Mechanical ventilation Ventilatory support

Noninvasive positive-pressure ventilation (NPPV) provides ventilatory support with oxygen and pressurized airflow using a face or nasal mask with a tight seal but without endotracheal intubation. In patients with acute respiratory failure due to COPD exacerbations, NPPV was associated with lower mortality, lower intubation rates, shorter hospital stays, and greater improvements in serum pH in 1 hour compared with usual care. Use of NPPV reduces the complications that often arise with invasive mechanical ventilation. NPPV is not appropriate for patients with altered mental status, severe acidosis, respiratory arrest, or cardiovascular instability. 

PSV is a type of mechanical ventilation that uses a patient s inspiratory effort and supplements to a select level of positive airway pressme. The patient controls the ventilatory rate and inspiratory assist time, whereas the PSV supplements inspiratory flow and tidal volume. Because PSV is heavily reliant on the patient s effort, this form of ventilation may not be optimal in patients unable to generate sufficient effort. However, it is the modality of choice in patients who are unable to synchronize with other modes of support. 

In a small number of patients whose respiratory center is depressed by long-term retention of carbon dioxide, injury, or drugs, ventilation is maintained largely by stimulation of carotid and aortic chemoreceptors, commonly referred to as the hypoxic drive. The provision of too much oxygen can depress this drive, resulting in respiratory acidosis. In these cases, supplemental oxygen should be titrated carefully to ensure adequate arterial saturation. If hypoventilation results, then mechanical ventilatory support with or without tracheal intubation should be provided. 

Positive pressure mechanical ventilation (PPMV) uses positive pressure tidal breaths to either totally or partially affect O2 and CO2 transport between the environment and the alveolar spaces. Positive pressure is also used to maintain alveolar patency during expiration. The desired effect of PPMV is to maintain appropriate levels of P02 and PCO2 in arterial blood while properly unloading the ventilatory muscles. Although a life-support technology, PPMV can also be harmful if used inappropriately. The discussion that follows, describes the principles of PPMV, its physiologic effects, the complications that can occur, and recent innovations. 

Brochard L, Rauss A, Benito S, et al. Comparison of three methods of gradual withdrawal from ventilatory support during weaning from mechanical ventilation. Am J Respir Crit Care Med 1994 150(4) 896-903. 

Only a small percentage of patients require ventilation for at least six hours per day for 21 days (2 These patients are often older, have more comorbid illnesses, and, liequendy, have underlying obstmetive lung disease (3,4). Patients who have had a shorter duration of mechanical ventilatory support during surgery are more likely to be successfully weaned (5,6). 

Recent advances in intensive care have resulted in an increased salvage of critically ill patients a number of patients have become dependent upon mechanical ventilation as a chronic form of life support (1). The increased use of prolonged mechanical ventilation (PMV) has led to greater intensive care unit (ICU) bed use, resource consumption, and costs (2,3). It is important to characterize such patients to define treatment goals and expectations, to establish ventilatory care units for their specialized care, and to provide prognostic information for overall survival, morbidities, and health-related quality of life. The goals of this chapter are to provide definitions of PMV, to characterize the patient population requiring this modality of treatment, and to briefly describe a multidiscipUnaiy approach to treatment. 

Prolonged mechanical ventilation (PMV) has been defined as the need for ventilatory support for >21 consecutive days for >6 hr/day (1). Patients who require PMV could be classified as chronically critically ill, as they depend on life support for survival. PMV is part of the continuum of critical care medicine. 

In summary, whereas for obvious reasons the immediate focus in patients with ventilatory failure is the prompt initiation of effective mechanical ventilatory support, it is also necessary for the health care professional to be mindful of the management of other aspects of their care, such as bronchodilators, steroids, antibiotics, and oxygen as well as issues such as secretion clearance, positioning, mobilization, and the potential for aspiration, especially among those patients ventilated through a tracheostomy. Many of these points are amplified elsewhere in this text. 

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. 

During the 1970s, more patients with respiratory failure due to neuromuscular disorders and chest wall deformities received long-term ventilatory assistance at home, either via tracheostomy or body ventilators, which provided effective nocturnal noninvasive ventilation (NIV) (5,6). In the 1970s, the development of home respiratory therapy companies improved support for home mechanical ventilation (HMV). Respiratory therapists could now set up ventilatory equipment, educate the patient and caregivers about using the equipment, and be available to deal with problems. 

NIV is not always preferred to tracheostomy ventilation. If patients lose their ability to protect their airway or if they develop vocal cord paralysis, invasive mechanical ventilation may be preferred, although some patients with severe impairment of speech and swallowing still respond favorably to NIV (32). Some patients feel more secure with invasive ventilation because of direct access for secretion clearance (4). Both approaches require skilled and dedicated caregivers, hut many patients requiring continuous ventilatory support elect for tracheostomy ventilation unless they are closely managed by a highly skilled team, staffed and experienced in NTV for patients with no ventilator-free time. 

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