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).

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

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.

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).

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

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

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].

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].

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).

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.