High Frequency Jet Ventilation during stereotactic ablation of liver tumours: an observational study on blood gas analysis as a measure of lung function during general anaesthesia

Introduction

Stereotactic ablation of tumours in solid organs is a promising, dynamically developing and potentially curative procedure in clinical oncology. High precision in targeting malignant lesions with the use of image fusion for pre-interventional diagnostic imaging is highly dependent on immobilisation of the operation field. Respiratory movements cause diaphragmatic shifts, displacing abdominal organs during stereotactic-guided procedures. Biro et al.¹ observed that breathing-related liver motion decreased from 20mm to 5mm using high frequency jet ventilation (HFJV) as compared to conventional ventilation.

HFJV is an attractive novel ventilatory strategy, minimizing movements in proximity to the lungs and keeping the abdominal and extraperitoneal target organ immobilised². The surgical ablation procedure usually requires at least 30 to 45 minutes to be performed and as the patient is anesthetised before ablation begins and a CT-scan is performed before the procedure can start, HFJV will thus be applied for up to at least an hour not uncommonly in elderly ASA Class 2-3 patients (ASA Class: American Association of Anesthesiologists Classification, ASA 2 being a patient with mild systemic disease and ASA 3 being a patient with severe systemic disease). The effects on oxygenation, carbon dioxide elimination, pH and lactate formation are not well-studied in this patient category during HFJV with the jet catheter placed through the endotracheal tube during liver ablation.

The primary aim of this explorative observational study was to study changes over time in arterial oxygenation, carbon dioxide elimination, pH and lactate formation in adult patients undergoing liver ablation under general anaesthesia who are ventilated with HFJV. The secondary aim was to describe the PaO₂/FiO₂ ratio and its relation to PaCO₂ during HFJV.

Methods

Results

All 25 patients had an uncomplicated perioperative course. Anaesthesia, surgery and early recovery was uneventful apart from one patient who experienced a minor pneumothorax associated with the surgical procedure and was left with no further intervention. Changes in the HFJV settings per the protocol for increasing PaCO₂ had to be initiated in one patient. HFJV did not need to be discontinued in any patients. One patient had a DP of 1,6 during the whole procedure. In one patient, oxygen was raised from 80% to 100% after 5 min of HFJV as saturation dropped to 93%. Saturation was then normalised.

A total number of 131 blood gases were analysed in this study.

Mean PaO₂ at baseline was 11.6 kPa (SD 2.3). All patients had a PaO₂ above the normal lower limit of 8 kPa except for one patient (7.9 kPa). The PaO₂ increased during HFJV and all patients had perioperative PaO₂>10 kPa (see Table 2). An inter-individual variation was seen in oxygenation during HFJV and the PaO₂/FiO₂ ratio significantly decreased but was restored at recovery (see Figure 1). During the stay in the recovery room, three patients required supplementary oxygen and in two cases values were not accessible. All remaining 20 patients breathing room air had a PaO₂ above the normal lower limit of 8 kPa (see Table 2).

Figure 1. PaO₂/FiO₂ ratio.

PaO₂/FiO₂ ratio at baseline, during high frequency jet ventilation and after extubation in the recovery room. An inter-individual variation was seen in oxygenation during HFJV and the PaO₂/FiO₂ ratio significantly decreased but was restored at recovery.

Mean PaCO₂ at baseline was 5.0 (SD 0.4). None of the patients had a PaCO₂ above the normal limit of 6 kPa. A significant raise was seen in mean PaCO₂ from baseline to t=1^st CT; 5.0 to 6.1 kPa (p=0.003). The mean PaCO₂ value increased further to peak mean PaCO₂ 7.1 kPa at 45’ (see Figure 2). The number of patients with a PaCO₂ >6 kPa was 14/24 at t=1^st CT, 15/24 at t=15’, 18/22 at t=30’, 15/19 patients at t=45’. Mean PaCO₂ during recovery (5,6 kPa) was not significantly different from the mean baseline. Five out of 23 patients had a PaCO₂ value >6 kPa at recovery, the highest value being 8.03 kPa.

Figure 2. PaCO₂.

PaCO₂ at baseline, during high frequency jet ventilation and after extubation in the recovery room. A significant raise was seen in mean PaCO₂ from baseline to 1^st CT. Five out of 23 patients had a PaCO₂ value >6 kPa at recovery, the highest value being 8.03 kPa.

