Ventilatory Modes and Settings of Ventilators

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1		Manel Lujàn	--	2348	2023-05-08 08:09:10	\|
2	format correct	Conner Chen	+ 14 word(s)	2362	2023-05-10 03:23:09	\|

This entry is adapted from the peer-reviewed paper 10.3390/jcm12082942

The choice of a ventilator model for a single patient is usually based on parameters such as size (portability), presence or absence of battery and ventilatory modes. In each mode, there is always a control or independent variable, which is programmed into the ventilator and remains constant throughout the inspiratory cycle, regardless of the variability in the patient’s ventilatory pattern. The control variables are usually pressure and volume. However, there are special cases, such as hybrid modes (average volume-assured pressure support -AVAPS (Average Volume-Assured Pressure Support)- or intelligent volume-assured pressure support—iVAPS-) in which the control variable (pressure) is modified in a predetermined pressure range depending on the estimation of a parameter (tidal volume).

trigger pressurization ventilatory modes

1. Volume-Limited or Volumetric Mode

The programmed volume remains constant as the independent variable and the pressure (dependent variable) changes depending on lung compliance, set volume, airway resistance and the patient’s inspiratory effort ^[1]. The main advantage of this ventilatory mode is that the volume is always delivered, regardless of airway resistance and lung compliance. The main drawbacks would be the efficiency loss in presence of leaks, as it does not compensate for them and the poor response in front of an increased patient’s effort (the delivered volume will not increase) ^[2]. Classically, it was the most widely used mode at the beginning of the home NIV era but has now been overshadowed by the pressure modes. The main parameters within this ventilatory mode are:

The tidal volume (VT) or volume delivered in each ventilatory cycle. In NIV, this is usually set at around 8–10 mL/kg of ideal patient weight to overcome the effect of potential leaks.
The basal respiratory rate (RR), is usually programmed 2–4 cycles below the patient’s spontaneous rate.
Inspiratory time (Ti). It should be noted that cycling (transition from inspiration to expiration) is always based on a time criterion in volumetric modes. A shorter Ti is usually used in patients with obstructive lung mechanics and a longer Ti in restrictive ones.
The shape of the flow waveform, which can be constant (flow is the same throughout the inspiratory cycle) or decelerating, more physiological, or with a higher flow at the beginning of inspiration.
The level of positive end-expiratory pressure (PEEP).
The trigger sensitivity.

2. Pressure-Limited or Barometric Mode

This is currently the most widely used ventilatory mode, due to its ability to compensate for leaks and its physiological mechanism, which allows the patient to maintain some control over tidal volume and inspiratory time (the latter in pressure support mode only) ^[2]. There are two main variants, pressure control (PC) and pressure support (PS). Both are based on a pre-set constant positive pressure at two different levels, inspiratory and expiratory, named Inspiratory positive airway pressure (IPAP) and Expiratory positive airway pressure (EPAP). The difference or gradient of pressures between the two is called “pressure support” (PS). The volume delivered in this ventilatory mode will depend, in addition to the pressure support gradient, on the patient’s lung mechanics and effort, which makes this mode more physiological ^[2]. These devices work by means of a turbine that will provide the necessary flow to reach the pre-set inspiratory and expiratory pressure values.

Moreover, some additional modes based on the possibility of a set backup respiratory rate can be found: assisted (or spontaneous “S”) where there is no safety backup respiratory rate; assisted with a backup respiratory rate (Spontaneous/timed “S/T”) or assisted-controlled (A/C) where there is a programmed safety frequency and in the event that the patient’s spontaneous frequency is lower than this, the ventilator starts to provide the cycles that will cycle by time in A/C and by flow in S/T and controlled (C).

Contrarily to volume-limited modes, an important feature of pressure modes that makes them particularly suitable for delivery as home NIV is their ability to compensate for moderate leaks. Additionally, patient-ventilator synchrony is usually much better in pressure-limited modes. The main drawback of pressure-limited modes is that they do not ensure a specific tidal volume (except for hybrid modes). Thus, the tidal volume will depend on the programmed pressure support (higher PS, higher VT), the impedance of the patient’s respiratory system (resistance and compliance) and the magnitude of the patient’s inspiratory effort. The main settings that can be modified in a barometric mode in a standard ventilator would be the following:

2.1. IPAP and EPAP Levels

The absolute values of both pressures will depend on several conditions: for example, EPAP level is usually set at a minimum pressure of 4 cm H₂O, to avoid rebreathing if a single limb with intentional leakage is used. If a double limb or a single limb with an active valve is used, the addition of EPAP is not strictly necessary. In addition, EPAP can be increased in certain conditions, such as upper airway obstructions in patients with obstructive apnea syndrome or expiratory flow limitation (EFL) in patients with COPD or obesity.

