Baroreflex sensitivity and carotid sinus dimensions in dogs with coarctation

Arterial hypertension and cardiovascular prognosis after successful repair of aortic coarctation: a clinical model for the study of vascular function

Despite successful surgical repair, aortic coarctation is associated with unfavourable prognosis mainly due to cardiovascular disease. Late timing of repair and arterial hypertension represent adverse prognostic factors. Arterial hypertension can recur after coarctation repair, despite the absence of residual obstruction, with a prevalence of up to 45%. Furthermore, even subjects with normal blood pressure values at rest may show an abnormal blood pressure elevation during exercise and daily life activities. The pathophysiology of such abnormal blood pressure behaviour is unclear. Different mechanisms have been proposed: resetting of the renin-angiotensin system, neurological dysfunction and impaired vascular reactivity and/or elastic properties. Several studies have supported these hypotheses, although the suggestion of a causative role of vascular dysfunction persisting late after coarctation repair has recently become more popular. Further studies are needed to investigate this issue; this particular syndrome may represent an important study model for the understanding of systolic hypertension.

Resetting of the cardiopulmonary baroreflex 10 years after surgical repair of coarctation of the aorta

OBJECTIVE

To characterise cardiopulmonary baroreflex responses and examine the effects of a 45 minute cycling bout late after successful repair of coarctation of the aorta.

SUBJECTS

10 young adults (mean (SEM) age 18.1 (2.6 years)) operated on for coarctation of the aorta 12.7 (3.5) years earlier, and 10 healthy controls.

DESIGN

Forearm blood flow (venous occlusion plethysmography) and vascular resistance, left ventricular internal diastolic diameter, and central venous pressure estimated from an antecubital vein were measured in the supine position at baseline and during five minute applications of lower body negative pressure (LBNP) at -15 mm Hg (LBNP(-15)) and -40 mm Hg (LBNP(-40)). Venous samples were obtained at baseline and during LBNP(-40) for noradrenaline (norepinephrine), adrenaline (epinephrine), renin activity, and aldosterone. The tests were repeated after 45 minutes of moderate exercise.

RESULTS

Baseline heart rate (78 (9) v 64 (6) beats/min), echocardiographic cardiac output (6.9 (1.1) v 5.0 (0.2) l/min), shortening fraction (41.7 (1.8)% v 33.3 (1.3)%), and forearm blood flow (3.4 (0.4) v 2.3 (0.3) ml/100 g/min) were higher in the coarctation group than in the controls (p < 0.05). Changes in forearm blood flow and forearm vascular resistance from baseline to LBNP(-40) were similar in both groups, but the relation between forearm vascular resistance and estimated central venous pressure or left ventricular internal diastolic diameter was shifted downward in the coarctation group. Plasma adrenaline was increased in the coarctation group (baseline: 3.2 (0.6) v 2.4 (0.3) pmol/l in controls; LBNP(-40): 687 (151) v 332 (42) pmol/l) (p < 0.05). Both groups showed a similar downward displacement of forearm vascular resistance (p < 0.05) after exercise.

CONCLUSIONS

There appears to be resetting of the cardiopulmonary baroreflex to a lower forearm vascular resistance in young adults operated on for coarctation of the aorta, associated with hyperdynamic left ventricular function. Raised circulating adrenaline could contribute to the lower forearm vascular resistance.

Baroreflex stabilization of the double product

We investigated whether the baroreflex control of heart rate (HR) stabilizes the product of arterial pressure (P(A)) and HR, called the double product (DP), an indirect indicator of left ventricular oxygen consumption. During pharmacological increases and decreases of P(A) in conscious rabbits, the mean (+/-SE) rate of change of the DP with respect to P(A) (dDP/dP(A)) was -88 +/- 36 and -20 +/- 36 DP units/mmHg, respectively. Regression analysis of all peak responses obtained in individual rats produced a dDP/dP(A) value of 15 +/- 16 DP units/mmHg. These estimates were significantly less than the dDP/dP(A) value predicted if HR were constant (184 +/- 7 DP units/mmHg) and were not significantly different from zero. We also compared values of baroreflex sensitivity (BRS) from the literature with those calculated to provide ideal stabilization of the DP. BRS values were significantly correlated with the calculated ideal values (R = 0.95; n = 14). BRS averaged 128 +/- 24% of the ideal value in all species and 148 +/- 28% in mammals and birds. Our results suggest that stabilization of the DP is a common consequence of the baroreflex control of heart rate.