Median pH decreased from baseline 7.44 to 7.31 at t=15’ (p=0.03). There was a small further drop in pH but with no statistical difference between time points (see Figure 3). Fourteen out of 24 patients had a pH <7.35 at t=1^st CT, 18/24 patients at t =15’, 17/22 at t =30’ and 14/19 at t=45’. Median pH during recovery (7.37) was not significantly different from the median baseline. Four out of 23 patients had a pH<7.35 at recovery, with the lowest value being 7.24 (see Figure 3).

All lactate values were within normal range during HFJV. One patient had a minor increase (2.3 mmol L^-1) during recovery.

There was no clear correlation between decrease in PaO₂/FiO₂ ration and the PaCO₂ increase (see Figure 4). Ten out of 16 blood gas analyses (62%) with a PaCO₂>8 had a PaO₂/FiO₂ ratio of >40. Thirteen out of 39 (33%) of the blood gas analyses with normal PaCO₂ had PaO₂/FiO₂ ratio <40.

Figure 3. pH.

pH at baseline, during high frequency jet ventilation and after extubation in the recovery room. There was a significant drop in pH from baseline during HFJV. Four out of 23 patients had a pH<7.35 at recovery, with the lowest value being 7.24.

Figure 4. PaO₂/FiO₂ ration vs PaCO₂.

Plotted blood gas pairs of PaO₂/FiO₂ ration vs PaCO₂ in the 25 patients studied. No clear correlation could be seen.

Discussion

There is sparse information about arterial blood gas changes during HFJV during abdominal surgery. No previous study has explicitly assessed oxygenation and carbon dioxide elimination during CT-guided stereotactic liver ablation with the jet ventilation catheter placed inside an ETT. Our study showed that this technique, high frequency jet ventilation by a catheter placed through the ETT, maintained adequate oxygenation in all patients. The gas exchange, the ratio of arterial oxygen partial pressure to fractional inspired oxygen (PaO₂/FiO₂), decreased and varied considerably. The carbon dioxide tension increased and pH decreased reasonably as an effect of the CO₂ retention. Furthermore, the impairment in oxygenation, decrease in PaO₂/FiO₂ ratio and CO₂ elimination was found somewhat surprisingly not to correlate, showing a scattered pattern. There was also only modest residual impairment of gas exchange during early recovery.

High frequency ventilation is not new³. However, most HFJV use is for airway interventions, for example during a bronchoscopy or laryngoscopy. This may not entirely translate to its use in abdominal surgery, in this case during puncture of the liver under CT-guidance, with the jet catheter placed through an endotracheal in patients who are under general anaesthesia and receiving muscle relaxants. The HFJV catheter in our study was inserted into the endotracheal tube and thus the explicit effective inspired oxygen fraction cannot be defined. The PaO₂/FiO₂ findings may thus not be entirely true. The open system may also impact the expired pressure as well as gas elimination. Pause pressure was measured via the tip of the jet cannula, providing the pressure at tracheal level between the jet inspirations. This is not a fully representative pressure for the whole lung but may still be seen as an indicator for built-up auto positive end expiratory pressure (PEEP). The HFJV technique is dependent on passive outflow, contrary to high frequency oscillation that possibly has a more active exhalation. There is indeed, with HFJV, an obvious risk of carbon dioxide retention as well as a potential for a build-up of intrinsic PEEP and subsequent risk for barotrauma.

All patients had total intravenous anaesthesia because of the open airway system and all had muscle relaxation to promote the stereotactic liver ablation, both factors that impact on lung function⁴. Age and supine position are also factors known to impair the lung function during anaesthesia⁵. In the present study a FiO₂ of 0.8 was used during the entire procedure except for one patient who had a FiO₂ of 1.0. High oxygen fraction may cause rapid absorption atelectasis and anaesthesia per se causes loss of muscle tone and reduced lung volume, factors having a negative effect on ventilation/perfusion matching⁶. It should also be acknowledged that preoxygenation was performed before induction, thus further promoting lung collapse when the high oxygen fraction is maintained⁷. Several of the patients had a history of pulmonary disease, further impairing ventilation/perfusion matching during anaesthesia⁸. The oxygenation as well as the arterial carbon dioxide tension was somewhat surprisingly restored by the arterial blood gas measurement in the recovery room. Lung recruitment manoeuvre is standard procedure following intubation but not during emergencies.

Conclusion

This study shows the feasibility of using HFJV through an ETT with an FiO₂ of 0.8 during liver tumour ablation. It did not cause any hypoxia or increase in lactate. It was associated with mild to moderate impairment in gas exchange that was restored during early recovery.

Data availability