Ventilator and Mode for Noninvasive Positive Pressure Ventilation

Ventilators use one of two basic methods: volume-preset and pressure-preset (10). With volume-preset, the ventilator always delivers the tidal volume which is set by the clinician, regardless of the patient's pulmonary system mechanics (compliance, resistance, and active inspiration). However, leaks at the skin-mask interface or through the mouth when using a nasal mask, reduce the volume received by the patient. Conversely, with pressure-preset, changes in pulmonary mechanics directly influence the flow and the delivered tidal volume (lower or higher) since the ventilator delivers the set pressure all along inspiration. Then leaks augment the flow and tend to maintain the tidal volume (19,20). It is important to understand that NIPPV is dominated both by rapid variations of nonintentional leaks and of the geometry and the resistance of the upper airway (21). Obviously, leaks and airway resistance partly interact. Facing these continuous changes the respective advantages and drawbacks of volume- and pressure-preset modes, which are opposite, make a predictable effect difficult. The way to begin and end inspiration is either initiated by the ventilator or in response to a patient effort to do so, allowing one to define the main modes: (i) control (ii) assist-control (iii) assist or spontaneous (possible only with pressure-preset). Most of the home ventilators function uniquely in volume or in pressure preset but modern ones may deliver inspiration according to the two modes. Besides the classical circuitry including two valves (on the inspira-tory and expiratory limbs) alternatively closing and opening, bilevel positive airway pressure (BPAP) ventilators are simpler and therefore lend themselves to home mechanical ventilation (22). Inspiratory and expiratory pressures are alternatively established in a single circuit incorporating an intentional, calibrated leak located close to the patient or even on the mask. The theoretical disadvantage with such a circuit is the risk of a variable CO2 rebreathing. Concern about the risk of CO2 rebreathing is not definitively documented (23-25) even if the trend is to consider it as negligible (26-28) provided positive expiratory pressure is applied in order to eliminate CO2 through the intentional leak (at least 2-4 cm H2O). Depending on the ventilator, all the different modes and refined settings and even closed-loop modes usually applied in the intensive care unit, are more or less available. Some ventilators may analyze ventilation in an on-going manner, keep it in an internal memory and provide the data for further assessment. The general objective is to provide many possible capabilities in order to have enough tools to adapt and optimize patient-machine synchronization. While conceptually attractive, sufficient studies have not been performed to document or refute the advantages of such complexity in the context of noninvasive home ventilation.

Choice of the Ventilator and Mode

Many clinicians currently prefer a pressure-preset ventilator in assist mode as the first choice with the view to offer the better synchronization (9). In fact, in the studies comparing volume- and pressure-preset ventilators no clear differences in the correction of hypoventilation in short-term studies (29-37) and in long-term outcomes (38-40) are shown. This is understandable since during NIPPV, leaks and resistance changes alternate very quickly and when the pressure target does well, the volume target does not do well, and conversely. However, it is important to remain flexible by trying alternative approaches if problems occur with one or the other type of ventilator. Besides, it should be noted that batteries are unavailable or they offer a short autonomy for BPAP ventilators and this would limit security and mobility of neuromuscular patients with hypoventilation and then drive the preference to the volume ventilator.

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