Issue 3. What Is the Definition of Weaning Success for the Patient Requiring PMV?
In the acute care ICU setting, weaning success is typically defined as extubation without the need for the reinstitution of ventilatory support (invasive or noninvasive) within the subsequent 48 to 72 h. This reflects the sentinel nature of removing the endotracheal tube and the likelihood that respiratory failure ensuing > 72 h after extubation in the acute ICU setting results from a new or unrelated process.
The 48- to 72-h criteria used in the acute ICU setting may not be appropriate in a PMV setting, where respiratory system recovery is slower and chronic comorbidities are prominent. Defining a time threshold for considering the tracheotomized patient with PMV successfully liberated from mechanical ventilation is thus less intuitive. Proposed definitions of weaning success in this setting include the acute ICU threshold of 48 h, 7 days, or 14 days without ventilatory support, freedom from ventilatory support at the time of hospital discharge, or at 6 months to 1 year after the onset of mechanical ventilation. Any definition of weaning success may be confounded by differences in patient population, discharge criteria, and institution specific characteristics (eg, transfers within the host hospital system). Moreover, the reinstitution of ventilatory support in a PMV patient may be required for factors that do not constitute a failure of the weaning process. Tracheostomy tube decannulation is not usually a prerequisite for defining weaning success. Although perhaps somewhat arbitrary, the importance of selecting a specific threshold for defining weaning success cannot be overstated, as it will allow assessment of the efficacy of weaning protocols, comparison between centers, and may drive reimbursement rules.
In patients with slowly resolving respiratory insufficiency, complete liberation from mechanical ventilation (or a requirement for only nocturnal NIV) for 7 consecutive days should constitute successful weaning. To further identify the optimal definition for weaning success, it is recommended that currently existing databases be interrogated to define the duration of liberation from the mechanical ventilator that best predicts long-term success (durability of weaning success). Prospective analysis of factors resulting in reinstitution of ventilatory support may also prove instructive in refining the definition of weaning success.
Issue 4. What Mechanisms Underlie the Need for PMV?
Identifying the etiology for ventilator dependence is likely to be important in designing strategies to liberate patients from mechanical ventilation. It is evident that numerous factors contribute to ventilator dependence (Table 1).
Systemic Disease Factors
A preliminary report from a multicenter observational study in 23 LTAC hospitals encompassing > 1,400 PMV patients found an average age of 72 years and a median APACHE III acute physiologic score of 36 on admission. These patients often had coexistent nonpulmonary diseases that are associated with poor outcome. Specifically, while 43% of these patients had COPD, 54% also had cardiac disease (coronary artery disease or congestive heart failure), and 20% had neurologic disease.
Other studies of PMV patient populations have also shown a high prevalence of comorbidities that adversely affected ventilator weaning. In one study of 52 patients requiring PMV and hemodialysis, none were successfully weaned and only 3 patients survived. In another investigation, PMV patients with severe renal dysfunction were also less likely to be successfully weaned from mechanical ventilation (13% vs 56%). Cardiac ischemia or left ventricular dysfunction can limit liberation from mechanical ventilation in the acute setting. Although detailed study of cardiac factors in prolonged mechanical ventilation have not been published, one preliminary report noted that successful diuresis and weight loss were associated with weaning success. Poor nutritional status contributes to PMV through several mechanisms: respiratory muscle dysfunction, attenuated ventilatory response to gas exchange abnormalities, and predisposition to infection. Weaning failure and PMV have also been associated with the presence of hypoalbuminemia, although this relationship may be as much a consequence of critical illness liver abnormalities as an effect of nutritional abnormalities.
Abnormal mental status has been linked to PMV.
As an example, a study of patients receiving PMV, and not receiving sedative infusions, observed that patients with a modified Glasgow coma scale score of < 8 were 6.5 times more likely to have weaning failure. The emotional stress and sleep deprivation associated with acute ICU care may also contribute to abnormal mental status.
The presence of these comorbidities likely contribute to the fact that a large fraction (23 to 48%) of PMV patients outside the STAC hospital ICU require transfer back to acute care at some time. This STAC ICU readmission rate is likely to increase as patients admitted to PMV-focused care sites have been noted to have increasingly higher severity of illness markers (and thus more comorbidities) in recent years. Over an 8-year period, Scheinhorn and colleagues noted a significant trend of higher LTAC hospital admission APACHE III acute physiologic score and a shorter duration of preadmission mechanical ventilation. More recently, regulatory changes have lengthened LTAC preadmission time, but illness severity has continued to increase.
Respiratory System Mechanical Factors (Load/Capacity Balance)
As in the acute care setting, an imbalance between excess patient breathing loads and diminished respiratory muscle capacity has been identified in many patients requiring PMV. Clinical indexes of excessive loading and/or impaired capacity include lower maximal inspiratory pressures (Pimax), and high respiratory frequency/tidal volume (f/VT) ratios during spontaneous breathing. An increased respiratory drive as manifest by high breathing frequencies or the airway pressure 100 ms after an inspiratory effort against a closed shutter (P0 1) may also indicate a load/capacity imbalance. Note, however, that the P0.1 has also been used as an index of ventilatory muscle strength.
