• Clinical science
  • Clinician

Mechanical ventilation

Summary

Mechanical ventilation is used to assist or replace spontaneous breathing to reduce the work of breathing and/or reverse life-threatening respiratory derangement in critically ill patients or to maintain respiratory function in those undergoing general anesthesia. It involves the application of positive pressure, which can be invasive (i.e., in intubated patients) or noninvasive (e.g., CPAP or BiPAP). Indications include hypoxemic and hypercapnic respiratory failure, hemodynamic compromise, and the need for close ventilatory control (e.g., therapeutic hyperventilation). Settings such as ventilation modes (e.g., assist-control, pressure-support) and parameters (e.g., tidal volume, respiratory rate, FiO2, PEEP) should be adjusted to patient needs in order to minimize complications and restore homeostasis. A variety of ventilation strategies have been described for treating different types of respiratory failure. These should be implemented in a critical care setting with close monitoring in collaboration with specialists, nurses, and respiratory therapists. Complications of mechanical ventilation include ventilator-induced lung injury and ventilator-associated pneumonia, as well as extrapulmonary complications such as GI ulcers and venous thromboembolism. A systematic approach to common problems in mechanical ventilation (e.g., sudden deterioration, problems with oxygenation and ventilation, hemodynamic compromise, patient-ventilator dyssynchrony, dynamic hyperinflation) is recommended to prevent morbidity and mortality. Once patients show sufficient spontaneous breathing, they are weaned off ventilation support. See also airway management.

Definition

  • Mechanical ventilation: use of a ventilator to assist or completely replace spontaneous breathing

Positive-pressure ventilation

Positive-pressure ventilation (PPV) is the underlying mechanism of modern mechanical ventilators [1]

Noninvasive positive-pressure ventilation

Types of noninvasive positive-pressure ventilation (NIPPV) [3]

Continuous positive airway pressure (CPAP)

Bilevel positive airway pressure (BIPAP)

CPAP only delivers one airway pressure, equivalent to PEEP. BIPAP cycles between two airway pressures and offers inspiratory pressure support.

Indications for NIPPV [6][7]

Contraindications for NIPPV [9][10]

  • Impaired/absent spontaneous breathing and/or patient not cooperative with NIPPV
    • Cardiac or respiratory arrest
    • Severe encephalopathy (GCS < 10)
    • Uncontrolled agitation
  • Impaired airway protection
  • Impaired mask seal
  • High risk of adverse effects from positive pressure

Uncontrolled agitation is a contraindication for NIPPV.

Procedure [3][7][10]

The following steps are pertinent to patients newly requiring NIPPV. Stable patients continuing home therapy (e.g., CPAP for longstanding OSA) should be started on their preferred interface and baseline settings.

  • Preparation
    • Obtain baseline ABG if feasible.
    • Make sure the patient is comfortable, alert, and in sitting position.
    • Monitor pulse oximetry, blood pressure, ECG, ABG, exhaled tidal volume, and respiratory mechanics.
    • Explain the procedure to the patient.
    • Prepare for intubation in case respiratory and clinical status deteriorates.
  • Choice of interface [10]
    • Oronasal mask
    • Total-face masks
    • Nasal mask or nasal pillows
    • Helmet
  • NIPPV settings [7]
    • BIPAP mode [3]
      • Low-high approach: Start with low pressures and adjust upward as needed and tolerated.
        • Start EPAP at 3–5 cm H2O.
        • Start IPAP at 10 cm H2O.
      • High-low approach: Start with high pressures and adjust downward as needed and tolerated.
        • Start EPAP at 5–8 cm H2O.
        • Start IPAP at 20–25 cm H2O.
    • CPAP mode: Set PEEP to 5–12 cmH2O.
    • Titrate FiO2 (between 30% and 50%) to the desired oxygenation target (e.g., SpO2 88–92% for COPD).
  • Monitoring and adjustments

Do not delay invasive mechanical ventilation (e.g., intubation) if a patient's condition is worsening with NIPPV.

