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Hypothermia is a decrease in the core body temperature in a patient below the reference limit and commonly

occurs during anesthesia of veterinary patients.1,2 Hypo-thermia develops secondary to alterations in thermoregu-lation and heat loss from radiation, conduction, convec-tion, and evaporation during the perianesthetic period.3 The ratio of body surface area to mass in domestic cats (Felis catus) is relatively large and may make them more susceptible to thermal loss and redistribution of core ther-mal energy, compared with larger cats. Domestic cats typi-cally have a body temperature of approximately 38.6°C (101.5°F).4 Body temperatures that are less than the refer-ence limit but > 36°C (96.8°F) are consistent with mild hypothermia, whereas body temperatures > 32°C (89.6°F) but < 36°C are consistent with moderate hypothermia.5

Body temperatures < 30°C (86°F) have been associated with substantial alterations to the CNS, resulting in states where no anesthetic agent is required.5

Development of perianesthetic hypothermia can alter coagulation, immune function, and cardiac conduction, resulting in potentially detrimental consequences.6–8 For example, hypothermic human patients undergoing ortho-pedic surgery develop greater hemorrhage, resulting in an increase in the number of transfusions, compared with normothermic patients.6,9 Similarly, perioperative hypo-thermia has been associated with increased infection rates

Effect of anesthetic breathing circuit type on thermal loss in cats during inhalation

anesthesia for ovariohysterectomy

Christopher K. Kelly, dvm; David S. Hodgson, dvm, dacva; Rose M. McMurphy, dvm, dacva, dacvecc

Objective—To compare the effects of a nonrebreathing circuit versus a reduced volume circle anesthetic breathing circuit on body temperature change in cats during inhalation anesthesia for ovariohysterectomy.Design—Randomized, controlled clinical trial.Animals—141 female domestic cats hospitalized for routine ovariohysterectomy.Procedures—Cats were randomly assigned to receive inhalation anesthetics from either a nonrebreathing circuit or a reduced volume circle system with oxygen flow rates of 200 and 30 mL/kg/min (90.9 and 13.6 mL/lb/min), respectively. Body temperatures were monitored throughout the anesthetic period via an intrathoracic esophageal probe placed orally into the esophagus to the level of the heart base.Results—No difference in body temperature was found between the 2 treatment groups at any measurement time. The duration of procedure had a significant effect on body tem-perature regardless of the type of anesthetic circuit used.Conclusions and Clinical Relevance—Duration of the procedure rather than the type of anesthetic circuit used for inhalation anesthesia was more influential on thermal loss in cats undergoing ovariohysterectomy. (J Am Vet Med Assoc 2012;240:1296–1299)

in both humans and rats,7,10 alteration in human immune cell function,11,12 and a longer duration of hospitalization, compared with human patients kept in a normothermic range.7 In hypothermic guinea pigs, evidence suggests a decrease in dermal resistance to Escherichia coli infection during anesthesia.13 Conversely, a study14 in dogs under-going surgical procedures showed that hypothermia did not play a substantial role in the development of postop-erative infection, and that duration of anesthesia was more influential. Changes in cardiac conduction because of hypothermia can also lead to alterations in QT intervals and increased potential for atrial arrhythmias.8,15,16 Taken individually or collectively, these detrimental effects indi-cate a need to minimize body temperature reductions dur-ing anesthesia.

Some forms of heat loss can be minimized to delay the onset and decrease the severity of hypothermia. Reducing conductive and convective losses by covering the patient and limiting the patient’s contact with cold surfaces can limit heat loss by > 30%.17,18 The use of circulating warm water pads or forced-air heating blankets, either separately or in combination, will further reduce heat loss.19,20 Nev-ertheless, losses of heat and fluid from the surgical site can occur,21,22 particularly when major body cavities such as

From the Department of Clinical Sciences, College of Veterinary Medicine, Kansas State University, Manhattan, KS 66506. Dr. Kelly’s present address is Department of Veterinary Clinical Sciences, Col-lege of Veterinary Medicine, Iowa State University, Ames, IA 50011.

Supported by a grant from the Kansas State University College of Vet-erinary Medicine Research Fund.

Address correspondence to Dr. Kelly (ckkelly@iastate.edu).

