Hypoxia or hypercapnia elicits cardiovascular responses associated with increased plasma catecholamine concentrations, whereas clonidine, an alpha(2)- adrenergic agonist, decreases plasma catecholamine concentrations. The authors examined whether systemic clonidine administration would alter the hemodynamic and catecholamine responses to hypoxia or hypercapnia in anesthetized dogs.
Pentobarbital-anesthetized dogs whose lungs were mechanically ventilated were instrumented for measurement of mean arterial pressure, heart rate, mean pulmonary artery pressure, right atrial pressure, cardiac output, left ventricular end-diastolic pressure, and the peak of first derivative of left ventricular pressure. The dogs were randomly assigned to receive an intravenous bolus injection of 10 microg/kg clonidine followed by continous infusion at a rate of 1 microg. kg (-1). min (-1)(clonidine-10 group, n = 7), an intravenous bolus injection of microg/kg clonidine followed by continuous infusion at a rate of 0.5 micro.kg(-).min(-1)(clonidine-5 group, n = 7), or an equivalent volume of 0.9% saline (control group = 7). Each dog underwent random challenges of hypoxia (PaO2 30, 40, and 50 mmHg) and hypercapnia (PaCO2 60, 80, and 120 mmHg). Measurements of hemodynamic and plasma norepinephrine and epinephrine concentrations were made after the loading dose of clonidine and the first and the second exposure of hypoxia or hypercapnia.
Although significant increases from prehypoxic values in mean arterial pressure (39 +/- 10 mmHg) and plasma norepinephrine (291 +/- 66 pg/ml) and epinephrine (45 +/- 22 pg/ml) concentrations were noted during hypoxia of PaO2 30, mmHg in the control group (P<0.05), such changes were absent in both clonidine groups. During hypercapnia of PaCo2 120 mmHg, changes from prehypercapnic values in mean arterial pressure, mean pulmonary artery pressure, the peak of first derivative of left ventricular pressure, and plasma norepinephrine and epinephrine concentrations in the clonidine-10 and clonidine-5 groups were significantly less than those in the control group. Plasma clonidine concentrations in the clonidine-10 and clonidine-5 groups were 16.8 +/- 1.7 and 8.9 =/- 1.0, 42.5 =/- 2.9 and 21.5 +/- 1.5, and 51.1 +/- 3.2 and 26.7 +/- 1.0 ng/ml after the loading dose of clonidine and the first and the second exposure of hypoxia or hypercapnia, respectively.
Systemic clonidine administration alter the hemodynamic changes associated with hypoxia or hypercapnia and suppresses plasma catecholamine responses in anesthetized dogs when a larger dose of clonidine is administered. catecholamines: epinephrine; norepinephrine.)
HYPOXIA or hypercapnia elicits cardiovascular responses usually associated with increased plasma catecholamine concentrations. [1–5]These changes are blunted by regional sympathetic blockade due to epidural anesthesia. [6,7]Based on these previous findings, more extensive sympathetic blockade, such as systemic administration of potent sympatholytic drugs, is likely to attenuate the cardiovascular and hormonal compensatory responses associated with hypoxia or hypercapnia.
Clonidine, a preferential alpha2-adrenergic agonist, has been widely used as an anesthetic adjuvant. However, because clonidine decreases sympathetic nervous activity, plasma norepinephrine concentration, and the sympathoadrenal responses, one should be concerned about the impairment of circulatory adjustments after systemic administration of clonidine, especially when administered in a larger dose. Under certain circumstances, a large dose of clonidine would be detrimental, because it could abolish the cardiovascular and hormonal adjustments in the face of hypoxia or hypercapnia. Alternatively, profound circulatory deterioration would occur during severe hypoxia or hypercapnia in humans or animals receiving pretreatment with a large dose of clonidine.
In the current experiment, we tested the hypothesis that systemic clonidine administration would alter the hemodynamic and plasma catecholamine changes associated with hypoxia or hypercapnia in anesthetized dogs.
