Abnormality of the renin/body-fluid-volume relationship in serially-studied inbred dogs with neonatally-induced coarctation hypertension

Angiotensin-converting enzyme inhibitors in cardiovascular anesthesia

Besides the long-term regulation of extracellular fluid volume, the RAS plays an important physiologic role in maintaining venous return and blood pressure during acute hemodynamic stresses. ACE inhibitors may therefore alter venous return and cardiac output regulation during anesthesia and surgery. This may be regarded as a drawback of ACE inhibition when other factors interfere with cardiovascular homeostasis; deleterious hemodynamic events may therefore occur when blood volume is decreased, which may be frequent during cardiovascular anesthesia and surgery. However, the alternative solution should not be to stop ACE inhibitors preoperatively. This would allow recovery of RAS control of blood pressure, but at the expense of some regional circulations. From this point of view, preliminary results from early studies during cardiovascular anesthesia and surgery showing redistribution of regional blood flow with inhibition of ACE are encouraging; whether postoperative outcome can be improved deserves further studies. At this time, the evidence is that ACE inhibition does not allow the anesthesiologist to be tolerant of hypovolemia.

The renin-angiotensin system and blood pressure regulation during infancy and childhood

The renin-angiotensin system plays multiple roles in the maintenance of normal blood pressure and renal function. The balance and integration of these roles change during development in ways that we do not yet fully understand. This article reviews the ways in which the renin-angiotensin system maintains normal cardiovascular homeostasis during development and its participation in physiologic and biochemical events.

Coronary vascular and myocardial lesions due to experimental constriction of the abdominal aorta

The development and evolution of arterial and myocardial lesions were morphologically evaluated in Wistar rats submitted to constriction of the abdominal aorta. The control (sham-operated) and operated groups were evaluated 1, 2, 4 and 6 weeks after surgery. The aorta-constricted groups developed arterial hypertension followed by myocardial hypertrophy evidenced from the first week onwards by the increase in ventricular weight and in the diameters of left and right ventricular myofibers. The histopathologic study of the myocardium revealed in aorta-constricted groups, since the first week, widespread necrotizing changes of the intramural coronary branches surrounded by multifocal areas of myofiber degeneration and necrosis. The lesions were more extensive in the wall of the right ventricle and were gradually replaced by scar tissue. At the 6th week patchy focal fibrotic scars were found scattered in the myocardium of both ventricles. There were no systemic lesions in aorta-constricted or sham-operated groups. The close association between the arterial and myocardial lesions shows that muscle necrosis and scars are due to ischemia. They may influence the contractile performance of the myocardium in this model of pressure-induced hypertrophy of the heart.

[Anesthetic consequences of hemodynamic effects of angiotensin converting enzyme inhibitors]

Angiotensin converting enzyme inhibitors (ACEI) are used increasingly to treat cardiovascular diseases, and so, therefore, the number of patients scheduled for surgery and treated preoperatively with these drugs. Haemodynamic instability has sometimes been observed during anaesthesia in these patients, leading some authors to discontinue ACEI administration before anaesthesia. However, recent physiological data concerning the renin angiotensin system (RAS) and ACEI pharmacological data may increase our understanding of the mechanisms of cardiovascular interaction between ACEI and anaesthesia. The RAS is involved in blood pressure regulation when extracellular fluid volume is decreased and in case of hypovolaemia, by inducing vasoconstriction and longterm volume regulation. Arterial vasoconstriction is the target for ACEI. However, venoconstriction may maintain venous return and cardiac output in spite of reduced blood volume. On the other hand, ACEI treatment impedes cardiac adaptation to acute changes in extracellular fluid volume. This effect may be increased by underlying pathology (especially in hypertension) as well as by anaesthesia. A combination of an increased sensitivity to acute changes in ventricular load due to treatment with ACEI and anaesthesia in hypertensive patients or in patients with cardiac failure may carry a high risk of hypotension. Specific studies on haemodynamic tolerance of anaesthesia in patients chronically treated with ACEI are required to assess the prevalence of this risk and how to manage it.

