Clinical application of high frequency jet ventilation in stereotactic liver ablations – a methodological study

Introduction

Thermal ablation of primary and secondary liver tumours is a potentially curative treatment, and an alternative for patients not eligible for surgical resection due to severe comorbidity or underlying liver disease. Its efficacy has been proven for tumours smaller than 30mm in diameter, especially in treatment of hepatocellular carcinomas¹. Adequate imaging of the tumour and precise guidance of the ablation device are crucial for accurate local ablative treatment. Accurate targeting is essential for an effective treatment, reducing the risk for local recurrence and need of retreatment^1,2.

Recent developments in image guidance systems, with robotic and computer-assisted navigation, may help correct needle placement and improved ablation efficacy. Needle navigation and placement is based on pre-interventional imaging. Early phantom and clinical experiences with navigation systems suggest good procedural accuracy, reduced procedure time and reduced patient radiation exposure compared to freehand techniques³.

The high frequency jet-ventilation technique (HFJV) was developed in the seventies by Klain and Smith, and mostly applied in the field of ear-nose-and-throat (ENT) surgery. It does not rely on conventional tidal volumes but uses a high frequency forced gas move⁴. The potential advantage of using HFJV in abdominal surgery is to minimise the amplitude of respiration-related upper-abdominal organs compared to conventional tidal volume lung ventilation (TV)^5–7.

The aim of this clinical methodological trial was to measure the liver movements during open surgery under general anaesthesia and compare HFJV with conventional ventilation.

Methods

Five consecutive patients who were scheduled for elective, open liver ablations were included in the clinical protocol.

Results

Patient demographics, medical status and extent of surgery is presented in Table 1.

Ventilator settings and readings are shown in in Table 2. The following parameters have been registered: end tidal CO₂ concentrations before and after HFJV phase, respiratory pressures on conventional tidal volume ventilation before and after HFJV, peak inspiratory pressure and mean airway before and after HFJV, mean airway pressure, dynamic lung compliance both before and after HFJV phase as well as tidal volumes on conventional lung ventilation, at liver displacement measurement point. HFJV ventilator settings: respiratory frequency and target driving pressure as well as the measured respiratory parameters: peak inspiratory pressure and mean airway pressure as well as maximum end tidal carbon dioxide tension on HFJV.

In one case (patient number 2) an increase in DP was needed, because the etCO2 rose to 8.3 kPa and the optimal value was set on to 1.5 bar where in other four cases it was set to 1.1–1.2 bar. One patient (number 5) turned out to have more than 20 metastases at time of surgery, but not on the imageing where the treatment was allocated. Rather than to close up, a new evaluation was done during surgery using intraoperative ultrasound and it was found that ablations could be done leaving an adequate volume of functioning liver parenchyma, why this was the course taken.

The mean Euclidean liver displacement was 0.80 (0.10 SD) mm and 2.90 (1.03 SD) mm for HFJV and TV respectively with maximum displacement going as far as 12 mm on standard ventilation (p=0.0001). Data shown in Figure 1.

Figure 1. Liver displacement and lung ventilation.

Displacement of measured point on liver surface during High-Frequency Jet Ventilation (HFJV) and standard ventilation (TV) Error bars mark standard deviation.

Discussion

One of the most important challenges the anaesthesiologist faces perioperatively is the maintaining of the patient’s homeostasis and the facilitating of the course of surgery. In certain clinical situations, such as in stereotactic ablative procedures, it can be difficult to establish since there is a demand for keeping respiratory organ displacement to a minimum.

The recent investigation provides evidence for the claim that respiration induced liver motion during intervention can be reduced by more than two thirds when using HFJV instead of TV. This is the only study measuring this effect dynamically. This can have a decisive bearing on the risk of local recurrence rates and risk for collateral damage after image guided stereotactic treatment of liver tumours. The benefits in terms of radiation dose and respiratory organ shifting, when using HFJV in interventional radiology has previously been reported from several groups^5,6,9, but these were all conducted in the setting of a CT-guided ablation with non-dynamic measurements of target organ displacement.

Stereotactic navigation is often performed on rigid registration between the intraoperative target organ with the images obtained before the surgery. With this setting, soft tissue deformation and patient motion will affect the navigation system and can cause significant inaccuracy¹⁰. The minimization of deformation-induced errors can be done in several ways. From experimental research point of view, the position of the moving target can be measured by implanted navigation aids or by using electromagnetic tracking devices^10,11. Implantation of invasive needles is however not prudent in a clinical setting due to high risk of haemorrhage, tumour seeding, and long-term risks with leaving foreign bodies in situ.

Another approach is the mathematical modelling of mechanical tissue properties and organ motion in order to predict the target location based on a statistical model derived from preoperative 4D CT. This approach is frequently used in intensity modulated radiation therapies (IMRT)¹². The relationship between the respiratory cycle and the movement of a target is however complex to predict and not possible in real time due to highly intensive computational requirements and obvious risks of differences in outcome during the acquisition of preoperative images and a situation with artificial respiration and an open or laparoscopically affected abdomen.

Therefore, respiratory gating methods that reliably reproduce a known breathing stage (temporarily disconnecting endotracheal tube in anaesthetized patients) seem to be a more reliable approach^12,13. An overall internal target movement of 1.41 ± 0.75 mm was reported. However, periods of apnea are usually limited to 1–2 minutes depending on the health condition of the patient. HFJV overcomes these restrictions.

Use of HFJV outside the ENT and Thorax suites have been the subject of several, but rather anecdotal reports. In minimally invasive oncological procedures HFJV have been used in percutaneous, laparoscopic as well as in open approaches^3,5,6,14. In cardiology it has been beneficial in catheter ablations¹⁵. In urology it can be helpful to minimize the numbers of shocks needed during ESWL-treatment^15,16.

The present study is small and though the liver displacement data is solid, further studies on the physiological effects of HFJV is needed to elucidate the limitations. Carbon dioxide control is one of the important aspects of perioperative management. In the treatment protocol established during the study, it remained even more challenging because of the “less is better” strategy, favouring relatively low respiratory driving pressures.

Introducing HFJV in the management of computer-assisted abdominal surgery to a wider extent remains promising. The wider use of this method is, however, limited by the equipment availability and staff experience. Nevertheless, in the scale of a highly specialized centre, the acceptable skill level can easily be achieved, and the overall cost of the equipment as well as materials and utilities remains reasonable. HFJV is a promising lung ventilation modality for patients undergoing stereotactic surgical procedures in general anaesthesia when reduction of target organ displacement is crucial.

Ethical considerations

All procedures performed were in accordance with the ethical standards of the institution at which the studies were conducted. Since this was a retrospective analysis of the clinical material collected before, the written consent has been obtained only from two patients still alive at the time when the decision of data analysis and publication have been made. Other three patients have already died.

Data availability

Dataset 1: Demographic data and ventilation readings. metodological_study_1.xls 10.5256/f1000research.14873.d207212¹⁷

Dataset 2: Liver positioning data. 14-09-01-open-liver-all.xlsx 10.5256/f1000research.14873.d207213¹⁸