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Vascular and Extravascular Volume Expansion of Dobutamine and Norepinephrine in Normovolemic Sheep

Stephens, Christopher T.; Uwaydah, Nabeel; Kramer, George C.; Prough, Donald S.; Salter, Michael; Kinsky, Michael P.

doi: 10.1097/SHK.0b013e318225b031
Basic Science Aspects
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In low-flow states, such as circulatory shock, both fluids and catecholamines are often coadministered. We have previously found that adrenergic agents alter volume expansion after a fluid bolus. The present study tested the volume expansion properties of dobutamine and norepinephrine in sheep treated with (series 1) and without (series 2) a fluid bolus. Series 1 (n = 6 per group): no drug (control), dobutamine (10 μg · kg−1 · min−1), or norepinephrine (1.0 μg · kg−1 · min−1) was begun 30 min before a 24-mL · kg−1, 20-min, 0.9% NaCl bolus. The effect of drug and fluid on plasma volume (ΔPV), urinary output (UOP), and extravascular volume (ΔEVV) was determined. Series 2: Identical protocol but no fluid bolus. Series 1: the fluid bolus resulted in a peak and sustained ΔPV expansion. Norepinephrine (7.5 ± 0.9 mL · kg−1) and dobutamine (9.5 ± 1.1 mL · kg−1) significantly increased ΔPV compared with control (3.8 ± 1.1 mL · kg−1). Cumulative UOP was reduced by dobutamine (3.8 ± 1.4 mL · kg−1) compared with norepinephrine (25.1 ± 3.9 mL · kg−1) and control (16.9 ± 4.0 mL · kg−1). Norepinephrine increased ΔPV, while reducing ΔEVV after bolus. Series 2: ΔPV was unchanged in the control group. Dobutamine and norepinephrine increased ΔPV over time, 5.1 ± 0.5 and 4.0 ± 0.5 mL · kg−1, respectively. At study end, UOP was lowest in dobutamine. Norepinephrine resulted in loss of ΔEVV fluid. data suggest a novel role for adrenergic receptors in regulating vascular and EVV expansion. β-Adrenergic agonists enhance vascular volume expansion, whereas α-adrenergic agonists eliminate extravascular fluid.

University of Texas Medical Branch, Galveston, Texas

Received 22 Mar 2011; first review completed 5 Apr 2011; accepted in final form 19 May 2011

Address reprint requests to Michael P. Kinsky, MD, Department of Anesthesiology, University of Texas Medical Branch, 301 University Blvd, Galveston, TX 77555. E-mail: mkinsky@utmb.edu.

This work was supported by funds associated with the Department of the Navy, Office of Naval Research (grants N00014-06-1-0300 and N00014-03-1-0363).

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INTRODUCTION

Fluid therapy is essential to support circulation and perfusion when vascular volume is lost. It is considered a cornerstone of care in the clinical practice of anesthesiology, emergency medicine, surgery, and critical care medicine. Intravenous fluids of varying types have been extensively studied in experimental models. Several different solutions and regimens have translated into clinical practice and are used to restore lost circulating volume and thereby organ perfusion. Despite differences in the physiological properties of fluids, balanced salt solutions or isotonic crystalloid solutions, e.g., lactated Ringer's and 0.9% NaCl, are still the most commonly used i.v. fluids to initially treat vascular volume deficits (1, 2).

A consistent limitation with isotonic crystalloid for fluid resuscitation is that it is an inefficient volume expander, and only a fraction of the infused volume remains in the intravascular space (3-5). Thus, large volumes of crystalloid are needed to maintain circulatory volume. As a result, a significant amount of fluid enters into the extravascular space, and an interstitial edema develops. Interstitial edema consequently can decrease wound healing, pulmonary gas exchange, cardiac and gastrointestinal function, and cerebral blood flow and function (2, 6, 7).

