Medical Ventilators

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 Medical Ventilators Ventilators

1.Introduction A medical ventilator (or simply ventilator in context) is a 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. insufficiently. While modern ventilators are computerized machines, patients can be ventilated Katrina,,  with a  a bag valve mask , a simple hand-operated machine. After Hurricane Katrina dedicated staff "bagged" patients in  in  New Orleans  Orleans hospitals for days with simple bag valve units attached to endotracheal tubes, a "ventilator" system which can be used with no definite time limit. Ventilators are chiefly used in  in intensive care medicine medicine,, home care care,, and and  emergency medicine  (as standalone units) and in  medicine in  anesthesia  anesthesia (as a component of an  an anesthesia machine)). machine Mechanical ventilators, which are often also called respirators, are used to artificially ventilate the lungs of patients who are unable to naturally breathe from the atmosphere. In almost 100 years of development, many mechanical ventilators with different designs have been developed [Mushin et al., 1980Philbeam, 1998]. The very early devices used bellows that were manually operated to inflate the lungs. Today’s respirators employ an array of sophisticated components such as microprocessors, fast response servo valves, and precision transducers to perform the task of ventilating the lungs. The changes in the design of ventilators have come about as the result of improvements in engineering the ventilator components and the advent of new therapy modes by clinicians. A large variety of ventilators are now available for short-term treatment of acute respiratory problems as well as long-term therapy for chronic respiratory conditions. It is reasonable to broadly classify today’s ventilators ventil ators into two groups. The first and indeed the largest group encompasses the intensive care respirators used primarily in hospitals to support patients following certain surgical procedures or assist patients with acute respiratory disorders. The second group includes less complicated machines that are primarily used at home to treat patients with chronic respiratory respirator y disorders. disorders.

2. Need of machine (Life-critical system): Because the failure of a mechanical ventilation system may result in death, it is classed as a  a life-critical system, system, and precautions must be taken to ensure that mechanical ventilation systems are highly reliable. This includes power-supply  provision. their  power-supply their Mechanical ventilators are therefore carefully designed so that no single point of  failure can endanger the patient. They may have manual backup mechanisms to enable hand-driven respiration in the absence of power (such as the mechanical ventilator integrated into an  an anaesthetic machine. machine. They may also have safety valves, which open to atmosphere in the absence of power to act as an anti-

 

suffocation valve for the spontaneously breathing patient. Some systems are also equipped with compressed-gas tanks, air compressors, and/or backup batteries to provide ventilation in case of power failure or defective defe ctive gas supplies, and methods to operate or call for help if their mechanisms or software fail.

3. Basic principle of Ventilation theory: The system delivers controlled volume or pressure breath profiles in response to clinician inputs.The ventilator is time cycled for controlled breaths and flow cycled with a time cycle override, for spontaneous breaths. The system will trigger on both pressure and flow, and will respond to a positive inspiratory trigger condition within eight ms. The system uses proportional flow control valves and an active exhalation valve in order to provide ventilation delivery. A controllable bias flow is maintained during ventilation delivery for use in detecting and responding to the spontaneous breath activity of the patient. The system incorporates monitoring of airway pressure, FiO 2, and exhaled volume monitoring that is independent of the ventilation delivery system. The system also includes an integrated nebulizer system employing electronic micropump technology for delivery of inhaled drugs. The system is a software controlled microprocessor based product that receives clinical control inputs and displays information via a graphical user interface display unit. The display unit communicates in real time with two other system microprocessors that control ventilation delivery and safety related ventilation monitoring. The display unit also communicates with Datex-Ohmeda monitoring modules in order to acquire and display additional monitoring information such as CO 2 and O2. 4. construction: The Ventilator Ventilator can be divided into two main units: 1.  Control unit (1). The control unit consists of the panel section (2) and the control section (3). 2.  Patient unit (4). The patient unit consists of the pneumatic section (5) and the power section (6). The patient unit and the control unit are connected to each other with the inter- connect connection ion cable cable (7).

 

 

 

 

Ventilators:    

Get oxygen into the lungs. Remove carbon dioxide from the body. (Carbon dioxide is a waste gas that can be toxic.)

