Adrenomedullin is a potent vasodilatory peptide originally identified in human pheochromocytomas . Plasma adrenomedullin concentration was reported to be increased in patients with cardiovascular diseases such as hypertension, acute myocardial infarction, congestive heart failure, and chronic renal failure [2-4]. Although the origin of plasma adrenomedullin has not been well elucidated, several studies have revealed that vascular endothelial and smooth muscle cells actively synthesize and secrete adrenomedullin [5,6], and specific receptors for adrenomedullin are expressed on these cells [7,8]. This suggests that these vascular cells may be major sources of plasma adrenomedullin that may act as a local modulator, as well as a circulating hormone in the control of vascular tone. Furthermore, adrenomedullin production in vascular smooth muscle cells is increased by some cytokines, such as tumor necrosis factor and interleukin-1 . Cardiopulmonary bypass (CPB) often induces systemic inflammatory responses and augments blood levels of chemotactic factors and proinflammatory cytokines . Based on these facts, Nagata et al.  investigated plasma concentration of adrenomedullin in patients undergoing cardiac surgery with CPB and reported that plasma adrenomedullin increases during and after CPB. However, they did not identify any sites for the augmented adrenomedullin production. Although they suggested that the pulmonary vasculature is the most likely source of adrenomedullin production, our recent study could not identify the pulmonary circulation as the main source of increased plasma adrenomedullin-related CPB . Neurological injury after CPB is well known, and cerebral ischemia, presumably due to embolism or hypoperfusion, may be a possible cause . A previous experiment showed that focal cerebral ischemia increases transcription of the adrenomedullin gene in the brain , and we previously demonstrated that plasma concentration of adrenomedullin is increased in patients with cerebral ischemic insult . In the present study, we determined whether the cerebral vasculature was a production site for adrenomedullin and whether cerebral metabolism of adrenomedullin occurred before, during, and after CPB in patients undergoing coronary artery bypass grafting (CABG).
This study was approved by our institutional human investigation committee, and informed consent was obtained from all participants. We studied 10 patients undergoing elective CABG for coronary artery disease. No patient had a history of liver, respiratory, or kidney disease; any history of stroke; or ischemic brain lesion on a computed tomographic scan.
All patients were premedicated with 0.02 mg/kg morphine and 0.006 mg/kg scopolamine injected intramuscularly 45 min before the induction of anesthesia. After routine monitors, including five-lead electrocardiogram, pulse oximetry, and automated noninvasive oscillometric blood pressure, were placed, general anesthesia was induced with 6-10 [micro sign]g/kg fentanyl and 0.05-0.1 mg/kg midazolam. Vecuronium 0.15 mg/kg was administered to facilitate tracheal intubation. A 20-gauge radial arterial catheter, a 7.5F pulmonary artery catheter, a 4F retrograde internal jugular bulb catheter, and a 7F triple-lumen central venous catheter were placed using sterile technique. Anesthesia was maintained with 40%-50% inspired oxygen in air, 0.5%-1.0% isoflurane, and intermittent bolus doses of 30-40 [micro sign]g/kg fentanyl and 0.2-0.3 mg/kg midazolam. A continuous infusion of 0.5-0.8 [micro sign]g [middle dot] kg-1 [middle dot] min-1 nitroglycerin for coronary vasodilation was started before the induction of anesthesia. Methoxamine 1 mg was given when the systolic arterial pressure was <85 mmHg, but no other vasopressor or inotropic drugs were used. Acetated or lactated Ringer's solution was infused at a rate of 10-15 mL [middle dot] kg-1 [middle dot] h-1 to maintain a central venous pressure (CVP) of 5-7 mm Hg and a pulmonary capillary wedge pressure (PCWP) of 8-12 mm Hg until CPB was introduced.
