High frequency jet ventilation (HFJV) in clinical practice

Abstract: Background: Surgery often requires general anaesthesia. During general anaesthesia, a ventilator is often used to secure the breathing of the patient. This is preferably done by mimicking normal ventilation. Conventional ventilation causes the lung to inflate and deflate which in turn makes the diaphragm move up and down in the craniocaudal direction. Therefore, all organs adjacent to the diaphragm will be affected by these breathing-related motions. During liver tumour ablation, stereotactic technique can be used. During stereotactic technique, radiological images are used to optimise needle placement in three dimensions, to reach the target tumour. It is of great importance that the target tumour does not move, ensuring that the tissue destruction is limited to the tumour and avoiding injury of healthy surrounding tissue. To meet the demand of target organ immobilisation, high frequency jet ventilation (HFJV) has become an interesting option. This method uses small tidal volumes at high frequencies that highly differ from normal physiological respiration in humans, contrary to conventional ventilation during surgery. HFJV has been used for decades especially for ventilation during airway procedures. To ventilate the patient while minimising abdominal organ movement and thereby improving surgical conditions during stereotactic ablative procedures is a novel way of using the benefits of HFJV. Aim: This doctoral thesis studied the feasibility and safety of using high frequency jet ventilation for the specific purpose of liver immobilisation during stereotactic ablation procedures. The aim of Study I was to study gas exchange during HFJV during stereotactic ablation of liver tumours. In Study II, post-operative hypertension and its relation to liver tumour ablation techniques and ventilation methods were studied. In Study III the formation of atelectasis during HFJV was studied. In Study IV the levels of carbon dioxide (CO2) were studied in two different groups randomised to different sizes of the endotracheal tube in which the jet-catheter was placed during HFJV in liver tumour ablation. In Study IV continuous transcutaneous carbon dioxide (TcCO2) monitoring was compared to intermittent measurement of arterial carbon dioxide (PaCO2). Methods: Study I is a prospective, observational study. Blood gas analysis was performed every 15 minutes for the first 45 minutes of HFJV in 24 patients undergoing liver tumour ablation. Study II is a retrospective, observational study. Medical chart records were collected and analysed for early post-operative hypertension for 134 patients receiving either HFJV or conventional ventilation (CV) and various ablation methods, microwave ablation (MWA) or irreversible electroporation (IRE). Study III is a prospective, observational study. CT-images over the lower part of the lung were taken in 25 patients every 15 minutes during the first 45 minutes of HFJV. The images were analysed for atelectasis formation during HFJV using the MatLab software program. Study IV is a randomised, prospective study. PaCO2 was measured during the first 45 minutes after initiation of HFJV in patients randomised to endotracheal tube (ETT) inner diameter (ID) 8 or 9 mm. TcCO2 was also measured during the same period and compared to gold standard PaCO2. Airway pause pressures, peak pressures and signs of intubation injuries were also studied. Results: In Study I blood gas analyses showed that none of the 24 patients experienced hypoxemia during the first 45 minutes of high frequency jet ventilation. A statistically significant rise in arterial carbon dioxide (PaCO2) was seen at all time points during HFJV compared to baseline. A further statistically significant rise in PaCO2 was seen during HFJV compared to T=0 at T=30 (p=0.006) and T=45 (p=0.003). A corresponding statistically significant decrease in pH was seen compared to baseline at T=15 (p=0.03) from a mean value of 7.44 to 7.31. A further small drop in pH was seen over time but with no significance between time points. During early recovery in the post anaesthesia care unit, PaCO2 and pH resumed spontaneous to baseline values. All lactate values were within normal range except for one value in one patient during recovery that was slightly raised to 2.3 mmol L-1. Study II showed that hypertension was common in post-operative care after liver tumour ablation. Patients receiving MWA under HFJV had the highest proportion of having at least one episode of severe hypertension (SAP >180 mmHg) when compared to patients receiving IRE under HFJV and MWA under CV. Multiple regression analysis showed increased odds for post-operative hypertension when MWA was used compared to IRE and when HFJV was used compared to CV. Study III showed that the formation of atelectasis increased over time during HFJV during the 45 minutes studied, from 5.6% to 8.1% of the total lung area. The increase in atelectasis was significant at T=30 (p=0.002) and T=45 (p=0.024). The area of normal ventilation was however unchanged. In a subgroup analysis with patients with a BMI<30, no significant difference in the amount of atelectasis could be seen between the time points. In Study IV PaCO2 increased in both groups, with ETT ID 8 and 9 mm, but no statistically significant difference between the two groups was seen (p=0.06). TcCO2 was measured and compared to PaCO2. A Bland-Altman plot and an ICC analysis showed a good correlation between the two methods. Conclusions: The overall result of this thesis indicates that high frequency jet ventilation is feasible and safe during stereotactic ablation of upper abdominal organs for up to 45 minutes. There is a risk of hypertensive events in the early recovery, following liver tumour ablation when MWA and HFJV are used. Atelectasis increases but the proportion of normally ventilated lung is preserved. PaCO2 increases but is rapidly reversed during recovery. An ETT ID 8 mm can be used in male patients for shorter procedures, regarding PaCO2. TcCO2 is a feasible technique when following the changes in carbon dioxide although blood gas analysis should be considered in patients in need for haemodynamic monitoring and risk of carbon dioxide retention.