Mechanical Ventilation

Commonly used ventilator modes:

Assist Control (AC): Used as initial mode of mechanical ventilation, patients receive breaths (mechanical) as per set parameters. Patients can take similar breaths (spontaneous) in addition to set rate but all other parameters remain the same. It can be set as volume-limited assist control ventilation (volume control, AC/VC) or pressure-limited assist control ventilation (pressure control, AC/PC). During AC/VC ventilation, inspiration ends after delivery of the set tidal volume whereas in AC/PC inspiration ends after delivery of the set inspiratory pressure.

Pressure Support: Pressure support ventilation (PSV) is frequently used during weaning from ventilator. Patient must be able to take spontaneous breaths as all breaths are supposed to be patient triggered. Patients receive a constant pressure (PEEP) and a supporting driving pressure (∆P) when the ventilator breath is triggered.

Standard ventilator measurements and settings:

Peak Inspiratory Pressure (PIP or Ppeak): Maximum pressure measured in airways at the end of inspiratory phase. (Target PIP < 35 cm H₂O)

Plateau Pressure (Pplat): Pressure measured by pressing inspiratory “pause/hold” button on ventilator, it is done to measure lung compliance. (Target Pplat < 30 cm H₂O)

Positive End Expiratory Pressure (PEEP): Applied positive pressure that remains in the alveoli at the end of exhalation and helps prevent atelectasis.

Intrinsic PEEP (iPEEP), or auto-PEEP: Pressure measured by pressing expiratory “pause/hold” button on ventilator, it is done to quantify air trapping due to incomplete exhalation (Target iPEEP < 5 cm H₂O)

Tidal volume (VT): Volume of gas that patient receives with each breath. Typical range is 6 to 8 mL/kg. In ARDS patients range is 4 to 8 mL/kg.

Respiratory rate or frequency (RR or f): Number of mandatory breaths delivered by the ventilator per minute. Typically 12 to 20 breaths/minute, can titrate up to 35 in certain cases.

Minute ventilation (VE): Tidal volume multiplied by the respiratory rate (VT x RR), it is the ventilation that patient receives in one minute.

Fraction of inspired oxygen (FiO2): Percentage of oxygen delivered by the ventilator during inspiration.

Typical initial settings for AC/VC (most frequently used mode in adult ICUs) mechanical ventilation

Tidal volume 6 mL/kg

Ventilator rate 14 breaths/min

PEEP 5 cm H2O

FiO2 100%

Inspiratory flow rate 60 L/min

General principles:

Ventilator parameters are based on predicted or ideal body weight (IBW) not the actual weight of patient.

Attempt to titrate down FiO2 to non-toxic levels (60 percent or below) as quickly as possible based on pulse oximetry (maintaining SpO₂ between 90 to 96%)

PEEP and FiO2 affect oxygenation whereas VT and RR affect CO2 removal

High PIP, low/normal Pplat suggests high resistance (impedance of flow in the tubing and airways) like in endotracheal tube obstruction or bronchospasm

High PIP, high Pplat suggests low compliance (lung’s ability to stretch and expand) like in interstitial pulmonary fibrosis, pneumonia, ARDS or pulmonary edema

Sedatives and analgesics:

Fentanyl: Analgesic-sedative. Good choice for analgesia and sedation for most patients. Start at 25 mcg/hour, can uptitrate to 200 mcg/hour.

Propofol: Hypnotic-sedative. Anticonvulsant but no analgesic effect. Watch for hypotension, bradycardia and monitor triglycerides. Start at 5 mcg/kg/minute, can uptitrate to 50 mcg/kg/minute (Use IBW in obese patients)

Dexmedetomidine (Precedex): Sympatholytic- anxiolytic-analgesic-sedative. Does not cause deep sedation, allowing patients to be easily awakened. Watch for hypotension, bradycardia and occasional hypertension. Start at 0.2 mcg/kg/hour, can uptitrate to 1.5 mcg/kg/hour. (Use IBW in obese patients)

Midazolam (Versed): Anxiolytic with rapid onset of action. Good choice for management of acute agitation. Start at 1 mg/hour, can uptitrate to 8 mg/hour

Neuromuscular blocking agents (Pancuronium, Rocuronium) are occasionally used in selected patients with persistent agitation despite being on multiple sedatives and analgesics.

Weaning from mechanical ventilation

Daily attempts should be made to reduce the level of sedation.

For patients receiving multiple sedatives, taper Fentanyl last so patients have some analgesic support when they wake up.

Objective criteria suggesting readiness for weaning:

1. Patient off pressors or on very low dose pressors

2. Arterial pH > 7.25

3. SpO₂ ≥ 90% or PaO₂/FiO₂ ≥ 150 on FiO₂ ≤ 40% and PEEP ≤ 5 cm H₂O

Rapid shallow breathing index (RSBI): One of the most commonly used weaning predictors. It is the ratio of respiratory frequency to tidal volume (f/VT). Weaning is initiated in patients with RSBI < 105 breaths/min/L

Spontaneous breathing trials (SBTs): Patients breath spontaneously through endotracheal tube (ETT) for 30 minutes to two hours with low level PSV support (∆P 5 to 8 cm H₂O, PEEP 5 cm H₂O and FiO₂ 40%). Daily SBTs are preferred for weaning, patients who tolerate SBT are evaluated for extubation.

Extubation: Patient is placed upright, head-end elevated and ETT removed during expiration in a single swift motion.