Ventilatory Modes and Settings During Noninvasive Ventilation

Abstract

Since the early 1980s, when noninvasive ventilation (NIV) showed efficacy in the management of some forms of respiratory failure [1–3], the number of patients receiving this treatment both in the acute setting and at home has steadily increased. This is explained by a growing number of indications in which the effectiveness of NIV has been proven, and also by major technological advances that led to the availability of high-performance portable ventilators and the development of a technical support infrastructure [4]. In the particular case of chronic respiratory failure patients, NIV applications were extended from conventional indications in chest wall and neuromuscular diseases to other more frequent conditions such as obesity hypoventilation and chronic obstructive pulmonary disease (COPD), although, in the latter, the role of NIV is still controversial [5–7]

Since the early 1980s, when noninvasive ventilation (NIV) showed efficacy in the management of some forms of respiratory failure [1–3], the number of patients receiving this treatment both in the acute setting and at home has steadily increased. This is explained by a growing number of indications in which the effectiveness of NIV has been proven, and also by major technological advances that led to the availability of high-performance portable ventilators and the development of a technical support infrastructure [4]. In the particular case of chronic respiratory failure patients, NIV applications were extended from conventional indications in chest wall and neuromuscular diseases to other more frequent conditions such as obesity hypoventilation and chronic obstructive pulmonary disease (COPD), although, in the latter, the role of NIV is still controversial [5–7].

NIV is predominantly applied at night. Sleep is a unique state that induces profound ventilatory changes, including modifications in ventilatory control, upper airway patency, and respiratory muscle recruitment. These sleep-related physiological changes, although without clinical consequences in healthy subjects, may represent an additional challenge when the respiratory system is in a disease state, exceeding the response capabilities of the system and leading to failure [8]. Furthermore, it has been well demonstrated that sleep-related hypoventilation is the first sign of ventilatory failure preceding daytime chronic respiratory failure [9].

Nocturnal NIV can have sustained effects in correcting arterial blood gases during the day. It has been hypothesized that this improvement is mediated by a number of possible mechanisms: (1) improving ventilatory mechanics, (2) resting fatigued respiratory muscles, or (3) enhancing ventilatory sensitivity to CO₂ [10]. In addition, improvement in sleep stage distribution may increase chemosensitivity and enhance sleep quality [11].

This chapter deals with the equipment and mechanisms for NIV and, in particular, the major ventilator modes and settings.

1 NIV: A Short Story

From an historical point of view, negative pressure ventilation (NPV) was the first mode applied to long-term mechanical ventilation. In NPV, a perithoracic subatmospheric pressure is applied to generate inspiration by using body ventilators (e.g., tank or cuirasses). Apart from some specialized centers, this treatment has been almost completely substituted for positive pressure ventilation. The main reasons for this trend were cumbersome NPV devices, the lack of accessibility by the patient, and the danger of inducing upper airway obstruction [12].

The first positive pressure ventilators were powered by a piston chamber, a rotary piston, or a standard compressor. They were large, encumbering, and not versatile and had limited capabilities to modify airflow. Most of them provided only a volume-targeted mode. In the mid-1990s, the development of blower-driven continuous positive airway pressure (CPAP) devices to treat obstructive sleep apnea (OSA) and the growing popularity of NIV influenced the evolution toward the first blower-driven devices providing bi-level positive pressure. These devices were originally conceived to improve the tolerance of patients with OSA and consisted of a CPAP blower modified with a magnetic valve to cycle between two different pressure levels, hence the name bi-level devices [13]. Since then, NIV devices have rapidly evolved with the development of microprocessor-controlled, high-performance, blower-driven micro ventilators. These new technology devices have some advantages, such as flexibility and variability in gas-delivery patterns, high leak compensation capabilities, and “intelligent” inspiratory and expiratory trigger mechanisms. Some of them also have the capability of interfacing with computer systems and built-in monitoring modules that can assess ventilatory parameters and store the data in memory for subsequent analysis.

2 Issues of Particular Importance During NIV: Leaks, Upper Airway Resistance, the Type of Exhalation Port, and NIV Ventilators

Compared with invasive ventilation, NIV has two unique characteristics: the non-airtight nature of the system that poses the potential risk of unintentional leaks, and the fact that the ventilator-lung assembly cannot be considered as a single-compartment model because of the presence of a variable resistance represented by the upper airway (UA). Both situations may compromise the delivery of an effective tidal volume. As a consequence, during NIV, increasing the delivered volume or the delivered inspiratory pressure do not necessarily result in increased effective ventilation reaching the lungs [14].