The IPAP level is usually set based on tidal volume monitoring. A value of around 7–8 mL/kg of ideal body weight is usually taken as a reference.

2.2. Backup Respiratory Rate (BURR)

BURR is defined as the number of controlled breaths delivered by the ventilator in one minute to cope with an eventual drop in the patient’s RR in the absence of patient effort. It is usually set 2–4 cycles/min below the patient’s spontaneous RF, as in volumetric modes. The lack of BURR was associated with an increased number of upper airway events, mixed and central, in patients with obesity-hypoventilation syndrome ^[3]. There are differences between manufacturers regarding the cycling criteria in controlled cycles when pressure support is used. In most models, the transition criterion from inspiration to expiration is time, although some manufacturers maintain the flow criterion for both assisted and controlled breaths. Finally, some hybrid modes, such iVAPS (Intelligent Volume Assured Pressure Support) and AVAPS (Average Volume-Assured Pressure Support) have auto backup rates with iVAPS targeting 2/3 of the set rate and increasing during central apnea and AVAPS based on recent breathing patterns (only in automatic mode) ^[4].

2.3. Trigger Sensitivity

It corresponds to the effort level at which a cycle is delivered in response to the patient’s demand, and controlling the transition from EPAP to IPAP. It is usually indicated as a numerical value in L/min or in an ordinal scale, corresponding the lower values to the most sensitive levels. It is a crucial parameter to ensure patient-ventilator synchronization. Since the introduction of trigger mechanisms 30 years ago, ventilator technology has improved, so the effort required by the patient to obtain an assisted cycle is considerably less in modern ventilators compared to those of the last decade of the last century. This technological progression has been accompanied by a redesign of the trigger variable (from older pressure trigger designs to newer sophisticated electronic trigger systems). As reflected in the review by Sinderby ^[5], the parameters that are usually taken as a reference for trigger designs are not measured directly but are the indirect consequences of the patient’s ventilatory drive in the circuit. Thus, pressure triggers react to a depressurization in the circuit because of patient effort and flow triggers react to increases in this parameter measured inside the ventilator. Only the NAVA (Neural Adjusted ventilatory assist) system uses a parameter directly measured in the patient, such as diaphragmatic electromyography, but its invasiveness (it requires placement of a nasogastric tube equipped with electromyographic sensors) and its high cost make it impractical to implement as a mode of home NIV ^[6].

Early ventilator models used the pressure trigger, in which the patient’s effort in front of a closed valve decreases the pressure in the circuit below a pre-set threshold (sensitivity) to receive the ventilator-assisted cycle. Apart from the decreased sensitivity compared with newer designs, the main drawback of the pressure trigger was the presence of nonintentional leakage, since the patient’s effort needs to be higher to compensate for these leaks and at the same time enough to decrease the pressure inside the limb ^[7].

Carteaux et al. ^[8] studied the performance of 19 ventilators under three conditions: no leak, continuous leak and inspiratory leak (using an underwater column). They found significant differences in the presence of trigger delay and autotriggering if the ventilators studied incorporated a specific non-invasive ventilation algorithm or not. Autotriggering was not observed in any of the NIV-specific ventilators. In contrast, Ferreira et al. ^[9], also on a bench test, found that most of the ventilators studied (specific to critical and acute non-invasive ventilation) required additional manual adjustments to avoid the presence of asynchronies in presence of leakage. A good surrogate for anticipating asynchronies is the increased work required to activate the mechanism in the presence of leaks (pressure-time product for the trigger -PTPtrig-), which would be in line with the effects of auto-adjusting algorithms, which decrease the sensitivity of the trigger in the presence of leaks ^[10].

Another complex trigger system is the so-called “Energy trigger” (Breas), based on the calculation of the first flow derivative ^[11]. In a bench study combining leakage and simulated obstruction, Zhu et al. ^[12] already demonstrated that in a ventilator with the same Energy trigger model (Vivo 60^®) the critical leakage level for triggering asynchronies was lower than in other ventilators.

