Mechanical Ventilation

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Mechanical Ventilation

• BY Jonathan Phillips

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Introduction

• Conventional mechanical ventilation refers to the
  delivery of full or partial ventilatory support
  by a volume cycled mechanical ventilator or by
  pressure support. It can include the maintenance
  of positive airway pressure at the end of
  exhalation i.e.. PEEP.

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Indications

• The decision to initiate mechanical ventilation
  entails potentially serious complications.
• The main indication for mechanical ventilation is
  ARF.
• The parameters needed include respiratory rate
  gt35, inspiratory force lt 25cm H2O, vital capacity
  lt 10-15ml/kg, PaO2 lt60 mm Hg with FIO2 gt60,
  PaCO2 gt50mmHg, with pH lt 7.35. and an absent gag
  or cough reflex.

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Common disorders for Mechanical ventilation

• Acute pulmonary parenchymal diseases, pneumonia,
  ARDS
• Cardiogenic pulmonary edema
• Neuromuscular disorders myasthenia gravis,
  Guillian Barre syndrome, poliomyelitis, and
  spinal cord trauma
• Systemic illnesses include shock and sepsis

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Volume cycled ventilation

• The controlled variables of tidal volume and
  inspiratory flow determine airway pressure and
  inspiratory time. Variations in airway resistance
  or lung compliance alter airway pressures but do
  not affect minute ventilation.
• Controlled mechanical ventilation
• Assist-control
• Intermittent mandatory ventilation

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Controlled mechanical ventilation

• Minute ventilation is completely dependent upon
  the rate and tidal volume set on the ventilator.
  Any respiratory efforts made by the patient do
  not contribute to minute ventilation.
• CV is required in patients who are not making
  respiratory effort, spinal cord injury, OD or
  pharmacologic paralysis

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CMV

• Combined neuromuscular paralysis and controlled
  mechanical ventilation can also be used to avoid
  volutrauma in patients with ARDS and to avoid
  baratrauma in asthmatics who are difficult to
  ventilate.
• In these settings hypercapnia is accepted
  provided that oxygenation is maintained.

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Assist control

• In A/C mode the ventilator senses an inspiratory
  effort by the patient and responds by delivering
  a preset tidal volume.
• Every inspiratory effort that satisfies the
  ventilators demand valve trigger threshold,
  initiates delivery of the preset tidal volume.
• A control mode back up rate is set on the
  ventilator to prevent hypoventilation
• Patient work is required to trigger the
  ventilator, and continues during inspiration. In
  the presence of auto-peep the effective trigger
  threshold is increased by the amount of auto-peep
  present.
• An ACCP consensus statement cautioned against the
  initial use of A/C in awake patients with
  obstructive airway disease, since this can lead
  to progressive hyperinflation.

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Intermittent Mandatory ventilation

• IMV, the degree of ventilator support is
  determined by the select IMV rate. At regular
  intervals, the ventilator delivers a breath based
  upon a preset tidal volume and rate.
• The patient is allowed to breathe spontaneously
  through the ventilator circuit at a tidal volume
  and rate according to need and capacity.

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IMV

• Most present day ventilators synchronize the
  intermittent mandatory ventilation breaths with
  inspirtatory effort by the patient, a modality
  termed synchronized IMV or SIMV.
• This modification requires a trigger modality,
  either a demand valve or flow-by, both of which
  need patient effort to trigger and therefore
  increase the work of breathing.

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Pressure Support Ventilation

• PSV is flow-cycled in that, once triggered by a
  demand valve, the preset pressure is sustained
  until the inspiratory flow tapers, usually to 25
  of its maximal value.
• PSV tends to be a comfortable ventilatory
  modality because the patients has greater control
  over ventilator cycling and flow rates.
• Close monitoring is required whenever PSV is used
  alone because neither tidal volume or minute
  ventilation is guaranteed.
• PSV is more appropriate during weaning from
  mechanical ventilation.

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Vent management

• FIO2-hypoxia is more dangerous than is brief
  exposure high levels of O2.
• The initial FIO2 should be 100
• FiO2 can be made to achieve a PaO2 greater than
  60mmHg or SaO2 gt90.
• Attempts should be made to utilize the lowest
  possible fraction of FIO2 that maintains the
  arterial oxygen saturation gt90 or PO2 gt60
• An FIO2 below 0.5 is preferable to minimize
  oxygen toxicity.

