Blood pressure response to thermoregulatory vasoconstriction during isoflurane and desflurane anesthesia

Effect of Prewarming on Perioperative Hypothermia in Patients Undergoing Loco-Regional or General Anesthesia: A Randomized Clinical Trial

Background and Objectives: Redistribution hypothermia occurs during anesthesia despite active intraoperative warming. Prewarming increases the heat absorption by peripheral tissue, reducing the central to peripheral heat gradient. Therefore, the addition of prewarming may offer a greater preservation of intraoperative normothermia as compared to intraoperative warming only. Materials and Methods: A single-center clinical trial of adults scheduled for non-cardiac surgery. Patients were randomized to receive or not a prewarming period (at least 10 min) with convective air devices. Intraoperative temperature management was identical in both groups and performed according to a local protocol. The primary endpoint was the incidence, the magnitude and the duration of hypothermia (according to surgical time) between anesthetic induction and arrival at the recovery room. Secondary outcomes were core temperature on arrival in operating room, surgical site infections, blood losses, transfusions, patient discomfort (i.e., shivering), reintervention and hospital stay. Results: In total, 197 patients were analyzed: 104 in the control group and 93 in the prewarming group. Core temperature during the intra-operative period was similar between groups (p = 0.45). Median prewarming lasted 27 (17-38) min. Regarding hypothermia, we found no differences in incidence (controls: 33.7%, prewarming: 39.8%; p = 0.37), duration (controls: 41.6% (17.8-78.1), prewarming: 45.2% (20.6-71.1); p = 0.83) and magnitude (controls: 0.19 °C · h-1 (0.09-0.54), prewarming: 0.20 °C · h-1 (0.05-0.70); p = 0.91). Preoperative thermal discomfort was more frequent in the prewarming group (15.1% vs. 0%; p < 0.01). The interruption of intraoperative warming was more common in the prewarming group (16.1% vs. 6.7%; p = 0.03), but no differences were seen in other secondary endpoints. Conclusions: A preoperative prewarming period does not reduce the incidence, duration and magnitude of intraoperative hypothermia. These results should be interpreted considering a strict protocol for perioperative temperature management and the low incidence of hypothermia in controls.

Effect of active warming on perioperative cardiovascular outcomes: a systematic review and meta-analysis of randomized controlled trials

PURPOSE

The objective of this study was to provide an updated review on the active warming effects on major adverse cardiac events, 30-day all-cause mortality, and myocardial injury after noncardiac surgery.

METHOD

We systematically searched MEDLINE, EMBASE, CINAHL, Cochrane CENTRAL, Web of Science, and Chinese BioMedical Literature Database. We included randomized controlled trials of adult population undergoing noncardiac surgeries that concentrate on the comparison of active warming methods and passive thermal management. Cochrane Collaboration's tool was applied for risk-of-bias assessment. We used trial sequential analysis to evaluate the possibility of false positive or negative results.

RESULTS

A total of 13,316 unique records were identified, of which only 19 with reported perioperative cardiovascular outcomes were included in the systematic review and nine of them were included in final meta-analysis. No statistically significant difference between active warming methods and routine care was found in major adverse cardiac events (RR 0.56, 95% confidence interval (CI) 0.14-2.21, I² = 71%, number of events 59 vs. 70), 30-day all-cause mortality (RR 0.81, 95% CI 0.43-1.54, I² = 0%, number of events 17 vs. 21), and myocardial injury after noncardiac surgery (RR 0.61, 95% CI 0.17-2.22, I² = 79%, number of events 236 vs. 234). Trial sequential analysis suggests that current trials did not reach the minimum information size regarding the major cardiovascular events.

CONCLUSIONS

Compared to routine perioperative care, we found that active warming methods are not necessary for cardiovascular prevention in patients undergoing noncardiac surgery.

