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Department of Intensive Care Medicine, Catholic University of Leuven, Leuven, Belgium
Address correspondence and reprint requests to Greet Van den Berghe, MD, PhD, Department of Intensive Care Medicine, Catholic University of Leuven, B-3000 Leuven, Belgium. Address e-mail to greta.vandenberghe{at}med.kuleuven.ac.be
| Abstract |
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| Introduction |
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-adrenergic actions gradually become important. Infusion rates of 10 to 20 µg · kg-1 · min-1 produce primarily
- and ß-adrenergic effects (2). It is important to note that these dose ranges are not cutoff values, at which one set of receptors is activated at the expense of another, but rather ranges within which the effects of one group of receptors predominate over another, with a huge variation between individuals. Furthermore, plasma DA clearance is much smaller in critically ill patients than in healthy people, again with considerable interindividual variation and little correlation between plasma levels and infusion rate (3). After the pioneering work by Goldberg (1) suggested a protective effect of low-dose DA (LDD) (<5 µg · kg-1 · min-1) on renal function, the drug became widely used in intensive care medicine for optimization of renal and splanchnic perfusion. Recently, these presumed protective effects on renal and splanchnic function have been repeatedly questioned, and an increasing number of side effects have been reported. Doubts are thus increasing about the place of DA in the treatment of critically ill patients.
In this review, we discuss the scientific evidence for and against the use of DA in terms of effect on several organ systems. In addition, we evaluate the scientific basis for the use of DA as a vasopressor.
| Renal Effects |
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In addition, DA also triggers natriuresis and diuresis through a direct effect on the tubular cell function. DA binds to DA-1 and DA-2 receptors in the proximal tubule, the thick ascending limb of the loop of Henle, and the cortical collecting ducts inhibiting Na+/K+-adenosine triphosphatase activity, thereby inducing natriuresis (8). Furthermore, activation of DA-2 receptors in the inner medullary collecting ducts stimulates prostaglandin E2 (PGE2) production, which antagonizes the effects of antidiuretic hormone, resulting in a washout of the medullary area and increased clearance of free water (9).
LDD induces regional re-distribution of blood flow within the kidney, preferentially increasing cortical blood flow (7), whereas PGE2 enhances blood flow in the inner medulla. LDD thus induces a shunting of blood away from the outer medulla. This is potentially detrimental in acute renal failure (ARF), given that the outer medulla, in particular, is highly metabolically active and thus very susceptible to ischemic injury (10,11).
As the result of the pioneering work of Goldberg (1), infusion of LDD was considered an attractive management option in patients with incipient or established ARF. However, at that time there were no clinical studies demonstrating the beneficial effect of LDD on renal function in critical illness and, indeed, although LDD may selectively increase renal blood flow in healthy volunteers (4), one cannot extrapolate this effect to critically ill patients.
Clinical trials have now investigated the use of LDD for the prevention of ARF in patients at risk, as well as its therapeutic use in patients with established ARF. Some of these studies have shown that LDD increases urine output (2,1217), whereas others found no effect (5,1820). The renal effect of DA also seems to decrease progressively with increasing severity of renal dysfunction and is lost in patients with a glomerular filtration rate less than 50 mL/min per 1.73 m2 (21). One study even suggested a detrimental effect of LDD on the tubular function, because LDD increases urinary excretion of retinol binding protein in patients undergoing coronary bypass surgery (18). As is often the case in the critical care setting, these inconsistencies are due to the heterogeneity of the studied patient population, the dose of DA used, or the timing and duration of DA administration.
Other problems are related to the study design, such as absence of concurrent control groups, absence of randomization, and relatively small sample sizes, which prevented adequate statistical analysis and conclusions. DA-induced diuresis may also be falsely reassuring because an increased urine output is not automatically associated with an improved renal function (16,17). Because renal hypoperfusion is a leading cause of ARF (10,11), one should be aware of the risk of inducing renal failure by increasing urine output in normovolemic and hypovolemic patients (20,22). Although several of these studies concluded that LDD influences renal function directly without significant effects on systemic hemodynamics, many did not report measurements of cardiac output. An opinion more recently gaining acceptance postulates that LDD increases urine output by enhancing cardiac output and thus not primarily by a direct renal effect (14,15,22).
