Materials and Methods

Animals—Ten female and 7 male 1- to 3-year-old Beagles weighing 5.5 to 11.8 kg were included in the study. Dogs were housed individually in an Institute of Laboratory Animal Resources-approved facility, fed a commercial diet^a twice daily, and provided water ad libitum. All dogs were assessed as healthy on the basis of results of physical examinations, CBCs,^b serum biochemical analyses,^c urinalyses that included evaluation of the urine protein-to-creatinine ratio,^d and results of a heartworm antigen test.^e In all dogs, diagnostic radiographic examinations, such as radiography^f of the abdomen and thorax, ultrasonography of the abdomen, and echocardiography,^g were performed to rule out abnormalities of the kidneys and confirm the previous assessments of health status. The study and procedures were approved by the Institute of Laboratory Animal Resources at Seoul National University.

Study design—Dogs were assigned by random number selection to 1 of 3 treatment groups to be administered thiopental (n = 5 dogs), propofol (7), or etomidate (5), respectively, during the induction of anesthesia; anesthesia was then maintained by administration of isoflurane. Food was withheld from all dogs for approximately 12 hours prior to the induction of anesthesia, but ad libitum access to water was permitted to prevent dehydration. Therefore, IV administration of fluids was not performed during anesthesia because of the hydration status of the dogs. No dog was premedicated with any drug prior to the induction of anesthesia. A 22-gauge catheter^h was inserted into each cephalic vein for the administration of contrast medium or an induction agent. Anesthesia was induced with 15 mg of thiopentalⁱ/kg, 6 mg of propofol^j/kg, or 2 mg of etomidate^k/kg and maintained with isoflurane^l evaporated in 100% oxygen. Oxygen flow rate was 1 L/min. After the induction of anesthesia, a 22-gauge catheter was placed in a dorsal pedal artery or femoral artery of each dog for the direct measurement of arterial blood pressure by use of a pressure transducer^m that had been equilibrated to the basal level of the heart while the dogs were positioned in dorsal recumbency on the imaging table of the CT machine. Heart and respiratory rates, arterial blood pressures (ie, SAP, DAP, and MAP), oxygen saturation, and partial pressure of carbon dioxide were monitored continuously by use of a patient monitoring systemⁿ during dynamic CT in all dogs.

Dynamic CT and CT-GFR—Images were obtained approximately 10 minutes after the induction of anesthesia. Hyperventilation procedures were performed 3 times during CT and included an initial renal helical scan for 40 seconds, a single-slice dynamic scan for 2 minutes, and a postcontrast helical scan for 40 seconds. Hyperventilation resulted when the respiratory rate was manually increased to approximately 30 to 40 breaths/min for 1 to 5 minutes to provide enough oxygen and to enable investigators to monitor the condition of each anesthetized dog. Total time spent for each CT-GFR evaluation was approximately 30 minutes from the induction of anesthesia to the conclusion of the postcontrast volume scan. Cardiovascular variables were recorded 2 to 3 times/procedure, which included initiation and completion of each of 4 scans (scout, initial precontrast helical-1, dynamic [cine], and post-contrast helical-2 CT). Scout CT scans are survey images obtained prior to dynamic CT. Initial precontrast helical-1 CT scans are images obtained prior to GFR measurement. Cine scans are images obtained during GFR measurement. Postcontrast helical-2 CT scans are images obtained after GFR measurement.

Imaging was performed with a single-detector CT scanner.^o Image acquisition settings included a helical and single-slice dynamic acquisition, a 512 × 512 matrix, a 25-cm field-of-view display, and a small-scan field of view. Initial precontrast and postcontrast volume scans of both kidneys and the abdominal portion of the aorta were performed by use of helical mode, 120 kVp, 50 mA, 3-mm slice thickness, pitch of 1.3, and 1-mm reconstruction interval for helical images. Single-slice dynamic CT was performed at 1 region with 1–mm-thick slices of the renal hilum of the right kidney at 1.5–second intervals for 2 minutes.