To prevent the untoward effects of fluid excess that accompany crystalloid resuscitation, inotropic and vasoactive drugs are often administered after fluid therapy fails to maintain cardiac output (CO), arterial blood pressure, and urine output to adequate levels (2, 8, 9). This adjunctive practice has become a mainstay of critical care, despite a paucity of data in the literature to support the use of specific pharmacological agents to maintain tissue perfusion along with crystalloid resuscitation. It is known that catecholamine infusions enhance cardiac contractility as well as vascular tone including the arteriolar, venous, and renovascular beds, thus influencing overall arterial, venous, and capillary pressures and blood flow (10). Specific catecholamines can modulate plasma volume expansion (ΔPV) after a crystalloid fluid bolus. Vane et al. (11) demonstrated that isoproterenol, a nonspecific β-adrenergic agonist, and dopamine, a mixed β-adrenergic and dopaminergic agonist, augmented ΔPV, whereas phenylephrine, an α-adrenergic agonist, attenuated ΔPV. In contrast, Kinsky et al. (12) found that esmolol, a β-adrenergic antagonist, decreased ΔPV and increased EVV. These studies suggest a potential role of adrenergic receptors in regulating the efficiency of blood-to-tissue transport of fluid.

Dobutamine and norepinephrine are often coinfused with and without fluid to support vascular tone and cardiac function in critically ill patients. The present study addressed the volumetric and hemodynamic effects of infusing either dobutamine or norepinephrine in conscious normovolemic sheep. This study was undertaken to specifically determine how each of these agents independently impacts vascular and EVV expansion.

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MATERIALS AND METHODS

The protocol and experimental procedures were reviewed and approved by the Animal Care and Use Committee of the University of Texas Medical Branch at Galveston with adherence to the Guide for Care and Use of Laboratory Animals.

We performed two series of experiments in sheep to evaluate the effects of norepinephrine and dobutamine on PV (vascular volume) expansion and fluid balance.

  • Series 1-conscious normovolemic sheep received norepinephrine (n = 6), dobutamine (n = 6), or control-no drug (n = 6), followed by a fluid bolus.
  • Series 2-conscious normovolemic sheep received norepinephrine (n = 6), dobutamine (n = 6), or control-no drug (n = 3) without a fluid bolus.
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Animals and surgical preparation-same for both series

Adult female Merino sheep weighing 29 to 40 kg were anesthetized with isoflurane, orotracheally intubated, and mechanically ventilated with 1.5% to 2.5% isoflurane anesthesia throughout surgery. After prepping and draping in a sterile fashion, vascular catheters were placed into both right and left femoral arteries and veins and advanced into the abdominal aorta and inferior vena cava, respectively. An introducer sheath with a pulmonary artery catheter was placed into the right common jugular vein and secured. All catheters were flushed with heparin sodium solution (1,000 U/mL) and secured to the fleece of the animal. Next, a splenectomy was performed via a left lateral subcostal incision. The surgical incisions were closed in multiple layers, and all catheters exteriorized. The animals were awakened and allowed to recover from surgery for 1 week before the experimental protocol. Postoperative analgesia included 0.3 mg i.m. buprenorphine to minimize postsurgical discomfort. The animals were maintained in metal cages with free access to food and water until 24 h before the experiments, when food and water were removed.

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Dose determination

There are limited data on the hemodynamic effects of dobutamine and norepinephrine in healthy sheep. Before the experimental studies, we determined the hemodynamic responses in two healthy sheep by administering escalating doses of dobutamine and norepinephrine. The specific physiological target was aimed at increasing basal CO by 50% for dobutamine and mean arterial pressure (MAP) by 20% for norepinephrine.

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Dobutamine

Sheep were incrementally administered 5, 10, and 20 μg · kg−1 · min−1 of dobutamine over a 1-h period. We found that 10 μg · kg−1 · min−1 of dobutamine resulted in a 50% increase in CO. This infusion dose was chosen for both series.

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Norepinephrine

Similarly, we performed a dose response for norepinephrine. We tested 0.1, 0.5, 1, and 2 μg · kg−1 · min−1. We found that 1 μg · kg−1 · min−1 resulted in a 20% increase in MAP. This infusion dose was chosen for both series.

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Experimental protocols

Measurements and calculations for both series were identical. On the day of the experiment, the animal's overall health was confirmed by absence of fever, abnormal vital signs, or signs of infection. After this assurance, the catheters were connected to saline-filled sterile pressure transducers (Baxter Pressure Monitoring Kit; Baxter Healthcare, Irvine, Calif) and a Hewlett-Packard (HP) Monitor model 78901A (Hewlett-Packard, Andover, Mass) for continuous hemodynamic monitoring. Electrocardiographic leads with adhesive pads were secured on the extremities and proximal trunk of the sheep and connected to the HP monitor. A urinary bladder catheter (14F; Sherwood Medical, St Louis, Mo) was placed in the bladder. A continuous CO pulmonary artery catheter (Vigilance System; Baxter Healthcare) was positioned in the pulmonary artery and confirmed by monitor waveform. A 1-h stabilization period was allowed before data collection.

