Continuous Versus Bolus Local Anesthetic Administration in Peripheral Nerve Blocks: Time to Relook

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We read the article by Khatibi et al1 with great interest. They found no evidence to support the hypothesis that bolus doses of ropivacaine given within the transversus abdominis plane (TAP) increased the spread of sensory deficit, as measured by cool roller along the mid-axillary line at 6 hours, compared with a continuous infusion. The authors suggested that both the bolus volume and the continuous infusion rate chosen were sufficient to have reached the anatomical limit of spread possible by the local anesthetic solutions within the TAP. They acknowledge that if they had chosen lower bolus volumes or lower basal infusion rates or a lower total dose, they may have observed a difference.
There are a few additional pertinent points in study design that need to be considered before their results are extrapolated to postoperative surgical patients. First, pain in surgical patients is a complex interplay of many factors that are not present in volunteers.2 Surgical patients demonstrate clinical pain in contrast to induced nociceptive pain elicited in volunteers. Clinical pain following surgery consists of prolonged, ongoing resting pain and stimulus-evoked pain (hyperalgesia and allodynia) caused by complex peripheral and central sensitization processes. The expectation of pain is itself a proven factor in determining the intensity of subsequently experienced pain and is influenced positively or negatively by the mood of surgical patient. In contrast, the healthy volunteer’s cortical activation patterns are exhibited on provoking nociceptive pain. Whether postoperative pain and cutaneous sensation are well correlated remains unknown, and this uncertainty complicates extrapolation of volunteer study observations to clinical practice.
Second, volunteers lack the actual sensation of pain and therefore lack the ischemic-like conditions including decrease in pH, oxygen tension, and an increase in lactate concentration, which correlate well with pain behaviors like guarding after incision in postoperative period. An increase in neutrophils in the wound tissue occurs shortly after surgery, peaks around 24 hours, and is associated with the release of proinflammatory mediators and endogenous opioids peptides. The proinflammatory mediators contribute to hypersensitization after incision. These mechanisms are not activated in volunteers.3
Third, earlier evidence of advantage of intermittent boluses over continuous infusion was provided by Hogan4 in cadavers using cryomicrotome imaging. They demonstrated a uniform spread of an ink in epidural space of cadavers after bolus dosing but nonuniform spread in rivulets for infusions. A larger volume and a higher infusion pressure produced a more uniform spread. This observation was supported in popliteal nerve block5 and femoral nerve block6 studies in which intermittent bolus technique was found to be superior to continuous infusion in surgical patients. Additional validation was provided in surgical patients receiving adductor canal block for postoperative pain relief by Thapa et al.7 Though the present study1 demonstrated the extent of spread, it did not comment upon uniformity of spread, an important factor in quality of analgesia in postoperative period. The role of continuous abdominal muscle movements during respiration in spread of drugs is also not known in TAP block.
Fourth, clinically intercepting and breaking the pain cycle in the initial postoperative period is important and may reduce the incidence of hyperalgesia and persistent postoperative pain. This cannot be assessed in a volunteer study.
In summary, we need to know whether bolus dosing or continuous basal infusion is best in immediate postoperative period where and when it is most important. The results of the study by Khatibi et al1 suggest a relook.

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