Lumbosacral Cerebrospinal Fluid Volume Is the Primary Determinant of Sensory Block Extent and Duration during Spinal Anesthesia 

After we obtained approval from the institutional review boards at the Virginia Mason Medical Center and at the Medical College of Wisconsin and informed consent, 10 healthy volunteers participated in the study (nine at Virginia Mason and one at the Medical College of Wisconsin). Lumbar puncture was performed (in the left lateral decubitus position) at the L2-L3 interspace with a 25-gauge Whitacre spinal needle through a 20-gauge introducer with the orifice of the spinal needle turned cephalad. Cerebrospinal fluid (0.2 ml), was aspirated and 50 mg 5% lidocaine in 7.5% dextrose was injected at a rate of approximately 0.25 ml/s. After injection, the volunteers were immediately placed supine and remained so for the rest of the study.

Dermatomal levels of analgesia to pinprick (18-gauge needle) were measured at 1, 2, and 5 min after intrathecal injection of lidocaine, every 5 min thereafter until 40 min after injection, and then every 10 min until recovery of sensation to pain prick at the S2 dermatomal level.

Transcutaneous electrical stimulation (TES) was used to estimate the onset and duration of surgical anesthesia because it has been shown to be as potent a noxious stimulus as surgical incision. [19]The TES leads were placed in the midline at the T10 and T12 dermatomes and bilaterally at L2-L3 (medial aspect above the knee) and L5-S1 (lateral aspect above the ankle). Five seconds of 50-Hz tetanus at 60 mA delivered with a commercially available nerve stimulator (model NS252; Fisher & Paykel, Auckland, New Zealand) was considered equivalent to surgical incision. [19]This particular unit was tested by our bioengineering department to verify sustained delivery of displayed currents. Tolerance to electrical stimulation was assessed 4 min after intrathecal injection of lidocaine and measured every 10 min thereafter by initially testing with 10 mA and then increasing in 10-mA increments to a maximum of 60 mA for 5 s. Absence of pain during the maximal stimulus was considered equivalent to “surgical anesthesia.”[19]Each TES location was tested in sequence moving from distal to proximal sites.

Motor strength in the right quadriceps and gastrocnemius muscles was assessed with a commercially available isometric force dynamometer (Micro FET; Hogan Health Industries, Draper, UT).# Strength was measured during a 5-s isometric maximal contraction of each muscle performed 0 and 10 min after injection of lidocaine and every 10 min thereafter until return to 90% of the baseline strength measurement. Recovery of 90% of baseline motor strength was identified as the duration of motor blockade. Measurements were performed in triplicate and then averaged at each measurement period. Isometric force dynamometry was shown previously to be a reliable quantitative measure of motor block during spinal and epidural anesthesia. [20,21] 

Low thoracic, lumbar, and sacral axial magnetic resonance images were obtained at 8-mm increments by fast-spin echo sequence, which highlights CSF (Figure 1). Details of the method have been described before. [18]Briefly, the dural sack and spinal cord areas were determined in a blinded manner for each image using video/digital analysis. Area of the sack minus area of the cord constituted the area of CSF and roots; this area multiplied by 8 mm resulted in CSF/root volume. The level of the disc between the 11th and 12th thoracic vertebrae was determined and the CSF/root volume was measured caudal from this site. Although this calculated volume includes the roots, it is hereafter referred to as the CSF volume.

Nine volunteers had spinal anesthesia performed before analysis of CSF volume. One volunteer had CSF volume measured before performing the spinal anesthesia.

Cerebrospinal fluid volumes were correlated using Kendall rank correlation with (1) the peak sensory block height to pinprick;(2) the time until two-segment regression from the peak sensory level;(3) the time until regression of pinprick analgesia to the L1 (duration of thoracic analgesia) and S2 (lowest dermatome tested) dermatomes;(4) the duration of surgical anesthesia at L2-L3, and L5-S1; and (5) the duration of motor blockade of the quadriceps and gastrocnemius muscles. Results are reported as means +/- SD.

Statistical correlations were assessed by Kendall-Tau analysis. Probability values <0.05 were considered significant. Correlation lines and r values for the figures were identified by linear regression analysis.

Spinal anesthesia was performed before magnetic resonance imaging in nine volunteers. The original statistical analysis revealed a probability value of 0.06 for the correlation between CSF volume and peak block height. Consequently, one volunteer from a previous study that measured CSF volume was approached and consented to receive spinal anesthesia. Data from this volunteer (#10) were added to those of the original nine volunteers and a new statistical analysis was performed.

