At the same time, the Vt will be watched by the second closed loop controller. Again, for paralyzed patients, the inspiratory pressure level is increased until the target Vt is achieved. As soon as the patient develops significant breathing activity, the Vt, at a given inspiratory pressure level, starts to rise. In response to this rise, the tidal volume controller reduces pressure support to maintain the target Vt, thereby weaning the patient from pressure support. If the patient becomes tachypneic, Vt usually drops and the tidal volume controller responds to this by increasing the pressure support level that increases Vt and decreases the respiratory rate. Thus, appropriate control of the volume will control also the respiratory rate. Here
It may be that a patient needs some challenge to start spontaneous breathing activity sufficient for a successful weaning. This can be achieved by reducing the V’gA target. In a pilot study on nine patients, we had determined that a target V’gA of one third of the measured value would create enough challenge yet still present a reasonable backup in case the wean would fail.
Twenty-seven patients who had required mechanical ventilatory support in our respiratory intensive care unit (RICU) for more than 24 h and who met the standard weaning criteria used in the unit, ie, vital capacity >10 ml/kg, respiratory rate <30 breaths/min on synchronized intermittent mandatory ventilation (SIMV) <10/min, Pa02 >8 kPa on FIo2 <0.4 and <10 cm H20 PEEP, PaC02 normal and hemodynamically stable, were entered into an open prospective study to evaluate the ALV controller.
Our institutional ethical research committee approval was obtained before commencing the study and informed consent was obtained from each patient prior to entering them into the study.