In this study, the effects of SHFJV on displacement of the diaphragm, bronchus, and mediastinum on the flooded lung side during OLF were compared with the effects of PCV. The key result was that SHFJV significantly reduced motion of all three structures.
OLF itself significantly reduces hemidiaphragm displacement on the flooded lung side during one-lung PCV. The saline-filled lung represents an incompressible mass that impedes movement of the ipsilateral hemidiaphragm. This study confirmed the results of our previous study, in which we found 78% reduction of diaphragm motion during OLF, compared with TLV. However, at that time we detected residual hemidiaphragm motion with a maximum displacement of 15 mm close to the ventilated lung during OLF [6]. This amount of displacement could be too high for safe and effective focused ultrasound ablation of tumours lying near the caudal mediastinum. The current study was therefore initiated to examine whether SHFJV could further reduce diaphragm motion on the flooded lung side.
The absolute end-expiratory and end-inspiratory motion values in the previous study [6] were lower than those in the current study because the right lung was flooded with a higher filling volume (15.5 mL/kg), and the animals were ventilated with a volume-controlled setting using a lower tidal volume (10 mL/kg). In the present study, the left lung was flooded with 12.5 mL/kg saline, and the contralateral lung was ventilated with PCV and a target tital volume of 15 mL/kg. Furthermore, during right lung flooding, the accessory lobe in the mediastinal recessus is filled, which arises from the right bronchus and lies directly on the diaphragm. Therefore, the area of the diaphragm covered by flooded lung is larger with right lung flooding than with left lung flooding. Additionally, motion of the right diaphragm is restricted by the underlying liver.
The passive breath hold method was used to overcome the motion problem during liver tumour ablation [12, 13]. Passive breath hold is however associated with a long treatment time and decreased efficiency, as the sonication is performed non-continuously.
HFJV has proven effective for reducing thoracoabdominal motion, but randomised controlled studies are needed to prove its superiority to standard ventilation [10]. There are many clinical settings, such as extracorporeal shockwave lithotripsy, cardiac ablation, and computed tomography–guided lung tumour ablation with radiofrequency, in which HFJV has been used to reduce target-lesion motion [14, 15, 16]. To prevent the possibility of CO2 retention under high-frequency jet ventilation [17] we used a jet ventilator that allows simultaneous application of two different jet streams (low frequency and high frequency), resulting in pulsatile bi-level ventilation. The low frequency is primarily responsible for CO2 elimination, while the high frequency is responsible for oxygenation. With SHFJV, superimposition of the low-frequency jet induces additional gas flow, resulting in thoracic inspiratory and expiratory movements similar to those during conventional ventilation. The low-frequency jet stream is responsible for lung and diaphragm movements and could be a disadvantage for immobilisation of the flooded contralateral lung. Nevertheless, our results showed that SHFJV led to marked reduction of diaphragm, bronchus, and mediastinum displacement when using a basic/outlet driving pressure of 0.9 bar for the low-frequency jet stream. The benefits of jet ventilation regarding motion reduction are negated if higher pressures are used. The empirically found jet parameters in the pilot study represent a compromise between tolerable gas exchange and effective movement reduction. In the future, we plan to examine the effects of SHFJV on haemodynamics and gas exchange during OLF in different animal positions.
Our results showed that SHFJV significantly reduced diaphragm, bronchus, and mediastinum displacement in all body positions. This is important because the required position for interventional targeting is based on available instrumentations and anatomic locations. Depending on both the tumour location and the method of ablation (magnetic resonance–guided or ultrasound-guided high-intensity focused ultrasound ablation) the flooded lung can be dependent or non-dependent.
The ability of SHFJV to reduce mediastinum and central bronchus movements is also an important result. Central tumours are usually located close to important functional structures, such as the heart, pulmonary vessels, and bronchi. Imprecise ablation of central tumours because of uncontrolled motion can cause life-threatening damage to these structures [18, 19].
This study had some limitations that should be addressed. For example, motion of the mediastinum and bronchus cannot be measured on the ventilated lung side by ultrasound, which prevented direct comparisons of displacement between the flooded and ventilated sides. Furthermore, we assume that the observed decreases in diaphragm, mediastinum, and bronchus movements are transferable to tumours. Ideally, tumour movement would be assessed, but an appropriate pig tumour model is not available. Additionally, although we attempted to reduce measurement variability between the two ventilation methods by using a fixed transducer position while switching between SHFJV and PCV within the same animal, the transducer position may have differed slightly between animals.