Ventilation management is an ever growing and changing the environment in which medical professionals, through positive patient outcomes can determine the best approach to patient care. This article will be focused specifically on pressure support ventilation and its role in assisting the spontaneous breathing patient population. Evidence-based practice will also be referenced regarding the pressure support mode and its role in liberating patients from mechanical ventilation. Pressure support breathing is a mode of ventilation that is made up of patient-triggered, pressure-limited, flow-cycled breaths. Each patient breath is supplemented with a set amount of positive pressure. Low levels of pressure support less than 5 cm H2O are often utilized to decrease resistance by overcoming ventilator accessory dead space such as the circuitry and its components. Higher levels of pressure support are introduced to alleviate work of breathing by introducing positive pressure to compliment the patient’s spontaneous effort. If pressure support levels of 10 to 12 mL/kg are utilized, all of the work of breathing is being assumed by the ventilator. It is valuable to note that the patient has consistent control over breath frequency, breath duration and flow while in a pressure support ventilation environment. The volume of each breath is a direct result of set pressures, patient effort, and potentially other mechanical settings that may oppose ventilation.
In the spontaneous breathing patient, pressure support mode is often used to aid in ventilator liberation. Pressure support mode is shown to be comfortable for patients who are weaning and can be easily titrated to manage a patient's breathing. It may also be initiated on patients without the intention of ventilation liberation. Patient populations requiring long-term mechanical ventilation may benefit from pressure support mode. This population may include patients with small artificial airways, patients with chronic obstructive pulmonary disease (COPD) and chronic muscle weakness.
In some respect, pressures support ventilation is similar in application to intermittent positive-pressure breathing (IPPB). An IPPB support device, much like pressure support is utilized to encourage higher quality breaths by promoting lung expansion. The concept of lung expansion therapy is that by increasing the alveolar pressure (positive pressure increases the transpulmonary pressure gradient by raising the pressure inside the alveoli. This mechanism of action reduces the partial pressure of carbon dioxide in the blood, as well as, decreasing the risk of pneumonia and increases overall pulmonary function.
The concept of pressure support is straightforward and logical in that increasing the pressure support results in increased ventilation and decreased carbon dioxide levels in the blood. Lung compliance, airway resistance, and patient synchrony play a notable role in the success of pressure support ventilation. A rise in airway resistance or a decrease in lung compliance results in a less than the desired outcome in pressure support ventilation.
As previously noted, pressure support ventilation is strictly a spontaneous mode. Patients must initiate his or her breaths. Apneic patients are required to be in a controlled breath environment in which the ventilator may fully sustain the respiratory function. It is essential that a patient be hemodynamically stable and have sufficient respiratory effort to qualify for pressure support mode. A patient who have a major acid-base abnormality, higher PEEP requirement (8 and more) and on 50% or more FIO2 on controlled setting should not be considered for pressure support mode.
A simple approach to understanding how resistance and compliance play a role in outcomes is by comparing two medical conditions with the same fictitious patient that may result in the initiation of pressure support ventilation. It may then be hypothesized how each scenario may respond to treatment. The first scenario is fictional patient. A post-surgical patient who is weaning from mechanical ventilation in pressure support mode with an unremarkable posteroanterior x-ray noted. Lung compliance is calculated to be 80 ml/cm H2O (within the normal range), and normocarbia is represented on the arterial blood gases. The patient’s ventilator is currently set with pressure support of 10 cm H2O yielding an average exhaled tidal volume of 550 ml. In comparison, the second scenario the patient's diagnosis deviated to postoperative pneumonia. The patient’s condition has improved and is being initiated on pressure support mode ventilation at 10 cm H2O to facilitate weaning. Posteroanterior x-ray noted fibrosis as a result of pneumonia. Lung compliance is calculated at 35 ml/cm H2O (low lung compliance) with slight hypercarbia based on blood gas results. The patient also presents with an exhaled tidal volume of 300 ml compared to 550 ml from the previous scenario. Based on this hypothesis, one may agree that the results of pressure support ventilation are directly related to the patients underlying diagnosis.