Hypertensive and Hypotensive Emergencies in Interventional Radiology

Hyper- and hypotensive emergencies represent some of the most severe clinical issues that can occur during or around an interventional radiology procedure. While some patients are known to be more predisposed to cardiovascular collapse, nearly all patients are at risk for such an outcome. This is particularly true of patients undergoing moderate sedation, with the possibility of cardiovascular compromise occurring not just due to the underlying pathology for which the patient is being treated, but as a complication of sedation itself. Understanding the underlying cause of hyper- or hypotension is paramount to performing an appropriate and timely intervention. While the underlying cause is being corrected-if possible-the changes in blood pressure themselves may need to be intervened upon to maintain cardiovascular stability in these patients. Interventional radiologists must be familiar with measures taken to correct hyper- or hypotensive emergencies, including the most commonly used medications to treat these disorders. This article discusses the most common etiologies of such clinical scenarios, and the most common interventions performed for these settings.

Effect of season and diet on heart rate and blood pressure in female red deer (Cervus elaphus) anaesthetised with medetomidine-tiletamine-zolazepam

Temperate zone ungulates like red deer (Cervus elaphus) show pronounced seasonal acclimatisation. Hypometabolism during winter is associated with cardiovascular changes, including a reduction in heart rate (fH) and temporal peripheral vasoconstriction. How anaesthesia with vasoactive substances such as medetomidine affect the seasonally acclimatised cardiovascular system is not yet known. We anaesthetised eleven healthy female red deer with medetomidine (0.1 mg/kg) and tiletamine/zolazepam (3 mg/kg) twice in winter (ad libitum and restricted feed) and in summer (ad libitum and restricted feed), with a two-week washout-period in-between, to test for the effect of season, food availability and supplementation with omega-3 or omega-6 polyunsaturated fatty acid (PUFA) on fH and arterial blood pressure (ABP) during anaesthesia. Six animals received pellets enriched with omega-6 fatty acids (FA), and five animals with omega-3 FA. Anaesthesia significantly decreased fH in summer but not in winter and ABP was lower in winter (p < 0.05). The combination of omega-6 FA enriched pellets and food restriction resulted in a lower fH and higher ABP during anaesthesia with more pronounced changes in winter (p < 0.001). Our results demonstrate that season, food availability and type of PUFA supplementation in red deer affect the cardiovascular system during anaesthesia.

Blood Pressure Profile and Trends in the Neonatal Transitioning Period: A Comparative Study of Term Asphyxiated and Healthy Newborns

Background. Extra-uterine transition difficulties including BP abnormalities have been documented following perinatal asphyxia. Aim. The study aimed to determine the BP changes in the early neonatal transitioning period. Method. This comparative, cross-sectional study was conducted at the Ekiti State University Teaching Hospital, Ado-Ekiti, Nigeria. Result. A total of 246 term babies were analyzed. Their mean (standard deviation) systolic, diastolic and mean arterial blood pressures (SBP, DBP, and MBP) at birth were 68.2 (9.0), 39.9 (7.0), and 49.4 (6.9) mmHg and 67.4 (7.9), 37.1 (6.1), and 47.2 (5.8) mmHg, respectively. Asphyxiated neonates had significantly higher initial DBP (P < .001) and MAP (P = .008), lower SBP (P < .001) and MAP (P = .024) at 6 hours and lower SBP at 12 hours after birth, statistically. Conclusion. This study showed that while the BP of healthy neonates steadily increases in the first 48 hours after birth, it fluctuated markedly within the first 12 hours of birth among asphyxiated babies.

Aggressive intraoperative warming versus routine thermal management during non-cardiac surgery (PROTECT): a multicentre, parallel group, superiority trial

BACKGROUND

Moderate intraoperative hypothermia promotes myocardial injury, surgical site infections, and blood loss. Whether aggressive warming to a truly normothermic temperature near 37°C improves outcomes remains unknown. We aimed to test the hypothesis that aggressive intraoperative warming reduces major perioperative complications.