A 1 mL/kg dosage of a 300 mg/mL solution of iohexol^p was administered at the rate of 3 mL/s by use of a previously inserted 22–gauge catheter and an injector system.^q The renal hilum of the right kidney was chosen as the region of interest. Regions of interest of the right kidney were manually drawn to exclude vessels and fatty tissues and highlight the abdominal portion of the aorta. Iohexol clearance per unit volume of the kidney was calculated by use of Patlak plot analysis.^20–22 Patlak plot analyses were performed by use of an in-house computer system that used a commercially available software program^r to plot ∫b(t)dt/b(t) against c(t)/b(t), where b(t) is the corrected CT number in the aorta at time t and c(t) is the corrected CT number for the kidney at time t. Slope and y-intercept of the line were used to indicate GFR (reported as the number of milliliters per second per unit volume of the kidney) and fractional vascular volume (reported as the percentage), respectively (Figure 1).²³

Representative Patlak plot analysis generated by plotting ∫b(t)dt/b(t) against c(t)/b(t) for the determination of CT-GFR for the right kidney of a Beagle administered propofol for induction of anesthesia that was maintained with isoflurane. Slope and y-intercept of the line indicate the GFR (number of milliliters per second per unit volume) and fractional vascular volume (percentage), respectively. b(t) = Corrected CT number in the aorta at time t. c(t) = Corrected CT number for the kidney at time t. HU = Hounsfield unit.

Citation: American Journal of Veterinary Research 72, 1; 10.2460/ajvr.72.1.146

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Representative Patlak plot analysis generated by plotting ∫b(t)dt/b(t) against c(t)/b(t) for the determination of CT-GFR for the right kidney of a Beagle administered propofol for induction of anesthesia that was maintained with isoflurane. Slope and y-intercept of the line indicate the GFR (number of milliliters per second per unit volume) and fractional vascular volume (percentage), respectively. b(t) = Corrected CT number in the aorta at time t. c(t) = Corrected CT number for the kidney at time t. HU = Hounsfield unit.

Citation: American Journal of Veterinary Research 72, 1; 10.2460/ajvr.72.1.146

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Representative Patlak plot analysis generated by plotting ∫b(t)dt/b(t) against c(t)/b(t) for the determination of CT-GFR for the right kidney of a Beagle administered propofol for induction of anesthesia that was maintained with isoflurane. Slope and y-intercept of the line indicate the GFR (number of milliliters per second per unit volume) and fractional vascular volume (percentage), respectively. b(t) = Corrected CT number in the aorta at time t. c(t) = Corrected CT number for the kidney at time t. HU = Hounsfield unit.

Citation: American Journal of Veterinary Research 72, 1; 10.2460/ajvr.72.1.146

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After completion of dynamic CT, a simple post-contrast helical scan was performed to assess the corresponding global renal volume. A kidney was continuously scanned from the cranial to caudal pole (slice thickness, 3 mm). Renal contours were identified within each CT section, and the area within these sections was then summed and multiplied by the slice thickness to calculate a global renal volume. The GFR per unit volume of the kidney (reported as the number of milliliters per minute per unit volume) was multiplied by the renal volume to yield the GFR (number of milliliters per minute) of a single kidney.¹² Iohexol clearance (reported as the number of milliliters per minute) was divided by the weight of the dog to allow for comparisons of a weight-adjusted GFR (reported as the number of milliliters per minute per kilogram).¹

Statistical analysis—Results of variables among treatment groups were reported as mean ± SD. Results were compared by use of a statistical analysis software program.^s Because of the small sample size and lack of normal distribution, Kruskal-Wallis tests were used to compare CT-GFR values among treatment groups. A Shapiro-Wilk normality test was used to analyze cardiopulmonary variables to satisfy the hypothesis of normality, and then a 1-way repeated-measures ANOVA was used for statistical analysis of cardiopulmonary data. When significant differences were detected, a Scheffé post hoc analysis was used to compare differences among treatment groups. A contrast test was used to evaluate significant differences in results over time. A value of P < 0.05 was used to indicate significance for all analyses.