2.1 Influence of Unintentional Leaks

Unintentional leaks are very common in NIV [11, 15]. Leakage may be absent or minimal when the patient is awake but may worsen during sleep as a result of the loss of voluntary control and decreased muscle tone. Leaks can take place at the mouth, between the skin and the mask but an amount of air can also be deposited at the oropharyngeal reservoir and even pass to the digestive tract (“internal” leaks) [16]. After having ruled out poor adaptation of the interface, leaks can be classified according to causal factor: as primary or “passive”, resulting from hypotonic masseter muscles and/or inability of the soft palate and the oral muscles to counterbalance the high inspiratory pressure insufflated by the ventilator, or secondary to closure of the upper airways occurring at the level of the oropharynx or the glottis [14, 16, 17].

Leaks can impair quality of both ventilation and sleep. They can largely affect ventilator triggering, pressurization, volume delivered, rate of inspiratory pressuring and cycling function and induce sleep fragmentation. A detail of leak-induced abnormalities can be seen in Table 7.1.

Table 7.1 NIV: impact of unintentional leaks (see text for details)

2.2 Influence of the Upper Airways: A to Component Variable Resistor

During NIV, a variable resistance constituted by the upper airway (UA) is interposed between the ventilator and the lung. This explains why a reduction of airway patency may occur, compromising delivery of an effective tidal volume. Intermittent obstruction of the UA is common during NIV and may be related to two mechanisms. The first corresponds to obstructive events at the oropharyngeal level because of UA collapse, as a result of insufficient expiratory positive airway pressure (EPAP). This mechanism may be present in patients with an unstable UA, such as patients with OSA [7, 18]. Another mechanism corresponds to episodes of intermittent obstruction at the glottal level, reflecting cyclic glottal closure induced by hyperventilation, a type of “ventilation resistance” reflex [19–22].

2.3 Influence of Type of Exhalation Device and Connecting Circuits

Whereas intensive care unit (ICU) ventilators classically use a double circuit with an integrated expiratory valve, two different types of circuits can be used to provide NIV. The first uses a similar assembly to those used in ICU devices and includes either single or double tubing, in which inspiration and expiration are separated and a true expiratory valve is present so that CO₂ rebreathing is not a significant problem (Fig. 7.1a). The other type of ventilators, like the CPAP devices that they were derived from, do not have a true exhalation valve and often use a single-limb circuit with a risk of rebreathing. To avoid rebreathing, this system includes a calibrated leak (called intentional leak) either at the mask level or in the circuit (Fig. 7.1b). Single-circuit pressure-targeted ventilators, provided with a calibrated leak (called bi-level ventilators), are most commonly used for NIV today. These devices cycle between a higher inspiratory positive airway pressure (IPAP) and a lower EPAP that can be independently adjusted. With these devices, a minimum mandatory EPAP level of 4 cm H₂O is needed to ensure an effective washout of CO₂, [23]. The use of specific “anti-rebreathing” valves may also diminish rebreathing, although their clinical relevance remains uncertain. Moreover, some of these devices increase expiratory work of breathing and may potentially lead to dynamic hyperinflation and patient discomfort [23, 24]. Interestingly, one study showed that the exhalation port position influences CO₂ rebreathing with a more efficient CO₂ washout when the leak is positioned within the mask [25].

Fig. 7.1

Type of circuits used in non-invasive ventilation (NIV) with (a) an expiratory valve and (b) intentional leak,Note that when a simple circuit with an intentional leak is used, the leak may be interposed in the circuit or incorporated at the mask

2.4 Influence of the Ventilator: Intensive-Care (ICU) Versus Home Devices

Both ICU and home ventilators can be used to deliver NIV. The main technical characteristic differentiating them is that, for the former, the driving pressure is supplied by compressed gas, whereas the latter incorporate their own pressure source. Nevertheless, the type of ventilatory support that they provide is similar

3 Patient Ventilator Synchrony: One Goal, Two Pumps

When delivering mechanical ventilation, there are two ventilatory pumps acting together: the ventilator, on the one hand, and the patient’s own respiratory muscles, on the other hand. These two pumps may work in harmony, but, in fact, they can interact in any number of ways, many of which will create problems rather than solving them. Hence, patient ventilator asynchrony is quite common in patients during NIV [26, 27]. Asynchronies may occur at two levels: during inspiratory triggering, in situations in which there is a mismatch between patient inspiratory effort and ventilator triggering (i.e., ineffective inspiratory effort, double triggering, or auto-triggering) or during cycling from inspiration to expiration, when ventilator cycling does not coincide with the end of patient effort (premature or delayed cycling) [27].

We will begin this section by reviewing the triggering and cycling modes.

3.1 How Do Inspiration and Expiration Start and Stop?

The patient can control the initiation (triggering) and the end (cycling) of inspiration or, on the contrary, neither of them, in which case the ventilator controls both the initiation and the end of inspiration.