METHODS

In this multicentre, parallel group, superiority trial, patients at 12 sites in China and at the Cleveland Clinic in the USA were randomly assigned (1:1) to receive either aggressive warming to a target core temperature of 37°C (aggressively warmed group) or routine thermal management to a target of 35·5°C (routine thermal management group) during non-cardiac surgery. Randomisation was stratified by site, with computer-generated, randomly sized blocks. Eligible patients (aged ≥45 years) had at least one cardiovascular risk factor, were scheduled for inpatient non-cardiac surgery expected to last 2-6 h with general anaesthesia, and were expected to have at least half of the anterior skin surface available for warming. Patients requiring dialysis and those with a body-mass index exceeding 30 kg/m² were excluded. The primary outcome was a composite of myocardial injury (troponin elevation, apparently of ischaemic origin), non-fatal cardiac arrest, and all-cause mortality within 30 days of surgery, as assessed in the modified intention-to-treat population. This study is registered with ClinicalTrials.gov, NCT03111875.

FINDINGS

Between March 27, 2017, and March 16, 2021, 5056 participants were enrolled, of whom 5013 were included in the intention-to-treat population (2507 in the aggressively warmed group and 2506 in the routine thermal management group). Patients assigned to aggressive warming had a mean final intraoperative core temperature of 37·1°C (SD 0·3) whereas the routine thermal management group averaged 35·6°C (SD 0·3). At least one of the primary outcome components (myocardial injury after non-cardiac surgery, cardiac arrest, or mortality) occurred in 246 (9·9%) of 2497 patients in the aggressively warmed group and in 239 (9·6%) of 2490 patients in the routine thermal management group. The common effect relative risk of aggressive versus routine thermal management was an estimated 1·04 (95% CI 0·87-1·24, p=0·69). There were 39 adverse events in patients assigned to aggressive warming (17 of which were serious) and 54 in those assigned to routine thermal management (30 of which were serious). One serious adverse event, in an aggressively warmed patient, was deemed to be possibly related to thermal management.

INTERPRETATION

The incidence of a 30-day composite of major cardiovascular outcomes did not differ significantly in patients randomised to 35·5°C and to 37°C. At least over a 1·5°C range from very mild hypothermia to full normothermia, there was no evidence that any substantive outcome varied. Keeping core temperature at least 35·5°C in surgical patients appears sufficient.

FUNDING

3M and the Health and Medical Research Fund, Food and Health Bureau, Hong Kong.

TRANSLATION

For the Chinese translation of the abstract see Supplementary Materials section.

Core/Shell Piezoelectric Nanofibers with Spatial Self-Orientated β-Phase Nanocrystals for Real-Time Micropressure Monitoring of Cardiovascular Walls

Implantable pressure biosensors show great potential for assessment and diagnostics of pressure-related diseases. Here, we present a structural design strategy to fabricate core/shell polyvinylidene difluoride (PVDF)/hydroxylamine hydrochloride (HHE) organic piezoelectric nanofibers (OPNs) with well-controlled and self-orientated nanocrystals in the spatial uniaxial orientation (SUO) of β-phase-rich fibers, which significantly enhance piezoelectric performance, fatigue resistance, stability, and biocompatibility. Then PVDF/HHE OPNs soft sensors are developed and used to monitor subtle pressure changes in vivo. Upon implanting into pig, PVDF/HHE OPNs sensors demonstrate their ultrahigh detecting sensitivity and accuracy to capture micropressure changes at the outside of cardiovascular walls, and output piezoelectric signals can real-time and synchronously reflect and distinguish changes of cardiovascular elasticity and occurrence of atrioventricular heart-block and formation of thrombus. Such biological information can provide a diagnostic basis for early assessment and diagnosis of thrombosis and atherosclerosis, especially for postoperative recrudescence of thrombus deep within the human body.