Abbreviations

MBS Miniature anesthetic circle breathing systemNRB Nonrebreathing anesthetic circuit systemTf Final body temperature at completion of

ovariohysterectomyTi Initial body temperature immediately after

anesthetic induction

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the thorax and abdomen are exposed.22 In addition, evap-orative heat loss from the respiratory tract can occur as a result of exposure to cold, dry carrier gases delivered by the anesthetic breathing circuit.21–25

Anesthetic equipment is known to influence the amount of heat lost from an anesthetized patient.23 The 2 major types of anesthetic breathing circuits commonly used for veterinary patients are the circle circuit and non-rebreathing circuit. In contrast to a circle circuit, a non-rebreathing circuit relies on high fresh gas flow rates to prevent rebreathing of carbon dioxide. The high fresh gas flow rate and the lack of rebreathing of warmed humidified gas will result in larger evaporative losses to the circuit.26 Fresh gas flow rates delivered to a circle breathing circuit vary greatly. However, lower flow rates permit rebreathing of gases with higher temperature and humidity.24,25

The purpose of the study reported here was to compare the effect of anesthetic breathing circuit type (MBS vs NRB) on thermal loss in cats during inhalation anesthesia for ovariohysterectomy. It was hypothesized that in cats undergoing an ovariohysterectomy, inhala-tion anesthetic delivery by use of an MBS would result in less thermal loss and the maintenance of a higher core body temperature than inhalation anesthetic deliv-ery with a conventional NRB.

Materials and Methods

Study protocol—One hundred forty-one female do-mestic cats (138 cats with an American Society of An-esthesiologists status of I [healthy patient] and 3 with an American Society of Anesthesiologists status of II [patient with mild systemic disease]) were admitted to the Kansas State University Veterinary Medicine Teach-ing Hospital for routine ovariohysterectomy. A thorough physical examination and appropriate clinicopathologic tests were performed on each cat and always included determination of PCV and plasma total protein concen-tration. The study protocol was approved by an institu-tional animal care and use committee; informed client consent was obtained for each patient.

By use of a random numbers table, a predetermined logbook was generated so that cats were randomly as-signed to 2 anesthetic maintenance treatment groups: ei-ther the MBSa group or the NRBb group. Anesthesia was maintained in 67 cats with an MBS (Figure 1), which is a reduced volume circle system. This system is a small version of the stack valve configuration found in current large animal anesthesia machines.c The MBS has a vol-ume of 1.08 L, including the 240-mL rebreathing bag, Y-piece, and breathing hoses. The carbon dioxide ab-sorber has a capacity of 300 g. The oxygen flow rate for the MBS was set at 30 mL/kg/min (13.6 mL/lb/min) and delivered by use of a precision calibrated flowmeterd (0 to 1,000 mL/min) to a precision isoflurane vaporizer.e Anesthesia was maintained in 74 cats with an NRB con-nected to a universal control arm.f The oxygen flow rate for the NRB was 200 mL/kg/min (90.9 mL/lb/min).

Anesthetic protocols were determined individually on the basis of patient history, physical examination, and clinicopathologic data. The cats were routinely premedi-cated, and anesthesia was induced with various combi-nations of the following drugs: acepromazine, atropine, glycopyrrolate, xylazine, medetomedine, buprenorphine,

meperidine, morphine, butorphanol, oxymorphone, fen-tanyl, thiopental, propofol, diazepam, and ketamine. Be-tween the 2 test groups, there were multiple combinations of drugs used for the anesthetic protocols; however, most cats were anesthetized with 1 of 4 combinations as follows: acepromazine, atropine, oxymorphone, and thiopental (NRB group, 24/74 [32%] cats; MBS group, 13/67 [19%] cats); acepromazine, atropine, butorphanol, and thiopen-tal (NRB group, 10/74 [13%] cats; MBS group, 7/67 [10%] cats); acepromazine, oxymorphone, and thiopental (NRB group, 11/74 [15%] cats; MBS group, 7/67 [10%] cats); and acepromazine, butorphanol, and thiopental (NRB group, 2/74 [3%] cats; MBS group, 6/67 [9%] cats).

