Mechanical Ventilation

Mechanical Ventilation is a topic covered in the Washington Manual of Medical Therapeutics.

To view the entire topic, please or .

The Washington Manual of Medical Therapeutics helps you diagnose and treat hundreds of medical conditions. Consult clinical recommendations from a resource that has been trusted on the wards for 50+ years. Explore these free sample topics:

-- The first section of this topic is shown below --

General Principles

Basic modes of ventilation: One can determine how the ventilator initiates a breath (triggering), how the breath is delivered, how patient-initiated breaths are supported, and when to terminate the breath to allow expiration (cycling).

  • Initiation of a breath: Triggering of a ventilator occurs after a period of time has elapsed (time triggered) or when the patient has generated sufficient negative airway pressure or inspiratory flow exceeding a predetermined threshold (patient triggered).
  • Modes of ventilation
    • Assist-control (AC) ventilation: Ventilator delivers a fully supported breath 
whether time or patient triggered. Primary mode of ventilation used in respiratory failure.
    • Synchronized intermittent mandatory ventilation (SIMV): Ventilator delivers a fully supported breath when time triggered. However, when the breath is patient triggered, the ventilator delivers a pressure-supported breath (at a level set by the clinician). The size of the patient-triggered breath depends on lung compliance and patient’s effort. This mode is commonly used in surgical patients.
    • Pressure support ventilation (PSV): Spontaneous mode of ventilation without a set respiratory rate. Delivers a clinician-determined inspiratory pressure during patient-triggered breathing. No respiratory rate is set, so there is no guaranteed minute ventilation.
  • Type of breath delivered
    • Volume control (VC): Ventilator delivers a clinician-determined tidal volume (VT) for each breath regardless of whether the breath was time or patient triggered. When predetermined VT is delivered, airflow is terminated and exhalation occurs.
    • Pressure control (PC): Delivers a practitioner-determined inspiratory pressure for each breath. When inspiratory time has elapsed, inspiratory pressure is terminated and exhalation occurs. The tidal volume varies based on lung compliance. PC ventilation does not deliver a guaranteed VT or minute ventilation and may lead to hypoventilation. However, PC may improve patient synchrony and comfort while on the ventilator.
  • Basic ventilator terminology and management: Flow-time and pressure-time tracings are demonstrated in Figure 8-1.
    • Minute ventilation: Defined as the product of VT and respiratory rate (VT × RR). Normally between 5–10 L/min in resting adults, but may be much higher in high metabolic states, e.g., septic shock.
      Figure 8-1 Flow–time and pressure–time tracings.
      Descriptive text is not available for this image

      A, Pressure–time curve for one breath. B, Flow–time curve for volume control ventilation. Pressure varies throughout inspiratory time, depending on lung compliance. C, Pressure–time curve for pressure control ventilation. Flow varies throughout inspiratory time, depending on lung compliance. D, Pressure–time curve demonstrating auto–positive end-expiratory pressure (auto-PEEP).

