The cardiovascular effects of isoflurane in lambs

Ultrasound images of the ascending aorta of anesthetized northern fur seals and Steller sea lions confirm that the aortic bulb maintains continuous blood flow

The increased size and enhanced compliance of the aortic bulb-the enlargement of the ascending aorta-are believed to maintain blood flow in pinnipeds during extended periods of diastole induced by diving bradycardia. The aortic bulb has been described ex vivo in several species of pinnipeds, but in vivo measurements are needed to investigate the relationship between structure and function. We obtained ultrasound images using electrocardiogram-gated transesophageal echocardiography during anesthesia and after atropine administration to assess the relationship between aortic bulb anatomy and cardiac function (heart rate, stroke volume, cardiac output) in northern fur seals (Callorhinus ursinus) and Steller sea lions (Eumetopias jubatus). We observed that the aortic bulb in northern fur seals and Steller sea lions expands during systole and recoils over the entire diastolic period indicating that blood flow is maintained throughout the entire cardiac cycle as expected. The stroke volumes we measured in the fur seals and sea lions fit the values predicted based on body size in mammals and did not change with increased heart rates, suggesting that greater stroke volumes are not needed for aortic bulb function. Overall, our results suggest that peripheral vasoconstriction during diving is sufficient to modulate the volume of blood in the aortic bulb to ensure that flow lasts over the entire diastolic period. These results indicate that the shift of blood into the aortic bulb of pinnipeds is a fundamental mechanism caused by vasoconstriction while diving, highlighting the importance of this unique anatomical adaptation.

An MRI approach to assess placental function in healthy humans and sheep

KEY POINTS

Human placental function is evaluated using non-invasive Doppler ultrasound of umbilical and uterine artery pulsatility indices as measures of resistance in placental vascular beds, while measurement of placental oxygen consumption ( V O 2 ) is only possible during Caesarean delivery. This study shows the feasibility of using magnetic resonance imaging (MRI) in utero to measure blood flow and oxygen content in uterine and umbilical vessels to calculate oxygen delivery to and V O 2 by the gravid uterus, uteroplacenta and fetus. Normal late gestational human uteroplacental V O 2 by MRI was ∼4 ml min^-1  kg^-1 fetal weight, which was similar to our MRI measurements in sheep and to those previously measured using invasive techniques. Our MRI approach can quantify uteroplacental V O 2 , which involves the quantification of maternal- and fetal-placental blood flows, fetal oxygen delivery and V O 2 , and the oxygen gradient between uterine- and umbilical-venous blood, providing a comprehensive assessment of placental function with clinical potential.

ABSTRACT

It has not been feasible to perform routine clinical measurement of human placental oxygen consumption ( V O 2 ) and in vitro studies do not reflect true metabolism in utero. Here we propose an MRI method to non-invasively quantify in utero placental and fetal oxygen delivery ( D O 2 ) and V O 2 in healthy humans and sheep. Women (n = 20) and Merino sheep (n = 10; 23 sets of measurements) with singleton pregnancies underwent an MRI in late gestation (36 ± 2 weeks and 128 ± 9 days, respectively; mean ± SD). Blood flow (phase-contrast) and oxygen content (T1 and T2 relaxometry) were measured in the major uterine- and umbilical-placental vessels, allowing calculation of uteroplacental and fetal D O 2 and V O 2 . Maternal D O 2 (ml min^-1  kg^-1 fetus) to the gravid uterus was similar in humans and sheep (human = 54 ± 15, sheep = 53 ± 21, P = 0.854), while fetal D O 2 (human = 25 ± 4, sheep = 22 ± 5, P = 0.049) was slightly lower in sheep. Uteroplacental and fetal V O 2 (ml min^-1  kg^-1 fetus; uteroplacental: human = 4.1 ± 1.5, sheep = 3.5 ± 1.9, P = 0.281; fetus: human = 6.8 ± 1.3, sheep = 7.2 ± 1.7, P = 0.426) were similar between species. Late gestational uteroplacental:fetal V O 2 ratio did not change with age (human, P = 0.256; sheep, P = 0.121). Human umbilical blood flow (ml min^-1  kg^-1 fetus) decreased with advancing age (P = 0.008), while fetal V O 2 was preserved through an increase in oxygen extraction (P = 0.046). By contrast, sheep fetal V O 2 was preserved through stable umbilical flow (ml min^-1  kg^-1 ; P = 0.443) and oxygen extraction (P = 0.582). MRI derived measurements of uteroplacental and fetal V O 2 between humans and sheep were similar and in keeping with prior data obtained using invasive techniques. Taken together, these data confirm the reliability of our approach, which offers a novel clinical 'placental function test'.

