Mechanical Ventilation (IBL-ICU 19092019)

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.

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):

• PEEP
• Protective lung ventilation
• Open lung approach to ventilation
• Transpulmonary pressure
• Driving pressure
• Ventilator Associated Lung Injury (VALI)
• Prone Position and Mechanical Ventilation

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):

• PaO2/FiO2 Ratio
• Post-intubation hypoxia
• Improving Oxygenation in ARDS
• Oxygen saturation targets in critical care

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):

• High airway and alveolar pressures
• Intrinsic PEEP

This question was covered in a separate equipment and procedures tutorial

Learn more (LITFL CCC):

• Patient-Ventilator Dyssynchrony
• Flow Volume Loops
• Dynamic Pressure-Volume Loops 
• Pressure vs Volume Loop

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 = V_T / (P_plat – PEEP_total)
• 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):

• Compliance