Cardiovascular effects of volatile anesthesia in rabbits: influence of chronic heart failure and enalaprilat treatment

The effects of recruitment of renal functional reserve on renal cortical and medullary oxygenation in non-anesthetized sheep

AIM

Recruitment of renal functional reserve (RFR) with amino acid loading increases renal blood flow and glomerular filtration rate. However, its effects on renal cortical and medullary oxygenation have not been determined. Accordingly, we tested the effects of recruitment of RFR on renal cortical and medullary oxygenation in non-anesthetized sheep.

METHODS

Under general anesthesia, we instrumented 10 sheep to enable subsequent continuous measurements of systemic and renal hemodynamics, renal oxygen delivery and consumption, and cortical and medullary tissue oxygen tension (PO2 ). We then measured the effects of recruitment of RFR with an intravenous infusion of 500 ml of a clinically used amino acid solution (10% Synthamin® 17) in the non-anesthetized state.

RESULTS

Compared with baseline, Synthamin® 17 infusion significantly increased renal oxygen delivery mean ± SD maximum increase: (from 0.79 ± 0.17 to 1.06 ± 0.16 ml/kg/min, p < 0.001), renal oxygen consumption (from 0.08 ± 0.01 to 0.15 ± 0.02 ml/kg/min, p < 0.001), and glomerular filtration rate (+45.2 ± 2.7%, p < 0.001). Renal cortical tissue PO2 increased by a maximum of 26.4 ± 1.1% (p = 0.001) and medullary tissue PO2 increased by a maximum of 23.9 ± 2.8% (p = 0. 001).

CONCLUSIONS

In non-anesthetized healthy sheep, recruitment of RFR improved renal cortical and medullary oxygenation. These observations might have implications for the use of recruitment of RFR for diagnostic and therapeutic purposes.

Mediators of Regional Kidney Perfusion during Surgical Pneumo-Peritoneum Creation and the Risk of Acute Kidney Injury—A Review of Basic Physiology

Acute kidney injury (AKI), especially if recurring, represents a risk factor for future chronic kidney disease. In intensive care units, increased intra-abdominal pressure is well-recognized as a significant contributor to AKI. However, the importance of transiently increased intra-abdominal pressures procedures is less commonly appreciated during laparoscopic surgery, the use of which has rapidly increased over the last few decades. Unlike the well-known autoregulation of the renal cortical circulation, medulla perfusion is modulated via partially independent regulatory mechanisms and strongly impacted by changes in venous and lymphatic pressures. In our review paper, we will provide a comprehensive overview of this evolving topic, covering a broad range from basic pathophysiology up to and including current clinical relevance and examples. Key regulators of oxidative stress such as ischemia-reperfusion injury, the activation of inflammatory response and humoral changes interacting with procedural pneumo-peritoneum formation and AKI risk will be recounted. Moreover, we present an in-depth review of the interaction of pneumo-peritoneum formation with general anesthetic agents and animal models of congestive heart failure. A better understanding of the relationship between pneumo-peritoneum formation and renal perfusion will support basic and clinical research, leading to improved clinical care and collaboration among specialists.

Renal oxygenation during the early stages of adenine-induced chronic kidney disease

To assess whether renal hypoxia is an early event in adenine-induced chronic kidney disease, adenine (100 mg) or its vehicle was administered to male Sprague-Dawley rats by daily oral gavage for 7 days. Kidney oxygenation was assessed by 1) blood oximetry and Clark electrode in thiobutabarbital-anesthetized rats, 2) radiotelemetry in unanesthetized rats, and 3) expression of hypoxia-inducible factor (HIF)-1α and HIF-2α protein. After 7 days of treatment, under anesthesia, renal O2 delivery was 51% less, whereas renal O2 consumption was 65% less, in adenine-treated rats than in vehicle-treated rats. Tissue Po2 measured by Clark electrode was similar in the renal cortex but 44% less in the medulla of adenine-treated rats than in that of vehicle-treated rats. In contrast, in unanesthetized rats, both cortical and medullary tissue Po2 measured by radiotelemetry remained stable across 7 days of adenine treatment. Notably, anesthesia and laparotomy led to greater reductions in medullary tissue Po2 measured by radiotelemetry in rats treated with adenine (37%) than in vehicle-treated rats (16%), possibly explaining differences between our observations with Clark electrodes and radiotelemetry. Renal expression of HIF-1α was less after 7 days of adenine treatment than after vehicle treatment, whereas expression of HIF-2α did not differ significantly between the two groups. Renal dysfunction was evident after 7 days of adenine treatment, with glomerular filtration rate 65% less and serum creatinine concentration 183% greater in adenine-treated rats than in vehicle-treated rats. Renal cortical tissue hypoxia may not precede renal dysfunction in adenine-induced chronic kidney disease and so may not be an early pathological feature in this model.