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Series 1

After establishing a 30-min baseline period without drug administration (T −60), a continuous infusion of norepinephrine (1 μg · kg−1 · min−1), dobutamine (10 μg · kg−1 · min−1), or control (30 mL · h−1 0.9% NaCl) was begun (T −30). The total volume and flow rate were equal for all groups. The amount of dobutamine or norepinephrine, which was added to the 250-mL (0.9% NaCL) infusion solution, was based on the animal's body weight. The final concentration for each drug was such that the volume being infused was 30 mL · h−1 for all experiments (with or without drug). We defined time zero (T0) as the start of the 0.9% NaCl fluid bolus, and T20, the end of the fluid bolus. The drug infusion began 30 min before fluid bolus (T −30) and was maintained for 150 min after the start of the fluid bolus, end of the experiment (T150). Blood samples for hemoglobin (Hb) determination and hemodynamic measurements were recorded at defined intervals.

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Series 2

Identical measurements and drug infusions of norepinephrine (1.0 μg · kg−1 · min−1), dobutamine (10 μg · kg−1 · min−1), or no drug (control of 0.9% saline at 30 mL · h−1) were begun 30 min (T −30) after a baseline stabilization period in each animal (T −60) and continued until study end (T150).

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Measurements and data collection

Hemodynamic variables, which included heart rate (HR), MAP, and CO, were continuously monitored and recorded at T −60, T −30, T0, T20, T30, T60, T90, T120, and T150 (T150 end of study). Fluid balance data, which included amount of fluid infused and cumulative urinary output (UOP), were recorded from T0 to T150. The effect of drug on fluid balance was recorded from T0 to T150. At T0, the urine collection container was emptied, and the fluid bolus was begun. Hemoglobin samples, measured by co-oximetry (482 Co-oximeter; Instrumental Laboratory Lexington, Mass), were used to determine PV expansion. Blood samples were collected in heparinized 1-mL syringes at T5, T10, T15, T20, T22, T24, T26, T28, T30, T60, T90, T120, and T150 min to determine Hb and total serum solute (TSS) concentration-an index of plasma protein (Schuco Clinical Refractometer 5711-2020; Schuco, Tokyo, Japan). Total Hb concentration [Hb] was measured in duplicate using the co-oximeter, with the mean value used for further calculations. To ensure that fresh circulating blood samples were being collected, samples were preceded by a 10-mL blood withdrawal from the catheter. After collection of each 1-mL sample, the 10 mL of blood was returned to the animal.

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Calculated variables

Baseline PV for each sheep was measured at T −45 using a 5-s injection of 12.5 mg of indocyanine green dye (Akorn Inc, Buffalo Grove, Ill) followed by 3-mL arterial blood samples collected every minute for a total of six samples. The samples were then processed to determine PV for each animal as previously described (11, 12).

Change in PV (ΔPV) was calculated based on Hb and red blood cell volume (RBCV) as previously described (11, 12). The time course of ΔPV was determined using the formula:

Blood volume changes (ΔBV) were derived from baseline PV and changes in [Hb] throughout each experiment at specific times using the formula:

where Hbt is the Hb at time (t) and Hb0 is the baseline Hb as previously described (11, 12). Red blood cell volume could be accounted for ΔPV = ΔBV.

Changes in EVV expansion (ΔEVV) over time were calculated using the following formula:

Distinguishing between PV expansion, EVV, and UOP was determined based on the assumption of mass balance including a constant RBCV and no change with respect to intracellular volume during isotonic fluid infusions. Isotonic 0.9% saline distributes within the extracellular compartments (PV and EVV). The extravascular expansion of 0.9% saline infusion likely represents interstitial volume expansion with minimal intracellular volume change.

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Statistical analysis

Data are presented as mean ± SEM. The hemodynamic and volumetric effects of three groups, dobutamine, norepinephrine, and control, were compared over time. Selected time points for volumetric calculations were T20, T30, T60, T90, T120, and T150. A one-way ANOVA was used to compare differences within groups followed by a Tukey multiple comparison test. GraphPad Prism (version 4.0; San Diego, Calif) was used for statistical analysis. Results from the statistical tests are displayed in the figure legends. Significance level, P, was set at P < 0.05. The following symbols were used to show statistical differences between groups: * for dobutamine versus control, ψ for dobutamine versus norepinephrine, and φ for norepinephrine versus control.

