Page 116 - Clinical Application of Mechanical Ventilation

82 Chapter 4

Learning Objectives

After studying this chapter and completing the review questions, the learner
should be able to:

Differentiate negative pressure ventilation and positive pressure ventilation.
Describe the characteristics and clinical application of the following terms or
modes of ventilation: spontaneous, positive end-expiratory pressure, continu-
ous positive airway pressure, bilevel positive airway pressure, controlled
mandatory ventilation, assist/control, intermittent mandatory ventilation, syn-
chronized intermittent mandatory ventilation, mandatory minute ventilation,
and pressure support ventilation.
Describe the characteristics and clinical application of the following terms
or modes of ventilation: adaptive support ventilation, proportional assist
ventilation, volume-assured pressure support, pressure-regulated volume
control, volume ventilation, pressure-controlled ventilation, airway pressure
release ventilation, biphasic positive airway pressure, inverse ratio ventila-
tion, pressure-controlled inverse ratio ventilation, automatic tube compensa-
tion, and neurally adjusted ventilator assist.

INTRODUCTION

This chapter provides an introduction to different operating modes available on most
mechanical ventilators. Since the information associated with mechanical ventilation
is immense, the reader should learn the operating modes and study them in the order
presented in this chapter. The definition and unique characteristics of each operating
mode are described here. The initiation and application of these operating controls
may be found in subsequent chapters.

NEGATIVE AND POSITIVE PRESSURE VENTILATION

Every ventilator must generate an inspiratory flow in order to deliver a tidal volume.
Mechanical ventilators Since gas flow requires a pressure gradient, a mechanical ventilator must produce
generate gas flow and volume
by creating either a negative a pressure gradient (i.e., pressure difference) between the airway opening and the
or positive pressure gradient. alveoli in order to produce inspiratory flow and volume delivery. The pressure gradi-
ent that must be generated between the airway opening and the alveoli is known as
the transairway pressure (Des Jardins, 2001).
Transairway Pressure (P ) 5 Pressure at Airway Opening (P ) 2 Alveolar
AO
TA
Pressure (P ALV )
At end-exhalation and prior to the beginning of inspiration, the pressures at
the airway opening and the alveoli are both equal to atmospheric pressure. Since these
two pressures are equal at this point, there is no pressure gradient and therefore no flow.

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