Pathophysiology of Ventilator Dependence

The most common causes of failure to wean include chronic obstructive pulmonary disease (COPD) exacerbations, neuromuscular diseases, h) oxic respiratory failure, post surgical complications (2), and heart failure. Weaning from the tracheostomy must consider the balance of respiratory muscle function and work of breathing. The work of breathing is determined by ventilatory demand, compliance of the lungs and chest wall, airway resistance, and intrinsic positive end-expiratory pressure (PEEPi). Adequacy of ventilatory drive and neuromechanical output can be assessed from the respiratory rate, airway occlusion pressure at 100 milliseconds (Po.i), maximum inspiratory pressure (MIP), and maximum voluntary ventilation (MW). 

Chest wall compliance may be reduced in kyphoscoliosis, fibrothorax, or spinal cord injury and lung compliance may be reduced in pulmonary edema, pulmonary fibrosis, and acute respiratory distress syndrome (ARDS) and COPD in the presence of hyperinflation. Airway secretions or bronchoconstriction may contribute to increased airway resistance. Respiratory drive and muscle function may be compromised by anesthetics, sedation, coma, or hypercapnia, and muscle dysfunction may occur in the presence of malnutrition, hypophosphatemia, disuse atrophy, sepsis, myopathies, or limited oxygen delivery (9). The factors that led to a tracheostomy must be optimized prior to decannulation. 

There is an important distinction between dependence on an artificial airway and mechanical ventilation, which can be provided noninvasively (9). The requirement for an artificial airway may reflect bulbar impairment as, in those with adequate bulbar function, noninvasive ventilation will sustain adequate ventilation even with veiy limited respiratory muscle function. Therefore, tracheostomized patients with preserved bulbar control can undergo decannulation. Airway secretions are important determinants of dependence on mechanical ventilation through an artificial airway, and aspiration pneumonia may result from an impaired level of consciousness, poor bulbar function, or inability to cough effectively. Such issues must be addressed by airway clearance techniques, prior to decannulation. 

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In this chapter we will discuss recommendations from the hterature regarding dec-aruiulation as well as our personal clinical experience. We will comment on the pathophysiology of ventilator dependence, the determination of candidates for weaning from ventilation and tracheostomy, and a stepwise approach to decannulation. Lastly, we will discuss the choices of noninvasive ventilatory supports and techniques that clinicians may utilize, such as lung volume recruitment (LVR), assisted coughing and mechanical airway clearance. 

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