Volunteers were young and healthy and of average height, weight, and body mass index (weight in kilograms divided by height in meters squared;Table 1). Peak sensory block height ranged from T12-T2. Time to achieve peak sensory block height ranged from 10–30 min (20 +/- 6 min). Time until two-segment regression of sensory analgesia to pinprick ranged from 30–80 min (57 +/- 16 min), regression to L1 ranged from 35–130 (104 +/- 29 min), and regression to S2 ranged from 90–180 min (144 +/- 30 min). Duration of surgical anesthesia assessed by TES at T10 ranged from 0–70 min (42 +/- 21), at T12 ranged from 0–110 min (68 +/- 29 min), at L2-L3 ranged from 10–130 min (80 +/- 32 min), and at L5-S1 ranged from 10–130 min (73 +/- 39 min). Two volunteers did not develop tolerance to maximal stimulation with TES at the T10 or T12 dermatomes. One volunteer did not develop a detectable motor block of the quadriceps and two did not develop motor block of the gastrocnemius muscles. In those volunteers in whom motor blockade developed, the duration in the quadriceps motor block ranged from 80–140 min (94 +/- 26 min) and in the gastrocnemius from 100–150 min (121 +/- 17 min).

Volumes of CSF ranged from 42.7 to 81.1 ml and were significantly correlated with the peak sensory block level to pinprick (Figure 2) and the duration of tolerance of TES (surgical anesthesia) in the L^5-S1dermatomes (Figure 3, Table 2). Volumes of CSF did not correlate with the onset of sensory analgesia or anesthesia, onset of motor blockade, or duration of motor blockade (Table 2).

This is the first study to determine directly whether variability in lumbosacral CSF volume can explain the wide range of spread of spinal anesthesia observed in clinical practice. We determined that volume of lumbosacral CSF is a critical determinant of the extent and duration of sensory blockade during spinal anesthesia. For example, differences in peak sensory block levels after injection of lidocaine appear to be explained largely by differences in CSF volume. Differences in CSF volume also correlate with differences in the duration of tolerance to tetanic electrical stimulation (Table 2). Similarly, serial intrathecal injections of local anesthetic into the same patient produces spinal anesthesia that is less variable in extent and duration than when the same dose of local anesthetic is injected into different patients. [3,4,6,20]Previous authors have suggested that variability in CSF volumes may be an important cause of variability in the spread of spinal anesthesia. [7,22]This suggestion is based on the fact that local anesthetic injected to produce spinal anesthesia will be diluted in the volume of lumbosacral CSF. Because the volume of CSF varies by a factor of three in humans, [18]variable dilution can be expected to have the important effect we observed on the qualities of neural blockade.

In contrast to the correlations between CSF volume and measures of sensory anesthesia, CSF volume did not correlate with the duration of motor blockade. The reason for this lack of correlation is unknown. One possible explanation may relate to our use of hyperbaric local anesthetic solutions in supine volunteers. The hyperbaric solution would be expected to preferentially distribute to the dorsal nerve roots, and the anterior roots should be exposed to less local anesthetic. Indeed, 2 of the 10 volunteers did not develop complete motor blockade in the lower extremities, and correlation analyses were limited to the remaining eight patients. Another explanation for the lack of statistical correlation may be that the number of patients included in this study was insufficient to identify correlations between CSF volume and blockade of motor fibers.

The method used for in vivo determination of CSF volume was validated in a previous study by measurement of known volumes and reliable duplication with repeated measurement. Although the volume measured included the cauda equina, this contributes only about 15% of the total volume of the dural sack at this level. [23]Consequently, uncertainty introduced by a variable nerve root volume should be a relatively small factor.

Our findings help explain the unpredictability of spinal anesthesia in clinical practice and provide valuable insight into mechanisms of spinal anesthesia. However, these findings will probably not be helpful for guiding clinical practice because CSF volume is not measured before anesthesia, is highly variable, and is not easily predicted by patient characteristics. Lumbosacral CSF volume has been shown to correlate with body mass index [18]; however, the correlation is relatively poor (R = 0.4) and, therefore, insufficient to reliably predict CSF volume. Because CSF volume is not easily measured; cannot be predicted by age, height, body mass index, or other readily available patient characteristics; and varies widely among individuals, our results cannot be expected to improve the ability to predict the spread or duration of spinal anesthesia in clinical practice.

In conclusion, variations in lumbosacral volume of CSF largely explain variability in the extent of the spread and duration of spinal anesthesia. Because CSF volume cannot be readily predicted, uncertainty in the extent and duration of spinal anesthesia is inevitable after intrathecal injection of a single local anesthetic dose.

# There is no relation between Hogan Health Industries and author Quinn Hogan.