Analysis of ventilator waveforms

This comes up a lot, being a part of the the bread and butter routine of ICU management. SAQs which have required the analysis of ventilator waveforms include the following:

• Question 21.1 from the first  paper of 2014 (gas trapping and bronchospasm)
• Question 5.1 from the first paper of 2012 (gurgling secretions in trachea)
• Question 27 from the second paper of 2009 (bronchospasm)
• Question 26.1 from the second paper of 2008 (bronchospasm)
• Question 30 from the first  paper of 2011(bronchospasm and cuff leak)

In short, its a popular topic. Usually the curves are those of a patient with high airway resistance, auto-PEEP and gas trapping; the college expect you to be able to identify this and make some comment as to how you would change the ventilator settings to improve the situation. Alternatively, the college might ask you to draw and label a diagram of a pressure-time curve for a patient with normal airways and a patient with bronchospasm. Either way, it seems bronchospasm is the major focus of these questions.

This is a brief summary, and will not go into great depth. Local long-form discussions of these matters include the following chapters:

• Intrinsic PEEP and the expiratory hold manoeuvre
• Interpreting the shape of the pressure waveform
• Interpreting the shape of the ventilator flow waveform
• Interpreting the shape of the pressure-volume loop

Ventilator waveforms associated with increased airway resistance

This waveform graphic is seen in Question 21.1 from the first  paper of 2014. Obviously, its not the college's own graphic (though they did use some of their own artwork in Question 26.1 from the second paper of 2008).

The candidate should be able to both identify the major features which are characteristic of bronchospasm, and to reproduce them on paper.

Specific features of increased airway resistance seen here are:

• High peak airway pressure, but a normal plateau pressure
• Slow return of the flow-time curve to baseline
• The flow-time curve does not reach baseline (indicating that emptying is incomplete)

After asking questions about waveform interpretation, the college typically goes on to ask further about what precisely one would do to manage such a problem. The answer typically revolves around increasing the I:E ratio, decreasing the respiratory rate, dropping the PEEP to zero, and so forth. This topic is explored in greater detail by the chapter on Intrinsic PEEP and dynamic hyperinflation.

Causes of increased airway resistance

Even though one's instincts might be strongly trending towards bronchospasm as the cause of such a picture, one must systematically consider all possible causes:

• Machine problems:
  • kinked ventilator tubing
  • "rain-out" in the ventilator tubing
  • old waterlogged HME
  • kinked or obstructed ETT
  • old expiratory filter in the ventilator
• Patient problems
  • biting and chewing on the tube
  • increased upper airway resistance due to some sort of sputum plug
  • bronchospasm (most likely)
  • increased chest wall rigidity, eg. due to massive fentanyl bolus, or hypothermia.