Work of breathing and breathing pattern

The most important effects are those related to the reduction in the work of breathing, and the changes induced in the characteristics of this work ( Brochard ei a/

1989). Pressure support tends to change the high-pressure and low-tidal-volume inspiratory efforts (which are usually observed in situations of acute respiratory failure) to inspiratory efforts producing lower pressures and higher tidal volumes (as usually occurs during normal breathing). Indeed, pressure support is not primarily directed to improving gas exchange. However, on increasing the levels of support, a decrease in Paco2 is usually observed.

Typical modifications in the breathing pattern induced by pressure support ventilation include an increase in tidal volume, a decrease in respiratory rate, and no major change in minute ventilation. Pressure support ventilation also induces a diminution in transdiaphragmatic pressure swings and in the pressure-time index of the diaphragm when compared with spontaneous breathing. Moreover, in patients who are difficult to wean, progressive increments of pressure support lead to parallel decreases in the work of breathing and oxygen consumption of the respiratory muscles, with disappearance of the electromyographic signs of impending diaphragmatic fatigue. These levels of support also allow accessory respiratory muscles (e.g. the sternocleidomastoid) to rest ( Biochard,,etM 1989).

Another important feature related to pressure support ventilation is its ability to overcome the extra work of breathing induced by the presence of the endotracheal tube and ventilator circuit and their related flow-resistive properties. The levels of pressure necessary to compensate for this extra work are variable and depend on several factors: the diameter of the endotracheal tube, the mean inspiratory drive, and the presence or absence of an intrinsic pulmonary disease. From a clinical point of view, levels of pressure support of about 8 cmH2O appear useful in overcoming this extra work of breathing.

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