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Principles of Mechanical Ventilation

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					Principles of Mechanical Ventilation
RET 2284
Module 5.0 Ventilator Management
     - Improving Ventilation/Oxygenation
Improving Ventilation / Oxygenation

   The first 30 – 60 minutes following initiation of ventilation
   are generally spent evaluating vital signs, breath sounds,
   ventilator parameters, lung compliance and resistance,
   the artificial airway, and documenting patient response to
   therapy

   After that initial phase, the RT is often concerned with
   improving ventilation and oxygenation and managing the
   patient-ventilator system
Improving Ventilation / Oxygenation

   Correcting PaCO2 Abnormalities
                      .
      A change in VE will often be needed when a
       patient is first placed on mechanical ventilation

      During ventilation a change in VT or rate (f) is
       needed to correct for respiratory alkalosis or
       acidosis
Improving Ventilation / Oxygenation

   Correcting PaCO2 Abnormalities
      Methods of Changing Ventilation Based on PaCO2 and pH


       If it is appropriate to keep rate (f) constant and change VT,
       the equations is as follows:



        Desired VT = Known PaCO2 x Known VT
                     Desired PaCO2
Improving Ventilation / Oxygenation

   Correcting PaCO2 Abnormalities
      Methods of Changing Ventilation Based on PaCO2 and pH

       If it is appropriate to keep VT the same and change rate (f),
       then the equations is as follows:


        Desired f = Known PaCO2 x Known f
                     Desired PaCO2
Improving Ventilation / Oxygenation

   Correcting PaCO2 Abnormalities
      Respiratory Acidosis
         Volume and Pressure Ventilation Changes
              When PaCO2 is elevated (>45 mm Hg) and pH is           .
               decreased (<7.35), respiratory acidosis is present and VA
               is not adequate
              Causes
                   PE, Pneumonia

                   Airway disease (e.g., severe asthma attack)

                   Pleural abnormalities (e.g., effusions)

                   Chest wall abnormalities

                   Neuromuscular disease

                   CNS problems
Improving Ventilation / Oxygenation

   Correcting PaCO2 Abnormalities
      Respiratory Acidosis
         Volume and Pressure Ventilation Changes


        Guideline:
               VT to 8 – 12 mL/kg ideal body weight (based on patient’s
                pulmonary problem)
               Maintain plateau pressure <30 cm H2O
               If VT is already high and/or Pplateau are already high, then f
                should be increased

                     Read example 1, 2 and 3: Respiratory Acidosis,
                      Increasing VT, page 259 – 260 (Pilbeam)
Improving Ventilation / Oxygenation

   Correcting PaCO2 Abnormalities
      Respiratory Alkalosis
         Volume and Pressure Ventilation Changes
              When PaCO2 is decreased (<35 mm Hg) and pH
               increases (>7.35), then respiratory alkalosis is present
               and alveolar ventilation is excessive
              Causes
                  Hypoxia with compensatory hyperventilation

                  Parenchymal lung disease

                  Medications

                  Mechanical ventilation

                  CNS disorders

                  Anxiety

                  Metabolic disorders
Improving Ventilation / Oxygenation

   Correcting PaCO2 Abnormalities
      Respiratory Alkalosis
         Volume and Pressure Ventilation Changes


        Guideline:
               Volume ventilation: f, and if necessary, VT
               Pressure ventilation: f, and if necessary, pressure

                     Read example 1 and 2: Respiratory Alkalosis,
                      Decreasing the rate, page 261 (Pilbeam)
Improving Ventilation / Oxygenation

   Correcting PaCO2 Abnormalities
      Metabolic Acidosis and Alkalosis

         Treatment of metabolic acidosis and alkalosis should
          focus on identifying those metabolic factors that can
          cause these acid-base disturbances
Improving Ventilation / Oxygenation

   Correcting PaCO2 Abnormalities
      Metabolic Acidosis and Alkalosis
         Metabolic Acidosis
               Causes
                   Ketoacidosis (alcoholism, starvation, diabetes)

                   Uremic acidosis (renal failure to excrete acid)

                   Loss of bicarbonate (diarrhea)

                   Renal loss of base following administration of

                    carbonic anhydrase inhibitors (e.g., Diamox)
                   Overproduction of acid (lactic acidosis)

