Isobaric spinal anesthesia is a simple and conventional technique. However, predicting the intrathecal spread of isobaric spinal anesthetics is still a challenge to anesthesiologists in clinical practice. Isobaric spinal anesthetics spread mainly through concentration gradients, whereas hyperbaric spinal anesthetics are affected mostly by gravity.1 This concentration gradient-dependent spread of isobaric spinal anesthetics within the intrathecal space often results in an unpredictable sensory block level, which is less common than with hyperbaric spinal anesthetics. Therefore, the physical properties of the cerebrospinal fluid (CSF) affect the intrathecal spread of isobaric spinal anesthetics more than that of hyperbaric spinal anesthetics. It is well known that the physical properties of the CSF, including the volume, density,2,3 and temperature,4,5 have significant effects on the sensory block level in isobaric spinal anesthesia. Higuchi et al.3 reported that the peak velocity of the CSF bidirectional oscillatory movement, which originates mainly from pressure changes during a cardiac cycle, is closely related to the regression time of the sensory block level in plain bupivacaine spinal anesthesia.
The effects of preanesthetic fluid administration (preload) to prevent hypotension after spinal anesthesia have been studied extensively over the last decades with the major focus on reducing the incidence and severity of spinal anesthesia-induced hypotension.6–8 However, whereas in our previous study we found no significant differences in the mean arterial blood pressure and heart rate between the preload and the nonpreload group, a preload with crystalloid delayed the time to reach the peak sensory block level in isobaric spinal anesthesia by affecting the pulsatile movement of the CSF at the midlumbar level.9 Intravascular volume expanders, crystalloid, and colloid differ in intravascular half-life,10,11 their contribution to intravascular osmotic pressure,12,13 and cardiac output.11,14–16 Intravascular osmotic pressure and intravascular volume shifting status caused by administration of either crystalloid or colloid may cause different responses in CSF movement. If different CSF movement occurred, it could affect the spread of the isobaric spinal anesthetic within the intrathecal spaces after it was injected. In this study, we examined whether a preload of either crystalloid or colloid can affect the intrathecal spread of isobaric spinal anesthetics in patients scheduled to undergo an elective transurethral resection of a bladder tumor. A cardiac gated, phase-contrast cine high-resolution magnetic resonance imaging (MRI) study was also performed to determine whether administration of crystalloid or colloid might result in different effects on the pulsatile movement of the CSF at the midlumbar level in young, healthy volunteers.
The IRB at our hospital approved this prospective study, and written informed consent was obtained from the patients and volunteers for their participation.
Clinical Study for Isobaric Spinal Anesthesia
Sixty-three patients (ASA physical status I and II; aged 70 years or younger; males and females) scheduled to undergo an elective transurethral resection of a bladder tumor under spinal anesthesia were enrolled in this study. The patients were allocated randomly into 1 of 2 groups using a stratified sealed envelope method: crystalloid preload group or colloid preload group. The exclusion criteria included patients who began their procedure after 11:00 AM, had a history of spinal disease or deformities, were unable to communicate because of neurological disease, or had other common contraindications for spinal anesthesia. All patients in both groups fasted for 8 hours before surgery. Intravenous administration of lactated Ringer solution was started at 6:00 AM at an infusion rate of 100 mL/h using a flow rate regulator (Dial-A-Flo; Abbott, Abbott Park, IL) and was maintained until the initiation of a preload of either crystalloid or colloid. No premedication was administered to the patients.
