Question 4

College Answer

Causes: consider increased expiratory resistance (prolonged expiratory time constant: eg. bronchospasm, narrow/kinked ETT, inspissated secretions, exhalation valves/HME/filters), increased minute ventilation (inadequate expiratory time), prolonged inspiratory time.

Consequences: consider increased intra-thoracic lung volume (with increased pressures for a given tidal volume and risks of barotrauma), increased intra-thoracic pressure (decreasing venous return, and increasing inspiratory work to trigger the ventilator).

Management: consider treatment of reversible factors (bronchospasm, secretions, expiratory devices), prolongation of expiratory time (decrease respiratory rate, increase inspiratory flow, decrease in inspiratory time) or decrease tidal volumes, application of exogenous PEEP (to 50 –85% of accurately measured intrinsic PEEP) can be used to decrease inspiratory triggering work in spontaneously breathing patients, and possibly to improve distribution of inspired gas.

Discussion

Intrinsic PEEP seems to be a favourite college topic. There are numerous questions, all of which ultimately ask the same thing: what is intrinsic PEEP, and how does one detect it, and which ventilator settings does one twiddle with in order to defeat it?

So, here we go again.

Causes

• Increased resistance to expiratory flow, due to:
  • Machine factors:
    • Blocked or faulty expiratory valve of the ventilator
    • kinked expiratory limb of the ventilator tubing
    • rain-out in the expiratory limb
    • clogged water-sodden HME
    • kinked ETT
    • ETT clogged with sputum
    • ETT being chewed on by the patient
  • Ventilator settings
    • Short expiratory time, eg. in very high respiratory rate
    • High I:E ratio
  • Patient factors
    • Bronchospasm
    • Increased respiratory rate

Consequences

• CO2 retention
• Increased work of breathing
• increased intrathroracic pressure, thus
  • decreased venous return, hemodynamic instability
  • decreased lymphatic return
  • decreased organ perfusion, organ oedema and decreased organ function

Management

• reverse reversible patient factors, eg. bronchospasm can be treated with bronchodilators
• correct machine factors, eg. empty the water out of the tubes, cheange the HME, change the ventilator valve
• Suction ETT, ensure patency
• Increase expiratory time by decreasing respiratory rate and decreasing I:E ratio
• Apply PEEP to counteract the increased work of breathing
• Decrease tidal volume