                   Toxin ingest that produce acidosis (salicylate,

                    ethylene glycol [antifreeze], methanol
Improving Ventilation / Oxygenation

   Correcting PaCO2 Abnormalities
      Metabolic Acidosis and Alkalosis
         Metabolic Acidosis
               Treatment should first deal with the cause of the acidosis

               Secondly, assess the need to reverse the acidemia with
                some form of alkaline agent
Improving Ventilation / Oxygenation

   Correcting PaCO2 Abnormalities
      Metabolic Acidosis and Alkalosis
         Metabolic Acidosis
               These patients are often struggling to lower their PaCO2
                to compensate for the metabolic acidemia. As a
                consequence, these patients are at risk for developing
                respiratory muscle fatigue
                                                                    .
               If the patient is losing the struggle to maintain high VE
                with spontaneous breathing, assisted ventilation may be
                necessary to avoid respiratory failure. It is then
                appropriate to keep the pH (7.35 – 7.45)
Improving Ventilation / Oxygenation

   Correcting PaCO2 Abnormalities
      Metabolic Acidosis and Alkalosis
         Metabolic Alkalosis
               Causes
                   Loss of gastric fluid and stomach acids (vomiting,

                    nasogastric suctioning)
                   Acid loss in the urine (diuretic administration)

                   Acid shift into the cells (potassium deficiency)

                   Lactate, acetate, citrate administration

                   Excessive bicarbonate loads (bicarbonate

                    administration)
Improving Ventilation / Oxygenation

   Correcting PaCO2 Abnormalities
      Metabolic Acidosis and Alkalosis
         Metabolic Alkalosis
         Treatment involves correcting the underlying cause and
          reversing those factors leading to the alkalosis. In sever
          cases, carbonic anhydrate inhibitors, acid infusion, and
          low bicarbonate dialysis my be required

               Only in rare circumstances does partial respiratory
                compensation of metabolic alkalosis occur – PaCO2 will
                usually not rise higher than 55 mm Hg (Remember that as
                the CO2 rises, the PaO2 falls)
Improving Ventilation / Oxygenation

   Correcting PaCO2 Abnormalities
      Mixed Acid – Base Disturbances
         Combined Respiratory Alkalosis and Metabolic Acidosis
              Read case studies: Pilbeam, pg. 262 – 263


         Combined Respiratory Acidosis and Metabolic Alkalosis
              Read case study: Pilbeam, pg. 263
Improving Ventilation / Oxygenation

   Correcting PaCO2 Abnormalities
      Increased Physiological Dead Space
         If pure respiratory acidosis persists even after alveolar
          ventilation has been increased, the patient may have a
          problem with increased dead space
               Causes
                   Pulmonary emboli

                   Low cardiac output  low pulmonary perfusion

                   High alveolar pressure (PEEP)   pulmonary blood

                    flow
                   Air trapping   pulmonary perfusion
Improving Ventilation / Oxygenation

   Correcting PaCO2 Abnormalities
      Increased Metabolism and Increased CO2
       Production
         Read case study: Pilbeam, pg. 264
                                 .
         Metabolic rate and VCO2 are increased in the following
          patients:
               Fever
               Sepsis
               Burns
               Multiple trauma and multiple surgical procedures
               Hyperthyroidism
               Seizures
Improving Ventilation / Oxygenation

   Correcting PaCO2 Abnormalities
      Increased Metabolism and Increased CO2
       Production      .
         In these patients VE is increased and WOB is elevated


        Treatment Options
               Increase machine rate to WOB: may cause auto-peep
               Add pressure support for spontaneous breaths to WOB
                through ET and circuit
               Switch to PC-CMV, use sedation to WOB
Improving Ventilation / Oxygenation

   Correcting PaCO2 Abnormalities
      Intentional Iatrogenic Hyperventilation
         Definition
               Deliberate hyperventilation in patients with acute head
                injury and increased intracranial pressure (ICP)

                     Hyperventilation reduces PaCO2 which causes
                      vasoconstriction of cerebral blood vessels and
                      decreases blood flow to the brain and is believed to
                      lower increased intracranial pressure ICP
Improving Ventilation / Oxygenation

   Correcting PaCO2 Abnormalities
      Intentional Iatrogenic Hyperventilation
         Current therapy guideline for head injuries with
          increased ICP do not recommend prophylactic
          hyperventilation (PaCO2 <25 mm Hg) during the first 24
          hours - may cause cerebral ischemia and cerebral
          hypoxemia
Improving Ventilation / Oxygenation