Determination of the minimum alveolar concentration of sevoflurane that blunts adrenergic responses and the effect of a constant rate infusion of ketamine in sheep

Minimizing sympathetic stimulation under anesthesia prevents activation of the neuroendocrine stress response. The minimum alveolar concentration blunting adrenergic responses in 50% of the population when exposed to a noxious stimulus is defined as MAC-BAR. The purpose of this study was to determine the MAC-BAR of sevoflurane (MAC-BAR_sevo) in sheep and the MAC-BAR sparing effects of ketamine. Thirteen healthy Dorset-cross adult ewes, 4 ± 1 year old and weighing 74 ± 9 kg, were enrolled in a randomized blinded crossover study design. Ewes were anesthetized twice for MAC-BAR_sevo determination. After face mask induction with sevoflurane, sheep received intravenous ketamine at 1.5 mg/kg and a constant rate infusion of 1.5 mg/kg/h or an equivalent volume of saline (placebo). After 8 day washout, the other treatment was administered. A bracketing technique was used for MAC-BAR_sevo determination and values were collected in duplicate. The mechanical stimulus (sponge forceps) was applied at the coronary band for 1 min and blood was collected for ketamine plasma concentrations. The MAC-BAR_sevo values of each treatment were compared using a paired t-test. Mean MAC-BAR_sevo of the ketamine and placebo were 2.73 ± 0.23% and 2.77 ± 0.31%, respectively and no significant difference was found (p = .638). Average ketamine plasma concentrations was 1.54 ± 0.18 μg/mL maintained through the study. Ketamine at 1.5 mg/kg, followed by 1.5 mg/kg/h, did not decrease the MAC-BAR_sevo in sheep. Further studies to determine the effect of higher doses of ketamine on inhalational anesthetic agents and their potential adverse effects are warranted.

Comparing anesthesia with isoflurane and fentanyl/fluanisone/midazolam in a rat model of cardiac arrest

Only one in ten patients survives cardiac arrest (CA), underscoring the need to improve CA management. Isoflurane has shown cardio- and neuroprotective effects in animal models of ischemia-reperfusion injury. Therefore, the beneficial effect of isoflurane should be tested in an experimental CA model. We hypothesize that isoflurane anesthesia improves short-term outcome following resuscitation from CA compared with a subcutaneous fentanyl/fluanisone/midazolam anesthesia. Male Sprague-Dawley rats were randomized to anesthesia with isoflurane ( n = 11) or fentanyl/fluanisone/midazolam ( n = 11). After 10 min of asphyxial CA, animals were resuscitated by mechanical chest compressions, ventilations, and epinephrine and observed for 30 min. Hemodynamics, including coronary perfusion pressure, systemic O2 consumption, and arterial blood gases, were recorded throughout the study. Plasma samples for endothelin-1 and cathecolamines were drawn before and after CA. Compared with fentanyl/fluanisone/midazolam anesthesia, isoflurane resulted in a shorter time to return of spontaneous circulation (ROSC), less use of epinephrine, increased coronary perfusion pressure during cardiopulmonary resusitation, higher mean arterial pressure post-ROSC, increased plasma levels of endothelin-1, and decreased levels of epinephrine. The choice of anesthesia did not affect ROSC rate or systemic O2 consumption. Isoflurane reduces time to ROSC, increases coronary perfusion pressure, and improves hemodynamic function, all of which are important parameters in CA models.

