Another important aspect related to the clinical application of pressure support is the adjustment of pressure levels. Modifications in the breathing pattern (increase in tidal volume and decrease in respiratory rate) induced by pressure support are very useful in adjusting ventilatory settings. Recent data suggest that, at least during the weaning period, levels of pressure support allowing a respiratory rate between 25 and 30 breaths/min would be adequate ( Jubran etal 1995). This goal is frequently achieved with pressure support of 15 to 20 cmH2O, but large interindividual variations in muscle unloading exist.
No additional beneficial physiological effects are obtained by increasing pressure levels more than required to unload the respiratory muscles ( Brochard etal 1989).
Higher pressure support levels will probably lead to an abnormal breathing pattern with excessively high tidal volumes, appearance of wasted inspiratory efforts, and even some periods of apnea. In the particular case of patients with chronic obstructive pulmonary disease who are difficult to wean, it should be remembered that addition of low levels of external positive end-expiratory pressure (PEEP) (about 5 cmH 2O) may be beneficial because this decreases the extra work of breathing induced by intrinsic PEEP which is due to dynamic hyperinflation with airflow limitation.
Once weaning has begun, reduction of support should be progressive, depending on the clinical tolerance of the patient (cardiovascular and neuropsychological status, gas exchange, and breathing pattern are the most important parameters to be assessed). The reduction is usually accomplished in one or two steps daily with a decrease in pressure support level of 2 to 4 cmH 2O in each step. When the patient tolerates levels of pressure support of approximately 10 cmH 2O (or lower in cases of tracheostomy), extubation is recommended.
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