Long-term measurement of renal cortical and medullary tissue oxygenation and perfusion in unanesthetized sheep

The role of renal cortical and medullary hypoxia in the development of acute kidney injury is controversial, partly due to a lack of techniques for the long-term measurement of intrarenal oxygenation and perfusion in conscious animals. We have, therefore, developed a methodology to chronically implant combination probes to chronically measure renal cortical and medullary tissue perfusion and oxygen tension (tPo2) in conscious sheep and evaluated their responsiveness and reliability. A transit-time flow probe and a vascular occluder were surgically implanted on the left renal artery. At the same operation, dual fiber-optic probes, comprising a fluorescence optode to measure tPo2 and a laser-Doppler probe to assess tissue perfusion, were inserted into the renal cortex and medulla. In recovered conscious sheep ( n = 8) breathing room air, mean 24-h cortical and medullary tPo2 were similar (31.4 ± 0.6 and 29.7 ± 0.7 mmHg, respectively). In the renal cortex and medulla, a 20% reduction in renal blood flow (RBF) decreased perfusion (14.6 ± 8.6 and 41.2 ± 8.5%, respectively) and oxygenation (48.1 ± 8.5 and 72.4 ± 8.5%, respectively), with greater decreases during a 50% reduction in RBF. At autopsy, minimal fibrosis was observed around the probes. In summary, we have developed a technique to chronically implant fiber-optic probes in the renal cortex and medulla for recording tissue perfusion and oxygenation over many days. In normal resting conscious sheep, cortical and medullary tPo2 were similar. The responses to and recovery from renal artery occlusion, together with the consistent measurements over a 24-h period, demonstrate the responsiveness and stability of the probes.

Losartan increases renal blood flow during isoflurane anesthesia in sheep

BACKGROUND

Inhaled anesthetics cause a transient reversible depression of renal function by direct renal effects or indirectly by changes in neurohumoral systems or cardiovascular performance. When the sympathetic nervous activity is decreased during anesthesia, other vasoactive systems like vasopressin (AVP) and particularly the renin angiotensin system (RAS) are of importance for blood pressure maintenance. Little is known about how the renal circulation is affected by angiotensin receptor blockade during isoflurane anesthesia.

METHODS

The study was performed on isoflurane anesthetized sheep equipped with flow probes (placed around a femoral and a renal artery) and a pulmonary artery catheter. During stable conditions the sheep were given one or more of the following substances: isotonic saline (NaCl); losartan (LOS) 10 mg x kg(-1); prazosin (PRAZ) 0.2 mg x kg(-1) and a vasopressin V1-receptor antagonist (AVP-a) 10 microg x kg(-1).

RESULTS

LOS and AVP-a did not affect mean arterial pressure (MAP), whereas PRAZ lowered MAP significantly (from 98+/-12 to 65+/-7 mmHg). Renal blood flow (RBF) increased after LOS treatment (148+/-34 to 222+/-33 ml x min(-1)). The other substances were without effect on RBF. Femoral blood flow remained unchanged after all treatments.

CONCLUSION

We conclude that the sympathoadrenal system is still the major determinant for blood pressure maintenance during isoflurane anesthesia in sheep. The apparently increased activity of the renin angiotensin system in this situation causes a reduction in renal blood flow, which is counteracted by angiotensin II AT1-receptor blockade.

Therapeutics for rabbits

A variety of pharmacologic agents are currently available to treat house rabbits. In many cases, dosages are based on extrapolation from other species or empirical data. Dosing in rabbits is further complicated by individual variation. An understanding of rabbit physiology and the pharmacology of prescribed medications helps ensure that the agents are used as effectively and safely as possible. In this article, basic rabbit pharmacobiology is reviewed and techniques for drug administration are described. A formulary for house rabbits is provided.

