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Introduction

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Objectives

  1. Describe causes of trigger asynchrony.

  2. Explain how intrinsic positive end-expiratory pressure can result in a failure to trigger.

  3. Describe flow asynchrony with volume control, pressure control, and pressure support ventilation.

  4. Describe cycle asynchrony with volume control, pressure control, and pressure support ventilation.

  5. Describe how the ventilator mode can contribute to asynchrony.

  6. Discuss approaches to remedy asynchrony.

  7. Assess dyspnea in mechanically ventilated patients.

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Asynchrony is a mismatch between the neural respiratory drive of the patient and the ventilator response. It is relatively common and has been associated with a longer stay on mechanical ventilation, although cause and effect has not been established. To the extent that asynchrony is stressful for the patient, it is generally agreed that good patient-ventilator synchrony is desirable. How a patient interacts with the ventilator is determined by many factors (Figure 15-1). These include the underlying disease process, the effects of therapeutic interventions, ventilator performance, and how the clinician sets the ventilator. In this chapter, the causes of asynchrony and appropriate clinical responses are described.

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Figure 15-1

Schematic representation of factors that influence patient-ventilator interaction. (Reproduced with permission from Pierson DJ. Patient-ventilator interaction. Respir Care. 2011; Feb; 56(2):214-228.)

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Trigger Asynchrony

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Trigger asynchrony occurs when the initiation of the inspiratory phase does not occur with the onset of the patient's inspiratory effort. In other words, there is a lack of synchrony between the onset of neural inspiration and the response of the ventilator. It can occur either because the ventilator autotriggers or because the patient has difficulty triggering the ventilator. The ventilator trigger sensitivity should be set as sensitive as possible without causing autotriggering. Although flow triggering is commonly used, there is little difference between flow triggering and pressure triggering on modern ventilators.

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Autotriggering causes the ventilator to trigger in response to an artifact. One such artifact is cardiac oscillations, in which the heart beating against the lungs produces sufficient flow or pressure change at the proximal airway to trigger the ventilator (Figure 15-2). This is addressed by adjusting the trigger sensitivity. Other causes of autotriggering include excessive water condensation in the ventilator circuit and leaks in the circuit. This is addressed by draining water from the circuit and correcting the leak. Leak compensation is useful during noninvasive ventilation to minimize autotriggering.

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Figure 15-2

(A) Cardiac oscillations triggering the ventilator at a rate of 24 breaths/min when the flow trigger is set at 2 L/min. (B) After changing the flow trigger to 8 L/min, the ventilator rate is 16 breaths/min, which is what is set on the ventilator. The cardiac oscillations are producing a flow of 4 to 6 L/min at the proximal airway.

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