Liquid ventilator
Encyclopedia
A liquid ventilator is similar to a medical ventilator
except that it should be able to ensure reliable total liquid ventilation with a breatheable liquid (a perfluorocarbon). Actually, liquid ventilators are prototypes used for animal experimentations but experts recommend continued development of a liquid ventilator toward clinical applications.
The pumping system is either a peristaltic pump
(in the simplest liquid ventilators) or two piston pump
s (in the most advanced liquid ventilators).
Because of the PFC viscosity
, the head loss in the airways requires a low negative driving pressure during the expiration phase that can collapse the airways. This is the choked flow phenomenon in TLV which compromises the minute ventilation and consequently the gas exchanges. To address this limitation, liquid ventilator integrates a control of the pumping system.
The liquid ventilator is always volume-controlled because the specified tidal volume
of PFC must be accurately delivered and retrieved. It is also pressure-limited because it must stop the expiratory or inspiratory phase when a too low, or a too large, driving pressure is detected.
However, during the expiratory phase, the expiratory flow can be commanded by an open-loop controller
or a closed-loop controller:
Also, during the inspiratory phase, the volume-controlled mode is realized by open-loop or closed loop control of the PFC flow.
(CO2) from the PFC by saturating it with oxygen
(O2) and medical air. This procedure can be performed with either a membrane oxygenator (a technology used in extracorporeal
oxygenators) or a bubble oxygenator.
The liquid ventilator heats the PFC to body temperature. This is performed with an heat exchanger
connected to the oxygenator or with dedicated heaters integrated in the oxygenator.
The oxygenator and the heater produce PFC vapor which is recuperated with a condenser
in order to limit the evaporation loss (the PFC is a greenhouse gas
).
A typical cycle is composed of 4 steps :
(ARDS). For example, total liquid ventilation could be used for newborns with severe neonatal respiratory distress syndrome
in which conventional treatment has failed. Typical cases are late preterm newborns who have an increased risk of intracranial hemorrhage
and for whom their small vessel size poses technical limitations for Extracorporeal membrane oxygenation
(ECMO).
). For example, literature data suggest a radical change in the treatment of meconium aspiration syndrome
(MAS) by considering the use of a liquid ventilator. The demonstration of its efficacy was performed in the neonatal lamb.
is an expected clinical application. For example, a study present that rapid cooling instituted by TLV improves cardiac and mitochondrial function.
Medical ventilator
A medical ventilator can be defined as any machine designed to mechanically move breatheable air into and out of the lungs, to provide the mechanism of breathing for a patient who is physically unable to breathe, or breathing insufficiently....
except that it should be able to ensure reliable total liquid ventilation with a breatheable liquid (a perfluorocarbon). Actually, liquid ventilators are prototypes used for animal experimentations but experts recommend continued development of a liquid ventilator toward clinical applications.
Driving liquid
In total liquid ventilation (TLV), the lungs are completely filled with a perfluorocarbon (PFC) liquid while the liquid ventilator renews the tidal volume of PFC. The liquid ventilator operates in mandatory mode: it must force the PFC in and out of the lungs with a pumping system.- During the inspiratory phase, the pump generates a positive driving pressure in the tracheaVertebrate tracheaIn tetrapod anatomy the trachea, or windpipe, is a tube that connects the pharynx or larynx to the lungs, allowing the passage of air. It is lined with pseudostratified ciliated columnar epithelium cells with goblet cells that produce mucus...
to ensure the PFC insertion of the tidal volume. - During the expiratory phase, the pump generates a negative driving pressure in the tracheaVertebrate tracheaIn tetrapod anatomy the trachea, or windpipe, is a tube that connects the pharynx or larynx to the lungs, allowing the passage of air. It is lined with pseudostratified ciliated columnar epithelium cells with goblet cells that produce mucus...
to ensure PFC withdrawal of the tidal volume.
The pumping system is either a peristaltic pump
Peristaltic pump
A peristaltic pump, or roller pump, is a type of positive displacement pump used for pumping a variety of fluids. The fluid is contained within a flexible tube fitted inside a circular pump casing . A rotor with a number of "rollers", "shoes" or "wipers" attached to the external circumference...
(in the simplest liquid ventilators) or two piston pump
Piston pump
A piston pump is a type of positive displacement pump where the high-pressure seal reciprocates with the piston. Piston pumps can be used to move liquids or compress gases.-Types:* Axial piston pump* Radial piston pump...
s (in the most advanced liquid ventilators).
Because of the PFC viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
, the head loss in the airways requires a low negative driving pressure during the expiration phase that can collapse the airways. This is the choked flow phenomenon in TLV which compromises the minute ventilation and consequently the gas exchanges. To address this limitation, liquid ventilator integrates a control of the pumping system.
Controlling liquid ventilator
The introduction of computers in liquid ventilators to control the pumping system provides different control modes, monitoring and valuable data for decision making.The liquid ventilator is always volume-controlled because the specified tidal volume
Tidal volume
Tidal volume is the lung volume representing the normal volume of air displaced between normal inspiration and expiration when extra effort is not applied.Typical values are around 500ml or 7ml/kg bodyweight.-Mechanical Ventilation:...
of PFC must be accurately delivered and retrieved. It is also pressure-limited because it must stop the expiratory or inspiratory phase when a too low, or a too large, driving pressure is detected.
However, during the expiratory phase, the expiratory flow can be commanded by an open-loop controller
Open-loop controller
An open-loop controller, also called a non-feedback controller, is a type of controller that computes its input into a system using only the current state and its model of the system....
or a closed-loop controller:
- when the expiratory flow is open-loop controlled, it is fast initially and slowing down progressively after to minimize the risk of collapse generation.
