Effect of Ventilator Flow Rate on Respiratory Timing in Normal Humans

Abstract

Respiratory rate (RR) increases as a function of ventilator flow rate (V). 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 (Vmin) to 2.5 (Vmax) L/s. V T was kept constant. The pressure developed by respiratory muscles (Pmus) was calculated with the equation of motion (Pmus = V. 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 V, from 1.44 +/- 0.34 s at Vmin to 0.62 +/- 0.26 s at Vmax (p < 0.00001). Changes in TEn were inconsistent and not significant. TIn/Ttot decreased significantly (0.30 +/- 0.06 at Vmin to 0.18 +/- 0.09 at Vmax; p < 0. 00001). We conclude that TI is highly sensitive to ventilator flow, and that the RR response to V is primarily related to this T In response. Because an increase in V 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 V over the range from 0.8 to 2.5 L/s.

Termination of inspiration by phase-dependent respiratory vagal feedback in awake normal humans

“…Thus we feel that the failure to demonstrate a reduction in TI during T assist with flow assist alone probably reflects the lower level of assist both early (since E assist did decrease TI) and over the entire breath (since combined assist of greater magnitude shortened TI in our previous study). Much larger steady-state increases in inspiratory flow in mechanically ventilated subjects also shorten TI (6,16,20). The results of the present study indicate that more subtle changes in inspiratory flow may also modulate the timing of inspiratory efforts on a breath-bybreath basis if the assist is given early during inspiration.…”

“…The reduction in TI is probably reflex in origin, and the afferent information is probably carried by the vagus nerve. Previous studies of steady-state increases in inspiratory flow demonstrated a decrease in TI (6) and an increase in respiratory rate during mechanical ventilation that was not affected by breathing route (oral or nasal) or upper airway anesthesia. This suggests that at least some of the receptors mediating the flow-related responses are located in the lungs or chest wall (9).…”

Effect of varying the pressurisation rate during noninvasive pressure support ventilation

“…flow or time cycled) are the most clinically used assisted mode of NIV [5]. Pressure support ventilation (PSV), for example, applies a constant level of positive airway pressure during spontaneous inspiratory efforts; the patient is not a "passive" part of the process, but may respond to the changes in ventilator settings via chemical, mechanoreceptor reflex and behavioural feedback systems [6][7][8].…”

Effects of controlled and pressure support mechanical ventilation on rat diaphragm muscle

“…The animals in our study, however, were monitored by flow and airway pressure waves. Alterations in airway pressure and flow waves suggest inspiratory efforts during MV, and have been show to coincide with electromyographic analysis 30 . Uniform airway pressure and flow waves analyzed by the ventilator display, together with the consistence of the respiratory frequency set by the investigator, indicate that passive ventilation was in fact achieved.…”