Pressure Support Ventilation

Anatomy and Physiology

Delivering mechanical ventilation requires establishing a closed circuit between the mechanical ventilator and the patient’s airways and lungs. This circuit includes the nasopharynx, oropharynx, trachea, bronchial tree, and alveoli. Invasive ventilation is achieved by endotracheal tube placement and inflation of an occlusive cuff in the trachea. Non-invasive ventilation utilizes the placement of a mask over the nose and mouth secured via a cushion and elastic straps around the head. If the endotracheal tube cuff or mask seal is not fully occlusive, this results in an air leak and decreased flow and tidal volume.[3] Upon establishing the closed circuit, the set PEEP increases pressure in the entire circuit to a value above atmospheric pressure, opposing passive emptying of the lungs; this overcomes resistance from the endotracheal tube, keeps the nasopharynx and oropharynx from collapsing (in non-invasive ventilation), and helps airways and alveolar sacs stay open during the expiratory phase.[4] When the driving pressure is delivered, the lungs fill with air, and the diaphragm is pushed downward and flattens. An additional respiratory effort by the patient will augment flow with diaphragmatic contraction. After delivering flow to the lungs, passive exhalation occurs, and the tidal volume returns to the ventilator.[2]

Indications

Pressure support ventilation is used to deliver oxygen and support ventilation in patients with hypoxemic, hypercapnic, and mixed respiratory failure. It is also used to perform a spontaneous breathing trial (SBT) to determine if an intubated patient on control mode ventilation is ready for extubation. The flow delivered by the driving pressure can provide a tidal volume and minute ventilation higher than the patient could achieve without ventilator support. This higher minute ventilation improves oxygen delivery and carbon dioxide offloading. PEEP improves oxygen delivery by keeping distal airways and alveolar sacs open during the expiratory phase, improving ventilation/perfusion (V/Q) matching in the lungs. Also, there is a reduction in oxygen consumption by decreasing the work of breathing.[5]

Contraindications

Pressure support ventilation is relatively contraindicated in patients who have a depressed respiratory drive, very high oxygen consumption, or elevated airway resistance. Because no mandatory breaths are given in PSV mode, minimum minute ventilation is not ensured. Patients with neurologic injury, encephalopathy from critical illness, or those receiving sedation may hypoventilate. Work of breathing and thus oxygen consumption is higher in PSV than in control modes of ventilation. Patients with shock or low cardiac output may need more respiratory support. High airway resistance in patients with obstructive lung disease limits peak flow and can result in small tidal volumes.[6]

Equipment

A mechanical ventilator and an external oxygen supply are required to deliver PSV. For invasive ventilation, an endotracheal tube, ventilator tubing, an endotracheal tube holder, and equipment for monitoring telemetry, blood pressure, and oxygen saturation are necessary. For non-invasive ventilation, CPAP tubing and a well-fitting full face or nasal mask are requirements.

Technique or Treatment

The initial settings for PSV are dependent on the indication. In a patient breathing at a consistent respiratory rate, setting a higher driving pressure will result in higher peak flow and a higher tidal volume. Minute ventilation will depend on the patient’s respiratory rate and inspiratory effort. After initiating PSV, the patient should be directly observed for several minutes to ensure that the goals of ventilation, oxygenation and patient comfort are met. Pulse oximetry, vital signs, the patient’s subjective response to therapy, and arterial blood gas (ABG) testing can be used to determine the effectiveness of the PSV settings.[7]

Non-invasive Ventilation in Patients with Acute Respiratory Failure

It is worth noting that many ventilators designed to provide non-invasive ventilation set an inspiratory positive airway pressure (IPAP) and expiratory positive airway pressure (EPAP) rather than a driving pressure and PEEP. Thus, the driving pressure is the IPAP minus the EPAP. It is important to set the driving pressure at a minimum of 5cmH2O to provide adequate tidal volume. Initial settings for non-invasive PSV are as follows: IPAP 10-15cmH20, EPAP 5-10cmH20, FiO2 100%.[8]

Invasive Ventilation for Patients with Acute Respiratory Failure

Mechanical ventilators designed to provide invasive ventilation set a driving pressure, PEEP, and FiO2. PSV is not an initial mode of ventilation for intubated patients due to respiratory depression following sedation given during intubation. Patients on control modes of ventilation who are meeting ventilation and oxygenation goals are candidates for PSV. Initial driving pressure should be tailored to approximate the patient’s tidal volume on control mode. PEEP and FiO2 settings should be at the same values as the previous control mode. For example, a patient on pressure control ventilation with a respiratory rate of 12, driving pressure of 15cmH20, PEEP of 8 cmH2O, and FiO2 of 40% would transition to PSV mode with driving pressure of 15 cmH20, PEEP of 8 cmH2O, and FiO2 of 40%.[5] Once on PSV, the patient requires direct observation with attention to signs of distress, changes in vital signs, and changes in minute ventilation. An automatic backup control mode ventilation should be set to initiate in case of prolonged apnea. Ventilator alarms for high and low minute ventilation, tidal volume, respiratory rate, and airway pressure should be in place.

