◆ Pressure support ventilation (PSV) is an assisted ventilatory mode that is patient-triggered, pressure-limited, and flow cycled. During PSV, airway pressure is maintained nearly constant during the entire inspiration.
◆ PSV allows the patient to maintain a certain degree of control over respiratory rate and tidal volume.
◆ The main use of the modality is in withdrawal from mechanical ventilation because it unloads respiratory muscles and allows a gradual reduction of support until extubation.
◆ If not properly used (usually due to excessive levels of support), this modality generates high and abnormal tidal volumes, and wasted inspiratory efforts.
◆ The closed-loop modality could have important clinical implications in withdrawal of mechanical ventilation in specific groups of patients. It appears to be as good as usual care performed by experts and skilled teams.
Pressure support ventilation (PSV) is an assisted ventilatory mode that is patient-triggered (by pressure, airflow, or both), pressure-limited, and flow cycled. In this modality, the airway pressure is maintained almost constant during the entire inspiration. The ventilator provides assistance when the patient makes a breathing effort, and then, when inspiratory flow reaches a certain threshold level, cycling to exhalation occurs. The use of PSV is common in all intensive care units (ICUs), and it is the most commonly used method to wean patients from mechanical ventilation .
In PSV, the clinician decides whether to use pressure or flow triggers to initiate ventilatory assist. The recommended pressure and flow triggers are, respectively, from –0.5 to –2.0 cmH2O, and from 1 to 2 L/min .
Several studies have compared the use of pressure versus flow triggers, without finding any significant differences between the triggers. However, Aslanian et al.  found that the flow trigger was more effective in reducing breathing effort when used in PSV versus volume-controlled modality. In patients with intrinsic positive end-expiratory pressure (PEEPi), the flow trigger can decrease inspiratory effort; moreover, low levels of external positive end-expiratory pressure (PEEP) are recommended to compensate for this PEEPi, the dynamic flow limitation and for decrease the work of breathing .
Once the ventilator is triggered, the machine provides an inspiratory flow (via a servo regulatory mechanism) to maintain the preset level of airway pressure (pressure support setting) nearly constant throughout the inspiration. The velocity of pressurization, which depends on the shape of the inspiratory flow waveform, is the time required for the ventilator to reach the pressure support setting at the onset of inspiration (rise time). Different pressurization rates have a profound effect on effort. Low pressurization rates produce a high inspiratory muscle effort, while high pressurization rates lower inspiratory muscle effort . Visual inspection of the ventilator waveforms may be used to guide this setting. Tidal volume depends on the preset level of pressure support, the inspiratory effort of the patient, the cycling-off threshold level, and the mechanical characteristics (resistance and compliance) of the patient’s respiratory system.
Cycling of expiration
During PSV, cycling from inspiration to expiration is triggered when the inspiratory airflow reaches a certain threshold value. The threshold value coincides, theoretically, with the end of inspiratory muscle effort. This flow value could be a percentage of peak flow (i.e. 25% of peak flow) or a fixed level (i.e. 5 L/min). The latest generation of ventilators allow the physician to set this flow threshold value . Modification of the cycling-off criteria can influence inspiratory effort and patient–ventilator synchrony. In patients with chronic obstructive pulmonary disease (COPD), setting the cycling-off at higher percentages of peak inspiratory flow can improve patient–ventilator synchrony and reduce inspiratory muscle effort . Thille et al.  have described a similar phenomenon in COPD and non-COPD patients. Tracings obtained during pressure support ventilation are shown in Fig. 97.1.
Breathing pattern and respiratory effort
PSV allows the patient to retain control over the respiratory rate and tidal volume, a process referred to as physiological ventilation. PSV induces changes in the breathing pattern that affect tidal volume and respiratory rate, without, however, inducing major changes in minute ventilation. As a result, in most patients, tidal volume rises and the ventilator respiratory rate decreases as the level of support is increased. Inappropriate low levels of support can generate low tidal volumes and a high respiratory rate, resulting in patient discomfort and hypercapnia; on the other hand, excessive levels of support may produce hyperinflation, the appearance of wasted inspiratory efforts, respiratory alkalosis, and even periods of apnoea.
Patient ventilator synchrony during PSV
Asynchronies can be present in all ventilator modalities and inappropriate ventilator settings can aggravate the frequency and severity of these asynchronies.
