Special Feature: Understanding the New Ventilator Modes and Related Features
By Dean R. Hess, PhD, RRT
Positive pressure ventilation is described by trigger, control, and cycle variables. The trigger initiates a breath. The control variable remains constant throughout inspiration regardless of changes in respiratory system impedance. Inspiration ends when the cycle is reached. The relationship between the various possible breath types and conditional variables is the mode of ventilation. A mode can include pressure and volume controlled breaths and can be sophisticated enough to switch from one control variable to the other. With each generation of ventilators, new modes and other features become available. The purpose of this article is to describe the technical aspects of new modes and related features of mechanical ventilators that recently have become available. Rather than citing individual published reports in this potentially confusing area, I provide references to the general topic and several important facets of it at the end.
The Ventilator Trigger
Although any signal may be used to trigger the ventilator, patient triggering is usually pressure- or flow-triggered. Pressure triggering requires sufficient patient inspiratory effort to cause airway pressure to fall from the set end-expiratory level to a threshold level (sensitivity) set by the clinician. With flow triggering, breath initiation is based on a flow change in the ventilator circuit beyond some predetermined threshold. From the available evidence, the following recommendations can be made:
- The trigger on current generation ventilators, whether pressure or flow, is superior to that which existed in the past.
- Flow triggering is superior to pressure triggering during continuous positive airway pressure (CPAP) and, therefore, spontaneous breathing trials should be performed using flow triggering.
- During pressure support, there is no clear superiority of flow triggering and pressure triggering. The choice of trigger type during pressure support should be based on patient response, using the trigger type that produces the best patient comfort.
Pressure-Controlled Ventilation
Pressure-controlled ventilation (PCV) is either patient- or ventilator-triggered and time cycled. Tidal volume during PCV is determined by a number of variables: the pressure control setting, airways resistance, respiratory system compliance, auto-PEEP, and patient effort. Inspiratory time also affects the tidal volume if the flow does not decrease to zero. Inspiratory flow is fixed with volume-controlled ventilation (VCV), whereas flow with PCV is variable. Because of this, PCV may be preferable to volume-controlled ventilation (VCV) if the patient is triggering the ventilator with a strong respiratory drive. PCV has been advocated by some authorities as a lung protective strategy. However, it is important to recognize that lung protection was achieved in the ARDSnet study (N Engl J Med. 2000;342:1301-1308) using VCV.
Pressure-Controlled Inverse-Ratio Ventilation
Early reports of improved oxygenation with pressure-controlled inverse-ratio ventilation (PCIRV) generated considerable enthusiasm for this method. Following the initial enthusiasm for this approach, a subsequent controlled study reported no benefit or marginal benefit for the use of PCIRV. Based on the available evidence, there seems to be no clear role for PCIRV in the management of patients with ARDS. The likelihood of an improvement in oxygenation using inverse ratio ventilation is small and the risk of auto-PEEP and hemodynamic compromise is great.
Pressure Support Ventilation
Pressure support ventilation (PSV) assists inspiratory muscles during invasive and noninvasive ventilation. PSV is patient-triggered and primarily flow-triggered. Secondary cycling mechanisms with PSV are pressure and time. Thus, PSV cycles to the expiratory phase when the flow decelerates to a ventilator-determined level, when the pressure rises to a ventilator-determined level, or when inspiratory time reaches a ventilator-determined limit. Although PSV is often considered a simple mode of ventilation, in reality it can be complex:
- The ventilator must recognize the patient’s inspiratory effort, which depends on the trigger sensitivity of the ventilator and the presence of auto-PEEP.
- The ventilator must deliver an appropriate flow at the onset of inspiration. A flow that is too high can produce a pressure overshoot, whereas a flow that is too low can produce patient flow starvation and dyssynchrony.
- The ventilator must appropriately cycle to the expiratory phase without the need for active exhalation by the patient.
As is the case with PCV, flow deceleration during PSV is largely a function of the resistance and compliance of the respiratory system. The flow at which the ventilator cycles can be either a fixed absolute flow, a flow based on the peak inspiratory flow, or a flow based on peak inspiratory flow and elapsed inspiratory time. Several studies have reported dyssynchrony with PSV in subjects having airflow obstruction (eg, COPD). With airflow obstruction, the inspiratory flow decelerates slowly during PSV, the flow necessary to cycle may not be reached, and this stimulates active exhalation to pressure cycle the breath. This problem increases with higher levels of PSV and with higher levels of airflow obstruction. Several approaches can be used to solve this problem.