3.1.1 Spontaneous Mode (S)

In this mode, the patient controls the beginning and end of inspiration. Inspiration starts when the ventilator is triggered by the patient. During the low-level expiratory pressure (EPAP), the patient’s inspiratory effort modifies the pressure and flow into the circuit, starting the change to the higher inspiratory pressure (IPAP). The pressure is maintained as long as a minimal preset inspiratory flow is occurring. The end of inspiratory assistance (cycling from inspiration to expiration) occurs when the declining inspiratory flow reaches a predetermined percentage of peak inspiratory flow and the pressure in the circuit reverses to EPAP. In this mode, a targeted inspiratory pressure, an inspiratory trigger sensitivity, and a percentage threshold of peak flow for cycling to expiration (see below) must be selected. In some ventilators, these can all be set by the clinician, whereas in others, only the inspiratory pressure can be set. Trigger sensitivity, peak flow, and the level of ventilatory support (IPAP – EPAP) are the main variables that determine the patient’s work of breathing Because each cycle is terminated by a flow criterion rather than by volume or time, the patient retains control of cycle length as well as its depth and flow profile. This mode is also called pressure support ventilation (PSV).

3.1.2 Assist Mode (A)

In A mode, the patient controls the onset of inspiration but the inspiratory length is regulated by the operator. In this mode, the clinician must select a targeted volume or pressure, an inspiratory-expiratory (I:E) ratio, or an inspiratory time and an inspiratory trigger sensitivity

3.1.3 Assist-Control Mode (A/C)

As in assist mode, in A/C mode the patient controls onset of inspiration but end of inspiration is time triggered and determined by the operator. As this mode also allows presetting a backup respiratory rate (RR), if patient RR is lower than the preset ventilator backup RR, the system moves to control mode. Then, this mode allows the patient to trigger the ventilator but also tries to grant a minimum minute ventilation by allowing a backup rate. In this mode, the clinician must select a backup rate, a targeted volume or pressure, an expiratory pressure, an I:E ratio, and inspiratory trigger sensitivity. Trigger sensitivity and peak flow are the main variables that determine the patient’s work of breathing.

3.1.4 Control Mode (C)

In the control mode, there is a preset automatic cycle based on time. The ventilator controls the beginning and end of inspiration and thus the RR. Therefore, one expects the ventilator to capture and inhibit the patient’s respiratory center and the patient to follow the settings imposed by the ventilator. In this mode, the clinician must select a targeted volume or pressure, a fixed RR, and an I:E ratio or inspiratory and expiratory durations. With this mode, the entire work of breathing is supposed to be performed by the ventilator. In some ventilators, this mode is also called timed (T) mode, but it is rarely used.

A particular combination of these modes is available in some NIV ventilators. This mode, called S/T is basically a PSV that provides a backup rate. In this particular mode, cycling from inspiration to expiration is flow limited in patient-triggered cycles and switched to time limited when the patient’s spontaneous RR falls below the backup rate. It also happens when, during S cycles, inspiratory time exceeds a predetermined maximal length (see below). A patient-triggered cycle can be seen in curves of ventilation as a negative inspiratory deflection in pressure and flow curves (see trace no 2 in both Fig. 7.2b, c).

Fig. 7.2

Flow and Paw dynamics during (a) spontaneous breathing, (b) pressure-targeted ventilation, (c) volume-targeted ventilation. 1 Controlled cycle, 2 assisted cycle. Dashed lines represent simultaneous theoretical spontaneous breathing kinetics. Note that, during PTM, flow contour remains close to physiological flow dynamics, facilitating a better adaptation to patient ventilatory needs and patient-ventilator synchrony (for details, see text)

3.2 How a Ventilator Acts and How Patient Ventilator Synchrony Is Achieved: The Ventilatory Cycle

Inadequate patient cooperation, mask intolerance, and patient selection criteria have been advocated as frequent causes of NIV failure, but little attention has been paid to settings and type of ventilator. However, appropriate settings are essential to obtain optimal patient-ventilator synchrony, a main condition to ensure a good quality of ventilation and a proper tolerance by the patient [28]. The most logical approach to explain how a ventilator acts and how patient ventilator synchrony is achieved is to analyze the different phases of a typical positive pressure ventilatory cycle (Fig. 7.3).

Fig. 7.3

The ventilatory cycle

3.2.1 Triggering

As described above, the beginning of inspiration can be triggered by time or patient effort. In the A and AC mode, the ventilator must recognize the patients’ inspiratory effort. This is called triggering function. Classically, NIV devices propose two types of triggers. The first, called a pressure-based trigger, present in older NIV ventilators, is based on a drop in proximal airway pressure and requires a closed circuit. The amplitude of this drop is a function of preset sensitivity and also of patient respiratory drive. A second, called a flow-based trigger, present in almost all newer NIV devices, is based on detection of an inspiratory flow in the presence of a continuous flow washing out the circuit during expiration.

