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Objectives

  1. Describe causes of trigger asynchrony.

  2. Explain how intrinsic positive end-expiratory pressure can result in trigger asynchrony.

  3. Describe double triggering and reverse triggering.

  4. Describe cycle asynchrony with volume-controlled ventilation (VCV), pressure-controlled ventilation (PCV), and pressure support ventilation (PSV).

  5. Describe patient-ventilator flow mismatch with VCV, PCV, and PSV.

  6. Describe how the ventilator mode can contribute to patient-ventilator interactions.

  7. Discuss approaches to improve patient-ventilator interactions.

  8. Assess dyspnea in mechanically ventilated patients.

Introduction

Asynchrony is a mismatch between the neural respiratory drive of the patient and the ventilator response. It is common in all ventilated patients, but its extent varies considerably and has been associated with a longer stay on mechanical ventilation, longer intensive care unit (ICU) stay, and increased mortality. However, cause and effect have not been established. Some types of asynchrony are injurious, whereas others may have limited impact on patient status. It is generally agreed that good patient-ventilator interaction is always 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, patient ventilator interactions are described.

Figure 15-1

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

Asynchrony

Trigger Asynchrony

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 auto-triggers or because the patient has difficulty triggering the ventilator. The ventilator trigger sensitivity should be set as sensitive as possible without causing auto-triggering. Although flow trigger is commonly used, there is little difference between flow trigger and pressure trigger on modern ventilators.

Auto-trigger causes the ventilator to trigger in response to an artifact rather than the inspiratory effort of the patient. 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 auto-triggering include excessive water condensation in the ventilator circuit, leaks in the circuit, or leaks through a bronchopleural fistula. This is addressed by draining water from the circuit and correcting the leak. Leak compensation is useful during noninvasive ventilation to minimize auto-triggering due to leaks around the interface.

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, ...

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