Pressure-volume and Flow-volume Loops
This comes up a lot, but unlike ventilator waveforms the loops are not a part of the ICU routine. In fact the analysis of pressure-volume and flow-volume loops adds little to one's management. For instance, unreliablility of pressure-volume loops has been demonstrated in the context of determining the ideal PEEP in ARDS. Usually, the college expects the candidates to draw the pressure-volume curves of a patient with reduced compliance (compared to normal), and the flow-volume graphics tend to be the "scooped out" loops of bronschospasm.
SAQs which have required the analysis of loops include the following:
- Question 13.1 from the first paper of 2014
- Question 13.2 from the first paper of 2014
- Question 26.1 from the second paper of 2008
- Question 26.2 from the second paper of 2008
- Question 4 from the second paper of 2000
This is a brief summary, and will not go into great depth. Locally, a long discussion of the shape of the pressure-volume loop and the flow-volume loop is available. For a formal discussion of loops, particularly as related to formal spirometry, one could do worse than the 1973 "A Current Perspective" by Hyatt and Black, from the American Review of Respiratory Disease. The ideal way to present this information is with actual spirometry diagrams, especially considering that the college SAQs on this topic typically either come in the form of diagrams, or call upon the candidate to draw diagrams.
What is the point of looking at loops?
In summary, the following useful information can be derived from them:
- graphical representation of lung compliance
- estimation of lower inflection point
- estimation of pressure required for complete alveolar recruitment
- adjusting PEEP to this may pervent derecruitment
- estimation of pressure which causes alveolar overdistension
- adjusting plateau pressure to this may prevent VILI
- estimation of the work of breathing
- estimation of the degree of airway obstruction
Limitations of the loops are as follows:
- Poor representation of heterogenous lung pathology
- In pressure-volume loops, there is inconsistent agreement among observers as to where the lower inflection point is
Pressure-volume loops in brief summary
Flow-volume loops in brief summary