   Correcting PaCO2 Abnormalities
      Intentional Iatrogenic Hyperventilation
         Hyperventilation may be needed for brief periods when
          acute neurological deterioration is present and ICP
          elevated

         Mild hyperventilation (PaCO2 30 – 35 mm Hg) may be
          used for longer periods in situation in which increased
          ICP is refractory to standard treatment

       The practice of iatrogenic hyperventilation still remains
       controversial
Improving Ventilation / Oxygenation

   Correcting PaCO2 Abnormalities
      Permissive Hypercapnia (PHY)
         Definition
               Deliberate limitation of ventilatory support to avoid lung
                overdistention and injury of lung
                   ARDS

                   Status asthmaticus

               PaCO2 values are allowed to rise above normal
                   ≥50 – 150 mm Hg

               pH values are allowed to fall below normal
                   ≥7.10 – 7.30

                   Most researchers agree pH ≥7.25 is acceptable
Improving Ventilation / Oxygenation

   Correcting PaCO2 Abnormalities
      Permissive Hypercapnia (PHY)
         PaCO2 accompanied PaO2
              O2 administration must be provided and monitored closely


         PaCO2 stimulates the drive to breath
              Appropriate to provide sedation to patients in whom PHY
               is being employed
Improving Ventilation / Oxygenation

    Correcting PaCO2 Abnormalities
      Permissive Hypercapnia (PHY)
          Procedures for Managing PHY
           1.   Allow PaCO2 to rise and pH to fall without changing
                mandatory rate or volume
                 a.  Sedate the patient
                 b.  Avoid high ventilating pressures
                 c.  Maintain oxygenation
           2.   Reduces CO2 production
                 a.  Paralyze
                 b.  Cool
                 c.  Restrict glucose
Improving Ventilation / Oxygenation

    Correcting PaCO2 Abnormalities
      Permissive Hypercapnia (PHY)
          Procedures for Managing PHY
           3.   Keep pH >7.25
                 a. Sodium bicarbonate
                 b. Tris-hydroxiaminomethane (an amino buffer)
                 c. Carbicarb (mixture of sodium carbonate and
                    bicarbonate
Improving Ventilation / Oxygenation

    Correcting PaCO2 Abnormalities
      Permissive Hypercapnia (PHY)
          Contraindications and Effects of PHY
              Head trauma
              Intracranial disease
              Intracranial lesions
Improving Ventilation / Oxygenation

    Correcting PaCO2 Abnormalities
      Permissive Hypercapnia (PHY)
          Relatively contraindicated in the following
              Cardiac ischemia
              Left ventricular compromise
              Pulmonary hypertension
              Right heart failure


       Because PHY may increase cardiac output (CO), a normal BP,
       and pulmonary hypertension, if a patient’s sympathetic response
       is impaired or blocked, or cardiac function is impaired, an
       increase in CO may not occur, allowing the vasodilatation to
       result in hypotension
Improving Ventilation / Oxygenation

    Correcting PaCO2 Abnormalities
      Permissive Hypercapnia (PHY)

       The use of PHY is restricted to situations in which the
       target airway pressure is at its maximum and the highest
       possible rates are being used

       The risks of hypercapnia are considered by some to be
       preferable to the high Pplat required to achieve normal CO2
       levels

       Read Case Study: Pilbeam, pg. 265 – 266
Improving Ventilation / Oxygenation

    Oxygenation Using FiO2 and PEEP
      Adjusting FiO2
          Every attempt should be made to maintain the FiO2
           <0.40 to 0.50 to prevent the complications of O2
           toxicity while keeping the PaO2 between 60 and 90
           mm Hg
                   .
          The SpO2 can then be used to titrate FiO2, with the
           goal of maintaining the SpO2 >90%
Improving Ventilation / Oxygenation

    Oxygenation Using FiO2 and PEEP
      Adjusting FiO2
          ABGs are obtained after mechanical ventilation is
           initiated and compared with FiO2 being delivered and
           the SpO2

          A linear .relationship exists between PaO2 and FiO2 as
           long as VE, CO, Shunt, VD/VT remain fairly constant
           (cardiopulmonary status)
Improving Ventilation / Oxygenation