NEW & NOTEWORTHY The preconditioning effect of volatile anesthetics in studies of ischemia-reperfusion injury has been demonstrated in several studies. This study shows the importance of anesthesia in experimental cardiac arrest studies as isoflurane raised coronary perfusion pressure during resuscitation, reduced time to return of spontaneous circulation, and increased arterial blood pressure in the post-cardiac arrest period. These effects on key outcome measures in cardiac arrest research are important in the interpretation of results from animal studies.

PARÂMETROS CARDIOPULMONARES E HEMOGASOMÉTRICOS EM OVINOS SOB ANESTESIA INALATÓRIA DURANTE ALTERNÂNCIAS DE DECÚBITO

Resumo Os rebanhos de ovinos do Centro-Oeste do Brasil, principalmente da raça Santa Inês, aumentaram muito nos últimos anos. Dessa forma, a casuística clínica também cresceu e a realização de cirurgias de grande porte sem dor e com segurança se faz presente, associada ao uso da anestesia geral inalatória. Este trabalho objetivou avaliar os parâmetros cardiopulmonares e hemogasométricos em ovinos sob anestesia inalatória com ventilação controlada, bem como avaliar a influência de alternâncias de decúbito nos mesmos parâmetros. Os animais foram sedados com acepromazina, induzidos com propofol, intubados e colocados sob anestesia inalatória com isoflurano durante manutenção anestésica. As frequências cardíaca e respiratória, pressão arterial e a concentração de gases sanguíneos foram aferidas a cada cinco minutos por um total de 45 minutos, durante os quais os animais foram alternados entre decúbito lateral direito, dorsal e esquerdo. Os parâmetros aferidos não apresentaram diferenças significativas entre os tempos e decúbitos, concluindo-se que a anestesia geral inalatória com isoflurano mostra-se viável e segura na espécie ovina, pois mantém os parâmetros cardiopulmonares e hemogasométricos em níveis seguros.

Clinical effects of isoflurane and sevoflurane in lambs

OBJECTIVE

To compare isoflurane and sevoflurane in lambs undergoing prolonged anaesthesia for spinal surgery.

STUDY DESIGN

Prospective randomised clinical study.

ANIMALS

Eighteen Scottish blackface lambs 3-6 weeks of age and weighing 10-17 kg.

METHODS

After intramuscular medetomidine, anaesthesia was induced and maintained with either isoflurane (group I) or sevoflurane (group S) delivered in oxygen. Meloxicam, morphine, a constant rate infusion of ketamine and atracurium were given intravenously (IV) during surgery. Lungs were ventilated to maintain normocapnia. with peak inspiratory pressures of 20-25 cmH(2) O. Ephedrine or dextran 40% was administered when mean arterial pressure (MAP) was <55 mmHg. Intrathecal morphine, and IV meloxicam and edrophonium were injected before recovery. Time to loss of palpebral reflex (TLPR) upon induction, cardiorespiratory variables, time at first swallowing and other movement, tracheal extubation, vocalisation, spontaneous head lifting (>1 minute), reunion with the ewe, and the number of MAP treatments were recorded. Statistical analysis utilised anova, Mann-Whitney, t-test or Pearson's correlation test as relevant. p < 0.05 was considered significant.

RESULTS

End-tidal carbon dioxide (mean ± SD) was significantly lower in group S (5.5 ± 0.6 kPa) than in group I (5.8 ± 0.5 kPa) while MAP (70 ± 11 mmHg) and diastolic arterial blood pressure (60 ± 11 mmHg) were higher in group S than in group I (65 ± 12 and 54 ± 11 mmHg, respectively). No differences were found with TLPR and MAP treatments. Time (median, range) from end of anaesthesia to ewe-lamb reunion was briefer (p = 0.018) in group S (48, 20-63 minutes).

