The main advantage of volume controlled ventilators is guaranteed minute ventilation. This is particularly important in the operating room, where lung compliance may be influenced by the type of surgery involved (abdominal or chest surgery), and in the ICU or in transit if patient’s tidal volumes are not being continuously monitored.
Early intensive care ventilators represented a continuation of operating room techniques, where the patient was heavily sedated and paralyzed until the disease process resolved. The problem, though, was how to get the patient off the ventilator before their muscles atrophied. This required some form of patient-ventilator interaction.
There is a considerable difference between mandatory and spontaneous breaths. In mandatory ventilation the patient is a passive object receiving gas as determined by the ventilator at a set rate and volume (or pressure). A spontaneously breathing (awake) individual demands gas at a flow and rate of their own choosing. Assisted ventilation thus requires a triggering device and a flow of gas to match the patient’s peak inspiratory demand (30 to 60 liters per minute). The two methods developed to overcome these problems were assist-control ventilation and intermittent mandatory ventilation.
Pressure Control refers to the type of breath delivered, not the mode of ventilation. Many different modes are pressure controlled (1). Conventionally, the term “pressure control” refers to an assist control mode (there is also a SIMV pressure control mode on some ventilators). In pressure control, a pressure limited breath is delivered at a set rate. The tidal volume is determined by the preset pressure limit. This is a peak pressure rather than a plateau pressure limit (easier to measure). The inspiratory time is also set by the operator. Again this is a trade off between short times with rapid inflow and outflow of gas, and long times with gas trapping. The flow waveform is always decelerating in pressure control: this relates to the mechanics of targeting airway pressure: flow slows as it reaches the pressure limit.
Gas flows into the chest along the pressure gradient. As the airway pressure rises with increasing alveolar volume the rate of flow drops off (as the pressure gradient narrows) until a point is reached when the delivered pressure equals the airway pressure: flow stops. The pressure is maintained for the duration of inspiration (2). Obviously, longer inspiratory times lead to higher mean airway pressures (the “i” time (Ti) is a pressure holding time after flow has stopped). The combination of decelerating flow and maintenance of airway pressure over time means that stiff, noncompliant lung units (long time constants) which are difficult to aerate are more likely to be inflated. Gas distribution in pressure control is like dropping a glass of water on the floor: the water trickles into every nook and cranny
It is known that decelerating flow patterns improve the distribution of ventilation in a lung with heterogeneous mechanical properties (as in acute lung injury) (4). Pressure control is also useful in patients whose airway cannot be fully sealed – children, patient with bronchopleural fistulae etc. The reason for this is that, although volume is lost through the leak, the ventilator will continue to attempt to pressurize the airway for the duration of the Ti: a constant flow pattern will be measured if the leak is large enough.
Patients can breath spontaneously on pressure control as long a the inspiratory time has not be unduly prolonged. The trigger mechanism is the same as in volume control. The key advantage of pressure targeted ventilation is unlimited flow in inspiration to satisfy the patient’s demands. The harder the patient draws in, the greater the pressure gradient, and the higher the flow.