Compared with control, during the first hour, resuscitation with both fluids increased heart rate, CO, CVP, and total peripheral conductance (TPC) (P < 0.05; Fig. 1). Both fluids also increased coronary blood flow (CBF) and conductance and mesenteric blood flow (MBF) and conductance (P < 0.05, Fig. 2). During the second and third hour after treatment, both fluids maintained a greater CO, CVP, TPC, and MBF (P < 0.05) than control, but the difference decreased and was small (Figs. 1 and 2). However, because of hemodilution, overall systemic DO2 did not change during the study period (Fig. 3). Coronary and mesenteric DO2 values were also unaffected (Fig. 3).
Despite the clear systemic and regional changes, neither treatment affected RBF or renal conductance (Fig. 4). Furthermore, although systemic DO2 remained constant, because of hemodilution, renal DO2 significantly decreased in the first hour (Fig. 4).
Although renal DO2 decreased, both normal saline and 3% saline significantly increased urine output (UO) and creatinine clearance (Fig. 5).
Infusion of 3% saline significantly increased serum sodium from 143.9 ± 3.8 to 150.0 ± 6.2 mmol/L over the first hour, followed by a rapid decrease to 145.9 ± 5.6 mmol/L 1 h later. In parallel, serum osmolarity also increased transiently (P < 0.05, Fig. 6). Normal saline infusion did not affect serum sodium level and osmolarity (P > 0.05).
We conducted a randomized, controlled, cross-over study of normal saline and hypertonic saline fluid resuscitation to assess and compare their effects on regional blood flow and DO2 in the normal mammalian circulation. We found that, as expected, both fluids increased CO, CVP, TPC, and CBF and MBF by approximately 50%. But we also found that these effects were mostly transient (first hour) and significantly decreased in magnitude over the following 2 h to approximately 10%. Because of hemodilution, both fluids actually failed to increase coronary or mesenteric DO2. More importantly, RBF was unaffected and, because of hemodilution, renal DO2 was decreased by approximately 20% in the first hour in both groups. Simultaneously, UO and creatinine clearance increased significantly during this period. Finally, we observed that 3% saline significantly increased serum sodium and osmolarity compared with control and saline also for the first hour.
The systemic changes reported in our study are expected and consistent with previous reports (18). However, for the first time we were able to assess the effects of these two fluids at equivalent amounts of sodium on CBF, MBF and RBF and DO2 in conscious animals. We found that both fluids increased flow to the coronary and mesenteric circulation and that the increase was associated with regional vasodilatation. Thus, the changes in these two regional beds mirrored systemic hemodynamics. However, because these fluids induced significant hemodilution, calculated DO2 to the heart and gut remained unchanged. The effects of saline on CBF have, to our knowledge, only been described in a model of severe sepsis (19). Similar to the effect in septic animals, in this study, saline administration increased CBF in normal sheep. This observation and the increase in coronary conductance and CO suggest that regional vasodilatation, possibly related to increased metabolic demand or to the effects of fluid itself on afterload through decreased blood viscosity (20), played a part in this change. The effects of hypertonic saline were essentially identical. No other information concerning the effect of fluids on coronary DO2 has been reported.
The effect of normal saline on MBF and conductance has not been assessed in controlled studies in conscious animals, except for a model of severe hyperdynamic sepsis (19). Consistent with our findings in normal animals, saline also increased MBF in septic animals. The effect of hypertonic saline on MBF was identical to that of normal saline. Both fluids, however, were associated with similar levels of hemodilution and no change in mesenteric DO2. These observations, in parallel with the observations made in septic animals for the coronary circulation, suggest that the physiological principles established by studying the normal mammalian circulation may apply to hemodynamic states altered by disease, such as hyperdynamic sepsis or during anesthesia. On the other hand, in a study of ventilated, anesthetized animals with hemorrhagic shock, normal and hypertonic saline have been shown to affect mesenteric DO2 differently, with a greater increment in association with hypertonic saline (12). Unfortunately, this study lacked a control population and only analyzed the effect of these fluids over a 60-min period—an important shortcoming given our findings on the transient effects of fluid administration. In a hypodynamic model of sepsis, other investigators (13) recently demonstrated similar transient effects of hypertonic saline and Ringer's solution on portal blood flow.
An important and clinically relevant finding of our study was that RBF and renal conductance did not change in either treatment group. In fact, because of this lack of response and the effect of hemodilution, renal DO2 decreased during the first hour. Consistent with these observations, in a study of hyperdynamic sepsis, we recently demonstrated that saline resuscitation did not affect RBF (19). In a model of hypodynamic sepsis, other investigators have recently reported only minor and transient changes in RBF with either Ringer's solution or hypertonic saline (13). No data on renal DO2 were included. In a model of hemorrhagic shock, Chiara et al. showed no difference in flow effects of these two fluids but a greater DO2 with hypertonic saline at 60 min. Unfortunately, no data were provided on a control group or on any independent effect beyond the initial first hour because of the addition of blood transfusion to the study protocol (12). Further, Prough et al. found similar and only transient effects of resuscitation with normal or hypertonic saline in a model of hemorrhagic shock (11). No DO2 data were reported. None of these investigations, however, reported functional changes.
