Does ventilator mode matter?

A pilot trial sets the stage to make pulmonologists interview for their jobs

Modern ventilators are complicated machines, but at its most basic level, a ventilator is simply a pump that generates pressure to push air (a mixture of gases) through a closed circuit at regular intervals.

The gas mixture behaves according to the ideal gas law:

PV = nRT

Or, [pressure multiplied by volume] equals [n, the amount of gas in moles] x [R, the universal gas constant] x [T, the gas’s temperature].

But because temperature and R are generally constant, inside closed loops (like a ventilator circuit) Boyle’s law suffices for most purposes:

PV = constant

What that means in practical terms can be illustrated by an example.

Imagine a man—let’s call him Robbie—blows up a balloon with exactly eight liters of air. It looks like this:

The pressure is measured without releasing any air. It’s 75 cm H2O over atmospheric pressure.

Now imagine that Robbie is worried the balloon might pop at higher pressures than that. He blows up another balloon, but this time, he carefully inflates it with a volume that reaches precisely 75 cm H2O over atmospheric pressure.

When the balloon reaches 75 cm H2O, Robbie has blown exactly eight liters, and it looks like this:

Volume Control ≈ Pressure Control Mode

This oversimplification captures the fundamental interdependence of volume and pressure, and by extension, the relative interchangeability of ventilator modes.

• In volume control mode, the tidal volume is set, and as the ventilator pumps that quantity of air in against resistance, it generates a certain pressure on the airways and alveoli.

• In pressure control mode, the inspiratory pressure (Pinsp) is set, which (because it’s greater than PEEP plus the resistance in the circuit plus the stiffness i.e. elastance of the lungs) causes a certain volume of gas to flow through the circuit, most of which inflates the lungs (i.e. the tidal volume).

Just like in our balloon example above,

• If in volume control mode a set 500 mL tidal volume with 5 cm H2O of PEEP is producing 20 cm H2O peak inspiratory pressure,

• Switching to pressure control mode with an inspiratory pressure of 15 cm H2O (over PEEP of 5 cm H2O) will deliver ~500 mL tidal volume and a peak inspiratory pressure of 20 cm H2O.

Yes, it’s more complicated than that (e.g., rate and inspiratory time also affect pressure and volume, and flow rates vary between the modes), but the gas law guarantees the fundamental equivalence of these two (and all) ventilator modes.

Of course, volume and pressure are uniformly interchangeable and interdependent only in simple systems like balloons. Injured alveoli are heterogeneous and variably sensitive to regional increases in pressure/volume.

Neither pressure control nor volume control have an inherent advantage in reducing ventilator-induced lung injury. But pressure control is regarded by some as safer for injured lungs, due to its limits on peak pressures and decelerating flow rate throughout inspiration.

The main pitfall of pressure control ventilation is that if lung compliance falls or airway resistance increases, the constant set inspiratory pressure becomes inadequate (as per the gas law) to produce airflow. Tidal volumes and gas exchange fall, often without immediate recognition by care teams, creating risk.

Ventilator companies have responded with “smart” ventilators that hybridize pressure control and volume control ventilation.

Adaptive Pressure Control ≈ Pressure-Regulated Volume Control ≠ Adaptive Support Ventilation

Adaptive Pressure Control and Pressure-Regulated Volume Control

In adaptive pressure control, the clinician sets a target tidal volume or minute ventilation. The vent measures expired tidal volume and adjusts inspiratory pressure on a breath-by-breath basis to achieve the target tidal volume.

Pressure-regulated volume control is another way of describing this process (semantically substituting volume for pressure, as specified by the gas law), and these two ventilator modes are essentially interchangeable.

Other names for this are volume control-plus or VC+, volume-targeted pressure control, or pressure-controlled volume guarantee.

Adaptive Support Ventilation

Adaptive support ventilation is substantially different. ASV is a complex, proprietary, more autonomous mode in which the desired “percent minute ventilation” is entered, and the vent dynamically adjusts pressure (and therefore volume) and respiratory rate to achieve that target. The “percent minute ventilation” target is calculated by the ventilator algorithm, not the clinician (who can dial it up or down).

Vents with this mode installed are marketed as supporting weaning by automatically reducing support as the patient’s spontaneous effort increases. (There is no good evidence that this is the case.)

In all “smart” modes, their autonomy permits them to deviate from set volumes and pressures, sometimes substantially. This produces a risk for ventilator-induced lung injury, but the magnitude or significance of this risk is unknown.