Complications [10]

Invasive mechanical ventilation

General principles [3][12]

Indications for invasive mechanical ventilation [13][14]

Contraindications

  • No absolute contraindications
  • Relatively contraindicated in patients with advanced directives against mechanical ventilation

Initiating mechanical ventilation in patients with severe obstructive lung disease, acidosis, and shock is associated with significant morbidity and mortality. These conditions require special care and preparation (see high-risk indications for mechanical ventilation).

Adjunctive care of ventilated patients

Sedation, analgesia, and muscle relaxants [16][17][18]

Sedation should be started immediately after intubation and should not be mistaken or confused for the paralysis caused by some induction agents. Paralysis without sedation should be avoided at all costs!

Bronchopulmonary hygiene [22]

Supportive care

  • Orogastric or nasogastric tube insertion
  • Pressure ulcer prophylaxis
  • VTE prophylaxis
  • Routine use of stress ulcer prophylaxis is controversial. [23][24][25][26]

Ventilator settings

Basic ventilation parameters [1][3][12][27][28][29]

  1. Tidal volume (Vt): the volume of air delivered to or taken by the patient per breath
    • Set by the clinician in volume-controlled modes (e.g., 8–12 mL/kg ideal body weight)
    • Measured by the ventilator in pressure-controlled or pressure-supported modes (e.g., PRVC and PSV)
  2. Respiratory rate (RR): breaths taken or delivered per minute
    • Set by the clinician in the absence of patient-initiated breaths (e.g., ∼ 10–15/min)
    • Set by the patient in spontaneous breathing modes
    • Commonly a combination of patient-triggered assisted breaths and a clinician-set backup RR
  3. Fraction of inspired oxygen (FiO2): the fraction of oxygen (by volume) in the inspired air
    • FiO2 of room air = 21%
    • FiO2 up to 100% can be delivered in perfectly sealed ventilator circuits.
  4. Positive end-expiratory pressure (PEEP):

Ventilator modes [1][3][12][27][28][29]

Mandatory modes

Mandatory modes are useful when respiratory musculature is weak/paralyzed and respiratory drive is impaired.

Spontaneous modes

Spontaneous modes are useful for when respiratory musculature is intact/weak and respiratory drive is intact.

  • Example: pressure-support ventilation (PSV)
  • Description:
    • Breath frequency and waveforms are mostly controlled by the patient.
    • The ventilator only provides inspiratory pressure to support breathing.
    • Almost all breaths are patient-triggered.
    • Breath volume and duration depend on patient effort and the degree of inspiratory pressure provided.

Comparison of commonly used ventilator modes

Comparison of ventilator modes [3][27][28][29]
Mode Programming Advantages Disadvantages

Volume-control ventilation (VCV)

  • Preset by clinician: Vt, RR, PEEP
  • Ventilator delivers fixed volume.
  • Variables to be monitored: inspiratory pressure
Pressure-control ventilation (PCV)
  • Preset by clinician: inspiratory pressure limit, RR, PEEP
  • Ventilator delivers airflow at a fixed inspiratory pressure.
  • Variables to be monitored: Vt
  • Volumes delivered are dependent on resistance and compliance in the circuit and can be unpredictable if these vary.
  • Traditional alarms need to be adjusted to monitor volumes.
Pressure-regulated volume-control (PRVC)
  • Hybrid of of VCV and PCV
  • Preset by clinician: inspiratory pressure limit, target Vt, RR, PEEP
  • Ventilator starts with PCV and adjusts inspiratory pressure breath-to-breath based on adaptive targeting of measured Vt.
  • Variables to be monitored: delivered Vt
Pressure-support ventilation (PSV)
  • Preset by clinician: IPAP, PEEP
  • Ventilator adds inspiratory and expiratory pressure to support spontaneous breaths.
  • Variables to be monitored: RR, Vt
  • Ideal for weaning
  • Ideal for spontaneously breathing patients with healthy lungs who were intubated for airway protection
  • Mandatory backup breaths in case of apnea are not automatic.
  • Hypoventilation can occur in patients with reduced respiratory drive or changes in circuit compliance or resistance.