In addition to the poor evidence for any beneficial effect of DA on renal function in the critically ill patient, evidence for any benefit on survival was also lacking until recently. A large study by the Australian and New Zealand Intensive Care Society group (23) clearly showed that LDD does not prevent or reverse ARF, nor does it improve outcome. It was the first large, prospective, randomized, double-blinded, placebo-controlled trial investigating the potential of LDD (2 µg · kg-1 · min-1) in critically ill patients with systemic inflammatory response syndrome and early renal dysfunction. In the 324 patients included, no differences in mortality, requirement for renal replacement therapy, renal recovery, or peak serum creatinine were found (23) (Fig. 1).
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Some authors suggest that the addition of LDD in patients who require norepinephrine may limit the vasopressor-induced adverse effects on renal function. Experimental studies have shown that infusion of LDD improves renal hemodynamics in healthy animals (28) and volunteers (5,6). However, LDD had no effects on renal flow in experimental animal models of septic shock treated with norepinephrine (29). Likewise, in patients with septic shock treated with catecholamines, LDD had no effect on renal function (16,22). Although the addition of LDD can increase urine output in patients with septic shock treated with norepinephrine (15,22), an effect that is probably directly related to its increase of cardiac output, this does not improve renal function (15). In sepsis, the renal vascular autoregulation and vasodilation are probably already maximal and not amendable by pharmacological vasodilators. There is thus no justification for the routine use of LDD to improve renal function in patients treated with large doses of catecholamines (16).
We can conclude that LDD may increase urine output in critically ill patients, but it neither prevents nor improves ARF. When DA does increase diuresis, it may actually increase the risk of ARF in normovolemic and hypovolemic patients.
Effects on Splanchnic Perfusion
The gut may be particularly susceptible to ischemia in shock, and disruption of the gut mucosal barrier is thought to play a key role in the development of multiple organ failure (30). Theoretically LDD may increase splanchnic blood flow by stimulation of the splanchnic dopaminergic receptors. The evidence, however, is poor.
In experimental animal models, DA increases splanchnic and hepatic blood flow (31,32). However, this is not necessarily accompanied by an improvement of mucosal perfusion (33). In dogs, LDD reduced intestinal oxygen uptake and mucosal blood flow (32), and in a porcine model of hemorrhagic shock, LDD hastened the onset of gut ischemia (34). A decrease in the ability of the gut to extract oxygen during infusion of DA could be explained by redistribution of blood flow within the gut, resulting in a reduced blood flow selectively in the mucosa (32,33). In other animal studies, however, LDD did improve mucosal blood flow, as well as oxygenation (31,35,36). These different effects can be explained, at least in part, by the different models and drug doses used (37).
Human data are even more controversial. LDD has been reported to increase splanchnic blood flow in septic shock and cardiac surgery patients (38,39), whereas other investigators found no effect (40). The observed increase in splanchnic blood flow in septic patients seems to vary between individuals, depending on the initial fractional splanchnic blood flow (38). Furthermore, LDD was recently shown to decrease the splanchnic oxygen consumption in septic patients despite an increase in systemic and splanchnic blood flow, an effect not seen in cardiac surgery patients (37). The same group of investigators previously showed a variable effect of the use of DA as a vasopressor on splanchnic oxygen consumption: 3 of 5 patients decreased and two increased splanchnic oxygen consumption (39). This controversy can be explained by several methodological differences between these two studies, such as the severity of illness on the start of DA and the timing and dose of DA infusion. Although LDD increases oxygen transport in septic patients, it decreases the gastric mucosal flow (41) and does not affect intramucosal pH (pHi), a surrogate marker of gut mucosal perfusion (17,4042). Larger doses of DA are associated with an even further decrease of pHi (43).
Thus, as for the renal effect of LDD, there is no evidence that LDD has beneficial effects on the splanchnic function or reduces the progression to multiple organ failure in sepsis. Recent data even suggest a potentially detrimental effect of LDD on splanchnic oxygen uptake.
