6. Stanek B, Frey B, Hulsmann M, et al: Prognostic evaluation of neurohumoral plasma levels before and during beta-blocker therapy in advanced left ventricular dysfunction. J Am Coll Cardiol 2001; 38:436 – 442
7. Charpentier J, Luyt CE, Fulla Y, et al: Brain natriuretic peptide: A marker of myocardial dysfunction and prognosis during severe sepsis. Crit Care Med 2004; 32: 660 – 665
8. Roch A, Allardet-Servent J, Michelet P, et al: NH2-terminal pro– brain natriuretic peptide plasma level as an early marker of prognosis and cardiac dysfunction in septic shock patients. Crit Care Med 2005; 33:1001–1007
Going on and on with NO?*
P
ulmonary artery vascular tone is mainly determined by the smooth muscle cell calcium balance (1). The most important vasoactive substances that regulate pulmonary vascular tone are derived from the endothelium. Key players in this process are endothelin-1 (ET-1), prostacyclin (PGI2), and nitric oxide (NO). The effects of ET-1 on pulmonary vascular resistance are mediated by two distinct receptor subtypes: the ETA receptor on the vascular smooth muscle cell is responsible for vasoconstriction (2), whereas the ETB receptor on the endothelial cell is responsible for vasodilation (3). There are clear developmental alterations in ET-1 receptor densities: In the fetal and newborn pulmonary circulations, exogenous ET-1 predominantly vasodilates (via the ETB receptor and subsequent NO release), whereas in the juvenile and adult pulmonary circulations, the predominant effect is vasoconstriction (via ETA receptor activation) (4). Arachidonic acid metabolites (eicosanoids) are among the most potent vasoactive substances. Arachidonic acid is degraded via the cyclooxygenase pathway to produce the primary prostaglandins, including PGI2 that is synthesized in endothelial cells. PGI2 subsequently activates adenylate cyclase in the adjacent smooth muscle cell. This process increases adenosine 3',5'-cyclic monophosphate (cAMP), which in turn initiates a cascade resulting in calcium efflux and subsequent smooth muscle relaxation (5). The biological action of PGI2 may be balanced by thromboxane, a vasoconstrictor that is synthesized in platelets and macrophages (6). NO, an inorganic,
*See also p. 1008. Key Words: inhaled nitric oxide; endothelin-1; pulmonary vascular smooth muscle Copyright © 2005 by the Society of Critical Care Medicine and Lippincott Williams & Wilkins DOI: 10.1097/01.CCM.0000163241.13478.88
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gaseous free radical, is a potent vascular smooth muscle relaxant with an important regulatory function for the pulmonary vascular bed (7). NO is synthesized from L-arginine catalyzed by nitric oxide synthase (NOS). In mammals, three isoforms of NOS are known: neuronal and endothelial NOS (nNOS and eNOS, respectively), whose activities are regulated by calcium and calmodulin, and inducible NOS (iNOS), which is largely calcium independent. Whereas expression of iNOS is induced by cytokines and other agents, eNOS may be activated in response to endothelial shear stress and, for example, bradykinin. NO, produced in the endothelial cell, diffuses into adjacent vascular smooth muscle cells and activates soluble guanylate cyclase, which catalyzes the production of guanosine 3',5'-cyclic monophosphate (cGMP). Finally, this cGMP initiates a cascade leading to smooth muscle relaxation. The intracellular levels of the cyclic nucleotide second messengers cAMP and cGMP, which play a central role in pulmonary vascular smooth muscle cell relaxation, are regulated by phosphodiesterases (PDE) (8). These enzymes, of which a large number of specific isoforms have been identified, are able to degrade the nucleotides. This underlies the vascular relaxation obtained by pharmacologic PDE inhibitors. Of note is that NO may also lead to smooth muscle relaxation by directly activating calcium-dependent potassium channels without requiring cGMP (9). Other important substances relevant for regulation of the pulmonary vascular tone include angiotensin II and atrial natriuretic peptide. The former has vasoconstrictive properties, and the latter causes pulmonary vasodilation (10). Important interactions are present: NO, for example, is involved in the regulation of vascular tone by the angiotensin II system by down-regulating the number and binding activity of angiotensin II receptors (11).
