Weaning from Mechanical Ventilation

Respiratory Ability vs Workload Balance

• The balance between respiratory ability and workload determines the need for mechanical ventilation.
• Total support is needed when the demands exceed the patient's capabilities, leaving no normal reserve.
• Partial support is needed when the patient has some respiratory ability but cannot handle the entire workload.
• Spontaneous ventilation is possible when the patient's respiratory ability exceeds the workload, indicating a tolerable load.

Demands on Respiratory System

• Pressure loads
  • CL (Compliance of the Lung Tissue; CL = \frac{ \Delta V }{ \Delta P })
  • \uparrow Raw (increased airway resistance)
• Ventilation loads
  • \uparrow VA (increased alveolar ventilation; VA = (VT - VD) \times f)
  • (TV{CO2}, \uparrow VO_2) Increased tidal volume of carbon dioxide & increased oxygen consumption
  • \uparrow VD (increased dead space ventilation; VD = \frac{PaCO2 - PECO2}{PaCO2} \times V_T)
• Imposed loads

Capabilities of Respiratory System

• Neural drive
• Muscle function
  • Strength
  • Endurance

Decision to Wean

• Weaning can be considered when the reason for required support has resolved.
• 80% of patients who require temporary support do not require weaning and may be simply extubated.
• Examples of patients who may not require weaning:
  • Post-operative patients
  • Trauma patients
  • Exacerbations of Asthma
  • Patients ventilated for less than a week
• Some patients require a longer slower process, “weaning.”
• Some require ventilatory support, or oxygen and PEEP to wean.
• Some only require an artificial airway.
• Some patients require two or more of the above types of support.

Most Common Weaning Modes

• SIMV – traditional mode, most ineffective.
• PSV – completely spontaneous, no mandatory breaths.
• T-piece – no ventilator.

SIMV (Synchronized Intermittent Mandatory Ventilation)

• Synchronized IMV breaths
• Theory – patient’s respiratory muscles would work during spontaneous breathing intervals between mandatory breaths.
• Weaning strategy – progressive reduction of mandatory breaths 1 – 2 as tolerated by patient.
• Pressure support – may be used to reduce the workload of the spontaneous breaths. 5-10 cmH_2O or until adequate tidal volume is achieved.
• PEEP – may also be added to increase FRC. FRC decreased by presence of OETT and when patient is lying in bed.
• Increased work of breathing – SIMV is associated with increased WOB because the respiratory center is unable to determine the nature of next breath, spont. or mandatory – increased dyssynchrony.
• When the mandatory VE less than 50% of required VE it may cause the patient to work as hard as if there was no support.
• SIMV – prolongs weaning.

PSV (Pressure Support Ventilation)

• In PSV the patient controls – f, timing (I:E), and depth of each breath, completely spontaneous.
• Safe mode due to sophisticated alarms.
• PS – level 5 – 15 cmH_2O or until adequate tidal volume and normal f are achieved.
• Inappropriate levels may lead to cp stress, paradoxical breathing and respiratory alternans.
• Weaning in this mode, is accomplished by gradually decreasing the PS level to about 5 cmH_2O
• 5 cmH_2O is considered adequate to overcome resistance of ET and circuit.

T-Piece

• Oldest method
• Involves removing ventilator from patient during t-piece trial.
• T-piece trial – may be performed as a typical SBT or as a prolonged weaning method in which weaning starts with short period of time, 5 - 10 minutes and lengthens as tolerated. At the end of each trial the patient is placed back on vent to rest. Trials repeated every 1 – 2 hours.
• Modern T-piece is a “Briggs-Adapter” connected to aerosol tubing and a large volume nebulizer set at required FIO_2 and a shorter piece of aerosol tubing connected to the other end of the T as a reservoir.
• Patient must be monitored very closely during t-piece weaning.
• Some RCP’s deflate OETT cuff during t-piece trials. However, HOB must be elevated to guard against aspiration.
• T-piece weaning can be accomplished through vent by using CPAP mode with no pressure support.
• May not be tolerated by certain patients.

Comparisons

• Although no one method is the best way to wean everyone, SIMV has been shown to take the longest and potentially cause the greatest work of breathing for the patient.
• If one mode does not work try a different one.

Newer Modes of Weaning

• Closed Loop – ATC, VS, automode, MMV, adaptive support, KSB.
• ATC – automatic tube compensation. Ventilator measures the precise level of PS required to overcome ET resistance. Several studies found ATC, in most cases is equivalent to CPAP +5/PS 5
• VS – volume support = PS with a volume target. Also called VPS – variable pressure support. The ventilator monitors lung characteristics and delivered tidal volume and adjust pressure support to achieve the target tidal volume.
• Automode – when activated can switch from PRVC to Volume Support when spontaneous breathing is detected.
• MMV – microprocessor monitors set parameters and either increases or decreases support to guarantee the selected VE.
• ASV – similar to MMV, vent monitors pt data and adjusts ventilation accordingly.
• AIS – “SmartCare/PS System. Only available on Drager XL and Infinity V500 models. Uses predetermined ranges for f, VT, and ETCO2 to automatically set and adjust insp. Pressure. Can predict right time for extubation.

Evidence based Weaning

• A lot of research has been performed to determine the best way to wean.
• A group task force made up of the AARC, ACCP, and the SCCM came up with a list of recommendations for weaning patients requiring more than 24 hours of ventilation. (See text for complete list of recommendations.)
• Three points must be addressed prior to any weaning attempt.
  • Pathology reversal – has there been a reversal of the cause for the need for mechanical ventilation?
  • Weaning criteria – are measured indicators of the patient’s stability acceptable or optimal?
  • SBT – if so, a spontaneous breathing trial must be performed to further assess the patient’s readiness to wean.

