Ventilator- and interface-related factors influencing patient-ventilator asynchrony during noninvasive ventilation

Abstract

Patient-ventilator asynchrony (PVA) is common in patients receiving noninvasive ventilation (NIV). This occurs primarily when the triggering and cycling-off of ventilatory assistance are not synchronized with the patient's inspiratory efforts and could result in increased work of breathing and niv failure. In general, five types of asynchrony can occur during NIV: ineffective inspiratory efforts, double-triggering, auto-triggering, short-ventilatory cycling, and long-ventilatory cycling. Many factors that affect PVA are mostly related to the degree of air leakage, level of pressure support, and the type and properties of the interface used. Careful monitoring and adjustment of these factors are essential to reduce PVA and improve patient comfort. In this article, we discuss the machine and interface-related factors that influence PVA during NIV and its effect on the respiratory mechanics during pressure support ventilation, which is the ventilatory mode used most commonly during NIV. For that, we critically evaluated studies that assessed ventilator- and interface-related factors that influence PVA during NIV and proposed therapeutic solutions.

Keywords: Air leak; humidity; interface; noninvasive ventilation; patient-ventilator synchrony.

Figure 1

This illustration shows the pressure and flow waves of pressure support. The delivered tidal volume is calculated by the area under the flow curve wave. The inspiratory airflow continuously decreases because, as air moves inside the lungs, the pressure in the alveoli builds up resulting in a reduced pressure difference between the machine and the alveoli (ΔP), and hence a decelerating flow wave. Cycling-off starts when airflow reaches a preset threshold, which is a preset percentage of maximal air flow (usually 25%). At this point, the ventilator stops to deliver inspiratory flow, the expiratory valve opens to allow passive exhalation, and pressure goes down to the set positive end-expiratory pressure. Increasing the cycling-off preset percentage of maximal airflow results in decreasing the inspiratory cycle. The illustration demonstrates two cycles; one of them has cycling-off preset percentage of maximal airflow of 25%, and the other is 50%. The inspiratory time is shorter with 50%, and hence the delivered tidal volume

Figure 2

It shows illustrations of pressure and flow waves of some of the common types of patient-ventilator synchrony during pressure support ventilation. (a) The figure illustrates pressure and flow waves during pressure support, where the patient tries to breathe, but he was unable to generate the needed effort required to trigger the ventilator. These minor efforts increase the work of breathing without generating enough tidal volume. (b) The figure illustrates pressure and flow waves during pressure support. When the ventilator does not meet the patients' demand for tidal volume, double-triggering may appear. In this example, two consecutive breaths occur with an interval of less than half of the mean inspiratory time. (c) The figure illustrates pressure and flow waves during pressure support. The machine delivers breaths in the absence of patient's effort, as inferred by the absence of a decrease in airway pressure prior to the machine-delivered breath. The illustration demonstrates the occurrence of more than three consecutive pressurizations at a ventilator frequency. (d) The figure illustrates pressure and flow waves during pressure support. The illustration shows an anomalous flow wave and an increased inspiratory time due to inspiratory air leak