Chapter 8: Advanced Modes of Mechanical Ventilation

With each generation of ventilators, new modes and variations on previous modes become available. There now exist numerous ventilator modes from a variety of manufacturers. The purpose of this chapter is to describe the technical and clinical aspects of advanced modes of ventilation that have recently become available. Although heavily promoted by their manufacturers, the clinical role of many of these modes remains unproven. Use of these modes is often based upon their availability and clinician's bias, rather than evidence that they are superior to traditional modes.

With dual-control modes, the ventilator can automatically switch between pressure control and volume control during a single breath. However, it is important to remember that the ventilator is controlling only pressure or volume at any given time, not both at the same time.

The proposed advantage of this mode is a reduced work-of-breathing (WOB) while maintaining a minimum minute volume and tidal volume (V_T). This approach operates during mandatory breaths or pressure-supported breaths to combine the high initial flow of a pressure-controlled breath with the constant volume delivery of a volume-controlled breath. Names for this approach are volume-assured pressure support, pressure augmentation, and machine volume. These modes are not commonly available on the newest generation of ICU ventilators.

The breath is patient or ventilator-triggered. The ventilator then attempts to reach the pressure setting as quickly as possible. This portion of the breath is pressure-controlled and associated with a variable flow. As this pressure is reached, the ventilator determines the volume that has been delivered and determines if the desired V_T will be delivered. If the delivered and set V_T are equal, the breath is a pressure support breath (Figure 8-1). If the patient's inspiratory effort is low, the breath changes from a pressure-controlled to a volume-controlled breath. Flow remains constant until the tidal volume has been delivered. During this time the pressure rises above the pressure setting. If pressure reaches the high-pressure alarm setting, the breath is pressure-cycled. The ventilator can allow the patient a V_T larger than that set.

Volume control on the Maquet Servo-i is an example of volume-controlled ventilation (VCV) with a dual-targeting scheme. Each breath begins as VCV, but if the patient makes an inspiratory effort sufficient to decrease airway pressure by 3 cm H₂O, the ventilator switches to pressure-controlled ventilation (PCV) within the breath. This allows the patient's effort to augment the set V_T. Depending on the intensity of the inspiratory effort, the ventilator may switch back to VCV with a volume-cycle criterion or end inspiration with a flow-cycle criterion, similar to a pressure support breath.

Adaptive pressure control is closed-loop PCV. Tidal volume is a feedback control for breath-by-breath adjustment of pressure control (Figure 8-2). All breaths are patient or ventilator-triggered, pressure-controlled, and time-cycled. This mode is available on most current ICU ventilators and has various names, dependent on the manufacturer, such as AutoFlow, pressure-regulated volume control (PRCV), volume control + (VC+), adaptive pressure ventilation, volume-targeted pressure control, and pressure-controlled volume guarantee. The ventilator delivers a test breath and calculates system compliance. A number of breaths are delivered to test the pressure control necessary to achieve the desired tidal volume based on the compliance calculation. The ventilator then increases or decreases the pressure on a breath-by-breath basis to deliver the desired V_T.

Perhaps the most important advantage of this mode is the ability of the ventilator to change inspiratory flow to meet patient's demand while maintaining a constant minute volume (Figure 8-3). An important disadvantage of this mode is that the tidal volume remains constant and peak alveolar pressure increases as the lungs become less compliant (eg, acute respiratory distress syndrome [ARDS]), which could result in alveolar overdistention and acute lung injury. With this mode, breaths can exceed set tidal volume in the presence of strong inspiratory efforts by the patient. When this occurs, the ventilator may excessively reduce the level of support, leading to asynchrony. On some ventilators a low-pressure limit as well as a high-pressure limit can be set.

Volume Support

Volume support (VS) is closed-loop control of pressure support ventilation (PSV). Tidal volume is used as feedback control to adjust the pressure support level. All breaths are patient-triggered, pressure-limited, and flow-cycled (Figure 8-4). A test breath with a low-pressure is applied. The delivered tidal volume (exiting the ventilator) is measured and compliance is calculated. A number of breaths are then delivered to test the calculated pressure to deliver the set tidal volume. The ventilator then attempts to maintain a constant delivered tidal volume on a breath-to-breath manner. Since VS is a variation on PSV, the breath is flow-cycled.

There are several potential issues with this mode. Auto-positive end-expiratory pressure (auto-PEEP) may occur if the pressure level increases in an attempt to maintain tidal volume in a patient with airflow obstruction. In the patient with a high ventilatory demand, ventilator support will decrease, which could be the opposite of the desired response. This mode of ventilation is available on most current generation ICU ventilators.

AutoMode

AutoMode allows the ventilator to switch between mandatory and spontaneous breathing modes. If the patient is apneic, the ventilator will provide VCV, PCV, or PRVC. If the patient triggers a breath, the ventilator switches from VCV to VS, from PCV to PSV, or from PRVC to VS. If the patient becomes apneic, the ventilator reverts to VCV, PCV, or PRVC.