- when the expiratory flow is closed-loop controlled, it is commanded in real-time to maintain a specified driving pressure. This is a pressure-regulated mode. Such approach automatically avoids collapse generation.
Also, during the inspiratory phase, the volume-controlled mode is realized by open-loop or closed loop control of the PFC flow.
Oxygenating and heating liquid
The liquid ventilator removes Carbon dioxideCarbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
(CO2) from the PFC by saturating it with oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
(O2) and medical air. This procedure can be performed with either a membrane oxygenator (a technology used in extracorporeal
Extracorporeal
An extracorporeal medical procedure is a medical procedure which is performed outside the body.-Circulatory procedures:A procedure in which blood is taken from a patient's circulation to have a process applied to it before it is returned to the circulation...
oxygenators) or a bubble oxygenator.
The liquid ventilator heats the PFC to body temperature. This is performed with an heat exchanger
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...
connected to the oxygenator or with dedicated heaters integrated in the oxygenator.
The oxygenator and the heater produce PFC vapor which is recuperated with a condenser
Condenser
Condenser may refer to:*Condenser , a device or unit used to condense vapor into liquid. More specific articles on some types include:*Air coil used in HVAC refrigeration systems...
in order to limit the evaporation loss (the PFC is a greenhouse gas
Greenhouse gas
A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...
).
Example
An example of a liquid ventilator is the Inolivent-4. It is composed of 2 independent piston pumps and integrated unit allowing for oxygenation of PFC, temperature control, and recovery of evaporated PFC. This liquid ventilator also includes volume and pressure control strategies to optimize the ventilatory cycle: it performs a pressure-regulated volume-controlled ventilation mode. It is designed for experimental research on animal models weighing between 0.5 kg to 9 kg.A typical cycle is composed of 4 steps :
- Inspiratory pump inserts a volume of PFC in the lungs (valve 1 open, valve 2 closed), and the expiratory pump pushes PFC in the oxygenator via the filter (valve 3 closed, valve 4 open).
- During the inspiratory pause (all valves are closed), the lung volume is at its maximal value. The measured pressure is the Positive End-Inspiratory Pressure (PEIP).
- Expiratory pump retrieves a volume of PFC in the lungs (valve 3 open, valve 4 closed), and the inspiratory pump draws PFC from the reservoir (valve 1 closed, valve 2 open).
- During the expiratory pause (all valves are closed), the lung volume is at its minimal value. The measured pressure is the Positive End-expiratory Pressure (PEEP).
Potential applications
Studies have shown both the efficacy and safety of liquid ventilation in normal, mature and immature newborn lungs. Overall, liquid ventilation improves gas exchange and lung compliance and prevents the lungs against ventilation-induced lung injury.Respiratory support
Studies suggest clear benefits of liquid ventilation in acute respiratory distress syndromeAcute respiratory distress syndrome
Acute respiratory distress syndrome , also known as respiratory distress syndrome or adult respiratory distress syndrome is a serious reaction to various forms of injuries to the lung....
(ARDS). For example, total liquid ventilation could be used for newborns with severe neonatal respiratory distress syndrome
Respiratory distress syndrome
There are two forms of respiratory distress syndrome:* ARDS, which is acute respiratory distress syndrome* Infant respiratory distress syndrome , which is a complication of premature birth, also known as hyaline membrane disease Also, respiratory distress can mean:* Shortness of breath*...
in which conventional treatment has failed. Typical cases are late preterm newborns who have an increased risk of intracranial hemorrhage
Intracranial hemorrhage
An intracranial hemorrhage is a hemorrhage, or bleeding, within the skull.-Causes:Intracranial bleeding occurs when a blood vessel within the skull is ruptured or leaks. It can result from physical trauma or nontraumatic causes such as a ruptured aneurysm...
and for whom their small vessel size poses technical limitations for Extracorporeal membrane oxygenation
Extracorporeal membrane oxygenation
In intensive care medicine, extracorporeal membrane oxygenation is an extracorporeal technique of providing both cardiac and respiratory support oxygen to patients whose heart and lungs are so severely diseased or damaged that they can no longer serve their function...
(ECMO).
Therapeutic lung lavage
Liquid ventilator can perform therapeutic lung lavage, the washout of endogenous and exogenous debris from the lungs, without suspension of ventilation support (without apneaApnea
Apnea, apnoea, or apnœa is a term for suspension of external breathing. During apnea there is no movement of the muscles of respiration and the volume of the lungs initially remains unchanged...
). For example, literature data suggest a radical change in the treatment of meconium aspiration syndrome
Meconium aspiration syndrome
Meconium aspiration syndrome is a medical condition affecting newborn infants. It occurs when meconium is present in their lungs during or before delivery...
(MAS) by considering the use of a liquid ventilator. The demonstration of its efficacy was performed in the neonatal lamb.
Therapeutic hypothermia with rapid cooling
The liquid ventilator with advanced control temperature of PFC allows the rapid cooling of the body. Consequently, therapeutic hypothermiaTherapeutic hypothermia
Therapeutic hypothermia, also known as protective hypothermia, is a medical treatment that lowers a patient's body temperature in order to help reduce the risk of the ischemic injury to tissue following a period of insufficient blood flow. Periods of insufficient blood flow may be due to cardiac...
is an expected clinical application. For example, a study present that rapid cooling instituted by TLV improves cardiac and mitochondrial function.