Papel de la microcirculación en el desarrollo de la lesión pulmonar aguda inducida por la ventilación mecánica

Effects of vascular flow and PEEP in a multiple hit model of lung injury in isolated perfused rabbit lungs

BACKGROUND

High vascular flow aggravates lung damage in animal models of ventilator-induced lung injury. Positive end-expiratory pressure (PEEP) can attenuate ventilator-induced lung injury, but its continued effectiveness in the setting of antecedent lung injury is unclear. The objective of the present study was to evaluate whether the application of PEEP diminishes lung injury induced by concurrent high vascular flow and high alveolar pressures in normal lungs and in a preinjury lung model.

METHODS

Two series of experiments were performed. Fifteen sets of isolated rabbit lungs were randomized into three groups (n = 5): low vascular flow/low PEEP; high vascular flow/low PEEP, and high vascular flow/high PEEP. Subsequently, the same protocol was applied in an additional 15 sets of isolated rabbit lungs in which oleic acid was added to the vascular perfusate to produce mild to moderate lung injury. All lungs were ventilated with peak airway pressure of 30 cm H2O for 30 minutes. Outcome measures included frequency of gross structural failure, pulmonary hemorrhage, edema formation, changes in static compliance, pulmonary vascular resistance, and pulmonary ultrafiltration coefficient.

RESULTS

In the context of high vascular flow, application of a moderate level of PEEP reduced pulmonary rupture, edema formation, and lung hemorrhage. The protective effects of PEEP were not observed in lungs concurrently injured with oleic acid.

CONCLUSIONS

Under these experimental conditions, PEEP attenuates lung injury in the setting of high vascular flow. The protective effect of PEEP is lost in a two-hit model of lung injury.

Mecanismos biofísicos, celulares y modulación de la lesión pulmonar inducida por la ventilación mecánica

Ventilator-induced lung injury (VILI) is associated to a high rate of mortality with an important social impact. Mechanical ventilation induces structural and ultrastructural alterations in all cell types of the lung and can derive in the transduction of intracellular signals, as well as in changes in the expression of genes, a process known as mechanotransduction. Some of the conditions involved, such as inflammation and/or coagulation, apoptosis/necrosis can lead to the propagation of the injury outside the lung, resulting in multiorganic failure. VILI can be modulated by means of diverse interventions as the use of protective ventilatory modes, therapeutic approaches based on vasoactive and antioxidative drugs, and more recently treatments based on the use of repairing substances of the surfactant like poloxamers among others. Knowledge of the mechanisms involved in VILI is definitive for a better approach to this condition.

Contributions of vascular flow and pulmonary capillary pressure to ventilator-induced lung injury

OBJECTIVE

To evaluate the influence of vascular flow on ventilator-induced lung injury independent of vascular pressures.

DESIGN

Laboratory study.

SETTING

Hospital laboratory.

SUBJECTS

Thirty-two New Zealand White rabbits.

INTERVENTIONS

Thirty-two isolated perfused rabbit lungs were allocated into four groups: low flow/low pulmonary capillary pressure; high flow/high pulmonary capillary pressure; low flow/high pulmonary capillary pressure, and high flow/low pulmonary capillary pressure. All lungs were ventilated with peak airway pressure 30 cm H2O and positive end-expiratory pressure 5 cm H2O for 30 mins.

MEASUREMENTS AND MAIN RESULTS

Outcome measures included frequency of gross structural failure (pulmonary rupture), pulmonary hemorrhage, edema formation, changes in lung compliance, pulmonary vascular resistance, and pulmonary ultrafiltration coefficient. Lungs exposed to high pulmonary vascular flow ruptured more frequently, displayed more hemorrhage, developed more edema, suffered larger decreases in compliance, and had larger increases in vascular resistance than lungs exposed to low vascular flows (p < .05 for each pairwise comparison between groups).

CONCLUSIONS

These findings suggest that high pulmonary vascular flows might exacerbate ventilator-induced lung injury independent of their effects on pulmonary vascular pressures.