The conventional Quincke-point needle used for the intrathecal injection of local anesthetic solution for spinal anesthesia has its opening at the end of the needle. Thus, there is no impediment to flow of the injectate, which is typically in a straight axial direction. This can be easily demonstrated in vitro by methylene blue injection into fluid (Figure 1). Unlike conventional Quincke needles, a pencil-point needle such as the Whitacre needle has an aperture on the side of the needle and a bullet-shaped needle tip. This diverts flow away from the axis of the needle and results in directional flow from the needle aperture (Figure 1). In studies performed in vitro, we have observed that injection through a 27-gauge Whitacre needle resulted in a laminar flow pattern characterized by a directional stream approximately 45 degrees from the longitudinal axis of the spinal needle.
Neigh et al.  studied the effects of needle aperture direction and injection speed on the resulting level and duration of spinal anesthesia. These investigators found that if the Whitacre needle aperture were oriented in a cephalad direction, a more cephalad sensory level resulted than if the same local anesthetic dose were injected with the aperture directed caudally. However, this study was performed with doses of 1% tetracaine that varied according to patient height, and the investigators used 22-gauge Whitacre spinal needles.
Isobaric preparations have spread characteristics different from those of hyperbaric solutions. This study was designed to prospectively evaluate the effects of 27-gauge Whitacre needle aperture direction on resulting sensory and motor anesthesia levels, as well as the duration of a standardized dose of isobaric lidocaine spinal anesthesia.
Data were recorded prospectively on 40 patients using a randomized double-blinded study design. All patients were ASA physical status I-II and were scheduled for elective ambulatory knee arthroscopic surgery. Patients were 18-60 years old and were free of contraindications to spinal anesthesia. Patients were studied subsequent to our institutional review board approval and written, informed consent. Study patients were randomly assigned to one of two study groups. Both groups received isobaric spinal anesthesia with intrathecal injection of 60 mg 2% plain isobaric lidocaine (Abbott, North Chicago, IL) using a 27-gauge Whitacre spinal needle (Becton-Dickinson, Franklin Lakes, NJ). Group I had the Whitacre needle aperture oriented in a cephalad direction throughout intrathecal injection. Group II had the aperture directed caudally.
Patients received no premedication, and sedation was limited to intravenous midazolam (maximum dosage 10 mg). No opioids or analgesics were administered intraoperatively. Usual monitoring was used.
After administration of 500 mL intravenous lactated Ringer's solution, spinal anesthesia was administered at the L3-4 interspace with the patient in the right lateral decubitus position. A midline approach was used in all patients, with a 20-gauge introducer needle preceding passage of the 27-gauge Whitacre spinal needle. Injection of the 3-mL dose was timed by digital stopwatch (Casio, Tokyo, Japan), to the nearest 1/10 of a second and recorded.
After injection, a single-blinded observer measured onset and offset of lumbar and thoracic sensory anesthesia by pinprick and measured motor block by Bromage scale . Sensory evaluation of onset and offset was defined by the patients' being able to perceive the pinprick as "sharp" in any given dermatome. Dermatomes were tested in a caudad to cephalad direction systematically. Motor evaluation was done utilizing a scale for which 0 = no paresis, full movement; 1 = partial paralysis, ability to flex knee only; 2 = partial paralysis, ability to flex foot only; and 3 = full paralysis, no movement. Perirectal sensory evaluation was not done. Measurements were made at 5, 10, 15, 20, 30, 40, 50, and 60 min and every 15 min thereafter until complete resolution of sensory anesthesia. Measurements extended from the preoperative to postoperative periods.
Times to completion of ambulatory milestones were measured and recorded by postanesthesia care unit nurses who were blind to group assignment. The following milestones were defined as the duration from the spinal injection until ability to sit in a chair, ability to spontaneously urinate, first oral intake, and discharge from the ambulatory unit by set criteria. Criteria for discharge were: 1) complete resolution of sensory anesthesia, 2) complete resolution of motor block, 3) return of vital signs to preanesthetic levels, 4) return of mental status to preanesthetic level, 5) adequate management of postoperative pain, 6) absence of nausea, 7) ability to urinate, and 8) ability to ambulate without assistance.
Statistical analysis was performed using unpaired t-test, chi squared, Mann-Whitney nonparametric, or simple regression analysis. To define differences in resolution of thoracic sensory, lumbar sensory, and lower extremity motor block, the time to full resolution of each of these was measured, and survival analysis was performed with between-group comparisons using a Cox regression test. Significance was defined as P < 0.05.
All 40 patients had satisfactory spinal anesthesia; general anesthesia was not required for any patient. There were no postdural puncture headaches. Patient demographic data are displayed in Table 1.
Results of mean spinal anesthetic durations for both groups are displayed in Table 2. There was a statistically significant difference in peak anesthetic sensory level to pinprick between groups. Whereas there was no significant difference between groups in the duration of thoracic sensory anesthesia, duration of lumbar sensory anesthesia was significantly longer in Group II.
Thoracic and lumbar sensory anesthesia as a function of time from injection of intrathecal local anesthetic is shown in Figure 2. In Group I, thoracic and lumbar sensory anesthesia tended to resolve together. By contrast, with caudad injections, lumbar sensory anesthesia outlasted thoracic anesthesia by 73 min (P < 0.01).