    Oxygenation Using FiO2 and PEEP
      Adjusting FiO2
          Because of the linear correlation between PaO2 and
           FiO2 the following equation can be used to select the
           desired FiO2 to achieve a desired PaO2:


       Desired FiO2 = PaO2 (desired) x FiO2 (known)
                         PaO2 (known)
Improving Ventilation / Oxygenation

    Oxygenation Using FiO2 and PEEP
      Adjusting FiO2

       Exercise
         After being supported on a ventilator for 30 minutes, a
         patient’s PaO2 is 40 mm Hg on an FiO2 of 0.50. Acid-
         base status is normal and all other ventilator
         parameters are within the acceptable range. What
         FiO2 is required to achieve a desired PaO2 of 60 mm
         Hg?
Improving Ventilation / Oxygenation

    Oxygenation Using FiO2 and PEEP
      Adjusting FiO2

       Desired FiO2 = PaO2 (desired) x FiO2 (known)
                          PaO2 (known)

       Desired FiO2 = (60 mm Hg) (0.50 FiO2)
                           40 mm Hg

       Desired FiO2 = 0.75
Improving Ventilation / Oxygenation

    Oxygenation Using FiO2 and PEEP
                                  _
      Selection of FiO2 or Adjustment of Paw
          Maintaining an FiO2 >60 may lead to:
              O2 toxicity
              Absorption atelectasis



        Lower limits of target PaO2 is 60 mm Hg
        Lower limits of target SpO2 is 90%
Improving Ventilation / Oxygenation

    Oxygenation Using FiO2 and PEEP
                                  _
      Selection of FiO2 or Adjustment of Paw

         When PaO2 remains very low on high FiO2, significant
         shunting, V/Q abnormalities , and/or diffusion defects
         are present - other methods to improve oxygenation,
         besides increasing FiO2, must be considered
           Paw
              _
                PEEP
                HFOV
                APRV
Improving Ventilation / Oxygenation

    Oxygenation Using FiO2 and PEEP
                                  _
      Selection of FiO2 or Adjustment of Paw
          _
          Paw can be used to increase the PaO2
                               _
          Factors that affect Paw during PPV
              PIP
              PEEP
              Auto-PEEP
              I:E ratio
              Respiratory rate
              Inspiratory flow patterns
Improving Ventilation / Oxygenation

    Oxygenation Using FiO2 and PEEP
                                  _
      Selection of FiO2 or Adjustment of Paw
         _
          Paw is a major determinant of oxygenation in patients
           with ARDS
              Mean alveolar pressure  oxygenation
              Alveolar recruitment  oxygenation
                                       _
          Typical method to increase Paw
              PEEP
                                      _
          Other methods to increase Paw
              HFOV
              APRV
Improving Ventilation / Oxygenation

    Oxygenation Using FiO2 and PEEP
                                  _
      Selection of FiO2 or Adjustment of Paw
          _
          Paw must be monitored closely to prevent:
              Air trapping
              Overdistention
              Barotrauma (e.g. pneumothorax)
              Venous return
              CO
Improving Ventilation / Oxygenation

    Oxygenation Using FiO2 and PEEP
      Positive End Expiratory Pressure (PEEP)
          Goals of PEEP
              Enhance tissure oxygenation
              Maintain a PaO2 above 60 mm Hg, and SpO2 ≥90% at
               an acceptable pH
              Restore FRC


          These goals my be accompanied by the opportunity to
           reduce the FiO2 to safer levels (<0.50) as PEEP
           becomes effective
              Must maintain cardiovascular function and avoid lung
               injury
Improving Ventilation / Oxygenation

    Oxygenation Using FiO2 and PEEP
      Positive End Expiratory Pressure (PEEP)
          Minimum or Low PEEP
              PEEP at 3 – 5 cm H2O to help preserve a patient’s
               normal FRC


          Therapeutic PEEP
              PEEP >5cm H2O
              Used in the treatment of refractory hypoxemia caused by
               increased intrapulmonary shunting and V/Q mismatching
               accompanied by a decreased FRC and pulmonary
               compliance
Improving Ventilation / Oxygenation

    Oxygenation Using FiO2 and PEEP
      Positive End Expiratory Pressure (PEEP)
          Optimal PEEP
              The level of PEEP at which the maximum beneficial
               effects of PEEP occur
                   O2 transport
                   FRC
                   Compliance
                   Shunt
Improving Ventilation / Oxygenation

