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Effect of Spontaneous Breathing Trial (SBT) Duration

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Published: Tue, 10 Apr 2018

Effect of Spontaneous Breathing Trial (SBT) Duration on the Clinical Outcomes of Mechanically Ventilated Patients Admittted at Medical Intensive Care Units of a Tertiary Government Hospital In Davao City

  1. INTRODUCTION

Background of the Study:

Mechanical ventilation (MV) is primarily used to support patients whose respiratory function is compromised due to a variety of reasons. However, several studies have shown that prolonged MV among intensive care patients is associated with adverse clinical outcomes. Thus, MV should be discontinued promptly as soon as patients are capable of breathing spontaneously.

Furthermore, patients who are dependent on MV stay longer in the intensive care unit, requiring dedicated care and frequent monitoring. The inadequate number of intensive care unit beds necessitates maximizing the use of limited resources in delivering essential care to critically ill patients. Discontinuing mechanical ventilation in a well-timed and safe manner should lead to desirable outcomes for both patients and clinicians. Hence, strategies that assist in early withdrawal of patients from mechanical ventilation should be investigated.

The process of liberating from mechanical support is known as weaning. In most studies, it was noted that nearly half of the total duration of mechanical ventilation is spent in the weaning process. Each day, a set of weaning predictors is tested and patients who meet the criteria proceeded to a spontaneous breathing trial (SBT). Several researches set the SBT at 120 minutes. The American Thoracic Society guidelines recommend SBT for 30 minutes to no longer than 120 minutes. In our institution, current practice involves an overnight duration of SBT. The optimal duration of SBT, however, is not known. The long duration of SBT requires close monitoring of a critically ill patient, which is challenging for the limited number of intensive care unit staff. Hence, this study will investigate the hypothesis that short (30 minutes), intermediate (120 minutes) and long (overnight) duration of SBT have similar clinical outcomes.

Review of Related Literature:

Mechanical ventilatory support is used when spontaneous ventilation is insufficient for the sustenance of life. The word supportis emphasized in this context since mechanical ventilation is not a cure for the underlying disease, but it is at best a type of support, offering rest to the patient while the disease processes are treated. A study by Esteban et al showed that half of the intensive care units in North America had at least 40% of adult patients receiving mechanical ventilation. This data is similar to that of a prospective study involving 20 countries in 2004, where it was reported that 33% of patients required mechanical ventilation.

Invasive mechanical ventilation is a risky, uncomfortable, and costly procedure that should only be utilized when indicated. Major indications for mechanical ventilation are: (1) partial pressure of oxygen in arterial blood (PaO2) cannot be maintained above 50 mm Hg despite high levels of delivered oxygen; (2) partial pressure of carbon dioxide in arterial blood rises above 50 mm Hg; (3) ventilation becomes inefficient and/or exhausted; (4) airway protection. According to Esteban et al (2002), the most common causes for mechanical ventilation were acute respiratory failure in the postoperative period (20.8%), pneumonia (13.9%), congestive heart failure (10.4%), sepsis (8.8%), trauma (7.9%), and acute respiratory distress syndrome (4.5%).

The goal of mechanical ventilation is to improve ventilation, oxygenation, and lung mechanics. However, as is the case with other medical therapies, the benefit of mechanical ventilation comes at a price. An Indian study revealed that 55 of the 100 mechanically ventilated patients admitted at a university hospital developed complications as follows: nosocomial pneumonia (29%), gastrointestinal hemorrhage (11%), airway complications, (10%), cardiovascular complications (8%), equipment failure (7%), and barotrauma (2%). The most common complication is ventilator-associated pneumonia (VAP) which occurs 48 to 72 hours or thereafter following endotracheal intubation. The incidence of VAP ranges from 9 to 27%, with mortality rate of between 33 to 50 %. A local private tertiary institution reports a lower incidence of VAP at 7.6%.In this institution, a total of 621 adult patients were intubated from September to December 2014. Of these patients, 13.3% developed ventilator-associated pneumonia.

Numerous studies report that weaning from mechanical ventilation after the underlying reason of respiratory failure has been resolved, account for more than half the total duration of mechanical ventilation. In some trials, however, weaning comprise only 40% of the whole duration of mechanical ventilation. Nevertheless, the duration of weaning is an important component that needs close attention. In a study by Coplin et al higher mortality, more cases of pneumonia and longer hospital admission was reported in patients who underwent more than 48 hours delay of liberation from mechanical ventilation. Hence, physicians should be encouraged to minimize the duration of weaning.

Researchers have long recognized the complications of mechanical ventilation. They have proposed multiple techniques to facilitate the transition to spontaneous ventilation. Successful weaning from MV at any time was reported to be associated with a higher survival rate. Generally, weaning has two components. The first component is a list of “readiness to wean” criteria based on clinical factors that help decide if a patient is ready to breathe spontaneously. Ely and colleagues developed a scoring system wherein all five criteria should be met to pass the screening test. The criteria are as follows: the ratio of the partial pressure of arterial oxygen to the fraction of inspired oxygen (PaO2/FiO2) had to exceed 200; the PEEP should not exceed 5 cm H20; there had to be an adequate cough during suctioning; the ratio of the respiratory frequency to the tidal volume should be less than 105 breaths per minute per liter; and no infusions of vasopressor agents or sedatives. The second component is the spontaneous breathing trial (SBT), a period of unassisted breathing during which the patient is observed for signs of respiratory failure.

Several studies have investigated the methodology for weaning. The commonly used techniques are T-piece, synchronized intermittent mandatory ventilation (SIMV), or Pressure Support Ventilation (PSV). The traditional mode of weaning is the T-piece weaning, which involves attaching the endotracheal tube to a T-piece such that one of the two remaining limbs of the T is connected to a humidifier, which supplies humidified oxygen while the third limb is left open to allow for exhalation. The primary disadvantage of this method of weaning is that apnea, low VE and airway pressure alarms are disabled, and close visual monitoring is required. However, this type of weaning provides an estimation of post-extubation breathing, resulting in rapid recognition of patients who are able to tolerate weaning.

One of the potential areas of study is improving processes that shorten the spontaneous breathing trial. Three previous studies conducted in years 1999, 2002 and 2003 demonstrate the equivalence of 30 minutes and 120 minutes SBT using both T-piece and PSV protocols. Based on these trials, Macintyre (2012) recommended that an SBT should be at least 30 min but no longer than 120 min to allow proper assessment of ventilator discontinuation Similarly, White reported that the length of an SBT should be approximately 30 minutes to 120 minutes. However, an overnight duration of SBT has been in practice in this institution for many years due to limited data on the success rate of extubation using a shorter SBT duration.

After the patient is able to sustain spontaneous breathing, the next step is to ascertain whether the patient can tolerate extubation. This is an important decision, as both delayed and failed extubation are associated with prolonged ventilation and mortality. Several factors may predict extubation failure after a successful SBT. The decision to extubate patients is largely based on the ability to clear secretions and protect the airway. A weak cough and moderate volume of secretions are also associated with extubation failure. Some studies suggest that a Glasgow coma score of ≤ 8-10 is correlated with extubation failure since increased risk of aspiration occurs in patients with reduced level of consciousness. Other characteristics recognized as risk factors for extubation failure are: older age, severity of illness on ICU admission, prolonged duration of ventilation prior to extubation, and continuous sedation


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