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. phases of a mechanical breath

Initiation phase

This starts at the end of expiration. At this stage, the breath is "triggered" - initiated either by the machine or by the patient.

Inspiratory phase

Once the breath is triggered, the inspiratory flow begins. This phase is defined by air flow into the patient.

Plateau phase

There may be an inspiratory pause, which would allow a plateau phase to form. This phase is defined by the absence of air flow.

Expiratory phase

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

ventilator waveform

The blow is a brief summary of these features. They are treated in greater detail elsewhere.

Peak pressure

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.

Airway pressure

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.

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.

PEEP

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)

Tidal volume

The tidal volume speaks for itself. It is a product of flow and time. This is the volume above FRC.

References

Most of this information comes from only two textbooks. With "Basic Assessment and Support in Intensive Care" by Gomersall et al (was well as whatever I picked up during the BASIC course) as a foundation, I built using the humongous and canonical "Principles and Practice of Mechanical Ventilation" by Tobins et al – the 1442 page 2nd edition.