The bypass circuit, involving a membrane oxygenator and an arterial filter, was primed with a mixture of lactated Ringer's solution and stabilized plasma protein solution. Heparin 250 [micro sign]/kg was injected to maintain an activated clotting time >400 s. Blood flow was nonpulsatile and was commenced at 2.2-2.4 L [middle dot] min-1 [middle dot] m-2 and titrated to maintain a systemic venous oxygen saturation >70%. Norepinephrine was used intermittently to maintain a perfusion pressure >50 mm Hg, and chlorpromazine or prostaglandin E1 was used to manage hypertension. Fresh oxygenator gas flow (blended oxygen/air mixture) was adjusted to maintain pH and PaCO2 in the normal range without adjustment for temperature (alpha-stat management). St. Thomas cold cardioplegia was induced with 15 mL/kg oxygenated blood-based cardioplegic solution via the antegrade aortic root and retrograde coronary sinus, followed by 10 mL/kg blood-based cardioplegic solution every 30 min. Mild hypothermia (30-33[degree sign]C nasopharyngeal temperature) was induced in all patients. The hematocrit was maintained at 20%-25% during CPB. A continuous infusion of dopamine and norepinephrine, if required was given to maintain cardiac output >2.5 L [middle dot] min-1 [middle dot] m-2 and a systolic arterial pressure >90 mm Hg during weaning from CPB. Nitroglycerin administration was resumed with the return of spontaneous cardiac rhythm for all patients. After separation from CPB, the remaining blood from the CPB circuit was processed with a cell saver device and subsequently reinfused into the patient.
Mean arterial pressure (MAP), CVP, mean pulmonary arterial pressure (MPAP), heart rate (HR), pulse wave oxygen saturation, and end-tidal carbon dioxide concentration were monitored continuously throughout the operation. PCWP was measured intermittently, and both radial and pulmonary artery blood was sampled to determine arterial pH, PaO2, PaCO2, HCO3-, base excess (BE), arterial oxygen saturation (SaO (2)), mixed venous oxygen saturation, hemoglobin concentration, and hematocrit, using a commercial blood gas analyzer (ABL-625; Radiometer, Copenhagen, Denmark). Cardiac output was measured using the thermodilution technique. All flow and resistance variables were calculated using standard formulas.
Cerebral blood flow (CBF) was measured using the argon (Ar) washin technique of Kety and Schmidt , and cerebral plasma flow (CPF) was calculated using the hematocrit value. Briefly, 30% Ar was introduced into the ventilator or oxygenator in an air/oxygen mixture. Five paired, simultaneously timed collections of radial and jugular bulb venous blood were drawn on the following schedule: arterial at 0.5, 1.5, 3.0, 6.0, and 14.0 min of Ar exposure; venous at 1.0, 2.0, 5.0, 10.0, and 15.0 min of Ar exposure. Each 1.5-mL sample was drawn anaerobically into heparinized syringes. The samples were immediately placed in ice, and the Ar concentrations in each sample were measured with a commercial analyzer (Medspect II; Allied Healthcare Products, St. Louis, MO). The CBF was calculated from arterial and jugular bulb uptake curves fit to the measured Ar concentrations and integrated to infinity. In addition, we calculated cerebral vascular resistance (CVR) according to the following formula: CVR = (MAP - CVP)/CBF, on the assumption that intracranial pressure of our patients was in the normal range.
In the present study, data collection and measurements of plasma adrenomedullin concentration were performed at the following intervals: 10 min after the start of stripping of the left internal mammary artery before CPB (A); 15 min after the commencement of cardiac arrest during CPB (B); and 10 min after the sternum was closed (C). Data collection included hemodynamic variables (MAP, CVP, MPAP, HR), cardiac output (before and after CPB) and pump flow (during CPB), CBF, body temperature, blood gas variables (pH, PaO2, PaCO2, BE, and SaO2), and hematocrit. Blood samples for plasma adrenomedullin concentration were obtained via the radial arterial and internal jugular bulb catheter simultaneously. Eight milliliters of blood was collected into chilled glass tubes containing disodium ethylenediaminetetraacetic acid (1 mg/ml) and aprotinin (500 U/mL) and was centrifuged at 4[degree sign]C for 15 min. Plasma samples were stored at -40[degree sign]C until extraction of adrenomedullin for assay.