Advanced settings

  1. Pressure support (PS): positive pressure added on top of PEEP during inspiration in pressure-supported ventilation modes (e.g., PSV)
    • Ranges from 5 cm H2O (minimal support) to 30 cm H2O (maximal support)
    • Work of breathing is mostly accomplished by the ventilator if PS > 20 cm H2O.
    • PS is typically increased to compensate for respiratory muscle fatigue, then gradually decreased during weaning from mechanical ventilation to allow patients to strengthen their respiratory musculature, with the goal of them breathing unassisted.
  2. Peak inspiratory pressure (PIP) limit/target: preset parameter in pressure-controlled and pressure-regulated ventilation modes
    • In other ventilator modes, PIP is a variable (see “monitoring” below).
    • Typically set at ∼ 20–25 cm H2O to prevent barotrauma
  3. Inspiratory flow rate (VI): the rate of airflow sent into lungs during the inspiratory phase
    • Adjustable; flow rate typically set at at 60 L/min
  4. Inspiratory:expiratory ratio (I:E ratio): the ratio of inspiratory time to expiratory time in a given breathing cycle
    • Usually expressed in whole numbers (e.g., 1:2, 1:3)
    • Can be adjusted directly or indirectly depending on the device
    • Target I:E ratios are desirable in certain conditions (e.g., 1:4–1:5 for obstructive lung disease).
    • Depends on other parameters
      • ↑ RR→ ↓ time available for passive expiration → ↑ I:E ratio
      • ↑ Vt → ↑ time required for inspiration → ↑ I:E ratio
      • ↑ VI → ↓ time required for inspiration → ↓ I:E ratio
      • Inspiratory time = Vt/VI
      • Expiratory time = [breath cycle time (1/RR) - inspiratory time]
      • I:E ratio = inspiratory time/expiratory time
  5. Trigger sensitivity (mechanical ventilation): the threshold of inspiratory pressure or flow-gradient at which the ventilator identifies the patient's attempt to initiate a breath
    • Typically standard to the device (1–3 cm H2O)
    • Not commonly adjusted by clinicians
      • Low sensitivity helps to decrease work of breathing
      • High sensitivity helps to decrease oversensing of breaths

Monitoring

General principles

All mechanically ventilated patients require close clinical, biomechanical, and laboratory monitoring in a critical care unit. This should include:

  • One-to-one nursing care
  • Continuous cardiac and hemodynamic monitoring
  • Respiratory monitoring: Sensors can be externally applied or built into modern ventilators.
  • Temperature monitoring
  • Blood gas analysis monitoring

External monitoring [31]

  • Pulse oximetry
  • Capnometry (portable): numerical measurement of end-tidal CO2 (EtCO2) [32]
    • Description
      • Externally-applied devices are being used with increasing frequency.
      • Expired air is sampled through a tube.
      • A transducer detects CO2 and the amount is displayed on the monitor.
    • Primary uses
      • Determining the adequacy of ventilation
      • Confirming ET tube placement
      • RR monitoring
    • Secondary uses
      • Short term guidance of hyperventilation therapy (e.g., treating ↑ ICP)
      • Diagnosing pulmonary embolism
      • Preventing hypercapnia, e.g., in brain-injured and post-cardiac arrest patients
      • Assessing CPR quality
    • Disadvantages: underestimates PaCO2 compared to ABG
  • Advanced monitoring to consider: especially for conditions exacerbated by positive pressure ventilation.
    • Hemodynamically unstable patients: CVP monitoring; IVC diameter and collapsibility
    • Patients at risk of abdominal compartment syndrome: abdominal compartment pressures

Ventilator-based monitoring [31][3][12]

The following are standard measurements built into most newer generation ventilators.

