Mechanism of Systemic Vasodilation During Normovolemic Hemodilution : Anesthesia & Analgesia

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Cardiovascular Anesthesia

Mechanism of Systemic Vasodilation During Normovolemic Hemodilution

Doss, Doss N. MD, PhD; Estafanous, Fawzy G. MD; Ferrario, Carlos M. MD; Brum, Jose M. MD; Murray, Paul A. PhD

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Abstract

Hemodilution is a well accepted technique during major surgery because it reduces the need for blood transfusions and it improves tissue perfusion and oxygenation [1,2]. The hemodynamic response to normovolemic hemodilution is characterized by an increase in cardiac output and a concomitant decrease in total peripheral resistance (TPR) [3-5]. Although the hemodynamic changes that occur during hemodilution have been well characterized, the mechanisms mediating those changes have not been fully elucidated. The goal of the present study was to investigate several mechanisms that could potentially mediate or modulate the systemic vascular response to normovolemic hemodilution. Specifically, we assessed the roles of the endothelium-derived relaxing factor, nitric oxide, and neural reflexes in the systemic vascular response associated with normovolemic hemodilution.

Methods

All surgical procedures and experimental protocols were approved by the Institutional Animal Research Committee. Male, Sprague-Dawley rats weighing 350-400 g were anesthetized with 1.2% halothane. Halothane anesthesia (1.2%) was maintained throughout the entire course of each experiment. A polyethylene cannula (PE-240) was inserted into the trachea. Ventilation was controlled with a positive pressure respirator (Model 681; Harvard Apparatus, South Natick, MA). The ventilatory rate was set at 50 breaths/min, and the tidal volume was adjusted according to body weight [6]. Positive-end expiratory pressure of 3 cm H2 O was applied after opening the chest. Blood gases were measured during each experiment and ventilatory variables were adjusted to maintain values within the normal range. A catheter (PE-50) was placed in the femoral vein for drug administration. Heparin (500 IU/kg), was administered to prevent coagulation. A catheter (PE-50) was inserted into the femoral artery and advanced to the descending aorta to measure systemic arterial pressure and to withdraw blood. After exposing the heart and great vessels, cardiac output was measured with an electromagnetic flowmeter by placing a flow probe (2.5 mm internal diameter) around the ascending thoracic aorta. The electrical outputs of the blood pressure transducer and the electromagnetic flowmeter were connected to a data acquisition system, which allowed continuous on-line measurements of systemic arterial blood pressure, cardiac output, and heart rate, and calculations of stroke volume and TPR. Hemodynamic measurements were displayed on a 4-channel Gould recorder (Model 2400; Gould, Inc., Cleveland, OH).

Three separate protocols were performed utilizing a total of 22 rats.

Protocol 1

Eight rats were studied in this protocol. Control measurements were made after a 30-min stabilization period. Normovolemic hemodilution was then produced by withdrawing a blood volume equivalent to 3.8% of total body weight while simultaneously administering an equal volume of 6% hydroxyethyl starch dissolved in 0.9% NaCl (Hespan Registered Trademark; Du Pont Pharmaceuticals, Wilmington, DE). Hespan Registered Trademark was administered via the right femoral vein at 1.5 mL/min over a 7-min period. This reduced the hematocrit from 49% +/- 1% to 23% +/- 1%. After a 30-min stabilization period, cardiovascular measurements were repeated during hemodilution. The rats then received the nitrous oxide (NO) synthase inhibitor, L-nitroarginine (LNA) 10 mg/kg intravenously (IV), dissolved in 0.9% NaCl to a total volume of 1 mL. Measurements were repeated 30 min after LNA administration, when cardiovascular variables had reached new steady state values. At that time hematocrit was 26% +/- 1%.

Protocol 2

Six rats were studied in this protocol. After control measurements were made, LNA (10 mg/kg IV) was administered. Measurements were repeated after a 30-min stabilization period. In order to reverse the systemic vasoconstrictor response to LNA, sodium nitroprusside (1-3 micro gram centered dot kg-1 centered dot min (-1) IV) was administered. When blood pressure was normalized (approximate 15 min), measurements were repeated, after which time the rats were hemodiluted as described in Protocol 1. Hematocrit was reduced from 51% +/- 1% to 22% +/- 1% during hemodilution.

Protocol 3

Eight rats were studied in this protocol. After making control measurements, the rats were "pithed" to abolish cardiovascular reflexes [7]. This was achieved by inserting a stainless steel wire (0.06 internal diameter; Small Parks, Inc., Miami, FL) into the spinal cord via the left orbit. Removal and reinsertion of the wire ensured complete destruction of the spinal cord. A 1.5-h period was allowed for recovery from the acute cardiovascular collapse caused by spinal cord destruction [7]. Experiments in five additional rats were terminated because of hemodynamic instability. After the 1.5-h stabilization period, cardiovascular variables were measured, after which time the rats were hemodiluted as described in Protocol 1. Hematocrit was reduced from 48% +/- 1% to 25% +/- 1% during hemodilution. After a 30-min stabilization period, measurements were made again, followed by the administration of LNA (10 mg/kg IV). Final measurements were made 30 min later when the hematocrit was 27% +/- 1%.

All data are presented as means +/- SEM. Absolute values of the measured variables are presented in the figures. Within group comparisons were made with one-way analysis of variance. When F-tests indicated a significant effect (P < 0.05), t-tests were used for post-hoc analysis. The Bonferroni inequality theorem was used to correct for multiple comparisons.

