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Ondansetron, a serotonin-3 receptor antagonist, reduces postoperative shivering. Drugs that reduce shivering usually impair central thermoregulatory control and may thus be useful for preventing shivering during induction of therapeutic hypothermia. We determined, therefore, whether ondansetron reduces the major autonomic thermoregulatory response thresholds (trigging core temperatures) in humans.
Ten healthy volunteers were studied on two days: Control and Ondansetron (intravenous infusion to plasma concentrations of 278 (57) ng mL−1, 234 (55) ng mL−1, and 243 (58) ng mL−1at the sweating, vasoconstriction, and shivering thresholds, respectively); this corresponded to ≈50 mg of ondansetron which is roughly ten times the dose used for postoperative nausea and vomiting. Each day, skin and core temperatures were increased to provoke sweating, then reduced to elicit peripheral vasoconstriction and shivering. We determined the core-temperature sweating, vasoconstriction, and shivering thresholds after compensating for changes in mean-skin temperature. Data were analyzed with paired t tests and presented as means (SD)s; P<0.05 was statistically significant.
Ondansetron did not change the sweating (Control: 37.4 (0.4)°C, Ondansetron: 37.6 (0.3)°C, P=0.16), vasoconstriction (37.0 (0.5) vs. 37.1 (0.3)°C; P=0.70), or shivering threshold (36.3 (0.5) vs. 36.3 (0.6)°C; P=0.76). No sedation was observed on either study day.
Ondansetron, therefore, appears to have little potential for facilitating induction of therapeutic hypothermia.
Overwhelming evidence from animal studies indicates that even mild hypothermia provides substantial protection against cerebral1 and myocardial ischemia.2 In humans, mild hypothermia improves outcome after cardiac arrest.3,
Many studies of therapeutic hypothermia target core temperatures between 33° and 34°C. The ability to induce even mild therapeutic hypothermia in patients having acute myocardial infarction or stroke may be compromised because small reductions in core temperature trigger aggressive thermoregulatory defences,4 such as shivering, which may be harmful. Various drugs have been shown to impair thermoregulatory defences,5,6 but even the best have limited efficacy and substantial side-effects. Drugs that moderate thermoregulatory defences against cold without causing serious side effects would thus be of considerable clinical interest.
Serotonin (5-hydroxytryptamine [5-HT]) is a critical thermoregulatory neurotransmitter.7–9 In humans, tramadol, a 5-HT reuptake inhibitor, prevents postanaesthetic shivering.10 and in non-anaesthetized individuals it decreases the core temperature triggering shivering (the shivering threshold) by roughly 0.9°C.11
The role of the 5-HT1A receptor subtype in 5-HT-mediated hypothermia is well established largely based on data using 8-hydroxy-[N-n-dipropyl-N-(3′-iodo-2′propenyl) amino] tetralin (8-OH-DPAT) as a ligand.12 Although the evidence of involvement of 5-HT3 receptor subtype in 5-HT-mediated thermoregulation is scarce, a serotonin 5-HT3 agonist can induce hyperthermia in rats,7 which is attenuated by pretreatment with a 5-HT3 antagonist. Furthermore, when given alone, 5-HT3 antagonists induce hypothermia.9 As one can expect from these effects in rats, 5-HT3 antagonists prevent postanaesthetic shivering in humans.13 They may thus inhibit shivering, perhaps enough to facilitate induction of therapeutic hypothermia. We, thus, tested the hypothesis that ondansetron, a prototypical 5-HT3 antagonist, comparably reduces the sweating, vasoconstriction, and shivering thresholds — and that the reduction is clinically important (i.e., ≥1°C).
With approval of the Human Studies Committee at the University of Louisville and written informed consent, we studied 10 healthy volunteers. None was obese, taking medication, or had a history of thyroid disease, dysautonomia, or Raynaud’s syndrome.
Each volunteer was studied on two randomly assigned days: 1) Control — no drug —and 2) ondansetron hydrochloride (GlaxoSmithKline, Research Triangle Park, NC) at a target plasma concentration of 250 ng mL−1. Volunteers were blinded as to which solution they received on each study day. The volunteers fasted 8 hours before the start of each study day. They dressed minimally and rested supine on a standard operating room table. Ambient temperature was maintained near 21°C and humidity at 25%.
A 14-gauge catheter was inserted in a right antecubital vein for blood sampling and an 18-gauge catheter was inserted in a left forearm vein for ondansetron or placebo administration. Blood for ondansetron analysis was sampled from the catheter inserted in right antecubital vein before drug administration (blank sample) and at each threshold. Blood was centrifuged, and the plasma was removed and stored at −40°C until analysis.