 

Help people breathe easier.

 

Breathe for people who have lost all ability to breathe on their own.









Air and supplemental  supplemental oxygen  oxygen is forced into the lungs through a medical ventilator. The ventilator helps patients improve oxygen levels in the lungs. It can also help remove carbon dioxide from the lungs and decrease how hard someone has to work to breathe. Mechanical ventilation ventilation  can be used to provide complete support, which means the ventilator is doing on the breathing for the patient. It can also be used to assist with breathing. This means the patient is still breathing on his own, but the ventilator is providing some assistance assistance..

Function In its simplest form, a modern positive pressure ventilator consists of a compressible air reservoir or turbine, air and oxygen supplies, a set of valves and tubes, and a disposable or reusable "patient circuit". The air reservoir is pneumatically compressed several times a minute to deliver room-air, or in most cases, an air/oxygen mixture to the patient. If a turbine is used, the turbine pushes air through the ventilator, with a flow valve adjusting pressure to meet patient-specific parameters. When overpressure is released, the patient will

exhale passively due to the lungs' elasticity, the exhaled air being released r eleased usually through a one-way valve within the patient circuit called the patient manifold. The oxygen content of 

 

the inspired gas can be set from 21 percent (ambient air) to 100 percent (pure oxygen). ox ygen). Pressure and flow characteristics can be set mechanically or electronically. Ventilators may also be equipped with monitoring and alarm systems for patient-related parameters (e.g. pressure, volume, and flow) and ventilator function f unction (e.g. air leakage, l eakage, power failure, mechanical failure), backup batteries, oxygen tanks, and remote control. The pneumatic system is nowadays often replaced by a computer-controlled  computer-controlled turbopump. turbopump.  Modern ventilators are electronically controlled by a small  small  embedded system  system to allow exact adaptation of pressure and flow characteristics to an individual patient's needs. Fine-tuned ventilator settings also serve to make ventilation more tolerable and comfortable for the therapists  are responsible for tuning patient. In Canada, and the United States,  States, respiratory therapists these settings while biomedical technologists are responsible for the maintenance. The patient circuit usually consists of a set of three durable, yet lightweight plastic tubes, separated by function (e.g. inhaled air, patient pressure, exhaled air). Determined by the ttype ype of ventilation needed, the patient-end of the circuit may be either noninvasive or invasive. Noninvasive methods, which are adequate for patients who require a ventilator only while sleeping and resting, mainly employ a nasal mask. Invasive methods require require  intubation intubation,,  which for long-term ventilator dependence will normally be a  a tracheotomy  tracheotomy cannula, as this is much more comfortable and practical for long-term care than is larynx or nasal intubation. Control describes how the ventilator delivers flow to the patient. In volume-controlled ventilators, the flow delivered is constant, the tidal volume is targeted, with a variable pressure delivered relative to compliance of the lung. In pressure-controlled ventilators, the flow decelerates through the breath to maintain the targeted pressure at the peak inspired pressure. The tidal volume delivered is determined by the t he compliance of the ventilated lung.

History The early history of mechanical ventilation begins with various versions of what was eventually called the  the iron lung, lung, a form of noninvasive negative pressure ventilator widely used during the  the polioepidemics polioepidemics of the 20th century after the introduction of the "Drinker Emerson  in respirator" in 1928, and the subsequent improvements introduced introduced by  by John Haven Emerson [1] 1931..  Other forms of noninvasive ventilators, also used widely for polio patients, 1931 primit ive positive pressure include  Biphasic Cuirass Ventilation, include Ventilation, the rocking bed, and rather primitive [1] machines..   machines In 1949, John Haven Emerson developed a mechanical assister for anesthesia with the cooperation of the anesthesia department at  at Harvard University. University. Mechanical ventilators began to be used increasingly in anesthesia and intensive care during the 1950s. Their development was stimulated both by the need to treat polio patients and tthe he increasing use of  muscle relaxants  relaxants during anesthesia. Relaxant drugs paralyze the patient and improve operating conditions for the surgeon, but also paralyze the respiratory muscles In the United Kingdom, the East Radcliffe and Beaver models were early examples, the later [2] using an automotive wiper motor to drive the bellows used to inflate the lungs lu ngs..  Electric