Average Volume-Assured Pressure Support

Average volume-assured pressure support (AVAPS) is a form of adaptive pressure control available on some ventilators for noninvasive ventilation. It maintains a V_T equal to or greater than the target V_T by automatically controlling the minimum and maximum inspiratory positive airway pressure (IPAP) setting. AVAPS averages V_T over time and gradually changes the IPAP over several minutes to achieve the target V_T. If the patient's effort decreases, IPAP is increased to maintain the target tidal volume. On the other hand, if the patient's effort increases, IPAP is reduced. As with other types of adaptive pressure control, there is a concern that the ventilator will inappropriately decrease support if respiratory drive increases.

SmartCare/PS is a mode that adjusts the level of PSV based on the patient's V_T, respiratory rate, end-tidal Pco₂, and preset parameters based on the patient's condition. SmartCare adjusts the PSV to maintain a normal range of ventilation (called the zone of comfort), defined as V_T > 300 mL, a respiratory rate 12 to 30 breaths/min, and end-tidal Pco₂ < 55 mm Hg (assuming the patient weighs > 55 kg, without chronic obstructive pulmonary disease [COPD] or neurologic injury). If the patient's ventilation falls outside of these parameters, SmartCare manipulates the PSV as often as every 5 minutes based on the current value, the clinician input parameters, and the patient's historical breathing pattern. SmartCare was designed to automatically wean patients from the ventilator. When the patient is weaned to PSV low enough, a spontaneous breathing trial is performed automatically. If the spontaneous breathing trial (SBT) is successful, the ventilator prompts the clinician to consider extubation.

Adaptive support ventilation (ASV) is based on the minimal WOB concept, which suggests that the patient will breathe at a tidal volume and respiratory frequency that minimizes the elastic and resistive loads while maintaining oxygenation and acid-base balance. This is described mathematically as:

where τ is the time constant (product of resistance and compliance), V̇_a is alveolar ventilation, and V̇_d is dead space ventilation. The ventilator attempts to deliver 100 mL/min/kg of minute ventilation, adjustable from 25% to 350%, which allows the clinician to provide full support or encourage spontaneous breathing.

When connected to the patient, the ventilator delivers a series of test breaths and measures compliance, resistance, and auto-PEEP. Lung mechanics are measured on a breath-to-breath basis and ventilator settings are altered to meet the desired targets (Figure 8-5). The ventilator adjusts the I:E ratio and inspiratory time of the mandatory breaths by calculation of the expiratory time constant (compliance × resistance) to maintain sufficient expiratory time (3 × τ). The breath types are adaptive pressure control or VS if the patient is triggering).

Spontaneous and mandatory breaths can be combined to meet the minute ventilation target (in other words, intermittent mandatory ventilation). If the patient is not triggering, the ventilator determines the respiratory frequency, tidal volume, and pressure required to deliver the tidal volume, inspiratory time, and I:E ratio. If the patient is triggering, the number of mandatory breaths decreases and the ventilator chooses a pressure support that maintains a tidal volume sufficient to ensure alveolar ventilation based on a dead space calculation of 2.2 mL/kg.

Intellivent

Intellivent expands on the concept of ASV by adding closed-loop control of oxygenation to closed-loop control of ventilation. Control of ventilation is primarily based on ASV, but with the option of additional control based on end-tidal Pco₂. End-tidal Pco₂ algorithms for normal lungs, ARDS, head injury, and COPD are available. Oxygenation is based on the ARDSNet PEEP/Fio₂ tables using the Spo₂ to adjust PEEP and Fio₂ (Figure 8-6).

This approach to ventilatory support takes control of gas delivery from the clinician and places it on the patient. The two modes of ventilation that fall under the classification of patient-controlled ventilation are proportional-assist ventilation (PAV) and neurally adjusted ventilatory assist (NAVA). With both of these modes, the clinician sets the proportion of work performed by the patient, but they do not force a ventilatory pattern. With these modes, patients can breathe rapid and shallow or slow and deep, based on the patient's breathing pattern. These modes may improve patient-ventilator synchrony and breathing variability.

PAV adjusts airway pressure in proportion to patient's effort. This is accomplished by a positive feedback control that amplifies airway pressure proportionally to instantaneous inspiratory flow and volume. With PAV, the amount of support changes with patient's effort, assisting ventilation with a uniform proportionality between ventilator and patient. Because inspiratory effort is a reflection of respiratory drive, this form of support may result in a more physiologic breathing pattern.

PAV is based on the equation of motion:

where P_aw is the total pressure applied at the airway, V is volume, C is compliance, R is resistance, and V̇ is flow. Airway pressure is amplified in proportion to the pressure developed by the respiratory muscles. Because flow and volume vary breath-by-breath, the airway pressure during PAV varies breath-by-breath (Figure 8-7). PAV allows the respiratory rate, inspiratory time, and inspiratory pressure to vary.

The newest algorithm for PAV, referred to as PAV+ on the Puritan-Bennett 840, automatically estimates compliance and resistance by performing a 300 milliseconds inspiratory pause every 8 to 15 breaths. Inspiratory V̇ is measured and instantaneously integrated to calculate volume. From the measured V̇ and pressure calculated from the equation of motion, WOB is calculated:

PAV is then set at a level of support that keeps the patient's WOB within the normal WOB range (0.3-0.7 J/L). Each breath is patient-triggered (pressure or flow) and flow-cycled.