Upon arrival at the waiting area of the operating room, an anesthesiologist who participated in neither isobaric spinal anesthesia nor in determining the sensory block level, administered either 15 mL/kg lactated Ringer solution (crystalloid preload group) or 5 mL/kg electrolytes-containing hetastarch (Hextend; Hospira, Inc., Lake Forest, IL) (colloid preload group) to the patients over a 10- to 15-minute period and then removed the solution from the patient's IV to maintain blinded study conditions. Subsequently, the patients were transferred to the operating room. Routine monitoring by electrocardiogram, noninvasive arterial blood pressure, and pulse oximetry were performed and the baseline measurements were recorded. Another anesthesiologist blinded to group assignment performed the spinal anesthesia with the patient in the right lateral decubitus position. In all patients, a midline approach was used at the midlumbar level (Tuffier's line) with a 25-gauge Whitacre spinal needle (BD Whitacre needle; BD Medical Systems, Franklin Lakes, NJ). A 0.5% isobaric tetracaine solution was prepared by dissolving 20 mg of crystalline tetracaine hydrochloride (Pantocainesterile; Daehan Medical Co., Seoul, Korea) into 4 mL of the CSF withdrawn through a Whitacre spinal needle. Tetracaine 12 mg (2.4 mL of the dissolved solution) was administered slowly into the intrathecal space over a 30-second period. The patient was placed in the horizontal supine position for 5 minutes and then in the lithotomy position. The sensory block levels were determined by a midline pinprick at 5-minute intervals for the first 30 minutes and then at 15-minute intervals until 90 minutes after the intrathecal injection of 0.5% isobaric tetracaine. Routine monitoring was recorded every 5 minutes. Ephedrine 5 mg was injected if hypotension (defined as a >30% decrease in the mean arterial blood pressure from baseline measurement) occurred, and 0.5 mg of atropine was injected in the case of bradycardia (heart rate <50 beats/min).
Volunteer Study for MRI
This study was performed to determine whether the administration of crystalloid has different effects on the pulsatile movement of the CSF at the L2-3 intervertebral intrathecal space and midportion of the aqueduct of Sylvius compared with colloid.
Twenty-three healthy male volunteers (age range, 21–26 years), who were free from neurological disease and spinal deformity with no history of medication, were enrolled in this study. All quantitative assessments of CSF movement by cardiac gated, phase-contrast cine high-resolution MRI were performed between 6:00 PM and 8:00 PM because the rate of CSF production in healthy adults is affected by the circadian rhythm.13 Each volunteer underwent 2 sequential MRI studies at 1-week intervals. The first MRI study involved crystalloid administration. The initial MRIs were taken for the baseline (vCRT0). Subsequently, 15 mL/kg lactated Ringer solution was administered over a 10- to 15-minute period and MRIs were taken 30 minutes (vCRT30) and 60 minutes (vCRT60) after the baseline MRIs had been taken. One week after the crystalloid study, the same volunteers underwent a colloid study receiving 5 mL/kg electrolytes-containing hetastarch (Hextend; Hospira, Inc.). The MRIs were obtained at baseline (vCOT0) and 30 minutes (vCOT30) and 60 minutes (vCOT60) later. The MRIs were taken at 30 and 60 minutes to match the interval of the intrathecal spread of isobaric spinal anesthetics from the initiation of preload in the clinical spinal anesthesia study, and to differentiate the effect of crystalloid and colloid according to their different intravascular half-lives. To investigate whether the rapid administration of crystalloid or colloid affects CSF pulsatile movement, a quantitative assessment of CSF pulsatile movement was performed at the midportion of the aqueduct of Sylvius and the L2-3 intervertebral intrathecal space as described previously.9,17,18 All cardiac gated, phase-contrast cine high-resolution sequence MRIs were performed using a 3T MR scanner (Achieva; Philips Medical Systems, Best, The Netherlands). For visualization of the anatomic landmarks, sagittal turbo spin echo T2-weighted images of the brain and thoracolumbar spine were obtained. The acquisition planes were selected perpendicular to the presumed direction of CSF movement through the midportion (at the level of the inferior colliculus) of the aqueduct of Sylvius and the L2-3 intervertebral intrathecal space. Flow images were acquired using 2-dimensional T1 fast field echo sequence quantitative-flow technique with peripheral gating. The imaging variables were as follows: repetition time/echo time = 13/7.9 ms; flip angle = 15 degrees; orientation = transverse; field of view = 150 mm; slice thickness = 10 mm; matrix size = 224 × 224; velocity encoding = 2 cm/s; cardiac phase = 30; and phase-contrast flow direction = foot to head. The measured flow of the cranial direction is presented as a positive value, and the caudal direction is reported as a negative value. The acquisition time was approximately 5 to 6 minutes, with slight variations according to each volunteer's heart rate. A radiologist, who was blinded to the choice of administered fluids, measured the variables of CSF movement on the axial slices using the circular region of interest method. Placement of the region of interest was performed in triplicate at different sessions, and the mean value was obtained. From the data acquired after processing, which was performed at a workstation using an analysis program (View Forum; Philips Medical Systems), the following variables of CSF pulsatile movement were obtained at the L2-3 intervertebral intrathecal space. The CSF production rate was measured by numerical integration of the CSF flow over the entire cardiac interval at the midportion of the aqueduct of Sylvius.19 We also investigated whether maximum amplitude measured at the midportion of the aqueduct of Sylvius had the correlation with CSF pulsatile movement at the L2-3 lumbar level.