Capnography

  • Description
    • Waveform version of capnometry displaying CO2 measurements in exhaled air over time
    • The EtCO2 is measured at the end of the expiratory phase of the breathing cycle.
    • Sensors are typically integrated into the ventilator but can be portable in rare cases.
  • Interpretation
    • Normal waveform: rapid increase of the CO2 concentration → plateaurapid decrease in the CO2 concentration during inspiration.
    • Loss of waveform
      • Cardiac arrest
      • Accidental extubation
      • Complete tube obstruction
    • Significant decrease in waveform amplitude
      • Partial tube obstruction
      • Airway leak
      • Hypotension

Pressure monitoring [14][33]

  • Peak inspiratory pressure (PIP):
    • Definition: the maximum pressure measured at any point throughout the inspiratory phase
    • Description: primarily reflects airway resistance; lung compliance reflected to a lesser degree
      • VCV
        • Measured PIP is greatly influenced by airway resistance.
        • Less reflective of alveolar pressure and risk of VILI
      • PCV/PRVC
        • PIP = inspiratory pressure target + PEEP
        • Accurately reflects alveolar pressure and correlates well with VILI risk
    • Interpretation: PIP > 35–40 cm H2O is generally considered elevated.
  • Plateau pressure (PPlat)
    • Definition: the maximum air pressure measured during a pause at the end of inspiration
    • Description: reflects lung compliance
      • VCV: more accurately reflects alveolar pressure and risk of VILI risk than PIP
    • Interpretation: Pplat > 30 cm H2O is considered elevated.
  • Auto-PEEP (intrinsic PEEP) [34]
    • Definition: PEEP remaining in the circuit at the end of expiration that is not delivered by the ventilator, i.e., generated by the patient
    • Description
      • Tends to occur in conditions in which the expiratory outflow is impaired (e.g., asthma)
      • Expiration is incomplete at the end of a breath cycle → air trappingdynamic hyperinflation
      • Estimated using expiratory hold maneuver
        • The circuit is paused for 3–5 seconds at the end of expiration.
        • The change in pressure waveform back to baseline is measured as it reaches equilibrium in the circuit, which is the total PEEP.
        • Auto-PEEP = total PEEP - extrinsic PEEP
    • Interpretation:

Laboratory monitoring [35]

Arterial blood gas monitoring

Interpretation of oxygenation using ABG in mechanically ventilated patients
Condition PaO2 (mm Hg)
Hypoxemia < 70
Normoxemia 70–120

Hyperoxia [36][37]

Mild 121–200
Moderate 201–300
Severe > 300

Venous blood gas [12]

Complications

Ventilator-induced lung injury (VILI) [3][31]

VILI refers to both pulmonary and extrapulmonary injuries resulting from any combination of the following.

Other complications

Intrapulmonary

  • VAP: See pneumonia.
  • Inspiratory muscle weakness and deconditioning [39]
  • Ventilator-associated pulmonary fibrosis: occurs in the subacute phase of ARDS [40][41]

Extrapulmonary [31]

Ventilation strategies

Overview of high-risk indications for mechanical ventilation [42][43]
Condition Cause of peri-intubation mortality Preventative measures
Obstructive lung disease (e.g., asthma)
  • Manual decompression prior to connecting to the ventilator

Severe acidosis

HAGMA conditions (e.g., salicylate toxicity)

CO2 narcosis (e.g., due to AECOPD)

Shock (e.g., sepsis)

  • Pre-intubation fluid optimization
  • Push-dose vasopressor pretreatment
  • Induction agent with stable cardiovascular response

Normal lung mechanics [12][14][46]

Permissive hypercapnia [47][48][49][50][51]

Ventilation strategy for obstructive lung disease [12][14][46]

These are conditions with a high-risk of peri-intubation mortality.

