Measuring regional anaesthesia

I think one of the more trying aspects of obstetric anesthesia is ensuring adequate regional anaesthesia for surgical delivery. Inadequate regional anaesthesia remains a top cause of medicolegal hassle, but surprisingly what little there is in the literature assessing for adequate anaesthesia is vague and imprecise. We read expressions such a "block to T4 with cold", or "T5 level to pin-prick". But does this mean the described dermatome is altered, or is it the first one unaffected? Is the sensory modality altered or absent at this dermatome? And what do we use: pin, cold or light touch? A local guru says only test with an ice block if the surgeon is going to operate with one, but we cannot test with a scalpel and scissors!
Most of the published work in recent years has been by Ian Russell in Hull, UK. He concluded in IJOA 1994 that no patient with a loss of light touch to a "Neurotip" at the T6 dermatome and above had pain or discomfort during CS. A light touch level can be several segments below a cold or pin-prick level, and Russell's own work finds that it takes over 20 minutes for light touch block to reach T5. His own words in this July 2004 paper are: "Surgery was allowed to start when T6 was either blocked to touch, or expected to be blocked to touch before surgery had reached the peritoneal cavity". So it seems Russell himself has an optimistic approach to his regional anaesthesia. Of his 102 subjects, 4 had intraoperative pain, and all 4 had low light touch levels with higher cold and pinprick levels, but his interquartile graph indicates that 25% had light touch levels below T5, and at 10 minutes (the mean time of surgical incision) 25% had light touch block below T7. So it seems to me that a failure of anaesthesia predicts a low light touch block, rather than the other way around.

Steve Yentis has a recent editorial in IJOA 2005, and also conributes to the topic, but to me he just makes it all a bit more confusing and uncertain. He describes his idea of assessing "wide-eyed" sensation to cold, and a "true" level at which sensation begins to change from normal. He rightly points out that personality, systemic and spinal drugs, the obstetrician, and the circumstances of delivery all contribute to how a patient will respond to surgery under regional anaesthesia. He concludes that he aims for a "true" level of T1, or higher. In practical terms this means sensation should be changing over the upper breast and even the axilla for effective regional anesthesia for CS.

So what do we do? It's all a bit confusing, and I know the local Guru often refers to the gods of regional anaesthesia. But I give a solid dose and aim for motor block of the lower limbs and alteration of pin and cold over the upper breasts and an absent pin up to and including T6 before the knife. In emergent conditions I rely on an ascending block, and with sensory changes occuring over the breasts and motor block of the legs I am optimistic of effective regional anesthesia.