In conclusion, the set of design differences, together with the manufacturer-specific trigger sensitivity levels, may account for the individual behaviors in the ventilators. It appears that flow triggers are less sensitive, mainly in the presence of leakage and probably because of the auto-adjusting sensitivity algorithm, while complex trigger systems tend to be more sensitive but may also be less specific.

2.4. Pressurization Ramp (“Rise Time”)

From a conceptual point of view, the ramp is the parameter that controls the time between the start of the inspiratory cycle and the point at which the prescribed IPAP is reached. The ramp or rise time can be set based on a time scale (usually ms) or in a numerical analogic scale, corresponding usually to the lower numbers to the fastest ramp and shortest time values. At the same time, these shortest time levels will also correspond to the highest flow values, so the ramp will be closely linked to the inspiratory muscle unloading. In the acute patient mainly shorter values should be set, whereas, in chronic home NIV, shorter values should be mainly reserved for the obstructive patient and longer values for the restrictive one. Typical values are between 50 and 500 ms. It should be noted that the rise time will influence the cycling sensitivity depending on whether the peak inspiratory flow is reached earlier or not.

However, the concept of pressurization ramp should be analyzed in-depth, since it was classically considered as a “time-set”. In other words, the time to reach IPAP was constant for each ramp level, irrespective of changes in pulmonary or rib cage mechanics or increases in the patient’s effort. Battisti et al. ^[13] studied 10 home ventilators in a bench model, setting two different pressure levels on the ventilators and four active effort levels on an active simulator. In this study, significant differences in ventilator response to increasing effort and leakage were also found in the pressure-time product (PTP) at 300 ms, Similar results were found in a group of intermediate respiratory care ventilators ^[14]. These differences are hard to explain if the ramp was time-limited.

Lalmolda et al. conducted a bench to-bedside study for evaluating the pressurization capabilities of nine different ventilators, two for the acute care setting and seven for home ventilation. They found important differences among studied ventilators in PTP300. In addition, the bedside study focused on COPD patients and used parasternal EMG as a surrogate of inspiratory muscle unloading, showed that the ventilators with worse performance in bench tests showed less muscle unloading at the same pressure support level. Finally, these authors concluded that the parameter controlling the ramp seems not to be the time, but the flow changes (mathematically, the first derivative of the flow) ^[15].

2.5. Cycling to Expiration

Cycling to expiration or expiratory trigger is related to the criterion used by the ventilators to control the transition from the inspiratory to expiratory phase. These criteria are a percentage of peak (maximum) flow in pressure support mode and a fixed inspiratory time in pressure control mode. In PS mode, an expiratory trigger can be set directly as a percentage of peak inspiratory flow or in a numerical (1–9) or nominal scale (sensitive, medium, low sensitive) depending on the manufacturer. In the first case (direct setting of the percentage of peak flow) it is usually set high if a short inspiratory time is desired (e.g., in COPD patients, where a short inspiratory time/total time ratio is desirable) and lower if a longer inspiratory time is desired (e.g., in restrictive patients). If a numerical scale is used, the lowest values (1 to 3) usually correspond to the highest percentages with respect to peak flow. Finally, on the nominal scale, the term “sensitive” is also related to the highest percentages relative to peak flow, and therefore, to the shortest inspiratory time values.

Some devices are equipped with more sophisticated systems, such as the “Auto-trak™ Respironics” which detects the patient’s breathing pattern based on an imaginary waveform and automatically adjusts the trigger sensitivity and cycling thresholds.

2.6. Maximum and Minimum Inspiratory Time

These parameters are considered “safety cycling” parameters. In the case of maximum inspiratory time, it works as an inspiratory time limiter in the case of important leaks, when the flow cycling criterion would never be reached (or reached too late for the neural inspiratory time of the patient) leaving the ventilator inadequately in the inspiratory phase. By contrast, the function of the minimum inspiratory time seems more controversial: its main role according to some manufacturers would be to ensure adequate inspiratory phase time, with improvement in alveolar ventilation.

2.7. Rise Fall or Expiratory Ramp

It is present only in some ventilator models. It acts in a similar way to the inspiratory ramp but in the transition from inspiration to expiration, which may be more abrupt or slower. There are no studies in the literature to support its usefulness. It should be considered that the use of slower than usual ramps may tend to increase the inspiratory time.

References

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Johnson, K.G. APAP, BPAP, CPAP, and New Modes of Positive Airway Pressure Therapy. Adv. Exp. Med. Biol. 2022, 1384, 297–330.
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