- PCV can be used, with the inspiratory time set short enough so that the patient does not contract the expiratory muscles to terminate inspiration.
- On newer generation ventilators (such as the Puritan-Bennett 840 and Hamilton Galileo), the clinician can adjust the termination flow at which the ventilator cycles.
- Systems are under development that produce automated setting of the expiratory cycle based upon respiratory mechanics.
In PSV, the flow at the onset of the inspiratory phase is determined by rise time—that is, the time required for the ventilator to reach the set PSV level at the onset of inspiration. The newer generation of ventilators (eg, Puritan-Bennett 840 and Dräger Evita 4) allow adjustments of the rise time during PSV. The rise time is adjusted to patient comfort, and ventilator graphics may be useful to guide this setting. In patients with a strong respiratory drive, a rapid rise time may decrease the work of breathing and the patient’s sensation of dyspnea. However, patient comfort may be compromised using rise times that are either too low or too high. Moreover, a high inspiratory flow at the onset of inspiration is not necessarily beneficial for several reasons. First, if the flow is higher at the onset of inspiration, the inspiratory phase may be prematurely terminated if the ventilator cycles to the expiratory phase at a flow that is a fraction of the peak inspiratory flow. Second, the existence of a flow-related inspiratory terminating reflex in the airway has recently been described. Activation of this reflex due to a higher inspiratory flow causes shortening of neural inspiration, which could result in brief, shallow inspiratory efforts.
Another issue with PSV is the presence of air leaks in the system (eg, bronchopleural fistula, cuffless airway, mask leak with noninvasive ventilation). If the leak exceeds the termination flow at which the ventilator cycles, either active exhalation will occur to terminate inspiration or a prolonged inspiratory time will be applied. With a leak, either PCV or a ventilator that allows an adjustable termination flow should be used.
Proportional Assist Ventilation
Proportional assist ventilation (PAV) was designed to increase or decrease airway pressure in proportion to patient effort, which should improve patient-ventilator synchrony. This is accomplished by a positive feedback control that amplifies airway pressure proportionally to inspiratory flow and volume, where respiratory elastance and resistance are the feedback signal gains. Unlike other modes of ventilatory support, which deliver a preset tidal volume or inspiratory pressure at the airway, with PAV the amount of support changes with patient effort, assisting ventilation with a uniform proportionality between ventilator and patient. The advantage of a proportional ventilatory support lies in its ability to track changes in ventilatory effort. To the extent that inspiratory effort is a reflection of ventilatory demand, this form of support may result in a more physiologic breathing pattern. Experience with this mode is limited in the United States, where the FDA has not yet approved it for commercial use. Although its physiologic basis is sound, the clinical role of PAV remains to be established.
Automatic Tube Compensation
Automatic tube compensation (ATC) is designed to compensate for endotracheal tube resistance via closed loop control of calculated tracheal pressure. This is available on the Dräger Evita 4 and the Puritan-Bennett 840. The proposed advantages of ATC are to overcome the work of breathing imposed by artificial airways, to improve patient-ventilator synchrony as a result of variable inspiratory flow commensurate with demand, and to reduce air trapping as a result of compensation for imposed expiratory resistance. This system uses the known resistive coefficients of the tracheal tube (tracheostomy or endotracheal) and measurement of instantaneous flow to apply pressure proportional to resistance throughout the total respiratory cycle. Because in vivo tracheal tube resistance tends to be greater than in vitro resistance, incomplete compensation for endotracheal tube resistance may occur. Additionally, kinks or bends in the tube as it traverses the upper airway and accumulation of secretions in the inner lumen will change the tube’s resistive coefficient and result in incomplete compensation.
Whether endotracheal tube resistance poses a clinical concern for increased work of breathing in adults is controversial. The imposed work of breathing through the endotracheal tube is modest at usual minute ventilations for the tube sizes most commonly used for adults. Several recent studies cast doubt on the importance of endotracheal tube resistance during short trials of spontaneous breathing. For example, similar outcomes have been reported when spontaneous breathing trials were conducted with PSV (7 cm H2O) or with a T-piece. Moreover, it has been reported that the work of breathing through the endotracheal tube amounted to only about 10% of the total work of breathing. The work for breathing during a 2-hour spontaneous breathing trial with a T-piece may be similar to the work of breathing immediately following extubation. Although prolonged spontaneous breathing through an endotracheal tube is not desirable due to the resistance of the tube, this may not be important for short periods of spontaneous breathing to assess extubation readiness.