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RESULTS

Series 1-drug + fluid bolus

Plasma volume expansion (ΔPV) (Fig. 1, top): The 24 mL · kg−1 0.9% saline bolus resulted in an increase in ΔPV (Fig. 1, top). In the control group (no drug), ΔPV peaked to 9.8 ± 1.4 mL · kg−1 at the end of fluid bolus (T20-T30) and resulted in a sustained ΔPV of 3.8 ± 1.1 mL · kg−1 by study end (T150). Dobutamine resulted in a significantly larger peak and sustained ΔPV compared with control, at all time points. The peak (T20) and sustained (T150-study end) values were 14.1 ± 0.9 vs. 9.8 ± 1.4 mL · kg−1 and 9.5 ± 1.1 vs. 3.8 ± 1.1 mL · kg−1 for dobutamine versus control, respectively. Norepinephrine resulted in peak ΔPV of 12.0 ± 0.9 mL · kg−1, which was not statistically different from dobutamine or control. Norepinephrine ΔPV was significantly higher than control after T90. At study end (T150), the ΔPV for norepinephrine was 7.5 ± 0.9 mL · kg−1.

Fig. 1

Fig. 1

Cumulative urine output (UOP) (Fig. 1; middle): A different diuretic pattern was observed among all groups. The cumulative UOP (T150) for control (no drug) was 16.9 ± 4.0 mL · kg−1 (∼70% of the infused fluid bolus). Dobutamine produced significantly less UOP (3.8 ± 1.4 mL · kg−1; ∼15% of infused bolus) compared with norepinephrine and control. In contrast, norepinephrine induced the highest UOP (25.1 ± 3.9 mL · kg−1), which was greater than the volume of the fluid bolus.

Extravascular volume expansion (ΔEVV) (Fig 1; bottom): The ΔEVV was calculated from the 24-mL · kg−1 fluid bolus, ΔPV, and UOP over time. Extravascular volume increased during fluid loading and then decreased, mostly due to urinary excretion. The ΔEVV in the control group had a lower trend compared with dobutamine. At study end, the ΔEVV in control sheep was 3.0 ± 3.8 mL · kg−1. Dobutamine ΔEVV at end of study was 10.6 ± 2.4 mL · kg−1. On the other hand, norepinephrine induced a substantial reduction in ΔEVV. The lower EVV was due to a greater diuresis with a concomitant PV expansion. Norepinephrine's effect was evident by the rapid decline in ΔEVV that was negative after T90. At study end, the ΔEVV for norepinephrine was −8.5 ± 3.0 mL · kg−1.

Total serum solute concentration (Table 1) decreased following the fluid bolus. The nadir in TSS occurred immediately at the end of the fluid bolus. The greatest decrease in TSS was observed with dobutamine, which was statistically lower than control at end of study (T150). A lower trend in TSS was observed in norepinephrine versus control.

TABLE 1

TABLE 1

Hemoglobin concentration (Table 2) decreased after the fluid bolus. The lowest [Hb] occurred at the end of the fluid bolus. The change (Δ) in [Hb] represents the effects of treatment on Hb over time. The Δ in [Hb] was also used to determine vascular volume expansion. At T20, the largest Δ in [Hb] was observed for dobutamine (−1.6 ± 0.1 g · dL−1) followed by norepinephrine (−1.5 ± 0.1 g · dL−1) and then control (−1.2 ± 0.1 g · dL−1). A sustained reduction in the Δ in [Hb] occurred for dobutamine, which was significantly lower than control after T20. Norepinephrine also resulted in a significantly lower Δ in [Hb], at end of study, compared with control.

TABLE 2

TABLE 2

Hemodynamics (Table 3): The fluid bolus in the control group resulted in a transient, albeit small, increase in MAP, which returned toward baseline after 60 min. A similar response in MAP was observed for dobutamine. There were no significant differences in MAP between dobutamine and the control group. In contrast, norepinephrine resulted in a significant and sustained increase in MAP compared with dobutamine and control before, during, and after fluid bolus. The fluid bolus, in the control group, minimally changed HR. Dobutamine was associated with highest HR before, during, and after the fluid bolus compared with control. Norepinephrine also caused HR to increase but lower than dobutamine. An increase in CO was observed during and immediately at the end of the fluid bolus in the control group. Thereafter, CO returned toward baseline after 60 min. Both dobutamine and norepinephrine treatment increased CO independent of the fluid bolus. The fluid bolus further augmented CO with dobutamine and norepinephrine and was significantly higher than control. The fluid bolus in the control and norepinephrine group did not alter systemic vascular resistance (SVR). Dobutamine significantly decreased SVR during and after the fluid bolus compared with control and norepinephrine at study end.