The patient–ventilatory synchrony achieved with PSV is good because it is able to recognize the beginning and the end of each spontaneous effort. However, asynchronies occur during PSV. Often, but not always, these asynchronies during PSV can be detected at the bedside by examining ventilator waveforms.
Thille et al.  found that assisted control modalities are associated with a higher prevalence of asynchronies compared with PSV. In their study, the most common asynchronies were ineffective triggering and double-triggering.
A study by Leung et al.  showed that high levels of support (above 60–70% of full support) generated wasted inspiratory efforts. A more recent study  found that the frequency of asynchronies can be decreased by lowering pressure-support levels.
Clinical usefulness and applications
Withdrawal of mechanical ventilation
In the process of withdrawal of mechanical ventilation, the support level should be reduced as quickly as the patient’s clinical tolerance will permit. This reduction, therefore, must be made on an individual basis. The support levels are usually lowered by one or two steps per day (between 2 and 4 cmH2O per step). A spontaneous breathing trial (SBT) should be conducted as soon as the physician suspects that weaning may be possible and the patient appears to be ready to breathe without ventilatory assistance. This trial can be performed by disconnecting the patient from the ventilator and attaching a T-piece to the endotracheal tube or, alternatively, the SBT can be performed by administering low levels of PSV with or without PEEP. Esteban et al.  showed that both methods (pressure support or a T-piece) are suitable for successful discontinuation of ventilator support. However, a recent study by Cabello and colleagues , showed that, in difficult to wean patients (those who had failed at least one SBT), the use of pressure support and PEEP modifies the breathing pattern, inspiratory muscle effort, and cardiovascular response when compared with the T-piece trial. In fact, of the 100% of difficult to wean patients who failed a T-piece trial, 79 and 57%, respectively, successfully completed subsequent PSV with PEEP and PSV without PEEP trials. For these reasons, it is still unclear as to which SBT is best to predict successful extubation, and therefore weaning strategies must be individualized.
Initial suggested settings
As occurs in other ventilation modalities, all PSV settings must be adjusted individually in each patient. However, we can provide some suggestions for the initial PSV settings. These settings should be checked several times during the day and/or whenever the patient requires an adjustment.
First, the pressurization rate should be fast (short rise time), and the support level should be adjusted to produce a respiratory rate of approximately 25–30 breaths/min depending on the patient’s comfort. Cycling off should be approximately 25% of peak inspiratory flow (a higher percentage is recommended in COPD patients). The FiO2 and PEEP must be adjusted according to gas exchange and PEEPi.
A closed-loop, knowledge-based system has been designed to help in withdrawal from mechanical ventilation. The system continuously analyses physiological data (respiratory rate, tidal volume, and end-tidal CO2 level) and adapts the level of pressure support to keep the patient within a ‘comfort zone’. This comfort zone is defined as a respiratory rate that can vary freely from 15 to 30 breaths/min (up to 34 breaths in patients with neurological disease). The tidal volume should be above a minimum threshold and an end-tidal CO2 level below a maximum threshold . The level of pressure support is periodically adapted by the system in steps of 2–4 cmH2O. The system automatically tries to reduce the pressure level to a minimal value, at which time a ‘spontaneous breathing trial’ with the minimal low-pressure support is performed by the system. Upon successful completion of this trial, a message on the screen recommends separation from the ventilator.
Lellouche et al. , showed that this system reduces the duration of mechanical ventilation and ICU stay compared with the usual intensive care weaning procedures. However, two recent studies have failed to fully confirm these results [13,14]. Rose et al.  reported that the automated system did not reduce weaning time in their study, in contrast to the positive findings of Lellouche and colleagues. However, this may be due to differences between the two studies, particularly in terms of patient severity, duration of ventilation, the patient–nurse ratio, and in ICU staffing levels. The study performed by Schadler et al.  also had several difference with the Lellouche et al. study. One important difference is that the Schadler study was performed in post-operative patients with nursing and medical staff who were skilled in the management of mechanical ventilation. The authors found that overall weaning times did not differ significantly between the control group and the experimental group, with the exception of a subgroup of 132 patients who had undergone cardiac surgery (24 hours in closed-loop versus 35 hours in control group, p = 0.035). Given the findings published to date, we can conclude that this closed-loop modality performs at least as well as experienced medical staff in weaning patients from mechanical ventilation.
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