The first generation of positive-pressure mechanical ventilators were simple high-pressure gas regulators on which clinicians could set the circuit pressure and the breathing frequency. In the middle of the twentieth century, more sophisticated devices appeared that allowed direct clinician control of flow and volume along with breath timing and expiratory pressure. As ventilator design improved and microprocessors became available, feedback mechanisms appeared that could provide automatic adjustments in these set variables depending upon a variety of conditions.1 A simple example was the use of a patient-effort sensor to adjust the number of mechanical breaths provided during assist-control modes or synchronized intermittent mandatory ventilation.2–4 A variation on this breath rate feedback mechanism was mandatory (or minimum) minute ventilation, which used minute ventilation to adjust the number of positive-pressure breaths delivered.5 At the same time, the development of flow control valves that could be adjusted based on a clinician-selected airway-pressure target appeared.6–9 This gave clinicians the choice of using either set flow-volume-targeted modes (volume assist-control ventilation; volume-targeted synchronized intermittent mandatory ventilation [volume SIMV]) or pressure-targeted modes (pressure-targeted assist-control ventilation [PACV]; pressure support; pressure SIMV). Taken together, these flow-targeted, volume-targeted, and pressure-targeted strategies comprise what is commonly referred to today as “conventional” mechanical ventilation.
In the late twentieth and early twenty-first century, increasingly complex and clever feedback mechanisms for these conventional breaths have been introduced. The behavior of these features is made more understandable if one considers that all positive-pressure breaths can be described by three variables: the trigger variable (what initiates a breath—usually an effort sensor or a machine timer); the target variable (what governs gas delivery—usually either a set flow or a set pressure target); and the cycle variable (what terminates the breath—usually a set time, flow, or volume).6,8 These newer feedback systems are designed to adjust one or more of these breath-delivery variables based on prescribed algorithms to provide more physiologically targeted and patient interactive ventilatory support. In addition, there has also been the development of feedback mechanisms to adjust the fractional inspired oxygen concentration (FIO2) and positive end-expiratory pressure (PEEP). These feedback mechanisms are often considered “partial” closed-loop conventional mechanical ventilation. The clinician, however, cannot expect these features to provide safe and effective support automatically. Rather, the clinician must understand their rationale, design principles, lung-protective targets, and evidence-based outcomes to apply them properly. These are the focus of the remainder of this chapter.
The advantage of a flow-targeted, volume-cycled breath is that a guaranteed volume is delivered with every breath (even though applied airway pressures may change). The advantages of a pressure-targeted breath (either time- or flow-cycled) are that an airway pressure limit is guaranteed (even though volumes may change), that the rapid initial flow may enhance gas mixing, and that the variable-flow pattern may enhance patient–ventilator synchrony.7,10,11
Over the last two decades, a number of engineering ...