TABLE 3

TABLE 3

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Series 2-drug alone

Plasma volume expansion (ΔPV) (Fig. 2, top): The sham group (saline vehicle only) did not demonstrate any appreciable change in ΔPV during the experiments (−1.0 ± 1.4 mL · kg−1 at T150). Dobutamine resulted in a sustained increase in ΔPV, reaching an apex by T120-T150 (5.0 ± 0.5 mL · kg−1). Similarly, norepinephrine resulted in a significant increase in ΔPV (4.0 ± 0.6 mL · kg−1) by study end (T150). The ΔPV for dobutamine was significantly higher than control after T60. The ΔPV for norepinephrine was significantly higher than control after T120.

Fig. 2

Fig. 2

Cumulative urine output (Fig. 2; middle) was similar through T90 for both control and norepinephrine but slightly higher, for the latter, at study end (7.8 ± 2.4 vs. 9.1 ± 1.9 mL · kg−1, respectively). Dobutamine resulted in a significantly lower UOP than sham and norepinephrine. At study end, only 1.4 ± 0.3 mL · kg−1 UOP was recorded.

Extravascular volume expansion (Fig. 2; bottom) was calculated from ΔPV and UOP. In the drug-alone or no-volume group, the mass balance technique could not be used (Fig. 2). Rather, the observed effect of the drug on PV expansion and UOP was used to calculate EVV. A net loss of EVV occurred over time for all groups. The sham and dobutamine groups resulted in similar ΔEVV, −7.4 ± 1.9 and −6.9 ± 0.2 mL · kg−1, respectively, at study end. For the sham group, the ΔEVV was due to urine production, whereas dobutamine's ΔEVV was primarily due to PV expansion. Norepinephrine was associated with a reduction in ΔEVV due to PV expansion and diuresis, −13.2 ± 1.5 mL · kg−1 at study end (Fig. 2, bottom).

Total serum solute concentration (Table 1) in sham-treated animals did not change compared with basal levels. The TSS concentration following norepinephrine treatment underwent very little change. Dobutamine reduced the TSS concentration after T30. A plateau in TSS occurred after T90, which was lower than sham-treated animals.

Hemoglobin concentration (Table 2): The [Hb] did not change in the sham-treated animals. However, the [Hb] in the dobutamine- and norepinephrine-treated animals decreased after the onset of drug infusion, as indicated by Δ in [Hb]. At study end, the Δ in [Hb] was −0.7 ± 0.1, −0.5 ± 0.1, and 0.1 ± 0.2 g · dL−1 for dobutamine, norepinephrine, and control, respectively. The Δ in [Hb] was significantly lower than control after T60 for dobutamine and study end for norepinephrine.

Hemodynamics (Table 4): Baseline hemodynamics were similar to series 1 animals. There was no appreciable change in MAP in either the sham or dobutamine groups throughout the experimental periods. Norepinephrine, on the other hand, demonstrated a significant sustained increase in MAP. Heart rate was unchanged in the sham group, whereas both dobutamine and norepinephrine resulted in a significantly elevated HR throughout the experimental course. The highest HR resulted from dobutamine. Cardiac output (CO) remained unchanged in the sham group, whereas both drug treatment groups developed a significant increase in CO after T0. Dobutamine was associated with a reduction in SVR compared with the sham group. Very little change in SVR was observed after norepinephrine infusion. The SVR in the sham group increased over time.

