Am. J. Respir. Crit. Care Med., Volume 159, Number 3, March 1999, 710-719

Effect of Ventilator Flow Rate on Respiratory Timing in Normal Humans

RAFAEL FERNANDEZ, MANUEL MENDEZ, and MAGDY YOUNES

Section of Respiratory Diseases, Department of Medicine, University of Manitoba, Winnipeg, Manitoba, Canada

Respiratory rate (RR) increases as a function of ventilator flow rate (). We wished to determine whether this is due to a decrease in neural inspiratory time (T In), neural expiratory time (TEn), or both. To accomplish this, we ventilated 15 normal subjects in the assist, volume cycled mode. Ventilator flow rate was varied at random, at four breaths with each step, over the flow range from 0.8 (min) to 2.5 (max) L/s. V T was kept constant. The pressure developed by respiratory muscles (Pmus) was calculated with the equation of motion (Pmus =  · R + V · E  Paw, where R = resistance, V = volume, E = elastance, and Paw = airway pressure). Electromyography of the diaphragm (Edi) was also done in five subjects. TIn and TEn were determined from the Pmus or Edi waveform. TIn decreased progressively as a function of , from 1.44 ± 0.34 s at min to 0.62 ± 0.26 s at max (p < 0.00001). Changes in TEn were inconsistent and not significant. TIn/Ttot decreased significantly (0.30 ± 0.06 at min to 0.18 ± 0.09 at max; p < 0.00001). We conclude that TI is highly sensitive to ventilator flow, and that the RR response to is primarily related to this T In response. Because an increase in progressively reduces T In/Ttot, and this variable is an important determinant of inspiratory muscle energetics, we further conclude that inspiratory muscle energy expenditure is quite sensitive to over the range from 0.8 to 2.5 L/s.

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