Article Notes
- Remifentanil infusions above 0.20-0.25 μg/kg/min are associated with hyperalgesia (OIH = Opioid Induced Hyperalgesia) and tolerance (AOT = Acute Opioid Tolerance) respectively.
- Some of these effects can be mitigated by multimodal analgesia (notably ketamine), and possibly by gradual weaning of a remifentanil infusion.
- The findings have been predominately identified in rats and volunteer human studies. The clinical and longterm significance is still uncertain.
- Although OIH and AOT arise from different physiological mechanisms, they are clinically difficult (if not impossible) to differentiate.
- The clinical priority for management is prevention.
Van Decar et al. on the diagnosis and management of intra-operative diabetes insipidus concludes:
For the average adult patient, urine output >125 mL/h is consistent with polyuria. Urinary osmolality and specific gravity should be obtained and levels <300 mOsm/kg and <1.003, respectively, are consistent with hypotonic urine.
It is prudent to rule out other causes of polyuria including hyperglycemia, uremia, or iatrogenic causes including diuretic or mannitol administration.
Serum electrolytes and osmolality should also be obtained, and a high sodium (>146 mmol/L) and plasma osmolality (>300 mOsm/kg) are typically seen with DI.
Treatment should focus on replacement of free water deficit with a balanced salt solution, pharmacotherapy including DDAVP or vasopressin as appropriate, and close monitoring of patient’s fluid and electrolyte status.
The association of anesthesia in the sitting beach-chair position with intra-operative stroke, continues to be controversial. Although some studies have identified this as a risk, it is still a rare complication, albeit devastating.
Expert opinion suggests intra-arterial blood pressure monitoring is best practice, but most importantly with consideration for actual cerebral perfusion pressure given the sitting position.
Some research suggests regional anaesthesia, possibly combined with spontaneous ventilation GA (rather than relaxation GA with IPPV) offers unique benefits that better maintain cerebral oxygenation, although the exact difference is unclear.
Similarly, the benefit and role of non-invasive cerebral perfusion monitoring has not been conclusively shown, although it appears logical that it may offer benefit in these patients.
Case studies of patients suffering cerebral ischaemia under beach-chair, do point to combinations of poor intra-operative blood pressure management and possibly pre-existing mild cardiovascular disease (eg. hypertension) as contributing to some degree.