MECHANICAL VENTILATION

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MECHANICAL VENTILATION

• Things I wish I knew when I was an Intern
• Amit Gupta, MD
• Internal Medicine
• North Mississippi Medical Center

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Mechanical Ventilation

• Indications for Intubation and Ventilation
• Principles of Mechanical Ventilation
• Patterns of Assisted Ventilation
• Ventilator Dependence Complications
• Liberation from Mechanical Ventilation Weaning
• Troubleshooting
• Arterial Blood Gases

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Indications for Mechanical Ventilation

• .An opening must be attempted in the trunk of
  the trachea, into which a tube or cane should be
  put You will then blow into this so that lung
  may rise again.And the heart becomes strong.
• -Andreas
  Vesalius (1555)

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Indications for Mechanical Ventilation

• 1. Thinking of Intubation elective v/s
  emergent
• 2. Act of weakness?
• 3. Endotracheal tubes are not a disease and
  ventilators are not an addiction
• 4. And the usual elective and emergent
  indications that you all know!

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Objectives of Mechanical Ventilation

• Improve pulmonary gas exchange
• Reverse hypoxemia and Relieve acute respiratory
  acidosis
• Relieve respiratory Distress
• Decrease oxygen cost of breathing and reverse
  respiratory muscle fatigue
• Alter pressure-volume relations
• Prevent and reverse atelectasis
• Improve Compliance
• Prevent further injury
• Permit lung and airway healing
• Avoid complications

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Strategies for Mechanical Ventilation
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Monitoring Lung Mechanics

• Proximal Airway Pressures (end-inspiratory)
• 1. Peak Pressure Pk
• Function of Inflation volume, recoil force of
• lungs and chest wall, airway resistance
• 2. Plateau Pressure Pl
• Occlude expiratory tubing at end-inspiration
• Function of elastance alone

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Use of Airway Pressures

• Pk increased Pl unchanged
• Tracheal tube obstruction
• Airway obstruction from secretions
• Acute bronchospasm
• Rx Suctioning and Bronchodilators

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Use of Airway Pressures

• Pk and Pl are both increased
• Pneumothorax
• Lobar atelectasis
• Acute pulmonary edema
• Worsening pneumonia
• ARDS
• COPD with tachypnea and Auto-PEEP
• Increased abdominal pressure
• Asynchronous breathing

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Use of Airway Pressures

• Decreased Pk
• System air leak Tubing disconnection, cuff leak
• Rx Manual inflation, listen for leak
• Hyperventilation Enough negative intrathoracic
• pressure to pull air into lungs may drop Pk.

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Compliance

• Static Compliance (Cstat)
• Distensibility of Lungs and Chest wall
• Cstat Vt/Pl
• Normal C stat 50-80 ml/cm of water
• Provides objective measure of severity of illness
  in a
• pulmonary disorder
• Dynamic Compliance
• Cdyn Vt/Pk
• Subtract PEEP from Pl or Pk for compliance
• measurement
• Use Exhaled tidal volume for calculations

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Patterns of Assisted Ventilation

• Assist Control
• Intermittent Mandatory Ventilation
• Pressure Controlled Ventilation
• Pressure Support Ventilation
• Positive end-expiratory ventilation
• Continuous Positive Airway Pressure

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Assist Control Ventilation

• Volume-cycled lung inflation
• Patient can initiate each mechanical breath or
  Ventilator
• provides machine breaths at a preselected rate
• Maintain IE ratio to 12 to 14. An increase in
  Peak flow
• decreases the time for lung inflation and
  increases the IE
• Ratio
• IE ratio of lt12 can cause hyperinflation by air
  trapping
• Diaphragmatic contraction continues during ACV
  and
• increases the work of breathing.

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Assist Control Ventilation

• Adverse effects
• In a tachypneic patientgtgtLead to overventilation
  and
• severe respiratory alkalosisgtgt Hyperinflation and
  
• Auto-PEEPgtgt Lead to Electromechanical
• dissociation

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Intermittent Mandatory Ventilation

• Delivers volume cycled breaths at a preselected
  rate with spontaneous breathing between machine
  breaths
• Less Alkalosis and Hyperinflation
• Synchronized IMV

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Intermittent Mandatory Ventilation

• Disadvantages
• Increased work of Breathing
• Spontaneous breathing through a high resistance
  circuit
• Solution Add Pressure support
• Cardiac Output Changes
• C O decreased by decreasing ventricular filling
• C O increased by reducing ventricular afterload
• More significant decrease in patients with LV
  dysfunction

