Liquid Ventilation and Anesthesia - Essay Example

Perfluorocarbons (PFC) are structurally similar to hydrocarbons with the hydrogens replaced by fluorine. The carbon chains vary in length and an additional moiety often is attached to the molecule which, together, give unique properties to each perfluorocarbon. In general, perfluorocarbons have excellent oxygen and carbon dioxide carrying capacity (50 ml O2/dl and 160-210 ml CO2/dl, respectively). They are clear, odorless, inert fluids that are immiscible in aqueous and most other solutions.

PHYSIOLOGY OF ADULT RESPIRATORY DISTRESS SYNDROME

Acute respiratory distress syndrome (ARDS; previously called adult respiratory distress syndrome) is a clinical syndrome characterized by a sudden and progressive pulmonary edema, increasing bilateral infiltrates on chest x-ray, hypoxemia refractory to oxygen supplementation, and reduced lung compliance. These signs occur in the absence of left-sided heart failure. Patients with ARDS usually require mechanical ventilation with a higher-than-normal airway pressure. A wide range of factors are associated with the development of ARDS (Fig.1), including direct injury to the lungs (eg, smoke inhalation) or indirect insult to the lungs (eg, shock). ARDS has been associated with a mortality rate as high as 50% to 60%. The major cause of death in ARDS is nonpulmonary multiple-system organ failure, often with sepsis.
Pathophysiology:

ARDS occurs as a result of an inflammatory trigger that initiates the release of cellular and chemical mediators, causing injury to the alveolar-capillary membrane. This results in leakage of fluid into the alveolar interstitial spaces and alterations in the capillary bed.
Severe ventilation-perfusion mismatching occurs in ARDS. Alveoli collapse because of the inflammatory infiltrate, blood, fluid, and surfactant dysfunction. Small airways are narrowed because of interstitial fluid and bronchial obstruction. The lung compliance
becomes markedly decreased (stiff lungs), and the result is a characteristic decrease in functional residual capacity and severe hypoxemia. The blood returning to the lung for gas exchange is pumped through the nonventilated, nonfunctioning areas of the lung, causing a shunt to develop. This means that blood is interfacing with nonfunctioning alveoli and gas exchange is markedly impaired, resulting in severe, refractory hypoxemia. Figure 1. shows the sequence of pathophysiologic events leading to ARDS.

Figure.1 Pathogenesis and pathophysiology of acute distress syndrome. Adapted from Farzan, S. (1997). A concise handbook of respiratory distress (4th ed.). Stanford. CT: Appleton & Lange.

MECHANISMS OF LIQUID VENTILATION:
Ventilators are used commonly in the operating theatre and in the ICU to deliver mechanical ventilation to the lungs. In the operating theatre, ventilation is in anesthetized and often pharmacologically paralyzed patients with predominantly normal lungs. These ventilators are relatively simple and are designed to deliver varying concentrations of oxygen, air, nitrous oxide, and volatile agents to patients through an anesthetic circuit. In ICU, ventilators provide respiratory support to patients with respiratory failure. Respiratory failure is a state in which the pulmonary oxygen uptake is so severely disturbed that the supply of oxygen to the tissues and/or the removal of carbon dioxide from them is inadequate.