 

motors were, however, a problem in the operating theatres of that time, as their use caused an ether  and and  cyclopropane cyclopropane.. In explosion hazard in the presence of flammable anesthetics such as  as ether the Westminster Hospital Hospital,, London, developed a ventilator which was 1952,  Roger Manley  1952, Manley of the  entirely gas driven, and became the most popular model used in Europe. It was an elegant design, and became a great favourite with European anesthetists for four decades, prior to the introduction of models controlled by electronics. It was independent of electrical power, and caused no explosion hazard. The original Mark I unit was developed to become the Manley Mark units. II in collaboration with the Blease whoan manufactured of lift a these Its principle of operation wascompany, very simple, incoming gasmany flow thousands was used to weighted bellows unit, which fell intermittently intermitt ently under gravity, forcing breathing gases into the patient's lungs. The inflation pressure could be varied by sliding the movable weight on top of the bellows. The volume of gas delivered was adjustable using a curved slider, which restricted bellows excursion. Residual pressure after the completion of expiration was also configurable, using a small weighted arm visible to the lower right of the front panel. This was a robust unit and its availability encouraged the introduction of positive pressure ventilation techniques into mainstream European anesthetic practice. The 1955 release of  Forrest Bird' Bird's "Bird Universal Medical Respirator" in the United States, changed the way mechanical ventilation was performed with the small green box becoming a [3] familiar piece of medical equipment. equipment.  The unit was sold as the Bird Mark 7 Respirator and no electrical informally called the "Bird". It was a  a  pneumatic  pneumatic device and therefore required no  power  source to operate. power Intensive care environments around the world revolutionalized in 1971 by the introduction of  900  ventilator (Elema-Schönander) Elema-Schönander). It was a small, silent and effective the first  first SERVO 900 system  controlling what had been set electronic ventilator, with the famous  famous SERVO feedback system and regulating delivery. For the first time, the machine could deliver the set volume in volume control ventilation. Ventilators used under increased pressure (hyperbaric) require special precautions and few [4] ventilators can operate under these conditions. conditions.  In 1979, Sechrist Industries introduced their [5] Model 500A ventilator which was specifically designed for use with  with hyperbaric chambers chambers..   300  ventilator series was introduced. The platform of the SERVO 300 In 1991 the  the SERVO 300 series enabled treatment of all patient categories, from adult to neonate, with one single ventilator. The SERVO 300 series provided a completely new and unique gas delivery system, with rapid flow-triggering response. A modular concept, meaning that the hospital has one ventilator model throughout the ICU department instead of a fleet with different models and brands for the different user needs, SERVO-i  in 2001. With this modular concept the ICU departments was introduced with  with SERVO-i could choose the modes and options, software and hardware needed for a particular patient category. Types:

 Negative-pressure  Negative-pre ssure ventilators 

The original ventilators used negative pressure to remove and replace r eplace gas from the ventilator chamber. Examples of these include the iron lung, the Drinker respirator, and the chest shell.

 

Rather than connecting to an artificial airway, these ventilators enclosed the body from the outside. As gas was pulled out of the ventilator chamber, the resulting r esulting negative pressure caused the chest wall to expand, which pulled air into the lungs. The cessation of the negative pressure caused the chest wall to fall f all and exhalation to occur. While an advantage of these ventilators was that they did not require insertion of an artificial airway, they were noisy, made nursing care difficult, and the patient was not able to ambulate.  Positive-pressure  Positive-pres sure ventilators 

Postive-pressure ventilators require an artificial airway (endotracheal or tracheostomy tube) and use positive pressure to force gas into a patient's llungs. ungs. Inspiration can be triggered either by the patient or the machine. There are four types of positive-pressure ventilators: volumecycled, pressure-cycled, flow-cycled, and time-cycled. ti me-cycled. VOLUME-CYCLED VENTILATORS. 