Patient ventilatory synchrony during the triggering phase needs a match between the three physiological variables characterizing spontaneous breathing (ventilatory drive, ventilatory requirements, and Ti/Tot, which is the ratio of inspiratory time/total time) and the three technological variables characterizing ventilator function (trigger function, gas delivery algorithm, and cycling criteria). Asynchrony during the inspiratory phase is quite common during sleep in patients under NIV, may compromise ventilatory efficacy and sleep quality, and is mainly influenced by the delay duration, trigger sensitivity, and amount of pre-inspiratory effort (which depends itself on respiratory drive and muscle strength) [26]. Therefore, the inspiratory trigger should have a short delay of response (i.e., a short time between onset of inspiratory effort and pressurization) and be sensitive enough to allow the patient to trigger easily without auto-triggering, even in the presence of leaks. It ideally should be <100 ms, inasmuch as higher values can increase work of breathing and lead to asynchrony or discomfort [29]. As opening a demand valve during pressure triggering can impose substantial effort [29], ventilators that use flow triggering have, in general, shorter trigger delays [30]. However, these triggers expose the patient to greater occurrence of auto-triggering [31, 32]. Other than intrinsic performance of the trigger system, triggering depends on type of circuit used (simple or double), the patient profile, the level of auto-positive end expiratory pressure (auto-PEEP), and the presence of leaks [33]. Leaks may greatly affect trigger function, either by precluding detection of patient inspiratory effort (leading to ineffective inspiratory effort) or by mimicking an “inspiratory flow” (when using flow triggering) or dragging EPAP level below trigger threshold (when using pressure triggering), with both of the latter situations possibly leading to autocycling. Finally, other frequent causes of asynchrony during the triggering phase are excessive pressure assistance (as high pressure generates dynamic hyperinflation due to larger tidal volume and shorter expiratory time, contributing to a new inspiratory effort occurring before an incomplete exhalation), additional resistance in the circuit, and dynamic hyperinflation [27, 33].

The newer technologies (microprocessors, servo valves, and fast blowers) have substantially improved trigger response. Moreover, an adjustable inspiratory trigger is an option presently available in most home ventilators. Automated complex trigger algorithms, are available, in which a flow-time waveform is used to trigger the ventilator. With these systems, triggering arises when patient inspiratory effort distorts the expiratory flow waveform and this signal crosses the flow shape signal. This method is said to be more sensitive than classical flow triggering, allows adjusting trigger sensitivity in presence of leaks, and can help to reduce ineffective efforts and autocycling. However, the respective advantages of this sophisticated trigger system have not been assessed in rigorous studies. It should be emphasized that some adjustable-trigger devices are scaled in arbitrary units (1–5 or even 1–10), which makes them difficult to use in real life

3.2.2 Pressurization

As the correct pressurization is essential to decrease inspiratory effort and improve synchronization, during this phase, inspiratory flow should be sufficient to match inspiratory demand [34]. Circumstances influencing pressurization are the level of ventilatory support, the amount of time required to reach the target pressure (pressurization slope, also called rise time), compliance and resistance of the respiratory system, and patient inspiratory effort. Studies comparing different ventilators also emphasize the influence of the type of device on pressurization, in particular in situations of high inspiratory demand [35].

A faster rise time has been shown to better unload the respiratory muscles [34]. As the slope becomes flatter, the machine delivers lower flow rates and the patient’s work of breathing increases [34]. In these situations, the device acts by creating an increasing hindrance to airflow, simulating a condition in which the patient breathes through a narrow circuit. However, it must be emphasized that if a slow pressurization can increase inspiratory work, an excessive peak flow can also have adverse effects as it may increase the sensation of dyspnea [36], induce double triggering [27], and lead to high peak mask pressure, which favors leaks. Moreover, leaks can themselves impair pressurization [35].

Some new ventilators offer an adjustable rise time, allowing an individual titration that can profoundly affect patient comfort and synchrony. In this context, it must be emphasized that even if the data published show that the steepest pressure ramp slope induced the lowest work of breathing in both obstructed and restricted patients [34], COPD patients tend to prefer relatively rapid rise time (0.05–0.1 s) whereas patients with neuromuscular diseases prefer a slower one (0.3–0.4 s)

Whereas the pressurization capacity of recent bi-level ventilators have shown improvements, NIV blower-powered devices are, in this aspect, clearly at a disadvantage when compared with proportional valve-powered ICU ventilators [35]. Moreover, studies comparing different home ventilators found major differences in terms of pressurization, even when tested at similar rise times, in particular in situations of high inspiratory demand [35]. Regardless, when considering long-term ventilation, this concern is probably not as important as it is in the acute setting because most patients do not have high inspiratory demands

3.2.3 Sustainment of Inspiratory Plateau

Inspiratory pressure level is one of the main determinants of efficacy of NIV. Determination of the optimal level can be the result of balancing two opposing aims: the desire to provide effective minute ventilation and the need to minimize leaks and discomfort caused by excessive inspiratory pressure. It must be emphasized that, even if newer ventilatory devices have great capabilities to compensate mild to moderate leaks, greater leaks may compromise the ability of the device to attain a desired level of inspiratory pressure. Because very high IPAP levels may favor leaks and the ability of these devices to compensate for them is limited, these two conditions will determine whether inspiratory pressure level remains stable or decreases. Even if there is no recognized gold standard for the level at which ventilatory support must be set, high IPAP levels must be avoided because, in addition to favoring leaks and discomfort, they can induce central apneas during sleep, leading to arousals and sleep fragmentation [37], and can also cause patient ventilatory asynchrony [27].