    Oxygenation Using FiO2 and PEEP
      Positive End Expiratory Pressure (PEEP)
          Optimal PEEP
              The level of PEEP is considered optimum because it is
               not associated with profound cardiopulmonary side
               effects
                   Venous return
                   CO
                   BP
                   Shunting
                   VD/VT
                   Barotrauma
                   Volutrauma
              Accompanied by safe levels of FiO2
Improving Ventilation / Oxygenation

    Oxygenation Using FiO2 and PEEP
      Positive End Expiratory Pressure (PEEP)
          Indications for PEEP Therapy
              Bilateral infiltrates on chest radiograph
              Recurrent atelectasis
              Reduced CL
              PaO2 <60 mm Hg on high FiO2 of >0.5
              PaO2/FiO2 ratio <200 for ARDS and <300 for ALI
              Refractory hypoxemia: PaO2 increases <10 with FiO2
               increase of 0.2
Improving Ventilation / Oxygenation

    Oxygenation Using FiO2 and PEEP
      Positive End Expiratory Pressure (PEEP)
          Specific clinical disorders that may benefit from PEEP
              ALI
              ARDS
              Cardiogenic PE
              Bilateral, diffuse pneumonia
Improving Ventilation / Oxygenation

    Oxygenation Using FiO2 and PEEP
      Positive End Expiratory Pressure (PEEP)
          Application of PEEP

              Increased in increments of 3 – 5 cm H2O in adults, 2 –
               3 cm H2O in infants

              Target acceptable PaO2/FiO2 ratio at a safe FiO2
                  >300 (e.g., PaO2 = 100, with FiO2 = 0.33
                   (optimal, but not always realistic)
Improving Ventilation / Oxygenation

    Oxygenation Using FiO2 and PEEP
      Positive End Expiratory Pressure (PEEP)
          Application of PEEP
              Patient Appearance
                   Color, level of consciousness, anxiety – a sudden
                    deterioration may indicate CV collapse or
                    pneumothorax
              Blood Pressure
                   BP of 20 mm Hg systolic drop is significant
              Breath Sounds
                   Barotrauma, e.g., pneumothorax
Improving Ventilation / Oxygenation

    Oxygenation Using FiO2 and PEEP
      Positive End Expiratory Pressure (PEEP)
          Application of PEEP
              Ventilator Parameters
                  VT, Flow, PIP, plateau pressure, VE
Improving Ventilation / Oxygenation

    Oxygenation Using FiO2 and PEEP
      Positive End Expiratory Pressure (PEEP)
          Application of PEEP
              Static Compliance (CS)
                   As PEEP progressively restores FRC, compliance
                    should increase
Improving Ventilation / Oxygenation

    Oxygenation Using FiO2 and PEEP
      Positive End Expiratory Pressure (PEEP)
          Application of PEEP
              Static Compliance (CS)
                   Too Much PEEP  Overdistention  CS
Optimized Lung Volume “Safe Window”
 Overdistension
    Edema fluid accumulation
    Surfactant degradation                                     Zone of
    High oxygen exposure                                    Overdistention
    Mechanical disruption
                                                              Injury

 Derecruitment, Atelectasis                          “Safe”
    Repeated closure / re-                          Window
     expansion                  Volume        Zone of
    Stimulation inflammatory              Derecruitment
                                           and Atelectasis
     response
    Inhibition surfactant                 Injury
    Local hypoxemia
    Compensatory                        Pressure
     overexpansion
Application of PEEP
Improving Ventilation / Oxygenation

    Oxygenation Using FiO2 and PEEP
      Positive End Expiratory Pressure (PEEP)
          Application of PEEP
              Arterial PO2, FiO2, and PaO2/FiO2
                   The usual approach to the management of FiO2
                    and PEEP is to start with high FiO2 and
                    incrementally decrease it as PEEP improves
                    oxygenation
Improving Ventilation / Oxygenation

    Oxygenation Using FiO2 and PEEP
      Positive End Expiratory Pressure (PEEP)
          Application of PEEP
              Arterial to End-Tidal Carbon Dioxide Tension Gradient
                   Normal P(a-et)CO2 gradient is 4.5 ± 2.5 (Pilbeam)
                   Is lowest when gas exchange units are maximally
                    recruited without being overdistended
                   If P(a-et)CO2 gradient increases above minimal
                    acceptable values, it signifies that too much PEEP
                    has been added and is producing a drop in cardiac
                    output and in increase in VD/VT
Application of PEEP
Improving Ventilation / Oxygenation