Hypertension in aortic valve disease and its response to valve replacement

We have investigated the prevalence of hypertension and the response of blood pressure to operation in 87 patients with lone aortic valve disease who underwent aortic valve replacement. In patients with aortic stenosis alone 26% were hypertensive pre-operatively (age and sex adjusted blood pressure greater than 160 systolic and or greater than 95 mmHg diastolic) and 24% were hypertensive post-operatively. In those with aortic regurgitation alone, hypertension was present in 65% before and 57% after valve replacement using the same criterion. For combined stenosis and regurgitation, the prevalence was 54% and 62%, respectively. The post-operative increase in systolic pressure in patients with aortic stenosis occurred mainly in those with a history of left ventricular failure. In those with aortic regurgitation or combined stenosis with regurgitation, diastolic pressure rose after valve replacement resulting in a prevalence of diastolic hypertension of 44% and 35%, respectively. Blood pressure changes were not predicted by the type of valve inserted nor its size. Our data show that despite severe symptomatic aortic valve disease, systolic hypertension was common in aortic stenosis and diastolic hypertension was found in aortic regurgitation. This underlines the importance of blood pressure monitoring in patients following aortic valve replacement.

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.

Operative factors, not hemodynamics, modify hormones in repair of coarctation

This study was designed to evaluate separately the influence of restoration of distal pulse pressure and the influence of factors related to operation upon hemodynamic and hormonal alterations accompanying repair of canine aortic coarctation. Eight normal adult dogs underwent a sham operation and served as controls. In 10 dogs the thoracic aorta was transfected and reanastomosed. In 10 dogs aortic coarctation was created within 1 week of birth by banding the aorta just proximal to the ductus ligament, thereby fixing lumenal diameter at 1 to 2 mm. Studies were performed in the latter animals 18 months after operation. These dogs were subdivided into two groups: one (n = 6) underwent surgical repair of the coarctation; in the other (n = 4), the coarctation was repaired and an occluder was placed on the aorta to maintain distal aortic pulse pressure at its diminished preoperative level. Normal distal pulse pressure was restored in these animals by releasing the occluder 1 week after recovery from the repair of coarctation. Each dog was studied in the conscious state before the definitive procedure and again 24 hr later. Plasma renin activity increased significantly after operation in the sham and in the transection groups. Renin activity and proximal blood pressure were significantly elevated and distal pulse pressure was diminished preoperatively in both the repair and the occluder groups. After their respective definitive procedures, the elevation in renin activity and in proximal pressure persisted in the repair group but decreased in the occluder group.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Altered baroreceptor function in children with systolic hypertension after coarctation repair

To determine whether altered baroreceptor function may contribute to systemic hypertension after coarctation of the aorta (C of A) repair, baroreceptor function was evaluated in 6 children with repaired C of A mild arm systolic hypertension. Data were compared with those from 7 normotensive control children with hemodynamically mild heart disease. Age at C of A repair averaged 9.9 +/- 3.1 years (mean +/- standard deviation [SD]). Arm systolic pressure was 143.8 +/- 2.9 mm Hg in the C of A repair group, compared with 118.3 +/- 9.9 for control subjects (p less than 0.001). At catheterization, steady-state sigmoidal baroreceptor function curves relating mean arterial pressure to R-R interval were derived by increasing and decreasing mean arterial pressure with small injections of phenylephrine and nitroprusside. Compared with control subjects, the baroreceptor function curves of children with repaired C of A (1) are reset about a higher baseline mean arterial pressure (108.8 +/- 6.6 versus 90.3 +/- 8.6 mm Hg, p less than 0.01), (2) have a decreased slope (7.9 +/- 3.7 versus 17.4 +/- 3.6 ms/mm Hg, p less than 0.001), and (3) have a diminished R-R interval range (246.7 +/- 81.5 versus 535.7 +/- 97.2 ms, p less than 0.001). Thus, in children with hypertension after C of A repair, the baroreflex is reset to an elevated mean arterial pressure level and has a diminished sensitivity to changes in arterial pressure.

