Positive End Expiratory Pressure

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Positive End Expiratory Pressure

• Dr Muhammad Asim Rana

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Description

• The ventilator applies a positive pressure at end
  exhalation, thus supra-atmospheric pressure is
  maintained throughout the breathing cycle.
• When applied during spontaneous breathing the
  term CPAP is used
• When applied to a mechanically ventilated patient
  the term PEEP is used
• When applied to end of exhalation the term
  END-EXPIRATORY POSITIVE AIRWAY PRRESSURE is used

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• From a physiological point of view PEEP, CPAP
  EPAP are identical
• Different acronyms are delivered from the
  controls of the equipment used to provide them.

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Advantages

1. Recruits stabilizes the collapsed alveoli
2. Increases the FRC prevents expiratory collapse
3. Improves oxygenation in conditions associated
   with diffuse alveolar collapse hypoxemia
4. Used to decrease inspiratory work of breathing
   with auto-PEEP
5. Decreases lung injury chances by maintaining
   minimum lung volume (minimizes shear forces
   associated with repetitive collapse recruitment
   of injured alveoli)

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• Positive end-expiratory pressure (PEEP) has lung
  protective effects during mechanical ventilation
  in isolated lungs, and in intact and open-chest
  animals.

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• In intact healthy rats, edema and hemorrhage from
  ventilation with excessive lung volumes were
  substantially reduced when PEEP was used
• In a dog lung injury model lung injury was caused
  by ventilation with large Vt and low PEEP.
• This injury was reduced in animals ventilated
  with smaller Vts and higher PEEPs despite similar
  EILVs.

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• The effect of end-expiratory atelectasis on lung
  injury was evaluated in a rabbit
  surfactant-deficient model. Rabbits ventilated
  with negative end-expiratory pressure
  demonstrated greater alveolar capillary
  permeability, reduced lung compliance, and worse
  gas exchange than rabbits ventilated with PEEP.

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• These and other studies provide convincing
  evidence that PEEP has lung protective effects
  during mechanical ventilation.
• However, PEEP also can contribute to lung injury
  by raising EILV unless Vt is simultaneously
  reduced.
• Moreover, PEEP may cause circulatory depression
  from increased pulmonary vascular resistance and
  decreased venous return.

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Disadvantages

1. Can impair cardiac output
2. Increases risk of barotrauma esp,gt15 cm
3. Increases intracranial pressure
4. Decreases renal portal blood flow
5. Can complicate data collection in pts of RHF
6. Increases extra vascular lung water
7. increases dead space if excessive
8. May increase inspiratory work of breathing if
   over distention occurs

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Indications

1. Hypoxia with FiO2 gt 0.5 in pts with diffuse B/L
   infiltrates (ARDS, Pul edema)
2. Cardiac surgery to prevent post operative
   mediastinal bleeding
3. Post operative atelactasis

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Determining the optimal level of PEEP

• Determining the optimal level of PEEP in
  individual patients represents a difficult and
  tenuous balance between potential lung protective
  effects and deleterious effects on the lung and
  other systems.

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• Some investigators have used static or
  quasi-static pressure-volume curves of the
  respiratory system to explain the effects of
  ventilation at low EELV, to predict the effects
  of ventilation with higher PEEPs and EELVs, and
  to identify the best PEEP to apply during
  Conventional Ventilation to achieve lung
  protection.

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• The slope of the pressure-volume relationship
  represents compliance of the respiratory system.
  Compliance in the lower portion of the curve
  increases as airway pressure and volume rise,
  representing gradual recruitment of atelectatic
  portions of the lung.

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• This interpretation is supported by improved
  arterial oxygenation53 and CT evidence of
  increased lung aeration in ARDS patients.

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Best PEEP

• The midpoint of the portion of the
  pressure-volume curve with increasing slope is
  frequently labeled Pflex and may represent the
  inspiratory airway pressure and volume where many
  lung units are open.

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• The mid-portion of the pressure-volume curve
  appears to be virtually rectilinear. This region
  of approximately constant compliance has been
  interpreted to represent a range of airway
  pressures and lung volumes in which little or no
  further recruitment occurs.

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• Some workers have advocated setting PEEP to
  approximately Pflex plus 2 cm H2O to prevent the
  closure of unstable lung units during expiration
  and, thus, to prevent injurious shear forces from
  ventilation with insufficient EELV.
• This recommendation is supported by the results
  of studies suggesting that CV with PEEP that is
  less than Pflex may cause VALI.

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Setting PEEP

• TECHNIQUE 1
• Begin at about 5 cm of H2O increase in steps of
  2 cm H2O until optimal PEEP is achieved i.e,
• PaO2gt60,FiO2lt0.5, hemodynamic stability
• Achieving max static compliance of lungs
• Intrapulmonary shunt fraction lt 15
• Maximum VO2 (O2 consumption per unit of
  time)

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TECHNIQUE 2

• Sedate/paralyze the patient. Patient should not
  be making any respiratory excrsions
• Suction the respiratory secretions
• Ensure tight endotracheal/TT cuff seal
• Increase FiO2 to 1.0
• Deliver single breath via ventilator to make
  patient achieve projected TLC

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• Disconnect patient from ventilator
• Allow patient to exhale to FRC
• Take a super-syringe (filled with 1.0 FiO2)
• Inflate lungs with 100 ml of O2 at a time pause
  for 2-3 seconds measure Pel.
• Keep inflating with 100 ml of O2 at each step
  until a volume of 25 ml/kg is injected or airway
  pressure of 35 cm of H2O is reached or SaO2
  starts falling lt 87
• An inflation curve plotting press vol is made
• Best PEEP is slightly above lower inflection
  point.

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Thank You