January 2002 / Volume 47 / Number 1 / Page 39
Evaluation of Work of Breathing in Spontaneously Breathing Animals During Partial Liquid Ventilation
Partial liquid ventilation (PLV) with perflubron is intended as an adjunct to conventional mechanical ventilation in patients with acute respiratory distress syndrome and neonates with surfactant deficiency. It uses the combination of gas delivery (via a conventional mechanical ventilator) and perflubron-filled lungs. Perflubron is a biochemically inert perfluorochemical that has a low surface tension, a positive spreading coefficient, and a high solubility for oxygen and carbon dioxide. Numerous animal studies using lung disease models have shown that PLV can improve gas exchange and lung mechanics such as pulmonary compliance.
Promising Phase I data have been published for the use of PLV in premature infants, and studies have been undertaken in both pediatric and adult patients with acute respiratory distress syndrome. Recently these studies have been completed, although questions persist concerning the ideal method for using mechanical ventilation during PLV. During the pediatric trial, patients were maintained on chemically induced paralysis for the duration of PLV treatment. However, during the adult trial, patients were allowed to breathe spontaneously during PLV. There are potential disadvantages to the use of neuromuscular blockade during mechanical ventilation, including prolonged muscle weakness, increased costs, reduced cardiac output and blood pressure, and lower functional residual capacity (FRC). Allowing patients to breathe spontaneously during PLV was discouraged during the pediatric trial because of a lack of data on PLV's effect on work of breathing (WOB). These concerns were based on the potential that WOB might be increased because of an increase in lung density during PLV. Also there may be concern about the patient's ability to trigger the ventilator across the liquid-gas interface during PLV. In contrast, there may be potential benefits from allowing the patient to breathe spontaneously during PLV. Previous studies with controlled mechanical ventilation found that neonatal patients breathing spontaneously in a patient-triggered mode (as compared to a non-patient-triggered mode) showed better gas exchange and needed a lower level of mechanical support. In addition, studies of neonates suggest that the use of patient-triggered ventilation increases the delivery of tidal volume (VT), shortens the length of mechanical ventilation, and decreases WOB by improving patient-ventilator synchrony.
Few data are available on the use of perflubron in spontaneously breathing animals or spontaneous ventilation during PLV in mechanically ventilated animals. No studies are available on the effects of pressure-support ventilation (PSV) during PLV in those animals.
To achieve normal ventilation, the body performs work, known as WOB, to overcome the elastic and frictional resistance of the lungs and chest wall. Total WOB (WOBT) is composed of the elastic work of breathing (WOBE) and the resistive work of breathing (WOBR). Elastic WOB represents physiologic work that includes the work to expand the lungs and chest wall. Resistive WOB is considered a measure of imposed WOB and includes work caused by the breathing apparatus, such as the endotracheal tube (ETT), the breathing circuit, and the ventilator's demand-flow system. The objective of this pilot study was to examine WOB in a spontaneously breathing animal model during PLV with and without the use of PSV.