Ondansetron was infused using a target-controlled infusion with a Stanpump program based on pharmacokinetic data from Colthup et al.14 and de Alwis et al.15 at a target plasma concentration of 250 ng mL−1. This corresponds to ≈50 mg of ondansetron over the approximately 2.5-hour study period. A similar volume of normal saline was infused on the control day. The infusion began at the start of thermal manipulation and continued until shivering was detected.
Skin and core temperatures were gradually increased with an Allon circulating-water garment (MTRE, North Industrial Park, Israel) and a whole-body convective-air warming coverlet and air heater-blower unit (Bair Hugger Model 250, Augustine Medical, Inc., Eden Prairie, MN) until sweating occurred. Subsequently, skin and core temperature were decreased with an Allon circulation-water garment and convective-air cooling coverlet (PolarAir, Augustine Medical Inc., Eden Prairie, MN) until shivering occurred. The volunteer’s body was insulated with the warming and cooling devices mentioned except for the head, and right forearm and hand, which were used for fingertip blood flow measurement.
Core temperature was measured at the tympanic membrane using Mon-a-therm thermocouples (Tyco-Mallinckrodt Anesthesiology Products, Inc., St. Louis, MO). The aural probes were inserted by the volunteers until they felt the thermocouple touch the tympanic membrane; appropriate placement was confirmed when volunteers easily detected a gentle rubbing of the attached wire. The aural canal was occluded with cotton and taped in place.
Mean skin-surface temperature and cutaneous heat transfer were calculated from measurements at 15 area-weighted sites. Temperatures were recorded at 1-minute intervals from thermocouples connected to calibrated Iso-Thermex® thermometers having an accuracy of 0.1°C and a precision of 0.01°C (Columbus Instruments, Corp., Columbus, OH).
Sweating was continuously quantified on the left upper chest using a ventilated capsule.4 We considered a sweating rate > 40 g·m−2·h−1 for at least 5 minutes to be significant.16 Absolute right middle fingertip blood flow was quantified using venous-occlusion volume plethysmography at 5-minute intervals.17 The vasoconstriction threshold was determined post hoc by an observer blinded to treatment and core temperature.
As in previous similar studies,16 we used systemic oxygen consumption to quantify shivering. A Deltatrac metabolic monitor (SensorMedics Corp., Yorba Linda, CA) was used in canopy mode. Initiation of the shivering threshold was determined post hoc by an observer blinded to treatment and core temperature. The same observer determined the shivering threshold for all subjects. End-tidal PCO2 was sampled from a catheter inserted into one nostril; gas removed from the catheter was returned to the canopy of the metabolic monitor.
Heart rate, end-tidal PCO2, and oxyhaemoglobin saturation (SpO2) were measured continuously using pulse oximetry, and blood pressure was determined oscillometrically at 5-minute intervals at the left ankle.
An investigator blinded to core temperature and treatment evaluated sedation using the responsiveness component of the Observer’s Assessment of Alertness/Sedation (OAA/S) score (Table 1)18 at several times: 1) 15 minutes before starting drug administration, 2) before thermal manipulation started, and 3) at each threshold.
Prior to extraction, plasma samples for ondansetron analysis were allowed to thaw at room temperature and vortexed. To 1 mL of plasma, 1 mL 0.1 M sodium phosphate buffer, pH 6.0 and internal standard (prazosine hydrochloride, Sigma-Aldrich, USA) was added and vortexed. Clean Screen solid phase extraction columns (CSDAU-203, World Wide Monitoring, United Chemical Technologies, Inc., Bristol, PA, USA) were preconditioned with 3 mL of methanol, 3 mL of deionised water and 3 mL of 0.1 M sodium phosphate buffer before the plasma samples were loaded (about 0.5 mL min−1). The cartridges were rinsed with 3 mL of deionised water, 2 mL of 1 M acetic acid, 3 mL of methanol and then dried under vacuum for at least 5 min. The extracts were then eluted from the cartridge with 3 mL of freshly prepared dichloromethane — isoproranol — ammonium hydroxide 25% (39:10:1; vol:vol:vol, prepared with sonication) by gravity filtration.