Pressure support ventilation as a key weaning strategy is noted in comparison to modes such as intermittent mandatory ventilation (IMV). Studies have shown that pressure support ventilation results in a decreased respiratory rate, increased tidal volume, reduced respiratory muscle activity, and decreased oxygen consumption than with IMV modes of ventilation. In 2000, an article from the United States National Library of Medicine noted benefits in utilizing pressure support to wean patients. The article states that a duration of 2 hours has been extensively evaluated, but the weaning outcome is the same when the duration is reduced to 30 min. Patients failing the initial spontaneous breathing trial need a gradual withdrawal of ventilator support. It is known that SIMV is the most ineffective method of weaning these patients. Pressure support mode can promote the strengthening of respiratory muscles by persistently allowing for the patient to spontaneously breath without controlled breaths. Based on this understanding, it can be determined that increased exposure to a consistent spontaneous breathing mode allows for respiratory muscle conditioning, resulting in an overall better outcome than in comparison to IMV based modes.
One may also compare additional weaning methods such as T-piece and trach collar trials when attempting to liberate patients from the ventilator. Studies show that patients were more successful when pressure support was utilized for spontaneous breathing trials rather than t piece for a simple wean. T-piece trials are noted to have a less successful outcome partially because the pressure equalization provided by pressure support is removed and the patient is solely reliant upon his or her ability to overcome the resistance of the endotracheal tube. The endotracheal tube is of less diameter than the natural airways and is static in its function. The natural airway is dynamic and dilates upon inspiration. Based on this knowledge it is easy to understand how an artificial static airway may be a disadvantage when breathing spontaneously without pressure compensation.
Patient-centered weaning is an important concept when utilizing pressure support as the primary mode of ventilation. Understanding that no two people are created equal will often determine the outcome of ventilation liberation. A ventilator liberation protocol that allows for an individualized approach has shown to have greater outcomes than with a one-size-fits-all approach. Instituting a pressure support level that stabilizes and patients work of breathing is valuable in determining a baseline point for ventilation. A slow, gradual withdrawal to increase muscle strength and endurance will result in more favorable outcomes than with IMV modes of weaning.
Understanding when to discontinue pressure support is just as important as when to initiate it. If a patient is transitioning from a breath rate controlled mode to pressure support, it is imperative to monitor the patient responsiveness to therapy. Assessing the rapid shallow breathing index (RSBI) is a good indicator of responsiveness. This calculation is simple. The patient's respiratory rate is divided by the average tidal volume in liters (L). If the number is greater than 105, weaning failure is virtually guaranteed. This quotient indicates that the patient is exhaling small tidal volumes at high frequency, indicating a struggle.
Many other factors play a role when pressure support is unsuccessful. Underlying issues such as congestive heart failure, chronic pulmonary disease, fluid overload, dehydration, or electrolyte abnormalities that result in hemodynamic compromise may all result in failed outcomes with pressure support weaning. Very low blood pressures may be a direct result of hypovolemia or potential sepsis. High blood pressure could be due to a cardiac, systemic condition or the result of the patient having distress due to intolerance of their spontaneous breathing efforts.
The patient’s presentation during the process can identify success in pressure support weaning. Monitoring of the RSBI and vital signs are a good indicator of patient tolerance. Although arterial blood gasses (ABG) may be drawn to determine tolerance, studies do not significantly reflect that an ABG altered the decision to extubate.
The introduction of systemic steroids has shown positive outcomes for those weaning from mechanical ventilation. Administration of steroids before and after extubation has demonstrated to aid in preventing upper airway obstruction and decrease the risk of reintubation in at-risk populations.
After extubation, monitoring a patient and mitigating risk of reintubation is imperative to success. According to a study, visual assessment of the extubated patient as well as the addition of non-invasive positive pressure ventilation via mask may significantly reduce the risk of reintubation for patients with chronic lung disease. Allowing the patient to transition from an artificial airway on pressure support mode to a noninvasive bi-level pressure to provide assistance increases positive outcomes relative to weaning.
Pressure support provides a set amount of pressure during inspiration to support the spontaneously breathing patient. Pressure support eases a patient's ability to overcome the resistance of the endotracheal tube and is often used during weaning because it decreases the effort of breathing. Pressure support has been deemed valuable for the weaning patient population. The importance of selecting the correct mode of ventilation based on the patient’s condition and ability is paramount to overall patient outcomes.
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