The flow setting on a mechanical ventilator estimates the delivered flow of inspired gas. During controlled mechanical ventilation (CMV) with an apneic patient, high flows shorten T I and may result in higher peak pressures and poor gas distribution. Slower flows may reduce peak pressures, improve gas distribution, and increase Paw at the expense of increasing T I . This may cause cardiovascular side effects and shorten T E , leading to air trapping.
A long T I (taking 3 to 4 time constants) has been shown to improve ventilation in nonhomogeneous lungs like those seen in ARDS. Fast flows (using fewer time constants) may benefit patients with increased airway resistance (Raw), as in COPD, providing longer T E and preventing or reducing air trapping (long T E of 3 to 4 time constants).
Flow rates up to 100 L/min may improve gas exchange in patients with COPD, by providing a longer T E . However, flows that are too high may affect distribution in the lung and cause immediate and persistent tachypnea, as well as increase peak inspiratory pressures.