Influence of ventilator settings in determining respiratory frequency during mechanical ventilation

During mechanical ventilation, changes in inspiratory flow and tidal volume (VT) have been shown to alter respiratory frequency (f). However, the chan ges in flow and VT have been accompanied by alteration in ventilator inspir atory time (T-I,T-vent), and it is not clear which variable is the primary determinant. To address this issue, we employed four protocols in 15 health y volunteers receiving assist-control ventilation. When VT was fixed and fl ow was delivered at 30, 60, and 90 L/min, f increased as a function of the increase in flow and the decrease in T-I,T-vent. When flow was held constan t and VT was changed among 0.5, 1.0, and 1.5 L, f increased as a function o f the decreases in VT and T-I,T-vent When flow was increased from 60 to 90 L/min and these changes were balanced with Vr settings of 1.0 and 1.5 L. to maintain a constant T-I,T-vent f did not change. When flow and VT were hel d constant and T-I,T-vent was varied by the application of inspiratory paus es (0 to 2 s), f decreased as a function of the increase in T-I,T-vent (p < 0.001). In conclusion, the imposed ventilator inspiratory time during mech anical ventilation can determine f independently of delivered inspiratory f low and Vr.