GWAS for urinary sodium and potassium excretion highlights pathways shared with cardiovascular traits

Urinary sodium and potassium excretion are associated with blood pressure (BP) and cardiovascular disease (CVD). The exact biological link between these traits is yet to be elucidated. Here, we identify 50 loci for sodium and 13 for potassium excretion in a large-scale genome-wide association study (GWAS) on urinary sodium and potassium excretion using data from 446,237 individuals of European descent from the UK Biobank study. We extensively interrogate the results using multiple analyses such as Mendelian randomization, functional assessment, co localization, genetic risk score, and pathway analyses. We identify a shared genetic component between urinary sodium and potassium expression and cardiovascular traits. Ingenuity pathway analysis shows that urinary sodium and potassium excretion loci are over-represented in behavioural response to stimuli. Our study highlights pathways that are shared between urinary sodium and potassium excretion and cardiovascular traits.

What is the predictor of the intraoperative body temperature in abdominal surgery?

PURPOSE

Inadvertent hypothermia is a relatively common intraoperative complication. Few studies have investigated predictors of body temperature change or the effect of the blanket type used with a forced-air warming device during the intraoperative period. We investigated the predictive factors of intraoperative body temperature change in scheduled abdominal surgery.

METHODS

We retrospectively reviewed the data from 2574 consecutive adult patients who underwent scheduled abdominal surgery in the supine position. Temperature data were collected from anesthesia records. Multiple regression analysis was performed at 60, 120, and 180 min after the surgical incision to identify the factors influencing body temperature change. We conducted nonlinear regression analysis using the equation ΔT = α (e^-γt-1) + βt, where ΔT represented the change in intraoperative core temperature (°C), t represented the surgical duration (minutes), and α, β, and γ were constants.

RESULTS

The intraoperative core temperature change was explained by the equation ΔT = 0.59 (e^- 0.018t - 1) + 0.0043t. Younger age, higher body mass index (BMI), male sex, laparoscopic surgery, and use of an underbody blanket were associated with increased core temperature at 1 or 2 h after surgical incision. Male sex and an underbody blanket remained strong predictive variables even 3 h after surgical incision, whereas BMI had little explanatory power at this timepoint. The difference in the heating effect of an underbody versus an overbody blanket was 0.0012 °C per minute.

CONCLUSIONS

The blanket type of the forced-air warmer, age, sex, laparoscopic surgery, and BMI are predictors of intraoperative core temperature change.

Effects of Thermal Protection in Patients Undergoing Body Contouring Procedures: A Controlled Clinical Trial

BACKGROUND

Hypothermia is common in many plastic surgery procedures, but few measures to prevent its occurrence are taken.

OBJECTIVES

This study evaluated the effect of hypothermia in patients undergoing plastic surgery procedures and the effect of utilizing simple and inexpensive measures to prevent patient hypothermia during surgery.

METHODS

A randomized controlled clinical trial was performed among 3 groups of patients who underwent body contouring surgery for longer than 3.5 hours. In group 1, no protective measures were taken to prevent hypothermia; in group 2, maneuvers were applied intraoperatively for the duration of the entire surgical procedure; and in group 3, measures were taken preoperatively and intraoperatively. The results were quantified and analyzed through a bivariate analysis, including degree of hypothermia, anesthesia recovery time, time spent in the recovery area, intensity of pain, cold perception, response to opioids, and nausea.

RESULTS

There were 122 patients included in the study: 43 in group 1, 39 in group 2, and 40 in group 3. All patients in group 1 had a higher degree of hypothermia, longer recovery time from anesthesia, longer overall recovery time, increased pain, increased feeling of cold, and more nausea. These patients also required a greater amount of opioids compared with the patients in groups 2 and 3. Many of the results were statistically significant.

CONCLUSIONS

The adoption of simple and inexpensive measures before and during plastic surgery can prevent patient hypothermia during the procedures, leading to a shorter anesthesia recovery time and avoiding the undesirable effects associated with hypothermia. In addition, these measures may have significant economic savings.

