Mandatory Minute Volume Ventilation

Abstract

Mandatory minute volume ventilation (MMV) is an original mode of mechanical ventilation introduced by Hewlett et al. in 1977. In this mode, the patient is guaranteed a predetermined (expired) minute volume (V_E), called the preset minute volume. If the patient is able to spontaneously breathe sufficiently to fit the preset minute volume, the ventilator does not deliver any mechanical breath. If the patient is unable to breathe spontaneously, the ventilator delivers mechanical breaths so that the patient receives a minute volume equal to the preset V_E. If the spontaneous breathing of the patient is inferior to the preset V_E, the remainder is automatically provided by the ventilator. The values of the preset V_E and of the mechanical tidal volume (V_T) are predetermined. The adjustments of the mechanical ventilation according to the changes in spontaneous breathing are usually (CPU 1) achieved by a modification of the rate of mechanical breaths. If the spontaneous ventilation exceeds the preset V_E, the ventilator progressively decreases the frequency of mechanical breaths and can possibly stop delivering mechanical ventilation. Conversely, if the spontaneous ventilation is less than the preset V_E, the ventilator progressively increases the rate of mechanical breaths. If no spontaneous breathing is present, all of the preset V_E will be provided by the ventilator with a frequency of mechanical breaths equal to the ratio of the preset V_E to V_T. With this mode of ventilatory support, the rate of mandatory breaths automatically changes according to the variations in the ability of the patient to breathe (Fig. 1). This ventilatory mode which combines spontaneous breaths and mechanical breaths is close to the intermittent mandatory ventilation (IMV). With IMV, the ventilator tidal volume and respiratory rate are predetermined and spontaneous breaths are permitted between mechanical breaths. However, if IMV is used with a low rate of mandatory breaths, any depression of spontaneous breathing (for instance, following injection of sedative drugs) may result in an alveolar hypoventilation with severe respiratory acidosis. Alveolar hypoventilation should not occur with MMV, at least if the preset V_E is high enough, because the patient is ensured to receive the preset V_E even if his spontaneous ventilation is markedly depressed. The selection of the preset V_E is of great importance: a low preset V_E will be associated with a mainly spontaneous ventilatory mode, but alveolar hypoventilation may occur. Conversely, with too high a preset V_E that largely