Results

Mean ± SD creatinine (0.75 ± 0.13 mg/dL), BUN (12.35 ± 3.67 mg/dL), phosphorus (6.44 ± 1.17 mg/dL), and albumin (2.91 ± 0.22 mg/dL) concentrations and urine protein-to-creatinine ratios (all ratios were < 0.5) were within reference ranges. Abnormal findings were not observed during the diagnostic imaging examinations. Renal function results of the right kidney obtained after the induction of anesthesia by the administration of thiopental, propofol, and etomidate were summarized (Table 1). Mean GFR results did not differ significantly (P = 0.791) among treatment groups.

Significant (P < 0.01) decreases in MAP over time during dynamic CT were detected, but significant differences were not detected among treatment groups (Table 2). Despite the lack of significant differences, MAPs were decreased in the thiopental treatment group, compared with MAPs in the propofol and etomidate treatment groups. Dogs in the etomidate treatment group had a minor decrease in MAP from 66 to 55 mm Hg during dynamic CT. Dogs in the propofol treatment group had a decrease in MAP from 71 to 55 mm Hg. Dogs in the thiopental treatment group maintained a low MAP of 50 to 53 mm Hg. The MAP results obtained during unenhanced helical scans were significantly decreased among treatment groups.

Significant differences in mean SAP were detected among treatment groups; mean SAP was significantly higher in dogs administered etomidate than in dogs administered thiopental during dynamic CT and CT-GFR (Table 2). All 3 groups had significant (P < 0.01) decreases in mean SAP over time throughout the entire CT study. Significant differences in mean DAP and MAC were not detected among treatment groups and time points analyzed.

Mean heart rate over time was significantly (P < 0.001) decreased in dogs of all treatment groups (Table 2). However, only a slight change in mean heart rate from 109 to 106 beats/min was observed in dogs administered etomidate. Dogs administered thiopental had a noticeable change in mean heart rate from 127 to 119 beats/min, compared with mean heart rate in dogs administered etomidate and propofol (109 to 108 beats/min and 110 to 103 beats/min, respectively).

Discussion

Anesthetic drugs may directly or indirectly affect renal blood flow and thus alter renal function via cardiovascular system changes (such as systemic hypotension and renal vasoconstriction), neuroendocrine activity, or a combination of these.²⁴ Therefore, all anesthetic drugs have the potential to decrease GFR. Effects on renal function can vary according to the anesthetic drug administered, the dose, and the relationship between anesthetic depth and duration of anesthesia. Factors influencing GFR during CT-GFR include the drug used as the induction agent, dose administered, contrast medium, hyperventilation procedure, isoflurane concentration, and total duration of anesthesia required. In the study reported here, effects of 3 anesthetic drugs on GFR were evaluated by use of dynamic CT and Patlak plot analysis; contrary to our hypothesis, remarkable differences in GFR values were not detected among treatment groups when results were combined and analyzed.

