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    WEANING FROM MECHANICAL VENTILATION

    Dr MEGHA JAIN

    University College of Medical Sciences & GTB

    Hospital, Delhi

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    HEADINGS

    Purpose of weaning and extubation. Rationale of predictive indices in weaning.

    Application of weaning parameters. Methods of weaning. Impediments to weaning.

    Extubation and terminal weaning.

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    Different stages in mech. Ventilated pts.

    1. Treatment of ARF

    2. Suspicion

    3. Assessingreadiness to

    wean

    4. SBT

    5. Extubation 6. Reintubation

    Admit Discharge

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    DEFINITIONS

    Weaning is the gradual reduction in the level of ventilatory support.

    Weaning success : effective spontaneous breathingwithout any mechanical assisstance for 24 hrs or more. Weaning failure: when pt is returned to mechanical

    ventilation after any length of weaning trial.

    Signs of weaning failure : abnormal blood gases,diaphoresis, tachycardia, tachypnea, arrythmias,hypotension.

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    Morbidity Associated With Prolonged

    Intubation and Mechanical Ventilation Vocal cord granulomas Ulceration of the true vocal

    cords Circumferential fibrous

    stenosis of trachea Epithelial damage, loss of

    cilia, and impairment of

    tracheal mucus clearance Risk factor for nosocomialpneumonia

    Precludes oral feeding

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    The assessment of weaning proceeds in twophases:

    Phase 1: To ensure that certain basic criteriaregarding initial reason for

    mechanical ventilation are satisfied

    Phase 2: Determine whether weaning is likelyto succeed on the basis of specifiedcriteria

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    READINESS FOR VENTILATOR WEANING

    Major determinants of ability to wean can be classifiedinto three categories:

    oxygenation ventilatory pump function

    neuropsychiatric status

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    OXGYGENATION

    Criteria of AdequacyPaO2 > 60 mmHg on FIO2 200

    Selected causes of failure: Hypoventilation: neurologic injury or drugs V/Q mismatch: severe CHF Anatomic (R-to-L) shunt (e.g. intracardiac, pulmonary A-V malformation).

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    VENTILATION

    Criterion of AdequacyPaCO2 < 50 mmHg or within 8 mmHg of baseline

    Selected causes of failure respiratory drive: sedation, drug overdose. resp bellows function: diaph weakness, N-m disease CO2 productionwithout compensatory in alveolar Ve:

    fever, hypermetabolism, carbohydrate overfeeding- dead space ventilationwithout compensatory alveolar Ve:

    PE, bullous emphysema

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    NEUROPSYCHIATRIC INTEGRITY

    Criteria of adequacy Awake, alert, cooperative, with intact gag and swallowing

    Selected causes of failure

    Cerebrovascular accidentSleep deprivation/ICU psychosisDrug therapyDepression

    Psychological dependency on ventilatory support

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    WEANING CRITERIA

    Used to evaluate the readiness of a patient for weaningtrial.

    Common weaning criteria :Ventilatory criteriaOxygenation criteriaPulmonary reservePulmonary measurementsOther factors

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    VENTILATORY CRITERIA

    PaCO2: < 50 mmhg with pH >/= 7.35. VC: > 10 to 15 ml/kg

    Spontaneous VT: > 5 to 8 ml/kg Spontaneous RR: < 30/min Minute ventilation: < 10 lts

    PaCO2 most reliable indicator VC and spon VT indicate mechanical cond of lungs

    A high spon RR and MV indicate WOB

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    OXYGENATION CRITERIA

    PaO2 without PEEP > 60 mmhg @FiO2 upto 0.4 PaO2 with PEEP > 100 mmhg @ FiO2 upto 0.4

    SaO2 > 90% @ FiO2 upto 0.4 Qs/Qt < 20% P(A-a)O2 < 350 mmhg PaO2/FiO2 > 200 mmhg

    Qs/Qt estimate wasted pulmonary perfusionP(A-a)O2 is related to degree of hypoxemia/shunt

    In pts with anemia or dysfunct Hb, PaO2 and SaO2 dont reflect true oxygenation statusSo arterial oxygen content should be measured

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    PULMONARY RESERVE AND MEASUREMENTS

    Pulmonary reserve:Max. voluntary ventilation 2min. vent@FiO2 upto 0.4

    Max. Insp. Pressure < -20 to -30 cmH2O in 20 sec. Pulmonary measurements :

    Static compliance > 30 ml/cm H2OVd/Vt < 60%

    Pulmonary reserve requires active pt cooperationPulmonary measurements indicate workload needed to support spont. ventilation

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    COMBINED WEANING INDICES

    Simplified weaning index: evaluates efficiency of gas exchange.= ( fmv (PIP PEEP)/MIP) PaCO2/40.should be < 9/min.

