Hemodynamic responses elicited by systemic injections of isotonic and hypertonic saline in hemorrhaged rats
Introduction
Moderate to severe hemorrhage (HEM) in humans and animals results in a fall in cardiac output (CO) and mean arterial blood pressure (MAP) but an increase in total peripheral resistance (TPR), which limits the fall in MAP (Baue et al., 1967, Baue et al., 1991, Boyd and Mansberger, 1968, Brooks, 1935, Drucker et al., 1981, Jarhult, 1973, Mittman et al., 1976, Pirkle and Gann, 1976, Traveso et al., 1987, Wade et al., 1997). The falls in CO and MAP during HEM are partially reversed by the movement of fluid and protein from the interstitium into capillaries (Boyd and Mansberger, 1968, Drucker et al., 1981, Guyton, 1965, Pirkle and Gann, 1976, Starling, 1896). The movement of fluid and proteins is initiated by a fall in capillary hydrostatic pressure, which promotes a rapid shift of interstitial fluid into the capillaries and a less rapid shift of interstitial albumin into the plasma, which helps to support plasma oncotic pressure (Drucker et al., 1981). Intracellular fluid drawn down an osmotic gradient facilitates restitution of plasma volume after HEM by replenishing interstitial fluid volume. The increase in interstitial volume and pressure provides the driving force for transcapillary movement of fluid and albumin into plasma (Drucker et al., 1981). The driving force responsible for movement of fluid into the interstitial space and transcapillary movement of albumin, is regulated by circulating factors and especially glucose derived from the splanchnic circulation. The release of these factors is triggered by HEM-induced increases in circulating hormones levels (Drucker et al., 1981, Friedman et al., 1982, Stone et al., 1977).
The major objective of providing fluid therapy to subjects with hemorrhagic shock is to restore circulating volume and thereby support tissue perfusion (Drucker et al., 1981, Dubick et al., 2013, Falk et al., 1983, Thongrong et al., 2013). Fluids such as physiological salt solutions, Ringer's lactate, hydroxyethyl starch, albumin and dextrans have been employed in clinical and experimental settings (Dubick et al., 2013, Falk et al., 1983, Moss and Gould, 1988, Shires et al., 1995). Administration of small volumes of hypertonic saline (H-saline) to subjects in hemorrhagic shock restores MAP by increasing circulating volume (Baue et al., 1991, Coimbra et al., 1997, De Felippe and Timoner, 1980, Dontigny, 1992, Nakayama et al., 1984, Shackford et al., 1998, Traveso et al., 1987), most likely due to the movement of intracellular fluid into the vascular space (Drucker et al., 1981). The use of reduced volume H-saline therapy has the advantage of reducing the potential development of third space fluid sequestration (such as cerebral edema in patients with head injury), which may occur with large volume therapy (Drucker et al., 1981). In addition, there are several reports that H-saline directly increased cardiac contractility in patients with moderate to marked HEM (Ogino et al., 1998). Several studies have also examined the effects of mild to severe HEM on MAP, CO and TPR in experimental subjects (Drucker et al., 1981). It should be noted however that in recent trials, low volume H-saline resuscitation did not improve outcomes in either hypovolemic shock or traumatic head injury patients (Dubick et al., 2013), thus calling into question the benefit of such therapy for these broadly defined patient populations. A more detailed understanding of the underlying pharmacology could aid in defining a patient population that might be better served by such therapy.
There is considerable information as to effects of small volumes of isotonic saline (I-saline) and H-saline on MAP, CO and TPR in low CO-induced hypotension during HEM (Barbosa et al., 1990, Barbosa et al., 1992, Brooks, 1935, Hannon et al., 1989, Maningas et al., 1986, Nakayama et al., 1984, Smith et al., 1985, Traveso et al., 1987). However, although the changes in systemic vascular resistances during HEM in animals have received attention (see Liu et al., 2003, Whalen et al., 2007), nothing is known about the changes in systemic resistances elicited by administration of I-saline or H-saline in these rats. Such vital data would help us to understand how vascular beds subserving different physiological roles respond to HEM and to H-saline. Moreover, the rat is an ideal species to perform pharmacological studies designed to develop therapeutic strategies to treat hemorrhagic shock and other conditions associated with severe hypotension.
The first aim of this study was to determine the changes in MAP and systemic vascular resistances resulting from mild HEM in pentobarbital-anesthetized rats, in which the falls in MAP during HEM are comparable to those of conscious rats (Soucy et al., 1995a, Soucy et al., 1995b). The second aim was to determine the changes in MAP and vascular resistances elicited by bolus injections of I- and H-saline in these HEM rats. The changes in hindquarter (HQR), renal (RR) and mesenteric (MR) vascular resistances elicited by mild HEM were examined because of the key roles these vascular beds play in the circulatory adjustments to hemodynamic challenges, and because endocrine cells in the kidneys and mesentery release factors known to directly affect fluid movement across capillary walls and overall body fluid homeostasis (see Drucker et al., 1981).
Section snippets
Rats and surgical procedures
All studies were carried out in accordance with the National Institutes of Health Guide for the Care and Use of Laboratory Animals (NIH Publications No. 80-23) revised in 1996. The protocols were approved by the University of Iowa Institutional Animal Care and Use Committee. Male Sprague–Dawley rats (250–300 g) from Harlan (Madison, WI) were anesthetized with sodium pentobarbital (50 mg/kg, i.p.). A catheter was placed into a femoral vein to administer drugs. Supplemental doses of pentobarbital (5
Hemodynamic responses produced by HEM
Resting hemodynamic parameters recorded prior to beginning the HEM protocol in the three groups are summarized in Table 1. As can be seen, there were no between-group differences in these parameters. A typical example of the responses during HEM (4.2 ml) in a rat, which did not receive subsequent injections of I- or H-saline, is shown in Fig. 1. HR, MAP and blood flow velocities began to fall about half way through HEM. At the completion of HEM (0 min post-HEM), there was a reduction in HR (− 22%)
Discussion
This study investigated the hemodynamic responses elicited by low-volume injections of I- or H-saline in pentobarbital-anesthetized rats subjected to HEM. The major novel findings were that (1) the injections of I-saline promoted the gradual recovery of MAP toward (but still substantially below) pre-HEM levels via mechanisms involving diminished hindquarter vasodilation, (2) injections of H-saline elicited initial decreases in MAP and hindquarter vascular resistance but minimal changes in RR
Disclosures
No conflicts of interest, financial or otherwise are declared by the authors.
Acknowledgments
This work was supported in part by NHLBI HL14388 and HL57472, NASA NAG5-6171, and the Office of Naval Research N00014-97-1-0145.
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Current address: Novartis Institutes for Biomedical Research, 100 Technology Square, Cambridge, MA 02139, USA.