    • Peak airway pressure: Composed of pressures necessary to overcome inspiratory airflow resistance, chest wall recoil resistance, and alveolar opening resistance. Does not reflect alveolar pressure.
    • Mean airway pressure: Mean pressures applied during the inspiratory cycle. Approximates alveolar pressure until overdistention occurs.
    • Plateau pressure: Reflects alveolar pressure. Checked by performing an end-inspiratory hold maneuver to allow pressures through the tracheobronchial tree to equilibrate.
  • Initial ventilator settings: One must decide on a ventilator mode (AC vs. SIMV), control (VC vs. PC), respiratory rate, FIO2, and PEEP. AC/VC is the most commonly used mode.
    • For VC, the following must be entered:
      • VT: Generally, begin at 6–8 mL/kg ideal body weight (IBW) to prevent barotrauma. There is growing evidence that low tidal volume ventilation may be beneficial in patients whether or not they have acute ARDS and should be routinely used whenever possible.1 IBW can be calculated as follows: Male IBW = 50 kg + 2.3 kg/in. × (Height in inches − 60) (imperial), 50 kg + 1.1 kg/cm × (Height in cm − 152.4) (metric); Female IBW = 45.5 kg + 2.3 kg/in. × (Height in inches − 60) (imperial), 45.5 kg + 1.1 kg/cm × (Height in cm − 152.4) (metric).
      • Inspiratory flow rate: May be constant (square wave) or ramp (decelerating). Recommend 60 L/min or greater. Higher flow rates increase expiration time, which may be important in obstructive lung disease to prevent auto-PEEP (ventilator delivers a breath before the patient has been able to fully expire).
    • FIO2: It is reasonable to start at 100%, but FIO2 should be weaned down quickly to maintain SaO2 >87% or PaO2 >55 mm Hg. There is growing evidence that tolerating hyperoxia after intubation may actually worsen patient survival.2 FIO2 can generally be quickly titrated down based on pulse oximetry alone.
    • PEEP: it is generally reasonable to start at 5–10; however, higher values are frequently used in the treatment of ARDS.
  • Advanced modes of ventilation: Advanced modes should generally only be used after discussion with higher level practitioners.
    • Pressure-regulated VC ventilation: Ventilator determines, after each breath, if inspiratory pressure was sufficient to achieve targeted VT; if insufficient or excessive, then ventilator will adjust inspiratory pressure to achieve desired VT.
    • Inverse-ratio ventilation (IRV): A pressure-controlled method of ventilation most commonly used in ARDS. Inspiratory time exceeds expiratory time to improve oxygenation at the expense of ventilation; patients are permitted to become hypercapnic to pH 7.20. If obstructive lung disease is present, can cause auto-PEEP and excessive hypercapnia.
    • Airway pressure release ventilation (APRV): An extreme version of IRV; inspiratory pressure (Phigh) applied for a prolonged period of time (Thigh) with a short expiratory time (Tlow, or release time)—usually <1 s—to allow for ventilation. Like IRV, patients are permitted to be hypercapnic to pH 7.20.
  • Mechanical ventilation principles for patients with ARDS: Owing to severe hypoxia associated with ARDS, oxygenation and prevention of barotrauma may have to be prioritized over ventilation, resulting in hypercapnia.
    • Hypercapnia resulting in a pH of 7.20–7.35 often is tolerated to sufficiently oxygenate the patient (“permissive hypercapnia”).
    • The plateau pressure should be checked and the tidal volume should be decreased down to 4 mL/kg of IBW as pH allows to achieve a plateau pressure ≤30 cm H2O.
    • There is growing evidence that driving pressure (ratio of VT/respiratory system compliance) is a key variable to optimize when ventilating patients with ARDS. The driving pressure can be calculated as the plateau pressure minus the PEEP and should be kept below 14 cm H2O when possible.3,4

-- To view the remaining sections of this topic, please or --

General Principles

Basic modes of ventilation: One can determine how the ventilator initiates a breath (triggering), how the breath is delivered, how patient-initiated breaths are supported, and when to terminate the breath to allow expiration (cycling).

  • Initiation of a breath: Triggering of a ventilator occurs after a period of time has elapsed (time triggered) or when the patient has generated sufficient negative airway pressure or inspiratory flow exceeding a predetermined threshold (patient triggered).
  • Modes of ventilation
    • Assist-control (AC) ventilation: Ventilator delivers a fully supported breath 
whether time or patient triggered. Primary mode of ventilation used in respiratory failure.
    • Synchronized intermittent mandatory ventilation (SIMV): Ventilator delivers a fully supported breath when time triggered. However, when the breath is patient triggered, the ventilator delivers a pressure-supported breath (at a level set by the clinician). The size of the patient-triggered breath depends on lung compliance and patient’s effort. This mode is commonly used in surgical patients.
    • Pressure support ventilation (PSV): Spontaneous mode of ventilation without a set respiratory rate. Delivers a clinician-determined inspiratory pressure during patient-triggered breathing. No respiratory rate is set, so there is no guaranteed minute ventilation.
  • Type of breath delivered
    • Volume control (VC): Ventilator delivers a clinician-determined tidal volume (VT) for each breath regardless of whether the breath was time or patient triggered. When predetermined VT is delivered, airflow is terminated and exhalation occurs.
    • Pressure control (PC): Delivers a practitioner-determined inspiratory pressure for each breath. When inspiratory time has elapsed, inspiratory pressure is terminated and exhalation occurs. The tidal volume varies based on lung compliance. PC ventilation does not deliver a guaranteed VT or minute ventilation and may lead to hypoventilation. However, PC may improve patient synchrony and comfort while on the ventilator.
  • Basic ventilator terminology and management: Flow-time and pressure-time tracings are demonstrated in Figure 8-1.
    • Minute ventilation: Defined as the product of VT and respiratory rate (VT × RR). Normally between 5–10 L/min in resting adults, but may be much higher in high metabolic states, e.g., septic shock.
      Figure 8-1 Flow–time and pressure–time tracings.
      Descriptive text is not available for this image

      A, Pressure–time curve for one breath. B, Flow–time curve for volume control ventilation. Pressure varies throughout inspiratory time, depending on lung compliance. C, Pressure–time curve for pressure control ventilation. Flow varies throughout inspiratory time, depending on lung compliance. D, Pressure–time curve demonstrating auto–positive end-expiratory pressure (auto-PEEP).

