Ventilator-associated lung injury
Encyclopedia
Ventilator-associated lung injury (VALI) is an acute lung injury that develops during mechanical ventilation
and is termed ventilator-induced lung injury (VILI) if it can be proven that the mechanical ventilation caused the acute lung injury. In contrast, ventilator-associated lung injury (VALI) exists if the cause cannot be proven. VALI is the appropriate term in most situations because it is virtually impossible to prove what actually caused the lung injury in the hospital.
. Severe injury to alveoli causes swelling of the tissues (edema) in the lungs, bleeding of the alveoli, loss of surfactant (increase in lung compliance) and complete alveoli collapse.
. Areas of the lung that are collapsed (atelectasis
) or filled with secretions will be underinflated, while those areas that are relatively normal will be overinflated. These areas will become over distended and injured. This may be reduced by using smaller tidal volumes.
During positive pressure ventilation, atelectatic regions will inflate, however the alveoli will be unstable and will collapse during the expiratory phase of the breath. This repeated alveolar collapse and expansion (RACE) is thought to cause VALI. By opening the lung and keeping the lung open RACE (and VALI) is reduced.
Possible reasons for predisposition to VALI include:
Preventing cyclic atelectasis – Applied positive end-expiratory pressure (PEEP) is the principal method used to keep the alveoli open and lessen cyclic atelectasis.
Open lung ventilation – Open lung ventilation is a ventilatory strategy that combines small tidal volumes (to lessen alveolar overdistension) and an applied PEEP above the low inflection point on the pressure-volume curve (to lessen cyclic atelectasis).
Non-conventional ventilation as a technique for management
High frequency ventilation is thought to reduce ventilator-associated lung injury, especially in the context of ARDS and acute lung injury
.
Mechanical ventilation
In medicine, mechanical ventilation is a method to mechanically assist or replace spontaneous breathing. This may involve a machine called a ventilator or the breathing may be assisted by a physician, respiratory therapist or other suitable person compressing a bag or set of bellows...
and is termed ventilator-induced lung injury (VILI) if it can be proven that the mechanical ventilation caused the acute lung injury. In contrast, ventilator-associated lung injury (VALI) exists if the cause cannot be proven. VALI is the appropriate term in most situations because it is virtually impossible to prove what actually caused the lung injury in the hospital.
Pathogenesis
Overdistension of alveoli and cyclic atelectasis are the primary causes for alveolar injury during positive pressure mechanical ventilationMechanical ventilation
In medicine, mechanical ventilation is a method to mechanically assist or replace spontaneous breathing. This may involve a machine called a ventilator or the breathing may be assisted by a physician, respiratory therapist or other suitable person compressing a bag or set of bellows...
. Severe injury to alveoli causes swelling of the tissues (edema) in the lungs, bleeding of the alveoli, loss of surfactant (increase in lung compliance) and complete alveoli collapse.
Etiology
One major causative factor is the over stretching of the airways and alveoli. During mechanical ventilation, the flow of gas into the lung will take the path of least resistancePath of least resistance
The path of least resistance describes the physical or metaphorical pathway that provides the least resistance to forward motion by a given object or entity, among a set of alternative paths. The concept is often used to describe why an object or entity takes a given path.In physics, the path of...
. Areas of the lung that are collapsed (atelectasis
Atelectasis
Atelectasis is defined as the collapse or closure of alveoli resulting in reduced or absent gas exchange. It may affect part or all of one lung. It is a condition where the alveoli are deflated, as distinct from pulmonary consolidation.It is a very common finding in chest x-rays and other...
) or filled with secretions will be underinflated, while those areas that are relatively normal will be overinflated. These areas will become over distended and injured. This may be reduced by using smaller tidal volumes.
During positive pressure ventilation, atelectatic regions will inflate, however the alveoli will be unstable and will collapse during the expiratory phase of the breath. This repeated alveolar collapse and expansion (RACE) is thought to cause VALI. By opening the lung and keeping the lung open RACE (and VALI) is reduced.
Risk factors
VALI is most common in patients receiving mechanical ventilation for acute lung injury or acute respiratory distress syndrome (ALI/ARDS).Possible reasons for predisposition to VALI include:
- An injured lung may be at risk for further injury
- Cyclic atelectasis is particularly common in an injured lung
Incidence
24 percent (24%) of all patients mechanically ventilated will develop VALI for reasons other than ALI or ARDS. The incidence is probably higher among patients who already have ALI/ARDS, but estimates vary widely. The variable estimates reflect the difficulty in distinguishing VALI from progressive ALI/ARDS.Prevention
Preventing alveolar overdistension – Alveolar overdistension is mitigated by using small tidal volumes, maintaining a low plateau pressure, and using pressure limited ventilation.Preventing cyclic atelectasis – Applied positive end-expiratory pressure (PEEP) is the principal method used to keep the alveoli open and lessen cyclic atelectasis.
Open lung ventilation – Open lung ventilation is a ventilatory strategy that combines small tidal volumes (to lessen alveolar overdistension) and an applied PEEP above the low inflection point on the pressure-volume curve (to lessen cyclic atelectasis).
Non-conventional ventilation as a technique for management
High frequency ventilation is thought to reduce ventilator-associated lung injury, especially in the context of ARDS and acute lung injury
Acute lung injury
Acute lung injury is a diffuse heterogeneous lung injury characterized by hypoxemia, non cardiogenic pulmonary edema, low lung compliance and widespread capillary leakage...
.