[The Myth of Predictable Spinal Anaesthesia: What Determines Block Spread?]

The spread of spinal anaesthesia remains only partially predictable despite its long-established clinical use. Although numerous physical, procedural and patient-related factors have been described, many are of limited relevance in routine practice. Dose, baricity and patient positioning represent the main controllable determinants of block spread. Current evidence indicates that lumbosacral cerebrospinal fluid volume is the only patient-specific factor, whereas demographic variables such as height, weight and sex do not reliably predict spread. Spinal anaesthesia should therefore be planned on the basis of modifiable technical variables, while acknowledging that its final spread cannot be predicted precisely in the individual patient.