The advantage of PSV for intubated patients is an improvement in comfort and ventilator synchrony. As the patient has more control over flow delivery and respiratory rate in PSV mode, there tends to be less ventilator desynchrony from patient-triggered breaths during inspiration or passive exhalation and less voluntary movement of the diaphragm in opposition to delivered breaths. Sedation can often be decreased for patients breathing comfortably on PSV mode, allowing for more awake interaction and participation in physical therapy.[9]

Spontaneous Breathing Trial

PSV mode is used during a spontaneous breathing trial (SBT) to determine a patient’s readiness for extubation.[10] Patients meeting the following criteria are candidates for SBT: the cause of respiratory failure has improved, FiO2 less than or equal to 40%, PEEP less than or equal to 8 cmH20, hemodynamic stability, arterial pH greater than 7.25, and the ability to initiate an inspiratory effort. The patient should ideally be alert or only lightly sedated and able to follow commands.[11] Initial settings for PSV with the purpose of SBT are as follows: driving pressure 5 to 8 cmH20, PEEP 5 to 8 cmH2O, and FiO2 less than or equal to 40%. As with PSV mode for respiratory support, an appropriate backup control mode and ventilator alarms are necessary. The patient should undergo direct observation with attention to signs of distress, changes in vital signs, and changes in minute ventilation. If the patient is breathing comfortably on PSV mode for 30 to 120 minutes with adequate tidal volumes and minute ventilation, the patient is favorable for extubation. The rapid shallow breathing index (RSBI) and ABG can provide additional information about the patient’s readiness for extubation. RSBI is the ratio of respiratory rate to tidal volume (f/VT). RSBI less than 105 is predictive of successful extubation and RSBI less than 65 is very favorable.[11] ABG values in the normal range after 30 to 60 of PSV are also positive indicators of successful extubation.

Complications

Complications of PSV include hypoventilation, hypoxemia, and the resultant changes in mental status and vital signs associated with these. It is imperative that a patient on PSV has adequate monitoring as described above to quickly identify these complications and change PSV settings or to a control mode of ventilation. Hypoventilation and hypoxemia can develop due to changes in mental status, airway resistance, and lung compliance. Conditions such as sedation effect, acute bronchospasm, and pulmonary edema are examples of physiologic changes in the patient that can result in decreased respiratory rate, decreased delivery of flow from the ventilator, or both.[2]

Clinical Significance

Pressure support ventilation is a common ventilator setting for both invasive and non-invasive ventilation. Healthcare workers involved in the care of patients on mechanical ventilation should be familiar with the advantages, disadvantages, monitoring, and complications of PSV.

Enhancing Healthcare Team Outcomes

Collaboration and communication between members of the healthcare team are essential to providing safe and effective PSV. [Level 5] Every team member involved in the patient’s care must be aware when PSV mode initiates for any indication. At least one team member should directly observe the patient for the first 5 to 10 minutes of PSV. Only the respiratory therapist should be allowed to make the ventilator changes, and they should document these in the logbook. Also, they should inform the nurse regarding the change. Both the respiratory therapist and the nursing staff should inform the treating physician of any significant changes or concerns that may arise.

This careful attention and open communication will allow the interprofessional healthcare team to avoid or quickly identify complications of PSV and initiate a ventilatory mode that meets the goals of ventilation, oxygenation, and patient comfort. Multiple randomized controlled trials show efficacy in protocol-based strategies to ensure communication provides safe PSV and SBT.[12][13] These trials have led to the development of care bundles that integrate methods of mechanical ventilation into patient-centered outcomes such as delirium prevention, length of stay, fall prevention, and a decrease in morbidity and mortality.[14]

Nursing, Allied Health, and Interprofessional Team Interventions

Monitor the patient's respiration status. Document the ventilator changes and assess for any respiratory difficulty.

Nursing, Allied Health, and Interprofessional Team Monitoring

Once a patient is on PSV, the nurse will monitor vital signs with attention to respiratory rate and pulse oximetry, telemetry with attention to arrhythmia and tachycardia, and the patient's comfort with the ventilator setting. Continuous monitoring by a member of the healthcare team is necessary during the first 5 to 10 minutes of starting PSV. Signs of respiratory distress, hypoxemia, and tachyarrhythmias are indicators that the patient is not tolerating PSV, and the nurse should contact the physician immediately. In addition, the bedside nurse should alert the respiratory therapist in the case of ventilator alarms sounding.