Plasma adrenomedullin was measured by radioimmunoassay after extraction and purification, as previously described . Cerebral adrenomedullin production was calculated as (adrenomedullinIJB - adreno-medullinRA) x (CPF) where adrenomedullinIJB and adrenomedullinRA are adrenomedullin plasma concentration in the internal jugular bulb and radial artery, respectively.
The data are expressed as mean +/- SD. Comparison of adrenomedullin concentrations among the three periods was assessed by using analysis of variance with repeated measurements followed by post hoc Scheffe's test. Differences in plasma adrenomedullin concentrations between radial artery and internal jugular bulb at each period were compared using paired t-test. Regression analysis was performed to examine the relationship between two variables. P < 0.05 was considered statistically significant.
Demographic characteristics of the patients included mean age 61.9 +/- 9.1 yr, body weight 58.5 +/- 9.0 kg, height 159.8 +/- 8.7 cm, CPB time 135.9 +/- 42.2 min, and aortic cross clamping time 74.2 +/- 26.1 min. Hemodynamic variables and temperature at the three study periods are shown in Table 1. Cerebral physiological data, including CPF calculated from the measured CBF are presented in Table 2. Adrenomedullin concentrations from the radial artery and the internal jugular bulb of each patient at the three study periods are presented in Figure 1. Plasma adrenomedullin concentrations did increase with time in blood from the radial artery, as well as from the internal jugular bulb, and reached the highest levels after CPB (Period C). Furthermore, plasma adrenomedullin concentrations were larger in the internal jugular bulb than in the radial artery after CPB (P < 0.05), although there was no difference in plasma adrenomedullin concentrations between the two sampling sites before and during CPB. The calculated cerebral production of adrenomedullin is shown in Table 3. Although cerebral adrenomedullin production was not noted before and during CPB (Periods A and B), significant cerebral adrenomedullin production was observed after CPB (Period C). The relationship between cerebral adrenomedullin production after CPB and aortic cross-clamping time is shown in Figure 2. There was a statistically significant correlation between the cerebral adrenomedullin production after CPB and the duration of aortic cross-clamping. There was no significant correlation between plasma adrenomedullin from the radial artery and intraoperative hemodynamic variables or between cerebral adrenomedullin production and CBF or CVR (data not shown).
The principal finding of the present study is that adrenomedullin production occurred most probably from the cerebral vasculature after CPB. The adrenomedullin production was correlated with the aortic cross-clamping time.
The clinical significance of adrenomedullin has not been well elucidated. Increased plasma adrenomedullin concentration has been reported in several clinical situations, such as hypertension, chronic renal failure, congestive heart failure, acute myocardial infarction, and septic shock [2,3,17]. Nagata et al.  documented that circulating adrenomedullin levels increase after CPB, and that this increase correlates with aortic cross-clamping time. Considering that endothelial and vascular smooth muscle cells actively produce adrenomedullin and that the gene transcription level of adrenomedullin in these cells is much higher than that in other tissue, such as the adrenal gland and the lung [5,6], CPB seems to facilitate adrenomedullin secretion from endothelial and vascular smooth muscle cells. However, the report of Nagata et al.  did not identify the sites at which secretion of adrenomedullin was facilitated. The authors suggested that the increase in plasma adrenomedullin associated with CPB was due, in part, to reperfusion injury in pulmonary circulation. However, our recent study did not support their hypothesis . Furthermore, as mentioned above, we recently demonstrated that plasma concentration of adrenomedullin is elevated in patients with cerebral ischemic insult . The present results show that adrenomedullin production from the cerebral circulation may contribute, at least partially, to the increase of adrenomedullin plasma concentration after CPB.
The mechanism of production of adrenomedullin by the cerebral circulation after CPB is not well understood. CPB induces a systemic inflammatory response that is attributed to the exposure of blood to artificial extracorporeal materials, and the levels of chemotactic factors and proinflammatory cytokines increase during CPB . However, laboratory studies indicate that adrenomedullin production in vascular smooth muscle cells and endothelial cells is augmented by stimulation with cytokines or lipopolysaccharide [5,9,18]. This evidence leads us to hypothesize that exposure of the cerebral vascular wall to these chemical factors during and after CPB facilitates adrenomedullin gene expression and adrenomedullin production. Our findings of a correlation between the cerebral production of adrenomedullin after CPB and the aortic cross-clamping time is consistent with this hypothesis.
