HIFU

High intensity focused ultrasound

Mechanisms

At the focal point, tissue destruction occurs by two main mechanisms:

coagulation necrosis from the thermal effect
- (temperatures can reach ∼60°C, with cell death occurring at ∼56°C for >1s)
cavitation-induced cellular damage
- (due to mechanical stress and thermal effect)

Real-time imaging during HIFU is essential because tissue inhomogeneities (blood vessels, fat, muscle), the presence of gas or bone, can distort the intended focal point as well as make the actual temperature rise at the focal point hard to predict. Hence real-time imaging is needed during every HIFU application for accurate targeting and monitoring of treatment effect, and to minimise collateral damage

Immobilisation

For ablation of intra-abdominal tumours (liver, pancreas, kidney) using extracorporeal HIFU, general anaesthesia is required as respiration needs to be controlled and patients must remain immobile. This facilitates localisation of the lesion and helps avoid accidental injury to the surrounding visceral organs

Immobilisation is required during HIFU targeting and ablation. For intra-abdominal lesions which move with respiration, ventilation must be stopped

Temperature

During HIFU ablation of intra-abdominal tumours, nasopharyngeal temperature monitoring is essential. Heat generated at the focal point can be absorbed, thus potentially raising body temperature, especially with long procedures or if the focal point is adjacent to vascular structures

The heart itself may be prone to thermal injury because of its proximity to the left lobe of the liver and we frequently have observed ventricular extrasystoles when temperature rises to near 38.5°C during ablation of tumours in this region

TIVA

If conventional ventilation with frequent respiratory disruption is used, propofol based TIVA can provide uninterrupted anaesthesia. It attenuates the effects on cerebral blood flow from the frequent Valsalva manoeuvres by preserving cerebral autoregulation and vascular reactivity, and reduces cerebral blood volume and intracranial pressure

propofol does not inhibit hypoxic vasoconstriction resulting in less V/Q mismatch compared to inhalation anaesthesia

Analgesia

For HIFU tumour ablation, analgesia is needed even though it is non-invasive. Coagulation necrosis of the lesion induces surrounding tissue oedema and swelling, causing mild to severe pain. Pain can also be due to superficial skin burns or pleural irritation if a hydrothorax is instilled. Good analgesia helps attenuate the sympathetic response during the procedure and afterwards and eases breathing in order to overcome alveolar atelectasis. Parenteral opioid analgesia is normally required during the procedure and may be continued into the post-procedural period as required

Prostate tumours

Often patients with prostate tumours are elderly. A rectal enema is given the night before the procedure. Most procedures are performed under regional anaesthesia but those undergoing palliative HIFU for prostate cancer with spinal metastasis may not be suitable for central neuraxial blocks and will require general anaesthesia.

After HIFU, the prostate swells and compresses the urethra; hence patients may need urinary catheterisation for up to 7days. The discomfort is usually mild to moderate with frequency and dysuria being the most common complaints. Prophylactic antibiotics and analgesia are prescribed postoperatively.

References

Anaesthesia for High Intensity Focused Ultrasound (HIFU) Therapy - BJA Ed