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IMV vs. ACV

• Switch to IMV for
• Rapid breathers with alkalosis and over-
• Inflation
• Switch to ACV for
• Patients with respiratory muscle weakness and
• LV dysfunction

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Pressure Controlled Ventilation

• Pressure cycled breathing, fully ventilator
  controlled
• Inspiratory flow rate decreases exponentially
  during lung inflation
• ()Reduces peak airway pressure and improves gas
  exchange
• (-)Inflation volume varies with changes in
  mechanical properties of the lungs.
• Suited for patients with neuromuscular diseases
  and normal lung mechanics

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Inverse ratio Ventilation

• PCV combined with prolonged inflation time
• Inspiratory flow rate is decreased
• IE ratio reversed to 21
• Helps prevent alveolar collapse
• (-) Hyperinflation, Auto-PEEP and decreased
  cardiac output
• Use ARDS with refractory hypoxemia or
  hypercapnia ?mortality benefit

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Pressure Support Ventilation

• Pressure augmented breathing
• Allows patient to determine the inflation volume
  and respiratory cycle duration
• Uses augment inflation during spontaneous
  breathing or overcome resistance of breathing
  through ventilator circuits (during weaning)
• Popular an a non-invasive mode of ventilation via
  nasal or face masks

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Positive end-expiratory pressure

• Alveolar pressure at end-expiration is above
  atmospheric pressure PEEP
• Extrinsic PEEP
• Auto PEEP

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Positive end-expiratory pressure

• EXTRINSIC PEEP
• Applied by placing pressure limiting valve in the
  expiratory limb of ventilator circuit
• Prevents end-expiratory alveolar collapse and
  recruits collapsed alveoli
• This decreases intrapulmonary shunting, improves
  gas exchange and improves lung compliance,
  allowing the FiO2 to be reduced to less toxic
  levels

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Positive end-expiratory pressure

• Cardiac Performance
• Greater reduction in cardiac filling and cardiac
  output (Q),
• irrespective of level of PEEP!
• It is a function of PEEP induced increase in mean
  
• intrathoracic pressure
• Oxygen transport Do2
• Do2 Q X 1.3 X Hb X SaO2
• Systemic O2 delivery may vary with the effect of
  PEEP on
• the Cardiac Output.

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Positive end-expiratory pressure

• Best PEEP Monitor Cardiac Output
• Another measure Venous Oxygen Saturation
• If VOS decreases after PEEP applied Drop CO
• Swan-Ganz catheter may be indicated in most
  patients on PEEP

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Positive end-expiratory pressure

• CLINICAL USES
• Reduce toxic levels of FiO2 (ARDS not pneumonia)
• Low-volume ventilation
• Obstructive lung disease (ExtrinsicOccult PEEP)

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Positive end-expiratory pressure

• CLINICAL MISUSES
• Reducing Lung Edema
• Routine PEEP
• Mediastinal Bleeding after CABG

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Continuous positive Airway Pressure

• Spontaneous breathing
• Patient does not need to generate negative
  pressure to receive inhaled gas
• CPAP replaced spontaneous PEEP
• Use Non-intubated patients (OSA, COPD)

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Occult PEEP

• Intrinsic or Auto-PEEP or Hyperinflation
• Incomplete alveolar emptying during expiration
• Ventilator Factors High inflation volumes, rapid
  rate, low exhalation time
• Disease factors Asthma, COPD
• Consequences Decreased CO/EMD, Alveolar rupture,
  Underestimation of thoracic compliance, increased
  work of breathing.
• If extrinsic PEEP does not increase Pk, then
  occult PEEP is present

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Complications of Mechanical Ventilation

• Toxic effects of Oxygen
• Decreased cardiac output
• Pneumonia and sepsis
• Psychological problems
• Ventilator dependence

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Complications of Mechanical Ventilation

• Purulent sinusitis
• Laryngeal Damage
• Aspiration Value of routine tracheal suctioning
• Tracheal Necrosis (pressure below 20mm water)
• Alveolar rupture Pneumothorax,
  pneumomediastinum, subQ emphysema,
  pneumoperitoneum
• Basilar and sub-pulmonic air collections in the
  supine position, as seen on X-ray

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Liberation from Mechanical Ventilation Weaning

