Phases of the Mechanical Breath
There are four phases to a mechanical breath: initiation phase, inspiratory phase, plateau phase, and expiratory phase.
Depicted below is the pressure waveform of a mechanical breath.
This starts at the end of expiration. At this stage, the breath is "triggered" - initiated either by the machine or by the patient.
Once the breath is triggered, the inspiratory flow begins. This phase is defined by air flow into the patient.
There may be an inspiratory pause, which would allow a plateau phase to form. This phase is defined by the absence of air flow.
The ventilator cycles from inspiration to expiration; the expiratory valve opens, and the patient exhales passively.
This phase is defined by air flow OUT of the patient.
The pressure which remains behind is the PEEP (positive end expiratory pressure).
Three graphs to rule them all
As mentioned previously, there are only 4 variables you need to think of: pressure, flow, volume, and time.
The first three are plotted over time on every ventilator screen
The blow is a brief summary of these features. They are treated in greater detail elsewhere.
This 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.
This 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.
This is the relationship between volume and compliance.
It 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.
Much of the time, you have some control over what the PEEP is. At the end of expiration, this is the alveolar pressure.
Peak inspiratory flow
This 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
This is a more interesting flow: It is generated by the elastic recoil of the patients 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)
The tidal volume speaks for itself. It is a product of flow and time. This is the volume above FRC.