TABLE 4

TABLE 4

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DISCUSSION

Pharmacological agents are often used in conjunction with fluids to support hemodynamics. We have previously reported that specific adrenergic agents and their antagonists alter the vascular volume expansion properties in normovolemic sheep administered a fluid bolus. Specifically, we found that isoproterenol, a nonspecific β-adrenergic agonist, enhanced vascular volume expansion after a fluid bolus, whereas phenylephrine and esmolol resulted in minimal vascular expansion and even a contraction of vascular volume following the fluid bolus (11, 12). However, isoproterenol was associated with potentially adverse hemodynamic effects that could make infusion of isoproterenol somewhat problematic. The present study addresses the hemodynamic and volume expansion properties of commonly used β- and α-adrenergic agonists, dobutamine and norepinephrine, respectively. We performed two series of experiments in which we measured the amount of vascular volume expansion of these agents with and without a fluid bolus. Our data demonstrate that dobutamine and norepinephrine expand the vascular compartment, even without fluid administration. These agents had unique effects on overall fluid balance. Dobutamine appeared to be a more powerful vascular volume enhancer, whereas norepinephrine increased vascular volume expansion and eliminated extravascular excess via diuresis.

Dobutamine is a relatively selective β-adrenergic agonist, primarily via the β1-adrenergic receptor (8-10). Dobutamine is used in many critical care settings to enhance systolic function in patients with heart failure or administered to patients in shock when the response to volume resuscitation is reduced or ineffective (8). Norepinephrine is a mixed adrenergic agonist. Norepinephrine acts on both the α- and β-adrenergic receptors (9, 13). Norepinephrine is used to maintain vascular tone in patients with reduced afterload conditions. Norepinephrine enhances organ perfusion and tissue oxygenation in low-perfusion/flow states by increasing MAP (8, 9, 13, 14).

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Dobutamine and fluid bolus

Plasma volume expansion (ΔPV) after a fluid bolus is transient. The coadministration of dobutamine with a fluid bolus resulted in a sustained increase in ΔPV throughout the experimental period. The β-adrenergic receptor may play a role in microvascular fluid regulation (15, 16). Adamson et al. (17) demonstrated that hydraulic conductivity of intact capillaries is reduced by the β-adrenergic agonist isoproterenol, likely via a cyclic adenosine monophosphate, resulting in less transvascular fluid flux and greater retention of vascular fluid. β-Adrenergic agonists have also been shown to reduce microvascular permeability by a similar mechanism, in heart failure and sepsis (15, 16, 18). Vane et al. (11) found that isoproterenol (a nonselective β-adrenergic agonist) after a saline bolus resulted in sustained vascular volume expansion and an associated antidiuresis compared with a saline bolus alone without isoproterenol. Our data, similarly, demonstrated an increase in vascular volume expansion (↑ΔPV) after a fluid bolus with dobutamine. The microcirculatory physiological determinants or Starling forces were not directly measured in our study. Total serum solute concentration, predominately plasma protein and an indirect assessment of plasma colloid osmotic pressure, was measured. Dobutamine plus fluid was associated with a larger reduction in TSS compared with saline alone. It is unlikely that dobutamine treatment caused a loss of protein or solute from the circulation, because [Hb] correspondingly decreased (Table 2). Parallel decreases in TSS and Hb support volume expansion. The mechanism(s) for dobutamine's enhanced volume retention could be due to reduced capillary pressure (19, 20). Other mechanisms that augment fluid reabsorption should be considered. A decrease in hydraulic conductivity (Lp) may explain the initial augmented PV and decreased EVV with bolus fluids in presence of dobutamine or β-adrenergic agonist. Adamson et al. (17) used intact single capillaries to show that β-adrenergic agonists reduce Lp. However, Lp is only a resistance term and would not be anticipated to greatly influence steady-state changes. Michel and Phillips (21) suggest a revised Starling hypothesis in which a change in filtration can be modulated in a few minutes by a change in the abluminal endothelial space. However, our data suggest a physiological significant sustained change for the 150 min following a bolus. Such a 2- to 3-h benefit could be clinically relevant. The discrepancy between our data and the theory of Michel and Phillips (21) may be explained by Zhang et al. (22, 23), who provide the model development and experimental evidence that pericytes may participate in fluid exchange. β-Adrenergic receptor regulation of these domains is a possibility. Others suggest a role for atrial natriuretic peptide (ANP)-dependent volume regulation (24). Cumulative UOP was significantly reduced with dobutamine, suggesting that β-adrenergic stimulation induces retention of intravascular and extravascular fluid (Fig. 1). In our study, the infusion of dobutamine (predominantly a β1-adrenergic receptor agonist) augmented vascular volume expansion. In the previous study performed by Vane et al. (11), isoproterenol (a nonspecific β-adrenergic receptor agonist) largely enhanced vascular volume expansion. These data suggest that both β-adrenergic agonists stimulate vascular volume expansion in normovolemic sheep. It is unclear if this effect on vascular volume expansion is species specific or if both β1- and β2-adrenergic agonists enhance the vascular retention of fluid.

