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INTRODUCTION

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Mechanical ventilation is provided to ensure adequate oxygenation and minute ventilation. The means to achieve these goals will vary depending upon the lung pathophysiology. Indications to improve oxygenation include pneumonia, pulmonary edema, and acute respiratory distress syndrome. Clinical situations requiring maintenance of minute ventilation include postoperative patients, asthma, and bronchiolitis. Several modes of mechanical ventilation are available to optimize patient recovery by normalizing pulmonary gas exchange while minimizing lung disease.

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CONVENTIONAL MECHANICAL VENTILATION (SEE TABLE 24-1)

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  • There are various ways to deliver a tidal volume to the patient, but understanding the basic terminology helps to understand the benefits or pitfalls of a particular mode.

    • Breaths are either mandatory and triggered by the machine (control mode) or on demand and triggered by the patient (assist mode).

    • Variables determining the breath

      • Trigger: the variable starting the breath. In a spontaneously breathing patient, a change in flow or pressure with an effort can trigger a breath; if the patient becomes apneic, the machine can deliver the breath based on a timed interval.

      • Cycle: the variable stopping the breath. In a volume mode of ventilation when the volume is delivered, the inspiratory phase will stop—it is volume cycled. In a pressure mode, the time the pressure is maintained will cycle the breath off—it is time cycled. The pressure support mode is flow cycled.

      • Limit: the goal of the delivered gas flow, either volume or pressure.

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VOLUME VENTILATION

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  • The primary input is volume to be delivered from the ventilator. Airway and alveolar pressures generated as a result of the volume delivered will vary depending upon the resistance to flow and the compliance of the respiratory system.

    • SIMV: synchronized intermittent mandatory ventilation (flow/pressure initiated, volume limited, volume/time cycled)

      • The volume set can be delivered in synchrony with a patient effort, decreasing the work of breathing.

      • With apnea, the rate set and the volume delivered will result in the minute ventilation the patient receives.

    • Assist control (flow/pressure/time initiated, volume limited, volume/time cycled)

      • Volumes delivered are consistent and synchronized with patient effort.

      • If there is no patient effort, the rate set and volume delivered determine the minute ventilation.

      • In contrast to SIMV, every patient effort results in the entire delivered tidal volume with each breath.

    • IMV: intermittent mandatory ventilation (time initiated, volume limited, volume/time cycled)

      • Rarely used pediatric mode delivering a volume of breath regardless of patient effort.

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TABLE 24-1

Ventilator Modes

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