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Principles of Mechanical ventilation

In the name of GOD. Principles of Mechanical ventilation. T erminology. f: rate of breathing Vt: Tidal volume VA: Alveolar ventilation VD: dead space = 2.2 ml/ kg FiO2: fraction of inspired O2 PEEP: Positive End Expiratory Pressure P ASB : Pressure Above Spontaneous Breathing

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Principles of Mechanical ventilation

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  1. In the name of GOD Principles of Mechanical ventilation

  2. Terminology • f: rate of breathing • Vt: Tidal volume • VA: Alveolar ventilation • VD: dead space = 2.2 ml/ kg • FiO2: fraction of inspired O2 • PEEP: Positive End Expiratory Pressure • PASB : Pressure Above Spontaneous Breathing • ASB : Assisted Spontaneous Breathing

  3. Basic pulmonary physiology • Air that moves in and out of a patient's lungs per minute that is 7-10 L/min  Minute volume (MV) • MV =Vt x f • Alveolar ventilation (VA)  in contact with the alveolar-capillary gas exchange interface • VA = (Vt - VD ) x f • Volume-pressure relation: P = V/C

  4. Plateau pressure (static pressure) • … is the pressure at the end of inspiration with a short breath hold • It should not be exceed 30 cmH2O • P plateau ~ 1/compliance (P = V/C) • volume = P plateau

  5. Pressure-time diagram forvolume controlled constantflow ventilation

  6. Peak Airway Pressure (dynamic pressure) • …. is pressure during inspiration • So related to both airway resistance and compliance • compliance and/or resistance  P peak • P peak – P plateau < 4 cm H2O (normal gradient) • P peak is a vital sign for mechanically ventilated patients. • P peak < 35 cmH2O

  7. Changes in inspiratory airway resistance

  8. Paw-peak increased but Plateau pressure unchanged: 1. Tracheal tube obstruction and kinking 2. Airway obstruction from secretions 3. Acute bronchospasm Rx: Suctioning and Bronchodilators

  9. Changes in compliance In this situation P-P gradient is fixid. increasing compliance → plateau and peak pressures fall decreasing compliance → plateau and peak pressures rise

  10. Paw-peak and Plateau pressure are both increased: 1. Pneumothorax 2. Lobar atelectasis 3. Acute pulmonary edema 4. Worsening pneumonia 5. ARDS 6. COPD with tachypnea and Auto-PEEP 7. Increased abdominal pressure 8. Asynchronous breathing

  11. Decreased Paw-peak: 1.Inadequate gas supply, inadvertent change in setting, system air leak, Tubing disconnection, cuff leak, unintended extubation and failure of the ventilator Rx: Manual inflation, listen for leak 2. Hyperventilation: Enough negative intrathoracic pressure to pull air into lungs may drop Paw-Peak

  12. Measurements

  13. Measurements

  14. P –T curve At the start of inflation, the airway pressure immediately rises because of the resistance to gas flow (A), and at the end of inspiratory gas flow the airway pressure immediately falls by the same pressure (A) to an inflexion point.

  15. P/T F/T V/T curves in VCV

  16. Increase in Ppeak–Pplat gradient • Increased airway resistance caused by heat and moisture exchanger (HME) • Patient biting endotracheal tube • Kinked or twisted endotracheal tube • Obstruction of endotracheal tube by secretions, mucus, blood • Bronchospasm • Obstruction of lower airways

  17. Schematic of two superimposed pressure-time curves showing a small increase in peak inspiratory pressures (Ppeak) with a greater increase in plateau pressures (Pplat). This is characteristic of decreased lung compliance

  18. Unchanged or decreasedPpeak–Pplat gradient • Pneumonia • Atelectasis • Mucus plugging of one lung • Unilateral intubation • Pneumothorax • Pulmonary edema (noncardiogenic and cardiogenic) • Abdominal distention/pressure

  19. Spontaneous Breathing

  20. Positive End Expiratory Pressure • … is the pressure at the end of expiration • Serial Elevated PEEP  P plateau and FRC

  21. 1- Extrinsic PEEP (applied PEEP by MV) • 3 - 20 cm H2O and be started on 5 cm H2O • It improves the oxygenation not CO2 removal • It may be increased 3-5 cmH2O Q 10-15 min • It has some side effects: biotraumas and hemodynamic compromise • What is the optimal PEEP? 1- increasing PEEP until a complication occurs 2- assessing P plateau

  22. Optimal PEEP • P-V curve monitoring • Cardiac Output monitoring and Venous Oxygen Saturation If SmvO2 decreases after PEEP application  Drop C.O. In this situation  PEEP and/or tidal volume

