Discuss the gas delivery patterns during volume-controlled ventilation (VCV) and pressure-controlled ventilation (PCV).
Describe the effect of varying rise time and inspiratory cycle criteria during pressure support ventilation and PCV.
Describe how an end-inspiratory plateau can be achieved with PCV.
Describe approaches to monitor gas delivery during PCV and VCV.
Contrast the advantages and disadvantages of PCV and VCV.
Controversy has always followed the introduction of new modes of ventilation. In the late 1970s it was assist/control (continuous mandatory ventilation [CMV]) versus intermittent mandatory ventilation (IMV). In the mid- to late-1980s, it was IMV versus pressure support ventilation. A debate today is whether gas delivery should be volume-controlled or pressure-controlled.
All of these first-generation ICU ventilators only provided VCV, and until the 1970s, it was without the option for patient-triggered breaths. Pressure-limited ventilators (eg, Bird and Puritan-Bennett machines) have been available since the 1950s, but they were not designed for continuous ventilatory support.
With VCV, the variable that is constant during each breath is tidal volume. With this approach, there is a variable inspiratory pressure. With changes in respiratory mechanics (eg, resistance, compliance) and patient's effort, airway pressure varies because volume delivery is constant. With VCV, the clinician sets tidal volume (VT), flow pattern, peak inspiratory flow, rate, and trigger sensitivity. In some ventilators, inspiratory time, minute volume, and I:E ratio are set instead of VT and flow. In other ventilators, both inspiratory time and flow are set; if the inspiratory time is longer than the time required to deliver the VT, an end-inspiratory pause will result. In practice, modern ventilators provide VCV by controlling the inspiratory flow. VCV can be applied as CMV or as synchronized IMV (CMV-IMV).
With pressure-controlled ventilation (PCV), a fixed pressure is applied during the inspiratory phase. In addition, inspiratory time, or I:E ratio, and trigger sensitivity are set. As respiratory mechanics (eg, resistance, compliance) and patient's effort change, VT must vary because pressure is constant. As noted in Table 7-1, the primary difference between these VCV and PCV is a fixed VT or a fixed peak inspiratory pressure (PIP), respectively. In newer generation ventilators, the clinician may also set the rise time, which is the time required for the set pressure to be reached. This occurs by varying the slope of the flow increase from baseline to peak flow.
Table 7-1PCV Versus VCV |Favorite Table|Download (.pdf) Table 7-1 PCV Versus VCV
| ||PCV ||VCV |
|VT ||Variable ||Constant |
|PIP ||Constant ||Variable |
|Pplat ||Constant ||Variable |
|Flow pattern ||Variable ||Set |
|Peak flow ||Variable ||Set |
|Inspiratory time ||Set ||Set |
|Minimum rate ||Set ||Set |
Pressure Support Ventilation
Pressure support ventilation (PSV) is ...