New dual ventilator modes: are we ready to perform large clinical trials?

Mechanical ventilation is usually achieved using either volume-controlled or pressure-controlled ventilation.1,2 Moreover, it can be applied as full ventilatory support, for which the total work of breathing (total WOB) is performed by the ventilator, without any active participation of the patient, or with a partial support, for which the total WOB is shared by the patient (patient WOB) and the ventilator (ventilator WOB), depending on the ventilator settings and the respiratory muscle capacity of the patient. Briefly, 4 key variables will influence the quality of the assistance delivered by the ventilator: volume-control versus pressure-control, and full support versus partial support. The clinician should choose between these variables to find the best combination for each patient. With volume control, a set tidal volume (VT) is delivered regardless of the respiratory-system compliance and airways resistance, but the airway pressure varies with respiratory system mechanics and patient effort. Therefore there is a risk of inducing barotrauma, such as pneumothorax, with high delivered pressures. With volume control, pressure will increase with worsening lung mechanics; this increase in pressure is not desirable. With volume control, increased respiratory drive (eg, ventilatory demand increase) can result in asynchrony, because flow is fixed. On the contrary, with pressure control without spontaneous breathing, the airway pressure is fixed, which reduces the risk of barotrauma, but the VT is variable. With pressure control, VT will decrease with worsening lung mechanics; this could cause hypoventilation, which may lead to alveolar hypoventilation and severe respiratory acidosis. With pressure control and spontaneous breathing, increased respiratory drive will result in an increase in VT. This may improve patient-ventilator synchrony but put the patient at increased risk for over-distention lung injury. Not surprisingly, the question, “Which is better: pressure or volume?” is often asked, and sometimes even expert physicians have difficulty answering. Obviously, the answer is delicate, because each mode has some advantages and some drawbacks. Probably part of the answer may be: “You should use the mode that you and your staff know well and have the most experience with. As such, it is probably the safer mode for your patient.” Indeed, “You can kill a lung with pressure mode and you can kill a lung with volume mode.”1 Probably the applied settings, monitoring, and skills of the clinician are more important factors than the ventilator type and the mode used.