Prone Positioning in ARDS
It has been shown that prone positioning improves oxygenation for 60% to 70% of ALI/ARDS patients, though the patients most likely to respond are not readily identified in advance. The mechanisms responsible for oxygenation improvements are uncertain, however, primary mechanisms likely responsible are:
• improved ventilation-perfusion matching
• changes in lung mass and shape
• alterations in compliance
Ventilation-perfusion matching may be severely impaired in patients with ALI/ ARDS, partially related to their inability to normally produce hypoxic pulmonary vasoconstriction. In turning a patient from supine to prone, more homogeneous gas distribution is achieved, thus contributing to improved ventilation-perfusion matching. Also, in the process of turning from supine to prone, the heart is no longer compressing the posterior aspects of the left lung, allowing for better aeration. Furthermore, the majority of lung tissue is posterior, and prone positioning allows this large amount of lung tissue to function as anterior regions with better aeration (though perfusion may not improve or worsen).
Concerns regarding prone positioning remain, primarily related to the safety of performing the prone maneuvers and providing patient care for patients in this position. Of 152 patients enrolled prone positioning oxygenation improved in approximately 70% of prone positioning patients, and no significant difference was noted in adverse events related to the prone positioning.
High concentrations of inspired oxygen should be minimized in the patient with ARDS. In most cases following the SaO2 to maintain a level > 90% is satisfactory. Levels less than 88% result in a steep decline in delivery as predicted by the oxygen-hemoglobin dissociation curve.
The predominant physiologic effect recognized from recruitment maneuvers is the increase in functional residual capacity of the lung, representing additional alveoli being opened and available for gas exchange. The most common method of recruitment includes PPEP and inverse ratio ventilation.
The Role of High-Frequency Ventilation (HFV) in ARDS
HFV (RR>60) may be defined as mechanical ventilation applied with high respiratory rates and low tidal volumes. There are a number of forms of HFV, including jet (100-200 Hz) ventilation and oscillatory ventilation. Early studies of HFV demonstrated improvements in oxygenation and in pathologic changes of lung injury or ARDS. Trials that have attempted to study the clinical effectiveness of HFV in ALI/ARDS patients have failed to observe any improvements. This may relate to pre-existing and ongoing lung injury, related to conventional ventilation strategies before the patient began HFV.
Use of Low Tidal Volume Ventilation
Traditional tidal volumes of 10-14 mL/kg can result in significant ventilator associated lung injury. Several studies have documented that low tidal volume ventilation, or permissive hypercapnea, is the only strategy proven to improve survival in ALI/ARDS patients. Mortality was decreased from 40% to 31%. Respiratory acidosis is tolerated well up to a pH of 7.20. Most institutions would reserve bicarbonate infusion for levels pH<7.10. There are many physiologic effects to be considered with respiratory acidosis and hypercarbia. These include myocardial depression, systemic vasodilatation, rightward shift of the oxygen-hemoglobin dissociation curve. This results in increased cardiac output, heart rate and decreased systemic vascular resistance. Relative contraindications to permissive hypercapnea include closed head injuries with increased intracranial pressures.
Strategies of ventilator support include lower tidal volumes in the range of 4-8 mL/kg as well as pressure limited settings.
ARDS is associated with a decrease in surfactant and increased alveolar surface tension. It has been theorized that the use of liquids with oxygen carrying capacity could be used to open and oxygenate alveolar spaces. The use of perfluorocarbons has been initiated in some trials. Total liquid ventilation has proven difficult and expensive opening the door to the use of partial liquid ventilation. In this process the lung is filled to FRC and ventilated with a conventional ventilator. Studies have shown it to be safe but no RCTs have shown it to be more effective.
1. Ashbaugh DG, Bigelow DB, Petty TL et al. Acute respiratory distress in adults. Lancet 1967; 2:319-323.
2. Cordingley JJ, Keogh BF. The pulmonary physician in critical care 8: Ventilatory management of ALI/ARDS. Thorax 2002; 57:729-734.
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