2000 OPEN FORUM Abstracts
Adaptive Support Ventilation (ASV)
Robert S. Campbell, RRT FAARC University of Cincinnati Medical Center, Cincinnati, Ohio
Adaptive Support Ventilation (ASV) is a form of closed-loop ventilation that is based on the minimal work of breathing concept described by Otis et al. in 1950. Advantages of ASV include automatic selection of initial ventilator parameters based on information gathered during a sequence of test breaths. Most ventilator parameters are then automatically titrated/adjusted based on changes in lung mechanics and patient effort.
Clinician input includes patient ideal body weight (IBW), PEEP, FiO2, high-pressure limit/alarm and a setting referred to as "percent minute volume" (% Min Vol.). In combination with IBW, the % Min Vol. determines the minimum minute volume to be delivered to the patient. With a % Min Vol. of 100%, the ventilator will attempt to deliver 100 ml/min/kg of minute ventilation to an adult and 200 ml/min/kg to a child. The % Min Vol. control is adjustable from 20 to 200% for the provision of full ventilatory support or incremental levels of partial ventilatory support.
The ASV algorithm uses the Otis formula along with IBW (which determines dead space) to adjust a number of ventilator variables including mandatory breath rate, tidal volume, inspiratory time, and inspiratory pressure. Input to the formula includes breath-by-breath ventilator measurement of system compliance, airway resistance and intrinsic PEEP using least square fitting technique. Dead space is determined as 2.2 ml/kg based on setting of IBW. Target values for tidal volume and respiratory frequency are determined and maintained by the ASV algorithm through manipulation of the above mentioned ventilator parameters. Algorithm priorities are to avoid/eliminate/treat: 1) apnea, 2) rapid/shallow breathing, 3) volu- or baro-trauma, and 4) intrinsic PEEP.
Breath delivery during ASV incorporates pressure limited, time-cycled mandatory breaths and pressure limited, flow-cycled patient controlled breaths. ASV may be viewed as an "automatic" ventilation mode for full ventilatory support, partial ventilatory support, and as a "weaning" mode. Patient safety is the main priority during ASV, although this safety is based on mechanics and not gas exchange. ASV will not eliminate the possibility of hypoxemia and/or hypercarbia, rather when properly set and monitored will minimize the incidence and severity of such events. Clinician understanding and interaction with the mechanical ventilator during ASV is paramount to success of this mode. ASV will certainly not eliminate the need for specially trained respiratory care practitioners in critical care, rather solidify the requirement for our presence in the ICU.