The therapeutic potential of exogenous, inhaled NO was recognized in the early 1990s. After inhalation, NO diffuses across the alveolar-capillary membrane into the smooth muscle of the pulmonary vessels and produces vasodilation via the same pathways as described for endogenous NO. Inhaled NO specifically increases blood flow to well-ventilated lung areas and thus improves ventilationperfusion matching. This effect is in contrast to intravenously administered vasodilators that produce diffuse dilation of the pulmonary vasculature including the nonventilated areas and thereby may increase intrapulmonary shunting. Furthermore, whereas intravenously infused vasodilators may cause systemic vasodilation and arterial hypotension, inhaled NO is largely selective for the lung vasculature because NO is scavenged by hemoglobin and inactivated when it diffuses into the blood. Currently, the main clinical applications of inhaled NO are the treatment of persistent pulmonary hypertension and respiratory distress syndrome in the premature newborn (12). Inhaled NO has also been applied during peri- and postoperative pulmonary hypertension after surgery for congenital heart disease (13), cardiac transplantation (14), and assist device insertion (15). Furthermore, clinical trials are reported with severe acute respiratory distress syndrome (16, 17) and chronic obstructive pulmonary disease (18). Potential negative side effects of inhaled NO include the toxicity of NO2 formed during NO breathing (19), and, in patients with severe left ventricular dysfunction, elevation of left ventricular filling pressure that may produce pulmonary edema. Furthermore, with prolonged use some degree of tachyphylaxis or tolerance may appear, and discontinuation of NO inhalation may cause severe rebound pulmonary hypertension, an increase in intrapulmonary right-toleft shunting, and a decreased PaO2 (20, 21). It has been suggested that this re1157
bound phenomenon is caused by downregulation of endogenous NO synthesis and elevated ET-1 levels during prolonged NO inhalation, but the underlying mechanisms remain to be elucidated (22, 23). In this issue of Critical Care Medicine, Dr. Lukaszewicz and colleagues (24) report on the use of NO inhalation in patients with acute lung injury or right ventricular failure. They specifically aimed to analyze the effects of NO inhalation on endogenous NO, ET-1, and renin-angiotensin pathways, because possible alterations in these pathways might interfere with prolonged NO therapy and have implications for NO withdrawal. Their results confirm the therapeutic benefits of inhaled NO on oxygenation, pulmonary resistance, and cardiac output found in previous studies and indicate that these effects were not attenuated during prolonged NO therapy (range, 0.5– 6.5 days). Furthermore, the beneficial effects were reported to disappear immediately after withdrawal of NO inhalation. These findings suggest that endogenous pathways were not importantly altered during prolonged NO inhalation. This suggestion was further supported by stable increased cGMP and NOx plasma concentrations during NO therapy, whereas plasma levels of vasoactive mediators, including ET-1, angiotensin II, angiotensin-converting enzyme, atrial natriuretic peptide, and renin activity, were unaffected by NO inhalation and withdrawal. The finding of unchanged ET-1 plasma levels appears to be at odds with the bulk of previous animal studies and with some previous patient studies. Potentially, a relatively high baseline plasma ET-1 concentration in the patients in Dr. Lukaszewicz and colleagues’ study may have played a role in the lack of alteration of endogenous ET-1. Furthermore, the results from two patient groups with different pathologies and potentially different mechanisms were pooled, which may have obscured some of the results. More in general, changes in receptor densities and affinities may occur during prolonged NO therapy (25) and, as mentioned by the authors, plasma levels may not adequately reflect tissular
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ET-1 production. However, despite the fact that some issues may require further study, the findings of Dr. Lukaszewicz and colleagues are very interesting and appear to support the possibility of prolonged use of inhaled NO. Paul Steendijk, PhD Department of Cardiology Leiden University Medical Center Leiden, The Netherlands
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