amount of man-power required for such manual intervention, mechanical positive-pressure ventilators became increasingly popular. positive-pressure ventilators work by increasing the patient's airway pressure through an endotracheal or tracheostomy tube. the positive pressure allows air to flow into the airway until the ventilator breath is terminated. then, the airway pressure drops to zero, and the elastic recoil of the chest wall and lungs push the tidal volume — the breath-out through passive exhalation. negative pressure main article: negative pressure ventilator negative pressure mechanical ventilators are produced in small, field-type and larger formats. the prominent design of the smaller devices is known as the cuirass, a shell-like unit used to create negative pressure only to the chest using a combination of a fitting shell and a soft bladder. in recent years this device has been manufactured using various-sized polycarbonate shells with multiple seals, and a high-pressure oscillation pump in order to carry out biphasic cuirass ventilation. its main use has been in patients with neuromuscular disorders that have some residual muscular function. the latter, larger formats are in use, notably with the polio wing hospitals in england such as st thomas' hospital in london and the john radcliffe in oxford. the larger units have their origin in the iron lung, also known as the drinker and shaw tank, which was developed in 1928 by j.h emerson company and was one of the first negative-pressure machines used for long-term ventilation. it was refined and used in the 20th century largely as a result of the polio epidemic that struck the world in the 1940s. the machine is, in effect, a large elongated tank, which encases the patient up to the neck. the neck is sealed with a rubber gasket so that the patient's face (and airway) are exposed to the room air. while the exchange of oxygen and carbon dioxide between the bloodstream and the pulmonary airspace works by diffusion and requires no external work, air must be moved into and out of the lungs to make it available to the gas exchange process. in spontaneous breathing, a negative pressure is created in the pleural cavity by the muscles of respiration, and the resulting gradient between the atmospheric pressure and the pressure inside the thorax generates a flow of air. in the iron lung by means of a pump, the air is withdrawn mechanically to produce a vacuum inside the tank, thus creating negative pressure. this negative pressure leads to expansion of the chest, which causes a decrease in intrapulmonary pressure, and increases flow of ambient air into the lungs. as the vacuum is released, the pressure inside the tank equalizes to that of the ambient pressure, and the elastic recoil of the chest and lungs leads to passive exhalation. however, when the vacuum is created, the abdomen also expands along with the lung, cutting off venous flow back to the heart, leading to pooling of venous blood in the lower extremities. the patients can talk and eat normally, and can see the world through a well-placed series of mirrors. some could remain in these iron lungs for years at a time quite successfully. some of the problems with the full body design were such as being unable to control the inspiratory to expiratory ratio and the flow rate. this design also caused blood pooling in the legs. intermittent abdominal pressure ventilator another type is the intermittent abdominal pressure ventilator that applies pressure externally via an inflated bladder, forcing exhalation, sometimes termed exsufflation. the first such apparatus was the bragg-paul pulsator. the name of one such device, the pneumobelt made by puritan bennett has to a degree become a generic name for the type. oscillator 3100a oscillator the most commonly used high frequency ventilator and only one approved in the united states is the 3100a from vyaire medical. it works by using very small tidal volumes by setting amplitude and a high rate set in hertz. this type of ventilation is primarily used in neonates and pediatric patients who are failing conventional ventilation. high frequency jet ventilation the first type of high frequency ventilator made for neonates and the only jet type is made by bunnell incorporated. it works in conjunction with a separate cmv ventilator to add pulses of air to the control breaths and peep.neonatal jet ventilator monitoring one of the main reasons why a patient is admitted to an icu is for delivery of mechanical ventilation. monitoring a patient in mechanical ventilation has many clinical applications: enhance understanding of pathophysiology, aid with diagnosis, guide patient management, avoid complications, and assess trends. in ventilated patients, pulse oximetry is commonly used when titrating fio2. a reliable target of spo2 is greater than 95%. the total peep in the patient can be determined by doing an expiratory hold on the ventilator. if this is higher than the set peep, this indicates air trapping. the plateau pressure can be found by doing an inspiratory hold. this shows the actual pressure the patient's lungs are experiencing. loops can be used to see what is occurring in the patient's lungs. these include flow-volume and pressure-volume loops. they can show changes in compliance and resistance. functional residual capacity can be determined when using the ge carestation. modern ventilators have advanced monitoring tools. there are also monitors that work independently of the ventilator which allow for measuring patients after the ventilator has been removed, such as a tracheal tube test. types of ventilators smart bag mo bag-valve-mask resuscitator ventilators come in many different styles and method of giving a breath to sustain life. there are manual ventilators such as bag valve masks and anesthesia bags that require the users to hold the ventilator to the face or to an artificial airway and maintain breaths with their hands. mechanical ventilators are ventilators not requiring operator effort and are