Pressure Support

Function

The PSV mode is often used in spontaneously breathing patients to aid ventilator liberation. This mode is comfortable for weaning patients and can be readily titrated to regulate a patient's breathing. In addition, PSV may be initiated in patients without the intention of ventilation liberation. Patient populations requiring long-term mechanical ventilation, such as those with small artificial airways, chronic obstructive pulmonary disease (COPD), and chronic muscle weakness, may benefit from pressure support mode.

In certain aspects, PSV shares similarities with intermittent positive-pressure breathing (IPPB). Similar to pressure support, an IPPB support device is used to enhance the quality of breaths by promoting lung expansion. The concept of lung expansion therapy is that the elevation of alveolar pressure (positive pressure) during lung expansion raises the transpulmonary pressure gradient by increasing the pressure within the alveoli. This mechanism of action lowers the partial pressure of carbon dioxide in the blood, decreases the risk of pneumonia, and increases overall pulmonary function.

The concept of pressure support is straightforward and logical, as elevating the pressure support leads to increased ventilation and reduced carbon dioxide levels in the blood. Lung compliance, airway resistance, and patient synchrony significantly influence the success of PSV. An increase in airway resistance or a decrease in lung compliance can lead to an outcome in PSV that falls short of the desired goal. 

When PSV is used as an NIV mode in the ICU, the settings differ from home NIV as inspiratory pressure is the sum of pressure support and PEEP. In contrast, expiratory pressure is equal to PEEP.[8] 

As previously noted, PSV is exclusively a spontaneous mode, requiring patients to initiate breaths. Apneic patients must be in a controlled breath environment where the ventilator may fully sustain respiratory function. A patient must be hemodynamically stable and have sufficient respiratory effort to qualify for pressure support mode. However, a patient with a significant acid-base abnormality, a higher PEEP requirement (8 and above), and an FiO₂ of 50% or more in a controlled setting should not be considered for pressure support mode.[9][10][11]

Numerous limitations for PSV extend beyond liberations from mechanical ventilation. The ideal level of pressure support for weaning may vary among patients depending on the tube size and lung compliance. In addition, prolonged periods of PSV use may lead to diaphragm weakness, delayed mechanical ventilation duration, and increased mortality.[12][13][14] Moreover, PSV can induce patient-ventilator asynchrony due to misalignment between patient effort and the pressure delivered by the mechanical ventilator in PSV mode.[15] When patient-ventilator asynchrony is noted, addressing the issue of ineffective triggering causing the asynchrony can be achieved by reducing cycle time or adjusting the level of pressure support.[15]

Issues of Concern

A simple approach to understanding the impact of resistance and compliance on outcomes is by comparing 2 medical conditions using the same fictitious patient, which could prompt the initiation of PSV. One can then speculate how each scenario might respond to treatment. In the initial scenario, we consider a fictional post-surgical patient undergoing weaning from mechanical ventilation in pressure support mode. The posteroanterior x-ray reveals no notable abnormalities. Lung compliance, calculated at 80 mL/cm H₂O, falls within the normal range, and arterial blood gases depict normocarbia.

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, in the second scenario, the patient's diagnosis shifts to postoperative pneumonia. Although the patient's condition has improved, they are initiated on pressure support mode ventilation at 10 cm H₂O to aid in the weaning process. A posteroanterior x-ray reveals fibrosis as a consequence of pneumonia. Based on blood gas results, lung compliance is calculated at 35 mL/cm H₂O, indicating low lung compliance with slight hypercarbia. The patient exhibits an exhaled tidal volume of 300 mL, contrasting with the 550 mL observed in the earlier scenario. Based on this hypothesis, one may reasonably infer that the outcomes of PSV are intricately linked to the patient's underlying diagnosis.[10]

PSV emerges as a pivotal weaning strategy when compared to IMV modes. Studies have shown that PSV leads to a reduced respiratory rate, increased tidal volume, diminished respiratory muscle activity, and lower oxygen consumption compared to IMV ventilation modes. In 2000, an article from the United States National Library of Medicine noted the benefits of utilizing pressure support to wean patients.[16] The report states that while a 2-hour duration has been extensively evaluated, similar weaning outcomes are achievable with a reduced duration of 30 minutes. A gradual withdrawal of ventilator support is recommended for patients failing the initial spontaneous breathing trial. 

Comparing various weaning methods, such as T-piece and trach collar trials, for liberating patients from the ventilator is crucial. Studies indicate higher success rates when utilizing pressure support for spontaneous breathing trials than T-piece trials for a simple wean. T-piece trials are noted to have a less successful outcome partially due to the absence of pressure equalization provided by pressure support, leaving the patient reliant solely on their ability to overcome the endotracheal tube's resistance.

In a significant 2019 trial published in JAMA, a comparison was made between 30 minutes of pressure support and 2 hours of T-piece weaning. Among the 1153 adults studied, the proportion of successfully extubated patients was 82.3% (n=557) in the 30-minute PSV group, whereas 74% (n=578) in the 2-hour T-piece ventilation group, which is a statistically significant difference.[17] 

The endotracheal tube, being smaller than natural airways, remains static, whereas the natural airway is dynamic and dilates during inspiration. This understanding underscores how an artificial static airway can pose a disadvantage when breathing spontaneously without pressure compensation.[18] 

One of the newer modes available in certain ventilator brands is adaptive support ventilation, characterized as volume-targeted PSV. In this mode, the machine dynamically adjusts respiratory rate, tidal volume, and inspiratory time in response to the patient's effort and respiratory mechanics.

