Author: Michael Vlessides
A new calibration methodology appears to have improved on existing nerve block needle pressure monitoring systems by calculating pressure gradient across the needle to estimate the pressure on tissue during injection. The new system may increase the sensitivity and specificity of detecting high injection pressures by eliminating the variability of needle selection from the measurement, according to the researchers.
As the investigators from Baylor Scott & White Health, in Temple, Texas, reported, current pressure monitoring devices measure injection pressure upstream of the needle tip, near the syringe. Nevertheless, previous research has shown that pressure gradients across nerve block needles for clinically relevant flow rates are both large and variable depending on the diameter and length of the needle. Such gradients may subsequently reduce sensitivity and specificity when it comes to detecting high needle tip pressures.
“The major study that set the 15 [pounds per square inch] threshold for possible intraneural or intrafascicular injection actually used opening pressure, defined by the start of flow through the needle,” said Ryan Lefevre, MD, an anesthesiology resident at the institution and lead author of the study (Anesthesiology 2014;120[5]:1246-1253).
As part of the trial, the researchers selected a variety of commonly used nerve block needles for testing. Next, they created a standard nerve block setup comprising the injection needle, short extension tubing and a syringe of local anesthetic. With this, the researchers added a digital manometer near the injection syringe and a mechanical syringe pump to ensure consistent flow rates across all tests.
The researchers then performed a series of injections into various-sized water columns to simulate different needle tip pressures, and while doing so, measuring and calculating the predicted pressure by subtracting the previously established zero value. At the same time, they found the actual pressure with an additional digital manometer.
As Dr. Lefevre reported at the 2018 World Congress on Regional Anesthesia and Pain Medicine and 43rd annual meeting of the American Society of Regional Anesthesia and Pain Medicine (abstract 5366), the researchers found that the calculated needle tip pressure correlated very well with actual pressure, with both the slope and the correlation coefficient approaching 1.0. This correlation held true across all tested injection pressures and every tested needle (Table).
| Table. Correlation Between Calculated Needle Tip Pressures and Actual Pressures | ||
| Needle | Trend Line Slope | Correlation Coefficient, r |
|---|---|---|
| 22 gauge × 2.5 in BD Quincke Spinal Needle | 1.0003 | 0.9998 |
| 22 gauge × 3.5 in BD Quincke Spinal Needle | 1.0184 | 0.9998 |
| 21 gauge × 4 in B. Braun Stimuplex A Needle | 1.0599 | 0.9993 |
| 22 gauge × 2 in B. Braun Stimuplex A Needle | 1.0446 | 0.9991 |
| 22 gauge × 3 1/8 in Havel’s AccuTarg Nerve Block Needle | 1.0019 | 0.9997 |
| 17 gauge × 9 cm B. Braun Tuohy Needle | 0.9987 | 1.0000 |
The researchers noted that these results demonstrate the validity of using an atmospheric pressure injection to calculate pressure gradient across a needle tip.
“Where would we like to go with this?” Dr. Lefevre asked. “Obviously you need to control the flow rate, as the gradient across the needle is proportional to flow. But if you could control or measure flow rate while performing an injection, then you can think of it as zeroing your injection system and thereby calculating the actual tissue pressure when you’re doing the injection. And this, we feel, is a much more accurate way to prevent intraneural and possibly intrafascicular blocks.”
For Jeff C. Gadsden, MD, the chief of the Division of Orthopaedics, Plastics and Regional Anesthesiology and associate professor of anesthesiology at Duke University School of Medicine, in Durham, N.C., the study further refines the research that he and his colleagues performed in 2014. “As the investigators point out, the problem with monitoring pressure the way we currently do is that once the flow starts, it’s very complicated to infer the pressure at the tip of the needle because factors like needle diameter, length and flow rate will all combine to influence it,” he said.
“So what we’re left with knowing is the pressure in the syringe and making the assumption that the pressure at the tissue level is lower,” noted Dr. Gadsden, who is a member of the Anesthesiology News editorial advisory board. “But I think creating this calibration factor allows us to calculate a fairly precise pressure at the tissue level, where the injectate is exiting the needle.”
The challenge of the calibration system, Dr. Gadsden added, is with the model’s assumed flow rate. “Because they’ve calibrated this to 15 mL per minute, everything they’ve done is only relevant to that flow rate. But if you can reliably limit the flow to 15 mL per minute or less, then you can use these factors with confidence.”