Although adrenomedullin causes vasodilation of cerebral arteries and increases regional CBF [19,20], we could not find a significant relationship between cerebral adrenomedullin production and CBF or CVR after CPB. Nonetheless, our result does not exclude a possible role for adrenomedullin as a cerebral vasodilator. A previous report by Nagata et al.  also failed to show a relationship between adrenomedullin concentration and either systemic or pulmonary hemodynamics after CPB. We administered several vasoactive drugs, such as nitroglycerin, chlorpromazine, dopamine, and norepinephrine, during and after CPB, these drugs might mask a vasodilatory effect of adrenomedullin.
In the present study, a significant increase of plasma adrenomedullin concentration was observed during and after CPB. However, the cerebral circulation did not contribute to the primary increase of adrenomedullin concentration during CPB. Because we concentrated exclusively on cerebral circulation in this study, the contribution of other organ sites to the increase in adrenomedullin plasma level during and after CPB cannot be determined. Further studies are needed to identify the other sites at which adrenomedullin secretion is facilitated during CPB.
The physiological roles of adrenomedullin in the cerebral circulation have not been well explained. Neurological injuries after cardiac surgery involving CPB are not rare [21,22]. Hypothermic CPB can facilitate drastic changes in CBF , and an imbalance of CBF and cerebral oxygen consumption during the rewarming period of CPB is a possible etiology . Dogan et al.  documented that the IV administration of adrenomedullin tends to suppress the reduction in regional CBF after occlusion of middle cerebral arteries and significantly decreases ischemic brain injury in rats. Thus, an increased concentration of plasma adrenomedullin during and after CPB might attenuate the reduction of local CBF and cerebral ischemic damage after cardiac surgery.
Although we designed the present study to examine the role of the cerebral circulation in adrenomedullin production in patients undergoing CABG, we acknowledge the limitations imposed by blood sampling from the radial artery instead of the carotid artery. Our measurement might not have reflected the real arteriovenous difference in the adrenomedullin concentration from the cerebral circulation. The patients in this study had atherosclerotic vascular disease, and cannulation of the carotid artery in such patients has the risk of cerebral microembolism. We therefore avoided cannulation of the carotid artery in our patients based on ethical considerations. The adrenomedullin concentration in the radial artery may be different from that in the carotid artery, because adrenomedullin is produced in peripheral vascular endothelial and smooth muscle cells.
In conclusion, adrenomedullin production in cerebral circulation is significantly enhanced after CPB in patients undergoing cardiac surgery and correlates with aortic cross-clamping time. The cerebral adrenomedullin production, as measured by differences in cerebral adrenomedullin levels measured in the internal jugular bulb and radial artery, may contribute to the increase of plasma levels of adrenomedullin after CPB.
The authors thank Dr. Sho Carl Shibata for his editorial assistance.
1. Kitamura K, Kangawa K, Kawamoto M, et al. Adrenomedullin: a novel hypotensive peptide isolated from human pheochromocytoma. Biochem biophys Res Commun 1993;192:553-60.
2. Nishikimi T, Saito Y, Kitamura K, et al. Increased plasma level of adrenomedullin in patients with heart failure. J Am Coll Cardiol 1995;26:1424-31.
3. Ishimistu T, Nishikimi T, Saito Y, et al. Plasma level of adrenomedullin, a hypotensive peptide, in patients with hypertension and renal failure. J Clin Invest 1994;94:2158-61.
4. Kobayashi K, Kitamura K, Hirayama N, et al. Increased plasma adrenomedullin in acute myocardial infarction. Am Heart J 1996;131:676-80.
5. Sugo S, Minamino N, Kangawa K, et al. Endothelial cells actively synthesize and secrete adrenomedullin. Biochem Biophys Res Commun 1994;201:1160-5.
