The role of inducible nitric oxide synthase in the evolution of myocardial (dys)function during resuscitated septic shock: The missing loop*
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athophysiologic generation of nitric oxide contributes to cardiovascular dysfunction during septic shock: Endotoxin- and cytokine-induced expression of inducible nitric oxide synthase (iNOS) in vascular endothelial and smooth muscle cells results in a massive release of nitric oxide causing profound vasodilation manifest as systemic hypotension, typically hyporesponsive to pressure agents. In addition, nitric oxide directly affects myocardial function (1, 2). Studies have shown a clear negative effect on systolic function, whereas diastolic function was found to be improved (3). The latter is evident from improved active relaxation and increased diastolic compliance. In this issue of Critical Care Medicine, Dr. Barth and colleagues (4) present an experimental study that evaluated these effects, in particular the role of iNOS, in a murine model of septic shock. The authors induced polymicrobial sepsis by ligating and puncturing the cecum in these animals and subsequently followed hemodynamics and cardiac function for 6 hrs starting at a time point 18 hrs after induction of sepsis. Although most experimental studies on sepsis use an endotoxin model (intraperitoneal injection of lipopolysaccharide), the cecal ligation and puncture model should be considered as a more realistic model of clinical sepsis (5). Moreover, the authors further mimicked the clinical situation by continuous intravenous colloid infusion and norepinephrine administration. The infusion rate of a combined colloid-norepinephrine mixture was titrated to main-
*See also p. 307. Key Words: sepsis; septic shock; inducible nitric oxide synthase; myocardial function; loading conditions; pressure-volume analysis The author has no financial interests to disclose. Copyright © 2006 by the Society of Critical Care Medicine and Lippincott Williams & Wilkins DOI: 10.1097/01.CCM.0000199072.99259.1E
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tain adequate mean arterial pressure (⬎65 mm Hg) throughout the 6-hr observation period. The role of iNOS was investigated by comparing the findings in normal (wild-type), untreated mice (control group) vs. iNOS⫺/⫺ knockout mice on the one hand and wild-type mice treated with a selective iNOS inhibitor on the other hand. Cardiac function was quantified by left ventricular pressures and volumes and derived variables. To this end, the animals were anesthetized, ventilated, and instrumented with infusion lines and a miniature left ventricular pressure-conductance catheter (6). The findings in the three groups were markedly different. First, subjectively, the control mice presented with more “clinical signs,” suggesting more severe sepsis than the other two groups. Second, compared with the control group, the norepinephrine infusion rate required for adequate resuscitation was only about half in the animals treated with the iNOS inhibitor and one fourth in the iNOS⫺/⫺ group. The same trend was present for the required amounts of colloids, but this effect was less pronounced and did not reach statistical significance. With regard to cardiac function, the authors summarize their finding by stating that genetic iNOS deletion or pharmacologic iNOS inhibition reduced norepinephrine requirements due to improved systolic function, whereas relaxation appeared to be impaired. The reduced norepinephrine requirement is clearly supported by the data, but it is not clear why the authors conclude that systolic function was improved. All reported “systolic” variables (arterial pressure, stroke volume, ejection fraction, cardiac output, and maximal change in pressure over time) are the same or, in most cases, even tend to be higher in the control animals than in the other two groups. The suggestion that the higher norepinephrine requirement in the control animals reflects that the underlying baseline systolic function was
more depressed cannot be proven by the present data. With regard to diastolic function, minimum change in pressure over time and may suggest a slightly slower relaxation in the iNOS⫺/⫺ and the iNOS-inhibited animals. However, these variables merely reflect the early, active part of diastole and, in fact, are strongly dependent on systolic function as well (7). Diastolic compliance, reflected by the end-diastolic pressurevolume point, presumably is more relevant in this situation, as a marker for adequate filling of the ventricle. The authors, indeed, mention a higher enddiastolic volume at comparable enddiastolic pressure in the control group, which suggests that diastolic compliance in this group was better (i.e., higher). Unfortunately, the authors did not fully exploit the acquired pressure-volume data. The major advantage of pressure-volume loop analysis over a more conventional description of ventricular function is that pressure-volume analysis may generate relatively load-independent indexes of systolic and diastolic function (8). This is a very useful approach particularly in situations where loading conditions are likely to be substantially altered. Furthermore, the pressure-volume framework enables quantification of the effective arterial elastance, which in combination with ventricular elastance can be used to determine (changes in) ventricular-arterial coupling, an important determinant of net cardiovascular performance and reserve function (9, 10). Theoretically, all variables, as presented in the tables and figures of Dr. Barth and colleagues’ article, are affected not only by changes in intrinsic myocardial function but also, to some extend, by changes in preand afterload. In the situation of septic shock with fluid resuscitation and catecholamine support, all of these factors are likely to be altered either directly or indirectly, and thus the conventional variables are rather difficult to interpret. In an attempt to illustrate this, Figure 1 shows schematic 545
Figure 1. Schematic pressure-volume loops in wild-type (WT), control mice, and inducible nitric oxide synthase (iNOS) ⍺/⍺ knockout mice, based on data reported by Barth et al. (4) in this issue of Critical Care Medicine. The symbols mark the end-systolic and end-diastolic pressure-volume points. Sham, sham-operated (i.e., nonseptic) mice; CLP, cecal ligation and puncture; ESPVR, end-systolic pressure-volume relation; EDPVR, end-diastolic pressurevolume relation; LV, left ventricular. A detailed description is given in the text.
