Courtney L. Dostal, Aditi Satti and Gerard J. Criner

Definition

Many studies employed differing definitions of prolonged mechanical ventilation (PMV). For example, the National Association of Medical Director of Respiratory Care (NAMDRC) consensus conference defined chronic ventilatory dependency as receiving mechanical ventilation for at least 6 hours a day for more than 21 days. However, it does not consider a patient ventilator dependent until the acute medical illness causing the respiratory failure has been adequately treated.

Prevalence

The prevalence of patients on PMV varies based on the definition and study population. One international study by Esteban et al examined the survival of 5183 patients on mechanical ventilation from 361 ICUs, observing that 25% of patients received mechanical ventilation for > 7 days. Survival rates also differed based on the patient population. Although the overall mortality rate was 31%, the rate was 52% in patients with the acute respiratory distress syndrome (ARDS) and but only 22% in chronic obstructive pulmonary disease (COPD) patients.

Predicting survival in PMV patients is difficult. In PMV patients in long-term acute care hospitals (LTACHs), Carson and co-workers found that the combination of older age and poor prehospitalization functional status conferred a higher risk of death at 1 year. Other studies revealed a higher incidence of poor functional status and quality of life scores in PMV patients.

Predictors of Weaning Success

The definition of weaning success in PMV is not as clear as in an acute care setting. In an acute setting, extubation within 48 to 72 hours typically defines success. In PMV, there is a slower respiratory recovery time with increased comorbidities. According to the NAMDRC consensus conference, weaning success is defined as removal from mechanical ventilation for 7 consecutive days or a requirement for only nocturnal non-invasive ventilation.

Traditional predictors of weaning are shown in Box 25.1. These parameters predict weaning success in patients ventilated < 7 days, but not in the PMV population. The reason for this discrepancy in predicting outcomes remains unclear. However, it probably relates to the acute-on-chronic respiratory failure affecting most PMV patients with a variety of diverse problems that affect pulmonary, respiratory muscle, and chest wall mechanics.

The poor performance of these traditional weaning parameters has encouraged development of a number of integrated physiologic parameters to predict weaning outcome in the PMV patient population. For example, Gluck et al evaluated a sophisticated scoring system to predict weaning outcome in patients ventilated for 3 weeks. The scoring system, incorporating compliance, resistance, dead space, rapid shallow breathing index, and PaCO₂, was more reliable than individual indices. However, this scoring system and similar approaches to predict weaning success in PMV patients have not been validated prospectively. More recently, Verceles et al demonstrated that an improving trend of the rapid shallow breathing index may predict weaning success.

The presence of “good” weaning parameters (traditional or integrated) provides objective support to proceed with weaning, but it does not guarantee a successful outcome or lessen the need for attention to the nonrespiratory factors required to discontinue mechanical ventilation (see Box 25.1). Likewise, “poor” weaning indices should engender caution about progressing and call attention to factors that might compromise the weaning process. However, they should not preclude carefully monitored attempts to begin gradually withdrawing mechanical ventilation.

Factors Associated with Prolonged Mechanical Ventilation

Weakness and deconditioning are widely prevalent in ICU patients requiring mechanical ventilation. Even in healthy persons, a short duration of bed rest adversely effects skeletal muscle performance, with decreased skeletal muscle force by 15% and 20% at days 14 and 35, respectively. Calf and thigh muscle volumes decreased, with a decrease in both slow- and fast-twitch muscle fiber size. Prolonged mechanical ventilation and immobility can weaken the diaphragm, just as occurs in other skeletal muscles. In an animal model, mechanical ventilation for 11 days caused a 25% decrease in maximum transdiaphragmatic pressure and endurance. Correspondingly, diaphragm-biopsy specimens obtained from human subjects on continuous mechanical ventilation for 18 to 69 hours showed atrophy of both slow- and fast-twitch muscle fibers. Diaphragmatic and accessory respiratory muscle weakness contributes to an overall decrease in functional status and impairs weaning from mechanical ventilation.

Neuromuscular weakness is also commonly found in the ICU. Critical illness via associated inflammatory mediators frequently results in critical illness myopathy as well as critical illness neuropathy (Chapter 48). These conditions may be exacerbated by the use of glucocorticoids and neuromuscular blocking agents (Chapter 6). These complications can prolong weaning duration and hospitalization in ICU patients. Although generally reversible, critical illness myopathy and neuropathy require intense and often prolonged rehabilitation.

The weakness and deconditioning that affect respiratory muscles also adversely impact nonrespiratory skeletal muscle of the limbs and oropharynx, and they limit the ability of PMV patients to ambulate, speak, and swallow as well as wean from mechanical ventilation. Multidisciplinary rehabilitation, addressing all of these issues, is required to successfully restore the patient’s functional status and ability to wean from PMV.

Factors That Increase Work of Breathing (WOB)

WOB may be increased by processes that raise airway resistance, decrease lung compliance, or stimulate respiratory drive beyond the normal range of minute ventilation (Box 25.2). Lumens of endotracheal tubes acquire an invisible biofilm over several weeks of use that markedly increases the tube’s airway resistance. Nonetheless, although the timing is debated, placing a tracheostomy tube facilitates weaning by decreasing resistive and elastic loads and thereby the WOB (see Figure 25.E1). This decreased WOB facilitates slow, progressive weans in deconditioned patients with lung mechanics impaired by a chronic underlying disease (e.g., chronic obstructive pulmonary disease [COPD]). Other benefits of tracheostomy include easier suctioning, improved patient comfort and mobility, and the potential for earlier initiation of speaking and oral feeding.

The ventilator circuit demand valve also increases airway resistance and thus increases the WOB. Even in assisted modes of ventilation, the WOB may equal or exceed that required for a normal spontaneous breath. Strategies for overcoming the intrinsic ventilator resistance include judicious titration of the flow rate, use of flow-triggered rather than pressure-triggered ventilators, and elimination of breathing circuit dead space.

Mild interstitial pulmonary edema, often unrecognized, may also prolong weaning. The transition from positive pressure ventilation to spontaneous breathing decreases intrathoracic pressures and increases venous return, such that patients with impaired left ventricular function may develop elevated left ventricular diastolic pressure and interstitial pulmonary edema. Also, the stress of weaning may precipitate coronary ischemia, resulting in ischemia-induced left ventricular diastolic dysfunction that further complicates the weaning process.

Rehabilitation in Patients Receiving PMV

Patients with chronic respiratory failure suffer from deconditioning, related to prolonged bed rest and the catabolic nature of their disease. The ability to sit, stand, and ambulate improves functional status and psychological outlook, prevents the complications of immobility, and facilitates weaning (Chapter 21).

Whole-body rehabilitation is an integral part in the care of the PMV patient. One study examined the efficacy of whole-body rehabilitation in 48 chronically ventilated patients. Physical therapy was initiated on admission to the ventilator rehabilitation unit and consisted of trunk control, active and passive extremity resistance training, and inspiratory muscle training. Initially deconditioned and bed-bound patients were able to sit, stand, and ambulate prior to discharge. Also, strengthening the pectoralis muscles, a muscle group of large mass with extensive thoracic attachments and dual inspiratory and expiratory functions, decreased weaning time and improved ventilatory mechanics. The salutary effects of pectoralis strengthening have been documented in other patient populations.