Phases of the mechanical breath

This chapter briefly describes the anatomy of a mechanical breath. As each waveform produced by the ventilator is dissected in great detail in later entries, this chapter is more of an introduction to the way one might classify a mechanical breath by the characteristics of its different parts. This is the gateway to understanding modes of ventilation, therefore seems like a fairly fundamental area. The whole idea of “mode” and what exactly a mode is elsewhere, even though the discussion of modes would flow logically from the discussion of phases and control variables.

The details of how a mechanical breath is organised have never appeared as a topic in the CICM Primary, nor has been listed in their syllabus, nor does it appear in the CICM WCA document “Ventilation”.  Fragments of this material have appeared in the Fellowship papers;  for example, Question 11.1 from the second paper of 2017 and in Question 18 from the second paper of 2015 either required the exploration of a phase variable (eg. ventilator triggering) or at least a vague understanding of what the terminology means.

In summary, a mechanical breath has four distinct phases, each of which has a governing variable which determines how that phase proceeds.

• Initiation phase, controlled by the trigger variable
• Inspiratory phase, controlled by the limit variable
• Cycling phase, controlled by the cycle variable
• Expiratory phase, governed by the PEEP variable

These concepts are best described with a diagram:

Phases of a mechanical breath

This system of separrating the mechanical breath into phases can apparently be traced back to William Mushin, who along with such rockstars as Mapleson (!) co-edited “Automatic Ventilation of the Lungs”, the Necronomicon for mechanical ventilation. The first edition came out in 1969. Apparently, they were not referred to as “phases” until Desautels (1985). In any case, this convention is a very convenient way to describe and classify mechanical  ventilation.

The initiation phase starts at the end of expiration. At this stage, the breath is "triggered" - initiated either by the machine or by the patient. The trigger variable controls how this occurs; the most common trigger variable settings are time, flow and pressure. There are advantages, disadvantages and specific applications for each method of breath triggering.

Inspiratory phase begins once the breath is triggered; the inspiratory flow begins. This phase is defined by airflow into the patient. The limit variable controls this phase by setting limits to the values each measured variable can attain during this phase (pressure, flow or volume). For example, in a continuous mandatory mode of ventilation, the patient's tidal volume is set at 500ml; the limit variable limits the volume so that each breath cannot exceed 500ml. Apart from volume, typical limit variables consist of pressure and flow.

Cycling phase is defined by the opening of the expiratory valve at the end of inspiration. This phase describes the brief instant during which airflow has ceased and expiration is about to begin. This phase is controlled by the cycling variable, which determines how and when the ventilator cycles from inspiration to expiration. Typical settings include time cycling, flow cycling and pressure cycling.

Expiratory phase begins when the ventilator cycles from inspiration to expiration; the expiratory valve opens, and the patient exhales passively. This phase is defined by airflow OUT of the patient. This phase is controlled by the PEEP variable.  PEEP is the pressure which remains after expiratory airflow has stopped; this pressure is generated by a continuous bias flow through the circuit and out through the expiratory solenoid valve.