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Dobutamine without fluid bolus

When dobutamine was infused without a saline bolus, there was a sustained increase in ΔPV. This increase was significantly higher than the sham group but not as marked when saline was coinfused with dobutamine. The vascular volume expansion induced by dobutamine without a fluid bolus was approximately 5 mL · kg−1. This volume was similar to the sustained vascular volume expansion observed following the saline-alone fluid bolus (≈5 mL · kg−1). These results suggest that dobutamine generates a similar sustained vascular volume expansion profile of a nontreated crystalloid fluid bolus. When dobutamine was added to a fluid bolus, there appeared to be an additive or synergistic effect on vascular volume expansion.

Urinary output was significantly diminished with dobutamine as compared with sham, again demonstrating the antidiuretic effect of dobutamine. This antidiuretic effect in a sheep model has been previously described and likely related to increased renin production (25, 26).

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Norepinephrine and fluid bolus

Norepinephrine resulted in a ΔPV that was greater than control but less than dobutamine. In contrast to dobutamine, norepinephrine resulted in a significant diuresis. As such, the ΔEVV diminished rapidly because of an increased UOP and vascular volume expansion. The finding that norepinephrine was able to maintain PV expansion notwithstanding a pronounced diuresis is intriguing and likely related to the mixed β- and α-adrenergic receptor agonist properties of norepinephrine. Our data suggest that β-adrenergic agonists increase PV, whereas the α-adrenergic properties of norepinephrine result in a pressure-mediated diuresis, thus ameliorating any significant increase in EVV from the fluid bolus (Fig. 1). In support of our findings, Vane et al. (11) demonstrated that phenylephrine, a selective α1 agonist, resulted in a diuresis similar to that of our findings with norepinephrine. Vane et al. concluded that phenylephrine must have caused a pressure diuresis in the sheep. A pressure diuresis and natriuresis via atrial natriuretic factor have been described in healthy volunteers receiving α agonists (27).

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Norepinephrine without fluid bolus

The effect of norepinephrine infusion without a coadministration of saline fluid bolus resulted in ΔPV similar to that observed with dobutamine. The effects of enhanced vascular volume expansion and diuresis are likely related to norepinephrine's mixed α- and β-adrenergic agonist properties. The β-adrenergic stimulation resulted in vascular volume expansion, whereas the α-adrenergic stimulation enhanced UOP (11).

The amount of cumulative UOP was similar to the sham group. Extravascular volume was lowest with norepinephrine because of vascular volume expansion and elimination of fluid via diuresis. We speculate that the effects of norepinephrine on blood to tissue transport of fluid are similar to those of dobutamine, because both agents activate cyclic adenosine monophosphate.

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Hemodynamic effects

We infused a fixed infusion dose for dobutamine and norepinephrine aimed at increasing CO and MAP, respectively. Sheep had a consistent response to the fixed infusion doses. In the present study, we observed that both dobutamine and norepinephrine caused an increase in CO, which was further augmented with a fluid bolus. This effect is likely related to the additional preload from the fluid bolus in the treatment groups. Mean arterial pressure was less affected in the dobutamine group, as expected, because dobutamine is a known weak peripheral vasodilator, attributed to its β2 agonistic properties. Somewhat surprisingly, norepinephrine did not increase SVR. This may be related to a species-specific response, because one would anticipate a greater increase in SVR with this dose in humans. The vascular pharmacological responses suggest that normovolemic sheep may have a functional predominance of β-adrenergic receptors (Tables 3 and 4).

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Limitations

Infusions of fluid and pharmacological agents into normovolemic sheep present several significant limitations. Sheep have physiological and pharmacological responses to the drugs that may not be the same as humans. We coinfused pharmacological agents with fluid into conscious normovolemic sheep, which is far different than infusing 0.9% saline and/or these agents into clinically ill patients. Our previous studies using normovolemic models suggest that the type of fluid and the pharmacological agent have varying effects on PV expansion (11, 12). We appreciate that fluids are usually infused in states of hypovolemia and volume contraction, whereas catecholamines are given to support the hemodynamics in patients with underlying clinical abnormalities such as shock and cardiac failure.