Screening Candidates for Weaning

• Some facilities use a weaning protocol or screen to determine who is a candidate for weaning. This process addresses some of the recommendations previously mentioned.
• This process is performed every morning and any other time when weaning ability is expected or anticipated.
• Patients must be on no sedation and only on minimal analgesics. – ideally a patient should be awake and alert when weaning is performed.
• Patients must have adequate cough and or gag. – a patient must be able to protect and clear their airway once extubated. The absence of either may point to neurological impairment.
• Patients must have an adequate cuff leak. – decreased or absent cuff-leak when cuff is deflated may indicate laryngeal edema.
• Hemodynamic Stability – Patient must be on no or minimal vasopressors. Hypotensive patients are not hemodynamically stable.
• Adequate Oxygenation – PaO2 > 60 mm Hg on an FIO2 < 0.4. PaO2/FIO2 ratio > 150 to 200 mm Hg, required PEEP < 5 to 8 cm H_2O. The patient must have and be able to maintain a normal oxygenation status in order to successfully wean.
• Patient must be capable of initiating an inspiratory effort.
• If all criteria are met or at the discretion of a physician, weaning parameters and a SBT can be initiated depending on the policy of your institution.

Weaning Parameters

• Vary from institution to institution.
• May be called parameters, or “off-volumes.”
• Should be performed with little or no help from vent;CPAP 0, PS 0. If not tolerated, CPAP +5 is usually acceptable.
• Weaning Parameters – typically consist of 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) should be < 105
  • NIF should be -20 to -30. The more negative the better.
  • VC should be > 15 ml/kg IBW. The bigger the better.
• Many things to consider during these measurements – patients should be as upright as possible. How big is OETT? Are there a lot of secretions. Is there anything that could possibly decrease the patient’s ability to perform these tests to the best of their ability?

RSBI (Rapid Shallow Breathing Index)

• f/VT.
• Indicator of ventilatory performance and predictor of fatigue and weaning failure.
• Can only be done when patient is breathing spontaneously.
• May be affected by patient anxiety.

NIF (Negative Inspiratory Force)

• Also called MIP, (maximum inspiratory pressure,) the maximum amount of negative pressure at patient can generate.
• Demonstrates muscle strength and a patient’s ability to take a deep breath.
• Patient cooperation is preferable.
• In most cases the Patient must be disconnected from ventilator to perform this test.
• May be affected by increased RAW, decreased CL or air- trapping.
• Patient must exhale completely for best reading.

VC (Vital Capacity)

• Maximum inspiration followed by maximum expiration.
• Also reflective of respiratory muscle strength, and patient’s ability to cough and clear their airway.
• VC – may be measured off the vent with a respirometer or through the vent by observation of the exhaled VT readout.
• Requires patient cooperation and may be affected by the same factors that affect NIF.

Other weaning Assessment Criteria

• Drive to breathe – P0.1, airway occlusion pressure. Airway occluded during first 100 msec of inspiration and pressure measured at upper airway. (0 to -2)
• Work of Breathing – no universally accepted measurement value. f/VT, others. C.R.O.P.

Physical Signs and Measurements of Increased WOB

• Use of accessory muscles
• Asynchronous breathing
• Nasal flaring
• Diaphoresis
• Anxiety
• Etc

SBT (Spontaneous Breathing Trial)

• Once patient has passed all screening criteria and weaning parameters, a spontaneous breathing trial may be initiated. Parameters for this vary from institution to institution. However CPAP 5cm H2O, or CPAP 0 and PS 5 are typically acceptable. In some cases, patients with COPD, CPAP + 5 cm H2O can be used in conjunction with PS 5 cm H_2O.
• Depending on patient type and toleration, SBT may be from 30 min. to 2 hours, followed by an ABG.

Ventilator Liberation

• In some cases ventilatory support may be discontinued but the patient is not extubated. Airway edema or excessive secretions may be reasons to leave the tube in a little longer. However in most cases these two are performed at the same time.

Extubation

• Patient may be extubated after successful SBT and ABG results are approved by the physician and an order is given.
• Patient may be placed on CAM, per AARC clinical practice guidelines, at the FIO_2 utilized during SBT or if there is no suspicion of laryngeal edema, patient may be placed directly on a nasal cannula.

SBT Failure

• Reason for failure must be determined and corrected if possible.
• SBT may be repeated as indicated by patient’s improvement or per physician’s order.
• Recognizing patient fatigue is very important in preventing SBT failure.
• Some patients may become very agitated, occasionally requiring mild anxiolytics or low-dose analgesics.
• A percentage of patients who fail a SBT may still be successfully extubated.
• NIPPV may be effective in assisting patients with the transition to unassisted spontaneous breathing.

Postextubation Difficulties

• Glottic Edema – may cause partial airway obstruction. Treatments include cool aerosol, racemic Epi and HeO_2.
• Laryngospasm – usually is transient, may respond to positive pressure with oxygen.
• Aspiration – associated with suppressed gag or cough reflex *see list of factors on pg. 459. Patients are typically maintained at an NPO status for several hours after extubation.

Tracheostomy Weaning

• Easier than weaning to extubate.
• Weaning Parameters still performed
• When the patient passes the parameters they are simply taken off the vent the cuff is deflated and they are placed on a trach-collar aerosol set at the same FIO2 as the set FIO2 on the ventilator.
• If patient fatigues or fails for any other reason, the patient may be placed back on the vent and support resumed.