Paradoxical hypertension after repair of coarctation of the aorta: a review of its causes

Correction of a coarctation of the aorta, an apparent simple cause of hypertension, paradoxically can provoke two hypertensive responses, one of which is potentially fatal. The first, limited to the first 24 hours, occurs in nearly one half of the patients. This is likely due to the high set of the carotid baroreceptors. The second, which may be associated with abdominal pain and, in some, with necrosis of the small bowel as a result of severe arteritis confined to arteries arising from the aorta below the coarctation, develops in about one half of the first responders. Norepinephrine excretion greatly increases for several days, whereas angiotensin levels are elevated for 3 to 4 days. The hypertension responds to beta-blockers, to arterial smooth muscle relaxants, and to angiotensin converting enzymes. A theory is advanced to explain the second response. It is the adaptation gone awry that ensures adequate flow to exercising muscles below the coarctation, above and beyond that delivered by increasing the systolic pressure. It could be a regionally controlled mechanism similar to the rationing of blood flow in diving mammals.

Renal function in different forms of renovascular hypertension in rats

This study examines renal function in different rat models of renovascular hypertension. Hypertension was induced by constriction of the aorta proximal to the renal artery (PAC), by PAC and nephrectomy (PAC + Nx) or by renal artery stenosis (RAS). PAC + Nx is equivalent to the Goldblatt 1 kidney-I clip hypertension model. The PAC rats were studied 3 weeks after surgery. Hypertension was by then well established. GFR, measured as the clearance of inulin, was significantly lower in PAC rats than in control (C) rats. GFR was the same in PAC + Nx rats as in C rats, but significantly lower in PAC + Nx than in Nx rats. Kidney weight was significantly higher in PAC + Nx rats than in C rats. Filtration fraction (FF), measured as the ratio between GFR and the clearance of PAH, was significantly higher in PAC and PAC + Nx rats than in C and Nx rats. In RAS rats hypertension was not established until 6 weeks after surgery, and RAS is equivalent to Goldblatt 2 kidney-I clip hypertension. Renal artery constriction was moderate as judged from the weight ratio between the stenosed and contralateral kidneys. The GFR in the stenosed kidney was not significantly lower in the contralateral kidney. FF was significantly higher in RAS rats than in C rats in both the stenosed and the contralateral kidneys, but the increase was less pronounced than in PAC and PAC + Nx rats.

Chronic MK421 fails to modify evolution of hypertension in neonatally coarcted pups

In inbred dogs with neonatally induced coarctation hypertension, prior serial studies during the first year after aortic banding showed extracellular volume excess with normal plasma renin activity (PRA). The present studies test the hypothesis that slowly evolving aortic constriction in this model will yield intrarenal angiotensin II excess, peripherally undetectable, with continuous slightly positive sodium balance, and thus that chronic blockade of angiotensin II formation will prevent generation of hypertension. Accordingly, we used MK421 (enalapril, 3 mg/kg twice daily), a long-acting angiotensin converting enzyme inhibitor, or placebo, administered orally, from the time of banding through 4 months after banding in sex-matched littermates randomly assigned to one of four groups: coarcted/MK421; control/MK421; coarcted/placebo; control/placebo. Results indicate that MK421 caused identical lowering of absolute forelimb systolic blood pressure in coarcted and control pups but failed to modify evolution of a significant (p less than 0.005) systolic blood pressure difference in coarcted versus control dogs. Thus, neither temporal course nor final magnitude of relative hypertension was altered by MK421. Efficacy of MK421 was documented by 83% inhibition of the pressor response to angiotensin I at nadir of drug effect and by sustained increases in angiotensin I and renin concentration throughout the period of study. Coarcted and control pups responded similarly to MK421 for all measured variables. Glomerular filtration rate and extracellular volume (measured by [14C]inulin disappearance) did not differ among groups. Thus, chronic administration of MK421 failed to prevent hypertension and did not impair maintenance of normal renal function in the evolving phase of neonatally induced coarctation hypertension. We conclude that, although angiotensin II may participate in the untreated model, it does not appear essential to generation of hypertension. We propose that the renal pressure-natriuresis mechanism regulates distal pressure, that stenosis-related resistance independently determines the proximal-distal difference, and that chronic converting enzyme inhibition lowers the set point of the former without influencing stenosis evolution, thus secondarily lowering proximal pressure by an equal degree.