Materials and Methods
Animal Preparation and Instrumentation
Investigations were carried out in 21 adult mongrel dogs of both sexes weighing 12–24 kg. The study protocol was approved by the Institutional Animal Care and Use Committee. Animals were anesthetized with 30 mg/kg intravenous pentobarbital, and the trachea was intubated using a cuffed tracheal tube. Anesthesia was maintained with continuous infusion of pentobarbital and pancuronium at a rate of 6 and 0.1 mg *symbol* kg sup -1 *symbol* h sup -1, respectively. The lungs were mechanically ventilated with air and supplemental oxygen if needed via a nonrebreathing circuit, using a volume-cycled animal ventilator (model 613, Harvard Respirator, South Natick, MA). Arterial oxygen tension (Pa sub O2) and carbon dioxide tension (PaCO2) were maintained above 70 mmHg and at approximately 40 mmHg during baseline measurements by adding supplemental oxygen and by adjusting respiratory rate between 14 and 20 breaths/min with 15 ml/kg of tidal volume, respectively. Inspiratory and expiratory gases were sampled via a sampling tube placed in the distal end of the tracheal tube to permit continuous monitoring of the inspiratory and end-expiratory concentrations of oxygen and carbon dioxide with a multigas analyzer (Capnomac ULTIMA, Datex, Helsinki, Finland). During all measurements, pulmonary arterial blood temperature was continuously measured by a thermistor of a pulmonary artery catheter connected to a cardiac output (CO) computer (model 7350, Arrow International, Reading, PA) and was maintained at 37+/-1 degree Celsius using a heating operating table. Arterial blood samples were collected into heparinized tubes for analysis of arterial blood gases. Arterial blood was analyzed for pHa, PaCO2, PaO2, and base deficit by a self-calibrating electrodes system (288 Blood Gas System, Ciba-Corning, Medfield, MA).
An intravenous cannula was placed in a forelimb vein for constant administration of lactated Ringer's solution at a rate of 5 ml *symbol* kg sup -1 *symbol* h sup -1. Lead II of an electrocardiogram (model 2236A, NEC San-ei Instruments, Tokyo, Japan) was monitored with subcutaneous electrodes in the legs. Heart rate was measured continuously on a beat-to-beat basis by a cardiotachometer (model 1321, NEC San-ei Instruments) triggered by lead II of the electrocardiogram. Arterial pressure measurements and samples for blood gas analysis were obtained from both femoral arterial catheters. A flow-directed, balloon-tipped pulmonary artery catheter (7-Fr Arrow Balloon Thermodilution Catheter, Arrow International) was inserted via the right external jugular vein, and its tip was advanced into the pulmonary artery for continuous monitoring of pulmonary artery (PAP) and right atrial pressures (RAP). CO was obtained by averaging triplicate thermodilution values obtained with 5 ml of 5% dextrose at room temperature. After an 8-Fr catheter-tipped transducer (model SPC-380, Millar Instruments, Houston, TX) had been soaked in 0.9% saline solution for 30 min, it was calibrated with 0.9% saline solution shielded from ambient light in vitro before insertion. This catheter was inserted through the right common carotid artery and positioned in the left ventricle to obtain left ventricular pressure, left ventricular end-diastolic pressure (LVEDP), and an instantaneous rate of rise in the left ventricular pressure (LV dP/dt) with the use of a differential amplifier (model 1323, NEC San-ei Instruments). Measurements of arterial pressure, PAP, and RAP were made with calibrated transducers (Uniflow, Baxter Health Care, Irvine, CA). The dogs were placed supine during the measurements, and the zero reference was leveled at the midchest. Both mean arterial pressure (MAP) and mean pulmonary artery pressure (MPAP) were determined electronically. Systemic vascular resistance (SVR) was calculated as (MAP - RAP)*symbol* CO sup -1 x 80, and pulmonary vascular resistance as (MPAP - LVEDP)*symbol* CO sup -1 x 80.
Pretreatment with Clonidine or Saline
After all dogs were allowed a stabilization period of at least 60 min after surgical preparation, baseline hemodynamic and plasma catecholamine measurements were made. The dogs randomly received an intravenous dose of 10 micro gram/kg clonidine (clonidine-10 group, n = 7), 5 micro gram/kg clonidine (clonidine-5 group, n = 7), or 1 ml/kg 0.9% saline (control group, n = 7) over 1 min. Clonidine hydrochloride (Sigma Chemical, St. Louis, MO) was dissolved in 0.9% saline solution to a concentration of 10 micro gram/ml for the clonidine-10 group or 5 micro gram/ml for the clonidine-5 group. The next set of measurements of hemodynamic and plasma catecholamine and clonidine concentrations was made after stable hemodynamic state was obtained for at least 10 min after the loading dose of clonidine or saline. These variables were adopted as baseline values before the first hypoxic or hypercapnic challenge as described below. Thereafter, clonidine was continuously infused at a rate of 1 and 0.5 micro gram *symbol* kg sup -1 *symbol* min sup -1 in the clonidine-10 and clonidine-5 groups, respectively, whereas the control group received continuous infusion of 0.9% saline at a rate of 0.1 ml *symbol* kg sup -1 *symbol* min sup -1.
