We have shown that the incidence of loss of normal arterial blood pressure waveform during flexion or extension of the hand is significantly lower with catheterization of the radial artery in the back of the hand (distal radial artery) than with catheterization of the radial artery at the wrist (proximal radial artery), with no significant difference in the success rate of insertion and in sampling arterial blood between the proximal and distal radial arterial catheterizations.

Previous studies [3, 7,8,9] have shown that the success rate of insertion of a catheter to the distal radial artery was high, and our previous observational study [3] has indicated that the “normal” arterial blood pressure waveforms were observed during general anesthesia in all the patients, and blood sampling was easy in a high proportion (97%) of patients [3]. The results of the current study are in agreement with the findings in these previous studies.

Advantages of catheterization of the distal radial artery over catheterization of the proximal radial artery may include improved patient comfort and easier hemostasis [7, 8]. In addition, catheterization of the distal radial artery can preserve the patency of the proximal radial artery, particularly if repeated catheterization of the radial artery is required, such as for coronary angiography and intervention. Furthermore, a catheter inserted to the distal radial artery is theoretically less likely to be kinked or dislodged, because the catheter inserted to the distal radial artery is supported by surrounding bony structures and is positioned relatively away from the movement of the wrist.

Because of high efficacies observed in the current study and because of these theoretical advantages, it would be reasonable to conclude that catheterization of the distal radial artery is feasible and would be a useful alternative to catheterization of the proximal radial artery during general anesthesia.

There may be several limitations in the study. First, the investigators were several anesthesiologists who had experience of catheterization of both the proximal and distal radial arteries. Therefore, the results may not be generalized, because insertion of a catheter to the distal radial artery may be difficult for doctors with insufficient experience. It is advisable to use ultrasonography when an attempt is made to insert a catheter to the distal radial artery, as it may frequently be difficult to locate the distal radial artery by palpation [3].

Second, the primary outcome measure was assessed by a non-blinded investigator, so that there may be potential for bias. Nevertheless, as we defined the objective criteria for assessment of the outcome measures, the effect of bias, if any, would be minimal.

Third, each investigator was allowed to insert a catheter to either the left or right radial artery, and thus it is not clear if the success rate of insertion and the incidence of complications is higher or lower between the left and right radial arteries. In theory, and in our experience, there would be little difference between the left and right sides, and insertion should be attempted to the side where the pulse is easily identified by palpation or the artery is easily identified by ultrasonography.

For the distal radial artery, each investigator was also allowed to insert a catheter either through the anatomical snuff box or through the back of the hand between the thumb and the index finger. In our experience, insertion of a catheter through the back of the hand between the thumb and the index finger would be more suitable than through the anatomical snuff box because the radial artery runs more close to the skin and the length of the radial artery is longer at the back of the hand than at the anatomical snuff box.

Fourth, we used wrist movements for a short period of time as a simulation to study the effect of repositioning of the patients (e.g., turning the patient from the supine to the prone) on the efficacy of artery blood pressure waveforms and on sampling the blood, so that it is not known if complications associated with repositioning of the patients are less likely to occur with catheterization of the distal radial artery than catheterization of the proximal radial artery. Nevertheless, as there was a marked difference in the incidence of complications between the two insertion sites even with the simulation test for a short period of time, and with the theoretical advantages of catheterization of the distal radial artery, complications associated with catheterization of the distal radial artery during repositioning of the patient would be fewer.

Fifth, we carried out a study only in adult patients, and thus the results may not be applicable to children. Nevertheless, in children, as in adults, catheterization of the distal radial artery has theoretical advantages over catheterization of the proximal radial artery. For example, in children, only a thin catheter (such as 24 gauge) can be inserted to the radial artery, and the length of the catheter inserted to the radial artery is much shorter in children than in adults. Compared with a catheter inserted to the proximal radial artery, a catheter in the distal radial artery is theoretically less likely to be kinked or dislodged by repositioning of the patient. In addition, catheterization of the distal radial artery may be advantageous particularly in small children, because arterial injury or occlusion could have a relatively greater impact on perfusion of the hand. On the other hand, the distal radial artery is generally smaller in caliber than the proximal radial artery, so that catheterization may frequently be difficult, particularly in small children. Therefore, although catheterization of the distal radial artery may be a useful option in children, its feasibility and its efficacy need to be studied formally.

In conclusion, we have shown that, in anesthetized adult patients, the incidence of loss of arterial blood pressure waveform during flexion or extension of the hand is significantly lower with catheterization of the radial artery in the back of the hand (distal radial artery) than with catheterization of the radial artery at the wrist (proximal radial artery), with no significant differences in the success rate of insertion and in sampling arterial blood. These findings suggest that the distal radial artery may be a useful alternative site to the proximal radial artery for catheterization, particularly in adult patients in whom positional change is required.