Perfusion index (PI) in the finger was empirically related to the temperature, and a higher PI was associated with a slightly higher SpHb. (Image source: Thinkstock)

Perfusion index (PI) in the finger was empirically related to the temperature, and a higher PI was associated with a slightly higher SpHb. (Image source: Thinkstock)

Noninvasive monitoring has come a long way.  First there was blood pressure, followed by oximetry and end-tidal carbon dioxide.  Now there are many more noninvasive blood tests, including noninvasive hemoglobin measurement, monitored continuously via spectrophotometric finger sensors (Masimo Corporation, Irvine, California, SpHb).  What factors affect the accuracy of this device?

Dr. Ronald D. Miller, Anesthesia and Perioperative Care, University of California, San Francisco, San Francisco, California, and colleagues examined how perfusion of a digit impacts the accuracy of noninvasive hemoglobin determination.  In their study “A Comparison of Lidocaine and Bupivacaine Digital Nerve Blocks on Noninvasive Continuous Hemoglobin Monitoring in a Randomized Trial in Volunteers,” the authors compared lidocaine to bupivacaine for digital block and assessed indices of enhanced perfusion of the finger – temperature, perfusion index, and SpHb.  Their results are published in this month’s issue of Anesthesia & Analgesia.

In a randomized, double-blinded fashion, the authors studied 12 healthy volunteers. SpHb was continuously monitored on either the third finger (3 volunteers) or fourth finger (9 volunteers) of each hand.  Digital nerve blocks were applied to both digits, randomized to 1% lidocaine or 0.25% bupivacaine used for the finger block on the right hand, with the alternative formulation applied to the finger on the left hand. Two mL was injected into each finger, 1 mL each at the base of the medial and lateral side of each finger, respectively.

Not surprisingly, the nerve block after either drug resulted in an increase in digital temperature. Not surprisingly, bupivacaine effects outlasted those of lidocaine.  Perfusion index (PI) was demonstrably related to the temperature, and a higher PI was associated with a slightly higher SpHb.  The authors infer that this relationship predicts that the SpHb would be more accurate, enough so to possibly determine meeting transfusion criteria in patients.

Should the reader then believe that a bupivacaine block is needed for the duration of time sensor measurements are made?  Would general anesthesia have the same effect as a block with a local anesthetic?  What about other patient comorbidities that might affect blood flow to the digit having an effect on device performance?  Would warming the hand help? It is not clear from this study whether SpHb is sufficiently accurate to guide clinical transfusion decisions.