Mechanical Ventilation-General

Indications for Intubation-Mechanical Ventilation

  • Airway Protection
    • Facilitation of Safe Ventilation
    • Obtundation/Coma with Inability to Protect Airway (see Obtundation-Coma, [[Obtundation-Coma]])
      • Intoxication
    • Peri-Operative Period with Associated Sedation or General Anesthesia
    • Procedural During Moderate Sedation
    • Protection Against Aspiration
    • Upper Airway Compromise
      • Example: Laryngospasm (see Laryngospasm, [[Laryngospasm]])
      • Example: Laryngeal Edema
      • Example: Laryngeal Stenosis
  • Excessive Work of Breathing
    • Severe Metabolic Acidosis (see Metabolic Acidosis-General, [[Metabolic Acidosis-General]]): with inability to maintain acid-base status
    • Shock of Any Etiology
    • Decreased Pulmonary or Chest Wall Compliance: ARDS, flail chest, etc
  • Pulmonary Toilet
    • Excessive Secretions
    • Massive Hemoptysis (see Hemoptysis, [[Hemoptysis]])
  • Severe Hypoxemia (see Hypoxemia, [[Hypoxemia]]): severe hypoxemia of any etiology, which is unresponsive to supplemental oxygen and/or NIPPV
  • Severe Hypercapnia
  • Failure of Noninvasive Positive-Pressure Ventilation (NIPPV) (see Noninvasive Positive-Pressure Ventilation, [[Noninvasive Positive-Pressure Ventilation]])

Technique

Types of Mechanical Ventilation

Volume-Cycled Ventilation

  • Mechanism of Delivery: delivers a pre-determined volume (VT) at a set inspiratory flow rate and respiratory rate (RR), allowing exhalation when volume is reached (resultingly, airway pressure is determined by lung mechanics or patient effort)
  • Representative Types of Volume-Cycled Ventilation
    • Assist-Control (AC)
    • Synchronous Intermittent Mandatory Ventilation (SIMV)
    • Pressure-Regulated Volume Control (PRVC)

Pressure-Cycled Ventilation

  • Mechanism of Delivery: delivers a pre-determined pressure (resultingly, VT is determined by lung mechanics or patient effort)
  • Representative Types of Pressure-Cycled Ventilation
    • Pressure-Control (PC)
    • Pressure Support (PS)

Ventilator Modes

  • Adaptive Servo Ventilation (ASV) (see Adaptive Servo Ventilation, [[Adaptive Servo Ventilation]])
  • Bilevel Positive Airway Pressure (BPAP) (see Bilevel Positive Airway Pressure, [[Bilevel Positive Airway Pressure]])
  • Continuous Positive Airway Pressure (CPAP) (see Continuous Positive Airway Pressure, [[Continuous Positive Airway Pressure]])
  • Neurally Adjusted Ventilatory Assist (NAVA) (see Neurally Adjusted Ventilatory Assist, [[Neurally Adjusted Ventilatory Assist]])
  • Pressure Control Ventilation (PCV) (see Pressure Control Ventilation, [[Pressure Control Ventilation]])
  • Pressure Control Ventilation (PCV) (see Pressure Support Ventilation, [[Pressure Support Ventilation]])
  • Proportional Assist Ventilation (PAV) (see Proportional Assist Ventilation, [[Proportional Assist Ventilation]])
  • Volume Assured Pressure Support Ventilation (VAPS) (see Volume Assured Pressure Support Ventilation, [[Volume Assured Pressure Support Ventilation]])

