Volume-cycled ventilators allow selection of rate and contour of inspiratory flow. Inappropriately rapid inspiratory flow rates may worsen the distribution of ventilation; the longer exhalation time may reduce air trapping in patients with airflow obstruction. Although peak airway pressure rises as flow rate increases, the mean airway pressure averaged over the entire ventilatory cycle may remain unchanged or even fall as flow rate increases.
On most volume-cycled ventilators, the inspiratory time percentage is usually set indirectly by specifying tidal volume, frequency, and inspiratory flow rate. In general, shorter inspiratory-to-expiratory (I:E) ratios reduce mean intrathoracic pressure. To avoid gas trapping, many ventilators provide a visual warning or auditory alarm when the I:E ratio exceeds 1:1 (duty cycle > 0.5). This threshold defines inverse ratio ventilation.
The extent to which the ventilator takes up the inspiratory work of breathing is a function of the triggering sensitivity and the margin by which flow delivery exceeds flow demand (Fig.2). Triggering sensitivity is generally set to the lowest value that does not result in inadvertent premature 'auto-cycling'. The flow metered by the ventilator must always satisfy the patient's inspiratory drive to breathe. Otherwise, the ventilator not only fails to reduce the work of breathing, but may also force the patient to overcome the resistance of the ventilator circuitry as well as his or her own internal impedance to airflow and chest expansion.
Fig. 2 Airway pressure tracing during volume-cycled ventilation with constant inspiratory flow. With inspiratory flow profile and tidal volume unchanged, the difference in machine work during assisted and controlled cycles quantifies the subject's work of breathing. Vigorous efforts may cause the subject to work against the external circuitry of the ventilator (green shaded region).
Comfortably rapid inspiratory flow rates are also desirable to ensure that the machine completes inflation before the patient's own ventilatory rhythm cycles into its exhalation phase. Delayed opening of the exhalation valve causes the patient to 'fight the ventilator'. As a general rule, the ventilator's average inspiratory flow should be approximately four times the minute ventilation. Peak flow should be set 20 to 30 per cent above this average value when a decelerating waveform is used. Because the patient's greatest demand for flow occurs in the very first part of the machine cycle, it makes sense to employ a decelerating-flow rather than a constant-flow delivery profile for the same tidal volume and inspiratory time settings. Decelerating-flow profiles also tend to improve the distribution of ventilation in a lung with heterogeneous mechanical properties (widely varying time constants).
To minimize the risk of barotrauma, maximum pressure (alarm and 'pop-off' pressure) should be set no more than 15 to 20 cmH 2O above the peak dynamic cycling pressure observed in a typical breath during constant flow. Unless there is severe airflow obstruction, the pop-off alarm should be set more closely than this (5-10 cmH2O) if a decelerating flow waveform or pressure control is used because end-inspiratory dynamic and static (plateau) pressures are not as widely separated.
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