ADAPTIVE LUNG VENTILATION (ALV) - EVALUAT ION OF A NEW CLOSED-LOOP ALGORITHM FOR SURGERY IN THE LATERAL DECUBITUS POSITION

The lateral decubitus position is the standard position for nephrectom ies. There is a lack of data about the effects of this extreme positio n upon respiratory mechanics and gas exchange. In 20 patients undergoi ng surgery in the nephrectomy position, we compared a new closed-loop- controlled ventilation algorithm, adaptive lung ventilation (ALV), whi ch adapts the breathing pattern automatically, to the respiratory mech anics with conventionally controlled mandatory ventilation (CMV). The aims of our study were (1) to describe positioning effects on respirat ory 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 b y the ALV controller. The respirator used was a modified Amadeus venti lator, which is controlled by an external computer and possesses an in tegrated lung function analyzer. In a first set of measurements, we co mpared parameters of respiratory mechanics and gas exchange in the hor izontal supine position and 20 min after changing to the nephrectomy p osition. In a second set of measurements, patients were ventilated wit h ALV and CMV using a randomized crossover design. The CMV settings we re a tidal volume of 10 ml/kg body weight, a respiratory rate of 10 br eaths/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 a nd PEEP to 3 cm H2O. During ALV a desired alveolar ventilation of 70 m l/kg KG . min was preset. All other ventilatory parameters were determ ined by the ALV controller according to the instantaneously measured r espiratory parameters. Positioning induced a reduction of compliance f rom 61.6 to 47.9 ml/cm H2O; the respiratory time constant shortened fr om 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 wi th ALV. In conclusion, we found that the effects of positioning for ne phrectomy are minor and may give rise to problems only in patients wit h restrictive lung disease. The novel ALV controller automatically sel ects ventilatory parameters that are clinically sound and are better a dapted to the respiratory mechanics of ventilated patients than the st andardized settings of CMV are.