NAVA increases or decreases airway pressure based on the electromyographic activity of the diaphragm (EAdi). What is set on the ventilator is the airway pressure applied for each microvolt change in EAdi. A specially designed nasogastric tube is placed in the esophagus. This tube has 4 EMG (Electromyography) electrodes. Proper placement requires two electrodes on either side of the patient's diaphragm. Maintaining proper placement of the nasogastric tube is a potential problem in the application of NAVA. Even a few centimeters' movement can alter the proper operation of NAVA. Tube position should be assessed regularly. NAVA can be used for invasive or noninvasive ventilation. An advantage of NAVA over PAV is that it operates efficiently in the presence of auto-PEEP.

Figure 8-8 shows the relationship between ventilator support and patient's effort for VCV, PCV, PAV, and NAVA. With VCV, as patient's effort increases ventilator pressure (work) decreases. With PCV, pressure (work) is constant regardless of effort. With PAV and NAVA, patient's effort and ventilator pressure (work) are related such that when patient's effort increases, there is an increased pressure applied by the ventilator.

Tube compensation (TC) continuously calculates tracheal pressure in intubated mechanically ventilated patients to allow breath-by-breath compensation of endotracheal tube resistance. TC compensates for endotracheal tube resistance via closed-loop control of calculated tracheal pressure. It uses the known resistive coefficients of the tracheostomy tube or endotracheal tube and measurement of instantaneous flow to apply pressure proportional to resistance throughout the total respiratory cycle (Figure 8-9).

Incomplete compensation for endotracheal tube resistance may occur because in vivo endotracheal tube resistance is greater than in vitro resistance. Additionally, kinks and accumulation of secretions in the tube change the resistive coefficient and results in incomplete compensation. Whether endotracheal tube resistance poses a clinical concern for increased WOB in adults is controversial. The imposed WOB through the endotracheal tube is modest at a usual minute ventilation for the tube sizes most commonly used for adults. Similar outcomes have been reported when SBTs were conducted with low-level pressure support, TC, or with a T-piece. It has also been reported that the WOB at the conclusion of an SBT is similar to the WOB 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 during an SBT to assess extubation readiness.

Airway pressure-release ventilation (APRV) uses long inflation periods (3-5 s) and short deflation periods (0.2-0.8 s). In addition to APRV, it is known as BiLevel, BIPAP, BiVent, BiPhasic, PCV+, and DuoPAP. Oxygenation is determined primarily by the high pressure level, which is typically set at 20 to 30 cm H₂O, and Fio₂. Ventilation is determined by the frequency with which the pressure releases to the lower pressure, the difference between the high pressure and the low pressure, and the magnitude of spontaneous breathing. The low pressure setting is usually 0 to 5 cm H₂O. Spontaneous breathing can occur at the high pressure and low pressure settings, although the time at low pressure is usually too short to allow spontaneous breathing (Figure 8-10). Because it combines mandatory and spontaneous breaths, APRV is technically intermittent mandatory ventilation.

Various time ratios for high-to-low airway pressure have been used with APRV, ranging from 1:1 to 9:1. To sustain optimal recruitment, the greater part of the total time cycle (80%-95%) occurs at the high airway pressure. To minimize de-recruitment, the time spent at low airway pressure is brief. Because the time at low airway pressure is short, exhalation is incomplete and alveolar recruitment due to auto-PEEP results. Creating auto-PEEP is, by design, required with the usual approach to APRV in which low airway pressure is set to 0 cm H₂O. With this approach, the time at low airway pressure is set such that the expiratory flow reaches 50% to 75% of the peak expiratory flow. Some ventilators allow the addition of PSV to the spontaneous breaths during APRV (Figure 8-11).

Spontaneous breathing during the high airway pressure phase of APRV has the potential to generate negative pleural pressures, which may add to the alveolar stretch applied from the ventilator. With APRV, there can also be very high exhaled V_T when the pressure is released from the high pressure to the low pressure. There is concern that these relatively large tidal volumes and transpulmonary pressures could contribute to the risk of ventilator-induced lung injury. The improvement in oxygenation that may occur with APRV must be balanced against the potential risk of lung injury.

Mandatory minute ventilation (MMV) is a mode intended to guarantee minute ventilation during weaning. If the patient's spontaneous ventilation does not match the target minute ventilation set by the clinician, the ventilator supplies the difference between the patient's minute ventilation and the target minute ventilation. If the patient's spontaneous minute ventilation exceeds the target, no ventilator support is provided. MMV is thus a form of closed-loop ventilation in which the ventilator adjusts its output according to the patient's response. MMV is only available on a few ventilator types used in the United States and its value to facilitate weaning is unclear. MMV can be provided by altering the rate or the tidal volume delivered from the ventilator. Some ventilators increase the mandatory breath rate if the minute ventilation falls below the target level, whereas others increase the level of pressure support when the minute ventilation falls below the target level.