Studies of the renal component of the hypertension in rats with aortic constriction. Role of angiotensin II

The object of this study was to investigate the renal component of hypertension in aortic constriction. In 40-day-old Sprague-Dawley rats the aorta were constricted either proximal (PAC) or distal (DAC) to the renal arteries. The rats were examined 3 weeks later together with control rats. The arterial pressure proximal to the constriction was elevated in the PAC group but not in the DAC group. In PAC rats the arterial pressure was also elevated distal to the constriction. There was a significant pressure gradient across the constriction in both PAC and DAC rats. The PAC rats had a significant decrease of renal blood flow, a significant increase in renal vascular resistance and a numerical but not significant decrease of glomerular filtration rate. Serum levels of angiotensin II were not significantly different in PAC and control rats. The pressor effect of a bolus dose of angiotensin II was significantly increased in PAC rats. Captopril, a converting enzyme inhibitor, decreased the arterial pressures and renal vascular resistance in PAC rats. The pressure elevating effects of angiotensin II and pressure lowering effect of captopril were more pronounced distal than proximal to the constriction. We conclude that the kidneys play a major role in the development of hypertension in PAC, and that the local effect of angiotensin II on the renal vascular bed is an important contributor to the renal component of the hypertension.

Canine neonatally induced coarctation hypertension in the second year. Variably hyperresponsive plasma renin activity

In canine neonatally-induced coarctation hypertension, we reported abnormally elevated plasma renin activity (PRA) during sodium restriction in 2-year-old dogs, but found normal PRA responses to sodium restriction +/- furosemide in coarcted dogs studied serially over the first year postaortic banding (PAB). To resolve this apparent discrepancy in PRA response, longitudinal studies were extended to 2 years PAB. In two separately-studied groups, each with three coarcted and three littermate controls, measurements of indirect forelimb blood pressure (BP) at 15 to 18 months, direct brachial arterial BP at 24 months, and serial measurement of PRA and extracellular volume (ECV, as 24Na space) were made over a 15- to 24-month age range during three sodium-volume levels: ad libitum sodium intake (NS), low-sodium diet alone (LS), and low-sodium plus Lasix (LS/Lasix). While PRA in coarcted dogs of both groups was comparable to controls at NS and LS, PRA in Group 1 coarcted dogs significantly exceeded that of littermates during LS/Lasix in both 18- and 24-month studies. In contrast, PRA in Group 2 coarcted dogs was not hyperresponsive to LS/Lasix as compared to simultaneously-studied littermates. The hyperresponsive PRA in Group 1 coarcted dogs could not be attributed to larger absolute or relative ECV deficits. Overall, ECV in coarcted dogs of each group was higher on the average but was not statistically different from controls. Results indicate that the hyperresponsive PRA in this canine model is: 1) a variable feature, developing secondarily in the late established phase; 2) reproducible for a given animal; and 3) not attributed to exaggerated ECV deficits during the LS/Lasix protocol.

Extracellular volume expansion and delayed resolution of hypertension after canine aortic coarctectomy

The hemodynamic changes occurring after surgical correction of thoracic aortic coarctation were studied in two neonatally coarcted dogs at six months of age and compared to sham surgery in two littermate controls. Excision of the tight iatrogenic aortic band with direct aortic reanastomosis abolished pressure gradients. Post-coarctectomy systemic pressure rose, after an early transient fall, to pre-operative proximal levels and was sustained for two to four weeks before decreasing to control normotensive values. Femoral systolic pressure (and renal perfusion pressure) rose by 47-57 mmHg in coarcted dogs (p less than .001). Extracellular volume (ECV) increased in both coarcted animals, peaking 28-32 cc/kg (7.1-7.6% increase) above precoarctectomy levels. Peak ECV expansion coincided with the peak post-coarctectomy blood pressure. Fluid administration, blood losses and plasma renin activity (PRA) were comparable in all animals. Post-coarctectomy tachycardia was also noted in coarcted dogs (p less than .001), whereas neither ECV nor heart rate changes occurred in control animals. We postulate that post-coarctectomy baroreceptor stimulation results in sympathetically-mediated renal sodium retention, not only preventing a pressure diuresis, but resulting in overt volume expansion. Failure of PRA suppression despite increased distal pressure and volume excess may also reflect sympathetic activation. Data are compatible with the view that sympathetic activation and consequent volume expansion transiently sustain hypertension in the post-coarctectomy setting until baroreceptor re-adjustments permit normalization of blood pressure.

