Evaluation of homogeneity of alveolar ventilation with electrical impedance tomography during anaesthesia and laparoscopic surgery

Introduction

After induction of anaesthesia and during abdominal surgery, homogeneity of ventilation is influenced by different factors such as compression and absorption atelectasis and change in abdominal pressure [1]. To assess the spatial change in ventilation by applying positive end-expiratory pressure (PEEP), we employed the dynamic centre of gravity index ycog [2], which is a new mathematical feature of electrical impedance tomography (EIT) measurement in a clinical study.

Methods

After approval of the local ethics committee and informed consent we prospectively randomized 32 consecutive patients (ASA physical status I/II) scheduled to undergo elective laparoscopic cholecystectomy. The patients were randomly assigned to the PEEP group (10 cmH₂O) or ZEEP group (0 cmH₂O). Patients obtained volume-controlled ventilation (8 ml/kg bw) and the minute volume was adjusted by increasing the respiratory rate but maintaining a PaCO₂ level between 35 and 45 mmHg. EIT (EIT evaluation KIT; Dräger Medical, Lübeck, Germany/GoeMF II system; University of Göttingen, Germany) was performed at an intercostal level of Th 6 in the supine position. Measurements were carried out preoperatively and intraoperatively at five different time points (T0–T4). We calculated the ventral/dorsal lung ycog [2] to investigate the differences in homogeneity of pulmonary ventilation. EIT data and gas exchange parameters were compared between the randomized groups. A t test and variance analysis by the GLM repeated-measures procedure (Greenhouse–Geisser) method were used for statistical analysis.

Results

Both study groups showed no differences in their preoperative characteristics. After induction of anaesthesia, oxygenation was reduced in the ZEEP group compared with the PEEP group and also the PaO₂/FiO₂ratio was lower during anaesthesia compared with T0 measurements. The PEEP-ventilated patients showed higher values of respiratory compliance. The ZEEP-ventilated patients showed a lower gravity index compared with the PEEP group (P = 0.018). Ventilation with PEEP showed no difference in ycog at T0.

Conclusion

The dynamic change of the homogeneity of ventilation after induction of anaesthesia and during surgery can be characterized by the calculation of the gravity index, which is a result of mathematical calculation of noninvasive EIT measurements. ZEEP ventilation resulted in a prominent reduction of oxygenation and a shift of the dynamic centre of gravity index compared with preoperative measurements and ventilation with PEEP. To optimize ventilation in anaesthetized patients, this new index can be of fundamental help.