Effects of pressure ramp slope values on the work of breathing during pressure support ventilation in restrictive patients

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Abstract
To investigate, in restrictive patients, the influence of pressure ramp slope values on the efficacy of pressure support ventilation. Prospective study. A university hospital medical intensive care unit. Twelve intubated restrictive patients. Patients were randomly assigned to four sequences in which the values of the slope of the pressure ramp increase were modulated so that the plateau pressure was reached within a predetermined time: 0.1, 0.50, 1, or 1.50 secs. The more rapidly the pressure plateau was achieved, the higher was the initial flow rate. For convenience, these four different ventilatory settings were termed T 0.1, T 0.5, T 1, and T 1.5. We measured the following parameters 10 mins after application of each pressure ramp slope: inspiratory work of breathing, breathing pattern, and intrinsic PEEP (PEEPi). Work of breathing was evaluated using Campbell's diagram, and expressed as a percentage of the values observed under spontaneous ventilation. A marked interindividual variation of the values for work of breathing was observed under spontaneous ventilation; the mean value for work of breathing was 1.97 +/- 0.82 joule/L, with a range of 1.22 to 4.10 joule/L. Comparison between the means for each sequence and each variable measured was performed by two-way analysis of variance with internal comparisons between sequences by Duncan's test. Between the first (T 0.1) and the last (T 1.5) sequence, the reduction of values of the pressure ramp slope induced a progressive increase in the values for work of breathing, regardless of the mode of expression (in joule, joule/L, or joule/min). The values for work of breathing (joule/L), expressed as a percentage of the values observed under spontaneous ventilation, increased from 44.2 +/- 14.4% to 78.3 +/- 17.8% (p < .001). In contrast, the reduction of the pressure ramp slope values and initial flow rate did not induce any significant change in tidal volume, respiratory frequency, and PEEPi. Among the four tested slope values, the steepest was that which induced the lowest possible work of breathing in restrictive patients ventilated by pressure support ventilation. In this type of patient, we therefore suggest that the programmed pressure value should be reached by using a steep pressure ramp slope. (Crit Care Med 1999; 27:715-722)

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