LEVEL OF EVIDENCE 2

The Cold Dusty Harmattan: A Season of Anguish for Cardiologists and Patients

Human health and disease often demonstrate seasonal patterns. Knowledge of these aspects aids anticipation and planning. Numerous studies have shown that hypertension and cardiovascular diseases demonstrate a seasonal pattern. The Harmattan, the cold dusty season in Sub-Saharan Africa, is the season of greatest concern in this regard. In this commentary, the author draws on his and other researchers' studies to explain the grounds for onset and worsening of existing cardiovascular diseases. As implied in the title, it is a season that puts great strain on the cardiologist and the health system, as well as greater disease burden on the patient. This should be taken into consideration in planning and pooling of resources for effective patient management and mitigation of impact of disease.

Perioperative thermoregulation and heat balance

Core body temperature is normally tightly regulated to within a few tenths of a degree. The major thermoregulatory defences in humans are sweating, arteriovenous shunt vasoconstriction, and shivering. The core temperature triggering each response defines its activation threshold. General anaesthetics greatly impair thermoregulation, synchronously reducing the thresholds for vasoconstriction and shivering. Neuraxial anaesthesia also impairs central thermoregulatory control, and prevents vasoconstriction and shivering in blocked areas. Consequently, unwarmed anaesthetised patients become hypothermic, typically by 1-2°C. Hypothermia results initially from an internal redistribution of body heat from the core to the periphery, followed by heat loss exceeding metabolic heat production. Complications of perioperative hypothermia include coagulopathy and increased transfusion requirement, surgical site infection, delayed drug metabolism, prolonged recovery, shivering, and thermal discomfort. Body temperature can be reliably measured in the oesophagus, nasopharynx, mouth, and bladder. The standard-of-care is to monitor core temperature and to maintain normothermia during general and neuraxial anaesthesia.

Relation Between Temperature Extremes and Symptom Exacerbation in Patients With Hypertrophic Cardiomyopathy

Warm temperatures induce peripheral vasodilation, decrease afterload, and may concurrently increase the left ventricular outflow tract (LVOT) gradient. We aimed to assess the impact of subjective ambient temperature on hypertrophic cardiomyopathy (HC) symptoms and determine whether they were associated with LVOT gradient, patient quality of life (QOL), and risk of sudden cardiac death (SCD). We identified consecutive patients with HC presenting to a tertiary referral center. Of the 173 patients in the study, 143 (83%) had HC symptoms, with ambient temperature change worsening symptoms for 72 patients (50%). Symptom exacerbation occurred only with heat for 57 (79%), whereas symptoms were exacerbated with cold only or with cold and heat equally for 15 (21%). Patients affected by any temperature exacerbation more commonly were women (p = 0.009), had a lower QOL (p = 0.04), had a family history of HC (p = 0.007), or underwent myectomy (p = 0.01). A greater proportion of patients with heat-only exacerbation had a family history of HC (p = 0.005) and SCD (p = 0.05). The presence of an LVOT gradient either at rest or with provocation was similar in all groups. In conclusion, although no appreciable difference in LVOT gradients were observed between patient groups, approximately half of the patients with HC reporting symptoms at baseline noted worsening of symptoms with temperature changes, with >75% describing heat-induced symptom exacerbation. Furthermore, affected patients more frequently were women, underwent surgical intervention and device implantation, and had an overall lower QOL.

Renal adaptation during hibernation

Hibernators periodically undergo profound physiological changes including dramatic reductions in metabolic, heart, and respiratory rates and core body temperature. This review discusses the effect of hypoperfusion and hypothermia observed during hibernation on glomerular filtration and renal plasma flow, as well as specific adaptations in renal architecture, vasculature, the renin-angiotensin system, and upregulation of possible protective mechanisms during the extreme conditions endured by hibernating mammals. Understanding the mechanisms of protection against organ injury during hibernation may provide insights into potential therapies for organ injury during cold storage and reimplantation during transplantation.

