Influence of tidal volume on ventilation distribution and oxygenation during one-lung ventilation

Abstract

Dear Editor, Incorrect ventilator settings have the potential to induce lung injury. For this reason, the individualized setting of tidal volume (VT) or variable VT could be more physiology-oriented and thus improve patient outcomes [1]. In thoracic surgery, where one-lung ventilation (OLV) is performed, the setting of VT is more challenging than during the ventilation of both lungs. VT, which is regarded as low during a normal two-lung ventilation scenario, could be too high during OLV, potentially inducing barotraumas. However, simply reducing VT to half during OLV may be insufficiently low to secure adequate gas exchange and result in hypoxemia [2]. We have conducted a study to explore the feasibility of titrating VT during OLV at the bedside, based on ventilation distribution and oxygenation. The rationale was to monitor the regional ventilation distribution to avoid volutrauma with acceptable levels of blood gasses. Nine consecutive patients with peripheral lung cancer requiring intubation with a double-lumen tube and subsequent OLV for thoracoscopic lobectomy were examined prospectively (2 male, 7 female; age 56 ± 7 yr (mean ± SD); body weight 63 ± 9 kg; height 163 ± 7 cm; ASA I–II). No emphysema, chronic obstructive pulmonary disease or restrictive pulmonary disease were present before surgery. The study was approved by the local ethics committee. Written informed consent was obtained from all patients before the study. After general anesthesia and tracheal intubation, mechanical ventilation was switched to OLV. FiO2 was set to 100%, and VT was gradually decreased from 8 ml/kg to 7 ml/kg, 6 ml/kg and 5 ml/kg ideal body weight, while ventilatory frequency was increased from 12/ml to 14/min, 16/min and 20/min, respectively. Each step lasted for 4 min. At the end of each step, PaO2 and electrical impedance tomography (EIT) measurements were performed. Fig. 1 summarizes the studied parameters at various VT steps. All values were normalized to the corresponding step of 8 ml/kg during OLV for intra-patient comparison. During OLV, global respiratory system compliance (Crs) measured with the ventilator was lower, since one of the lungs was not aerated. Significant differences in the global Crs were found among different VT steps during OLV (8 ml/kg vs. 6 ml/kg, p < 0.01; 8 ml/kg vs. 5 ml/kg, p < 0.01; Fig. 1 top-left). Maintaining VT at the level of two-lung ventilation might prevent the drop in PaO2 (Fig. 1 top-right); however, it would simultaneously induce a higher degree of ventilation inhomogeneity (EIT-based index [3]; Fig. 1 bottom-right). In fact, both high VT and low VT could result in high ventilation inhomogeneity (Fig. 1, bottom-right). Maximal regional Crs (measured with EIT [4]) showed high variations, suggesting the need for individual titration (Fig. 1 bottom-left). Studied parameter values at various tidal volume steps in nine patients. All values were normalized to the corresponding step of 8 ml/kg during one-lung ventilation. Global Crs, global dynamic respiratory system compliance derived from the ventilator; PaO2, partial pressure of oxygen in arterial blood; GI, the global inhomogeneity index derived from EIT; Regional Crs, regional compliance derived from EIT. T: ventilation of two lungs; O: one-lung ventilation. The boxes mark the quartiles, while the whiskers extend from the box out to the most extreme data value within 1.5*the interquartile range of the sample; red + indicates an outlier. EIT was proposed to monitor ventilation distribution in the lungs to prevent tidal recruitment and overdistension [5]. Concerns were raised if EIT could be applied during thoracic surgery. Based on our experience, EIT can monitor ventilation during minimally invasive thoracic surgery without intrusion in the surgical field. When the electrotome is on, the EIT cable should be disconnected. Our study shows the feasibility of using EIT in combination with PaO2 to identify suitable VT with respect to ventilation distribution and gas exchange at the bedside during OLV. A suitable VT should be selected (with a trade-off) by minimizing the decrease in the PaO2 and maximizing the improvement in the regional Crs and ventilation homogeneity. This work was financially supported by the Shanghai Committee of Science and Technology, China (16411967600) and the Shanghai Hospital Development Center Foundation (SHDC12014241). Clinical trial registration number: ChiCTR-RPC-17010715.