ventilator

                         

VENTILATOR

A medical ventilator (or simply ventilator in context) is a machine designed to mechanically move breathable air into and out of the lungs, to provide the mechanism of breathing for a patient who is physically unable to breathe, or  While modern ventilators are computerized machines, patients can be ventilated with a bag valve mask, a simple hand-operated machine. After Hurricane Katrina, dedicated staff "bagged" patients in New 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 intensive care medicine, home care, and emergency medicine (as standalone units) and in anesthesia (as a component of an anesthesia machine).

Medical ventilators are sometimes colloquially called "respirators," a term which stems from commonly used devices in the 1950s (particularly the "Bird Respirator"). However, in modern hospital and medical terminology, these machines are never referred to as respirators, and use of "respirator" in this context is now a deprecated anachronism which signals technical unfamiliarity.

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 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). 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 (e.g. air leakage, power failure, mechanical failure), backup batteries, oxygen tanks, and remote control. The pneumatic system is nowadays often replaced by a computer-controlled turbopump.

Modern ventilators are electronically controlled by a small embedded 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 patient. In Canada, and the United States, respiratory therapists are responsible for tuning 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 type 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 intubation, which for long-term ventilator dependence will normally be a tracheotomy cannula, as this is much more comfortable and practical for long-term care than is larynx or nasal intubation.

Life-critical system

Because the failure of a mechanical ventilation system may result in death, it is classed as a life-critical system, and precautions must be taken to ensure that mechanical ventilation systems are highly reliable. This includes their power-supply provision.

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 anaesthetic 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 gas supplies, and methods to operate or call for help if their mechanisms or software fail.

History

The early history of mechanical ventilation begins with various versions of what was eventually called the iron lung, a form of noninvasive negative pressure ventilator widely used during the polio epidemics of the 20th century after the introduction of the "Drinker respirator" in 1928, and the subsequent improvements introduced by John Haven Emerson in 1931.^[ Other forms of noninvasive ventilators, also used widely for polio patients, include Biphasic Cuirass Ventilation, the rocking bed, and rather primitive positive pressure machines.

In 1949, John Haven Emerson developed a mechanical assister for anesthesia with the cooperation of the anesthesia department at Harvard 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 the increasing use of muscle relaxants during anesthesia. Relaxant drugs paralyze the patient and improve operating conditions for the surgeon, but also paralyze the respiratory muscles

Biphasic Cuirass Ventilation

Biphasic Cuirass Ventilation (BCV) is a method of ventilation which requires the patient to wear an upper body shell or cuirass, so named after the body armor worn by medieval soldiers. The ventilation is biphasic because the cuirass is attached to a pump which actively controls both the inspiratory and expiratory phases of the respiratory cycle. This method has also been described as 'Negative Pressure Ventilation' (NPV), 'External Chest Wall Oscillation' (ECWO), 'External Chest Wall Compression' (ECWC) and 'External High Frequency Oscillation' (EHFO). BCV may be considered a refinement of the iron lung ventilator. Biphasic Cuirass Ventilation was developed by Dr Zamir Hayek, a pioneer in the field of assisted ventilation. Some of Dr Hayek's previous inventions include the Hayek Oscillator, an early form of the technology

Modes of mechanical ventilation

Modes of mechanical ventilation are one of the most important aspects of the usage of mechanical ventilation. The mode refers to the method of inspiratory support. In general, mode selection is based on clinician familiarity and institutional preferences, since there is a paucity of evidence indicating that the mode affects clinical outcome. The most frequently used forms of volume-limited mechanical ventilation are IMV and CMV.  There have been substantial changes in the nomenclature of mechanical ventilation over the years, but more recently it has become standardized by many respirology /pulmonology groups. Writing a mode is most proper in all capital letters with a dash between the cycle and the strategy (i.e. PC-IMV, or VC-MMV etc.)

Cycle

Cycling is the method for how a ventilator knows to give a breath and stop a breath. Cycling is the governing system for how a breath will ultimately be applied. Parameters vary but rate (), I:E and other similar parameters are almost always set by the clinician alongside the cycle

Volume controlled

Flow-volume loop

Volume controlled systems of ventilation are based on a measured volume variable that is set by the clinician. When the ventilator detects the set volume  having been applied, the ventilator cycles to exhalation. This is measured various ways by each brand and model. Some ventilators  measure using a flow sensor at the circuit wye while some measure where the expiratory circuit plugs into the expiratory port on the v entilator body, which means that the machine will give all support to take breath to the patient.

Pressure-controlled

Pressure-controlled cycling is based on an applied positive pressure that is set by the clinician. In pressure-controlled modes, the total volume is variable, as the ventilator is using only the pressure as a measurement for cycling. Most ventilators calculate pressure at the expiratory circuit though some measure near the circuit with a proximal pressure line.

Spontaneously controlled

Spontaneously controlled cycling is a flow-sensed mode dependent on a spontaneously breathing patient to cycle. Spontaneously controlled ventilation is typically only in reference to continuous spontaneous ventilation, also called continuous positive airway pressure (CPAP).

Negative pressure-controlled

Negative pressure ventilation cycles by producing a negative pressure around the chest and abdomen. Negative pressure moves across the chest and diaphragm and causes air to move into the lungs in the normal fashion.^] When the negative pressure stops being applied, the chest returns to atmospheric pressure and the inspired air then is exhaled.

Strategy

Airway pressure release ventilation

Airway pressure release ventilation graph

Airway pressure release ventilation is a time-cycled alternant between two levels of positive airway pressure, with the main time on the high level and a brief expiratory release to facilitate ventilation

Airway pressure release ventilation is usually utilized as a type of inverse ratio ventilation. The exhalation time (T_low) is shortened to usually less than one second to maintain alveoli inflation. In the basic sense, this is a continuous pressure with a brief release. APRV currently  the most efficient conventional mode for lung protective ventilation.