Blood pressure response to patterns of weather fluctuations and effect on mortality

Very few studies have looked at longitudinal intraindividual blood pressure responses to weather conditions. There are no data to suggest that specific response to changes in weather will have an impact on survival. We analyzed >169 000 clinic visits of 16 010 Glasgow Blood Pressure Clinic patients with hypertension. Each clinic visit was mapped to the mean West of Scotland monthly weather (temperature, sunshine, rainfall) data. Percentage change in blood pressure was calculated between pairs of consecutive clinic visits, where the weather alternated between 2 extreme quartiles (Q(1)-Q(4) or Q(4)-Q(1)) or remained in the same quartile (Q(n)-Q(n)) of each weather parameter. Subjects were also categorized into 2 groups depending on whether their blood pressure response in Q(1)-Q(4) or Q(4)-Q(1) were concordant or discordant to Q(n)-Q(n). Generalized estimating equations and Cox proportional hazards model were used to model the effect on longitudinal blood pressure and mortality, respectively. Q(n)-Q(n) showed a mean 2% drop in blood pressure consistently, whereas Q(4)-Q(1) showed a mean 2.1% and 1.6% rise in systolic and diastolic blood pressure, respectively. However, Q(1)-Q(4) did not show significant changes in blood pressure. Temperature-sensitive subjects had significantly higher mortality (1.35 [95% confidence interval, 1.06-1.71]; P=0.01) and higher follow-up systolic blood pressure (1.85 [95% confidence interval, 0.24-3.46]; P=0.02) compared with temperature-nonsensitive subjects. Blood pressure response to temperature may be one of the underlying mechanisms that determine long-term blood pressure variability. Knowing a patient's blood pressure response to weather can help reduce unnecessary antihypertensive treatment modification, which may in turn increase blood pressure variability and, thus, risk.

Strategies for therapeutic hypometabothermia

Although therapeutic hypothermia and metabolic suppression have shown robust neuroprotection in experimental brain ischemia, systemic complications have limited their use in treating acute stroke patients. The core temperature and basic metabolic rate are tightly regulated and maintained in a very stable level in mammals. Simply lowering body temperature or metabolic rate is actually a brutal therapy that may cause more systemic as well as regional problems other than providing protection. These problems are commonly seen in hypothermia and barbiturate coma. The main innovative concept of this review is to propose thermogenically optimal and synergistic reduction of core temperature and metabolic rate in therapeutic hypometabothermia using novel and clinically practical approaches. When metabolism and body temperature are reduced in a systematically synergistic manner, the outcome will be maximal protection and safe recovery, which happen in natural process, such as in hibernation, daily torpor and estivation.

Can personal exposures to higher nighttime and early-morning temperatures increase blood pressure?

Environmental temperatures are inversely related to BP; however, the effects of short-term temperature changes within a 24-hour period and measured with high accuracy at the personal level have not been described. Fifty-one nonsmoking patients living in the Detroit area had up to 5 consecutive days of 24-hour personal-level environmental temperature (PET) monitoring along with daily cardiovascular measurements, including BP, performed mostly between 5 pm and 7 pm during summer and/or winter periods. The associations between hour-long mean PET levels during the previous 24 hours with the outcomes were assessed by linear mixed models. Accounting for demographics, environmental factors, and monitoring compliance, systolic and diastolic BP were positively associated with several hour-long PET measurements ending from 10 to 15 hours beforehand. During this time, corresponding mostly to a period starting from between 1 am and 3 am to ending between 7 am and 9 am, an increase of 1°C was associated with a 0.81 mm Hg to 1.44 mm Hg and 0.59 mm Hg to 0.83 mm Hg elevation in systolic and diastolic BP, respectively. Modestly warmer, commonly encountered PET levels posed a clinically meaningful effect (eg, a 6.95 mm Hg systolic pressure increase per interquartile range (4.8°C) elevation at lag hour 10). Community-level outdoor ambient temperatures were not related to BP. The authors provide the first evidence that personal exposure to warmer nighttime and early-morning environmental temperatures might lead to an increase in BP during the ensuing day.

Unintended perioperative hypothermia

BACKGROUND

Hypothermia, defined as a core body temperature less than 36°C (96.8°F), is a relatively common occurrence in the unwarmed surgical patient. A mild degree of perioperative hypothermia can be associated with significant morbidity and mortality. A threefold increase in the frequency of surgical site infections is reported in colorectal surgery patients who experience perioperative hypothermia. As part of the Surgical Care Improvement Project, guidelines aim to decrease the incidence of this complication.

