Positive End Expiratory Positive End Expiratory PressurePressure

Dr Muhammad Asim RanaDr Muhammad Asim Rana

DescriptionDescription

• The ventilator applies a positive pressure at The ventilator applies a positive pressure at end exhalation, thus supra-atmospheric end exhalation, thus supra-atmospheric pressure is maintained throughout the pressure is maintained throughout the breathing cycle.breathing cycle.

• When applied during spontaneous breathing When applied during spontaneous breathing the term CPAP is used the term CPAP is used

• When applied to a mechanically ventilated When applied to a mechanically ventilated patient the term PEEP is usedpatient the term PEEP is used

• When applied to end of exhalation the term When applied to end of exhalation the term END-EXPIRATORY POSITIVE AIRWAY END-EXPIRATORY POSITIVE AIRWAY PRRESSURE is usedPRRESSURE is used

• From a physiological point of view From a physiological point of view PEEP, CPAP & EPAP are identical PEEP, CPAP & EPAP are identical

• Different acronyms are delivered Different acronyms are delivered from the controls of the equipment from the controls of the equipment used to provide them.used to provide them.

Advantages Advantages

1.1. Recruits & stabilizes the collapsed alveoliRecruits & stabilizes the collapsed alveoli

2.2. Increases the FRC & prevents expiratory Increases the FRC & prevents expiratory collapsecollapse

3.3. Improves oxygenation in conditions Improves oxygenation in conditions associated with diffuse alveolar collapse & associated with diffuse alveolar collapse & hypoxemiahypoxemia

4.4. Used to decrease inspiratory work of Used to decrease inspiratory work of breathing with auto-PEEPbreathing with auto-PEEP

5.5. Decreases lung injury chances by maintaining Decreases lung injury chances by maintaining minimum lung volume (minimizes shear minimum lung volume (minimizes shear forces associated with repetitive collapse & forces associated with repetitive collapse & recruitment of injured alveoli)recruitment of injured alveoli)

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

• 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.

•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.

• 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.

Disadvantages Disadvantages

1.1. Can impair cardiac outputCan impair cardiac output

2.2. Increases risk of barotrauma esp,>15 cmIncreases risk of barotrauma esp,>15 cm

3.3. Increases intracranial pressureIncreases intracranial pressure

4.4. Decreases renal & portal blood flowDecreases renal & portal blood flow

5.5. Can complicate data collection in pts of Can complicate data collection in pts of RHFRHF

6.6. Increases extra vascular lung waterIncreases extra vascular lung water

7.7. increases dead space if excessive increases dead space if excessive

8.8. May increase inspiratory work of May increase inspiratory work of breathing if over distention occursbreathing if over distention occurs

Indications Indications

1.1. Hypoxia with FiO2 > 0.5 in pts with Hypoxia with FiO2 > 0.5 in pts with diffuse B/L infiltrates (ARDS, Pul diffuse B/L infiltrates (ARDS, Pul edema)edema)

2.2. Cardiac surgery to prevent post Cardiac surgery to prevent post operative mediastinal bleedingoperative mediastinal bleeding

3.3. Post operative atelactasisPost operative atelactasis

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.

•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.

•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.

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

• 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.

•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.

•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.

Setting PEEPSetting PEEP• TECHNIQUE 1TECHNIQUE 1

• Begin at about 5 cm of H2O & increase in Begin at about 5 cm of H2O & increase in steps of 2 cm H2O until optimal PEEP is steps of 2 cm H2O until optimal PEEP is achieved– i.e,achieved– i.e,

PaO2>60,FiO2<0.5, hemodynamic PaO2>60,FiO2<0.5, hemodynamic stabilitystability

Achieving max static compliance of Achieving max static compliance of lungslungs

Intrapulmonary shunt fraction < 15%Intrapulmonary shunt fraction < 15%

Maximum VO2 (O2 consumption per Maximum VO2 (O2 consumption per unit of time)unit of time)

TECHNIQUE 2TECHNIQUE 2

• Sedate/paralyze the patient. Patient Sedate/paralyze the patient. Patient should not be making any respiratory should not be making any respiratory excrsionsexcrsions

• Suction the respiratory secretionsSuction the respiratory secretions

• Ensure tight endotracheal/TT cuff sealEnsure tight endotracheal/TT cuff seal

• Increase FiO2 to 1.0Increase FiO2 to 1.0

• Deliver single breath via ventilator to Deliver single breath via ventilator to make patient achieve projected TLCmake patient achieve projected TLC

• Disconnect patient from ventilatorDisconnect patient from ventilator

• Allow patient to exhale to FRCAllow patient to exhale to FRC

• Take a super-syringe (filled with 1.0 FiO2)Take a super-syringe (filled with 1.0 FiO2)

• Inflate lungs with 100 ml of O2 at a time pause Inflate lungs with 100 ml of O2 at a time pause for 2-3 seconds & measure Pel.for 2-3 seconds & measure Pel.

• Keep inflating with 100 ml of O2 at each step Keep inflating with 100 ml of O2 at each step until a volume of 25 ml/kg is injected or airway until a volume of 25 ml/kg is injected or airway pressure of 35 cm of H2O is reached or SaO2 pressure of 35 cm of H2O is reached or SaO2 starts falling < 87%starts falling < 87%

• An inflation curve plotting press & vol is madeAn inflation curve plotting press & vol is made

• Best PEEP is slightly above lower inflection Best PEEP is slightly above lower inflection point.point.

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