Hypoxic or Hypercapnic Challenge
After baseline measurements during normoxia (after loading dose of clonidine or saline), hypoxic hypoxia was induced by adding nitrogen (0.2–3.3 l/min) to the inspiratory gases, so that arterial oxygen partial pressure (PaO2) attained approximately 30 (hypoxia-30), 40 (hypoxia-40), and 50 mmHg (hypoxia-50). The same set of hemodynamic and catecholamine measurements was repeated after equilibrating for at least 10 min at each level of hypoxia (PaO230, 40, and 50 mmHg).
Hypercapnia was induced by adding carbon dioxide (0.05–1.2 l/min) to the inspiratory gases, so that arterial carbon dioxide partial pressure (PaCO2) attained approximately 60 (hypercapnia-60), 80 (hypercapnia-80), and 120 mmHg (hypercapnia-120). During hypercapnic challenge supplemental oxygen (0.05–0.5 l/min) was added to inspiratory gases to maintain normoxia (PaO2> 70 mmHg). The same set of hemodynamic and catecholamine measurements was repeated at each level of hypercapnic (PaCO260, 80, and 120 mmHg) after obtaining an at least 10 min-stabilization period.
The sequences of hypoxic or hypercapnic challenge and those of three hypoxic or hypercapnic periods were randomized in each dog. After completion of three periods of hypoxic or hypercapnic challenge, another set of hemodynamic and plasma catecholamine measurements was made again during normoxia or normocapnia, and these values served as the baseline values before the next hypercapnic or hypoxic challenge in each dog. Plasma clonidine concentrations were also measured after the first and the second exposure of hypoxia or hypercapnic.
Measurement of Plasma Catecholamine and Clonidine Concentrations
Arterial blood samples were collected into tubes containing sodium ethylenediamine tetraacetic acid and immediately placed on ice for analysis of plasma catecholamine and clonidine concentrations. Plasma for catecholamine and clonidine assays was quickly separated by centrifugation and stored frozen at -40 degrees C until assayed in a week.
Free concentrations of plasma catecholamines were analyzed by a fully automated high-performance liquid chromatography using a diphenylethylenediamine condensation method (model HLC-725 catecholamine analyzer, Tosoh, Tokyo, Japan). Six hundred microliters of plasma was mixed with 300 micro liter of a 6% perchloric acid solution by a vortex-mixer for deproteination. After adding 200 micro liter of 1.5 mol/l sodium acetate, the mixture was stirred and centrifuged (14,000 rpm) at 4 degrees C for 20 min. The clear supernatant was applied to the autosampler of the high-performance liquid chromatography analyzer. Absorbed catecholamine was eluted and delivered to analytical column (TSK gel, Catecholpak 6 x 150 mm). The separated catecholamines were delivered to a reaction coil (90 degrees C) with fluorogenic reagent D (diphenylethylenediamine in 50% ethanol solution) as well as reagent E (ethanol, potassium hexacyanoferrate, boric acid, ascorbic acid), and the catecholamines were converted to diphenylethylenediamine derivatives. The fluorescence intensity of the eluate from the reaction unit was measured in a detector at 470 nm wavelength with an excitation wavelength at 355 nm. The detection limit of both norepinephrine and epinephrine was 5 pg/ml. The coefficients of variation for measurements of norepinephrine and epinephrine were 1.77% and 1.37%, respectively.