    CROP index: evaluates pulmonary gas exchange and balanceb/w respiratory demands and respiratory neuromuscular reserve.

    = ( Cd MIP PaO2/PAO2)/f.Should be > 13 ml/breath/min.

    RSBI:should be < 105 cycles/min/lt.= f/Vt.Most accurate test to predict weaning success.

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    RSBI

    First described by Yang and Tobin in 1991. Its a one min. trial of unassisted breathing measured during the

    T piece trial.

    Main defect: excessive false +ves Should not be measured until sedative and narcotic effects haveadequately abated and the pt. triggers 2 to 3 breaths/min aboveventilator set rate.

    Measure RR and MV for 1 min. during unassisted breathing( 0

    PEEP/5 cmH2O PSV). At end of 1 min. divide MV by RR to calculate avg. tidal vol. Divide RR by TV to obtain RSBI.

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    OTHER FACTORS

    Metabolic factors: * Inadequate nutrition protein catabolism* Overfeeding - CO2 production * Phosphate, ? Magnesium deficiency -respi pump functn* Impaired O2 delivery -respi pump functn.

    Renal function :* Patient should have adeq renal output (> 1000 ml/day)* Monitor electolytes to ensure adequate respi msl functn

    Cardiovascular function * Ensures sufficient O2 delivery to tissues*Cardiac rate, rhythm, BP, CO and CI should be optimal

    with minimal pressure support CNS assessment

    * Assess for LOC, anxiety, dyspnea, motivation

    * CNS should be intact for protection of airway.

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    Weaning methods

    Spontaneous breathing trial SIMV with pressure support.

    PSV Rapid ventilator discontinuation: pt.on vent for < 72 hrs., hasgood spont RR, MV, MIP, f/Vt

    SBT for 30 to 120 min.

    EXTUBATEif no other limiting factor

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    Spontaneous Breathing Trial

    T-Tube trial:allows spont. breathing several times per dayinterspersed with periods of ventilatory support.

    Initial SBTs may last only 5 to 30 min. Resume mechanical ventilation at night or if distress occurs.

    ADVANTAGES Tests pts spon breathing ability Allows periods of work and rest

    Weans faster than SIMV

    DISADVANTAGES

    Abrupt transition difficult for sm ptsNo alarms, unless attached to vent.

    Requires careful observn.

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    Weaning protocol for a SBT with a T-Tube

    Verify that pt is a candidate for vent. discontinuation

    Primary cause reversed

    Pt is afebrile, awake, alert and free of seizures HD stable with adeq. Hb, S.E.

    Oxygenation and ventilation adequatePaO2>/= 60mmhg on FiO2 7.35

    Other indices : - f/VT < 105 - MIP < -20 cm H2O- f < 30 and > 6/min. - VC > 10 to 15 ml/kg

    - TV > 5 ml/kg

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    Weaning protocol for a SBT with a T-Tube

    Prepare for T-Tube trial

    3 min. screening trial

    Measure TV,RR

    Measure MIP thrice selecting the best

    Adequate staff, equipment, no sedatives

    Formal SBT of upto 2 hrs.

    MIP < -20 cm H20TV spon. > 5 ml/kg

    RR spon. < 35/min.

    Continue trial for 30 120 min.

    Extubate if no signs of intolerance

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    Signs of intolerance of SBT

    Agitation, anxiety, diaphoresis or change in mental status RR > 30 to 35/min

    SpO2 < 90% > 20% or in HR or HR > 120 to 140/min SBP > 180 or < 90 mmhg.

    Such pts are returned to full ventilatory support for 24 hrs. to

    allow the ventilatory msls. to recover.

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    Weaning with SIMV

    Involves gradual reduction in machine rate based on ABG andclinical assessment.

    Rate is generally adjusted in increments of 2 breaths/min.followed by pt assessment.

    ADVANTAGESGradual transition

    Easy to useMinimum MV guaranteed

    Alarm system may be usedShould be used in comb. with PSV/CPAP

    DISADVANTAGES

    Pt. ventilator asynchronyProlonges weaningMay worsen fatigue

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    Pressure Support Ventilation (PSV)

    Patient determines RR, VE, inspiratory time a purely spontaneous mode Parameters

    Triggered bypts own breath Limited by pressure Affects inspiration only

    Uses Complement volume-cycled

    modes (i.e., SIMV) Does not augment TV but

    overcomes resistance createdby ventilator tubing PSV alone

    Used alone for recoveringintubated pts who are not quite

    ready for extubation Augments inflation volumes.

    PSV is most often used together with other volume-cycled modes. PSV provides sufficient pressure to overcome the resistance of th