We did not infuse these agents to a specific target end point for each individual experiment. The dose-response data for these agents were collected during preliminary experiments. We selected the doses of 10 and 1 μg · kg−1 · min−1 for dobutamine and norepinephrine to increase basal CO by 50% and MAP by 20%, respectively. These physiological responses were accomplished in both series, with satisfactory physiological measurements in all animals. Our aim was to evaluate the effects of these common pharmacological agents on PV under normovolemic healthy conditions. It is probable that the effects we have noted in this animal model may be different under models of sepsis or hemorrhage. It will be important to examine this protocol in animal models of sepsis or hemorrhage, where animals are actively being resuscitated using these adrenergic agonists with fluids. Furthermore, we acknowledge that the cardiac physiology could be altered during non-euvolemic states, leading to different outcomes with respect to infusing β-agonists. The outcome may indeed be different under pathological conditions such as hypovolemia, cardiac failure, or septic shock. Further studies will be needed to determine if these agents, at clinical doses, have similar resuscitation effects in these pathological conditions.

We made assumptions that the infused fluid is capable of moving only within three spaces: the vascular space (PV), UOP, or extravascular space (EVV). The maintenance infusion volume, although uniform for all groups, was not accounted for in our calculations. The total maintenance volume could add an additional 10% more fluid on top of the fluid bolus. However, the maintenance fluid was also administered to offset insensible water losses (estimated 30 mL · h−1). We acknowledge that the unaccounted volume (maintenance infusion − insensible water losses) could alter the UOP and therefore EVV calculations. We also appreciate that adrenergic agents could influence insensible water losses; however, because of the large size of the fluid bolus compared with the potential insensible water loss and maintenance fluid gain, we suspect that this offset would be minimal.

Most of the infused isotonic crystalloid moves into the extravascular space, resulting in UOP or extracellular water (11). To measure our baseline PV and ΔPV, we used our previous well-established technique of indocyanine green indicator and Hb dilution measurements. These techniques have been used extensively with reproducible results (12, 28-30).

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Clinical implications

These data support that, in addition to enhancing contractility and afterload, dobutamine and norepinephrine, respectively, increase circulating volume. These findings are consistent with other reports of using adrenergic agonists to treat inflammatory conditions (15-17). This common pathway may explain the findings of previous studies looking at the effects of adrenergic agonists and antagonists on volume expansion (11, 12).

Isotonic crystalloid solutions, such as lactated Ringer's or 0.9% normal saline, are the most common type of i.v. fluid administration in the United States. The results of our study suggest that β-adrenergic agonists enhance vascular volume expansion. These results suggest that the β-adrenergic receptor participates in transvascular fluid flux. Specifically, we have shown that β-adrenergic agonists result in vascular volume expansion [Vane et al. (11) and in the present study], whereas β-adrenergic antagonists result in vascular volume contraction (12). Norepinephrine resulted in a modest vascular volume expansion and diuresis. This effect is likely due to norepinephrine's mixed adrenergic agonist properties. The enhanced vascular expansion observed with norepinephrine is consistent with its β-adrenergic activity. The elimination of fluid (diuresis) is likely related to norepinephrine's α-adrenergic activity, since Vane et al. (11) demonstrated that phenylephrine, an α1 agonist, reduced vascular volume expansion but resulted in a prominent diuresis.

The present study shows that dobutamine and norepinephrine, commonly used agents for patients requiring hemodynamic support, result in vascular volume augmentation. Clinical scenarios involving various shock states, cardiac failure, and multiple organ failure frequently require the use of fluids and inotropic and vasopressor therapies. These data suggest that adrenergic receptors, in part, regulate vascular and EVV after a fluid bolus. The volemic status of patients in need of vascular support using these agents should be accounted for, because there is some synergy when these agents are coinfused with i.v. fluid. Patients at risk for large fluid shifts could benefit from adrenergic agents.

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CONCLUSIONS

Dobutamine and norepinephrine augmented vascular volume expansion with and without a crystalloid fluid bolus in conscious normovolemic sheep. The adrenergic receptor appears to modulate intravascular volume depending on the type of adrenergic agent occupying the receptor. Further studies are needed to determine if the vascular and extravascular expansion properties of dobutamine and norepinephrine occur in pathological conditions. A more comprehensive knowledge of the therapeutic effects of these agents in the critically ill could afford the clinician better judgment in the management of these challenging patients.

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Keywords:

Fluid therapy; adrenergic agonists; vascular volume expansion; catecholamine infusions; dobutamine; norepinephrine; sheep

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