One study in patients with COPD receiving mechanical ventilation for > 21 days found lower f/VT ratios and higher P0.1 values were associated with weaning success. Another study evaluated 39 PMV patients (intubated for > 3 weeks), 28 with COPD and 11 who had undergone cardiac surgery (CS) complicated by diaphragmatic dysfunction. Compared to a nonventilator dependent COPD control group, the ventilator-dependent COPD group demonstrated reduced tidal volume, minute ventilation, maximal transdiaphragmatic pressure, and Pimax, while showing increased respiratory frequency, P0 1, significant intrinsic positive end-expiratory pressure, and elevated airway resistance. Compared to a nonventilator-dependent CS patient control group, ventilator-dependent CS patients had reduced tidal volume and Pimax while demonstrating higher respiratory frequency and P01 values. Similar to investigations in the acute setting, the majority of these PMV patients had ventilatory muscle tension time indexes values > 0.15 reflecting a high load-to-muscle-capacity ratio.
Specific abnormalities in respiratory muscle function appear common in patients requiring PMV and having difficulty weaning. As an example, one study found that 62% of patients with weaning failure had electromyographic evidence of new neuromuscular disease (chronic illness polyneuropathy or myopathy). Another study found 96% of patients receiving ventilation for > 7 days had either electromyographic or muscle biopsy evidence of neurogenic abnormalities or myopathic changes. Not surprisingly, the presence of these abnormalities correlates with longer duration of mechanical ventilation. In one study, patients requiring a period of paralysis in the ICU took twice as long to be subsequently weaned from mechanical ventilation. The role of respiratory muscle dysfunction is further suggested by investigations demonstrating improvement over time in Pimax when comparing patients failing weaning and then later at the time of weaning success.
Trigger asynchrony (the inability of a patient’s effort to trigger the ventilator because of either weak muscles or inspiratory loading from intrinsic positive end-expiratory pressure) can be an important manifestation of additional load/capacity imbalances in PMV weaning difficulties. In one LTAC study, patients with and without trigger asynchrony were compared. In the 19 trigger asynchrony patients, only 3 patients (16%) were successfully weaned after 70 to 108 days, compared to 181 patients without trigger asynchrony, who were successfully weaned 56% of the time after a median 33 days. From a simple monitoring perspective, failure to appreciate trigger asynchrony may lead to an underestimation of the respiratory rate and therefore the f/VT ratio.
A number of iatrogenic factors have been associated with PMV. First among these is the failure to appreciate ventilator liberation capabilities by not using appropriately frequent assessments or using inappropriately slow reductions in support strate-gies. Inappropriate ventilator settings can also slow the withdrawal process through imposed muscle loading or patient-ventilator dyssynchrony. A corollary to this is the use of unnecessary sedation either because of the dyssynchrony or because of inappropriate monitoring of actual sedation needs. As noted above, stresses related to being in an ICU along with sleep deprivation may result in mental status changes that also prompt excessive sedation use ordered via Canadian Health&Care Mall.
Abnormalities of the upper airway resulting from complications of the artificial airway can also contribute to ventilator dependence. Ten percent of patients with PMV had tracheal injury despite the use of artificial airways with low-pressure, high-volume cuffs. Indeed, in one study 37 of 756 patients (5%) receiving PMV had evidence of distal tracheal obstruction contributing to ventilator dependence.
Tracheostomies also promote swallowing dysfunction and aspiration in PMV patients. Depending on the method of detection, 30 to 83% of patients receiving PMV have swallowing dysfunction (often silent aspiration), a factor that may contribute to failure to liberate from mechanical ventilation.
Process of Care Issues
Increasing duration of mechanical ventilation has been associated with increasing risk of complications, including nosocomial infection, in the acute ICU setting. In a preliminary report from a prospective, multicenter, observational study of patients requiring PMV, infection appeared common (ie, there was evidence for treatment of urinary tract infection in 32%, lower respiratory tract infection in 28%, Clostridium difficile infection in 18%, and central line infection in 12%). Importantly, length of stay and time to wean were significantly longer for patients with apparent infection defeated by Canadian Health&Care Mall.
The capability to successfully liberate patients from mechanical ventilation is directly linked to the caregiver’s skills with patients requiring PMV. Indeed, some investigators report increasing rates of weaning success over time as caregivers acquired experience in managing patients with prolonged mechanical ventilation. Absence of an organized strategy for weaning (eg, weaning protocol) may also increase the number of patients requiring mechanical ventilation. As an example, patients weaned by protocol were less likely to require > 21 days of mechanical ventilation when compared to patients weaned by a traditional approach. Additionally, it has been demonstrated that decreases in effective nursing force can lead to dramatic increases in the duration of mechanical ventilation (from 7 to 38 days).
Predicting Weaning Success Based on Dependency Factors
Clinical studies evaluating weaning predictors in the PMV population are far fewer than in the acute ICU setting. As noted above, a number of respiratory system assessments are associated with weaning outcomes. However, as also noted above, the etiology of ventilator dependence is often multifactorial in the PMV population. Thus, an approach that combines not only respiratory factors but also nonrespiratory factors into sophisticated scoring systems may prove superior in predicting weaning outcome for patients receiving PMV. Although several such approaches have been published, none of these multidimensional strategies to predicting weaning success have been independently validated.
Although numerous factors contributing to ventilator dependence have been identified, their relative frequency among cohorts of PMV patients has not been defined. Greatest emphasis should be placed on identifying factors that are potentially reversible, especially iatrogenic factors.