The effects of isoflurane and halothane on left ventricular afterload in dogs with dilated cardiomyopathy

The effects of volatile anesthetics, including isoflurane (ISO) and halothane (HAL), on determinants of left ventricular (LV) afterload have not been comprehensively described in experimental models of, or patients with, heart failure. We tested the hypothesis that ISO and HAL produce beneficial alterations in LV afterload when evaluated with aortic input impedance and interpreted using a three-element Windkessel model in dogs before and after development of pacing-induced cardiomyopathy. Hemodynamics and aortic pressure and blood flow waveforms were recorded in the conscious state and during 1.1- and 1.5-minimum alveolar anesthetic concentration (MAC) ISO and HAL anesthesia on separate days in chronically instrumented dogs (n = 6). Dogs were then paced at 220-240 bpm for 20 +/- 3 days (mean = SEM) to develop cardiomyopathy, and the experiments were repeated after pacing had been temporarily discontinued. ISO decreased mean arterial pressure (MAP), mean aortic blood flow (MAQ), and total arterial resistance (R) and increased total arterial compliance (C) and characteristic aortic impedance (Zc) in dogs before pacing. HAL decreased MAP and MAQ and increased C but did not alter R and Zc. Chronic rapid LV pacing increased HR and LV end-diastolic pressure and decreased MAP, LV systolic pressure, and the peak rate of increase of LV pressure. MAQ, C, R, and Zc were unchanged. ISO and HAL decreased arterial pressure but did not affect C and Zc in the presence of LV dysfunction. HAL, but not ISO, increased R at 1.1 MAC, which indicates that this drug increases resistance to LV ejection. In contrast to findings in normal dogs, these results indicate that neither ISO nor HAL reduce arterial hydraulic resistance to LV ejection or favorably improve the rectifying properties of the aorta in dogs with pacing-induced cardiomyopathy.

IMPLICATIONS

Isoflurane and halothane produce favorable alterations in the determinants of left ventricular afterload before, but not after, the production of experimental left ventricular dysfunction by sustained, rapid cardiac pacing in chronically instrumented dogs.

Impaired antioxidant defence in guinea pig heart tissues treated with halothane

PURPOSE

To investigate the effects of halothane and halothane plus vitamin E treatment on myocardial free radical metabolism in guinea pigs.

METHODS

Four groups of seven animals were studied: control, halothane, halothane plus vitamin E and vitamin E groups. In the halothane group, halothane 1.5% in oxygen was given for 90 min over three days. In the halothane plus vitamin E group, 300 mg.kg-1.day-1 vitamin E im was started three days before the first halothane treatment and continued for three days. Following sacrifice, the hearts were assayed for superoxide dismutase (SOD), glutathione peroxidase (GSH-Px) and catalase (CAT) and malondialdehyde (MDA) level was determined. Electron spin resonance (ESR) analysis and electron microscopy (EM) were also performed.

RESULTS

In the halothane group, SOD activities and MDA concentrations were increased compared with control and GSH-Px and CAT activities were decreased. In the halothane plus vitamin E group, there were no differences in enzyme activity compared with halothane alone but the MDA level was decreased. In the vitamin E group, enzyme activities were increased compared with control. Mainly the CF3CHCl radical was identified by ESR analysis in heart tissues exposed to halothane and the concentration of this radical was reduced by vitamin E. Electron microscopy showed cytoplasmic vacuolisation and dilation in sarcoplasmic reticulum in the heart tissues exposed to halothane: both were prevented by vitamin E.

CONCLUSION

Although halothane causes impairment in enzymatic antioxidant defence potential, due to lowered GSH-Px and CAT activity, and accelerates peroxidative reactions in the tissues affected, no subcellular damage occurred. Vitamin E may protect tissues against free radical attack by scavenging toxic free radicals formed in heart tissue during halothane anaesthesia.

