Adaptive lung ventilation (ALV) - Evaluation of a new closed-loop algorithm for surgery in the lateral decubitus position

The lateral decubitus position is the standard position for nephrectomies. There is a lack of data about the effects of this extreme position upon respiratory mechanics and gas exchange. In 20 patients undergoing surgery in the nephrectomy position, we compared a new closed-loop-controlled ventilation algorithm, adaptive lung ventilation (ALV), which adapts the breathing pattern automatically, to the respiratory mechanics with conventionally controlled mandatory ventilation (CMV). The aims of our study were (1) to describe positioning effects on respiratory mechanics and gas exchange, (2) to compary ventilatory parameters selected by the ALV controller with traditional settings of CMV, and (3) to assess the individual adaptation of the ventilatory parameters by the ALV controller. The respirator used was a modified Amadeus ventilator, which is controlled by an external computer and possesses an integrated lung function analyzer. In a first set of measurements, we compared parameters of respiratory mechanics and gas exchange in the horizontal supine position and 20 min after changing to the nephrectomy position. In a second set of measurements, patients were ventilated with ALV and CMV using a randomized crossover design. The CMV settings were a tidal volume of 10 ml/kg body weight, a respiratory rate of 10 breaths/min, an I:E ratio of 1:1.5, and an end-inspiratory pause of 30% of inspiratory time. With both ventilation modes FiO2 was set to 0.5 and PEEP to 3 cm H2O. During ALV a desired alveolar ventilation of 70 ml/kg KG . min was preset. All other ventilatory parameters were determined by the ALV controller according to the instantaneously measured respiratory parameters. Positioning induced a reduction of compliance from 61.6 to 47.9 ml/cm H2O; the respiratory time constant shortened from 1.2 to 1.08 s, whereas physiological dead space increased from 158.9 to 207.5 ml. On average, the ventilatory parameters selected by the ALV controller resembled very closely those used with CMV. However, an adaptation to individual respiratory mechanics was clearly evident with ALV. In conclusion, we found that the effects of positioning for nephrectomy are minor and may give rise to problems only in patients with restrictive lung disease. The novel ALV controller automatically selects ventilatory parameters that are clinically sound and are better adapted to the respiratory mechanics of ventilated patients than the standardized settings of CMV are.