The definitions of the measured variables are as follows:
- Stroke volume (mL): average of the CSF volume moving caudally during systole and cranially during diastole
- Regurgitant fraction (%): ratio of caudal to cranial flow of CSF per single stroke volume
- Absolute stroke volume (mL): integral over time for the volumetric flow rate
- Mean flux (mL/s): mean velocity × area or the CSF volume that passes the contour per second
- Stroke distance: average of the distance of the CSF moving caudally during systole and cranially during diastole
- Mean velocity (cm/s): mean value of the distance of CSF movement over time during each cardiac phase
- Maximum amplitude (mL/s): difference between the maximum systolic CSF flux and maximum diastolic CSF flux
Statistical analysis was performed using SAS software (version 9.13; SAS Institute, Cary, NC). The comparisons were performed using a paired t test with a Bonferroni correction for continuous normally distributed variables and a Wilcoxon signed rank test with a Bonferroni correction for non-normally distributed data. The incidence of hypotension and bradycardia between the 2 groups in the clinical study was compared using a χ2 test. The correlation between the maximum amplitude of the CSF at the midportion of the aqueduct of Sylvius and at the L2-3 intervertebral intrathecal space was performed by Spearman correlation analysis.
Based on a previous study,9 a minimum of 30 patients in each group in the clinical spinal anesthesia study were required to detect a significant difference in the time to reach the peak sensory block level between the 2 groups (α = 0.05, β = 0.8). A pilot study showed that the MRI study would require 23 volunteers in each group to detect a significant difference in the regurgitant fraction (>20%) measured at 30 minutes (T30) between the 2 groups (α = 0.05, β = 0.8). A P value <0.05 was considered significant.
Clinical Study for Isobaric Spinal Anesthesia
Among the 63 patients, 2 patients (1 in each group) were excluded because of inadequate spinal anesthesia. One patient in the colloid preload group was excluded because general anesthesia was also induced after spinal anesthesia at the surgeon's request. Therefore, 60 patients (n = 30 in each group) were included in the analysis.
Table 1 lists the demographic data of the clinical study for isobaric spinal anesthesia. The patients' characteristics in the 2 groups were comparable in terms of age, height, body weight, gender, and body mass index. The median sensory block levels of the crystalloid preload group at 15 minutes (T10; range, T7–T12; P < 0.05) and 20 minutes (T9.5; range, T7–T11.5; P < 0.05) were significantly lower than those of the colloid preload group (T8; range, T6–T10 at 15 minutes, and T7; range, T6–T9, respectively) (Fig. 1). More time was necessary to reach peak sensory block level in the crystalloid preload group (27.2 ± 17.8 minutes; P < 0.01) than in the colloid preload group (13.9 ± 7.0 minutes). The peak sensory block level was lower (but not statistically significant [P = 0.057]) in the crystalloid preload group (T8; range, T3–T11) than in the colloid preload group (T6; range, T2–T11). Regression of the 2 dermatomes from the peak sensory block level was observed at a similar time interval in the 2 groups. There were no significant changes in mean arterial blood pressure and heart rate between the 2 groups throughout the study. In addition, the incidence of hypotension and bradycardia were also similar in the 2 groups (Table 2).
