Mechanical ventilation
In architecture, mechanical engineering,HVAC, mechanical ventilation isusepowered equipment, e.g. fanssblowers,move air; as contrasted withnatural ventilation provided by convectionwinds.
In medicine, mechanical ventilationemployedassist, orsome cases replace, spontaneous breathing. Mechanical ventilation can be life-savingismainstayresuscitation, intensive care medicine,anesthesia.
Clinical use
Mechanical ventilationused when natural (spontaneous) breathingabsent (apnea) or insufficient. This may becasecasesintoxication, circulatory arrest, neurological disease or head trauma, paralysis ofbreathing muscles duespinal cord injury, oreffectanesthetic or muscle relaxant drugs. Various pulmonary diseases or chest trauma, cardiac disease such as congestive heart failure, sepsisshock may also necessitate ventilation.Depending onclinical situation, mechanical ventilation may be continued forfew minutes or many months. While returningspontaneous breathingrarelyproblemroutine anesthesia, weaning an intensive care patient from prolonged mechanical ventilation can take weeks or even months. Some patients do not regainability breathe by themselves sufficientlytherefore require permanent mechanical ventilation. Thisoftencasesevere brain injury, spinal cord injury, or neurological disease.
Techniques
Positivenegative pressure ventilation
Whileexchangeoxygencarbon dioxide betweenbloodstream andpulmonary airspace works by diffusionrequires no external work, air must be moved intoout oflungsmakeavailable togas exchange process. In spontaneous breathing, an underpressurecreated inpleural cavity bymusclesrespiration, andresulting gradient betweenatmospheric pressure andpressure insidethorax generatesflowair. Thisimitated bynegative-pressure ventilation thatemployediron lungs. An iron lung works by creating an underpressure inchamber which enclosesbodyis sealed atneck. Withpatient's airways open,resulting gradient toatmospheric pressure servesinflatelungs.All other techniquesventilationpositive pressure ventilation techniques, meaning that airforced intolungs by an external overpressure.
Mouth-to-mouthbag-mask systems
Arguablysimplest formmechanical ventilation ismouth-to-mouth or mouth-to-nose technique thatusedbystander cardiopulmonary resuscitation. This techniquehowever limited as itnot possibleventilatepatientoxygen-enriched air: oncontrary, only approximately 16 percent oxygen (in contrast21 percentambient air or up100 percent by mechanical ventilators) can be achieved. Therealsopossible riskdisease transmission through exchangebody fluids. Mechanical devices such asbag-mask-valve systemtherefore preferred where available.A bag-mask-valve system consists offace mask thatpressed overpatient's nosemouthachievetight seal, an elastic bag that can be manually compresseddeliver air topatient, andvalvedirect air flow. A sourceoxygen can be connected toreservoir attached tobagachievehigher concentrationoxygen than thatambient air. This simple technique can be sufficientmaintain ventilation (and consequently,lifean apneic patient)upseveral hours.
Mechanical ventilators
In anesthesiaintensive care, mechanical ventilatorsroutinely used.Ventilators allow various modesmechanical ventilation ranging from assisted spontaneous breathingfully controlled ventilation. In some cases,patient can breathe almost naturally, receiving only an occasional "push"airaugment individual breaths. Thistermed assisted (or augmented) ventilation. Assisted ventilation modesusedanesthesiainprocessweaningpatient from controlled ventilation.
In sicker patients,degreeventilator-driven respiration can be increased,if necessary,ventilator can take overworkbreathing entirely (controlled ventilation). Modern ventilators allowcontinuous adaptation ofdegreemechanical assistance according topatient's individual demands.
The lungsventilated patients havetendencycollapse partially, leadingimpaired gas exchange. Therefore, many ventilation modes allowusePEEP (positive end-expiratory pressure). With PEEP, there isresidual overpressure atend ofbreathing cycle that keepslungs inflated.
