Chapter 8: Advanced Modes of Mechanical Ventilation

Introduction

With each generation of ventilators, new modes and variations on previous modes become available. There now exist numerous ventilator modes from a variety of manufacturers. The purpose of this chapter is to describe the technical and clinical aspects of advanced modes of ventilation that have recently become available. Although heavily promoted by their manufacturers, the clinical role of many of these modes remains unproven. Use of these modes is often based upon their availability and clinician's bias, rather than evidence that they are superior to traditional modes.

Dual-Control Modes

With dual-control modes, the ventilator can automatically switch between pressure control and volume control during a single breath. However, it is important to remember that the ventilator is controlling only pressure or volume at any given time, not both at the same time.

The proposed advantage of this mode is a reduced work-of-breathing (WOB) while maintaining a minimum minute volume and tidal volume (V_T). This approach operates during mandatory breaths or pressure-supported breaths to combine the high initial flow of a pressure-controlled breath with the constant volume delivery of a volume-controlled breath. Names for this approach are volume-assured pressure support, pressure augmentation, and machine volume. These modes are not commonly available on the newest generation of ICU ventilators.

The breath is patient or ventilator-triggered. The ventilator then attempts to reach the pressure setting as quickly as possible. This portion of the breath is pressure-controlled and associated with a variable flow. As this pressure is reached, the ventilator determines the volume that has been delivered and determines if the desired V_T will be delivered. If the delivered and set V_T are equal, the breath is a pressure support breath (Figure 8-1). If the patient's inspiratory effort is low, the breath changes from a pressure-controlled to a volume-controlled breath. Flow remains constant until the tidal volume has been delivered. During this time the pressure rises above the pressure setting. If pressure reaches the high-pressure alarm setting, the breath is pressure-cycled. The ventilator can allow the patient a V_T larger than that set.

Volume control on the Maquet Servo-i is an example of volume-controlled ventilation (VCV) with a dual-targeting scheme. Each breath begins as VCV, but if the patient makes an inspiratory effort sufficient to decrease airway pressure by 3 cm H₂O, ...