CONCLUSION

Isoflurane and sevoflurane are both suitable for maintaining general anaesthesia in lambs although sevoflurane, as used in this study, allows a more rapid reunion with the ewe.

CLINICAL RELEVANCE

The principal advantage of sevoflurane over isoflurane during prolonged anaesthesia in lambs is a more rapid recovery.

The effects of age, isoflurane and sevoflurane on atracurium in lambs

OBJECTIVE

To determine the effects of age, sevoflurane and isoflurane on atracurium-induced neuromuscular blockade in 3-16 week-old lambs.

STUDY DESIGN

Prospective randomized experimental trial.

ANIMALS

Twenty-six Scottish blackface ewe-lambs were anaesthetized for spinal surgery when either 3-6 (mean age 4.6 weeks; n = 18) or 12-16 weeks (mean age 13.7 weeks; n = 15) of age; seven animals were anaesthetized at both ages.

METHODS

After intramuscular injection of medetomidine (10 μg kg(-1)) anaesthesia was induced in the younger lambs either with isoflurane or sevoflurane in oxygen delivered by mask, and in the older lambs with ketamine (4 mg kg(-1)), and midazolam (0.2 mg kg(-1) ) administered intravenously (IV). In both groups anaesthesia was maintained with fixed end-tidal concentrations of either sevoflurane (2.8%) or isoflurane (1.8%) delivered in oxygen. Before surgery meloxicam (0.6 mg kg(-1)), morphine (0.5 mg kg(-1)) and ketamine (1 mg kg(-1) followed by 10 μg kg(-1) minute(-1) ) were administered IV. The lungs were ventilated mechanically to maintain normocapnia. Neuromuscular block was achieved with a loading dose (LD) of atracurium (0.5 mg kg(-1) IV). The peroneal nerve was stimulated (train-of-four every 12 seconds). Evoked responses in the digital extensor muscles were evaluated by palpation and observation. Maintenance doses (MD) of atracurium (0.17 mg kg(-1) IV) were administered when the first twitch (T1) returned. The onset and duration of LD action (T1 absent) and the duration of MD were recorded. Data were analysed using Student's t test, Mann-Whitney U test, repeated-measures anova, Wilcoxon's matched pairs test or Pearson correlation coefficient as relevant (p < 0.05).

RESULTS

Onset of LD action developed significantly (p < 0.05) more rapidly in isoflurane compared with sevoflurane-anaesthetized lambs (55 ± 18 cf. 80 ± 37 seconds). Duration of action of LDs and MDs was longer (p < 0.05) in lambs aged 12-16 than 3-6 weeks (33 ± 5.4 cf. 25 ± 6.4 and 26 ± 4.2 cf. 18 ± 5.5 minutes) but were independent of the anaesthetic used.

CONCLUSIONS AND CLINICAL RELEVANCE

The effect of atracurium is age-dependent in lambs being prolonged in older animals. The onset of neuromuscular blockade is more rapid in isoflurane compared with sevoflurane-anaesthetized lambs.

The effects of isoflurane anesthesia and mechanical ventilation on renal function during endotoxemia

BACKGROUND

Isoflurane is a common anesthetic agent used in human surgery and in animal models of sepsis. It has been suggested to have beneficial anti-inflammatory properties and to protect kidney function. Here, we investigated the effect of isoflurane on the development of kidney injury and dysfunction during 48-h endotoxemia in sheep.

METHODS

Before the experiments, the sheep (n=16) were surgically equipped with transit-time flowprobes around the renal, femoral and superior mesenteric artery. The animals were randomized to either be anesthetized with isoflurane and mechanically ventilated or to remain conscious while they received intravenous Escherichia coli lipopolysaccharide (LPS) for 48 h (25 ng/kg/min). In two animals in each group, the LPS was excluded to investigate any effect of isoflurane per se over time.