  23. P-V curve Adequate PEEP Inadequate PEEP

  24. PEEP

  25. PEEP Disadvantages: • 1. Decrease BP & CPP 2. Increase PCO2 3. because alveolar injury is often heterogeneous, appropriate PEEP in one region may be suboptimal in another and excessive in another

  26. 2- Intrinsic PEEP • …is incomplete alveolar emptying during expiration due to air trapping • Ventilator Factors: High inflation volumes, rapid rate, low exhalation time • Disease factors: Asthma, COPD (collapsed airway) • And has some effects: • Decreased C.O. • Alveolar rupture • increased work of breathing

  27. Int. PEEP • Int. PEEP may be detected in two ways: (1) evaluating the flow-time trace (exp. Flow not returned to zero before next breath) (2) disconnecting the patient from the ventilator and listening for additional exhaled gas after an exhalation has occurred.

  28. Pressure cycled • Volume cycled

  29. Indications for mechanical ventilation

  30. No absolute contraindications • Loss of airway anatomy • Loss airway protection • Respiratory and cardiac Failure • Apnea / Respiratory Arrest • Inadequate ventilation (acute vs. chronic) • Inadequate oxygenation • Eliminate work of breathing • Reduce oxygen consumption • Neurologic dysfunction • Central hypoventilation/ frequent apnea • Comatose patient, GCS < 8 • Inability to protect airway

  31. Control Mechanisms

  32. Control mechanisms • Spontaneous breathing (PSV) • Pressure targeted ventilation • Volume targeted ventilation

  33. 1. Spontaneous breathing: • Setting: FiO2 and PEEP • Flow and f is dictated by patient

  34. 2. Pressure-Cycled (targeted): Alters gas flow and volume  fixed preset airway pressure (Paw) for the duration of a preset inspiratory time (Ti ) • Advantage: fixed pressure  limit or eliminate alveolar over-distention and barotraumas • One problem: changes in compliance or resistance with fixed Paw  variable received volume

  35. 3 . Volume/ Flow-Cycled (targeted) • Deliver a preset volume of gas ( VT)  Paw is variable • Advantage: delivery a constant VT changes in compliance or resistance with fixed volume  changes airway pressure

  36. There are no clinical outcome studies showing benefit of one breath-targeting strategy over the other. • Pressure-targeting provide a variable flow tends to synchronize better with patient effort.

  37. Modes

  38. 1. Controlled Mechanical Ventilation /Assist Control (CMV/AC) 2. Intermittent Mechanical Ventilation (IMV)/Synchronized IMV (SIMV). 3. Continuous Positive Airway Pressure (CPAP) /pressure support ventilation (PSV)

  39. Controlled Mechanical Ventilation (CMV) Assist Control (AC)

  40. Control Mode Ventilation • Continuous mandatory ventilation • Continuous mechanical ventilation • Controlled mandatory ventilation • Intermittent Positive Pressure Ventilation (Dräger) • …is a full support mode (machine breaths) • All breaths are supported regardless of initiation of breathing and can be set to VCV and PCV • Used for: apneic patients, respiratory muscle weakness and LV dysfunction

  41. (CMV/AC ) • Sensitivity pressure  0.5 to 2 cm H2O (1-3 cm H2O Tintinalli) • The higher sensitivity  the greater work of breathing • After spontaneous breath  the ventilator`s timer resets from this time

  42. (CMV /AC) • CMV  preferred and most commonly used initial mode for acute phase of respiratory failure in ED • but CMV : 1. Poor toleration in awake patients 2. Worsening of volume retention in COPD / asthma

  43. A/C mode

  44. 2. Intermittent Mechanical Ventilation (IMV) Synchronized IMV (SIMV)

  45. SIMV (Dräger, Hamilton) • SIMV (VC) + PS (Maquet) • VCV-SIMV (Puritan-Bennett, Respironics) • Volume SIMV (Viasys) • Intermittent demand ventilation • IMV  combination of spontaneous vent. and AC • IMV  is a partial support mode • This ventilator mode provides breaths at a preset rate (machine breath) similar to the AC mode • Can be set to PCV and VCV

  46. (IMV / SIMV ) • But in spontaneous breath  only receive a spontaneous VT withno support from the ventilator and has a high work of breathing • The synchronized version of IMV  coordination spontaneous and machine breaths • SIMV: 1. To prevent excess VT delivered (stacking) decreased hyperinflation, barotrauma 2. During exhalation from a spontaneous breath exhalation compromised

  47. 3. Continuous Positive Airway Pressure (CPAP) pressure support ventilation (PSV)

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