3.2.4 Cycling from Inspiration to Expiration

Switching from inspiration to expiration can be time cycled or flow cycled. In the time-cycled mode, ventilators use a time criteria chosen by the clinician. In the flow-cycled mode, cycling occurs as inspiratory flow decreases to a preadjusted percentage of the peak inspiratory flow, which is supposed to indicate the end of inspiratory effort. The criteria used to end inspiration may have a clinically relevant impact on quality of ventilation. Ideally, cycling should coincide with the end of patient effort. However, synchronization between end of neural inspiration and ventilator expiratory triggering is mainly determined by respiratory mechanics moving from a premature cycling in restrictive patients to a late cycling in obstructive ones [35, 38]. Moreover, when flow cycling is used, leaks may also delay switching to expiration because flow rate is maintained, in an attempt to maintain pressure, above the level at which cycling into expiration occurs (Fig. 7.4). Both these conditions may lead to patient ventilator expiratory asynchrony, a common condition in patients with COPD [38]. Moreover, this late cycling may aggravate auto-PEEP, also leading to ineffective inspiratory triggering [38]. As with other components of the ventilatory cycle, leaks may profoundly modify I to E cycling, either by advancing or delaying expiratory triggering. In the latter case, increasing the ventilator flow for leak compensation may counterbalance the decrease of inspiratory flow under the preadjusted threshold level, thus impeding recognition of the end of inspiration. This results in abnormal prolongation of inspiratory time that may lead to asynchrony, as patients exhale against the machine (aggravating auto-PEEP, in particular, in obstructive patients) or even inhale without receiving any ventilatory support (inspiratory hang-up) [38].

Fig. 7.4

Impact of leaks on I to E cycling during PSV (S in bi-level devices) mode. (a) Pressure support ventilation without leaks. (b) With leaks. Note that during leaks, the cycle switches to a time mode

In older ventilators, expiratory trigger is fixed at 25 % of peak flow, but newer ventilators offer adjustable expiratory triggers. Some of them use arbitrary units, but others allow defining a known percentage of peak flow. These adjustable expiratory triggers may allow tailoring settings to the patient’s underlying condition. For instance, Tassaux et al. [39] demonstrate in a COPD population under invasive ventilation that increasing the expiratory trigger from 10 to 70 % of peak flow (this means shortening inspiration to allow a greater expiratory time) was associated with a marked reduction in delayed cycling and intrinsic PEEP. Whether the same is true in patients under NIV remains to be elucidated.

To improve patient ventilator expiratory synchrony, some bi-level ventilators provide intelligent flow-based algorithms that, by “copying” previous ventilatory cycle patterns and by using moving signals, are able to modify cycle thresholds to automatically adjust breath-to-breath inspiratory time. These algorithms are supposed to be useful, in particular to adjust inspiratory time during leaks.

Finally, additional mechanisms proposed by some ventilators can improve cycling to prevent undesired inspiratory time prolongation. Sudden increases in pressure (that can be assumed as secondary to an active expiratory effort) produce, in almost all the devices, early cycling to expiration. Another mechanism is to limit maximal inspiratory time. This maximal inspiratory time (called also Timax) is generally fixed at 3 s but may be adjustable for some devices. The aim of Timax is to switch to a time criterion to terminate the breath to prevent an unsuitable lengthening of inspiratory time (in particular when leaks are present).

3.2.5 PEEP Level

PEEP is an above-atmospheric (positive) pressure applied during expiration. When positive pressure is applied during machine breaths, the term PEEP is maintained but when applied during spontaneous breathing the term CPAP is used. With both, the positive pressure is maintained throughout the entire cycle. Providing an external PEEP (called EPAP in bi-level devices) during NIV has many theoretical advantages. Other than flushing dead space CO₂ and preventing rebreathing, EPAP can, in some obstructive patients, reduce dynamic hyperinflation by offsetting intrinsic PEEP [40], thereby reducing inspiratory work required to trigger assisted inspiration. Moreover, an optimal PEEP level can preserve the airway patency in patients with unstable upper airway during sleep. Additional advantages are alveolar recruitment, which increases functional residual capacity and decreases the tendency to microatelectasis. In the three latter situations, higher levels of EPAP (>6 cm) may be needed [41]. Unnecessary increases in EPAP levels should be avoided because inspiratory pressure must be increased in parallel if inspiratory assistance is to be maintained, which can lead to intolerance and favor leaks. As a result and regarding the ventilator category (ICU or home ventilator), the PEEP setting may interfere with either pressure support or IPAP levels. In fact, ICU ventilators propose PEEP and pressure support settings, however, PEEP and IPAP settings are usually associated with home ventilators. Thus, PEEP setting increases IPAP level on ICU ventilators, and PEEP setting decreases pressure support level on home ventilators. Moreover, high EPAP levels may, in some cases, increase work of breathing if lung volume increases to a point where EPAP induces overdistension and increases elastic impedance [42]. A further concern is that the application of a high level of EPAP can result in hemodynamic impairment.

