The aim of the present prospective randomised study is to compare differences in levels of carbon dioxide between two different endotracheal tube-sizes.

Background: High Frequency Jet Ventilation (HFJV) has become an important technique for ventilation during stereotactic near-diaphragm ablation to minimise breathing related movements. Thus, avoiding risks associated to movements and subsequent dislocation of the target tumour. HFJV is being increasingly used during CT-guided, percutaneous liver tumour ablation. As shown previously, breathing related movements in upper abdominal organs, are decreased significantly when compared to conventional ventilation. When HFJV is being used in the setting for tumour ablation the jet cannula is inserted into an ordinary endotracheal tube (ETT). The HFJV catheter is placed in the ETT and is during jet ventilation lying freely within the tube. Right placement is confirmed by a harmonic curve on the screen of the jet ventilator. It is of great importance that the ETT is not obstructed as this type of ventilation is dependent on passive outflow of expired air, through the tube, passing from the lungs around the catheter within the tube into the surrounding air. If the lumen around the catheter within the ETT is becoming too small, there is a risk for intrinsic PEEP built up, and subsequent risk for barotrauma, pneumothorax. The space between the catheter and the inner lumen of the ETT also governs the exhalation of gas, the passage of expired gas containing CO2 from the lung. A small inner diameter of the ETT may thus likewise increase the risk for a raise in arterial carbon dioxide. To minimise these risks, an ETT one size larger than usual can be used to lower the risk for pressure increase and improve gas exchange. One should consider the risks, even though very low, of mechanical injury in the airway when larger ETT's are being used. Previous studies on arterial blood gas in liver tumour was performed using a ETT one size larger (size 9 in men). Tadié et al showed that laryngeal injury was related to the length of intubation, the non-use of muscle relaxant drugs and the height/ETT diameter ratio. This was a study made in an ICU and all patients were intubated \>24 hrs and therefore not fully applicable to the OR-setting. Jaensson et al describe a significant lower risk for post operative sore throat (POST) using a smaller ETT size in women during elective surgery. Myles et al describe women being 1.5 times more likely to report having POST compared to men. To defend the superiority of a larger tube size further studies assessing the effect of the ETT size on the arterial CO2 tension is needed.

Methods: After written, informed consent, 26 (13 in each group) consecutive, male patients planned for liver tumour ablation with HFJV will be included in the study.

Participants will be randomized to ETT size 8 or 9. Randomisation will take place in an earlier stage with the help of a computerised randomisation generator where 26 envelopes are prepared in which the information of tube size is to be found. The envelopes are on the outside anonymous. The envelopes will be opened by the anaesthetic nurse in the operation room, who will then prepare the ETT.

An arterial line will be placed before the start of anaesthesia and a transcutaneous measuring device will be placed on the forehead of the patient. After preoxygenation, induction and ETT placement, conventional ventilation will be started and a normal tcCO2 value reached (tcCO2 4,5-5 before the start of HFJV). HFJV will have pre-set settings with the same values for all patients. Driving pressure (DP) will be 1,2 bar and frequency 220/min. Measurements from the three different methods; arterial pCO2, transcutaneous CO2 and etCO2 will then be recorded at the start of HFJV (t=0) and then every 15' until t=45 minutes of HFJV. In the case of rapidly increasing CO2, a cut off limit of 10 kPa will be used. When this level is reached, settings on the ventilator will be changed. That means increasing DP and, if needed, lower the frequency. If this does not lead to a satisfactory CO2-level HFJV has to be changed to conventional ventilation until the CO2-level is within normal limits. Any injury during intubation and post extubation stridor (PES) will be recorded. The patient will be asked for post operative sore throat (POST) and any signs of postoperative hoarseness (PH) will be noted in the protocol before leaving the post anaesthetic care unit (PACU) and also approximately 24 hrs after extubation. If symptoms still occur after 24 hrs, follow up will take place approximately 72 hrs and 96 hrs after extubation. If symptoms are still present by this point the patient will be offered a doctor's appointment.

Statistics: The power calculation was based on data from previous studies. The base-line PaCO2 among 12 male patients, at the start of HFJV, was 4.9 kPa (SD 0.5) with an increase to mean PaCO2 of 6.7 (SD 1.5) at 15 minutes with HFJV and a tube size of 9. With the hypothesis that the PaCO2 would increase to a mean of 8.0 kPa after 15 minutes of HFJV ventilation, with a tube size 8, groups of 11 patients are needed to verify the difference at a p\<0.05 with a power of 80%. To compensate for potential drop-outs, groups of 13 patients is set as the study population. Data will be presented as mean, SD, median and range where applicable. One-way ANOVA will be used on normal distributed data and ANOVA on ranks will be used on non-normal distributed data. Bonferroni's test will be used to compare differences between the different timepoints. A p-value of \<0,05 will be considered as statistically significant.

Importance of the study: This study is conducted to if the larger endotracheal tube size has benefits in gas exchange when HFJV is being used in liver tumour ablation procedures. Normally it is desirable to use as small ETT as possible as this minimises the risks for airway injury. The risks of air entrapment and even pneumothorax and raise in carbon dioxide levels are also risks that are potentially harmful for the patients and must be avoided. The small risk of using a one size larger ETT would therefore be accepted if the benefits can be shown in this study.