Airway Pressure-Release Ventilation
Airway pressure-release ventilation (APRV) produces alveolar ventilation as an adjunct to CPAP. Airway pressure is transiently released to a lower level, after which it is quickly restored to reinflate the lungs. Because the patient is allowed to breathe spontaneously at both levels of CPAP, the need for sedation may also be decreased. Tidal volume for the APRV breath depends on lung compliance, airways resistance, the magnitude of the pressure release, the duration of the pressure release, and the magnitude of the patient’s spontaneous breathing efforts. Of concern is the potential for alveolar derecruitment during the release of pressure with APRV. A modification of APRV is the situation in which the inspiration-to-expiration (I:E) ratio is not reversed. This is available on some ventilators as "PCV+" (called "BIPAP" in Europe) or "Bilevel." Without spontaneous breathing, PCV+ is similar to PCV, and APRV is similar to PCIRV.
One potential advantage of these modes is that the exhalation valve is active during both the inspiratory and expiratory phase. Prior to the current generation of ventilators, the exhalation valve was active during the expiratory phase, but closed completely during the inspiratory phase. An active exhalation valve during the inspiratory phase will open as necessary to maintain a constant inspiratory pressure.
One use of PCV+ (or Bilevel) is to provide sighs during PCV or CPAP. With this technique, several periods (2-4/min) of elevated airway pressure (20-40 cm H2O) are used periodically as a sigh (1-3 seconds at the higher pressure level). This approach differs from the sighs that were available in older generation of ventilators in several ways: 1) They are provided more frequently; 2) They are pressure limited; 3) They are applied for a time longer than the typical inspiratory times set on the ventilator; and 4) Due to the active exhalation valve, the patient can continue to breathe spontaneously at the higher pressure. Although this strategy is attractive in spontaneously breathing patients prone to develop atelectasis, its benefit to date is anecdotal. APRV and PCV+ are available on the Dräger Evita 4, and Bilevel is available on the Puritan-Bennett 840.
Dual Control Modes
Recently developed modes allow the ventilator to control pressure or volume based on a feedback loop (dual control). It is important to appreciate, however, that the ventilator can only really control either pressure or volume—not both at the same time. Dual control within a breath describes a mode where the ventilator switches from pressure control to volume control during the breath. Dual control breath-to-breath is simpler because the ventilator operates in the either PCV or PSV, and the pressure limit increases or decreases to maintain the selected tidal volume.
Dual Control Within a Breath
The proposed advantage of this approach is a reduced work of breathing while maintaining constant minute volume and tidal volume. Examples include volume-assured pressure support (VAPS) (available on the Bird 8400Sti and Tbird) and pressure augmentation (PA) (on the Bear 1000). VAPS and PA are meant to combine the high initial flow of a pressure-limited breath with the constant volume delivery of a volume-limited breath. When the breath is triggered, the ventilator attempts to reach the pressure support setting as quickly as possible. This portion of the breath is pressure-controlled and is associated with a high variable flow that may reduce the work-of-breathing. As this pressure level is reached, the ventilator determines the delivered volume, compares this to the desired tidal volume, and determines whether the minimum desired tidal volume is reached. If the delivered volume is greater than or equal to the set tidal volume, the breath is pressure-supported. If the delivered tidal volume is less that the set tidal volume, as the flow decreases to the set peak flow, the breath changes from a pressure-controlled to a volume-controlled breath. Flow remains constant, increasing the inspiratory time until the volume has been delivered. During this time, pressure rises above the pressure support setting.
Dual Control Breath-to-Breath— Pressure-Limited Flow-Cycled Ventilation
Breath-to-breath dual control is available as Volume Support (VS) (Siemens 300) and Variable Pressure Support (Cardiopulmonary Venturi). Its proposed advantages are to provide the positive attributes of PSV with a constant minute volume. This is closed-loop control of PSV, wherein tidal volume provides feedback control for continuously adjusting the pressure support level. All breaths are patient triggered, pressure limited, and flow cycled. The pressure support level varies breath-to-breath to maintain a constant tidal volume. The maximum pressure change is < 3 cm H2O and can range from 0 cm H2O above PEEP to 5 cm H2O below the high-pressure alarm setting.