Effects on Gastrointestinal Motility
DA-2 are present in the human enteric nervous system, and DA-antagonistic drugs, such as metoclopramide and domperidone, improve the anteroduodenal coordination, which is required for optimal gastric emptying. One can thus suspect that infusion of DA will interfere with the gastrointestinal motility. Indeed, in healthy volunteers short-term DA administration has been shown to interrupt the fed gastrointestinal motility pattern (44). In critically ill patients, the use of DA (2.55 µg · kg-1 · min-1) was found to be the most significant factor associated with poor gastric emptying (45), and it was clearly shown that LDD adversely affects gastroduodenal motility in mechanically ventilated critically ill patients both during fasting and nasogastric feeding (46). Hence, the use of DA cannot be reconciled with the current recommendation to preferentially use enteral rather than parenteral nutrition in critically ill patients. Indeed, because DA may aggravate digestive intolerance to enteral feeding, its use is not advisable.
Respiratory Effects
There are two potentially detrimental effects of LDD on respiratory function that are often overlooked. First, DA has been shown to impair the ventilatory drive in response to hypoxemia and probably hypercapnia by depressing the carotid body (47). Second, DA reduces arterial oxygen saturation by impairing regional ventilation/perfusion matching in the lung (48). Both mechanisms are synergistic and can usually be counterbalanced by a larger oxygen supplement. In mechanically ventilated patients, depression of the ventilatory drive by DA does not impose a clinically relevant problem. However, such problems can arise when the patient is taken off mechanical ventilatory support, because LDD blunts the conscious discomfort evoked by arterial hypoxemia and hypercapnia (47). Thus, when a patient is being weaned from ventilatory support while still receiving DA, he or she may not be able to give the physician important symptomatic feedback about impaired gas exchange (47,49). Paradoxically, patients receiving LDD may be easier to wean, but with the potential danger of precipitating respiratory failure (49). In summary, also from a respiratory point of view, the use of LDD cannot be advised.
Endocrine and Immunological Effects
The anterior pituitary gland plays a crucial role in metabolic and immunologic homeostasis; the corticotropic, lactotropic, gonadotropic, and thyrotropic axes are key determinants of normal growth, metabolism, and host defense. Critical illness is associated with various alterations in these neuroendocrine axes, depending on the phase (acute versus chronic) of illness (50). In the initial stress response, the release of all anterior pituitary hormones is stimulated, whereas in more prolonged critical illness, a uniform suppression of the hypothalamic-pituitary axes ensues while cortisol secretion remains increased through a peripheral drive. Although the functional implications and clinical relevance of these changes are unclear, it is conceivable that the acute changes are part of the bodys protective mechanism against disease. However, in prolonged critical illness, hypothalamic hypopituitarism has been found to evoke inappropriate and harmful metabolic changes (51). DA has been shown to further suppress the secretion and function of a number of key anterior pituitary hormones (Fig. 2) and may thus aggravate impairment of anabolism and cellular immune function.
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Other neuroendocrine hormonal axes are also affected by DA administration during critical illness. Growth hormone (GH) is a typical stress hormone that has high levels in response to several types of insults, whereas in prolonged critical illness, GH secretion is blunted and insulin-like growth factor-I (IGF-I) levels are low (50). This conceivably contributes to impaired anabolism, because it has been shown that the co-infusion of GH secretagogues and thyrotropin releasing hormone (TRH) restores pulsatile GH secretion and normalizes serum IGF-I levels, which results in anabolism in peripheral lean tissues, such as bone (51). Prolonged DA infusion further suppresses pulsatile GH secretion and is associated with even lower levels of IGF-I (52). Dehydroepianosterone sulfate, an anabolic and immunostimulating steroid secreted by the adrenal cortex, is also suppressed by LDD administration during critical illness (62) (Fig. 3).
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When given in the acute phase of illness, DA induces the pituitary impairment seen in prolonged critical illness. When used in the chronic phase of critical illness, infusion of DA further aggravates the suppression of circulating concentrations of pituitary hormones. Although the full effect of the endocrine abnormalities seen in intensive care patients remains unclear, the additional suppressive effects of DA, especially when given for more than a few days, are unlikely to be beneficial and may be harmful (55).