• Weaning Gradual withdrawal of mechanical
  ventilation
• Misconceptions
• Duration- longer duration, harder to wean
• Method of weaning determines ability to wean
• Diaphragm weakness is a common cause of
  failed weaning
• Aggressive nutrition support improves
  ability to wean
• Removal of ET tube reduces work of breathing

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Bedside Weaning Parameters
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Bedside Weaning Parameters
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Maximal Inspiratory Pressure

• Pmax Excellent negative predictive value if less
  than 20 (in one study 100 failure to wean at
  this value)
• An acceptable Pmax however has a poor positive
  predictive value (40 failure to wean in this
  study with a Pmax more than 20)

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Frequency/Volume ratio

• Index of rapid and shallow breathing RR/Vt
• Single study results
• RR/Vtgt105 95 wean attempts unsuccessful
• RR/Vtlt105 80 successful
• One of the most predictive bedside parameters.

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T-Piece Weaning

• On-off toggle switch that circulates between on
  and off the ventilator
• Inhaled gas is delivered at a high flow rate
• Varied protocols like 30min-2hr on and off, or
  keep as long as possible and if tolerated for
  gt2-4hr. Deemed successsful (RR, TV, HR,
  diaphoresis, sat)
• Failed T piece Resume Vent support till
  comfortable, 24h

vent
Airflow with CPAP
patient
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T-Piece with Ventilator

• Drawback increased resistance due to vent tubing
  and actuator valve in circuit
• Provide minimum pressure support (PSV) Pmin
• Pmin PIFR X R
• PIFR is during spontaneous breathing
• R is airflow resistance during mech ventilation
• R Pk-Pl/Vinsp
• (Vinspinspiratory flow rate delivered by the
  vent)

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IMV Weaning

• Gradual decrease in no of machine breaths in
  between the spontaneous breaths
• False security It does not adjust to patients
  ventilatory demands to maintain constant MV
• End point in IMV weaning is the T-piece trial
• Most important to recognize when a patient is
  capable of spontaneous unassisted breathing
• T-piece more rapid than IMV

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Complicating Factors

• DYSPNEA
• Anxiety and dyspnea are detrimental (low dose
  haloperidol or morphine)
• CARDIAC OUTPUT
• Increased LV afterload can reduce CO, impair
  diaphragm function, promote pulmonary edema
• (Use Swan to monitor CO, may use dobutamine)
• ELECTROLYTE DEPLETION
• OVERFEEDING

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The Problem Wean

• RAPID BREATHING Check TV
• Low TVgtgt Resume vent support
• TV not low.. Check arterial pCO2
• Arterial pCO2 decreasedgtsedate (anxiety)
• Arterial pCO2 not decreasedgt Resume vent

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The Problem Wean

• ABDOMINAL PARADOX
• Inward displacement of the diaphragm during
  inspiration is a sign of diaphragmatic muscle
  fatigue
• HYPOXEMIA
• May be due to low CO and MVO2
• HYPERCAPNIA
• Increase in PaCO2-PetCO2 increase dead space
  ventilation
• Unchanged gradient Respiratory muscle fatigue or
  enhanced CO2 production

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Tracheal Decannulation

• Successful weaning is not synonymous with
  tracheal decannulation
• If weaned and not fully awake or unable to clear
  secretions, leave ETT in place
• Contrary to popular belief, tracheal
  decannulation increases the work of breathing due
  to laryngeal edema and secretions
• Do not perform tracheal decannulation to reduce
  work of breathing

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Inspiratory Stridor

• Post extubation inspiratory stridor is a sign of
  severe obstruction and should prompt reintubation
• Laryngeal edema (post-ext) may respond to
  aerosolized epinephrine in children
• Steroids have no role
• Most need reintubation followed by tracheostomy

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ARDS and Low Volume Ventilation

• ARDS Network trial NEJM May 4, 2000 p1301-08
• Traditional TV 10-15ml/kg, keep plateault50cm
  water
• Low TV ventilation TV 6ml/kg, keep plateault30cm
  water
• Need high RR in Low TV group to prevent acidosis
• Permissive hypercapnia tolerated well, if needed,
  use IV bicarb to maintain pH
• May add PEEP in addition to the low TV group to
  prevent atelectrauma (open-close alveoligtgt
  alveolar fracture)
• Results Lower mortality in the Low TV group (31
  v/s 39.8 plt0.007) Higher days without vent use
  and lower average plateau pressures in low TV
  group.

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