We found that, during the decrease in DO2in the first hour, UO and creatinine clearance increased. This observation is similar to observations made in hyperdynamic sepsis (19); namely, that UO and creatinine clearance are dissociated from, and cannot be clinically used as, predictors of changes in RBF. This paper extends such observations to renal DO2. It highlights that, if the intent is to increase DO2 to the kidney with fluid resuscitation, the opposite might happen under circumstances of stable RBF (MAP within the autoregulatory range). This is likely explained by fluid-induced hemodilution and stable RBF when the kidney is operating within its autoregulatory state (20). Moreover, even in models of hemorrhagic shock or hypodynamic shock, the renal effects of fluid resuscitation with either normal or hypertonic saline are transient (11,13). Because the glomerular filtration rate is partly regulated by plasma oncotic pressure, the dilutional effect of fluids on protein concentration can be reasonably expected, in the setting of unchanged blood flow, to increase both UO and creatinine clearance, as previously noted (21,22) and as seen in this experiment. It is important to appreciate that this increased glomerular filtration rate will lead to increased solute presentation to the tubules (esp. the thick ascending loop of Henle) and increased renal medullary oxygen consumption. If this occurs while DO2 is decreased (as seen here with saline or 3% saline loading), the renal medulla is then exposed to an increased risk of ischemia (23). These observations on the limited and temporarily negative effect of crystalloid fluid loading on RBF and DO2 also stand in clear contrast with the marked increase seen when normal animals receive norepinephrine infusion at moderate doses (24)
Finally, we observed a differential effect of hypertonic saline on serum sodium and osmolarity, as expected. The magnitude and duration of these effects has been previously assessed by Cooper et al. using 7.5% saline but in the setting of multiple other interventions (6). We found these effects to be moderate in size and transient in duration. The maintenance of a high osmolarity state for longer than 1 h would likely require repeated administration of hypertonic saline.
Our study has both strengths and limitations. The possible strengths include the presence of a control group (a feature absent in all other relevant studies identified in the literature), the use of an identical dose of sodium chloride, the study of conscious animals, the assessment of renal function (also a feature absent in all other relevant studies), and the measurement of study variables over an extended period. The limitations of our study relate to its assessment of regional flows without simultaneous measurement of regional oxygen extraction. This makes the assessment of the effect of changes in DO2 difficult. However, we wished to study conscious animals, which makes it technically extraordinarily difficult to maintain cannulation of regional veins. Further, in our study, the chronic implantation of such cannulas in the sheep leads to vascular thrombosis. We could only calculate total regional values for conductance (the inverse of resistance). Changes in global organ conductance in any given direction may mask within-organ changes in the opposite direction such that, for example, in the kidney, changes in cortical and medullary conductance may differ. This aspect of our study needs to be considered when interpreting our findings. We did not induce hemorrhagic shock before fluid administration, making our observations less clinically relevant to one possible setting in which such fluids might be given. However, we consider that the first fundamental step in a planned research program dedicated to understanding the effect of such fluids should be to study them in the normal mammalian circulation. Our observation that in normal animals, the findings for normal saline mirror those reported in hyperdynamic sepsis, supports the relevance of such an initial approach, and the findings concerning the effect on the renal circulation are likely to apply to most, if not all, situations where RBF is operating within the limits of autoregulation (MAP between approximately 60 and 120 mm Hg). We now plan to study these fluids further in a variety of simulated disease states. We used 3% hypertonic saline instead of 7.5% hypertonic saline as others have used. However, on the basis of our observations, we consider it unlikely that, provided an equal dose of sodium chloride is given over the same period, the regional flow findings would differ. The increase in serum osmolarity, however, might be greater with 7.5% saline (6). Finally, we did not compare these two crystalloid solutions to a colloid solution. This would be clinically important, and accordingly, such studies are now under way in our laboratory.
In summary, both normal and hypertonic saline significantly, but only transiently, increase CO, CVP, TPC, CBF and MBF. Yet, they do not affect RBF, and they transiently significantly decrease renal DO2 while increasing tubular solute presentation. In addition, although 3% saline significantly increases serum sodium and osmolarity, this effect lasts only for approximately 1 h. Knowledge of the direction, magnitude and nature of these changes may assist clinicians in appreciating the likely physiological consequences of their interventions.
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