Richard C, Monnet X. Beneficial hemodynamic effects of prone positioning in patients with acute respiratory distress syndrome. Am J Respir Crit Care Med. 2013 Dec 15;188(12):1428-33.

Rationale: The effects of prone positioning during acute respiratory distress syndrome on all the components of cardiac function have not been investigated under protective ventilation and maximal alveolar recruitment.

Objectives: To investigate the hemodynamic effects of prone positioning.

Methods: We included 18 patients with acute respiratory distress syndrome ventilated with protective ventilation and an end-expiratory positive pressure titrated to a plateau pressure of 28-30 cm H2O. Before and within 20 minutes of starting prone positioning, hemodynamic, respiratory, intraabdominal pressure, and echocardiographic data were collected. Before prone positioning, preload reserve was assessed by a passive leg raising test.

Measurements and Main Results: In all patients, prone positioning increased the ratio of arterial oxygen partial pressure over inspired oxygen fraction, the intraabdominal pressure, and the right and left cardiac preload. The pulmonary vascular resistance decreased along with the ratio of the right/left ventricular end-diastolic areas suggesting a decrease of the right ventricular afterload. In the nine patients with preload reserve, prone positioning significantly increased cardiac index (3.0 [2.3-3.5] to 3.6 [3.2-4.4] L/min/m(2)). In the remaining patients, cardiac index did not change despite a significant decrease in the pulmonary vascular resistance.

Conclusions: In patients with acute respiratory distress syndrome under protective ventilation and maximal alveolar recruitment, prone positioning increased the cardiac index only in patients with preload reserve, emphasizing the important role of preload in the hemodynamic effects of prone positioning.

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Body position changes redistribute lung computed-tomographic density in patients with acute respiratory failure: impact and clinical fallout through the following 20 years. (Stockwell)

Gattinoni L, Pesenti A, Carlesso E. Body position changes redistribute lung computed-tomographic density in patients with acute respiratory failure: impact and clinical fallout through the following 20 years. Intensive Care Med. 2013 Nov;39(11):1909-1915.

ABSTRACT: In patients with acute respiratory distress syndrome (ARDS), in supine position, there is a decrease of inflation along the sternum vertebral axis, up to lung collapse. In 1991 we published a report showing that, in ARDS patients, shifting from supine to prone position led immediately to the inversion of the inflation gradient and to a redistribution of densities from dorsal to ventral lung regions. This led to a “sponge model” as a wet sponge, similar to a heavy edematous lung, squeezes out the gas in the most dependent regions, due to the weight-related increase of the compressive forces. The sponge model accounts for density distribution in prone position, for which the unloaded dorsal regions are recruited, while the loaded ventral region, collapses. In addition, the sponge model accounts for the mechanism through which the positive end-expiratory pressure acts as counterforce to oppose the collapsing, compressing forces. The final result of proning was that the inversion of gravitational forces, together with other factors such as lung-chest wall shape-matching and the heart weight led to a more homogeneous distribution of inflation throughout the lung parenchyma. This is associated with oxygenation improvement as the dorsal recruitment, for anatomical reasons, prevails on the ventral de-recruitment. The more homogeneous distribution of inflation (i.e. of stress and strain) decreases/prevents the ventilator-induced lung injury, as consistently shown in animal experiments. Finally, and a series of clinical trials led to the conclusion that in patients with severe ARDS, the prone position provides a significant survival advantage.

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Effects of prone positioning on lung protection in patients with acute respiratory distress syndrome. (Fortenberry)

Am J Respir Crit Care Med. 2013 Jan 24. [Epub ahead of print] PMID: 23348974

Rationale: Positive end-expiratory pressure (PEEP) and prone positioning may induce lung recruitment and affect alveolar dynamics in acute respiratory distress syndrome (ARDS). However, whether there is any interdependence between the effects of PEEP and prone positioning on these variables is unknown. Objectives: To determine the effects of high PEEP and prone positioning on lung recruitment, cyclic recruitment/derecruitment and tidal-hyperinflation, and how these effects are influenced by lung recruitability. Methods: Mechanically ventilated patients (VT 6 ml/kg IBW) underwent whole-lung computed tomography (CT) during breath-holding sessions at airway pressures of 5, 15, and 45-cmH2O, and Cine-CTs on a fixed thoracic transverse slice at PEEP 5 and 15-cmH2O. CT-images were repeated in supine and prone. A recruitment maneuver at 45- cmH2O was performed before each PEEP change. Lung recruitability was defined as the difference in percentage of non-aerated tissue between 5 and 45-cmH2O. Cyclic recruitment/derecruitment and tidal-hyperinflation were determined as tidal changes in percentage of non-aerated and hyperinflated tissue, respectively Main Results: 24 ARDS patients were included. Increasing PEEP from 5 to 15-cmH2O decreased non-aerated tissue (501±201 to 322±132grs, p<0.001) and increased tidal-hyperinflation (0.41±0.26 to 0.57±0.30%, p=0.004) in supine. Prone positioning further decreased non-aerated tissue (322±132 to 290±141grs, p=0.028), and reduced tidal-hyperinflation observed at PEEP 15 in supine (0.57±0.30 to 0.41±0.22%). Cyclic recruitment/derecruitment only decreased when high PEEP and prone were applied together (4.1±1.9 to 2.9±0.9%, p=0.003), particularly in patients with high lung recruitability. Conclusions: Prone positioning enhances lung recruitment and decreases alveolar instability and hyperinflation observed at high PEEP in ARDS patients.

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Intraocular pressure in pediatric patients during prone surgery. (Chandler)

Anesth Analg. 2013 Apr 4. PMID: 23558834

BACKGROUND: Intraoperative intraocular pressure (IOP) in the prone position and IOP changes over time have not been evaluated in pediatric surgical patients. We sought to determine time-dependent changes in IOP in children undergoing surgery in prone position.

METHODS: Thirty patients undergoing neurosurgical procedures in prone position were included. Using a pulse-mode pneumatonometer, IOP was measured in supine position after induction and before emergence of anesthesia and in prone position before the start and after the end of surgery. IOP changes over time in the prone position were assessed with a linear mixed model (i.e., random slope and intercept model) to adjust for the within-patient correlation.

RESULTS: IOP in prone position increased by an average of 2.2 mm Hg per hour (P < 0.001). Sixty-three percent of patients (95% confidence interval [CI], 46%-81%) had at least 1 IOP value exceeding 30 mm Hg, and 13% (95% CI, 1%-25%) had at least 1 IOP value exceeding 40 mm Hg while prone. Mean IOP increased 7 mm Hg (95% CI, 6-9) during the position change from supine to prone (P < 0.001) and decreased 10 mm Hg (95% CI, 9-12) after changing the position from prone back to supine (P < 0.001).

CONCLUSIONS: Changing position from supine to prone significantly increases IOP in anesthetized pediatric patients. Moreover, the IOP continued to increase during surgery and reached potentially harmful values, especially when combined with low mean arterial blood pressures that are common during major surgery.

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