Synchronized Intermittent Mandatory Ventilation With and Without Pressure Support Ventilation in Weaning Patients With COPD From Mechanical Ventilation
The most critical time for patients with chronic obstructive pulmonary disease (COPD) mechanically ventilated for acute respiratory failure (ARF) is the weaning period. Asthma inhalers online Link Patients with hyperinflation and/or bad nutritional status2 are obviously exposed to difficulties in recovering sustained spontaneous breathing. Indeed, patients with COPD often do not tolerate discontinuation of mechanical ventilation (MV) due to the combination of a number of factors. During ARF, the increase in both inspiratory and expiratory flow resistances results in an increased mechanical load for the respiratory muscles, and leads to intrinsic positive end-expiratory pressure (PEEP) which acts as an inspiratory threshold load. Concurrently, the hyperinflation induces a flattening of the diaphragm which then operates on a less efficient portion of its force-length curve. So, COPD patients in ARF have to cope with an increased work of breathing that has to be overcome by respiratory muscles which are in a disadvantageous position. Furthermore, MV itself may aggravate intrinsic PEEP, may increase the mechanical load by the resistances of endotracheal tube and respirator cricuitry, and can be so considered as an additional burden for the respiratory muscles.
There is actually no gold standard procedure to wean COPD patients from MV. Several ventilatory modalities have been proposed to facilitate the recovery from MV and to reduce the weaning period. Spontaneous breathing trials via a T piece are still used under close monitoring. Minute mandatory ventilation is a volume-assisted ventilatory mode that did not appear to be well tolerated by patients suffering from airflow limitation. The assist mode and the intermittent mandatory ventilation (IMV) mode are demand systems based on a one-way valve trigger that opens when a given negative pressure is generated during the patient’s inspiratory effort. It has been shown that such systems may have a deleterious effect on the mechanics of breathing and may impair the recovery of patients and thereby limit weaning success. Nevertheless, IMV, and especially “synchronized” IMV (SIMV), remains largely employed for weaning patients who fail to come off the respirator.
Some other ventilatory modalities are now available on ‘ ‘new-generation’ ’ respirators despite the lack of scientific studies on their potential clinical benefits in humans. Pressure support ventilation (PSV) is a recent mode used alone or in association with other modes in patients requiring a ventilatory assistance. PSV acts by maintaining through the respirator circuitry a constant preset positive airway pressure during spontaneous inspiration. As in IMV, spontaneous breathing with PSV requires the patient to open the demand valve which might increase the work of breathing. However, Brochard and coworkers have shown that PSV reduces significantly the work imposed on the respiratory muscles. Therefore, an association of PSV and SIMV could be of some interest in weaning COPD patients. The aim of this work was to compare in such patients the effects of two weaning modalities— SIMV alone vs PSV added to SIMV—on the following: (1) the duration of the weaning period; (2) the oxygen cost of breathing; and (3) the respiratory pattern.