Ch. 20: Weaning from Mechanical Ventilation

Respiratory Ability vs Workload Balance

• Partial support: spontaneous ventilation needed, but tolerable load is possible.
• Total support: spontaneous ventilation not possible due to intolerable load.
• Balance achieved by increasing reserve.
• Demands: pressure loads (increased CL and R{aw}), ventilation loads (increased VA, TVCO2, VO2, and VD), and imposed loads.
• Capabililites: Neural drive and Muscle function (strength and endurance).

Decision to Wean

• Consider when the reason for support has resolved.
• 80% of patients needing temporary support may be extubated without weaning.
• Some patients require a slower weaning process with ventilatory support or oxygen/PEEP.

Most Common Weaning Modes

• SIMV: Traditional, often ineffective.
• PSV: Completely spontaneous.
• T-piece: No ventilator.

SIMV (Synchronized Intermittent Mandatory Ventilation)

• Theory: Respiratory muscles work during spontaneous breathing intervals between mandatory breaths.
• Weaning: Progressive reduction of mandatory breaths, 1-2 as tolerated.
• Pressure support: 5-10 cmH2O to reduce workload of spontaneous breaths.
• PEEP: May be added to increase FRC.
• Increased WOB: Can cause dyssynchrony and prolong weaning.
• Mandatory VE less than 50% of required VE may cause increased WOB.

PSV (Pressure Support Ventilation)

• Patient controls rate, timing, and depth of each breath.
• PS levels: 5-15 cm H2O until adequate tidal volume and normal rate are achieved.
• Weaning: Gradually decrease PS level to about 5 cm H2O.
• 5 cm H2O is considered adequate to overcome resistance of ET and circuit.

T-Piece

• Oldest method, involves removing the ventilator.
• Trial: Start with short periods (5-10 minutes) and lengthen as tolerated, repeating every 1-2 hours.
• Modern T-piece: Briggs-Adapter with aerosol tubing and a large volume nebulizer.
• Can also use CPAP mode with no pressure support.

Comparisons

• SIMV shown to take the longest and cause the greatest work of breathing.
• If one mode does not work try another.

Newer Modes of Weaning (Closed Loop)

• ATC (Automatic Tube Compensation): Measures PS needed to overcome ET resistance.
• VS (Volume Support): PS with a volume target, adjusting pressure to achieve the target tidal volume.
• Automode: Switches from PRVC to Volume Support when spontaneous breathing is detected.
• MMV: Microprocessor adjusts support to guarantee selected VE.
• ASV: Similar to MMV, adjusts ventilation accordingly.
• AIS: "SmartCare/PS System," automatically sets and adjusts inspiratory pressure.

Evidence-Based Weaning

• Task force recommendations for patients ventilated >24 hours.
• Three points to address:
  • Pathology reversal.
  • Acceptable weaning criteria.
  • SBT (Spontaneous Breathing Trial).

Screening Candidates for Weaning

• Screen to determine candidates.
• No sedation, minimal analgesics.
• Adequate cough and/or gag reflex.
• Adequate cuff leak.
• Hemodynamic stability (minimal vasopressors).
• Adequate oxygenation: PaO2 > 60 mm Hg on FIO2 < 0.4, PaO2/FIO2 ratio > 150-200 mm Hg, PEEP < 5-8 cm H2O.
• Capable of initiating an inspiratory effort.

Weaning Parameters

• Vary from institution to institution.
• Typically include f, VT, VE, RSBI, NIF, and VC.
  • f < 35 breaths/min.
  • VT 4-6 ml/kg IBW.
  • VE 10-15 L/min.
  • RSBI (f/VT) < 105.
  • NIF -20 to -30 (more negative is better).
  • VC > 15 ml/kg IBW (bigger is better).

Weaning Parameters Details

• RSBI: Rapid shallow breathing index (f/VT), predictor of fatigue and weaning failure.
• NIF/MIP: Maximum inspiratory pressure, demonstrates muscle strength.
• VC: Maximum inspiration followed by maximum expiration, reflects respiratory muscle strength and ability to cough.

Other Weaning Assessment Criteria

• Drive to breathe: P0.1 (airway occlusion pressure, 0 to -2).
• Work of Breathing: Using f/VT, others.

SBT (Spontaneous Breathing Trial)

• After passing screening criteria and weaning parameters.
• CPAP 5 cm H2O, or CPAP 0 and PS 5 acceptable.
• Duration: 30 min to 2 hours, followed by ABG.

Ventilator Liberation

• Discontinuing ventilatory support without extubation.

Extubation

• After successful SBT and ABG results.
• Place on CAM or nasal cannula.

SBT Failure

• Determine and correct the cause.
• May be repeated with improvement.
• Recognize patient fatigue.
• Consider NIPPV for transition.

Postextubation Difficulties

• Glottic Edema: Cool aerosol, racemic Epi, HeO2.
• Laryngospasm: Positive pressure with oxygen.
• Aspiration: Maintain NPO status.

Tracheostomy Weaning

• Easier than weaning to extubate.
• Perform weaning parameters, then place on trach-collar aerosol.