Plasma clonidine concentrations were determined by a specific radioimmunoassay using3Hydrogen-clonidine (Nippon Boehringer Ingelheim Kawanishi Institute). Briefly, 0.1 M phosphate buffer (pH 7.4), rabbit anticlonidine antibody containing 0.5% bovine serum albumin, and sup 3 Hydrogen-clonidine (NEN) were added to 200 micro liter volumes of calibration standards (25–1,000 pg/ml) or plasma samples containing unknown concentrations of clonidine. They were mixed in a vortex-mixer, and the mixture was incubated overnight at room temperature. After adding 400 micro liter of saturated ammonium sulfate solution (pH 7.4), the mixture was centrifuged for 15 min at 3,000 rpm. The supernatant was added to 5 ml of scintillator (aquazol-2, NEN), and the radioactivity was measured by a liquid scintillation counter (LS 5801, Beckman, Fullerton, CA). The minimal detectable concentration of clonidine was 0.05 ng/ml. The coefficient of variation for measurements of clonidine was 4.87%.
All recordings were made on the eight-channel recorder (model RECTI-HORIZ-8K, NEC San-ei Instruments) throughout the investigation. All values were expressed as mean+/-SEM. Continuous variables (hemodynamic and blood gas data) were analyzed by repeated-measures analysis of variance followed by paired Student's t test in each group for comparison among prehypoxic or prehypercapnic values and hypoxic or hypercapnic values of three degrees. A two-way analysis of variance for repeated measures was used for intergroup comparisons during the same degree of hypoxia or hypercapnia. When analysis of variance indicated a significant difference, Bonferroni's multiple-comparisons test was used to determine which groups were significantly different from each other. Because values of plasma catecholamine concentrations are not normally distributed data, nonparametric analysis of variance by Friedman ranks was used. Individual differences were assessed by the Wilcoxon's signed-rank test. Results were considered statistically significant at P < 0.05.
The average body weight of dogs was comparable among groups (15.8 +/-1.6, 15.1+/-0.3, and 15.6+/-0.9 kg in the control, clonidine-10, and clonidine-5 groups, respectively).
Effects of Loading Doses of Saline or Clonidine
After clonidine administration, heart rate, CO, LV dP/dt, plasma norepinephrine and epinephrine concentrations, and PaO2decreased, whereas RAP, LVEDP, and SVR increased (Table 1). Significant increases in MPAP after loading doses of clonidine and saline were noted in the clonidine-5 and control groups, but not in the clonidine-10 group, because MPAP decreased along with a great reduction in CO in one dog. However, there was no difference in any variable after clonidine administration between the two clonidine groups.
Responses to Hypoxic Challenge
The prehypoxic values of heart rate, MPAP, CO, LVEDP, and plasma norepinephrine concentrations in the clonidine groups were different from those in the control group (Table 2). The magnitudes of PaO2reductions during hypoxia were similar among groups (Table 3).
During hypoxia, dose-dependent changes in MAP were noted in the control group (P < 0.05 among periods), and MAP increased by 39+/- 10 mmHg during the hypoxia-30 period (Figure 1). However, the pressor response during the hypoxia-30 period was absent in both clonidine groups (P < 0.05 vs. the control group). MPAP increased during the hypoxia-30 and -40 periods in the control and clonidine-10 groups, and the MPAP change was greater during the hypoxia-30 period than during the hypoxia-50 period in the control group (P < 0.05). SVR fell during the hypoxia-30 and -40 periods in the clonidine groups, but it remained unchanged during hypoxia in the control group. The SVR change during the hypoxia-30 period was greater in the clonidine-10 group than in the control group. CO increased during the hypoxia-30 period in both clonidine groups but not in the control group, because CO remained unchanged in one dog. There were no differences between the two clonidine groups in changes of any hemodynamic variables during each period of hypoxia (Figure 1).
Plasma norepinephrine concentrations increased in a dose-dependent fashion during hypoxia in the control group (P < 0.05 among periods) and increased during the hypoxia-30 period in the clonidine-5 group (Figure 2). However, such an increase was not observed in the clonidine-10 group, resulting in significant differences among groups. Although plasma epinephrine concentrations also increased in a dose-related manner during hypoxia in the control group (P < 0.05 among periods), they showed no change or slight increases in the clonidine groups, resulting in significant intergroup differences.
Responses to Hypercapnic Challenge
Because progressive hypotension and cardiac arrest developed in one dog of each clonidine group immediately after inhalation of carbon dioxide, the remaining six dogs from each clonidine group were subjected to data analysis. The prehypercapnic values of MPAP, SVR, LVEDP, and plasma norepinephrine concentrations in the clonidine groups were different from those in the control group (Table 4). The magnitude of Pa sub CO2increases during hypercapnia was similar among groups, except during the hypercapnia-120 period (Table 5).
