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Kingdom of Saudi Arabiaking Fahd HospitalJeddahIntensive Care Unit


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Mechanical ventilationDr.Wail Bajhmoom

15.3.2005 A.D 5.2.1425A.H


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Background.Definitions.Classifications.Indications.


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Physiologic aspects.Ventilator mode.Complications.Weaning.


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Background. 1929 was one of the firstnegative-pressure

ventilators widely used formechanical ventilation. Thismetal cylinder completely

covered the patient up tothe neck.


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known as the iron lung, thisdevice used negative

pressure to cause a drop inthe intrapulmonary pressureand to allow ambient airflow

into the patient's lungs.


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In the 1950s, the intensiveuse of mechanical ventilation

started during the polioepidemic in Scandinavia andthe United States. In

Copenhagen, Denmark.


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ventilating patients withpolio and respiratoryparalysis by manually forcing50% oxygen through atracheostomy reduced the

mortality rate from 80% to25%.


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Definition:Ventilators are: specially

designed pumps that cansupport the ventilatorfunction of the

respiratory system.


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They improve oxygenation

through application of highoxygen content gas andpositive pressure.


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Classifications. Modern ventilators areclassified by the method ofcycling from the inspiratory

phase to the expiratory

phase.


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Classifications. The signal to terminate themachine's inspiratory activity

can be: (volume-cycled ventilator), (pressure-cycled ventilator), (time-cycled ventilator).


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Indications: Mechanical ventilation shouldnot be initiated without

thoughtful considerationbecause intubation and

positive-pressure ventilation

could have potentially harmfuleffects.


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Indications: Many factors are consideredin the decision to institutemechanical ventilation.

Respiratory failure is the

primary indication


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Indications: Apnea with respiratory arrest Acute lung injury

-R.R more than 30 BPM

-Minute ventilation>10 L/min

-PaO2, with supplemental (FIO2)less than 55 mm Hg


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Chronic obstructive lung disease

-Blood gases - Persistenthypoxemia, PCO2 (acutely) greater than

50 mm Hg with pH less than 7.25

_ Clinical deteriorationRespiratory muscle fatigue, coma,

hypotension, or tachypnea or bradypnea


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Neuromuscular disease

Clin ical judgm ent shou ld be used ;

An increasing sever i ty of the i l lness is

a sign that should alert the cl in ic ianto cons ider inst i tu t ing mechanicalvent i lat ion.


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Physiologic aspects. Most modern mechanical ventilatorsfunction by providing warmed and

humidified gas to the airwayopening in conformance with variousspecific volume, pressure, and timepatterns. The ventilator serves asthe energy source for inspiration,


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replacing the muscles of thediaphragm and chest wall.

Expiration is passive, driven bythe recoil of the lungs andchest wall


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Ventilator mode: This setting specifies the

manner in which ventilatorbreaths are triggered,cycled, and limited .


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Assist Control Mode Ventilation(ACMV)

An inspiratory cycle is initiatedeither by the patient's inspiratoryeffort or, if no patient effort isdetected within a specified timewindow,


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Every breath delivered consists ofthe operator-specified tidalvolume.

ACMV is the recommended mode forinitiation of mechanical ventilationbecause it ensures a backup minuteventilation in the absence of an intactrespiratory drive.


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Synchronized IntermittentMandatory Ventilation(SIMV)The major difference between SIMV

and ACMV is that in the formerthe patient is allowed to breathespontaneously, i.e., withoutventilator assist


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If the patient fails to initiatea breath, the ventilatordelivers a fixed-tidal-volume

breath and resets the internaltimer for the next inspiratorycycle.


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SIMV is a useful mode of

ventilation for both supporting andweaning intubated patients

SIMV may be difficult to use inpatients with tachypnea becausethey may attempt to exhale duringthe ventilator-programmed

inspiratory cycle.


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Continuous Positive Airway Pressure(CPAP)

.This is not a true support-mode of

ventilation, since all ventilationoccurs through the patient'sspontaneous efforts.


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CPAP is used to assess extubationpotential in patients who have beeneffectively weaned and are

requiring little ventilator support


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Pressure-Control Ventilation(PCV)

This form of ventilation is time triggered,time cycled, and pressure limited.During the inspiratory phase, a givenpressure is imposed at the airwayopening, and the pressure remains atthis user-specified level throughout

inspiration


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PCV is the preferred mode ofventilation for patients withdocumented barotrauma, since

airway pressures can belimited,


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Pressure-Support Ventilation(PSV)This form of ventilation is patient

triggered, flow cycled, andpressure limited; it is specificallydesigned for use in the weaning

process.


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PSV is well tolerated by mostpatients who are being weaned:PSV parameters can be set in sucha way as to provide full or nearly

full ventilatory support and can bewithdrawn slowly over a period ofdays in a systematic fashion to

gradually load the respiratorymuscles.


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COMPLICATIONS: Endotracheal intubation and positive-pressure mechanical ventilation havedirect and indirect effects on several

organ systems Including:

-the lung and upper airways, the

cardiovascular system, and the gastrointestinalsystem.


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barotrauma, nosocomial pneumonia,oxygen toxicity, tracheal stenosis,and deconditioning of respiratorymuscles.

emphysema, pneumomediastinum,subcutaneous emphysema, orpneumothorax.


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Patients intubated for longer than 72 hare at high risk for nosocomialpneumonia as a result of aspiration fromthe upper airways via small leaks around

the endotracheal tube cuff enteric gram-negative rods,

Staphylococcus aureus, and anaerobicbacteria.


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Oxygen toxicity is a potentialcomplication when an FIO2 of 0.6 orgreater is required for more than 72 h.

Hypotension resulting fromelevated intrathoracic pressureswith decreased venous return


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Gastrointestinal effects ofpositive-pressure ventilation includestress ulceration and mild tomoderate cholestasis. It is common

practice to provide prophylaxis withH2-receptor antagonists orsucralfate for stress-relatedulcers.


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Weaning. Many approaches to weaning patients

from ventilator support have beenadvocated.

T-piece and CPAP weaning are besttolerated by patients who haveundergone mechanical ventilation for

brief periods


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Weaning. SIMV and PSV are best forpatients who have been intubated

for extended periods and requiregradual respiratory-musclereconditioning.


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