Preoperative and postoperative renin levels in coarctation of the aorta

We studied plasma renin activity (PRA) in eight children before and after surgical correction of aortic coarctation. These eight children underwent a combination of low-sodium diet and diuresis before surgery, and PRA was measured shortly thereafter. Thirty-two to 51 months after successful surgical correction, PRA was measured again. The mean PRA was 21.4 +/- 1.3 ng/ml/hour (+/- SD) preoperatively and 5.5 +/- 1.5 ng/ml/hour postoperatively. These findings provide further evidence of the significance of increased renin-angiotensin activity in patients with aortic coarctation.

Acute responses to arterial pressure and plasma renin activity to converting enzyme inhibition (SQ 20,881) in serially studied dogs with neonatally-induced coarctation hypertension

In six inbred dogs with neonatally-induced coarctation hypertension, and in seven littermate controls, acute responses of proximal arterial pressure and plasma renin activity (PRA) to converting enzyme inhibitor (CEI; SQ 20,881, 0.5 mg/kg i.v.) were serially examined. Studies were performed at 2, 6, and 12 months post-aortic banding under sodium-replete and -deplete conditions. Both in normotensive controls and in coarcted dogs, depressor responses (pre- minus post-CEI values) were positively correlated, not only with initial (pre-CEI) PRA, but also independently with initial blood pressure. Although absolute depressor responses in coarcted dogs exceeded those of the control group, there were no significant group differences when, by analysis of covariance, depressor responses were adjusted for the physiologic influence of initial pressure. Similarly, depressor responses expressed as a percent of initial pressure were comparable in coarcted and control groups. Initial PRA and PRA response to CEI in coarcted dogs were also comparable to control dogs; the PRA response correlated with initial PRA in both groups. CEI did not significantly diminish the magnitude of blood-pressure difference between coarcted and control dogs. Thus, in neonatally-induced coarctation hypertension, under both sodium replete- and -deplete conditions: 1) acute depressor and PRA responses to CEI are modulated by the same factors that influence responses of normotensive controls; 2) larger absolute depressor responses to CEI appear to be a physiologic function of higher initial pressure; and 3) blood pressure excess over littermate controls is largely sustained by CEI-resistant factors, potentially including the known volume excess in coarcted dogs.

In vitro determinants of plasma renin activity in serially-studied inbred dogs with neonatally induced coarctation hypertension: renin reactivity, renin substrate, and renin concentration

Increased renin activity of plasma, suggesting an excess of circulating accelerators and/or deficit of inhibitors of the renin reaction, has been reported in a number of hypertensive states; however, its contribution to genesis and/or maintenance of hypertension is unknown. To longitudinally assess the evolution of plasma renin reactivity in relation to blood pressure in neonatally-induced coarctation hypertension, we have made serial observations in 6 coarcted dogs and in 7 littermate controls over 1-12 months post-aortic-banding during varied steady-state sodium intake. Measurements of renin activity (defined as the increment of angiotensin I-generation rate following addition of exogenous renin to plasma), renin substrate concentration (RS), and plasma renin activity (PRA), together with calculation of plasma renin concentration (PRC) (as PRC = PRA divided by renin reactivity) provided estimates of the three major determinants of PRA. RS values were adjusted for variability due to assay-control and to age via covariate analysis. Results indicate no difference in adjusted RS between coarcted and control dogs, thus obviating the influence of RS differences on renin reactivity results. Renin reactivity and PRC in coarcted dogs were also comparable to control values. Furthermore, responses of RS, renin reactivity and PRC to dietary sodium manipulation were similar in coarcted and control animals. We conclude that circulating modifiers of the renin reaction play no role in the genesis or in the first-year maintenance of neonatally-induced coarctation hypertension.