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MECHANICAL VENTILATION

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  1. MECHANICAL VENTILATION • Phunsup Wongsurakiat, MD, FCCP • Division of Respiratory Disease and TB • Department of Medicine, Siriraj Hospital

  2. Mechanical Ventilation • Invasive (intubation) • Non-invasive (other interface such as face mask): • - negative pressure • - positive pressure

  3. DISTENDING PRESSURES Plat P - Base P (PEEP) = pressure to distend resp system (lung + chest wall) Plat P = peak alveolar pressure Transpulmonary pressure = Plat P – Pleural pressure

  4. Influence of chest wall stiffness 35 cm H2O 35 cm H2O 5 cm H2O 10 cm H2O 30 cm H2O 25 cm H2O

  5. FLOW Resistance (R) Peak P - PlatP = pressure for flow R = Flow/(PeakP - Plat P)

  6. Physiology of Respiration • O2 consumption (VO2) ≈ 250 mL/ min • CO2 production (VCO2) ≈ 200 mL/ min • Cardiac output (CO) ≈ 5 L/min • Minute ventilation = RR X VT ≈ 5 - 8 L/min • VT ≈ 5 - 8 mL / kg • RR ≈ 12 - 20 bpm • PaCO2 = 35-45 mmHg • pH = 7.35-7.45

  7. Physiology of Respiration • PAO2 = PIO2 –PaCO2 / R [PIO2 = FIO2 x (barometric pressure – 47)] • PaCO2 = k x VCO2 / VA

  8. Mechanical Ventilation • Variables • Mode • Objectives • Clinical settings • Complications

  9. Mechanical VentilationVariables • Tidal volume • Rate • Total time (inversely related to rate): - inspiratory time (adjustable) - expiratory time (total time - inspiratory time) • Flow (tidal volume / inspiratory time) • Minute ventilation

  10. Mechanical VentilationVariables • Trigger sensitivity • FiO2 • Pressure: - peak pressure - plateau pressure • Compliance

  11. Mechanical VentilationModes Limit • variables rise no higher than some preset value and increase to preset value before inspiration ends = limit variable Cycle • Variable that terminate inspiration = cycle variable

  12. Breath characteristicsGas Delivery

  13. Mechanical Ventilation Bird • Limit: pressure, flow • Cycle: pressure Bennett 7200 (volume assist-control) • Limit: volume, flow • Cycle: volume Pressure support • Limit: pressure • Cycle: flow

  14. Mechanical VentilationModes • Volume-targeted: Pre-set tidal volume • Pressure-targeted: Pre-set inspiratory pressure • Mandatory breaths: Breaths that the ventilator delivers to the patient at a set frequency, volume/pressure, flow/time • Spontaneous breaths: Patient initiated breath

  15. Mechanical VentilationModes Volume-targeted • Controlled mechanical ventilation (CMV) • Assist/control (A/C) mechanical ventilation • Synchronized intermittent mandatory ventilation (SIMV)

  16. Modes of Ventilation (CMV) • Background: • Full preset tidal volume at a fixed preset rate • Rate and minute ventilation cannot  by patient effort • Trigger sensitivity is locked out, need sedated or paralyzed

  17. Modes of Ventilation (CMV) Advantages: • Near complete resting of ventilatory muscles • Respiratory muscle rest, secured minute ventilation Disadvantages: • “Stack" breaths (air trapping) and develop barotrauma • “AutoPEEP" with barotrauma or hypotension • Need sedated or paralyzed • Respiratory muscle atrophy Uses: • apnea, little breathing effort, unstable

  18. Modes of Ventilation (Assist/Control) • Background: • Full preset tidal volume at a minimum preset rate • Additional full tidal volumes given if the patient initiates extra breaths

  19. Modes of Ventilation (Assist/Control) • Advantages: • Near complete resting of ventilatory muscles • Comfortable, respiratory muscle rest, secured minute ventilation • Effectively used in awake, sedated, or paralyzed patients • Disadvantages: • Hyperventilate and become alkalotic • “Stack" breaths (air trapping) and develop barotrauma • “AutoPEEP" with barotrauma or hypotension • Uses: • apnea, little breathing effort, unstable

  20. Modes of Ventilation (SIMV + Pressure support) • Background: • Preset tidal volume at a fixed preset rate • Ventilator waits a predetermined trigger period • Patient can take additional breaths but tidal volume of these extra breaths is dependent on the patient's inspiratory effort

  21. Modes of Ventilation (SIMV + Pressure support) • Advantages: • Improved venous return: intermittent negative pressure (spontaneous) breaths • More comfortable: more control over their ventilatory pattern and minute ventilation Disadvantages: • Can result in chronic respiratory fatigue if set rate is too low; • Uses: • Weaning, bronchopleural fistula

  22. Mechanical VentilationModes Pressure-targeted • Pressure support ventilation (PSV) • Pressure-control ventilation (PCV) • Pressure-targeted assist-control (A/C-PC) • Pressure-targeted SIMV (SIMV-PC)

  23. Pressure support Ventilation • Background: • Patient triggers, a preset pressure support is delivered • Terminate inspiration by flow rate • Tidal volume and minute ventilation are dependent on the preset pressure • and patient’s lung-thorax compliance

  24. Pressure support Ventilation • Advantages: • Avoids patient-ventilator asynchrony • More comfortable: full control over ventilatory pattern and minute ventilation • Avoids breath stacking and autoPEEP (especially in patients with COPD) • Disadvantages: • Required patient’s triggering, cannot be used in heavily sedated, paralyzed, or comatose patients • Respiratory muscle fatigue if pressure support is set too low

  25. Pressure Control Ventilation • Background: • Ventilator control predetermined pressure in a • fixed predetermined time and rate

  26. Pressure Control Ventilation • Advantages: • Pressure and time are controlled • High flow rate • Disadvantages: • Tidal volume variable • Produced autoPEEP if inspiratory time too long • Uncomfortable mode for most patients

  27. Pressure-targeted assist-control (A/C-PC) • All breaths machine-delivered at a preset inflation pressure • Patients can  rate by triggering additional machine breaths if desired • Same as assist/control volume targeted mode

  28. Pressure-targeted SIMV(SIMV-PC) • Fixed rate of machine-delivered breaths at a preset inflation pressure • Patients can breathe spontaneously between machine-delivered breaths if desired • Same as SIMV volume targeted mode

  29. Mechanical Ventilation • Objectives: • Physiology • Comfortable • Least complications

  30. Mechanical Ventilation • Usual settings • Minute Ventilation = RR X VT ( 5 - 8 L/min) • VT = 5 - 8 mL / kg • RR = 12 - 20 bpm • PaCO2 = 35-45 mmHg • pH = 7.35-7.45

  31. Mechanical Ventilation • Triggering: sensitivity of ventilator to patient’s respiratory effort. • Flow or pressure setting that allows ventilator to detect patient’s inspiratory effort • Allows ventilator synchronize with patient’s spontaneous respiratory efforts • Improving patient’s comfort during mechanical ventilation. • Setting: lowest but not self cycling • Less work in flow triggering

  32. Mechanical Ventilation • Flow rate: • adjust to patient’s comfort • Usually > 60 L/min • Pressure: • Plateau pressure < 30 cmH2O

  33. Positive End Expiratory Pressure(PEEP) •  Functional residual capacity • Move fluid from alveoli into interstitial space • Improve oxygenation