METHODS

We review the physiology of temperature regulation, mechanisms of hypothermia, effects of anesthetics on thermoregulation, and consequences of hypothermia and summarize recent recommendations for maintaining perioperative normothermia.

RESULTS

Evidence suggests that prewarming for a minimum of 30 minutes may reduce the risk of subsequent hypothermia.

CONCLUSIONS

Monitoring of body temperature and avoidance of unintended perioperative hypothermia through active and passive warming measures are the keys to preventing its complications.

Defeating normal thermoregulatory defenses: induction of therapeutic hypothermia

Therapeutic hypothermia may be useful in various circumstances including stroke. However, core body temperature is normally tightly regulated. Even mild hypothermia in conscious subjects thus provokes vigorous thermoregulatory defenses which are potentially harmful in fragile patients. Furthermore, thermoregulatory responses are effective, which reduces the rate at which hypothermia can be induced. Drugs are thus often given to blunt normal thermoregulatory defenses. General anesthetics profoundly impair thermoregulatory control, but prolonged general anesthesia is rarely practical or appropriate. A variety of other drugs have therefore been evaluated. Most opioids only slightly impair thermoregulatory defenses, but meperidine is considerably more effective than equipotent doses of other opioids. The central alpha-2 agonists clonidine and dexmedetomidine are also useful. However, the best overall approach to inducing thermal tolerance appears to be a combination of buspirone and meperidine, which reduces the core temperature triggering shivering to about 33.5 degrees C in doses that maintain adequate ventilation.

Thermoregulatory defense mechanisms

Core body temperature is normally tightly regulated by an effective thermoregulatory system. Thermoregulatory control is sometimes impaired by serious illness, but more typically remains intact. The primary autonomic defenses against heat are sweating and active precapillary vasodilation; the primary autonomic defenses against cold are arteriovenous shunt vasoconstriction and shivering. The core temperature triggering each response defines its activation threshold. Temperatures between the sweating and vasoconstriction thresholds define the inter-threshold range. The shivering threshold is usually a full 1 degrees C below the vasoconstriction threshold and is therefore a "last resort" response. Both vasoconstriction and shivering are associated with autonomic and hemodynamic activation; and each response is effective, thus impeding induction of therapeutic hypothermia. It is thus helpful to accompany core cooling with drugs that pharmacologically induce a degree of thermal tolerance. No perfect drug or drug combination has been identified. Anesthetics, for example, induce considerable tolerance, but are rarely suitable. Meperidine-especially in combination with buspirone-is especially effective while provoking only modest toxicity. The combination of buspirone and dexmedetomidine is comparably effective while avoiding the respiratory depression association with opioid administration.

Repeated anaesthesia with isoflurane and xylazine/levomethadone/fenpipramide premedication in female Beagle dogs: Influence on general health and wellbeing

Summary

Beagle dogs continue to be used in experimental studies and preclinical and clinical trials, many of which address the usage of anaesthesia. In order to reduce the number of animals, researchers tend to conduct several experiments on a single animal. The question arises, however, as to whether or not this frequent usage involves more than simply additional stress and discomfort for the individual animal. Within the framework of an existing study involving six female Beagle dogs, we investigated the effects of repeated (5) isoflurane anaesthesia with xylazine/levomethadone/fenpipramide premedication carried out at short intervals (2 weeks) and compared these with the effects of two treatments intermitted by a longer resting period (8 weeks). To verify our hypothesis that frequent anaesthesia affects the dog's wellbeing more than the occasional anaesthesia, the following parameters were measured at regular intervals: body weight, body temperature, respiratory rate, blood pressure, reflexes and heart rate, both at rest and during a treadmill exercise. In addition, recovery behaviour subsequent to anaesthesia was monitored for one hour. Our observations indicate that the anaesthetic effects are most prominent 24 h after the anaesthetic treatment. However, crossover analysis of our data cannot show that there is no statistical difference of whether dogs were anaesthetized occasionally or frequently. In our study, it appears that frequent anaesthesia within a two-week period did not affect the wellbeing and general health of Beagle dogs in a super-additive manner and that a minimum of two-week testing-free period is sufficient to ensure complete recovery from the unwanted effects induced by anaesthesia.

