The modern-day concept of anaesthesia revolves around making it tailormade as per the requirement of the patient so as to ensure best results with minimal side effects. Caudal anaesthesia is amongst the common blocks given in children for intraoperative and post operative pain relief. It accounts for 30–40% of regional techniques being used in pediatric patients.[10] Thereby, ensuring an adequate level of analgesia with minimal side-effects is of utmost importance, which further requires, calculation of an adequate amount to cater the same.
Armitage formula was submitted as a letter to the editor in 1979, based on the literary findings that the epidural space volume increases continuously from caudal to cranial, thus a volume of 0.5 ml kg− 1 may be expected to reach sacral, 1.0 ml kg− 1 lumbar, and 1.25 ml kg− 1 mid-thoracic dermatomes.3
However, it just gives an estimate of the volume required to reach these levels. Further research followed and there were attempts to determine the volume dosing of caudal blocks using fluoroscopy guidance which largely failed to verify the results of these earlier publications.[11, 12] Rather, these studies mostly established that the maximum height under fluoroscopic visualisation rarely goes beyond T11 level. Though the clinical correlation, suggested that a midthoracic level is achieved clinically secondary to the phenomenon of cerebrospinal fluid rebound.[4] Also, the use of fluoroscopy is not dynamic and is limited by radiation exposure, cost, and special space requirement. Thus, none of the evidence till now has been clearly able to define the exact volume required to reach a desired lumbar or thoracic levels.
Rather, literary evidence for knowing the minimum drug volume effective for reaching the desired spinal level and ensuring adequate surgical anesthesia and postoperative pain relief is also scarce. We, therefore, used the dynamic guidance of ultrasound to determine the minimum volume required to reach a desired level. Our results clearly showed that the volume required to reach T10 level under real time visualisation is significantly less than that determined by Armitage formula. Our results are in conjunction with a previously published study by Brenner et al,[5] who under dynamic ultrasound guidance measured the distance from conus medullaris to the level achieved by a fixed drug volume and found that one can reliably expect that inguinal hernia surgery can be successfully performed even using a volume as low as 0.7 ml/kg.
Regarding the maximum height achieved, there was no significant difference between ultrasound guided vs formula-based group and clinically adequate anaesthesia (T10 level) was achieved in all patients which is consistent with the established literature that dermatomal level achieved is higher than actually visualized level.[4]
Also, we through long term research in every other field of anaesthesia have learned that great individual variations exist in dosing and effect of anaesthetic drugs. The basic rule of anaesthesia that we should use minimum effective drugs in all patients, is more applicable to paediatric patients who have low range of safety to over dosing profile. Further it has been seen that high caudal doses can even lead to increase intracranial pressures as well as decreased flow in cerebral vessels, hence it is imperative to use just required doses rather than as much as possible.[13] Furthermore to stress this point it is also seen that paediatric patients suffer from LA toxicity even in doses which are well within the recommended LA dosing and that too in upto 41% of these cases.[14] This further reiterates our hypothesis of using minimum effective dosing in caudal anaethsia. Thus, individual patient based techniques are being universally applied in different domains of anaesthesia, from the use of BIS monitoring, to even concentration based drug infusion, objective patient based techniques are being universally practiced. But somehow caudal anaesthesia dosing is still relying on traditional non objective methods. As needle insertion in caudal space is now being done through ultrasonography in many centres and also epidual space is being visualized by same method, we through this study recommend the amalgamation of both to visualise spread of drug to epidural spaces for the purpose of estimate minimum effective dosing.
We therefore advocate that real time ultrasonography should be used to place as well as to ascertain drug dosing via epidural space dilation/doppler color flow in caudal paediatric anaesthesia. We also advocate that a volume of 0.7 ml/kg of local anaesthetic in caudal block for infraumblical surgeries in pediatric patients compared to conventionally used volume of 1 ml/kg which is adequate to achieve surgical anaesthesia and post operative pain relief.
Real time ultrasonography can give objective individual based minimum effective dosing for caudal anaesthesia in paediatric patients and we found that a volume of 0.7 ml/kg of local anaesthetic in pediatric caudal block is sufficient to achieve a target of T10 level for infraumblical surgeries, and the same results in effective intraoperative and post operative analgesia without any side effects.