Leaks, if significant, may make it impossible to maintain the set EPAP level. If the device uses a pressure-based inspiratory trigger, this may lead to autocycling because the EPAP levels fall below the trigger threshold.

Measures aimed at improving patient-ventilator synchrony under NIV are detailed in Table 7.2.

Table 7.2 Measures aiming to improve patient-ventilator synchrony under NIV

4 Modes of Ventilation

When NIV was introduced, there were a limited number of modalities and types of ventilators with only a few possible settings. We now have more than 30 brands offering numerous options for settings [43]. Moreover, ventilators are not submitted to stringent medical regulations. This leaves manufacturers free to give different names to the same ventilator modalities and settings and even to “create” new modalities that correspond frequently only to small modifications of a known class. This explains the wide variety of existing terminology describing NIV modalities.

Theoretically, NIV can be delivered using all the modalities used in invasive ventilation. In practice, this is not the case, because the circumstances of ventilation and target population are different but also because available equipment is often limited. Because most ventilators used for NIV deliver either volume- or pressure-targeted modes, and the place of other anecdotally proposed modes for some NIV devices, such as synchronized intermittent mandatory ventilation (SIMV), proportional assisted ventilation (PAV) or other “hybrid” modalities, is not yet clear, this chapter focuses only on the former two modes.

4.1 Volume-Targeted Mode

In the volume-targeted mode (VTM), also called the flow-limited mode, the ventilator delivers a fixed volume during a given time and with whatever pressure is necessary to achieve this. Pressure in the airways (Paw) results from the interaction between ventilatory settings, compliance and resistance of the respiratory system, and spontaneous inspiratory efforts (Fig. 7.2b). It should be emphasized that any inspiratory effort will not lead to changes in delivered volumes or flows but will only result in a decrease in Paw.

Because each breath is delivered with the same predetermined flow time profile and the area under the flow time curve defines volume, the advantage of this mode is the strict delivery, in the absence of leaks, of the preset volume, whatever the values of C and R. A major disadvantage is, precisely, that delivery of this fixed ventilatory assistance does not allow taking into account the patient’s varying requirements. Another inconvenience is that, if there is a leak, there will be no increase in flow rate to compensate and the generated pressure will be lower, so that the effectively delivered volume will be reduced proportionally.

Ventilators that deliver VTM use a simple or double circuit with an integrated expiratory valve. Nevertheless, a new ventilator (Trilogy, Philips, Koninklijken Netherlands) incorporates a volume mode using a simple circuit with intentional leak. To ensure tidal volume, this device includes a complex algorithm to compensate for total leaks. Most older ventilators delivering this mode deliver volumes via a piston or bellows, however, newer devices are blower driven and capable of providing internal PEEP adjustment.

4.2 Pressure-Targeted Mode

In the pressure-targeted mode (PTM), also called the pressure-limited mode, the ventilator is set to deliver airflow by generating a predefined positive pressure in the airways for a given time. Therefore, airflow is adjusted so as to establish and maintain a constant Paw according to the preset pressure. Constant analysis of the flow rate and airway pressure determines the flow variations necessary to maintain a flat or “square wave” pressure. Flow is brisk at the beginning of inspiration when the gradient between the circuit pressure and pressure target is large (Fig. 7.2c). As this gradient narrows, flow decelerates until driving flow no longer exists and flow ceases. Thus, for a given patient, the volume delivered is not fixed and will depend on the interaction between the preset pressure, patient inspiratory effort, the physical characteristics of the respiratory system (R and C), and inspiratory time. PTM ventilators can use circuits with or without an expiratory valve. An important advantage of PTM is the ability to compensate for mild to moderate leaks. Simple-circuit PTM ventilators without an expiratory valve (called bi-level ventilators) are used the most for NIV. These devices are provided with a calibrated leak and cycle between IPAP and EPAP. The mathematical difference between both pressures corresponds to a pressure support in S mode or a pressure control in T mode. With these devices, the patient’s effort determines flow and, when switching to EPAP, the device delivers a lower positive pressure that splints and maintains a positive alveolar pressure. Therefore, IPAP and EPAP level can be independently adjusted both to augment alveolar ventilation and maintain upper airway patency during sleep. In S mode (and in ST mode above backup RR), the patient can control inspiratory and expiratory time, inspiratory flow, tidal volume, and respiratory rate, with th the ventilator providing only a preset pressure. Therefore, this mode is comfortable and provides a suitable patient ventilator synchrony.