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Vent Management

• Respiratory rate of 10-15 breaths per minute to
  begin.
• In COPD patients, minute ventilation should be
  adjusted to achieve baseline PaCO2 and not
  necessarily a normal PaCO2.
• Hyperventilation with resultant metabolic
  alkalosis in these patients may be associated
  with serious electrolytes shifts and arrhythmias.
• Initial tidal volumes usually can be set at 10-12
  ml/kg
• Patients with decreased ling compliance (ARDS)
• Often need smaller lung volumes 6-8 to minimize
  peak airway pressures.

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Vent managementPEEP

• Positive end-expiratory pressure is defined as
  the maintenance of positive airway pressure at
  the end of expiration.
• It can be applied in both CPAP and continuous
  positive pressure pressure ventilation.
• PEEP increases lung compliance and oxygenation
  while decreasing the shunt fraction and the work
  of breathing.

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Vent ManagementPEEP

• The main goal of PEEP is to achieve a PaO2
  greater than 55-60 mm HG with a FIO2 less than or
  equal to 60.
• PEEP is applied in 3-5 cm H2O increments.
• PEEP gt10 should not have their PEEP removed
  abruptly, because removal can result in collapse
  of distal lung units, worsening of shunt, and
  potentially life threatening hypoxemia.

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Vent managementInspiratory flow rate

• Low flow rates can be associated with prolonged
  inspiratory times that can lead to the
  development of auto-PEEP.
• The resultant hyperinflation can affect patient
  hemodynamics adversely by impairing venous return
  to the heart.

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Vent ManagementTrigger sensitivity

• Pressure triggering to initiate either a machine
  assisted breath or to permit spontaneous
  breathing between IMV breaths, or during trials
  of CPAP.
• The patient must generate a decrease in the
  airway circuit pressure equal to the selected
  pressure sensitivity.
• The smallest trigger sensitivity should be
  selected, allowing the patient to initiate
  mechanical or spontaneous breaths without causing
  the ventilator to autocycle.

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Vent ManagementFlow-by

• Refers to triggering of the ventilator by changes
  in airflow as opposed to changes in airway
  pressure.
• ? Flow sensitivity, the rate of inhaled flow that
  triggers the ventilator to switch from base flow
  to either a machine delivered or a spontaneous
  breath.
• ?Flow by triggering requires less work of
  breathing when used with patients receiving CPAP.
• ?However it offers no advantage over demand valve
  triggering when using pressure support
  ventilation.

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Weaning Strategies

• The level of support ventilation is decreased
  gradually, and the patient assumes more of the
  work of ventilation.
• IMV allows a change from mechanical ventilation
  to spontaneous breathing by decreasing the
  ventilator rate gradually.

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Weaning strategies

• T-Tube intersperses periods of unassisted
  spontaneous breathing through a T-Tube with
  periods of ventilator support.
• Short daytime periods 5-15 minutes 2-6 times a
  day with increasing periods.
• Extubation may be appropriate when the patient
  can tolerate more than 30-90 minutes of T-tube
  ventilation

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Weaning strategies

• PSV is used when respiratory muscle weakness
  appears to be compromising weaning success.
• A decrease in respiratory rate with achieved
  tidal volumes of 10-12 ml/kg signals that the
  optimal PSV level has been reached. At this point
  PSV can be reduced.
• Once PSV level 5-8 cm H2O is reached the patient
  can be extubated.

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Extubation

• Should be performed early in day
• Patient should be told to cough
• Elevate head to 30-45 degrees
• Oropharynx should be suctioned
• Cuff deflated
• Patient extubated and face mask placed
• Patient encouraged to deep breath and cough with
  HHN to follow

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Competency Exam
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Questions

• 1. Which is the main indication for vent
  management?
• A. Acute respiratory failure
• B. Severe COPD
• C. CO2 retention
• D. Severe pneumonia

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Questions

• 1. Which is the main indication for vent
  management?
• A. Acute respiratory failure
• B. Severe COPD
• C. CO2 retention
• D. Severe pneumonia

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Questions

• 2. Which is not a guideline for withdraw of
  mechanical ventilation?
• A. PaO2 gt60
• B. FIO2lt50
• C.PEEP gt10
• D.Vital capacity gt10

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Questions

• 2. Which is not a guideline for withdraw of
  mechanical ventilation?
• A. PaO2 gt60
• B. FIO2lt50
• C.PEEP gt10
• D.Vital capacity gt10

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Questions

• 3. An FIO2 below this value is preferable to
  minimize oxygen toxicity?
• A.80
• B.70
• C.60
• D.50

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Questions

• 3. An FIO2 below this value is preferable to
  minimize oxygen toxicity?
• A.80
• B.70
• C.60
• D.50

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End of Lecture