Anesth Analg 2003;97:244-246
© 2003 International Anesthesia Research Society
NEUROSURGICAL ANESTHESIA
Intrathecal Temperature Is Closely Reflected by the Aortic, but Not by the Rectal, Temperature in a Rabbit Model of Spinal Cord Ischemia
Erich Knolle, MD*,
Ursula Windberger, DVM
,
Marek Ehrlich, MD
,
Georg Heinze, PhD
,
Matthias J. Oehmke, MD*,
Edvin Turkof, MD||,
Udo Losert, DVM, and
Hans Georg Kress, MD PhD*
*Department of Anesthesiology and General Intensive Care (B),
Institute of Biomedical Research,
Department of Cardiothoracic Surgery,
Department of Medical Computer Sciences, and
||Department of Plastic and Reconstructive Surgery, University of Vienna, Austria
Address correspondence and reprint requests to Erich Knolle, MD, Department of Anesthesiology and General Intensive Care (B), University of Vienna, Waehringer Guertel 18-20, 1090 Vienna, Austria. Address e-mail to erich.knolle{at}univie.ac.at
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Abstract
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IMPLICATIONS: Experimental ischemic lesions in the lumbar spinal cord of the rabbit can be induced by inflating the balloon of a Swan-Ganz catheter in the abdominal aorta. The intrathecal temperature is significantly better reflected by the temperature from the thermistor of the catheter than by the rectal temperature.
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Introduction
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In the rabbit, experimental ischemic lesions in the lumbar spinal cord can be induced by aortic occlusion distal to the renal arteries (1) because the arterial perfusion of the lumbar spinal cord in this species depends on segmental arteries leaving the infrarenal aorta (2,3). Because hypothermia has a protective effect against ischemic damage of the spinal cord (4–7), it is impossible in experimental studies to differentiate, without temperature monitoring, between the neuroprotective effect of a tested drug and neuroprotection by means of induced hypothermia (8). However, direct measurement of lumbar intrathecal temperature is difficult and introduces the risk of spinal cord damage and infection (9) because the spinal cord of the rabbit ends at the second sacral segment (10). In the rabbit, the rectal temperature is normally used to estimate the body temperature and with it the intrathecal temperature. In this study of rabbits, we evaluated whether the temperature measured by the thermistor of a Swan-Ganz catheter (Baxter Healthcare Corp, Irvine, CA) used for aortic occlusion (11) correlates more closely with the intrathecal than does the rectal temperature.
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Materials and Methods
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All rabbits received humane care in compliance with the Guide for the Care and Use of Laboratory Animals published by the National Institutes of Health (publication 85–23, revised 1985). After approval by the institutional Animal Care and Use Committee, 8 New Zealand white rabbits (3.7 to 4.2 kg) were anesthetized with ketamine at a rate of 50–70 mg · kg-1 · h-1 and xylazine at a rate of 0.25–0.5 mg · kg-1 · h-1. After tracheal intubation, normocapnia was maintained (Servo 900®; Siemens AG, Munich, Germany) based on the monitoring of arterial blood gases collected from a catheter (22-gauge) placed in an ear artery.
A right femoral arteriotomy (art. fem. superficialis) was performed 2 cm distal to the inguinal ligament, and a 4F Swan-Ganz catheter was advanced into the abdominal aorta and the balloon placed exactly distal to the left renal artery. The correct location of the catheter was fluoroscopically controlled by injection of 2–5 mL of a contrast medium (iopamidol). The thermistor of the catheter was used for continuous measurement of intraaortic temperature (Vigilance Monitor; Baxter Healthcare). The rabbits were laid on the right side for all the following procedures.
On the right side of the seventh lumbar spinous process, a 18-gauge atraumatic Sprotte® needle (length, 120 mm; Pajunk, Geisingen, Germany) was percutaneously introduced at an angle of approximately 20 degrees in the cephalic direction targeting the interlaminar space of L5-6. After the needle passed through the ligamentum flavum, resistance to its advancement stopped, and it was firmly held by the ligament as described by Hughes et al. (12). After the stylet was removed, a polyvinyl catheter (outer diameter, 0.85 mm; Perifix®, Braun, Melsungen, Germany) was inserted and advanced 2 cm after it passed through the orifice at the tip of the Sprotte® needle. When the resistance against advancement was low and muscular fasciculation occurred, we regarded the catheter to be intrathecally positioned (13), and we attempted to aspirate cerebrospinal fluid through the catheter. When we felt a rubber-like resistance against advancement and no cerebrospinal fluid could be aspirated, we assumed the catheter to be located in the epidural space. In this case, the catheter was removed, the Sprotte® needle was advanced a few millimeters further, the catheter was again introduced, and an aspiration test followed. When the position of the Sprotte® needle was considered to be intrathecal, the catheter was removed, and a temperature probe (YSI 511, Yellow Springs Inc, OH) was introduced. Finally, we removed the Sprotte® needle while keeping the position of the temperature probe unchanged.