When comparisons were made among the 3 anesthetic drugs, etomidate was expected to cause minimal changes to the cardiovascular system and to have the highest CT-GFR values because it does not depress the cardiovascular and respiratory systems.¹⁵ Observed effects on the cardiovascular system were as expected and included a minimal decrease in blood pressure and heart rate; however, GFR in dogs administered etomidate was similar to that of dogs administered propofol and thiopental. Mild decreases in heart rate and arterial blood pressures were observed as anticipated in dogs administered propofol, but no significant difference in GFR was detected among treatment groups. An MAP < 55 mm Hg, an SAP < 85 mm Hg, and a heart rate that was initially increased were observed in dogs administered thiopental. Investigators of several studies^17,25,26 have reported that thiobarbiturates induce tachycardia in dogs, but marked decreases in MAP were observed in the present study. Thiopental has been reported²⁴ to maintain or slightly decrease GFR; however, GFR in dogs administered thiopental in the study reported here was within the reference interval. Regardless of the observed differences in systemic hypotension among treatment groups, significant differences in CT-GFR values were not detected. This lack of significant differences could have been the result of autoregulation of blood flow by the kidneys, which reportedly^27–29 maintains a constant GFR under variable systemic blood pressures (70 to 180 mm Hg) by controlling vasoconstriction or vasodilation of afferent and efferent arterioles of the glomeruli.

Administration of a nephrotoxic contrast medium could influence renal function during anesthesia.²⁴ Although the pathogenesis of contrast medium-induced nephropathy is not fully understood, this is most likely the result of prolonged vasoconstriction of arterioles and impaired autoregulation of blood flow that predisposes the renal medulla to hypoxia in combination with direct cytotoxic effects.³⁰ Contrast medium-induced nephropathy has been reported³¹ to result in dose-dependent reversible vasoconstriction in dogs. The median effective dose (ie, dose that induces the desired effect in 50% of a population) for vasoconstriction of the renal artery of dogs is 56.2 mg of iohexol/mL. Therefore, the dose of iohexol administered in the present CT-GFR evaluation had the potential to cause contrast medium-induced nephropathy. However, it has been reported³² that dogs have a lower incidence of contrast medium-induced nephropathy, compared with the incidence of this condition in humans.

Performing a hyperventilation procedure prior to CT-GFR is necessary to prevent motion during the imaging procedure that could lead to the collection of inaccurate data over multiple time points. An artificially increased respiratory rate was expected to alter renal function in combination with secondary effects on the cardiopulmonary system. However, according to the results of another study,³³ passive hyperventilation that leads to a short duration of respiratory alkalosis does not appear sufficient to alter renal hemodynamics, to result in changes of GFR, or to affect renal plasma flow. Three 3- to 5-minute hyperventilation periods were performed during the present study. However, we believe this short-term hyperventilation period did not directly affect CT-GFR results.

Isofurane was used for the maintenance of anesthesia during dynamic CT. The inhalation of isoflurane has been reported²⁴ to cause decreases in renal blood flow and GFR in a dose-dependent manner. Although isoflurane has little effect on renal blood flow, it is reported³⁴ to decrease GFR and urine output. In the study reported here, a significant decrease in cardiovascular system variables (such as heart rate, MAP, and SAP) over time was detected among treatment groups. Although there was a slight difference in the concentration of isoflurane delivered to the dogs, significant differences in MAC were not detected among treatment groups.

Limitations exist for the measurement of GFR by the use of dynamic CT and Patlak plot analysis. The single-slice CT technique has limitations because it requires the extrapolation of GFR values from a small region to provide the GFR for an entire kidney.^1,12,20,21 Furthermore, thin slice thickness of the CT-GFR dynamic scan was a limitation observed during the study reported here.

In conclusion, analysis revealed that dynamic CT of the kidneys can be a valuable procedure for the evaluation of the effects of anesthetic drugs on renal function in dogs. Therefore, CT could be used widely because of the advantages of its functional application in veterinary medicine and reproducible measurement of GFR. Significant differences in CT-GFR values were not detected among treatment groups in the study reported here. However, thiopental may induce hypotension after administration. Etomidate is recommended for the induction of anesthesia in geriatric patients or patients with cardiovascular disease because it has minimal effects on cardiopulmonary variables. However, the use of etomidate may be associated with a higher number of adverse events during induction and a more difficult recovery from anesthesia.¹⁹ Propofol is recommended for the induction of anesthesia in young healthy dogs undergoing CT-GFR because of its ability to cause smooth and rapid induction and uncomplicated recovery.