Hemodynamic effects of anesthesia in patients with ischemic heart failure chronically treated with angiotensin-converting enzyme inhibitors

Anesthesia may induce hemodynamic instability in patients treated with angiotensin-converting enzyme inhibitors (ACEIs). To assess the hemodynamic effects of anesthesia in patients treated (n = 9) or not treated (n = 9) with ACEIs for ischemic left ventricle dysfunction after myocardial infarction, we studied 18 patients scheduled for elective coronary artery bypass graft surgery. Induction of anesthesia with fentanyl (5 micrograms/kg), flunitrazepam (30 micrograms/kg), and pancuronium (100 micrograms/kg) was followed by a significant decrease in mean arterial blood pressure in both groups (-18.6% +/- 8.1% in controls and -25.7% +/- 7.8% in ACEI-treated patients, P = 0.01). In controls, cardiac index and systemic vascular resistance were not significantly altered (-11.2% +/- 9.4% and -16.2% +/- 4.6%, respectively, not significant [NS]). In ACEI-treated patients, cardiac index decreased significantly (-27.3% +/- 11.6%, P = 0.01 from baseline and P = 0.03 when compared with controls), and systemic vascular resistance was unchanged (1.0% +/- 18.7%, NS from baseline and P = 0.04 when compared with controls). Two patients from each group experienced a transient severe hypotensive episode. ACEI treatment in patients with infarction-induced myocardial dysfunction does not increase the incidence of severe hypotension after induction of anesthesia.

Effects of isoflurane on cardiovascular system and sympathovagal balance in New Zealand white rabbits

We investigated the effects of isoflurane on the rabbit cardiovascular system at several end-tidal concentrations. Furthermore, because isoflurane has been reported to produce tachycardia while reducing sympathetic nervous activity and baroreflex function, we evaluated whether the chronotropic effects of isoflurane could be due to a vagal withdrawal. ECG, mean arterial pressure (MAP), and heart rate (HR) were obtained in rabbits the conscious, unsedated state and during isoflurane anesthesia by telemetric device. Measurements of pH, oxygen, carbon dioxide, plasma catecholamines, baroreflex sensitivity, and spectral analysis of HR variability were made in nonanesthetized and anesthetized animals. Isoflurane caused an increase in HR at 0.5, 1, and 1.5 minimum alveolar concentration (MAC) and a decrease in systolic and diastolic blood pressure (SBP, DBP) and MAP at 1 and 1.5 MAC. Biochemical analysis showed that isoflurane-mediated cardiovascular effects were not accompanied by any significant changes in plasma norepinephrine (NE) and epinephrine (Epi) levels. Neither were any significant differences in plasma catecholamine levels noted between anesthetized and awake animals. The analysis of spectral components of HR variability and baroreflex function indicated that isoflurane induced a marked reduction in the low- and high-frequency spectral power of HR variability and in baroreflex sensitivity. Tachycardia under isoflurane was suppressed dose dependently by the administration of clonidine or atenolol and was not influenced by bilateral vagotomy. Collectively, our results indicate that cardiovascular effects induced by isoflurane in smaller animals such as rabbits are similar to those observed in humans and other animal species. We showed that isoflurane-induced tachycardia is mainly the result of vagal withdrawal rather than a baroreflex response, even though a marginal role of baroreflex in heart response to higher concentrations of isoflurane cannot be excluded.

Angiotensin-converting enzyme inhibitors in cardiovascular anesthesia

Besides the long-term regulation of extracellular fluid volume, the RAS plays an important physiologic role in maintaining venous return and blood pressure during acute hemodynamic stresses. ACE inhibitors may therefore alter venous return and cardiac output regulation during anesthesia and surgery. This may be regarded as a drawback of ACE inhibition when other factors interfere with cardiovascular homeostasis; deleterious hemodynamic events may therefore occur when blood volume is decreased, which may be frequent during cardiovascular anesthesia and surgery. However, the alternative solution should not be to stop ACE inhibitors preoperatively. This would allow recovery of RAS control of blood pressure, but at the expense of some regional circulations. From this point of view, preliminary results from early studies during cardiovascular anesthesia and surgery showing redistribution of regional blood flow with inhibition of ACE are encouraging; whether postoperative outcome can be improved deserves further studies. At this time, the evidence is that ACE inhibition does not allow the anesthesiologist to be tolerant of hypovolemia.