The eluate was evaporated to dryness under nitrogen stream in a water bath at about 40°C. The residues were redissolved in 200 μL mobile phase mixture, briefly mixed in an ultrasonic bath, and loaded into auto-sampler vials. Samples were analyzed by HPLC (Merck-Hitachi, L6200A and AS-2000A, Hitachi, Ltd., Tokyo, Japan) with UV detection (Merck-Hitachi, DAD L-4500) at 250 nm. Aliquots of 20 μL were injected onto a Nucleodur 100-5 CN-RP (Macherey-Nagel, CH), 250mm, 4 mm ID column with an 8 mm precolumn. The mobile phase consisted of 40% acetonitrile, 60% 20 mM bisodiumphosphate buffer, pH 5.4. The flow rate was 1.5 mL min−1 and operating temperatures were 35°C. The data were processed with proprietary HPLC control software (Merck-Hitachi, Model D-7000, Hitachi Instruments, Inc., San Jose, CA, USA).
Retention times of the internal standard and ondansetron (Zofran, GlaxoSmithKline plc, Middlesex, UK) were 2.96 and 3.36 min, respectively. Calibration curves with spiked and extracted plasma passed through the origin and were linear in the calibration range of 150 to 400 ng mL−1 with correlation coefficient values of r2 ≥ 0.99. The intraday and interday coefficients of variation were 2.5% and 3% for quality control samples containing 250 ng mL−1 ondansetron, respectively. For ondansetron the limit of quantification (S/N = 10) was 15 ng mL−1 and the recovery was ≥ 95%.
The cutaneous contribution to the thermoregulatory responses — sweating,19 vasoconstriction, and shivering — is linear.20 We thus used measured skin and core temperatures in °C at each threshold to calculate the core-temperature threshold that would have been observed had skin been maintained at a single designated temperature. For this purpose, we used Equation 1 that corrects core temperature for cutaneous temperature, providing response thresholds that would have been observed at a designated core temperature.
We have previously described the derivation and validation of this equation.16 We used a ß of 0.1 for sweating19 and a ß of 0.2 for vasoconstriction and shivering.20 The designated skin temperature was set at 34°C, a typical intraoperative value.
We determined that a sample size of 10 would detect a 0.5°C difference in shivering thresholds between control and drug days with 90% power. Based on a similar study conducted with the same design in our laboratory with doxapram,21 we assumed that the difference in shivering thresholds on the two study days would have a standard deviation of 0.44 and correlation of 0.75.
Oxygen consumption and calculated respiratory quotient were averaged during baseline (before commencement of drug infusion), during the cooling phase before shivering, and during shivering, respectively. The averaged values were then compared with two-way ANOVA (two factors; drug, period) and post-hoc Bonferroni/Dunn tests.
Ambient temperature, humidity, mean arterial pressure, heart rate, SpO2, and end-tidal PCO2 on each study day were averaged within each volunteer across the warming and cooling periods; the resulting values were then averaged among volunteers. Results for each study day were compared using Wilcoxon signed-rank tests or paired t-tests. Plasma concentrations of ondansetron at each threshold were compared using one-way ANOVA. All results are presented as means ( SD) or median [range], as appropriate; P < 0.05 was considered statistically significant.
Seven of the participants were male; three were female. They were 22 [19, 31] years old, weighed 71 [57, 97] kg, and were 176 [157, 183] cm tall (median, [minimum, maximum]). Among the factors that might have influenced the thermoregulatory thresholds, ambient temperature was significantly higher on ondansetron days than on the control days, but the average difference was only by about 0.7°C (P=0.004; Table 2).
Plasma ondansetron concentrations were similar at each threshold, ≈250 ng mL−1. The median dose of ondansetron was 49 mg and ranged from 36 to 70 mg. None of the study participants was sedated at anytime on either study day. That is, OAA/S scores remained 5 for all volunteers throughout the study.
The sweating thresholds were similar on the control (37.4 (0.4)°C) and ondansetron (37.6 (0.3)°C) days (P = 0.16) (mean (SD)). Ondansetron did not alter either the vasoconstriction threshold (control: 37.0 (0.5)°C; ondansetron: 37.1 (0.3)°C; P = 0.70) or the shivering threshold (control: 36.3 (0.5)°C; ondansetron: 36.3 (0.6)°C; P = 0.76) (Table 3 and Fig. 1).
Plasma ondansetron concentrations were similar at each of the three thermoregulatory thresholds and were near our target concentration of 250 ng mL−1. The Stanpump program, based on pharmacokinetic data of Colthup et al.14 and de Alwis et al.,15 thus proved reliable. According the to 2003 edition of the Physician’s Desk Reference, a single 32-mg bolus of ondansetron —the dose used for prevention of chemotherapy-induced nausea and vomiting — yields a maximum plasma concentration of 264 ng mL−1. We thus chose a target ondansetron plasma concentration of 250 ng mL−1. Our modelling suggested that the total dose after a 5-hour infusion would be 60 mg in a 70-kg volunteer, which is less than that used for prophylaxis of chemotherapy-induced nausea and emesis in clinical trials.22 In fact, the total dose we used averaged 49 (12) mg.