RESULTS

Endotoxemia caused cardiovascular changes typical for hyperdynamic sepsis and, although renal hyperemia occurred, impaired renal function in both groups. Compared with conscious animals, isoflurane significantly (P<0.05) reduced urine output, renal creatinine clearance, fractional sodium excretion and renal blood flow during endotoxemia. Furthermore, the plasma concentrations of urea and creatinine increased more in the anesthetized animals. Isoflurane anesthesia also enhanced neutrophil activity and accumulation in the kidney during endotoxemia. N-acetyl-β-D-glucosaminidase was significantly increased, with no inter-group difference as an indication of tubular injury.

CONCLUSIONS

The results of the current study suggest that isoflurane anesthesia (minimum alveolar concentration 1.0) with mechanical ventilation aggravates renal dysfunction during 48 h of endotoxemia and does not significantly reduce the inflammatory response or signs of tubular damage.

Clinical, cardiopulmonary, hematological and serum biochemical effects of sevoflurane and isoflurane anesthesia in oxygen under spontaneous breathing in sheep

Effects of sevoflurane and isoflurane anesthesia in oxygen on clinical, cardiopulmonary, hematological, and serum biochemical findings were compared in sheep breathing spontaneously undergoing minor surgical operations during short-term (60-80min) or long-term (3-4h) anesthesia. All sheep were premedicated with atropine sulfate (0.1mg/kg) intramuscularly, and 10min later, induced to anesthesia by intravenous infusion of sodium thiopental (mean 14.1+/-3.4 S.D. mg/kg). After intubation, they were anesthetized with either isoflurane or sevoflurane in oxygen at a total gas flow rate of 1.5l/min. The results revealed that recovery time with sevoflurane was more rapid than with isoflurane. Respiration rates, tidal volume, minute ventilation and heart rates during sevoflurane anesthesia were similar to those during isoflurane anesthesia. The degree of respiratory acidosis during sevoflurane anesthesia was also similar to that during isoflurane anesthesia. There were no significant differences between sevoflurane and isoflurane anesthesia in hematological and serum biochemical values.

Chronic cocaine administration does not modify haemodynamic responses to isoflurane anaesthesia in sheep

PURPOSE

Cocaine use is epidemic in the developed world, resulting in numerous patients presenting for surgery and anaesthesia with a history of chronic cocaine exposure. The purpose of this study was to determine the effect of chronic cocaine exposure on the cardiovascular response to isoflurane general anaesthesia.

METHODS

The changes in mean arterial pressure (MAP), heart rate (HR), cardiac output (CO), central venous pressure (CVP), pulmonary capillary wedge pressure (PCWP) and systemic vascular resistance (SVR) with increasing concentration of isoflurane (1%, 1.7%, and 2.4% end tidal) were determined at baseline in six sheep. The animals then received a continuous cocaine infusion (0.2 mg.kg-1.hr-1) and twice daily cocaine boluses (4 mg.kg-1) for 17 days followed on day 18 by a cocaine binge consisting of eight cocaine boluses (4 mg.kg-1) administered at one hour intervals. The haemodynamic studies conducted at baseline prior to cocaine exposure were then repeated on days 15 and 18.

RESULTS

Increasing concentrations of isoflurane produced the expected dose-dependent cardiovascular depression, but this was not altered by cocaine exposure.

CONCLUSION

Although chronic cocaine exposure has been shown to increase isoflurane minimum alveolar concentration by 25% in sheep; chronic cocaine exposure does not result in tolerance of the cardiovascular depression produced by isoflurane.

General anesthetic techniques in ruminants

Sedation, anesthesia, protection of the airway during general anesthesia, and control of pain in the perioperative period are important considerations in the management of sheep, goats, and cattle. Though ruminants are classically considered farm animals and are often intended for the production of food and fiber, these species are used extensively in research and teaching and they are increasingly important as companion animals. Whatever their use may be, anesthetic and analgesic drugs and techniques should be used to ensure minimal stress and discomfort during the perioperative period.