This type delivers a preset tidal volume then allows passive expiration. This is ideal for patients with acute respiratory distress syndrome (ARDS) or bronchospasm, since the same tidal volume is delivered regardless of the amount of airway rresistance. esistance. This type of  ventilator is the most commonly used in critical care environments. PRESSURE-CYCLED VENTILATORS. 

These ventilators deliver gases at a preset pressure, then allow passive expiration. The benefit of this type is a decreased risk of lung damage from high inspiratory pressures, which is particularly beneficial for neonates who have a small lung capacity. The disadvantage is that the tidal volume delivered can decrease if the patient has poor lung compliance and increased airway resistance. This type of ventilation is usually used for short-term therapy (less than 24 hours). Some ventilators have the capability to provide both volume-cycled and pressurecycled ventilation. These combination ventilators are also commonly used in critical care environments.

FLOW-CYCLED VENTILATORS.  Flow-cycled ventilators deliver oxygenation until a preset flow rate is achieved during inspiration. TIME-CYCLED VENTILATORS.  Time-cycled ventilators deliver oxygenation over a preset time period. These types t ypes of  ventilators are not used as frequently as the volume-cycled and pressure-cycled ventilators.

 

Modes of ventilation

Mode refers to how the machine will ventilate the patient in relation r elation to the patient's own respiratory efforts. There is a mode for nearly every patient situation; plus, many different types can be used in conjunction with each other.

CONTROL VENTILATION (CV). 

CV delivers the preset volume or pressure regardless of the patient's own inspiratory i nspiratory efforts. This mode is used for patients who are unable to initiate a breath. If it is used with spontaneously spontaneous ly breathing patients, they must be sedated and/or pharmacologically paralyzed so they don't breathe out of synchrony with the ventilator. ASSIST-CONTROL VENTILATION (A/C) OR CONTINUOUS MANDATORY VENTILATION (CMV). A/C or CMV delivers the preset volume or pressure in response to the patient's inspiratory effort, but will initiate the t he breath if the patient does not do so wit within hin a preset amount of time. This mode is used for patients who can initiate a breath but who have weakened respiratory muscles. The patient may need to be sedated to limit the number of spontaneous breaths, as

hyperventilation can occur in patients with high respiratory rates. SYNCHRONOUS INTERMITTENT MANDATORY VENTILATION (SIMV).  SIMV delivers the preset volume or pressure and preset respiratory rate while allowing the patient to breathe spontaneously. The vent initiates each breath in synchrony with the patient's breaths. SIMV is used as a primary mode of ventilation as well as a weaning mode. (During weaning, the preset rate is gradually reduced, allowing the patient to slowly regain breathing on their own.) The disadvantage of this mode is that it may increase the effort of  breathing and cause respiratory muscle fatigue. (Breathing spontaneously through ventilator tubing has been compared to breathing through a straw.) POSITIVE-END EXPIRATORY PRESSURE (PEEP). 

PEEP is positive pressure that is applied by the ventilator at the end of expiration. This mode does not deliver breaths but is used as an adjunct to CV, A/C, and SIMV S IMV to improve oxygenation by opening collapsed alveoli at the end of expiration. Complications from the increased pressure can include decreased cardiac output, lung rupture, and increased intracranial pressure.

CONSTANT POSITIVE AIRWAY PRESSURE (CPAP).  CPAP is similar to PEEP, except that it works only for patients who are breathing spontaneously. spontaneous ly. The effect of CPAP (and PEEP) is compared to inflating a balloon but not letting it completely deflate before inflating it again. The second inflation is easier to perform because resistance is decreased. decreased. CPAP can also be administered using a mask and CPAP

 

machine for patients who do not require mechanical ventilation but who need respiratory support (for example, patients with sleep apnea). PRESSURE SUPPORT VENTILATION (PSV).  PS is preset pressure which augments the patient's spontaneous inspiration effort and decreases the work of breathing. The patient completely controls the t he respiratory rate and tidal volume. PS is used for patients with a stable respiratory status and is often used with SIMV S IMV

during weaning. INDEPENDENT LUNG VENTILATION (ILV).  This method is used to ventilate each lung separately in patients with unilateral lung disease or a different disease process in each lung. It requires a double-lumen endotracheal tube and two ventilators. Sedation and pharmacologic pharmacologic paralysis are used to facilitate optimal ventilation and increase comfort for the patient on whom this method is used. High-frequency ventilation (Active)

The term active refers to the ventilators forced expiratory system. In a HFV-A scenario, the ventilator uses pressure to apply an inspiratory breath and then applies an opposite pressure to force an expiratory breath. In high-frequency hi gh-frequency oscillatory ventilation (sometimes abbreviated HFOV) the oscillation bellow and piston force positive pressure in and apply negative   pressure to force an expiration.

