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Alternative modes of mechanical ventilation: A review for the hospitalist

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ABSTRACTNewer ventilators can be set to modes other than the pressure-control and volume-control modes of older machines. In this paper, the authors review several of these alternative modes (adaptive pressure control, adaptive support ventilation, proportional assist ventilation, airway pressure-release ventilation, biphasic positive airway pressure, and high-frequency oscillatory ventilation), explaining how they work and contrasting their theoretical benefits and the actual evidence of benefit.

KEY POINTS

  • The alternative modes of ventilation were developed to prevent lung injury and asynchrony, promote better oxygenation and faster weaning, and be easier to use. However, evidence of their benefit is scant.
  • Until now, we have lacked a standard nomenclature for mechanical ventilation, leading to confusion.
  • Regardless of the mode used, the goals are to avoid lung injury, keep the patient comfortable, and wean the patient from mechanical ventilation as soon as possible.


 

References

Technologic advances and computerized control of mechanical ventilators have made it possible to deliver ventilatory assistance in new modes. Driving these innovations is the desire to prevent ventilator-induced lung injury, improve patient comfort, and liberate the patient from mechanical ventilation as soon as possible.

We call these innovations “alternative” modes to differentiate them from the plain volume-control and pressure-control modes. Some clinicians rarely use these new modes, but in some medical centers they have become the most common ones used, or are being used unknowingly (the operator misunderstands the mode name). The information we provide on these modes of ventilation is by no means an endorsement of their use, but rather a tool to help the clinician understand their physiologic, theoretical, and clinical effects.

We focused on two goals:

  • Explain what the mode does
  • Briefly review the theoretical benefits and the actual evidence supporting these alternative modes of ventilation.

STANDARD NOMENCLATURE NEEDED

Since its invention, mechanical ventilation has been plagued by multiple names being used to describe the same things. For example, volume-control ventilation is also called volume-cycled ventilation, assist-control ventilation, volume-limited ventilation, and controlled mechanical ventilation. Similarly, multiple abbreviations are used, each depending on the brand of ventilator, and new acronyms have been added in recent years as new modes have been developed. The vast number of names and modes can confuse even the most seasoned critical care physician.

Efforts to establish a common nomenclature are under way.1

WHAT IS A MODE?

A mode of mechanical ventilation has three essential components:

  • The control variable
  • The breath sequence
  • The targeting scheme.

Similar modes may require more detailed descriptions to distinguish them, but the basic function can be explained by these three components.

The control variable

In general, inspiration is an active process, driven by the patient’s effort, the ventilator, or both, while expiration is passive. For simplicity, in this article a mechanical breath means the inspiratory phase of the breath.

The machine can only control the volume (and flow) or the pressure given. The breaths can be further described on the basis of what triggers the breath, what limits it (the maximum value of a control variable), and what ends (cycles) it.

Figure 1. Volume control (top) and pressure control (bottom) are modes of continuous mandatory ventilation. Each mode is depicted as patient effort increases. Notice that the mode’s control variable (volume or pressure) remains constant as patient effort increases. Contrast these findings with those in Figure 2.

Therefore, a volume-controlled breath is triggered by the patient or by the machine, limited by flow, and cycled by volume (Figure 1). A pressure-controlled breath is triggered by the patient or the machine, limited by pressure, and cycled by time or flow (Figure 1).

The breath sequence

There are three possible breath sequences:

  • Continuous mandatory ventilation, in which all breaths are controlled by the machine (but can be triggered by the patient)
  • Intermittent mandatory ventilation, in which the patient can take spontaneous breaths between mandatory breaths
  • Continuous spontaneous ventilation, in which all breaths are spontaneous (Table 1).

The targeting scheme

The targeting or feedback scheme refers to the ventilator settings and programming that dictate its response to the patient’s lung compliance, lung resistance, and respiratory effort. The regulation can be as simple as controlling the pressure in pressure-control mode, or it can be based on a complicated algorithm.

In the sections that follow, we describe some of the available alternative modes of mechanical ventilation. We will explain only the targeting schemes in the modes reviewed (Table 1, Table 2), but more information on other targeting schemes can be found elsewhere.1,2 We will focus on evidence generated in adult patients receiving invasive mechanical ventilation.

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