Different perceptions of this mode may exist around the globe. While 'APRV' is common to users in North America, a very similar mode, biphasic positive airway pressure (BIPAP), was introduced in Europe. The term APRV has also been used in American journals where, from the ventilation characteristics, BIPAP would have been perfectly good terminology.

 But BiPAP(tm) is a trademark for a noninvasive ventilation mode in a specific ventilator (Respironics Inc.).

Other manufacturers have followed with their own brand names (BILEVEL, DUOPAP, BIVENT). Although similar in modality, these terms describe how a mode is intended to inflate the lung, rather than defining the characteristics of synchronization or the way spontaneous breathing efforts are supported.

Continuous mandatory ventilation

Continuous mandatory ventilation (formerly known as Assist Control or AC) is a mode of ventilation where breaths are delivered based on set variables. The patient may initiate breaths by attempting to breathe. Once a breath is initiated, either by the patient or by the ventilator the set tidal volume is delivered. Continuous mandatory ventilation used to also be called Volume Control or Assist Control Volume Control (AC/VC), though this is no longer recommended. Since nomenclature of mechanical ventilation is only recently standardized there are many different names that historically were used to reference CMV but now reference Assist Control Names such as: volume control ventilation, and volume cycled ventilation in modern usage refer to the Assist Control mode.

Controlled mechanical ventilation in its original form had no patient sensitivity. A breath set was a breath delivered. Continuous mandatory ventilation was created out of the need for patient-initiation in breaths. Fundamentally, Continuous mandatory ventilation is controlled mechanical ventilation (CMV) with a sensitivity for patient breathing. The use of controlled mechanical ventilation requires the patient be completely unconscious, either pharmacokinetically or otherwise in a coma.

Continuous mandatory ventilation (formerly Assist Control or AC) is associated with profound diaphragm muscle dysfunction and atrophy. Continuous mandatory ventilation is no longer the preferred mode of mechanical ventilation.

Intermittent mandatory ventilation

Intermittent mandatory ventilation is similar to continuous mandatory ventilation in two ways: The minute ventilation (V_E) is determined (by setting the respiratory rate and tidal volume); and the patient is able to increase the minute ventilation. However, IMV differs from continuous mandatory ventilation in the way that the minute ventilation is increased. To be specific, patients increase the minute ventilation by spontaneous breathing rather than patient-initiated ventilator breaths. The ventilator breaths are synchronized with patient inspiratory effort IMV with pressure support is the most efficient and effective mode of mechanical ventilation.

Intermittent mandatory ventilation has not always had the synchronized feature, so the division of  modes were understood to be SIMV (synchronized) vs IMV (not-synchronized). Since the American Association for Respiratory Care established a nomenclature of mechanical ventilation the "synchronized" part of the title has been dropped and now there is only IMV.

Mandatory minute ventilation

Mandatory minute ventilation (MMV) allows spontaneous breathing with automatic adjustments of mandatory ventilation to the meet the patient’s preset minimum minute volume requirement. If the patient maintains the minute volume settings for V_T x f, no mandatory breaths are delivered.

If the patient's minute volume is insufficient, mandatory delivery of the preset tidal volume will occur until the minute volume is achieved. The method for monitoring whether or not the patient is meeting the required minute ventilation (V_E) differs by ventilator brand and model, but, in general, there is a window of monitored time, and a smaller window checked against the larger window (i.e., in the Dräger Evita® line of mechanical ventilators there is a moving 20-second window, and every 7 seconds the current tidal volume and rate are measured) to decide whether a mechanical breath is needed to maintain the minute ventilation.

MMV is an optimal mode for weaning in neonatal and pediatric populations and has been shown to reduce long-term complications related to mechanical ventilation.

Pressuated volume controlre- regul

Pressure-regulated volume control is an IMV based mode. Pressure-regulated volume control utilizes pressure-limited, volume-targeted, time-cycled breaths that can be either ventilator- or patient-initiated.

The peak inspiratory pressure delivered by the ventilator is varied on a breath-to-breath basis to achieve a target tidal volume that is set by the clinician.

For example, if a target tidal volume of 500 mL is set but the ventilator delivers 600 mL, the next breath will be delivered with a lower inspiratory pressure to achieve a lower tidal volume. Though PRVC is regarded as a hybrid mode because of its tidal-volume (VC) settings and pressure-limiting (PC) settings fundamentally PRVC is a volume-control mode.

Continuous positive airway pressure

continuous positive airway pressure (CPAP) is a non-invasive positive pressure mode of ventilation (NPPV). CPAP is simply a pressure applied at the end of exhalation to keep the alveoli open and not fully deflate. This mechanism for maintaining inflated alveoli helps increase partial pressure of oxygen in arterial blood, an appropriate increase in CPAP increases the PaO₂.

Bilevel positive airway pressure

Bilevel positive airway pressure (BPAP) is a mode used during noninvasive positive pressure ventilation (NPPV). First used in 1988 by Professor Benzer in Austria it delivers  a preset inspiratory positive airway pressure (IPAP) and expiratory positive airway pressure (EPAP). BPAP can be described as a Continuous Positive Airway Pressure system with a time-cycled change of the applied CPAP level.^[17] CPAP, BPAP and other non-invasive ventilation modes have been shown to be effective management tools for chronic obstructive pulmonary disease and acute respiratory failure.

Often BPAP is incorrectly referred to as "BiPAP". BiPAP® is the name of a portable ventilator manufactured by Respironics Corporation; it is just one of many ventilators that can deliver BPAP.

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 oscillatory ventilation (sometimes abbreviated HFOV) the oscillation bellow and piston force positive pressure in and apply negative pressure to force an expiration.