Patient-centered weaning is important when utilizing pressure support as the primary ventilation mode. Recognizing the uniqueness of each individual will often determine the outcome of ventilation liberation. A ventilator liberation protocol that allows for an individualized approach yields superior results compared to a one-size-fits-all approach. Increased pressure support levels are introduced to alleviate the work of breathing (WOB) by supplementing a patient's spontaneous effort with positive pressure. Instituting a pressure support level that stabilizes patients' WOB is valuable in determining a baseline point for ventilation. A gradual, slow withdrawal to enhance muscle strength and endurance leads to more favorable outcomes than weaning modes, such as IMV. When using pressure support levels between 10 and 12 mL/kg, the ventilator takes over all the WOB.

Understanding when to discontinue pressure support is just as important as when to initiate it. When transitioning a patient from a breath rate-controlled mode to pressure support, monitoring their responsiveness to therapy becomes imperative. Assessing the rapid shallow breathing index (RSBI) is a valuable indicator of responsiveness. This calculation is straightforward: the patient's respiratory rate is divided by the average tidal volume in liters (L). If the resulting number exceeds 105, weaning failure is virtually guaranteed. This quotient indicates that the patient is exhaling small tidal volumes at a high frequency, indicating respiratory distress and a potential 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 leading to hemodynamic compromise can all contribute to unsuccessful outcomes. Very low blood pressure may stem from hypovolemia or potential sepsis, whereas high blood pressure could result from cardiac or systemic conditions or the patient's distress due to intolerance of their spontaneous breathing efforts.

The patient's presentation during the process can identify success in pressure support weaning. Monitoring the RSBI and vital signs is a good indicator of patient tolerance. Although arterial blood gasses may be drawn to determine tolerance, studies do not significantly reflect that an arterial blood gas altered the decision to extubate.[19]

Systemic steroids yield favorable outcomes for individuals undergoing mechanical ventilation weaning. Administering steroids both before and after extubation proves beneficial in preventing upper airway obstruction and reducing the risk of reintubation, particularly in at-risk populations.

After extubation, vigilant patient monitoring and proactive measures to mitigate the risk of reintubation are crucial for success. A study suggests that visually assessing the extubated patient and implementing noninvasive positive pressure ventilation through a mask can significantly decrease the risk of reintubation, especially for patients with chronic lung disease. Allowing a smooth transition from an artificial airway on pressure support mode to noninvasive bi-level pressure assistance enhances positive outcomes in the weaning process.[20][6][16][21][7]

Managing PSV in cases of COPD and asthma can be challenging due to substantial variability among patients and varying airway resistance. Furthermore, other factors should be considered, such as expiratory and inspiratory time, airway resistance, lung compliance, and WOB.[22] The use of PSV can cause a decrease in CO₂ levels, which can induce central apneas, especially in susceptible patients with spinal cord injury and heart failure.[23][24][25]

Clinical Significance

Pressure support provides a set amount of pressure during inspiration to support the spontaneously breathing patient. This mode eases a patient's ability to overcome the resistance of the endotracheal tube and is frequently used during weaning, as it reduces the effort of breathing. Pressure support is considered valuable for the weaning patient population. The critical aspect of ensuring optimal patient outcomes is selecting the appropriate ventilation mode based on the patient's condition and abilities.[21][18]

Other Issues

When a patient requires ventilator weaning in pressure support mode, activating the backup mode on the ventilator is recommended. This mode will be triggered if the patient encounters difficulty breathing or exhibits shallow breaths.

Enhancing Healthcare Team Outcomes

PSV is recognized as a crucial weaning strategy, distinguishing itself from other modes, such as intermittent mandatory ventilation (IMV). Studies have shown that PSV results in a decreased respiratory rate, increased tidal volume, reduced respiratory muscle activity, and decreased oxygen consumption compared to IMV ventilation modes. Therefore, an interprofessional healthcare team approach to care involving clinicians, nursing staff, and respiratory therapists is essential for effectively implementing and monitoring PSV, ultimately contributing to improved patient outcomes.

Nursing, Allied Health, and Interprofessional Team Interventions

Ventilator weaning with PSV necessitates a collaborative, multidisciplinary effort involving close cooperation and coordination among the nurse, respiratory therapist, and the healthcare team. Typically, these patients are initiated on PSV, and their sedation is paused.

To avoid complications and self-extubations, the healthcare team should be readily available, and the patient must be closely monitored. Incorporating daily spontaneous breathing trials and sedation vacations, which are part of the ABCDEF bundle, into the best practices of all ICUs, can enhance patient-centered outcomes, including reducing ventilator days and mitigating delirium in the ICU.

Nursing, Allied Health, and Interprofessional Team Monitoring

During the weaning trial on PSV, it is crucial to closely monitor the patient for any signs of respiratory compromise. Respiratory therapists, nurses, and other healthcare team members should collaborate to vigilantly observe the patient throughout the weaning process.