Mechanical Ventilation

Neil MacIntyre, M.D.
Talal Dahhan, M.D.

Introduction to mechanical ventilation

June 2022

Brief review on mechanical ventilation

Indications

• Improve gas exchange
• ↑ oxygenation
• ↑ alveolar ventilation and/or reverse acute respiratory acidosis
• Relieve respiratory distress
• ↓ work of breathing (can account for up to 50% of total oxygen consumption)
• ↓ respiratory muscle fatigue
• Apnea
• Airway protection
• Pulmonary toilet in association with weakness

Great overview videos by Dr. MacIntyre are below to discuss how to set it the ventilator when we have parenchymal lung disease vs obstructive airway diseases

Please Take a look at the review article by Dr. Macintyre in Respiratory Care (Jan 2011). It describes basic concepts and modes to understand mechanics and how respirators (ventilators) work.

Basic Modes

Steps to think about when trying to set up a ventilator:

• Pick ventilator mode
• Institutional preference and patient’s comfort determine choices.

• Please remember;

• • Do no harm: when you mechanically ventilate patients, check every time the following when changing or reviewing a mechanically ventilated patient:

• • 1. V_T to be between 4 to 8 ml/kg of ideal body weight
    2. P_plateau [ Alveolar pressure obtained in inspiratory pause ideally in paralyzed patients ] < 30 cm H₂O.
    3. FiO₂: Helps to correct hypoxemia, not to exceed 0.6 (or 60%) to avoid oxygen toxicity.
    4. Lower driving pressures [ P_plateau – PEEP ] improve survival in ARDS patients.

• • To improve oxygenation: increase FiO2 and/or Positive End Expiratory Pressure (PEEP)
  • To improve CO₂ removal, increase the Minute Ventilation: V_T x RR. These parameters what you need to work with to increase washing out PaCO₂.
  • Be aware and avoid “Auto-PEEP” on the ventilator flow waveform or the alarm. Increasing RR reflexively, can kill the patient.

Reasons for changing the mode:

• Dyssynchrony
• Gas Exchange disorders.
• Patient care: as in sedation, weaning or lung protection.
• Lung Protection: might not be able to achieve 4-8 ml/kg VT target in all modes!
• Alarms are set up to volume or pressure, flow.

Set and/or check the remaining variables;

• FiO2: Helps to correct hypoxemia, not to exceed 0.6 (or 60%) to avoid oxygen toxicity.
• Positive End Expiratory Pressure (PEEP): helpful in severe ARDS, not much in R to L intracardiac shunt or mild ARDS,
• Inspiratory:Expiratory (I:E) time: where you increase the ratio from 1:3, to 1:2 or 1:1.5.

The following FiO₂ / PEEP tables should guide your choice if PaO₂:FiO₂ ratio is > 200.

The following FiO₂ / PEEP tables should guide your choice if PaO₂:FiO₂ ratio is < 200.

Weaning of Mechanical Ventilation Review

The key is to perform daily assessment of readiness for sponaneous awakening (SAT) and spontaneous breathing trials (SBT)

Clinical screening criteria:

• Stable vital signs (ideally completely off inotropes, can have some)
• Minimal secretions.
• Adequate cough (assessed with minimal sedation).
• Cause of respiratory failure or previously failed SBT have been reversed.
• Ventilator parameters:
• PaO₂/FiO₂ >200,
• PEEP ≤5,
• Rapid Shallow Brething Index (RSBI) <105.
• If > 105, negative predictive value of 0.95.
• Minute Ventilation (V_E) <12 L/min,
• Vital Capacity (VC) >10 mL/kg of ideal body weight

 Daily awakening trial (Spontaneous Awakening Trial – SAT):

• Minimize sedation, when clinically possible.
• Patient is able to open eyes, comfortable as possible and following simple commands.
• Absence of agitation, Tachypnea (where RR >35), Hypoxemia SpO₂ <88% and arrhythmias.

 Successful SAT = All above

If failed, restart sedation at half of the prior dose.

Sontaneous Breathing Trial (SBT)

• Setting the ventilator on PSV with 0-5/5
• Have that between 30–120 min.

Does it help to have daily SAT and SBT? Yes, NNT was 7 to save humans off the ventilators! Girard et al, Lancet 2008.