During hypercapnia, MAP, SVR, and LV dP/dt decreased markedly in both clonidine groups (Figure 3, P < 0.05 vs. the control group). In both clonidine groups, there were dose-related changes in MAP (P < 0.05 among periods), SVR (P < 0.05 between the hypercapnia-120 or -80 and -60 periods), and LV dP/dt (P < 0.05 between the hypercapnia-120 and -80 or -60 periods). RAP increased during hypercapnia in all groups, and the changes were dose-related in the control group (P < 0.05 between the hypercapnia -120 and -80 or -60 periods) and in the clonidine-10 group (P < 0.05 among periods). MPAP increased during hypercapnia in the control group (P < 0.05 among periods) but not in the clonidine groups (P < 0.05 vs. the control group). In the control group, LVEDP increased during the hypercapnia-80 and -60 periods (P < 0.05 between the control and clonidine groups) but not during the hypercapnia-120 period because LVEDP decreased in one dog. There were no differences between the two clonidine groups in changes of any hemodynamic variables during each period of hypercapnia except in MAP changes during the hypercapnia-60 period (Figure 3).
Plasma norepinephrine concentrations increased in a dose-related fashion during hypercapnia in the control group (Figure 4, P < 0.05 among periods). However, such increases in plasma norepinephrine concentrations were absent or suppressed to a great extent in both clonidine groups (P < 0.05 vs. the control group). Similarly, there were significant increases in plasma epinephrine concentration during the hypercapnia-120 period in all groups and during the hypercapnia-80 period in the control and clonidine-10 groups. The changes in the control group were dose-related (P < 0.05 among periods), and significant differences were observed during the hypercapnia-120 period between the control and clonidine groups. However, no differences were noted in changes of plasma catecholamine concentrations during any hypercapnic periods between both clonidine groups.
Plasma Clonidine Concentrations
There was a significant difference between both clonidine groups in plasma clonidine concentrations after the bolus injection of clonidine (Table 6). Thereafter, plasma clonidine concentrations further increased after the first and second hypoxic or hypercapnic exposure. The plasma clonidine concentrations in the dogs that experienced cardiac arrest during hypercapnic challenge were 24.7 and 49.6 ng/ml and 13.9 and 30.2 ng/ml after the bolus injection of 10 and 5 micro gram/kg clonidine and the first hypoxic challenge, respectively. These values were greatest in each group.
The major findings in our study are that pretreatment with a large dose of clonidine (10 micro gram/kg intravenously, followed by continuous infusion at a rate of 1 micro gram *symbol* kg sup -1 *symbol* min sup -1 ) resulted in (1) blunted pressor and plasma catecholamine responses to hypoxia and (2) profound hypotension due to systemic vasodilation with suppression of plasma catecholamine increases during hypercapnia. The alterations in cardiovascular and plasma catecholamine responses to hypoxia or hypercapnia in dogs given a half dose of clonidine were fundamentally similar to but of lesser degree than those in animals receiving a large dose of clonidine.
Effects of Clonidine Pretreatment
The cardiovascular changes after clonidine administration in the current study (Table 1) were similar to previous experiments. [11,12]reporting the effects of intravenous 15 micro gram/kg clonidine in anesthetized dogs or sheep. Although MAP remained unchanged after clonidine in our study, clonidine is shown to cause an initial increase with or without a subsequent long-lasting moderate decrease in MAP. [11,12]The decrease in the sympathetic tone and/or the increase in the vagal tone seem to be involved in the hemodynamic changes associated with clonidine administration, [11,13–15]in addition to direct vasoconstrictor action of clonidine on peripheral blood vessels. Because MPAP value is more likely to be susceptible to changes in CO, alveolar pressure, and LVEDP and the significant decreases in CO were noted after clonidine in the clonidine groups, the significant MPAP increase after clonidine in the clonidine-5 group may be caused primarily by LVEDP elevation. The increase in MPAP solely due to administration of an equivalent volume of 0.9% saline (1 ml/kg) in the control group was much smaller.
The small but significant reduction in PaO2observed after clonidine administration in the current results coincides with the previous investigations in sheep, [12,18]which showed increases in intrapulmonary shunt and deadspace ventilation after clonidine and suggested transient platelet aggregation and pulmonary embolism as an etiology of clonidine-induced decrease in PaO2. .