High-frequency ventilation (Passive)

The term passive refers to the ventilators non-forced expiratory system. In a HFV-P scenario, the ventilator uses pressure to apply an inspiratory breath and then simply returns to atmospheric pressure to allow for a passive expiration. This is seen in High-Frequency Jet Ventilation, sometimes abbreviated HFJV. INVERSE RATIO VENTILATION (IRV).  The normal inspiratory:expiratory ratio is 1:2, but this is reversed during IRV to 2:1 or

greater (the maximum is 4:1). This method is used for patients who are still hypoxic, even with the use of PEEP. Longer inspiratory time increases the amount of air in the lungs at the end of expiration (the functional residual capacity) and improves oxygenation by reexpanding collapsed alveoli. The shorter expiratory time prevents the alveoli from collapsing again. This method requires sedation and therapeutic paralysis because it is very uncomfortable for the patient.

Ventilator settings  settings 

Ventilator settings are ordered by a physician and are individualized for the patient. Ventilators are designed to monitor most components of the patient's respiratory status. Various alarms and parameters can be set to warn healthcare providers that the patient is having difficulty with the settings.

 

RESPIRATORY RATE.  The respiratory rate is the number of breaths the ventilator will deliver to the patient over a specific time period. The respiratory rate parameters are set above and below this number, and an alarm will sound if the patient's actual rate is outside the desired range. TIDAL VOLUME.  Tidal volume is the volume of gas the ventilator will deliver to the patient with each breath. The usual setting is 5-15 cc/kg. The tidal volume parameters are set above and below this

number and an alarm sounds if the patient's actual tidal volume is outside the desired range. This is especially helpful if the patient is breathing spontaneously between ventilatordelivered breaths since the patient's own tidal volume can be compared with the desired tidal volume delivered by the ventilator. OXYGEN CONCENTRATION (FIO2).  Oxygen concentration is the amount of oxygen delivered to the patient. It can range from 21% (room air) to 100%. INSPIRATORY:EXPIRATORY (I:E) RATIO.  As discussed above, above, the I:E ratio is normally 1:2 or 1:1.5, unless inverse ratio ventilation is desired.  PRESSURE LIMIT.  Pressure limit regulates the amount of pressure the volume-cycled ventilator can generate to deliver the preset tidal volume. The usual setting is 10-20 cm H2O above the patient's peak  inspiratory pressure. If this limit is reached the ventilator stops the breath and alarms. This is often an indication that the patient's airway is obstructed with mucus and is usually resolved with suctioning. It can also al so be caused by the patient coughing, biting on the endotracheal tube, breathing against the ventilator, or by a kink in the ventilator tubing. FLOW RATE.  Flow rate is the speed with which the tidal volume is delivered. The usual setting is 40-100 liters per minute. SENSITIVITY/TRIGGER.  Sensitivity determines the amount of effort required by the patient to initiate inspiration. It can be set to be triggered by pressure or by flow. SIGH.  The ventilator can be programmed to deliver an occasional sigh with a larger tidal volume. This prevents collapse of the alveoli (atelectasis) which can result from the patient constantly inspiring the same volume of gas.