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pressure-volume loops in the various conditions for the control (wild-type) group and the iNOS⫺/⫺ group. The schematic loops are based on the reported (median) end-diastolic pressures and volumes and on the estimated end-systolic pressures and volumes. Unfortunately, the latter were not reported, and therefore mean arterial pressure was substituted for end-systolic pressure and end-systolic volume was calculated as end-diastolic volume minus stroke volume. Furthermore, only a single-beat analysis could be done; therefore, a simplified approach assuming linear end-systolic and end-diastolic pressure-volume relations with a fixed zero-volume intercept was used. Despite these limitations, this analysis may provide additional insights in the findings. First, Figure 1A and 1B, respectively, shows the loops in septic (cecal ligation and puncture) control and iNOS⫺/⫺ animals at the 18-hr time point in relation to the corresponding shamoperated (i.e., nonseptic) animals. The different response in control vs. iNOS⫺/⫺ is obvious: In the control animals, resuscitated septic shock resulted in enhanced systolic function, evident from a steeper endsystolic pressure-volume relation, and improved diastolic function, represented by a downward shift of the diastolic pressurevolume relation. Even without relying on the simplified pressure-volume relations, the finding of an increased systolic pressure at a smaller end-systolic volume is clear evidence for increased systolic function, and a similar reasoning holds for the improved diastolic function. In contrast, in the iNOS⫺/⫺ group, both diastolic function and systolic function are virtually unchanged (or, if anything, systolic function is even decreased in the resuscitated septic animals). The evolution in time (18, 21, 24
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hrs) is shown in Figure 1E and 1F. In the control group (Fig. 1E), a continuous decrease in systolic function is evident, whereas diastolic function appears unchanged. The iNOS⫺/⫺ loops (Fig. 1F) show an initial decrease in systolic function at 21 hrs but no further changes at 24 hrs. Diastolic function appears to be gradually improved somewhat. Finally, the middle panels (Fig. 1C and 1D) point to potential limitation of the model: The time evolution of ventricular function in the shamoperated animals shows a gradual change in both systolic and diastolic function, presumably related to the prolonged anesthesia. In fact, this effect may partly explain the decreased systolic function in the septic shock groups. Obviously, these new results should be interpreted cautiously because they were obtained by simply combining the reported median results for the various variables, rather than from an analysis of the individual pressure-volume data. Moreover, the observed changes may not be statistically significant. In conclusion, Dr. Barth and colleagues should be complimented for a very interesting study, which provides new insight into the controversial issue of iNOS inhibition in septic shock. The developed murine model closely mimics the clinical situation, and future studies, preferably including a detailed analysis of pressure-volume loops and relations, should reveal the complex interaction between alterations in loading conditions and intrinsic myocardial function. This information ultimately should help to develop more effective therapies for septic shock. Paul Steendijk, PhD Department of Cardiology Leiden University Medical Center Leiden, The Netherlands
REFERENCES 1. Parrillo JE, Burch C, Shelhamer JH, et al: A circulating myocardial depressant substance in humans with septic shock. Septic shock patients with a reduced ejection fraction have a circulating factor that depresses in vitro myocardial cell performance. J Clin Invest 1985; 76:1539 –1553 2. Parrillo JE, Parker MM, Natanson C, et al: Septic shock in humans. Advances in the understanding of pathogenesis, cardiovascular dysfunction, and therapy. Ann Intern Med 1990; 113:227–242 3. Paulus WJ, Vantrimpont PJ, Shah AM: Acute effects of nitric oxide on left ventricular relaxation and diastolic distensibility in humans. Assessment by bicoronary sodium nitroprusside infusion. Circulation 1994; 89: 2070 –2078 4. Barth E, Radermacher PL, Thiemermann C, et al: Role of inducible nitric oxide synthase in the reduced responsiveness of the myocardium to catecholamines in a hyperdynamic, murine model of septic shock. Crit Care Med 2006; 34:307–313 5. Villa P, Ghezzi P: Animal models of endotoxic shock. Methods Mol Med 2004; 98:199 –206 6. Georgakopoulos D, Mitzner WA, Chen CH, et al: In vivo murine left ventricular pressurevolume relations by miniaturized conductance micromanometry. Am J Physiol 1998; 274:H1416 –H1422 7. Brutsaert DL, Sys SU: Relaxation and diastole of the heart. Physiological Reviews 1989; 69: 1228 –1315 8. Burkhoff D, Mirsky I, Suga H: Assessment of systolic and diastolic ventricular properties via pressure-volume analysis: A guide for clinical, translational, and basic researchers. Am J Physiol Heart Circ Physiol 2005; 289: H501–H512 9. Kass DA: Age-related changes in ventriculararterial coupling: Pathophysiologic implications. Heart Fail Rev 2002; 7:51– 62 10. Kass DA: Ventricular arterial stiffening: Integrating the pathophysiology. Hypertension 2005; 46:185–193
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