    Oxygenation Using FiO2 and PEEP
      Positive End Expiratory Pressure (PEEP)
          Application of PEEP
              Arterial-to-Venous Oxygen Difference (C(a-v)O2)
               reflects O2 utilization by the tissues
                   Normal value is 5 vol%
                   Increases in C(a-v)O2 with increases in PEEP may
                    indicate hypvolemia, cardiac malfunction,
                    decreased venous return to the heart, and
                    decreased cardiac out from PEEP
Application of PEEP
Improving Ventilation / Oxygenation

    Oxygenation Using FiO2 and PEEP
      Positive End Expiratory Pressure (PEEP)
          Application of PEEP
              Mixed Venous O2 Tension or Saturation
                  Normal PvO2 = 35–40 mm Hg
                   (minimal acceptable is 28 mm Hg)
                  Normal SvO2 = 75%
                   (minimal acceptable is 50%)
                  PEEP usually improves PvO2 and SvO2
                  When PvO2 and/or SvO2 decrease, with a increase
                   C(a-v)O2 increase, this indicates a decrease in
                   cardiac output – TOO MUCH PEEP
Application of PEEP
Improving Ventilation / Oxygenation

    Oxygenation Using FiO2 and PEEP
      Positive End Expiratory Pressure (PEEP)
          Application of PEEP
              Cardiac Output
                  Cardiac output provide key information about the
                   body’s response to PEEP
                  PEEP improves V/Q  Oxygenation  CO
                  Too much PEEP  Overdistention  Venous
                   return  CO
Application of PEEP
Improving Ventilation / Oxygenation

    Oxygenation Using FiO2 and PEEP
      Positive End Expiratory Pressure (PEEP)
          Application of PEEP
              Pulmonary Vascular Pressure Monitoring
                  When using PEEP >15 cm H2O, it is important to
                   closely evaluate the patient’s hemodyamic status,
                   which may require the placement of a pulmonary
                    artery catheter
                   If pulmonary artery occluding pressure (PAOP),
                    also known as “wedge pressure,” rises markedly as
                    PEEP is increased, the lungs may be overinflated
                   On the other hand, when PEEP rises, PAOP may be
                    markedly decreased because of pulmonary blood
                    flow is reduced as a result of decreased venous
                    return to the right side of the heart
Application of PEEP
Improving Ventilation / Oxygenation

Data From a Patient with ARDS on MV 24 Hours after Admission

VT: 1100         f: 6     VE: 6.6         FiO2: 0.8

PEEP     BP      HR       PCWP      CO    CS     PIP   PaO2    PVO2
0    130/65      130      16        4.8   28     50    40      27
5    120/55      135      13        4.2   31     58    45      37
10 135/65        125      18        5.8   33     60    50      35
15 130/70        120      19        5.9   36     55    115     37
20 110/50        130      25        4.1   27     63    150     29


Can you find the optimal PEEP level?
Improving Ventilation / Oxygenation

    Oxygenation Using FiO2 and PEEP
      Positive End Expiratory Pressure (PEEP)
          Weaning From PEEP
              Patient should demonstrate an acceptable PaO2 on an
               FiO2 of <0.40
              Must be hemodynamically stable and nonseptic
              Lung conditions should have improved
                    CS, PaO2/FiO2 ratio
              Reduce PEEP in 5 cm H2O increments
              Evaluate SpO2 within 3 minutes to determine effect – if
               it falls <20% from previous PEEP level, the patient is
               ready to tolerate lower PEEP level. If SpO2 drops
               >20% place PEEP at previous level
Improving Ventilation / Oxygenation

    Oxygenation Using FiO2 and PEEP
      Positive End Expiratory Pressure (PEEP)
          Weaning From PEEP
              Wait between reductions in PEEP and reevaluate the
               initial criteria. If the patient is stable, reduce PEEP by
               another 5 cm H2O. This may take 1 hour or may
               require as long as 6 hours or more
              When the patient is at 5 cm H2O, an additional
               evaluation is necessary. If reducing the PEEP to zero
               result is a worsening of the patient, then it may be
               appropriate to leave the patient at 5 cm H2O until
               extubation

				
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