Anesthesia was maintained with either isoflurane or halothane in oxygen in both groups. Anesthetic monitor-ing included a continuous ECG, systolic blood pressure measurement via Doppler ultrasonography,g and cardiac auscultation with an esophageal stethoscope. Additional monitoring, as deemed appropriate, included mainstream capnography and pulse oximetry. All cats were placed on a circulating warm water padh set to a temperature of 38°C (100.4°F). Core body temperature was monitored with a dig-ital thermistor probei that was factory calibrated and periodi-cally checked during the study with an American Society for Testing and Materials–certified mercury thermometer. The probe was placed orally into the esophagus to the level of the heart base as determined by measurements prior to an-esthesia to the fourth to fifth intercostal space. Temperature readings were recorded immediately after anesthetic induc-tion (Ti) and every 15 minutes until the completion of the ovariohysterectomy, at which time the Tf was recorded. The temperature difference was then calculated on the basis of the temperature change from Ti at each 15-minute interval as well as from Ti to Tf.

Statistical analysis—The response variable in this study was the temperature change during the use of 2 breathing circuits. Measurements of temperature change were analyzed with a repeated-measures analysis by means of statistical software.j The appropriate covari-ance matrix structure in this completely randomized design was determined by comparing models with dif-ferent covariance structures, from the most complex to

Figure 1—Photograph of an MBSa used to maintain anesthesia in 67 cats undergoing ovariohysterectomy. This system is a small version of the stack valve configuration found in large animal anesthesia machines.c The stack valve configuration allows for a decrease in circuit volume through close proximity while main-taining a unidirectional flow of gas. A decreased volume set of breathing tubes and a Y-piece are also illustrated. The absorbent chamber allows for adequate absorption capabilities with mini-mized resistance and volume.

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the simplest. The complex model fit best with the covari-ance structure in this study. The unstructured covariance matrix (most complex) used to model this structure did not place any restrictions on the variance and covariance estimates of the different time points. The 0.05 level of probability was used as the decision point for all tests of statistical significance.

Results

Demographics and anesthetic duration for all cats included in this study were summarized (Table 1). No significant difference in change in body temperature was found between cats during inhalation anesthesia via the MBS and NRB at any of the time intervals measured. There was no circuit by time interaction (P = 0.25), and there was no circuit effect (P = 0.36). The Tf of the 2 circuits (NRB, 35.26°C [95.47°F]; MBS, 35.43°C [95.77°F]) had a mean ± SE difference of 0.11 ± 0.12°C (0.20 ± 0.22°F). There was a significant (P < 0.001) effect of time on tem-perature difference independent of the circuit type. Mean ± SE change in body temperature in cats during inhalation anesthesia via the NRB was 2.76 ± 0.11°C (4.96 ± 0.20°F) at 60 minutes with an additional loss of 0.07 ± 0.43°C (0.12 ± 0.77°F) at 90 minutes. Mean ± SE change in body temperature in cats during inhalation anesthesia via the MBS was 2.82 ± 0.34°C (5.07 ± 0.61°F) at 60 minutes with an additional loss of 0.27 ± 0.48°C (0.49 ± 0.86°F) at 90 minutes.

Discussion

The MBS was postulated to have a more favorable effect on limiting temperature reduction in anesthetized cats, compared with an NRB, because of the small volume and lower gas flow rates. However, the statistical analysis did not detect any significant difference in temperature change for the cats in either of the 2 study groups. The lower flow rates and recirculation with the MBS would be expected to allow for rebreathing of heated, humidified gas. As the gas within the anesthetic circuit becomes more humidified and closer to core body temperature, less pa-tient expenditure of energy is required to maintain core temperature.24,25 A study27 of infants undergoing anesthe-sia with a pediatric circle and a low fresh gas flow rate (0.6 L/min) revealed no difference in thermal loss but demon-strated a greater degree of humidification within the cir-

cuit than when a flow rate of 6 L/min was used. Although these studies24,25,27 support the possibility of a warming or humidification effect in a small-volume recirculating sys-tem, any increase that might have been induced by the use of the MBS in the present study was not of sufficient magnitude to produce a difference between circuit types. In future studies, it would be useful to measure both tem-perature and humidity in the recirculating system to pro-vide further insight into any potential response.

Although the type of system (MBS vs NRB) used did not result in a difference in body temperature between cat groups, a significant (P < 0.001) temperature reduction from Ti to Tf occurred in both groups. Human studies28 have demonstrated the greatest decrease in core body temperature occurs during the first hour of anesthesia, fol-lowed by a plateau. Findings in the present study showed a similar pattern of decrease and plateau of body tempera-ture in anesthetized cats, and it is possible that time has a major influence on thermal loss. The period of accelerated heat loss included the time for induction of anesthesia and surgical preparation. In the present study, ovariohysterec-tomies were performed by fourth-year veterinary students, and anesthetic and surgical times were likely longer than those of more experienced veterinarians. However, the data acquired from these longer procedure times are appli-cable to patients anesthetized for more prolonged, compli-cated procedures. In the present study, it is also likely that abdominal visceral exposure would be greater than that in an ovariohysterectomy performed by an experienced veterinarian. This may account for an additional source of heat loss in our subjects. Because most heat is rapidly lost in the first hour, immediate and sustained thermal preser-vation efforts remain critical for short procedures.