    • Peak airway pressure: Composed of pressures necessary to overcome inspiratory airflow resistance, chest wall recoil resistance, and alveolar opening resistance. Does not reflect alveolar pressure.
    • Mean airway pressure: Mean pressures applied during the inspiratory cycle. Approximates alveolar pressure until overdistention occurs.
    • Plateau pressure: Reflects alveolar pressure. Checked by performing an end-inspiratory hold maneuver to allow pressures through the tracheobronchial tree to equilibrate.
  • Initial ventilator settings: One must decide on a ventilator mode (AC vs. SIMV), control (VC vs. PC), respiratory rate, FIO2, and PEEP. AC/VC is the most commonly used mode.
    • For VC, the following must be entered:
      • VT: Generally, begin at 6–8 mL/kg ideal body weight (IBW) to prevent barotrauma. There is growing evidence that low tidal volume ventilation may be beneficial in patients whether or not they have acute ARDS and should be routinely used whenever possible.1 IBW can be calculated as follows: Male IBW = 50 kg + 2.3 kg/in. × (Height in inches − 60) (imperial), 50 kg + 1.1 kg/cm × (Height in cm − 152.4) (metric); Female IBW = 45.5 kg + 2.3 kg/in. × (Height in inches − 60) (imperial), 45.5 kg + 1.1 kg/cm × (Height in cm − 152.4) (metric).
      • Inspiratory flow rate: May be constant (square wave) or ramp (decelerating). Recommend 60 L/min or greater. Higher flow rates increase expiration time, which may be important in obstructive lung disease to prevent auto-PEEP (ventilator delivers a breath before the patient has been able to fully expire).
    • FIO2: It is reasonable to start at 100%, but FIO2 should be weaned down quickly to maintain SaO2 >87% or PaO2 >55 mm Hg. There is growing evidence that tolerating hyperoxia after intubation may actually worsen patient survival.2 FIO2 can generally be quickly titrated down based on pulse oximetry alone.
    • PEEP: it is generally reasonable to start at 5–10; however, higher values are frequently used in the treatment of ARDS.
  • Advanced modes of ventilation: Advanced modes should generally only be used after discussion with higher level practitioners.
    • Pressure-regulated VC ventilation: Ventilator determines, after each breath, if inspiratory pressure was sufficient to achieve targeted VT; if insufficient or excessive, then ventilator will adjust inspiratory pressure to achieve desired VT.
    • Inverse-ratio ventilation (IRV): A pressure-controlled method of ventilation most commonly used in ARDS. Inspiratory time exceeds expiratory time to improve oxygenation at the expense of ventilation; patients are permitted to become hypercapnic to pH 7.20. If obstructive lung disease is present, can cause auto-PEEP and excessive hypercapnia.
    • Airway pressure release ventilation (APRV): An extreme version of IRV; inspiratory pressure (Phigh) applied for a prolonged period of time (Thigh) with a short expiratory time (Tlow, or release time)—usually <1 s—to allow for ventilation. Like IRV, patients are permitted to be hypercapnic to pH 7.20.
  • Mechanical ventilation principles for patients with ARDS: Owing to severe hypoxia associated with ARDS, oxygenation and prevention of barotrauma may have to be prioritized over ventilation, resulting in hypercapnia.
    • Hypercapnia resulting in a pH of 7.20–7.35 often is tolerated to sufficiently oxygenate the patient (“permissive hypercapnia”).
    • The plateau pressure should be checked and the tidal volume should be decreased down to 4 mL/kg of IBW as pH allows to achieve a plateau pressure ≤30 cm H2O.
    • There is growing evidence that driving pressure (ratio of VT/respiratory system compliance) is a key variable to optimize when ventilating patients with ARDS. The driving pressure can be calculated as the plateau pressure minus the PEEP and should be kept below 14 cm H2O when possible.3,4

There's more to see -- the rest of this entry is available only to subscribers.