  • Humidification of the Ventilator Circuit

    • Rationale for Humidification
      • The Upper Airway Provides 75% of the Heat and Moisture Supplied to the Alveoli
      • When the Upper Airway is Bypassed with an Endotracheal Tube, the Humidifier Needs to Supply the Missing Heat/Moisture
    • Types of Humidification
      • Active Humidification Via a Heated Humidifier: actively increase the heat and water vapor content of inspired gas
      • Passive Humidification Via a Heat and Moisture Exchanger: operate passively by storing heat and moisture from a patient’s exhaled gas (and subsequently releasing it into the inhaled gas)
    • Clinical Efficacy
      • Review and Clinical Practice Guideline for the Use of Humidification with Mechanical Ventilation (Respir Care, 2012) [MEDLINE]
    • Recommendations (Respir Care, 2012) [MEDLINE]
      • Humidification is Recommended for Every Patient on Invasive Mechanical Ventilation (Grade 1A Recomedation)
      • NIPPV: active humidification is recommended (Grade 2B Recommendation), as it may improve adherence and comfort
        • Passive Humidification is Not Recommended for NIPPV (Grade 2C Recommendation)
      • Invasive Mechanical Ventilation
        • When Providing Humidification to Patients with Low Tidal Volumes (i.e. Those on Lung-Protective Ventilation Strategies), Heat and Moisture Exchangers are Not Recommended Because They May Contribute to Additional Dead Space, Which Can Increase the Ventilation Requirement and pCO2
      • Heat and Moisture Exchanger Should Not Be Used for the Prevention of Ventilator-Associated Pneumonia (VAP) (Grade 2B Recommendation)

    Pressures on the Ventilator

    Peak Inspiratory Pressure (PIP)

    • Definition: maximal airway pressure (as measured at ventilator) achieved during gas delivery
    • During Volume-Cycled Ventilation (where tidal volume is manually set): PIP is dependent on static compliance, airway resistance, tidal volume, and inspiratory flow rate/pattern
    • During Pressure-Cycled Ventilation (where Delta P is manually set): “PIP” = Delta P

    Plateau Pressure

    • Definition: end-inspiratory pressure at point of no airflow

    Mean Alveolar Pressure

    • Most important determinant of oxygenation
    • Correlates with risk of ventilator-associated barotrauma
    • Not able to measure clinically, but best estimated by Plateau Pressure

    Mean Airway Pressure

    • Definition: airway pressured averaged over the entire ventilatory cycle
    • Usually underestimates the Mean Alveolar Pressure, but correlates with oxygenation

    End-Expiratory Pressure

    • Definition: airway pressure at end of expiration

    Minute Ventilation (VE)

    Physiologic

    • VE = VA + VD
      • Minute Ventilation (VE): expressed in L/min
      • Alveolar Ventilation (VA): expressed in L/min
      • Dead Space Ventilation (VD): expressed in L/min
        • Not on the Ventilator: VD is equivalent to approximately 1/3 of resting tidal volume (or approximately 2.2 ml/kg)
        • On the Ventilator with Lung Pathology: VD/VT ratio is variable

    Clinical

    • VE = RR x VT
      • Minute Ventilation (VE): expressed in L/min
      • Respiratory Rate (RR): expressed in breaths/min
      • Tidal Volume (VT): expressed in L/breath

    General Ventilator Settings

    Initial Settings for Volume-Cycled Ventilation Modes (AC, VC/PRVC, SIMV)

    • Respiratory Rate (RR)
    • Tidal Volume (VT): since tidal volume is manually set, PIP that occurs will depend on lung/chest wall compliance
    • Positive End-Expiratory Pressure (PEEP): typically set to +5
    • Inspired Oxygen (FIO2): usually initially set to 100% FIO2
    • Peak Inspiratory Flow Rate (PIF): usually set between 60-100 L/min

    Initial Settings for Pressure-Cycled Ventilation Modes (PC)

    • Respiratory Rate (RR)
    • Driving Pressure (Delta P): since driving pressure is manually set, tidal volume that occurs will depend on lung/chest wall compliance
    • Positive End-Expiratory Pressure (PEEP): typically set to +5
    • Inspired Oxygen (FIO2): usually initially set to 100% FIO2
    • Peak Inspiratory Flow Rate (PIF)

    Goals of Ventilator Settings (For Volume-Cycled Ventilation)

