Phase variables in mechanical ventilation are parameters which control the phases of a mechanical breath. Triggering controls the initiation of inspiration, cycling controls the initiation of expiration, and limits are set to maintain control over the three main parameters while inspiration is taking place. PEEP is also viewed as a phase variable, for lack of a better classification, and is the variable which reigns over the otherwise very boring expiratory phase. CICM have shown some interest in these matters over the years, and the Fellowship Exam papers have occasionally included questions about triggering and cycling (for example Question 11.1 from the second paper of 2017 and in Question 18 from the second paper of 2015). However, each variable is treated with sufficient attention in a dedicated chapter of its own. This chapter is something of an overview to lend some structure to this topic. Depending on time pressure, an exam candidate may find it easier to skim through the point-form summary.
- The trigger variable determines how and when the ventilator ends exhalation and commenced inhalation.
- The trigger setting can be time, flow, pressure or volume
- This variable determines whether a mode of ventilation is “mandatory” (machine triggered) or “spontaneous” (patient-triggered)
- The limit variable restrict the maximum value which the parameters can achieve during inspiration.
- Reaching the limit variable during inspiration does not abort the inspiratory phase
- This is distinct from the alarm variables, which are activated whenever their values are breached, and which abort inspiratory flow or open the expiratory valve
- Several limit variables can be selected simultaneously.
- The cycling variable is measured during the inspiratory phase; it is the mechanism used to end inspiration and commence expiration
- Time and flow are the most common settings for this varible, though volume-cycled and pressure-cycled ventilation is also possible
- Only one cycling variable can be set at any given time
- The PEEP variable is the pressure setting which determines the pressure maintained by the circuit bias flow during expiration.
The trigger variable determines when a breath is delivered. This variable distinguishes “mandatory” from “spontaneous” modes of ventilation; where “mandatory” refers to the fact that the ventilator decides when you take a breath, usually according to a timer. In the dark ages of critical care, this was the only sort of ventilation available to the slightly comatose patient.
The limit variable is the unimaginative name given to the limits of the mechanical breath. One usually has only minimal control over these, as they are integral parts of the selected mode of ventilation. In short, a limit variable is the maximum value a variable can attain during inspiration; and this is something distinct from the alarm settings, which can also be viewed as “limits”. Alarm settings tend to terminate the inspiration by opening the expiratory valve, whereas the main characteristic of the limit variable is that the inspiratory phase continues even after the limit has been reached.
Specifically, the term “limit variable” refers to the inspiratory phase: during inspiration, the ventilator won’t let that parameter (flow, volume, pressure, etc) get beyond its limit value. That is not to say that those parameters are completely ignored during the other phases: there are still limits in place but they fall into the territory of safety parameters. For example, let us take this pressure control mode of ventilation as depicted below.
The PEEP is set as 10 cm H2O, and the pressure control variable is 20 cm H2O. Thus, the pressure limit during inspiration is 30 cm H2O. The pressure will get no higher than this during inspiration. In addition to this limit, there is also as safety limit which is set as one of the ventilator alarms, which is (conventionally) set as 40 cm H2O. This is not unique to the inspiratory phase- breaching this limit during any phase of ventilation will abort the delivery of flow to prevent injury. This is clearly a confusing distinction: both values can be referred to as “limits”. Because of this, the ISO has moved away from referring to this phase variable as “limit” – they would prefer us to call it a “target” variable, whereas the term “limit” should be restricted to the abovementioned safety alarms.
Limit variables can include flow, volume and pressure. Time is obviously not a limit variable because it would defy logic. All limit variables can be active simultaneously, i.e. one can have a mechanical breath which is pressure-limited flow-limited and volume-limited.
The cycling variable determines how and when the ventilator transitions from inspiration to expiration. The ventilator measures this variable during the inspiration phase. The cycling variable is distinct from the limit variable, in that it ends the inspiratory phase when it is reached. When the set parameter for this variable is achieved, the ventilator opens the expiratory valve, and expiration may begin. Typical methods of ventilator breath cycling include:
Unlike the limit variable (of which several can be active simultaneously), there can only be one cycling variable. In general, time and flow are the most popular settings. Volume and pressure cycled ventilation is something of a historical footnote.
The expiratory phase in modern ventilators is generally a fairly passive and unexciting time, where the only thing happening is a slow bias gas flow seeping out of the circuit via the expiratory solenoid valve. This phase generally only has one variable applied to it, which is PEEP. In the absence of any pressure, one should probably refer to it as ZEEP (zero end-expiratory pressure). Historically, during the Dark Age of critical care many physicians held heretical beliefs regarding the need to actively assist patients with expiration, and so NEEP (negative end-expiratory pressure) was used as a means of promoting expiratory airflow. A representative example from that era is this article by Hill et al (1965), instructing people on the correct use of a -4 cm H2O NEEP for post-operative cardiac surgery patients via the Engstrom respirator. Around the 1960s-1970s people finally realised that this practice was insane, and ventilator manufacturers stopped incorporating sub-ambient pressure into their equipment.
The control variables are the independent limit variables in a mode of ventilation. In essence, the control variable is the constant to which all other variables are enslaved. There are only two possible control variables: pressure and flow. These are discussed in another chapter because they seemed like something fundamentally important and therefore deserving of a separate page.