4.3 NIV: Volume or Pressure Targeted?

Most of the initial studies concerning NIV used VTM ventilators [1, 44]. However, PTM ventilators were increasingly prescribed and surpassed VTM ventilators at the end of the 1990s. Although studies published showed no significant differences in terms of clinical efficacy or arterial blood gas results [45, 46], a European survey showed that more than 75 % of home-ventilated patients use PTM ventilators and that, in fact, VTM indications were restricted to patients with neuromuscular disease [47]. There are several technological and financial reasons for this trend. Ventilators providing PTM, in particular those of the bi-level type, are smaller, quieter, lighter, more compact, cheaper, and easier to adjust. Moreover, PTM provides better synchronization because this mode (and, in particular, PSV) was primarily designed to facilitate the patient’s effort to breathe and flow contour and volume can be varied on a breath-by-breath basis. In addition, as a consequence of the decelerated flow rate, PTM can provide the same delivered tidal volumes by generating lower mean and peak airway pressures [48, 49], allowing less mask tightness and reducing the likelihood of leaks and side effects. An additional benefit of PTM is a better compensation for mild-to-moderate unintentional leaks because the capabilities of these devices to increase inspiratory flow to compensate for a leak-induced pressure drop [50].

A characteristic of NIV is that the magnitude of leaks changes continuously with movement, posture, and sleep stages changes and with the amount of pressure, which may provide a more stable ventilatory support. In fact, some of the modern home PTM devices can achieve peak inspiratory flow rates up to 180 l∙min^–1. A further advantage of small, portable PTV ventilators is the absence of unnecessary alarms, which controls costs and maximizes portability. Finally, a further advantage of PTM concerns the situations in which O₂ must be added. As most of the available portable ventilators do not have an O₂ blender, supplemental O₂ is provided by an additional admission in the ventilator tube. When VTM is used, this flow is added to minute volume delivered to the patient. Therefore, tidal volume needs to be proportionally reduced to impede overventilation. On the contrary, as in PTM, the ventilator targets a preset pressure, the addition of an external flow will not modify tidal volume and then no additional adjustments are needed.

On the other hand, PTM ventilators are less able to compensate for changes in compliance and resistance and the blowers powering most bi-level devices have limited maximal pressure-generation capacities. This may lead to insufficient ventilation. Therefore, PTM ventilators are less reliable than piston-driven volume ventilators when higher insufflation pressures are needed. Moreover, during PTM, delivered tidal volume differs substantially among different pathological entities and also between ventilators, despite similar settings, even in the absence of leaks. This is related to differences in inspiratory flow, rates, inspiratory duration, and even actual pressure delivered [50]. For that, VTM may be preferred in patients with changing respiratory impedance to ensure a given tidal volume and for some situations characterized by reduced thoraco-pulmonary compliance. An additional advantage of VTM is to provide the possibility of air stacking for assisted coughing and increasing the voice volume during NIV. Finally, VTM has been shown to produce more complete offloading of respiratory muscles, but at the expense of comfort [51]. Therefore, as has been suggested, VTM may offer advantages in patients with some conditions, such obesity hypoventilation, chest wall restriction, and neuromuscular disease, who may require high insufflation pressure or in those in whom adequate control is not achieved with PTM [52, 53]. Also, VTM is suitable in more dependent patients needing alarms (as VTM ventilators have better alarm capabilities than simple bi-level ventilators) or built-in battery backup systems (because blower-based technology used by bi-level devices consumes considerably more energy than piston-powered devices, large internal batteries are needed that counterbalance the benefits of light weight and compact size).

A comparative analysis of the two modalities and of corresponding flow and pressure patterns is summarized in Table 7.3.

Table 7.3 Comparison between pressure- and volume-targeted ventilators

4.4 Volume Targeting Pressure Mode

A limitation of pressure ventilation is that it cannot guarantee a tidal volume delivered to the patient. Volume targeting is a feature available in some bi-level ventilators that can allow overweighting of this limitation. In this hybrid mode, which combines the advantages of the pressure and volume modes, the ventilator estimates the delivered tidal volume and adjusts parameters to achieve a target tidal volume (Vt). Some of them adjust a target volume intracycle (each breath starts as a pressure-limited breath and if the set Vt is not reached, the breath converts to a flow-cycled mode by prolonging the inspiratory time), but most progressively adjust the pressure support level along several cycles within a preset range to provide a Vt as close as possible to the target volume set by the clinician. Whether this feature can improve ventilation effectiveness is not yet clear; as has been demonstrated, a higher level of pressure support is not necessarily associated with a decrease in diaphragmatic energy expenditure because wasted efforts were more common [54]. Interestingly, two studies using of this mode in patients with obesity hypoventilation syndrome (OHS) showed a significant improvement in nocturnal and daytime PaCO₂ compared with usual bi-level ventilation, but at the expense of impaired objective quality of sleep [55, 56]. Moreover, there are no differences between the two modes in terms of health-related quality of life.