DA as a Vasopressor
Which vasopressor to use when hypotension persists after volume resuscitation in distributive shock remains an unanswered question. Both norepinephrine and DA in the appropriate dose are excellent
-adrenergic agents (vasopressor) with an associated lesser degree of ß-adrenergic activity (inotrope). Because of the fear of excessive vasoconstriction, norepinephrine is still considered by many to be deleterious, causing end-organ hypoperfusion and severe ischemia of vital organs; this explains its nickname ("Lethalphed"). Therefore, DA is still often preferred as the first-choice vasopressor when hypotension persists despite adequate fluid resuscitation (64). However, for patients in whom DA fails to restore an adequate hemodynamic status, norepinephrine has been found to be beneficial. In recent years, an increasing number of favorable reports on the use of norepinephrine in effectively volume-resuscitated septic shock patients have shown that the fear of deleterious effects of norepinephrine is unwarranted. Martin et al. (65) reported a faster decline of circulating lactate levels in patients with septic shock treated with norepinephrine as compared with DA. In a septic setting, DA seems to act largely by increasing cardiac output, whereas norepinephrine more specifically increases vascular resistance without compromising flow (43).
The use of norepinephrine, as compared with DA, prevents delay in the restoration of mean arterial blood pressure and reduces the time to resuscitation. When excessively large doses of norepinephrine are avoided and volume resuscitation is adequately performed by using optimized monitoring, the adverse effects of norepinephrine can be effectively minimized or even avoided. Animal data suggest that there may even be a protective effect of norepinephrine on renal blood flow in septic shock (66). Human studies have also shown an improvement of urine output in patients treated with norepinephrine (65). Furthermore, no deleterious effects of norepinephrine on the splanchnic circulation have been demonstrated in animal models of endotoxin shock (67), as well as in patients with septic shock (68). In patients with septic shock, norepinephrine may even increase pHi, whereas DA tends to decrease the pHi further (43). In line with these data, it was recently shown that DA (4 µg · kg-1 · min-1) reduces hepatosplanchnic oxygen uptake in septic patients despite an increase in systemic and regional perfusion, an effect not seen with dobutamine (37). Hence, there is actually no evidence that reversing hypotension with norepinephrine compromises mesenteric or renal flow after adequate volume resuscitation, and the fear that norepinephrine might contribute to the progression of multiple organ failure has been vitiated (69). One study even observed a survival advantage of norepinephrine treatment as compared with other vasopressors in septic shock patients (70).
There are strong arguments for the addition of an inotrope to the vasopressor therapy in patients with septic shock whenever suppressed myocardial function is suspected, to optimize regional perfusion and oxygen delivery (71). However, the theoretical advantage of DA to also exert inotropic effects through ß-agonism does not annihilate its side effects. Instead, adding a more specific ß-agonist, such as dobutamine, not only abrogates these side effects but also, through separate titration, allows a more targeted intervention tailored by the patients condition. Therefore, although there are few valid studies comparing the use of DA versus norepinephrine, norepinephrine may be emerging as the vasopressor of choice for severe hypotension in adequately volume-resuscitated septic shock patients.
| Conclusion |
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- and ß-adrenergic effects. These effects are believed to be dose dependent and vary considerably between patients. DA is used to support cardiac output and blood pressure in patients with cardiac failure and distributive shock. Because of the work of Goldberg (1), which suggested that LDD induces protective effects on renal and splanchnic function, and because of the general belief that LDD is relatively free of side effects, the drug also became widely used in intensive care to protect kidney and gastrointestinal function in critically ill patients. However, these assumptions were proven incorrect. There is indeed no evidence that low "renal" dose DA has any beneficial effect on renal function or on the outcome of patients with ARF. Furthermore, there is no evidence that LDD has beneficial effects on hepatosplanchnic circulation, and recent data suggest that DA may even have detrimental effects on splanchnic oxygen uptake. In addition to the lack of efficiency, side effects of LDD have increasingly been reported and documented. It has been clearly shown that DA suppresses the secretion and function of anterior pituitary hormones, aggravating the impairment of anabolism and cellular immune function. DA also aggravates the digestive tolerance of enteral feeding and suppresses the ventilatory drive.
We conclude that there is now ample scientific evidence demonstrating that LDD is ineffective and has several serious side effects. The widespread use of LDD for renal protection can thus no longer be justified. In view of its side effects, there are clearly much better alternatives to use as first-choice vasopressors or inotropes in patients with septic shock.
| Acknowledgments |
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| References |
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