Securingpatient's airways
Mechanical ventilation will be unsuccessfuldangerous unlesspatient's airwayspatent, meaning air can flow unimpeded backforth intolungs. Italso necessaryavoid air leakage so that air flowpressuremaintained atvalues set.Another great riskthataspiration pneumonia. Aspirationwhen stomach contents come back upesophagusentertracheaenterlungs. When stomach contents get intolungs,patient can actually drown due tovolumegastic material, or,less material, suffer damage tolung tissue due toacid content ofstomach. Measuresprevent aspiration depend onsituation andindividual patient - endotracheal intubationoften necessaryprotect against this.
Therevarious proceduresmechanical devices that provide protection against airway collapse, air leakage,aspiration:
- Face mask - In resuscitationfor minor procedures under anesthesia,face maskoften sufficientachieveseal against air leakage. Airway patency ofunconscious patientmaintained either by manipulation ofjaw or byusenasopharyngeal or oropharyngeal tubes. Thesedesignedprovidepassageair topharynx throughnose or mouth, respectively. A face mask does, however, not provide protection against aspiration. Face masksalso used"non-invasive ventilation"conscious patients. Non-invasive ventilationaimed at minimizing patient discomfortventilation-related disease. Itoften usedcardiac or pulmonary disesase.
- Larygeal mask airway - Another device islaryngeal mask airway (LMA), which consists oftubean inflatable cuff thatinserted intopharynx. It causes less paincoughing thantracheal tube; however, sealing against aspirationinferiortracheal tubes, making careful patient evaluationselection mandatory. The LMAusedanesthesiasometimesemergency medicine.
- Tracheal intubation or, colloquially, "intubation"often performedmechanical ventilationhours'weeks' duration. A tubeinserted throughnose (nasotracheal intubation) or mouth (orotracheal intubation)advanced intotrachea. In most cases tubesinflatable cuffsusedprotection against leakageaspiration. Intubation withcuffed tubethoughtprovidebest protection against aspiration. Tracheal tubes inevitably cause paincoughing. Therefore, unlesspatientunconscious or anesthetizedother reasons, sedative drugsusually givenprovide tolerance oftube.
- Tracheostomy - When mechanical ventilationrequiredmore than days orfew weeks, tracheostomy providesmost convenient access topatient's airways. A tracheostomy issurgically created access totrachea. Tracheostomy tubeswell toleratedoften do not necessitate any usesedative drugs.
Ventilation-associated lung injuryprotective ventilation
In most casesmechanical ventilation,patient's prognosisdetermined byunderlying diseaseits reponsetreatment. However, ventilation itself can cause significant problems that may prolong intensive caresometimes leadpermanent injurydeath. Ittherefore desirablelimit mechanical ventilation toshortest appropriate time.Infectious complications, particularly pneumonia, occurmany patients who remain intubatedmore thanfew days. Tracheal intubation interferes withnatural defenses against lung infection, particularly withprocess"mucociliary clearance". This iscontinuous transportairway secretions fromlungs toupper airways that servesremove bacteriaforeign bodies. Itthought thatintubation-related disruptionthis transport mechanism ismajor factor indevelopmentpneumonia.
Thereevidence that oxygenhigher concentrations may contributeinjurylung tissueventilated patients. Ittherefore recommendedset ventilatorsdeliverlowest appropriate concentrationoxygen. However,patientsseverely impaired pulmonary gas exchange, high oxygen concentrations may be necessarysurvival.
Most techniquesventilation rely on an overpressure being applied tolungs. In diseased lungs this may leadfurther tissue injury caused by excessive mechanical stress (overdistension, shear forces, high peak pressure)aggravated by inflammatory processes. Such mechanically induced lung injury can leadsevere impairment ofpulmonary gas exchange, thereby necessitating even more aggressive ventilation.
"Protective ventilation" iscollective termstrategiesminimize ventilation-associated lung injury, manywhich rely on sophisticated ventilator settingsreduce overdistension oflungs.
History
The iron lung was used through much of20th century, mostlylong-term ventilation.
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