Another factor that might have impacted the results of the present study was the administration of opioids, ketamine, acepromazine, and inhalation anesthetics. All of these drugs are known to have effects on thermoregula-tion.29–34 In the present study, opioids were administered to each cat in both treatment groups. Opioids have shown differing effects on thermoregulation among species.29–32 In experimentally induced environments ranging from 20° to 30°C (68° to 86°F), mice given opioids were more hypothermic than were untreated mice.29 Posner et al30 showed that opioids, as part of an anesthetic protocol, may cause a hyperthermic response in some cats, consis-tent with previous results for cats given morphine31 and

Anesthetic Age Body weight Surgical time Anesthetic time circuit type Breed No. of cats (mo) (kg) (min) (min)

NRB DSH 54 13.14 6 1.67 3.05 6 0.07 72.63 6 2.86 109.53 6 3.21 DMH 9 DLH 10 SIA 1 MBS DSH 54 9.95 6 0.56 2.98 6 0.06 72.34 6 2.66 113.61 6 3.33 DMH 5 DLH 7 SIA 1 All DSH 108 11.63 6 0.92 3.02 6 0.05 72.50 6 1.96 111.44 6 2.31 DMH 14 DLH 17 SIA 2

DLH = Domestic longhair. DMH = Domestic medium hair. DSH = Domestic shorthair. SIA = Siamese.

Table 1—Mean ± SE demographic and surgical-anesthetic data for 141 cats during inhalation anesthe-sia (with an NRB or MBS) for ovariohysterectomy.

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hydromorphone.32 Posner et al30,32 also suggest that peri-operative hypothermia may be associated with postop-erative hyperthermia. There are few data to correlate most anesthetic induction protocols directly with hypothermia. Propofol, a known vasodilator, has been shown to increase core temperature loss, compared with induction of anes-thesia via inhalation of sevoflurane.33 Isoflurane, another known vasodilator, has been shown to have minimal ef-fects on cutaneous thermal loss.34

The present study was conducted in a teaching hos-pital, and routine ovariohysterectomies were performed by senior veterinary students. The subjects were client-owned, not purpose-bred, animals, and the students were asked to develop anesthetic protocols on the basis of their understanding of pharmacology, individual patient his-tory, and physical examination findings. Drug protocols differed; however, similar anesthetic protocols were used for more than half the cats of both groups. We have no evi-dence to suggest that the drug protocols used influenced the results, but we are unable to definitively address this question with the data available. Doing so would require a separate and specifically designed study. Future investiga-tions involving the use of a standard or limited number of drug protocols could help to determine whether this is an influencing factor.

In summary, an MBS with a fresh gas flow of 30 mL/kg/min did not result in a significant reduction in heat loss, compared with the NRB used in this study. Veterinar-ians should continue to rely on other means to limit heat loss, such as circulating warm water blankets, forced air warmers, and limiting anesthetic and surgical durations.

a. Miniature circle breathing system, Department of Clinical Sci-ences, College of Veterinary Medicine, Kansas State University, Manhattan, Kan.

b. Bain Breathing Circuit, 60 inches, Hudson RCI, Research Tri-angle Park, NC.

c. LDS 3000 Large Animal Anesthesia Machine, SurgiVet Inc, Waukesha, Wis.

d. Model F-150 B1557, Porter Instrument Inc, Hatfield, Pa.e. Ohmeda Tec 3, BOC Healthcare, Holmfirth, West Yorkshire,

England.f. Universal Control Arm No. 00-115, Anesthesia Associates Inc,

San Marcos, Calif.g. Doppler model 811-B, Parks Medical Electronics Inc, Aloha, Ore.h. TP-500 heat pump, Gaymar Industries Inc, Orchard Park, NY.i. YSI tele-thermometer, YSI Inc, Yellow Springs, Ohio.j. SAS, version 6.12, SAS Institute Inc, Cary, NC.

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