Volunteer Study for MRI
There were significant differences in age, height, and body mass index between the volunteers in the MRI study and the patients in the clinical study undergoing isobaric spinal anesthesia (Table 1).
The comparisons of the variables measured at the L2-3 intervertebral intrathecal space of both vCRT0 and vCOT0 were not significantly different. In group vCR, the stroke volume, absolute stroke volume, and mean flux at vCRT30 decreased significantly from the vCRT0 and returned at vCRT60 within the boundary of not exceeding the values of the vCRT0. The regurgitant fraction of vCRT30 showed a significant increase compared with vCRT0. However, group vCO showed no significant changes in the variables of CSF movement throughout this study. There were significant differences in stroke volume, regurgitant fraction, absolute stroke volume, mean flux, and mean velocity between the 2 groups at vCRT30 and vCOT30, but not at vCRT60 and vCOT60 (Table 3).
There were no significant correlations between the maximum amplitudes of the CSF pulsatile movement measured at the midportion of the aqueduct of Sylvius and those at the L2-3 intervertebral intrathecal space, regardless of the choice of fluid administration. However, the CSF production rate significantly increased at 30 minutes (637 μL/min, P < 0.05) after crystalloid preload compared with the baseline measurement (448 μL/min), and then slightly decreased (609 μL/min) at 60 minutes. These values are within the range of previous reports.20,21 In the colloid preload group, the CSF production rate was not statistically significant compared with the baseline measurement (464, 512, and 542 μL/min at baseline, 30 minutes, and 60 minutes, respectively) (Fig. 2).
Isobaric spinal anesthetics render concentration gradient-dependent kinetic properties that make it more difficult to predict the sensory block level than hyperbaric spinal anesthetics. More than 25 factors affect the sensory block level in spinal anesthesia, but there is limited information on predictions of the cephalad spread of isobaric spinal anesthetics in clinical practice.22 In this clinical study of isobaric spinal anesthesia, patients receiving crystalloid (15 mL/kg) showed a significantly longer mean time to reach the peak sensory block and a lower median sensory block at 15 and 20 minutes than those receiving colloid (5 mL/kg). In the MRI study, the administration of crystalloid decreased CSF flow in the cranial direction significantly and attenuated pulsatile movement of CSF at the L2-3 intervertebral intrathecal space. However, this was not observed with colloid.
Quantitative assessment of CSF movement by cardiac gated, phase-contrast cine high-resolution MRI revealed pulsatile movement of the CSF within the craniospinal axis, and its propelling force into the spinal canal originated mainly from changes in pressure during the cardiac cycle. Cardiac cycle–related periodic pulsations produce craniocaudal CSF movement during systole and caudocranial CSF movement during diastole.23–26 CSF shifts rapidly from the intracranial intrathecal space into the cervical canal during an early systole.27 The changes in CSF pulsatile movements measured by cardiac gated, phase-contrast cine high-resolution MRI were used to evaluate selective cases of hydrocephalus or myelopathy.28–31 Among the variables of CSF pulsatile movement at the lumbar level, the mean velocity and stroke distance represent directional shifts in CSF pulsatile movement whether or not the main CSF flow moves cranially or caudally. Given that the analysis program for CSF pulsatile movement in this MRI study was set as a positive value for the caudocranial direction, a significant decrease in the positive values of the mean velocity may indicate reduced CSF flow in the cranial direction after crystalloid administration. Because the mean velocity is reported as a positive value, the regurgitant fraction may represent the craniocaudal movement of CSF flow. An increased regurgitant fraction was observed only after crystalloid administration, which may be considered reduced caudocranial CSF flow. The stroke volume, absolute stroke volume, mean flux, and maximum amplitude may provide information on the intensity of pulsatile movement of the CSF at the L2-3 intervertebral intrathecal space. Decreases in stroke volume, absolute stroke volume, and mean flux measured at 30 minutes after crystalloid administration may indicate an attenuation of CSF pulsatile movement at the L2-3 intervertebral intrathecal space (Fig. 3). The changes in pulsatile movement of CSF at the L2-3 intervertebral intrathecal space after crystalloid administration coincide with a previous study.9 Remarkably, colloid administration did not cause any significant changes in pulsatile movement of CSF at the L2-3 intervertebral intrathecal space, which suggests that colloid administration has no effect on CSF pulsatile movement at the lumbar intrathecal space and does not match any preload conditions.