Consistent with several previous reports, [19–23]plasma norepinephrine and epinephrine concentrations decreased after clonidine administration in the current study, but a wide individual variability was noted in plasma catecholamine concentrations before clonidine administration, especially in plasma epinephrine concentration. The cause of the variability in plasma catecholamine concentrations is unclear but contributes to the resulting finding that clonidine could not further suppress levels in those dogs in whom the baseline plasma catecholamine concentrations were already low.
Prehypoxic values of hemodynamic variables and plasma norepinephrine concentrations in both clonidine groups were different from those in the control group (Table 2), indicating alterations of hemodynamic and sympathetic environment by clonidine. Because no dog showed pHa value less than 7.35 and the arterial blood gas values were similar among groups during any hypoxic periods, the different pHa values are unlikely to have affected the current results.
In humans, arterial pressure is unchanged or increased during hypoxia via stimulation of chemoreceptor in the carotid bodies. The increase in arterial pressure during chemoreceptor stimulation has been shown to be related to an increase in SVR as well as increased CO and LV dP/dt in animal experiments. [25–28]The magnitude of increases in MAP (39 mmHg) and heart rate during the hypoxia-30 period in the control group (Figure 1) was consistent with that in the previous experiment using awake dogs. The failure to observe a significant increase in CO during the hypoxia-30 period in the control group was related to the results that CO remained unchanged in one of seven dogs. Power analysis of the data indicated that, if the number of animals in the control group was increased to 44, statistical significance of P < 0.05 would have been achieved, provided the variability remained the same. In both clonidine groups, the significant increases in CO during the hypoxia-30 period appear to have been offset by the reductions in SVR, resulting in unchanged MAP. Although acute hypoxia elicits pulmonary vasoconstriction, [29–32]the reason for no significant increases in MPAP during the hypoxia-30 or the hypoxia-40 period in the clonidine-5 group remains unclear, despite the significant increases in MPAP in the control and clonidine-10 groups. A possible explanation is that a critical element in determining the pulmonary pressor response to hypoxia may be the preexisting state of tonus in the responding vessels, [17,28,33]in addition to some individual variabilities in hypoxic pulmonary vascular response. [34,35].
Among the three groups, only the control group showed significant increases in both plasma norepinephrine and epinephrine concentrations during all hypoxic periods (Figure 2). Because the chemoreceptor response to hypoxia is enhanced by circulating norepinephrine, one can assume that the modulation of cardiovascular responses to acute hypoxia by systemic clonidine is, for the most part, mediated by suppression of increases in plasma norepinephrine concentration during hypoxia. Based on the findings that the changes in plasma norepinephrine concentrations during the hypoxia-30 period differed between both clonidine groups, the suppressive effect of clonidine on increased plasma norepinephrine concentration during severe hypoxia was minimal in three of seven dogs of the clonidine-5 group. Power analysis of the data indicated that, if the number of animals in each group was increased to 528, statistical significance of P < 0.05 would have been achieved in the changes in plasma norepinephrine concentration during the hypoxia-30 period between the control and clonidine-5 groups, provided the variability remained the same. Alternatively, this finding implies that 10 micro gram/kg intravenous clonidine or more is sufficient, whereas a half dose of clonidine is insufficient for complete suppression of increased plasma norepinephrine concentration during severe hypoxia in the dog. On the other hand, both doses of clonidine appear enough to suppress the plasma epinephrine response to hypoxia, because the increases in plasma epinephrine concentration were suppressed to a similar extent during all hypoxic periods in both clonidine groups. Finally, the previous report showing the dose-related hypoxic hemodynamic responses is in agreement with the current results that most hemodynamic and plasma catecholamine alterations during hypoxia were of lesser magnitude as the severity of hypoxia was alleviated (Figure 1and Figure 2).
Prehypercapnic values of hemodynamic variables in both clonidine groups were different from those in the control group (Table 4); this finding indicates alterations of hemodynamic and sympathetic environment by clonidine. Because the greater pHa value during the hypercapnia-120 period due to lower PaCO2and the lesser pH value during the hypercapnia-60 period due to lower base excess were noted in the clonidine-10 group (Table 5), we cannot exclude the possibility that the difference in pHa values among groups may have affected the current results to a certain extent. However, the differences in PaOxygen2among groups is unlikely to have influenced the current results, because no dogs showed PaOxygen2< 70 mmHg.
