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Synopsis: This study of the level of comfort of three modes of assisted ventilation used during weaning used healthy volunteers who found that assisted spontaneous breathing (pressure support) was the most comfortable and synchronized IMV was the least comfortable of the three.
Source: Russell WC, et al. Crit Care Med 2000;28:3645-3648.
Russell and colleagues at the Leicester royal Infirmary recruited 24 healthy adult volunteers and had them breathe spontaneously through a mouthpiece on 5 cm H2O of continuous positive airway pressure (CPAP) from a Drager Evita II ventilator, until they were at ease with the environment and the apparatus. Then, in random order, Russell et al had the subjects breathe for three minutes on three different modes available for use during weaning, and subsequently to assess the relative comfort of each mode using a visual analog scale. The modes tested were all used with 5 cm H2O positive end-expiratory pressure (PEEP). The first was synchronized intermittent mandatory ventilation (SIMV), with tidal volume 5 mL/kg, inspiratory flow 60 L/min, and mandatory rate 8 breaths/min. Second was assisted spontaneous breathing (equivalent to pressure support), with an inspiratory pressure of 10 cm H2O above PEEP, flow cycled at 25% peak flow. Third was biphasic positive airway pressure, a mode available on the Evita II which is essentially the same as airway pressure release ventilation without inversed inspiration:expiration ratio, with set inspiratory pressures of 5 and 10 cm H2O, a rate of 8 breaths/min, and an inspiration:expiration ratio of 1:2.
Individual scores for the three ventilation modes among the 24 subjects ranged between extremes of 0.3 and 9.3 arbitrary units on the visual analog scale. There was a clear separation in mean scores for the three modes: assisted spontaneous breathing (pressure support) 2.03; biphasic positive airway pressure 4.12; and SIMV 5.38. These scores were significantly different with P < 0.001. In terms of subjective preference, 20 subjects ranked assisted spontaneous breathing first and only one ranked it third; in contrast, no subject ranked SIMV first and 19 ranked it third in preference. The preferences for biphasic positive airway pressure were intermediate between the other two, and individual comparisons between pairs of modes all showed statistically significant differences.
Under the conditions of this study, healthy volunteers without a previous history of being mechanically ventilated considered assisted spontaneous breathing most comfortable, and SIMV least comfortable, of the three modes examined. The differences in comfort level were considerable.
This study was designed to get some idea of what patients experience during the transition from full ventilatory support to completely spontaneous ventilation. It is hard to know what the experiences of these healthy volunteers, breathing for three minutes on each mode through a mouthpiece, has to do with the experience of mechanical ventilation by critically ill patients. However, it makes sense that completely spontaneous breathing, with each breath boosted by positive pressure at very high inspiratory flow, would be less distressing than having to accommodate to a fixed rate of pressure changes or of preset small tidal volumes at relatively low inspiratory flow. I have always considered SIMV, when used to provide partial ventilatory support during weaning, to be an unnatural and most likely highly uncomfortable arrangement for an alert patient, particularly one who is dyspneic. Thus, the results of this study are in keeping with my long-held bias on the subject. I wish Russell et al had included a "T-piece" arm in their evaluation, and would be interested to know whether these normal volunteers would have liked completely unassisted breathing most of all.