With volume control ventilation (VCV), the inflation (tidal) volume is preselected, and the lungs are inflated at a constant flow rate until the desired volume is delivered. The inspiratory flow rate is adjusted so that the
time for lung inflation is no more than one-third of the respiratory cycle (i.e., I:E ratio of 1:2).

With pressure control ventilation (PCV), the inflation pressure is preselected, and high flow rates are used at the onset of lung inflation to achieve the desired inflation pressure quickly. The flow rate decelerates during lung inflation, and the inspiratory time is adjusted to allow the flow rate to fall to zero at the end of inspiration.

With VCV, the airway pressure at the end of inspiration (peak pressure) is the pressure needed to overcome both airway resistance, and the elastic recoil force of the lungs and chest wall. These two components can be separated by briefly holding the inflation volume in the lungs, as demonstrated in Figure 19.2.

With PCV, there is no airflow at the end of the inspiration, so the airway pressure at end-inspiration is equivalent to the alveolar pressure (end-inspiratory Paw = Palv).

It is the elastic recoil pressure of the lungs and chest wall (Pel in Figure 19.2); as such, it can be used to compute the compliance (C) of the thorax (lungs and chest wall) at a particular tidal volume (V_T); i.e.,

It is a reflection of the stress imposed on the walls of the alveoli by the inflation (tidal) volume. An increase in the end-inspiratory alveolar pressure to >30 cm H₂O creates a risk for stress fractures in the alveolar-capillary interface, which results in ventilator-induced lung injury (described in Chapter 17, Section II-A) (2,4). Alveolar injury from overdistension is called volutrauma.

It is a reflection of the tendency for overt alveolar rupture, with escape of air into the lung parenchyma or pleural space (i.e., barotrauma).

One advantage of VCV is the ability to maintain a constant level of alveolar ventilation, despite changes in the mechanical properties of the lungs. With PCV, alveolar ventilation will decrease if there is an increase in airways resistance (e.g., from secretions) or a decrease in lung compliance (e.g., from atelectasis or worsening of infiltrative lung disease).

Another advantage of VCV is the ability to use the lung protective ventilation protocol (see later).

A major advantage of PCV is patient comfort, which promotes synchronous breathing with the ventilator and reduces the work of breathing (5). This has been attributed to the high initial flow rates used during PCV (which are more likely to match the high flow demands of patients with respiratory failure), and the decelerating flow pattern (which promotes more even ventilation of the distal airspaces). A decelerating flow pattern is available for VCV, and has been shown to improve patient comfort (6).

Another stated advantage of PCV is the lower peak airway pressures. However, as shown in Figure 19.1, the end-inspiratory alveolar pressure is the same with PCV and VCV (at the same tidal volume), so the lower peak airway pressures with PCV do not reduce the risk of alveolar overdistension and lung injury. This only occurs when the tidal volume is reduced during PCV.

The ventilator breath on the left begins with a negative pressure deflection, which represents a spontaneous inspiratory effort by the patient. This is a patient-triggered ventilator breath.

The ventilator breath on the right does not begin with a negative pressure deflection, indicating the absence of a spontaneous inspiratory effort by the patient. This is a time-triggered ventilator breath, which is delivered at a preselected rate.