Cardiovascular alterations associated with hypercapnia in the normal condition include no changes or increases in MAP, heart rate, and MPAP; increases in RAP, CO, pulmonary vascular resistance, and LVEDP; and decreases in SVR and LV dP/dt. [2,3,7,36–40]Certainly, both direction and magnitude of their changes appear to depend primarily on the basal sympathetic activity before hypercapnia, [7,39]as well as the degree of hypercapnia per se. Under most circumstances, the net cardiovascular response to hypercapnia results from both a direct action of carbon dioxide (depression of myocardial contractility and peripheral vasodilation) and an indirect, stimulatory effect on the sympathetic nervous system. In the current study, the decreases in MAP of approximately 60 mmHg during the hypercapnia-120 period in both clonidine groups (Figure 3) were greater compared with the previous report in which MAP decreased only by 3–38 mmHg during hypercapnia of the same degree in dogs receiving epidural anesthesia. This finding may indicate the difference in the extent of sympathetic blockade between the two studies; i.e., sympathetic depression by systemic clonidine administration appeared more marked than that by epidural anesthesia. The profound reductions in MAP during hypercapnia in both clonidine groups are attributable to potent peripheral vasodilation by carbon dioxide, [2,7]because the significant decreases in SVR were noted without significant changes in CO (Figure 3). The significant differences in MPAP response to hypercapnia among groups may be explained mainly by the dissimilar changes in LVEDP. Likewise, the significant reductions in LV dP/dt in the clonidine groups appear to be secondary to those in MAP according to the Anrep effect..
In the control group, plasma norepinephrine concentration increased in proportion to the severity of hypercapnia (Figure 4). The increases in plasma norepinephrine concentrations during hypercapnia were almost completely inhibited by clonidine pretreatment, except during the hypercapnia-120 period in the clonidine-5 group. The increase in PaCOsub 2 causes a progressive increase in chemoreceptor activity, [42,43]which elicits cardiovascular response, as well as in plasma catecholamine concentrations, [3,4,7]which further increase chemoreceptor discharge. Therefore, systemic clonidine administration appears to disclose the cardiovascular depressive actions of carbon dioxide on the myocardium and peripheral vasculature by preventing the increase in circulating catecholamines during hypercapnia, thereby withdrawing the sympathoadrenal support of circulation.
Limitations of the Current Study
The depth of anesthesia was not similar among groups, because all dogs received the same doses of pentobarbital throughout the study, despite the existence of anesthetic sparing effect of clonidine. [19,45]However, it remains unknown to what extent the doses of clonidine administered in our study would reduce pentobarbital requirement. In addition, chemoreceptor stimulation elicits more intense cardiovascular responses in the conscious state than during pentobarbital anesthesia. Studies in conscious animals were not possible in our institution.
The depth of anesthesia was not constant during the experiment, particularly in both clonidine groups, because plasma clonidine concentrations increased progressively over time, presumably due to a relatively high continuous infusion rate of clonidine. However, this finding is unlikely to have affected the current results, because the sequences of hypoxic or hypercapnic challenge and those of three hypoxic or hypercapnic periods were randomized in each dog. The plasma clonidine concentrations in the current study (approximately 9–50 ng/ml) were greater than those of 1–5 ng/ml in humans receiving therapeutic doses of clonidine. [47–49]According to a clinical report that clonidine poisoning-related symptoms were severe when the administration dose exceeded more than 20 micro gram/kg, the ranges of plasma clonidine concentrations observed in our study may represent overdose or intoxication. Our data nevertheless suggest that intravenous clonidine can suppress catecholamine responses to severe hypoxia or hypercapnia in the anesthetized dog. This assumption can be partly supported by a recent experiment demonstrating that somatosympathetic reflexes and spontaneous sympathetic activity were depressed by intravenous clonidine in a dose-dependent manner, and they were completely or almost abolished after total doses of 600 micro gram (approximately 24–32 micro gram/kg) in dogs. .
In conclusion, our results show that systemic clonidine administration alters the hemodynamic changes associated with hypoxia or hypercapnia and suppresses plasma catecholamine responses in anesthetized dogs. This finding may imply that a large dose of clonidine medication may impair the compensatory circulatory adjustments during hypoxia or hypercapnia.
The authors thank M. Asakura, Division of Biochemistry, Nippon Boehringer Ingelheim Kawanishi Institute, for help in the measurement of plasma clonidine concentration.