There are two possible explanations for these findings. One is that pressure variations may induce sleep stages changes and/or arousals. But it is also possible that this altered sleep quality is a consequence of an impairment in patient ventilator synchrony secondary to glottic apneas triggered by increasing inspiratory pressure [21]. Interestingly, one study shows that the respiratory disturbance index was greater when patients were ventilated with volume-targeted mode, supporting the latter hypothesis [56].

5 Combined or Dual Ventilators

Some manufacturers of home ventilators have developed a category of ventilators that combine some features of bi-level, home VTM, and ICU ventilators. These high-performance micro blower-driven devices, called intermediate ventilators, can provide PTM, VTM, and hybrid modes (such as volume targeting) and have been designed to bridge the gap between bi-level devices, volume-targeted home ventilators, and ICU ventilators. They can guarantee high inspiratory pressures (up to 40 cmH₂O), have adjustable pressure rise time and maximum inspiratory time, and some also have minimal inspiratory time and sophisticated monitoring systems. Some of these devices offer the possibility to choose between either a single-limb circuit with calibrated leak or a single or double limb with expiratory valve and are able to automatically detect the type of circuit. The role of these hybrid ventilators has not yet been sufficiently explored. Their attraction is the availability of different modes in the same ventilator and the possibility to ensure the best tailored settings for each patient using only one device.

A summary of technical characteristics of NIV ventilators is provided in Table 7.4.

Table 7.4 Summary of technical characteristics of NIV ventilators

6 Choice of Ventilator

In spite of the increasing sophistication and wide difference in available options present in commercial home ventilators, there are, to date, no formal recommendations concerning preference of one particular ventilator. Moreover, technical issues determining the actual quality of devices are neither standardized nor regulated by guidelines. Both ventilator choice and optimal settings are poorly defined and selection is left to the perceptions of the clinician and patient. This underlines the importance of patient-tailored prescription and emphasizes the need for nocturnal monitoring of NIV efficacy. The choice of NIV ventilator should depend on both patient condition and device characteristics, including (1) clinical situation and underlying diagnosis (in particular, the degree of ventilator dependency and mobility); (2) patient comfort; (3) versatility and, if needed, availability of different ventilatory modes; (4) device performance; (5) mechanisms of leak compensation; (6) quality and accuracy of monitoring (asynchronies and unintentional leak detection); and (7) experience of the clinical team. At the same time, as those ventilators can be used regularly and long term at home, devices should be simple and easy to handle. Therefore, the ergonomics of home ventilators must be taken into account as benefits derived from good performance can sometimes be outweighed by a confusing user interface. In a trial evaluating user friendliness of home ventilators frequently used in clinical practice, Gonzalez et al. [57] tested the capacity of physicians experienced in mechanical ventilation to perform a series of standardized usual tasks. They showed major differences in accessibility of settings between the different devices with potentially dangerous delays necessary to accomplish even the simplest maneuver.

In general, patients eligible for home ventilation are in a stable condition. Therefore, in contrast to invasive mechanical ventilation or acute NIV, patients under long-term NIV were not threatened by temporary disruption of ventilation. Consequently, with the exception of patients with little autonomy, home ventilators may not need sophisticated technical systems such as external batteries, supplemental O₂, pressure or volume monitoring, or surveillance devices or alarms that are more of an annoyance than a necessity and must be optional rather than imperative. Moreover, as NIV is generally used at night, these alarms when activated may disrupt sleep quality.

Finally, mechanisms of leak detection and leak compensation capabilities are critical issues that dictate ventilator choice because they determine how well a NIV device performs. Moreover, because knowing the actual Vt is essential to providing the desired ventilatory support, another important aspect is how the device estimates Vt. In a system characterized by leaks, measuring effective ventilation is impossible, thus, Vt can only be estimated from appropriate mathematic algorithms integrating flow and pressure. These algorithms, usually not disclosed by manufacturers, are crucial in determining the quality of a given ventilator as they determine not only the ability of the device to guarantee the prescribed ventilatory support but also because they serve to adjust inspiratory and expiratory triggers. Fortunately, nearly all newer home ventilators perform as least as well as ICU ventilators and are capable of meeting high ventilatory demands [52]. Conditions to be met by an “ideal” ventilator are listed in Table 7.5

Table 7.5 The ideal ventilator