Control variables: volume and pressure

Control variables in mechanical ventilation are independent parameters which are targeted by the ventilator mechanism, and upon which all other variables are dependent. Along with phase variables, these are among the most important characteristics of a mode of ventilation. In terms of exam relevance, CICM have never asked about this topic directly, perhaps because it is so foundational that all trainees are assumed to have an intuitive grasp of it.

In short:

Definition of the “control” variable

The word "control" here implies that the person setting the ventilator is most interested in that specific variable, and wants to control that variable exclusively (i.e. without letting the respiratory system make any decisions about it). Pilbeams’ Mechanical Ventilation defines the control variable as “the primary variable the ventilator adjusts to achieve inspiration”. A better way to define it practically is probably the way it is done by Garnero et al (2013):

            “the variable which the ventilator uses as feedback signal for controlling inspiration”

This definition has a certain pragmatic appeal. Consider that the ventilator is usually a flow and pressure regulator. The flow and pressure are measured continuously, at millisecond increments. The valves which control this pressure are constantly receiving feedback from the sensors, and in this fashion, the desired variable (pressure or volume) can be achieved and maintained as prescribed.

From a scientific and theoretical standpoint, the most accurate definition is probably the one offered in the  Chatburn chapter from Tobins (3rd ed), which defines the control variable in terms of the equation of motion:

Thus, if one of these variables is pre-determined, it becomes the independent variable in the equation, making all the other variables dependent upon it. Volume and pressure are the only variables which can be pre-determined in this way.  Flow cannot be a control variable because flow is volume over time, and thus when volume is controlled, flow is controlled indirectly.

Extending the metaphor for control brings with it the question, why would one choose one specific control variable over another? Under which circumstances would one want to ventilate the patient with a pressure controlled mode, instead of volume, and vice versa? This is actually a fundamentally important question which often determines the choice of ventilator mode. Modes of ventilation are usually defined primarily by how they handle the control variable.

Advantages and disadvantages of pressure and volume control strategies

In brief, the main differences between these control variables are seen in the following domains:

• Reliability of minute volume achieved
• Mean airway pressure achieved
• Recruitment achieved
• Patient comfort and synchrony

It is important to make sure one is clear about the separate discussion of pressure as the control variable from the discussion of the pressure control mode of ventilation (usually abbreviated as PCV) which also has several other characteristics. PCV tends to be created with a square pressure waveform in mind, which has several advantages.  For example, the mean airway pressure is higher, the inspiratory flow rate is higher, and the risk of barotrauma due to inadvertent overdistension is also higher. However, it also can be challenging to achieve the desired minute volume, particularly if the respiratory system characteristics change erratically. In any case, all of these are actually advantages and disadvantages of PCV, not of the pressure control variable. As such, advantages and disadvantages of each mode of ventilation will be discussed elsewhere. Here it will suffice to say that controlling the pressure variable is beneficial because it maintains a stable pressure in the face of fluctuating respiratory performance,  which prevents lung injury from excess pressure. Similarly, using volume as the control variable gives one a more stable minute volume, which keeps the PaCO2 at the desired level, but which may play havoc with respiratory system pressures.