Patients with obstructive lung disease are at high risk of peri-intubation mortality.

Asthma and anaphylaxis [57][58]

  • Indications: asthma exacerbation.
  • Typical settings
    • Mode
      • Use caution with continuing AC/VCV.
      • Consider switching to PCV or PRVC.
    • Vt: 4–8 mL/kg, then adjust according to ABG
    • RR: Adjust according to permissive hypercapnia.
      • Increase if pH < 7.25.
      • Acceptable ranges: 8–10 breaths/min
    • FiO2
      • Initially 100%; rapid titration to < 30–50%
      • Target minimum required to maintain SpO2 88–92%
    • PEEP: Keep < 80% of auto-PEEP value (0–5 cm H2O).
  • Additional parameters
    • Maximize expiratory time (e.g., consider I:E ratio 1:4 rather than 1:2).
    • VI: 60–80 L/min
    • Auto-PEEP: Keep to a minimum.

AECOPD [14][46]

  • Indication: COPD exacerbation.
  • Typical settings
    • Mode:
      • Use caution with continuing AC/VCV.
      • Consider switching to PCV or PRVC.
    • Vt: 6–8 mL/kg
    • RR: Adjust according to permissive hypercapnia.
      • Increase if pH < 7.25.
      • Acceptable ranges; 10–12 breaths/min
    • FiO2
      • Initially 100%; rapid titration to < 30–50%
      • Target minimum required to maintain SpO2 88–92%.
    • PEEP: Keep < 80% of auto-PEEP value.
  • Additional parameters
    • Maximize expiratory time (e.g., consider I:E ratio 1:4 rather than 1:2).
    • VI: 60–80 L/min
    • Auto-PEEP: Keep to a minimum.

Use the I:E ratio to maximize expiratory time and prevent auto-PEEP in obstructive lung disease.

Ventilation strategy for severe acidosis [12][14]

These are conditions with a high-risk of peri-intubation mortality.

Optimizing the tidal volume and respiratory rate to match the patient's respiratory compensation is critical in patients with severe acidosis to avoid cardiac arrest.

Lung-protective ventilation strategy [12][14][46][61][62][20][63][35][64]

This is used in very high-risk and challenging clinical situations. Expert consultation is crucial during initiation and adjustment.

Ventilation strategy for elevated ICP [15][66]

Elevated ICP is a high-risk condition primarily due to the adverse effects of induction medications and laryngoscopy.

  • Common use
    • High ICP refractory to other lowering measures
    • Should only be used as a temporizing measure
    • Avoid in the first 24 hours after head injury. [12][67]
  • Typical settings
    • Mode: AC/VCV
    • FiO2: 100%
    • PEEP: ≤ 5 cm H2O
    • Vt/RR
      • First 30 minutes
        • Set initial Vt ≥ 8 mL/kg.
        • Adjust RR to target PaCO2 30–35 mm Hg.
      • Subsequently can reduce Vt to 6–8 mL/kg and adjust RR to target normocapnia (i.e., 35–40 mm Hg).
  • Additional parameters: Raise the head of the bed.

Ventilation strategy for neuromuscular weakness and chest wall injury [12][14]

Ventilator weaning

  • Definition: the process of easing a patient off mechanical ventilatory support
  • Typical settings
    • Mode: PSV
    • Vt/RR: set by patient
    • FiO2: Begin at level from prior ventilatory mode (e.g., AC/VCV) and reduce to < 40%.
    • PEEP: Begin at level from prior ventilatory mode (e.g., AC/VCV) and reduce to 3–5 cm H2O.
    • Pressure support (PS)
      • Range: 5–20 cm H2O
      • Initially choose the amount required to match the Vt delivered on the previous ventilation mode.
      • Progressively reduce, as tolerated by the patient's clinical status and ventilatory parameters.
  • Spontaneous breathing trial
    • A test that is used to determine whether a mechanically ventilated patient is ready to breathe without a ventilator
    • Criteria for extubation
      • Patient is able to spontaneously initiate an inspiratory effort (i.e., good neuromuscular function).
      • Underlying lung disease is stable/resolving.
      • Normal PaO2 and SpO2 on minimal support (e.g., PS ≤ 5 cm H2O)
      • pH ≥ 7.35
      • Patient is hemodynamically stable with little or no vasopressor therapy.