Results

Protocol 1

The summarized data are illustrated in Figure 1. Hemodilution resulted in an increase in cardiac output and stroke volume, and a decrease in TPR. Hemodilution had no effect on heart rate or systemic blood pressure. Administration of LNA during hemodilution returned cardiac output, stroke volume, and TPR to control values.

F1-6
Figure 1:
Summarized data from Protocol 1. Hemodynamic values are presented at control, during acute normovolemic hemodilution (HMD), and during combined hemodilution and nitric oxide synthase inhibition with L-nitroarginine (LNA).

Protocol 2

The summarized data are presented graphically in Figure 2. In rats with a normal hematocrit, LNA increased systemic blood pressure and TPR. Administration of sodium nitroprusside returned systemic blood pressure and TPR to control values. In this setting, hemodilution failed to have an effect on any of the measured cardiovascular variables.

F2-6
Figure 2:
Summarized data from Protocol 2. Hemodynamic values presented at control, after L-nitroarginine (LNA), after sodium nitroprusside (SNP), and after combined LNA plus SNP and hemodilution (HMD).

Protocol 3

The summarized data are shown in Figure 3. Spinal cord destruction with pithing decreased cardiac output, heart rate, and systemic blood pressure. In this setting, hemodilution increased cardiac output and stroke volume, and decreased TPR to the same extent as that observed in intact rats (Protocol 1). Administration of LNA during hemodilution returned cardiac output, stroke volume, and TPR to control values.

F3-6
Figure 3:
Summarized data from Protocol 3. Hemodynamic values presented at control, after spinal cord destruction by pithing (PITH), after hemodilution (HMD), and after combined HMD and L-nitroarginine (LNA).

Discussion

The hemodynamic effects of acute normovolemic hemodilution are characterized by an increase in cardiac output and a decrease in TPR, which serves to maintain systemic blood pressure at control values [3,8-11]. The mechanisms responsible for the decrease in TPR during hemodilution have not been clearly elucidated, but are thought to involve decreased blood viscosity, reflex vasodilation via the autonomic nervous system, and local vasoregulatory factors. Our results directly suggest that the autonomic nervous system does not modulate TPR during hemodilution, because the decrease in TPR was still apparent after spinal cord destruction. Our results also support the concept that endogenously released NO mediates the decrease in TPR during hemodilution, because the decrease in TPR was abolished by NO synthase inhibition.

In the presence of hemodilution, LNA returned TPR to control values Figure 1. Although this result supports a role for endogenous NO in mediating the decrease in TPR during hemodilution, it could also be a nonspecific effect of LNA secondary to unmasking endogenous vasoconstrictor mechanisms. We attempted to address this latter possibility in Protocol 2 by administering sodium nitroprusside in an amount that returned TPR to control values in the presence of LNA. Our objective was to continuously administer sodium nitroprusside, an endothelium-independent activator of vascular smooth muscle guanylate cyclase, while at the same time inhibiting the endogenous production of NO via LNA. This allowed us to assess the role of endogenous NO during hemodilution without the confounding effect of nonspecific vasoconstrictor stimuli unmasked by LNA. In this setting, hemodilution failed to decrease TPR, which supports a role for endogenous NO in mediating the response.

The "pithed" rat preparation allowed us to assess the effects of hemodilution in the absence of centrally mediated cardiovascular reflex activity [12]. After spinal cord destruction, hemodilution had the same effect on TPR as that observed in intact animals. Thus, reflex neural modulation of TPR is not a primary mechanism in the systemic vasodilator response to hemodilution.

The endothelium acts as a signal transducer, sensing changes in intraluminal blood flow [13,14] and the chemical milieu [15-18]. Endothelial cells possess a high capacity for NO production, which permits rapid adjustments of NO release in response to changes in blood flow and shear stress. In rats, acute normovolemic anemia increases gastric mucosal blood flow, an effect that is mediated by the endogenous release of NO [19]. In the present study, the increase in cardiac output and peripheral blood flow associated with hemodilution could have acted as the stimulus for endogenous NO release and a further reduction in TPR. Alternatively, the reduction in hematocrit associated with hemodilution could have reduced the NO scavenging capacity of the blood. In this regard, there appears to be a tonic release of NO in many vascular beds, such that inhibition of NO synthesis results in vasoconstriction [17,20,21]. Consistent with this, we observed an increase in TPR after LNA in intact rats Figure 2. Both hemoglobin and artificial hemoglobin solutions have been shown to cause vasoconstriction by inactivating NO [22,23]. Although speculative, it is possible that hemodilution effectively increases the concentration of endogenous NO by reducing the scavenging capacity of blood to inactivate NO.

We did not investigate the role of viscosity in the peripheral vascular response to hemodilution in this study. The increase in blood flow caused by hemodilution has been shown to be independent of plasma viscosity in dogs [24]. Based on previous work [25], it is likely that our use of Hespan Registered Trademark as a normovolemic replacement for blood had little or no effect on viscosity.

In conclusion, our results suggest that reflex neural mechanisms do not modulate the peripheral vasodilator response to hemodilution. However, our results are consistent with a possible role of endogenous NO as a mechanism for the decrease in TPR associated with acute normovolemic hemodilution in the rat.

The authors sincerely thank Ronnie Sanders for her secretarial skills in preparing this manuscript.

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