Notes on Mechanical Ventilation, Weaning, and Spontaneous Breathing Trials

MECHANICAL VENTILATION

Weaning & Spontaneous Breathing Trials

Overview
  • Topic covers the process and techniques surrounding withdrawing mechanical ventilatory support from patients.
  • Focuses on the transfer of work of breathing (WOB) from a mechanical ventilator back to the patient.

AGENDA

  • Duration
  • Complications
  • Weaning criteria
  • Weaning modes
  • Extubation processes
  • Terminal weaning transitions

PROCESS OF WEANING

  • Definition: The process of withdrawing mechanical ventilatory support involves gradually transferring the work of breathing from the machine to the patient.

FACTORS THAT MAY HINDER WEANING

Patient’s Prior Condition

  • Patient's health status before being placed on ventilation.
  • Recovery status of the patient.
  • Any conditions that may interfere with WOB, including:
    • Acid-base imbalance
    • Electrolyte imbalance
    • Nutritional deficiency
    • Anemia

Pathophysiology Issues

  • Conditions impacting the ability to wean:
    • Fever
    • Sepsis
    • Renal failure
    • Sleep deprivation
    • Infection

Remember:

  • Success rate of weaning is dependent on the patient population.
  • Duration of weaning attempts varies significantly between patients.

CARDIAC ISSUES

Complications Relevant for Weaning

  • Blood pressure variations: High or low
  • Cardiac output variations: High or low
  • Fluid imbalance states including overload and dehydration
  • Cardiac arrhythmias

WEANING CRITERIA

Important Patient Assessments for Weaning Success

  • Improving chest X-ray results
  • Electrolyte and acid/base stability
  • Stable chest wall
  • Intact cough and gag reflex
Successful Weaning Indicators
  • Patients should ideally meet many of the following criteria:
    • Psychologically ready (eager to have the tube removed)
    • No abdominal distention
    • Adequate nutritional status
    • General body strength

WEANING CRITERIA: VENTILATION

Patient Needs to Maintain Adequate Ventilation

  • Key indicators to assess:
    • PaCO2: Normal for the patient (with normal pH)
    • Respiratory Rate (RR): Should be < 25
    • Spontaneous Tidal Volume (VT): Must be > 5 mL/kg
    • Vital Capacity (VC): Should be > 10 mL/kg
    • Minute Ventilation (MV): Must be < 10 LPM
    • Negative Inspiratory Force (NIF): Must be > -20 cmH2O over 20 seconds
    • Dead Space to Tidal Volume Ratio (VD/VT): Should be < 60%
    • Forced Expiratory Volume in 1 second (FEV1): Should be > 10 mL/kg
    • Static Compliance (CStat): Should be > 0.030 L/cmH2O (i.e. > 30 mL/cmH2O)
    • Rapid Shallow Breathing Index (RSBI): Must be < 100

WEANING CRITERIA: OXYGENATION

Key Measures for Oxygenation Readiness

  • PaO2: Must be > 60 mmHg on FiO2 40% or less
  • PEEP: Should be < 5 cmH2O
  • PaO2/FiO2 ratio: Should be > 200
    • PaO2 of 80 on 50% FiO2 = 160, indicating readiness.
    • PaO2 of 80 on 30% FiO2 = 267, indicating readiness due to lesser oxygen requirement for satisfactory oxygenation.
  • Oxygenation may also be met off the ventilator using other oxygen devices:
    • QS/QT: Should be < 20%
    • A-a gradient (AaDO2): Should be < 300 mmHg on 100% FiO2
    • SaO2: Should be > 90% on FiO2 40% or less

WEANING CRITERIA: CARDIOVASCULAR

Cardiovascular Stability Indicators

  • Must have normal blood pressure
  • Should have a normal and regular pulse
  • Controlled arrhythmias (should not exceed 4 PVCs per minute)
  • Adequate hemoglobin levels
  • Stable cardiac output

WEANING MODES

Types and Criteria for Weaning Modes

Short (Rapid) Wean
  • For patients who have been on ventilation less than 48 hours.
  • Mode adjustments involve quickly reducing settings like RR, FiO2, PS, etc.
Longest Wean
  • For patients on ventilation for more than 30 days.
  • Requires adequate nutrition and psychological support.
  • Likely to have respiratory muscle atrophy.
Weaning Vent Modes: SIMV
  • Criteria:
    • Remove mechanical deadspace
    • Eliminate sighs
    • Ensure adequate flow
    • Gradually decrease the rate
  • Advantages:
    • Alarms available, reduces time on the vent as muscle utilization is promoted.
    • Simplifies the weaning process without extra equipment.
  • Disadvantages:
    • Patient may not be monitored closely due to reliance on alarms.
    • Potentially too rapid weaning.
Weaning Vent Modes: CPAP/PSV
  • Patient should have good respiratory drive and can wean slowly.
  • Back-up ventilation kicks in if the patient becomes apneic.
Weaning Devices: T-Piece
  • The patient breathes spontaneously with gradually extended periods off the ventilator.
  • Advantages:
    • Cost-effective and simple.
    • Optimizes use of respiratory muscles.
  • Disadvantages:
    • Time-consuming and requires close monitoring.
    • Increased WOB may lead to psychological setbacks if the wean fails.