For rectal temperature measurement, a rectal probe (YSI 401; Yellow Springs Inc) was inserted 7 cm. To prevent cooling, the rabbits were laid on a heating pad through which flowed water heated to a constant 38°C. Because of the decubitus position, the distances from the three temperature probes to the heating pad—and thus the heat received—were nearly the same. After the temperatures were recorded during nonischemic conditions for 20 min, the balloon was inflated with 0.5 mL of air for aortic occlusion lasting 40 min. We verified the occurrence of complete aortic occlusion by monitoring aortic pressure via the proximal orifice of the catheter. Twenty minutes after the catheter was deflated, the experiment was concluded and the rabbits killed with a lethal injection of pentobarbital.
Blood pressure, measured at the pressure-monitoring port proximal to the balloon of the Swan-Ganz catheter, and temperature were recorded digitally every 10 s (Hellige/Biosys, Vienna, Austria). Median values and quartiles of the distribution of absolute differences between aortal and intrathecal temperature values (ADspin-aort) and between rectal and intrathecal temperature values (ADspin-rect) were calculated for the entire study period of 80 min. For this, we used the blocked bootstrap resampling procedure (14) with blocks defined by rabbits assuming the temperature values of each rabbit correlated. The median absolute difference (MAD) of intrathecal and aortal temperature measurements (MADspin-aort) was compared with the MAD of intrathecal and rectal measurements (MADspin-rect) using a paired t-test with the standard error of MADspin-rect - MADspin-aort obtained by the blocked bootstrap. A significantly smaller value of one measure of MAD would indicate that the intrathecal temperature was better reflected by that temperature measurement. To eliminate a specific effect of aortic occlusion on the temperature measurements, MADs were additionally compared among the experimental time periods with and without aortic occlusion using paired t-tests based on the blocked bootstrap. P < 0.01 was considered significant.
To confirm that the intrathecal probe did not damage the spinal cord and that spinal cord ischemia occurred after the balloon of the Swan-Ganz catheter was inflated, transcranial-motor-evoked potentials (tc-MEPs) were monitored before and after insertion of the temperature probe as well as during the early phase of aortic occlusion starting immediately before the occlusion. They were recorded in the m. quadriceps femoris and m. biceps femoris of the right lower limb (Viking IV; Nicolet, Madison, WI).
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Results
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Box plots of the intrathecal, aortic, and rectal temperature values determined are shown in Fig. 1. In all experiments, the intrathecal temperature values were better reflected by the aortic than by the rectal values: The median of ADspin-aort was 0.13°C (range, 0.06°C–0.27°C). This was significantly smaller (P = 0.0074) than the median of ADspin-rect, which was 0.56°C (range, 0.35°C–0.82°C). In seven rabbits, the rectal temperature values were less than the intrathecal and aortic values (Fig. 2).
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Figure 1. Box plots of temperature values determined intrathecally, within the abdominal aorta, and rectally in eight rabbits. The box plots represent all values recorded every 10 s during time periods of 20 min before, 40 min during, and 20 min after aortic occlusion. Assuming that the temperature values of each rabbit correlated at the different measurement points, the median absolute difference of intrathecal and aortal temperature measurements proved to be significantly smaller than the median absolute difference of intrathecal and rectal measurements.
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Figure 2. Time courses of temperature values registered at different measurement points during aortic occlusion in one of eight experiments. The rectal temperature values were less than the intrathecal and aortal values, which was characteristic of seven of the experiments.
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The absolute temperature values varied within the experiments, but they showed no characteristic changes whether or not the aorta was occluded (Fig. 1). The medians of ADspin-aort with and without aortic occlusion were 0.15°C (range, 0.06°C–0.31°C) and 0.12°C (range, 0.06°C–0.24°C), respectively, with no significant difference. The respective medians of ADspin-rect with and without aortic occlusion were 0.68°C (range, 0.44°C–1.09°C) and 0.56°C (range, 0.32°C–0.94°C), respectively.
In all experiments, myogenic tc-MEPs were present before and after intrathecal insertion of the temperature probe. They disappeared within 3 min after aortic occlusion.
Aortic occlusion resulted in a decrease of aortic pressure distal to the balloon to <20 mm Hg in all experiments. In one experiment, the time of aortic occlusion was only 17 min because the balloon ruptured.
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Discussion
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We have demonstrated that during aortic clamping, the intrathecal temperature can be closely estimated from the temperature measured by the thermistor of a Swan-Ganz catheter used for aortic occlusion.
The correlation between the intrathecal and rectal temperature proved to be less reliable because in seven experiments, the rectal temperature was less than the intrathecal temperature, whereas in one experiment, the rectal temperature exceeded intrathecal temperature values, possibly because the probe was surrounded by feces (15). Whether the spinal cord temperature is also well reflected by the esophageal temperature, as stated by Miyamoto et al. (16), has to be proven by further studies.
In the present study, a percutaneous insertion technique was used for the first time to introduce a temperature probe into the intrathecal space without injuring the motor neuron function of the spinal cord as confirmed by maintenance of tc-MEPs. The method was similar to that of Taguchi et al. (17) developed for epidural catheterization. We believe that the different results of catheterization (intrathecal versus epidural) can be explained mainly by the different tips and insertion techniques of the epidural needles used by Taguchi et al. (17) and in our study.
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References
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Accepted for publication February 25, 2003.
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Abstract
Introduction