Considerable speculation, but little data, suggests that VS will wean the patient from pressure support as patient effort increases and lung mechanics improve. If the pressure level increases in an attempt to maintain tidal volume in the patient with airflow obstruction, auto-PEEP may result. In cases of hyperpnea, as patient demand increases, ventilator support will decrease. This may be the opposite of the desired response. Additionally, if the minimum tidal volume chosen by the clinician exceeds the patient demand, the patient may remain at that level of support and weaning may be delayed.
Dual Control Breath-to-Breath— Pressure-Limited Time-Cycled Ventilation
This approach is available as Pressure-Regulated Volume Control (PRVC) (Siemens 300), Adaptive Pressure Ventilation (APV) (Hamilton Galileo), Auto-Flow (Dräger Evita 4), or Variable Pressure Control (Venturi). This approach provides the positive attributes of PCV with a constant minute volume. This mode is a form of pressure-limited, time-cycled ventilation that uses tidal volume as a feedback control for continuously adjusting the pressure limit. All breaths are ventilator or patient triggered, pressure limited, and time cycled. The pressure increases or decreases by £ 3 cm H2O per breath to deliver the desired tidal volume. The pressure limit fluctuates between 0 cm H2O above the PEEP level to 5 cm H2O below the upper-pressure alarm setting.
The proposed advantage of PRVC is that it maintains the minimum peak pressure that provides a constant set tidal volume and automatic weaning of the pressure as the patient improves. Perhaps the greatest advantage of this mode is the ability of the ventilator to change inspiratory flow to meet patient demand while maintaining a constant minute volume. PRVC and similar modes are attractive with implementing lung protective strategies (such as the ARDSnet protocol), because the tidal volume can be set to 6 mL/kg and the peak pressure can be set to 30 cm H2O. However, only anecdotal support of this approach is currently available.
Automode
The proposed advantages of AutoMode (Servo 300A) are automatic weaning from pressure control to pressure support and automated escalation of support as patient effort diminishes. The ventilator provides PRVC if the patient is making no breathing efforts. If the patient triggers 2 consecutive breaths, the ventilator switches to VS. If the patient becomes apneic, the ventilator switches back to PRVC. AutoMode also switches between PCV and PSV or VCV and VS.
Adaptive Support Ventilation
Adaptive Support Ventilation (ASV) is available on the Hamilton Galileo. Its proposed advantages are automated escalation or withdrawal of support based on changes in patient effort and lung mechanics and automated selection of initial ventilatory parameters. ASV is based on the minimal work-of-breathing concept. The clinician inputs the patient’s ideal body weight, the high-pressure alarm, PEEP, FIO2, and rise time and flow cycle for PSV. The ventilator attempts to deliver 100 mL/min/kg of minute ventilation for an adult. This is adjusted by the % minute volume control, which can be set from 20 to 200%.
This allows the clinician to provide full ventilatory support or encourage spontaneous breathing and facilitate weaning. The ventilator measures compliance, airways resistance, and auto-PEEP using a least squares fit technique. It then uses the clinician input and measured respiratory mechanics to select a respiratory frequency, inspiratory time, I:E ratio, and pressure limit for mandatory and spontaneous breaths. These variables are measured on a breath-to-breath basis and altered by the ventilator’s to meet the desired targets. The ventilator adjusts the I:E ratio and inspiratory time to prevent air trapping and auto-PEEP. ASV can provide pressure-limited time-cycled ventilation, add dual control of those breaths on a breath-to-breath basis, allow for mandatory breaths and spontaneous breaths (dual control SIMV + PSV) and eventually switch to pressure support with dual control breath-to-breath (variable pressure with each pressure supported breath). This is a new technique with a lack of either clinical evaluation or clinician experience.
Conclusions
New ventilator modes and related features have become available over the past decade, with the claim that they improve the efficiency and safety of mechanical ventilation. Some also claim that these modes facilitate the weaning process. The decision to apply a particular mode of ventilation, however, should also be based upon an understanding of the underlying physiology. Just because a new mode does what it claims does not mean it will be more useful than existing modes. Unfortunately, there are few clinical outcomes data upon which to base a decision regarding the choice of ventilator mode. The choice of a particular mode is often based on clinician experience and bias, institutional preferences, and the capabilities of the ventilators available at that institution.
References
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