Extubation with inadequate weaning (i.e., without proper muscle restrengthening) can lead to respiratory failure and require reintubation.

Pitfalls and troubleshooting

Approach to mechanically-ventilated patient in respiratory distress [3][12][42][31][14]

  • If hemodynamically unstable: Disconnect the patient from the ventilator and switch to manual ventilation with 100% FiO2.
  • If hemodynamically stable: Increase FiO2 to 100% until further assessment is complete.
  • Perform focused clinical evaluation.
  • Check monitors and alarms: Review PIP and PPlat.
  • Inspect ventilator equipment and tubing.
  • Perform bedside ultrasound or order portable CXR, as guided by clinical suspicion (e.g., to rule out pneumothorax).

In patients who are hemodynamically unstable, disconnect them from the ventilator and start manual ventilation with 100% FiO2.

Causes of sudden deterioration post-intubation [68][69]

  • Displacement/disconnection
    • See airway management for ET tube troubleshooting.
    • Check for disconnection in the circuit.
  • Obstruction: See airway management for causes.
    • Deep suctioning with saline flushes
    • Undo kinks in tubing.
    • Evaluate patients biting ET tubes for causes of pain/discomfort.
      • Adjust sedation and analgesia accordingly
      • Apply a bite block.
  • Pneumothorax
  • Equipment failure
    • If suspected, disconnect the patient from the ventilator and manually ventilate with 100% FiO2 until it can be replaced.
    • Troubleshoot with a respiratory therapist.

Causes of sudden deterioration after intubation can be recalled using the DOPE mnemonic: Displacement/Disconnection, Obstruction, Pneumothorax, Equipment failure.

Improving oxygenation in mechanically ventilated patients

  • Especially challenging in patients with hypoxemic respiratory failure
  • Adjust FiO2 and PEEP.
    1. Increase FiO2 to rapidly deliver a higher oxygen concentration.
      • Avoid prolonged exposure to high FiO2 levels (> 60%) to prevent hyperoxia/oxygen toxicity. [36][37]
        • Avoid SpO2 ≥ 95% for prolonged periods, unless specifically indicated.
        • If SpO2 is persistently 100%, check PaO2 on ABG (see “monitoring” above).
    2. Increase PEEP if FiO2 > 60% is required. [70][71][72][73]
      • Titrate up as needed to allow FiO2 to be reduced to less risky levels.
      • Avoid prolonged exposure to high PEEP to minimize VILI.
      • Aim for the lowest PEEP tolerated to maintain oxygenation target.
  • Treat reversible underlying causes of hypoxemia.
  • Consider lung recruitment maneuvers for refractory hypoxemia.

Improving ventilation in mechanically ventilated patients

Hemodynamic compromise in mechanically ventilated patients [2][3][4]

Patient-ventilator dyssynchrony [74]

  • Description
    • A mismatch between patient and equipment
      • Airflow: The flow delivered by the ventilator is inconsistent with the patient's demand.
      • Phase: The timing and cycle of ventilator-delivered breaths are misaligned with the patient's respiratory drive.
  • Recognition
    • Often identified by nursing staff or respiratory therapists
    • Possible clues [74]
      • Biological: unstable vital signs, pressure/flow waveform patterns, changes in ABG parameters, EtCO2
      • Behavioral: coughing, signs of increased respiratory effort , agitation
  • Aggravating factors