The mechanical ventilator, secondary to its role as the deliverer of flows and the regulator of pressures, is also a complex measurement device for monitoring the behaviour of the respiratory system it has been connected to. It collects a vast amount of data from each breath and makes this knowledge available to the user by means of numerical output and graphical representations. Of these, the graphs of variables over time are the most informative.
Apart from the pragmatic need to understand what you're doing, this material has some exam relevance. CICM have often offered interesting ventilator waveforms for the exam candidates to interpret. Weirdly, there is no mention of ventilator waveforms in the 2017 version of the CICM primary syllabus, but by the time they are ready for the Part II exam the trainees are expected to have some considerable mastery of this topic (judging by the complex waveforms they need to interpret in SAQs such as Question 11.3 from the second paper of 2017). There is also nothing in the CICM WCA ("Ventilation") to suggest that looking at the ventilator screen is ever expected of a trainee in the middle of their training. In short, there appears to be something of a void in the training program where it comes to ventilator waveform interpretation.
This void does not go unrecognised by the fields' giants. Dean Hess (2005) complained that there is little formal study of this area in medical education, that "much information scrolls by on the ventilator screen without receiving much notice" and that "ventilator graphics are seldom afforded the detailed pattern recognition that is commonly devoted to the electrocardiogram", which are completely valid statements. The linked article is a foreword to a massive issue of Respiratory Care which was dedicated to exactly this topic. The articles from this issue were used to fashion a section on ventilator waveform interpretation. Its main value was to the author, as a means of organising this information and to address the shortcomings of whatever formal instruction he had received on the subject. Any subsequently discovered relationship between these notes and exam questions is therefore purely coincidental.
Three graphs to rule them all
Most ventilators have some way of displaying a graph of pressure, flow and volume over time. However, as you can plainly see, neither the colour scheme nor the waveform arrangement are standardised in any meaningful way.
The decision of how to array their variables which variable to display and how to identify the important components of a breath appear to have been left to the whims of marketing executives. If there is some art or science behind these, it is difficult to find the evidence of it. It makes some sort of primitive macaque-level sense that pressure is an angry red on the screen of the Bellavista 1000, but was this intentional? And if yes, then why white for flow, why yellow for tidal volume? Why did Drager decide that grey was the colour of spontaneous respiration? And why did nobody care about any of these things when designing the Puritan Bennett 840?
In short, the manufacturers each had their own ideas about how to best represent the data produced by their devices, and as there are no rules to guide them it is understandable that each took their own slightly different path to the same goal. Interestingly, in a phenomenon akin to parallel evolution, most of the UI designers had for some reason decided to put pressure at the top. Volume is almost always at the bottom, and that leaves flow in the middle. The ventilator graphics in this learning resource follow this unspoken convention.
Here is a schematic representation of ventilator pressure, flow and volume waveforms:
Below is a brief summary of these features. They are treated in greater detail elsewhere.
- Peak pressure is the pressure due to the sum of airway pressure and alveolar pressure.
- A rising peak pressure alerts one to the possibility of airway narrowing in some sense, be it the endotracheal tube being kinked (or chewed on), or the ventilator tubing being full of fluid, or the heat and moisture exchanger being waterlogged, or the secretions building up on the inside of the endotracheal tube. Or, the patient might actually be having some sort of bronchospasm.
- Airway pressure is the pressure due to the resistance of the airways. It is only present while flow is occurring. As soon as flow stops, the pressure due to airway resistance drops to zero. Thus, one can estimate the airway pressure (and thus the degree of airway resistance) from the difference between peak pressure and plateau pressure.
- Plateau pressure is the pressure in the lung which results from the insufflation of the controlled volume. It is unrelated to flow; this is the pressure in the circuit which prevails when the breath is "held", i.e. the tidal volume is inside the patient without any flow going in or out.
- PEEP is the pressure during the expiratory phase, over which you have some control (presumably, you set it)
- Peak inspiratory flow is the flow generated during the inspiratory phase. Much of the time it is irrelevant from the diagnostic point of view, as it is something wholly machine-related. The ventilator generates this flow.
- Peak expiratory flow is a more interesting flow: It is generated by the elastic recoil of the patient's lungs and chest wall. In the same way as peak flow measurements can be used to assess an asthmatic, so can the expiratory peak flow of a ventilated patient inform you about the airway resistance. A low expiratory flow obviously suggests you have an airway obstruction (or, more freakishly, an abnormally over-compliant chest wall)
- Tidal volume speaks for itself. It is a product of flow and time. This is the volume above FRC.