BEDSIDE MONITORING IMPORTANCE

  • Key parameters to monitor for fatigue signs:
    • RR: Respiratory Rate
    • VT: Tidal Volume
    • MV: Minute Ventilation
    • pH: Blood pH level
    • PaCO2: Partial pressure of carbon dioxide in arterial blood
  • Recognize and address:
    • Impaired neuromuscular function
    • Loss of muscle mass due to poor nutrition
    • Accumulation of metabolic byproducts (e.g., lactic acid)
    • Increasing energy demands that exceed muscle work capability
    • Increasing airway resistance (RAW) and decreasing lung compliance
    • Symptoms of hypoxemia exacerbating the condition.

RESPIRATORY MUSCLE FATIGUE

Symptoms and Treatment

Signs of Increased WOB
  • Increased respiratory effort (WOB)
  • Elevated respiratory rate (RR)
  • Decreased tidal volume (VT)
  • Symptoms can include:
    • Diaphoresis (sweating)
    • Tachycardia (fast heart rate)
  • Intervention strategies:
    • Maintain higher ventilatory support modes
    • Adjust settings for sensitivity or flow
    • Implement pressure support (PS)
    • Engage in muscle exercises between rest periods
    • Repeat assessments of NIF and VC to monitor progress.

SPONTANEOUS BREATHING TRIALS

Weaning Procedure Steps

  • Gradual reduction of support:
    • Mode reduced to least support.
    • Adjust CMV, AC, SIMV, CPAP, or T-Piece settings.
    • Lower FiO2 to < 40%%; PEEP to 5 cmH2O; PS minimized to lowest tolerated levels.
    • Conduct weaning parameters assessments (e.g., ABGs, RR, VT, MV, VC, NIF, RSBI).
  • If parameters are within normal limits, extubation is considered.
  • If the patient shows fatigue at any assessment stage, revert to the last stable setting.

WEANING FAILURE SYMPTOMS

Indicators of Increased Work of Breathing (WOB)

  • Symptoms include:
    • Accessory muscle usage, retractions, nasal flaring, diaphoresis
    • Respiratory rate ≥ 25 - 30
  • Causes of increased WOB may be:
    • Inappropriately sized or kinked endotracheal tube (ETT)
    • Abdominal distention
    • Elevated metabolic rate
    • Poor sensitivity or flow settings

EXTUBATION

Procedure and Indicators

  • Extubation involves the removal of the endotracheal tube (ETT) from the trachea.
  • Early extubation benefits include:
    • Shortened hospital stays
    • Reduced healthcare costs
    • Alleviated patient discomfort
    • Decreased risk of ventilator-associated pneumonia (VAP) or related injuries.
  • Predictors of successful extubation include:
    • Favorable arterial blood gases (ABGs)
    • Meeting weaning parameters
    • Absence of:
    • Congestive heart failure (CHF)
    • Pneumonia
    • Unexplained tachycardia
    • Arrhythmias
    • Chest retractions
    • Abdominal distention.
  • Predictors of re-intubation include:
    • SIMV or AC rate > 6
    • Most recent pH > 7.45
    • Most recent PaO2/FiO2 < 200
    • Heart rate > 120 in the preceding 24 hours, indicating potential cardiac insufficiency
    • The presence of three or more medical disorders
    • Patient's alertness prior to extubation
    • The initial reason for intubation was not post-operative or procedural.

UNPLANNED EXTUBATION

Causes and Management

  • Main causes for unplanned extubation:
    • Tubing becoming entangled during patient maneuvering
    • Patient's hands not secured
    • Patients waking and inadvertently pulling the tube.
  • If a tube is removed prematurely, the management steps include:
    • Quickly assess the patient’s stability.
    • If patient is stable, provide supplemental oxygen and monitor.
    • If patient is unstable, activate emergency response for bagging or re-intubation.

TERMINAL WEAN

Ethical Considerations

  • The process of withdrawing support from patients not expected to survive must be carefully discussed:
    • Discussions should involve the patient, family, and designated healthcare advocates.
    • These wishes can be pre-expressed through advanced directives (living wills).
  • Must be approached systematically, objectively, and compassionately:
    • Provide the family comprehensive understanding of the patient’s prognosis without fostering false hopes.
    • Healthcare teams should guide families through decision-making processes without placing the burden solely on them.
    • Respiratory care practitioners (RCP) should be comfortable with the decision-making process, and they can refuse participation if there are moral or ethical dilemmas.
    • Morphine is the drug of choice post-extubation for patient comfort; it is a respiratory depressant and provides significant comfort at the end of life.

APNEA TEST

Objective

  • Conducted to determine brain death by verifying the absence of brainstem activity.
  • Patients generally undergo other tests before this, such as MRI and EEG, to confirm brain death.
  • Terminal weaning may be initiated following a successful diagnosis of brain death through the apnea test.