Intermittent Demand Ventilation (IDV): A New Technique for Supporting Ventilation in Critically Ill Patients

Abstract

The control mode of positive-pressure mechanical ventilation (MV) can increase mean intrathoracic pressure enough to embarrass venous return and cardiac output in patients with compromised cardiovascular reserves. The classic answer in MV has been to use large tidal volumes and slow ventilation rates to minimize increases in mean intrathoracic pressure, but frequently this requires undesirable sedation or paralysis. The assist mode of MV also is inadequate for most critical patients because of their rapid and erratic ventilation. A recently used alternative is intermittent mandatory ventilation (IMV), which allows the patient to breathe ambient gas at his own spontaneous rate, with timed mechanical breaths delivered intermittently by a ventilator. This is fine for weaning patients from ventilators, and it may be useful for ventilatory support when used with high PEEP in some patients. But we find that many patients with cardiovascular instability do not tolerate IMV to our satisfaction, perhaps because some timed mandatory breaths come at inappropriate moments and combine with spontaneous breaths to cause variable increases in intrathoracic pressure, leading to hemodynamic instability. Therefore we have been experimenting with a variant mode called intermittent demand ventilation (IDV), in which the mechanically augmented breath is delivered on patient demand, not on a timed mandatory schedule. IDV theoretically enhances ventilatory support by causing minimal elevation of mean intrathoracic pressure and minimal consequent embarrassment of venous return. We use the Searle VVA ventilator, which has an independent exhalation pressure line and a ventilation mode controller (VMC) to activate that line and cause the ventilator to deliver a positive-pressure breath. The VMC can be set to deliver anything from 1 augmented breath out of every 2 breaths to 1 augmented breath out of 100. When the VMC is activated, it waits for the patient's next inspiratory effort and interprets it as a demand signal, and the ventilator delivers the next tidal volume mechanically, as part of the patient's own rhythm of breathing. Experimental use of IDV on volunteers has shown that the IDV breathing pattern is more like spontaneous breathing than is IMV, and IDV mean intrathoracic pressures are lower than those in IMV. After limited clinical experience with IDV we think it probably offers advantages in support of patients who can do reasonable amounts of breathing work and that further clinical studies are warranted.