6. Sugo S, Minamino N, Shoji H, et al. Production and secretion of adrenomedullin from vascular smooth muscle cells: augmented production by tumor necrosis factor-alpha. Biochem Biophys Res Commun 1994;203:719-26.
7. Shimekake Y, Nagata K, Ohta S, et al. Adrenomedullin stimulates two signal transduction pathways, cAMP accumulation and Ca2+
mobilization, in bovine aortic endothelial cells. J Biol Chem 1995;270:4412-7.
8. Eguchi S, Hirata Y, Kano H, et al. Specific receptors for adrenomedullin in cultured rat vascular smooth muscle cells. FEBS Lett 1994;340:226-30.
9. Sugo S, Minamino N, Shoji H, et al. Interleukin-1, tumor necrosis factor and lipopolysaccharide additively stimulate production of adrenomedullin in vascular smooth muscle cells. Biochem Biophys Res Commun 1995;207:25-32.
10. Hall RI, Smith MS, Rocker G. The systemic inflammatory response to cardiopulmonary bypass: pathophysiological, therapeutic, and pharmacological considerations. Anesth Analg 1997;85:762-85.
11. Nagata N, Kitamura K, Kato J, et al. The effect of hypothermic cardiopulmonary bypass on plasma adrenomedullin in adult cardiac surgical patients. Anesth Analg 1997;84:1193-7.
12. Nishikimi T, Hayashi Y, Iribu G, et al. Increased plasma adrenomedullin concentrations during cardiac surgery. Clin Sci 1998;94:585-90.
13. Maclean RF, Wong BI, Naylor CD, et al. Cardiopulmonary bypass, temperature, and central nervous system dysfunction. Circulation 1994;90(Part 2):II-250-5.
14. Wang X, Yue T, Barone FC, et al. Discovery of adrenomedullin in rat ischemic cortex and evidence for its role in exacerebating focal brain ischemic damage. Proc Natl Acad Sci USA 1995;92:11480-4.
15. Kikumoto K, Kubo A, Hayashi Y, et al. Increased plasma concentration of adrenomedullin in patients with subarachnoid hemorrage. Anesth Analg 1998;87:859-63.
16. Kety SS, Schmidt CF. The determination of cerebral blood flow in man by the use of nitrous oxide in low concentration. Am J Physiol 1945;143:53-66.
17. Nishio K, Akai Y, Murao Y, et al. Increased plasma concentrations of adrenomedullin correlate with relaxation of vascular tone in patients with septic shock. Crit Care Med 1997;25:953-7.
18. Isumi Y, Shoji H, Sugo S, et al. Regulation of adrenomedullin production in rat endothelial cells. Endocrinology 1998;139:838-46.
19. Lang MG, Paterno R, Faraci FM, Heistad DD. Mechanisms of adrenomedullin-induced dilatation of cerebral arterioles. Stroke 1997;28:181-5.
20. Dogan A, Suzuki Y, Koketsu N, et al. Intravenous infusion of adrenomedullin and increase in regional cerebral blood flow and prevention of ischemic brain injury after middle cerebral occlusion in rats. J Cereb Blood Flow Metab 1997;17:19-25.
21. Breuer AC, Frulan AJ, Hanson MR, et al. Central nervous system complications of coronary artery bypass graft surgery: prospective analysis of 421 patients. Stroke 1983;14:682-7.
22. Furlan AJ, Breuer AC. Central nervous system complications of open heart surgery. Stroke 1984;15:912-5.
23. Schell RM, Kern FH, Greeley WJ, et al. Cerebral blood flow and metabolism during cardiopulmonary bypass. Anesth Analg 1993;76:849-65.
© 1999 International Anesthesia Research Society
24. Nakajima T, Kuro M, Hayashi Y, et al. Clinical evaluation of cerebral oxygen balance during cardiopulmonary bypass: online continuous monitoring of jugular venous oxyhemoglobin saturation. Anesth Analg 1992;74:630-5.