Similar patterns were noted in the duration of lumbar motor blockade. Group I had a mean duration of motor block of 118 +/- 26 min compared with 150 +/- 23 min for Group II, P < 0.001 (Figure 3).
The results of cumulative survival analyses for regressions of sensory and motor blocks are shown in Figure 4. The durations of lumbar sensory and lower extremity motor block by survivorship analysis were significantly longer in Group II (P < 0.01 and P < 0.001, respectively).
The mean times from intrathecal injection to achievement of the various ambulatory milestones are listed in Table 1. Times to achievement of all milestones (except oral intake) were prolonged in Group II. The mean time from spinal injection to first spontaneous urination and mean time to discharge from the ambulatory unit were approximately 30 min longer for patients in Group II, which is clinically significant.
Speed of injection ranged from 15.4 to 23.4 s with a mean of 20.0 +/- 2.3 s. This range in injection time did not result in any correlation (r = 0.17) between peak level and injection time (Figure 5) for either caudad and cephalad injections.
There were no statistically significant differences in the incidences of hypotension (systolic <80 mm Hg) or bradycardia (heart rate <50 beats/min) between groups (Table 1). Twenty percent of patients in Group I developed hypotension compared to 0% in Group II. However, this was not statistically significant.
Orientation of the aperture of a 27-gauge Whitacre needle in a cephalad compared with a caudad direction resulted in a marked difference in the clinical characteristics of isobaric lidocaine spinal anesthesia. Orienting the aperture of the needle directly caudally prolonged several measurable outcome variables, including durations of sensory anesthesia to pinprick and motor block, as well as times to ambulate, urinate, and discharge from the ambulatory unit. These results can presumably be explained on the basis of altered distribution of intrathecal local anesthetic in the subarachnoid space. This is presumably due to a greater amount of the local anesthetic being concentrated near the lower lumbar and sacral nerve roots when the injection is directed caudally. The increase in spinal anesthetic duration in this study was of similar magnitude to the effects of adding epinephrine to intrathecal lidocaine [3,4].
Our study is not the first to find evidence that mechanisms of injection can alter intrathecal local anesthetic distribution. Other investigators have found that several factors influence the intrathecal distribution of local anesthetics . These factors include needle direction [1,6,7], temperature , and speed of injection . Angle of insertion of a 25-gauge spinal needle was found to result in a two-segment difference in peak sensory level of anesthesia by Stienstra et al. .
Neigh et al.  studied the effect of orientation of a 22-gauge Whitacre needle on hyperbaric spinal anesthesia with tetracaine. These investigators found that the direction of a 22-gauge Whitacre needle tip had a significant influence on sensory level of spinal anesthesia but found no differences in duration of spinal anesthesia. However, in this study, dose of anesthetic, a factor shown to influence duration [9-11] and peak sensory level , was also altered. Injections were done in the sitting position and with a hyperbaric solution of local anesthetic, both of which have a major impact on intrathecal local anesthetic distribution, duration, and sensory level. When using hyperbaric drugs, factors such as position or operating room Table orientationmay have greater influence on distribution of intrathecal anesthetic  and, thus, the effects of needle direction on duration may have been somewhat masked in the study by Neigh et al. . By contrast, this study examined a single dose of plain intrathecal lidocaine.
By using a plain anesthetic with isobaric characteristics , the local anesthetic is more likely to remain where directed. A greater amount of anesthetic in a given area of the subarachnoid space may therefore result in a prolonged duration of sensory anesthesia in the corresponding dermatomes, as well as prolonged motor block of the motor nerves from this area. Deliberate, slow injection of hyperbaric local anesthetic exhibits a prolonged duration by this mechanism . A range of normal injection rates was found to have no significant influence in our study. This agrees with the findings of Neigh et al.  when using hyperbaric tetracaine.
Cephalad injections resulted in significantly greater numbers of thoracic dermatomes being anesthetized. Although we noted no statistically significant difference in hypotension between groups, four patients in the Group I became hypotensive, whereas none in Group II did. Since extensive thoracic neural blockade produces circulatory effects, it is possible that the incidence of hypotension is greater when using needles in which the aperture is in a cephalad direction.
Aperture orientation may also play a role in neurotoxicity by directing the local anesthetic to affect a certain area of nerves in the cauda equina. Beardsley et al.  recently reported two cases in which caudad orientation of a Whitacre needle aperture during injection of lidocaine resulted in prolonged sacral sensory neural deficits. However, these investigators used hyperbaric lidocaine anesthesia. We have not noted this phenomenon in the patients in this study, in which isobaric lidocaine was used. However, this study was not adequately designed to assess potential neurotoxic effects of lidocaine with regard to aperture orientation. Nevertheless, this is another theoretical reason for deliberate orientation of the aperture of a noncutting spinal needle.
We conclude that Whitacre needle aperture orientation exerts a major influence on sensory level and duration of isobaric lidocaine spinal anesthesia. Caudad orientation of the aperture prolonged lumbar sensory and motor nerve block. Studies of spinal anesthesia using pencil-point needles must control for needle tip orientation if meaningful conclusions are to be drawn.
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