    • Set RR and Tidal Volume: aim to achieve pH 7.4 and plateau pressure <35 cm H2O
      • Current recommendations for VT are 6-8 ml/kg of predicted body weight (PBW): older literature suggested 10-12 ml/kg of PBW
      • Note: VT <6 ml/kg may cause hypercapnia (due to an increase in relative dead space): permissive hypercapnia may be useful in patients with obstructive airways disease
      • Plateau pressure <35 cm H2O is believed to decrease risk of barotrauma to lungs (since plateau pressure is the best clinical estimate of mean alveolar pressure)
    • Set PEEP (if necessary): aim to decrease FIO2 to <50%
      • PEEP is generally used for diffuse lung processes (multi-lobar pneumonia, ARDS, pulmonary edema, etc), but it less useful in localized processes (due to PEEP transmission to more compliant lung regions)
      • The addition of PEEP will generally increase the PIP by an equivalent amount, except in the presence of auto-PEEP: as a matter of fact, the failure of the PIP to increase with application of extrinsic PEEP is evidence for presence of auto-PEEP
    • Set Peak Inspiratory Flow Rate: consistent with patient comfort and desired inspiratory:expiratory (I:E) ratio
      • Typical I:E ratio during non-ventilated breathing is 1:2
      • Patients with obstructive airways disease (asthma, COPD, etc) may require ventilator I:E ratio of up to 1:5
      • Most COPD patients prefer flow rates of aprroximately 60 L/min [MEDLINE]
        • Use of high flow rates has been shown to increase respiratory rates [MEDLINE]

    Strategies to Improve Oxygenation in the Ventilated Patient


    Permissive Hypercapnia

    • History and Rationale: historically, was first used in status asthmaticus
      • Purposeful underventilation (using lower respiratory rate) which allows pCO2 to rise, with associated beneficial effect of prolonging expiratory time (and preventing the development of auto-PEEP) (see PEEP+Auto-PEEP, [[PEEP+Auto-PEEP]])
        • Ultimate goal of technique: maintaining plateau pressure <35 cm H2O to minimize risk of barotrauma
    • Indications
      • Asthma Exacerbation/Status Asthmaticus Requiring Mechanical Ventilation
      • COPD Exacerbation Requiring Mechanical Ventilation
    • Technique: underventilation allows to pCO2 to increase to as much as 70-100 mm Hg
      • During permissive hypercapnia, bicarbonate (or tris-hydroxymethyl aminomethane, THAM) may be used to maintain pH >7.2
        • However, bicarbonate may not be effective in increasing the pH

    Adverse Effects/Complications of Mechanical Ventilation

    Pulmonary Adverse Effects/Complications


    References

    • American Association for Respiratory Care Consensus Group. Essentials of Mechanical Ventilators. Respir Care 1992; 37:1000-1008
    • Classification for mechanical ventilators. Respir Care 1992; 37:1009-1025
    • Increased initial flow rate reduces inspiratory work of breathing during pressure support ventilation in patients with exacerbation of chronic obstructive pulmonary disease. Intensive Care Med. 1996 Nov;22(11):1147-54 [MEDLINE]
    • The treatment of acidosis in acute lung injury with tris-hydroxymethyl aminomethane (THAM). Am J Respir Crit Care Med. 2000 Apr;161(4 Pt 1):1149-53 [MEDLINE]
    • High-frequency oscillatory ventilation for acute respiratory distress syndrome in adults: A randomized, controlled trial. Am J Respir Crit Care Med 2002;166:801-808
    • Effect of inspiratory time and flow settings during assist-control ventilation. Curr Opin Crit Care. 2003 Feb;9(1):39-44 [MEDLINE]
    • High-frequency oscillatory ventilation in adults: The Toronto experience. Chest 2004;126:518 [MEDLINE]
    • Humidification during invasive and noninvasive mechanical ventilation: 2012. Respir Care. 2012 May;57(5):782-8. doi: 10.4187/respcare.01766 [MEDLINE]