Because we targeted a plasma ondansetron concentration that might be used for treatment of chemotherapy-induced nausea and vomiting, our volunteers received roughly ten times the dose normally used for prevention of post-operative nausea and vomiting.23 But even at this high concentration, there was no perceptible effect of ondansetron on the sweating, vasoconstriction, or shivering thresholds. It is thus apparent that ondansetron alone cannot be used to facilitate induction of therapeutic hypothermia — and there is little reason to believe that it will prove a useful adjunct to other anti-shivering medications.
Meta-analysis indicates that meperidine 25 mg, clonidine 150 μg, ketanserin 10 mg, and doxapram 100 mg, are effective anti-shivering agents.24 Nalbuphine also prevents post-operative shivering25 and has been formally evaluated for shivering threshold suppression.26 Several drugs that are unquestionably effective for postoperative shivering actually reduce the shivering threshold only slightly. For example, clonidine and doxapram reduce the shivering threshold only about 0.5°C, but are nonetheless effective anti-shivering agents. The reason, presumably, is that many postoperative patients have core temperatures only slightly below the normal shivering threshold, which is about 35.5°C.27 Other effective anti-shivering drugs reduce the shivering threshold even more, typically about 1°C (Table 4). Ondansetron thus appears to be unique in reportedly reducing incidence of post-operative shivering13 but not in reducing the shivering threshold, even at doses ten times those used for treatment of post-operative nausea and vomiting.
Postoperative rhythmic shivering-like muscular activity is a complex phenomenon. Most such activity is thermoregulatory, and most is normal shivering. However, some is expressed as tonic stiffening that occurs when residual concentrations of volatile anaesthesia are 0.2–0.4 of the minimum alveolar concentration. Other tremor is manifested as a clonic tremor similar to pathological clonus or physiological action tremor; it appears to be triggered by low concentrations (i.e., 0.1 minimum alveolar concentration) of volatile anaesthetics.28 Other shivering-like tremor is not thermoregulatory at all,29 and appears to be facilitated by pain.30 It is thus possible that 5-HT3 antagonists inhibit non-thermoregulatory shivering-like activity.
A limitation of our study was that the mean ambient temperature was significantly greater on the ondansetron day than the control day. However, the temperatures differed by only 0.6ºC. More importantly, skin temperature — except for the head and the right forearm — was deliberately manipulated throughout the study period. Skin temperature contributes 10% to autonomic control of sweating19 and 20% to control of vasoconstriction and shivering.20 And thermal sensitivity of the face is greater than that of remaining skin surface.31 However, the lower arm and face make up a small portion of the total skin surface. We have previously demonstrated that neither arm nor face warming influences the shivering threshold in non-anaesthetised human volunteers.32 The small difference in ambient temperature on the two study days is thus unlikely to compromise the thermoregulatory threshold outcomes.
In summary, ondansetron at a plasma concentration of ≈250 ng mL−1 — a dose near that used to treat chemotherapy-induced nausea and emesis — did not significantly alter the core-temperature thresholds triggering sweating, vasoconstriction, or shivering. The drug, therefore, appears to have little potential for facilitating induction of therapeutic hypothermia.
Supported by NIH Grant GM 061655 (Bethesda, MD), the Gheens Foundation (Louisville, KY), the Joseph Drown Foundation (Los Angeles, CA), and the Commonwealth of Kentucky Research Challenge Trust Fund (Louisville, KY). Mallinckrodt Anesthesiology Products (St. Louis, MO) donated the thermocouples we used in this study; MTRE Advanced Technologies, Ltd. (North Industrial Park, Israel) donated the Allon circulating-water system; Arizant Medical, Inc. (Eden Prairie, MN) donated the forced-air warming and cooling systems.
We thank Gilbert Haugh, M.S., for statistical analyses of the data and Nancy Alsip, Ph.D., for editorial assistance (both of the University of Louisville).
Received from the Outcomes Research Institute and Department of Anesthesiology and Perioperative Medicine, University of Louisville, Louisville, KY; the Department of Outcomes Research, The Cleveland Clinic, Cleveland, OH; and the Department of Anaesthesiology, University of Bern, Bern, Switzerland.