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.

Different reactivity and structure of the prestenotic and poststenotic aorta in human coarctation. Implications for baroreceptor function

In eight humans with coarctation, fresh aortic tissue was examined pharmacodynamically. In four of these patients, and in 12 additional patients, the aorta above and below the coarctation was studied morphologically and compared with eight control aortas. By in vitro stimulation with potassium (127 mM), noradrenaline (18 microM), and prostaglandin F2 alpha (28 microM), postcoarctational aortic ring preparations showed a significantly greater contractility than precoarctational rings (p less than 0.05). Volumetric analysis showed significantly more collagen (P less than 0.01) and les smooth muscle mass (p less than 0.01) in the aorta above than below the coarctation. No significant differences were found between sections from the arch and distal to the ligamentum arteriosum in the normal aortas. We conclude that the precoarctational aortic wall is more rigid than the postcoarctational wall. This may influence baroreceptors in the upper vascular bed in such a way as to tolerate a higher pressure. This would explain the preoperative proximal hypertension, the paradoxic hypertension and the frequent lack of normalization of blood pressure postoperatively.

Coarctation of the aorta and baroreceptor resetting. A study of carotid baroreceptor stimulus-response characteristics before and after surgical repair in the dog

We studied baroreceptor function in dogs before and after surgical repair of coarctation of the aorta by direct recording of multifiber carotid sinus (CS) nerve activity (NA) during alteration of pulsatile arterial pressure with systemic phenylephrine and nitroprusside, and during static pressure changes using a CS pouch preparation. Coarctation was induced by banding the proximal thoracic aorta in ten 3- to 5-day old puppies. One and one-half years later, five of these coarctated animals were studied before, and five were studied 3-7 months after, surgical repair. Five adult animals also were studied 4-6 months after the proximal thoracic aorta had been banded. Controls were eight normal adult dogs. Threshold pressure at which NA began, saturation pressure at which NA reached a maximum, and slope (% Max NA/mm Hg) of the linear portion of the stimulus-response curve were determined. Pulsatile manipulations of pressure elicited normal sensitivity (slope) in dogs with coarctation but static nonpulsatile pressure changes showed depressed sensitivity compared to controls. After surgical repair, threshold and saturation returned toward normal; sensitivity determined with static pressure manipulations returned to control value. Coarctation reset CS baroreceptors to operate at higher pressures in both puppies and adult dogs and repair of coarctation returned function toward normal. We conclude that resolution of hypertension after repair of coarctation may depend upon baroreceptor readaptation.

Histology and ultrastructure of the carotid sinus in experimental hypertension

Histological studies including electron microscopy were performed on the carotid sinuses of dogs with renal hypertension of 17 to 82 days duration or hypertension of two years secondary to aortic coarctation. Physiological abnormalities in the baroreceptors including resetting had been demonstrated before death. Histologically there were no qualitative or quantitative differences in the intrasinus nerve fibers of the hypertensive animals compared tp controls with normal blood pressures, and there was no evidence that baroreceptor degeneration occurred as a consequence of the hypertension. Structural lesions confined to the intima were evident in the sinus walls of some of the renal hypertensive animals whereas the dogs with coarctation showed more advanced sinus wall changes including medial calcification and enlargement of the vascular lumen.