Influence of desflurane, isoflurane and halothane on regional tissue perfusion in dogs

The actions of desflurane, isoflurane and halothane on regional tissue perfusion were studied using radioactive microspheres in dogs chronically instrumented for measurement of arterial and left ventricular pressure, global (left ventricular dP/dtmax) and regional (percent segment shortening) contractile function, and diastolic coronary blood flow velocity. Systemic and coronary haemodynamics and regional tissue perfusion were measured in the conscious state and during anaesthesia with equihypotensive concentrations of desflurane, isoflurane, and halothane. All three volatile anaesthetics (P < 0.05) increased heart rate and decreased mean arterial pressure, left ventricular systolic pressure, and left ventricular dP/dtmax Myocardial perfusion was unchanged in subendocardial midmyocardial, and subepicardial regions by the administration of either dose of desflurane. No redistribution of intramyocardial blood flow (endo/epi ratio) was observed during desflurane anaesthesia. Although regional myocardial perfusion was reduced (P < 0.05) in a dose-related fashion by halothane and by isoflurane at high concentrations, redistribution of intramyocardial blood flow was not observed during halothane or isoflurane anaesthesia. All three volatile anaesthetics reduced blood flow to the renal cortex, but only desflurane produced a decrease in renal cortical vascular resistance. Hepatic blood flow decreased in response to halothane but not desflurane or isoflurane. Concomitant decreases in hepatic resistance were observed during administration of desflurane and isoflurane. Dose-related decreases in intestinal and skeletal muscle blood flow were observed during halothane and isoflurane but not desflurane anaesthesia. The results suggest that desflurane maintains myocardial, hepatic, intestinal, and skeletal muscle blood flow while halothane and isoflurane decrease regional tissue perfusion in these vascular beds to varying degrees during systemic hypotension in the chronically instrumented dog.

Clinical investigations of halothane and isoflurane for induction and maintenance of foal anesthesia

Fifty-eight foals were divided into two groups for study of aspects of the clinical anesthetic management of foals and to characterize effects of halothane (n = 30) and isoflurane (n = 28) in foals. There were no significant differences (P greater than 0.05) in the demographics of the two groups. Results of hemograms and biochemical analysis of venous blood samples before and after anesthesia were either not influenced or only mildly (clinically unimportant) affected by either agent. Like adult horses, foals have an increased PaCO2 when anesthetized with inhaled anesthetics. We could detect no difference in the magnitude of increase in PaCO2 with either anesthetic. Anesthetic induction and recovery was most rapid with isoflurane. The quality of induction and recovery was similarly acceptable with either agent. Heart rate during isoflurane was not significantly different from conscious conditions but during halothane, heart rate was significantly less than control except at 91-120 min when statistical significance was not detected. These results support the clinical impression that foals can be safely and reliably anesthetized with either agent.

Hemodynamic alterations and regional myocardial blood flow during supraceliac aortic occlusion in dogs with a critical coronary stenosis

The hemodynamic consequences and myocardial blood flow alterations associated with cross-clamping of the thoracic aorta were studied during pentobarbital (control), halothane (1 MAC), and isoflurane (1 MAC) anesthesia in dogs with a critical stenosis of the left circumflex coronary artery. Aortic clamping at the level of the diaphragm resulted in significant and equivalent increases in mean aortic pressure and left atrial pressure during the control clamp, halothane clamp, and isoflurane clamp periods. Likewise, aortic clamping resulted in a significant and equivalent decrease in cardiac output during control-clamp, halothane clamp, and isoflurane clamp. Myocardial contractility as assessed by dP/dt was depressed during halothane and isoflurane anesthesia when compared with control, but no further change in contractility was associated with aortic clamping. No significant alterations in regional or transmural myocardial blood flow were found with halothane or isoflurane anesthesia, or with aortic clamping during halothane or isoflurane anesthesia. It is concluded that there are significant hemodynamic consequences associated with aortic clamping, that neither halothane nor isoflurane anesthesia alters these consequences when compared with pentobarbital anesthesia alone, and that the deterioration in myocardial function observed during aortic clamping with halothane and isoflurane anesthesia cannot be attributed to any maldistribution of myocardial blood flow.