typically computer-controlled or pneumatic-controlled. mechanical ventilators typically require power by a battery or a wall outlet (dc or ac) though some ventilators work on a pneumatic system not requiring power. there are a variety of technologies available for ventilation, falling into two main (and then lesser categories), the two being the older technology of negative-pressure mechanisms, and the more common positive-pressure types. common positive-pressure mechanical ventilators include: transport ventilators—these ventilators are small and more rugged, and can be powered pneumatically or via ac or dc power sources. intensive-care ventilators—these ventilators are larger and usually run on ac power (though virtually all contain a battery to facilitate intra-facility transport and as a back-up in the event of a power failure). this style of ventilator often provides greater control of a wide variety of ventilation parameters (such as inspiratory rise time). many icu ventilators also incorporate graphics to provide visual feedback of each breath. neonatal ventilators (bubble cpap, hfjv, hfov)—designed with the preterm neonate in mind, these are a specialized subset of icu ventilators that are designed to deliver smaller volumes and pressures to these patients. these may be conventional or high frequency types. positive airway pressure ventilators (pap) — these ventilators are specifically designed for non-invasive ventilation. this includes ventilators for use at home for treatment of chronic conditions such as sleep apnea or copd and in the icu setting. breath delivery mechanisms trigger the trigger, either flow or pressure, is what causes a breath to be delivered by a mechanical ventilator. breaths may be triggered by a patient taking their own breath, a ventilator operator pressing a manual breath button, or based on the set respiratory rate. cycle the cycle is what causes the breath to transition from the inspiratory phase to the exhalation phase. breaths may be cycled by a mechanical ventilator when a set time has been reached, or when a preset flow or percentage of the maximum flow delivered during a breath is reached depending on the breath type and the settings. breaths can also be cycled when an alarm condition such as a high pressure limit has been reached. limit limit is how the breath is controlled. breaths may be limited to a set maximum pressure or volume. breath exhalation exhalation in mechanical ventilation is almost always completely passive. the ventilator's expiratory valve is opened, and expiratory flow is allowed until the baseline pressure (peep) is reached. expiratory flow is determined by patient factors such as compliance and resistance. artificial airways as a connection to the ventilator main article: artificial airway there are various procedures and mechanical devices that provide protection against airway collapse, air leakage, and aspiration: face mask — in resuscitation and for minor procedures under anaesthesia, a face mask is often sufficient to achieve a seal against air leakage. airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway. these are designed to provide a passage of air to the pharynx through the nose or mouth, respectively. poorly fitted masks often cause nasal bridge ulcers, a problem for some patients. face masks are also used for non-invasive ventilation in conscious patients. a full-face mask does not, however, provide protection against aspiration. non-invasive ventilation can be considered for epidemics of covid-19 where sufficient invasive ventilation capacity is not available (or in some milder cases), but pressurized protection suits for caregivers are recommended due to the risks of poorly fitting masks emitting contaminating aerosols. tracheal intubation is often performed for mechanical ventilation of hours to weeks duration. a tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea. in most cases, tubes with inflatable cuffs are used for protection against leakage and aspiration. intubation with a cuffed tube is thought to provide the best protection against aspiration. tracheal tubes inevitably cause pain and coughing. therefore, unless a patient is unconscious or anaesthetized for other reasons, sedative drugs are usually given to provide tolerance of the tube. other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis. supraglottic airway — a supraglottic airway (sga) is any airway device that is seated above and outside the trachea, as an alternative to endotracheal intubation. most devices work via masks or cuffs that inflate to isolate the trachea for oxygen delivery. newer devices feature esophageal ports for suctioning or ports for tube exchange to allow intubation. supraglottic airways differ primarily from tracheal intubation in that they do not prevent aspiration. after the introduction of the laryngeal mask airway (lma) in 1998, supraglottic airway devices have become mainstream in both elective and emergency anesthesia. there are many types of sgas available including the esophageal-tracheal combitube (etc), laryngeal tube (lt), and the obsolete esophageal obturator airway (eoa). cricothyrotomy — patients requiring emergency airway management, in whom tracheal intubation has been unsuccessful, may require an airway inserted through a surgical opening in the cricothyroid membrane. this is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access. tracheostomy — when patients require mechanical ventilation for several weeks, a tracheostomy may provide the most suitable access to the trachea. a tracheostomy is a surgically created passage into the trachea. tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs. tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease, or in any patient expected to be difficult to wean from mechanical ventilation, i.e., patients with little muscular reserve. mouthpiece — less common interface, does not provide protection against aspiration. there are lipseal mouthpieces with flanges to help hold them in place if patient is unable.