April 2002 / Volume 47 / Number 4 / Page 416
Pressure-Controlled Versus Volume-Controlled Ventilation: Does It Matter?
IntroductionVolume-controlled ventilation (VCV) and pressure-controlled ventilation (PCV) are not different ventilatory modes, but are different control variables within a mode. Just as the debate over the optimal ventilatory mode continues, so too does the debate over the optimal control variable. VCV offers the safety of a pre-set tidal volume and minute ventilation but requires the clinician to appropriately set the inspiratory flow, flow waveform, and inspiratory time. During VCV, airway pressure increases in response to reduced compliance, increased resistance, or active exhalation and may increase the risk of ventilator-induced lung injury. PCV, by design, limits the maximum airway pressure delivered to the lung, but may result in variable tidal and minute volume. During PCV the clinician should titrate the inspiratory pressure to the measured tidal volume, but the inspiratory flow and flow waveform are determined by the ventilator as it attempts to maintain a square inspiratory pressure profile. Most studies comparing the effects of VCV and PCV were not well controlled or designed and offer little to our understanding of when and how to use each control variable. Any benefit associated with PCV with respect to ventilatory variables and gas exchange probably results from the associated decelerating-flow waveform available during VCV on many ventilators. Further, the beneficial characteristics of both VCV and PCV may be combined in so-called dual-control modes, which are volume-targeted, pressure-limited, and time-cycled. PCV offers no advantage over VCV in patients who are not breathing spontaneously, especially when decelerating flow is available during VCV. PCV may offer lower work of breathing and improved comfort for patients with increased and variable respiratory demand. Key words: mechanical ventilation, pressure control, volume control.
Definitions and Technical Descriptions
Other Forms of Pressure-Controlled Ventilation
Volume Control Versus Pressure Control: Does It Matter?
Patient Work of Breathing and Comfort
The goal of mechanical ventilation is to provide or improve ventilation, oxygenation, lung mechanics, and patient comfort while minimizing any associated complications. The earliest mechanical ventilators used on humans were pressure controllers. They were not truly pressure-limited; rather they were pressure-cycled, terminating the inspiratory phase when a set pressure was achieved. Pressure pre-set ventilation fell from favor, however, because of the inability to monitor delivered tidal volume and control minute ventilation. In an effort to overcome those limitations, new ventilators were developed that used volume-control. This allowed clinicians better control and regulation of both delivered tidal volume and control minute ventilation. There was renewed interest in the pressure-limited approach in the early 1980s for the management of severe respiratory distress. Priority was placed on limiting high inflation pressures, which were known to cause or worsen lung injury. Debate over the most efficient and safest control mode has continued ever since, often reaching religious fervor.
This review offers detailed definitions and descriptions of the various modes of volume-controlled and pressure-controlled ventilation. Proposed advantages and disadvantages of each approach to ventilatory support are compared, and evidence from the available literature is considered.