Regarding both the clinical study for isobaric spinal anesthesia and the MRI study, a preload with colloid does not affect the pulsatile movement of CSF at the lumbar subarachnoid space nor does it decrease CSF flow cranially. Therefore, it does not interfere with the spread of isobaric spinal anesthetics. In contrast to a colloid preload, crystalloid administration decreased CSF pulsatile movement at the L2-3 intrathecal space, which may result in a lower peak sensory block level and delay the time to reach the peak sensory block level in isobaric spinal anesthesia.
Before launching the current study, we assumed that the maximum amplitude of brain pulsatile movement might be changed differently according to the administration of different fluids. However, there was no significant correlation between the maximum amplitude measured at the midportion of aqueduct of Sylvius and CSF pulsatile movement at the L2-3 lumbar level.
One putative explanation for the attenuation of the CSF pulsatile movement at the L2-3 level after crystalloid preload in contrast to colloid preload may be attributed to increased CSF production rate. The CSF production rate in crystalloid and colloid preload groups increased by 189 and 48 μL/min, respectively, at 30 minutes from the baseline measurement. An increase of CSF volume after crystalloid preload may increase the pressure within the closed spinal canal, which may in turn attenuate CSF pulsatile movement at the lumbar level.
There are reports that replacement of CSF volume after unintentional dural puncture reduces the severity of postdural puncture headache and the incidence requiring epidural blood patch.32,33 If we consider the difference in CSF production rate according to the crystalloid or colloid preload, we may deduce that administration of colloid has a minimal effect in reducing the severity of postdural puncture headache.
This study had 2 limitations. First, there were significant differences in age between the subjects in the clinical and MRI studies. Stoquart-ElSankari et al.34 reported that their elderly group had significantly lower cervical CSF pulsatile movement than the younger group. Age-related different responses in CSF pulsatile movement after the administration of fluids can affect the interpretation of this study. Second, the change of intrathecal CSF volume presumably affected by significant increases in CSF production rate after crystalloid preload could neither be verified nor quantified in this study, because we only applied a phase-contrast cine high-resolution MRI method.
In conclusion, we observed that a crystalloid preload resulted in a rapid increase of CSF production, and we believe that this finding may explain the decreased CSF pulsatile movement in crystalloid preload, in contrast to a preload of colloid, which may be comparable to a no-preload condition. Decreased CSF pulsatile movement in return may be the reason for the delayed time to reach the peak sensory block level in isobaric spinal anesthesia. Therefore, different preload solutions may be a determining factor in the spread of isobaric local anesthetics within the spinal canal.
Name: Byung Seop Shin, MD.
Contribution: Study design, conduct of study, data analysis, and manuscript preparation.
Name: Chung Su Kim, MD.
Contribution: Conduct of study.
Name: Woo Seok Sim, MD.
Contribution: Conduct of study.
Name: Chul Joong Lee, MD.
Contribution: Conduct of study.
Name: Sung Tae Kim, MD.
Contribution: Data analysis.
Name: Gunn Hee Kim, MD.
Contribution: Conduct of study.
Name: Si Ra Bang, MD.
Contribution: Conduct of study.
Name: Sang Hyun Lee, MD.
Contribution: Manuscript preparation.
Name: Sun Ji Hyun, MD.
Contribution: Data analysis.
Name: Gaab Soo Kim, MD.
Contribution: Study design, data analysis, and manuscript preparation.
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