Mechanical Ventilation (IBL-ICU 19092019)

Notes scribed and collated by Dr Chandraa Balakrishnan and reviewed by Dr Rahul Costa-Pinto.

Here are the notes from the IBL-ICU discussion of the daily questions on the topic of Mechanical Ventilation: that took place on 20th November 2019 at ICU Junior Medical Staff Teaching.

Q1.
Review the ventilator settings on your intubated patients during your ward round. Are they receiving “protective lung ventilation”? What is the justification for the choice of mode, tidal volume and PEEP?

Types of ventilation:

  • Invasive or non-invasive
  • Invasive ventilation – mandatory or spontaneous modes
  • Mandatory ventilation – volume control or pressure control

Protective lung ventilation

  • Low tidal volume ventilation (4-8 mL/kg)
  • Often includes permissive hypercapnia
  • Prevents ventilator-induced lung injury (VILI)
    • Volutrauma (hyperinflation and shearing injury)
    • Barotrauma (alveolar rupture and pneumothorax)
    • Biotrauma (release of inflammatory mediators)
  • Evidence
    • ARDSnet/ARMA trial 2000
    • Non-ARDS patients – meta-analysis of 20 studies (Serpa Neto et al 2012)
    • SUMMARY: best to use in all mechanically ventilated patients; set a tidal volume of 6 mL/kg based on predicted body weight (PBW) and target plateau pressures < 30 cmH2O

Learn more (LITFL CCC):

Q2.
Assess an intubated patient and determine their PF ratio. What does the PF ratio tell you? What ventilator and non-ventilator strategies can be used to improve oxygenation?

PF ratio (Carrico index)

  • At sea level PF ratio ~ 500 mmHg – can roughly estimate if there’s a significant A-a gradient
  • Can differ in different altitudes due to barometric pressures; Dependent on O2 flux which can vary in severe sepsis, hypercatabolic state etc.
  • Should only be used if PaCO2 normal – No shunt suspected
  • Can be used in SMART-COP risk score; classifying severity of ARDS; decision-making for V-V ECMO (i.e. PF ratio < 60); research studies to assess baseline characteristics of patients
  • In ARDS, mild (PF ratio 200-300) = mortality 27%; moderate (PF ratio 100-200) = mortality 32%; severe (PF ratio < 100) = mortality 45%

Ventilator strategies to improve oxygenation

  • Increase FiO2
  • Increase mean alveolar pressure
  • Increase mean airway pressure
  • Increase PEEP
  • Increase I:E ratio
  • Minimise dead space ventilation (remove excess tubing)

Non-ventilator strategies to improve oxygenation

  • Regular suctioning, chest physiotherapy
  • Drain pleural effusions
  • Sedation and neuromuscular blockade
  • Optimise fluid balance – aim negative
  • Optimise haemoglobin – PRBC transfusion
  • Inhaled nitric oxide
  • Prone positioning
  • V-V ECMO
  • Reduce intra-abdominal pressure or deflation of gas-filled stomach if significant

Learn more (LITFL CCC):

Q3.
Perform an inspiratory hold and an expiratory hold on a stable mechanically ventilated patient. What information does this give you about your patient’s respiratory condition?

Inspiratory hold

  • Allows measurement of the plateau pressure – safe pressures < 30 cmH2O
  • Reflects pressure within the alveoli (in the absence of flow in a paralysed patient) –  the equilibrated pressure within the respiratory circuit (lungs and ventilator) after a static volume challenge
  • Suggested for 2 second hold to take into account different time constants for alveoli
  • The pressure generated in the lung during the inspiratory pause is the pressure required to overcome lung and chest wall compliance

Expiratory hold

  • Measurement of PEEPi (intrinsic PEEP or auto-PEEP)
  • PEEPi is measured by performing an end expiratory pause or hold manoeuvre – expiratory circuit occlusion for 3-5 seconds allows alveolar pressure to equilibrate with airway pressure
  • Causes of increased PEEPi
    • Increased expiratory resistance – bronchospasm (e.g. asthma, COPD), narrowed/kinked ETT, secretions, exhalation valves, HME filter
    • Impaired elastic recoil – emphysema
    • Increased minute ventilation – inadequate expiratory time
  • Consequence of untreated elevated PEEPi is dynamic hyperinflation and haemodynamic instability

Learn more (LITFL CCC):

Q4.
Review a patient who is intubated and receiving mechanical ventilation. What can you learn from the ventilator waveforms?

This question was covered in a separate equipment and procedures tutorial

Learn more (LITFL CCC):

Q5.
Review a patient who is intubated and receiving mechanical ventilation. How can you assess their lung compliance on the ventilator? What factors affect lung compliance?

Compliance

  • Change in volume for a given change in pressure, measured in mL/cmH2O
  • Due to the tendency of a tissue to resume its original position after removal of an applied force
  • It is the inverse of ​elastance​, which is the force at which the lung recoils for a given distension
  • A decreased compliance means the transpulmonary pressure must change by a greater amount for a given volume, which increases elastic work of breathing
  • Compliance of the respiratory system​ as a whole is ~ 100 mL/cmH2O
  • Can be static or dynamic

Static compliance

  • Compliance of the system at a given volume when there is no flow i.e. no pressure component due to resistance
  • Static compliance is a function of elastic recoil of the lung and surface tension of alveoli
  • When paralysed and mechanically ventilated, peak airway pressure is the force required to overcome resistive and elastic recoil of the lung and chest wall – to distinguish resistive from elastic recoil-related pressures requires an introduction of an end-inspiratory circuit occlusion after VT delivery.
  • Peak pressure will decrease down to a stable plateau pressure (3 second hold) – this corresponds to the elastic recoil pressure
  • “Quasi-static” Compliance = VT / (Pplat – PEEPtotal)
  • When patient spontaneously breathing, compliance becomes uncertain (can decrease the pause time to 1 second but is difficult to measure)

Dynamic compliance

  • The compliance measured during respiration, using continuous pressure and volume measurements i.e. ​includes the pressure required to generate flow​ by overcoming resistance forces
  • Dynamic compliance is always less than static compliance​, as there will always be a degree of airway resistance
  • It is reduced in lung units with unequal time constants at high respiratory rates
  • Normal dynamic compliance during mechanical ventilation is 50-100 mL/cmH2O

Changes with pathology

  • Increased Lung Compliance – Normal aging, Asthma, Emphysema
  • Decreased Lung Compliance – Pneumonectomy/Lobectomy, Atelectasis, Pneumonia, ARDS, APO, Hyaline Membrane Disease, Pulmonary fibrosis
  • Increased Chest Wall Compliance – Collagen disorders
  • Decreased Chest Wall Compliance – Obesity, Spastic paralysis of chest wall musculature, Kyphosis/Scoliosis, Circumferential burns), Prone positioning

Learn more (LITFL Part One):

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