BIPHASIC POSITIVE AIRWAY PRESSURE (BIPAP) - A NEW FORM OF ASSISTED VENTILATION

  • 1 September 1989
    • journal article
    • research article
    • Vol. 38  (9) , 452-458
Abstract
Two modes of combining spontaneous breathing and mechanical ventilation are already in use periodic mechanical support always followed by a period of spontaneous breathing (intermittent mandatory ventilation; IMV) and mechanical support of each spontaneous breath (inspiratory assistance; IA). Biphasic positive airway pressure (BIPAP), in contrast, is based on neither of the above mentioned principles. It is rather a mixture of pressure controlled (PC) ventilation and spontaneous breathing, which is unrestricted in each phase of the respiratory cycle. The BIPAP circuit switches between a high (Phi) and a low (Plo) airway pressure level in an adjustable time sequence. At both pressure levels the patient can breathe spontaneously in a continuous positive airway pressure system (CPAP). The volume displacement caused by the difference between Phi and Plo and the BIPAP frequency (F) contribute the mechanical ventilation to total ventilation. Duration of he Phi and the Plo phases can be independently adjusted. Similar to the I:E ratio during controlled ventilation, the phase time ratio (PhTR) is calculated as the ratio between the durations of the two pressure phases. A PhTR>1:1 is called IR-BIPAP. A BIPAP system can be set up either as a continuous flow system, or as a demand valve system. A continuous-flow BIPAP system consists of a high-flow CPAP system, a reservoir bag, and a pneumatically controlled membrane valve in the expiratory limb. A magnetic valve operated by an impulse generator switches between Phi and Plo, controlling the pop-off pressures of the exspiratory valve. To avoid large volume displacements of the reservoir bag as a result of the difference in BIPAP pressure, an additional magnetic valve driven by the impulse generator and a check valve are added in the inspiratory limb. In the demand valve system the setting of Phi and Plo not only controls the expiratory/PEEP valve but is also used as reference pressure for the demand valve. BIPAP does not fit into the conventional classification system for ventilatory techniques. If the patient does not breathe spontaneously, the BIPAP system starts to work according to the familiar pattern of PC ventilation. While a patient is breathing only at the low pressure level BIPAP is essentially an IMV, whereas with the onset of spontaneous breathing at both pressure levels a new ventilatory pattern is established. Featuring all modes of ventilation at the same time with no necessity for changing the setting of a ventilator, BIPAP is a useful aid in the process of early weaning.