 

Operation

Many ventilators are now computerized and have a user-friendly user-friendl y control panel. To activate the various modes, settings, and alarms, the appropriate key need only onl y be pressed. There are windows on the face panel which show settings and the alarm values. Some ventilators have dials instead of computerized keys, e.g., the smaller, portable ventilators used for transporting patients. The ventilator tubing simply attaches to the ventilator on one end and to the patient's artificial airway on the other. Most ventilators have clamps that prevent the tubing fr from om draping across the patient. However, there should be enough slack so that the artificial airway isn't accidentally pulled out if the patient turns. Ventilators are electrical equipment so they must be plugged in. They do have battery b back  ack  up, but this is not designed for long-term use. It should be ensured that they are plugged into an outlet that will receive generator power if there is an electrical power outage. Ventilators are a method of life-support. If the ventilator should stop working, the patient's life will be in  jeopardy. There should be a bag-valve-mask bag-valve-mask device device at the bedside bedside of every patient receiving receiving mechanical ventilation so they can be manually ventilated if needed.

Other ventilation modes and strategies

Liquid ventilation

Liquid ventilation  ventilation is a technique of mechanical ventilation in which the lungs are insufflated with an oxygenated perfluorochemical liquid rather than an oxygen-containing gas mixture. The use of perfluorochemicals, rather than nitrogen, as the t he inert carrier of oxygen and carbon dioxide offers a number of theoretical advantages for the treatment of acute lung injury, including: 







 

Reducing surface tension by maintaining a fluid interface in terface with alveoli

Opening of collapsed alveoli by hydraulic pressure with a lower risk of barotrauma Providing a reservoir in which oxygen and carbon dioxide can be exchanged with pulmonary capillary blood   Functioning as a high efficiency heat exchanger

   

Despite its theoretical advantages, efficacy studies have been disappointing and the optimal [26] clinical use of LV has yet to be defined. defined.   Total liquid ventilation In total liquid ventilation (TLV), the entire lung is filled with an oxygenated PFC liquid, and a liquid tidal volume of PFC is actively pumped into and out of the lungs. A specialized apparatus is required to deliver and remove the relatively dense, viscous PFC ti tidal dal volumes,   and to extracorporeally oxygenate and remove carbon dioxide from the liquid. l iquid.

 

 Partial liquid ventilation In partial liquid ventilation (PLV), the lungs are slowly filled with a volume of PFC equivalent or close to the FRC during gas ventilation. The PFC within the lungs is oxygenated and carbon dioxide is removed by means of gas breaths cycling c ycling in the lungs by a conventional gas ventilator.

Maintenance When mechanical mechanical ventilation is initiated, the ventilator goes through a self-test self- test to ensure it is working properly. The ventilator tubing should be changed every 24 hours and another selftest run afterwards. The bacteria filters should be checked for occlusions or tears and the water traps and filters should be checked for condensation or contaminants. These should be emptied and cleaned every 24 hours and as needed.

Ventilator Circuits

The ventilator circuit is a system s ystem of tubing that connects the patient’s airway to the ventilator  itself. Most ventilator circuits are disposable, but reusable circuits are still in use. Regardless of the type, all circuits include inhalation tubing, exhalation tubing, ports for pressure detectors, a Yconnector, an elbow, a universal adaptor, and some sort of humidification and heating system. The circuit may also include an adaptor for an in-line suction system or adapters for delivery of aerosol medications.

Trouble Shooting of ventilator

The goal of the ventilator alarm system is to warn of events. The alarms are individualized for each Events are categorized into patient. into mechanical/technical mechanical/technical and patient generated events. events. They are categorized according to levels of priority. malfunction n (life threatening)  Level 1 - Ventilator malfunctio • No gas delivery to patient • Excessive gas delivery to patient • Exhalation valve failure • Loss of electric power   Level 2 - Ventilator malfunction malfunctio n (not immediately life threatening) threa tening) • Blender failure • Loss of PEEP or excessive PEEP

• Autocycling

 

• Circuit leak  • Circuit partially occluded • Inappropriate I:E ration • Inappropriate heater/humidifier function  Level 3  –  Patient event affecting ventilator-patient interface • Change in ventilatory dr ive ive (CNS, peripheral nerves or muscle function) • Change in compliance/resistance (air trapping, barotrauma)

• Auto PEEP  Level 4  –  Patient event not affecting ventilator-patient interface • Change is gas exchange (Capnograph, oximeter, ) • Change in respiratory system impedance • Change in muscle function • Change in cardiovascular function.  function. 

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