Temperature monitoring and perioperative thermoregulation

Most clinically available thermometers accurately report the temperature of whatever tissue is being measured. The difficulty is that no reliably core-temperature-measuring sites are completely noninvasive and easy to use-especially in patients not undergoing general anesthesia. Nonetheless, temperature can be reliably measured in most patients. Body temperature should be measured in patients undergoing general anesthesia exceeding 30 min in duration and in patients undergoing major operations during neuraxial anesthesia. Core body temperature is normally tightly regulated. All general anesthetics produce a profound dose-dependent reduction in the core temperature, triggering cold defenses, including arteriovenous shunt vasoconstriction and shivering. Anesthetic-induced impairment of normal thermoregulatory control, with the resulting core-to-peripheral redistribution of body heat, is the primary cause of hypothermia in most patients. Neuraxial anesthesia also impairs thermoregulatory control, although to a lesser extent than does general anesthesia. Prolonged epidural analgesia is associated with hyperthermia whose cause remains unknown.

Perioperative normothermia depends on intraoperative warming procedure, extent of the surgical intervention and age of the experimental animal

The maintenance of a physiological body temperature during and early after surgical interventions in experimental animals such as rodents is often neglected. Therefore the positive influence of an adequate use of warming blankets (WB) on the rectal body temperature in rats was investigated during two different surgical interventions, with a special focus on possible differences between young adult (2.5+/-0.14 months) and adult animals (9.3+/-0.13 months). Anesthesia was induced with isoflurane short inhalation and maintained with ketamine and domitor intramuscularly. Animals were divided into ten groups according to (a) the age of the animals, (b) the temperature of the WB and (c) the kind of surgical intervention (either an intravenous [i.v.] cannulation of the right external jugular vein or an intra-aortal implantation of a telemetric transmitter or both). Results clearly show that the surface temperature of the WB has a major impact on the perioperative thermoregulation. The rectal body temperature of animals operated on a cooler WB dramatically decreased depending on the age of the rat and also on the extent of the surgical intervention. The opening of the abdominal cavity in older rats resulted in a severe hypothermia: they lost 5.6 degrees C compared to 3.2 degrees C in the young adult rats. The implantation of the i.v. catheter had no serious effect on the thermoregulation. In conclusion, the results clearly show that an adequate perioperative warming system positively influences the postoperative outcome in young adult and most notably in adult rats and thus enables early postoperative experiments without effects on measured parameters.

Physiology and clinical relevance of induced hypothermia

Experimental evidence and clinical experience suggest that mild hypothermia protects numerous tissues from damage during ischemic insult. However, the extent to which hypothermia becomes a valued therapeutic option will depend on the clinician's ability to rapidly reduce core body temperature and safely maintain hypothermia. To date, general anesthesia is the best way to block autonomic defenses during induction of mild-to-moderate hypothermia; unfortunately, general anesthesia is not an option in most patients likely to benefit from therapeutic hypothermia. Induction of hypothermia in awake humans is complicated by both the technical difficulties related to thermal manipulation and the remarkable efficacy of thermoregulatory defenses, especially vasoconstriction and shivering. The most effective thermal manipulation devices are generally invasive and, therefore, more prone to complications than surface methods. In an effort to inhibit thermoregulation in awake humans, several agents have been tested either alone or in combination with each other. For example, the combination of meperidine and buspirone has already been applied to facilitate induction of hypothermia in human trials. However, pharmacological induction of thermoregulatory tolerance to cold without excessive sedation, respiratory depression, or other serious toxicity remains a major focus of current therapeutic hypothermia research.