Eric D. Irwin

Prolonged Activation of the Baroreflex Produces Sustained Hypotension

Abstract—The role of baroreflexes in long-term control of arterial pressure is unresolved. To determine whether chronic activation of the baroreflex produces sustained hypotension, we developed a method for prolonged activation of the carotid baroreflex in conscious dogs. This was achieved by chronically implanting electrodes around both carotid sinuses and using an externally adjustable pulse generator to electrically activate the carotid baroreflex. Control values for mean arterial pressure (MAP) and heart rate were 93±3 mm Hg and 64±4 bpm, respectively. After control measurements, the carotid baroreflex was activated bilaterally for 7 days at a level that produced a prompt and substantial reduction in MAP, and for day 1 MAP was reduced to 75±4 mm Hg. Moreover, this hypotensive response was sustained throughout the entire 7 days of baroreflex activation (day 7, MAP=72±5 mm Hg). During prolonged baroreflex activation, heart rate decreased in parallel with MAP, although the changes were not as pronounced (day 7, heart rate=51±3 bpm). Prolonged baroreflex activation was also associated with ≈35% reduction in plasma norepinephrine concentration (control=87±15 pg/mL). After baroreflex activation, hemodynamic measures and plasma levels of norepinephrine returned to control levels. Interestingly, despite the pronounced fall in MAP, plasma renin activity did not increase during prolonged baroreflex activation. These data indicate that prolonged baroreflex activation can lead to substantial reductions in MAP by suppressing the sympathetic nervous system. Furthermore, sustained sympathoinhibitory effects on renin secretion may play an important role in mediating the long-term hypotensive response.

Chronic Baroreceptor Activation Enhances Survival in Dogs With Pacing-Induced Heart Failure

Much of the current pharmacological therapy for chronic heart failure targets neurohormonal activation. In spite of recent advances in drug therapy, the mortality rate for chronic heart failure remains high. Activation of the carotid baroreceptor (BR) reduces sympathetic outflow and augments vagal tone. We investigated the effect of chronic activation of the carotid BR on hemodynamic and neurohormonal parameters and on mortality in dogs with chronic heart failure. Fifteen dogs were instrumented to record hemodynamics. Electrodes were applied around the carotid sinuses to allow for activation of the BR. After 2 weeks of pacing (250 bpm), electrical carotid BR activation was initiated in 7 dogs and continued for the remainder of the study. The start of BR activation was used as a time reference point for the remaining 8 control dogs that did not receive BR activation. Survival was significantly greater for dogs undergoing carotid BR activation compared with control dogs (68.1±7.4 versus 37.3±3.2 days, respectively; P<0.01), although arterial pressure, resting heart rate, and left ventricular pressure were not different over time in BR-activated versus control dogs. Plasma norepinephrine was lower in dogs receiving BR activation therapy 31 days after the start of BR activation (401.9±151.5 versus 1121.9±389.1 pg/mL in dogs not receiving activation therapy; P<0.05). Plasma angiotensin II increased less in dogs receiving activation therapy (plasma angiotensin II increased by 157.4±58.6 pg/mL in control dogs versus 10.1±14.0 pg/mL in dogs receiving activation therapy; P<0.02). We conclude that chronic activation of the carotid BR improves survival and suppresses neurohormonal activation in chronic heart failure.

Prolonged Activation of the Baroreflex Abolishes Obesity-Induced Hypertension

Prolonged electrical activation of the carotid baroreflex produces sustained reductions in sympathetic activity and arterial pressure in normotensive dogs. The main goal of this study was to assess the influence of prolonged baroreflex activation on arterial pressure and neurohormonal responses in 6 dogs with obesity-induced hypertension. After control measurements, the diet was supplemented with cooked beef fat for 6 weeks, whereas sodium intake was held constant. After 4 weeks of the high-fat diet, there were increments in body weight from 25.8±0.7 to 38.6±1.0 kg, mean arterial pressure from 97±2 to 110±3 mm Hg, heart rate from 67±3 to 91±4 bpm, and plasma norepinephrine concentration from 141±35 to 280±52 pg/mL. Plasma glucose and insulin concentrations were elevated, but increases in plasma renin activity during the initial weeks of the high-fat diet were not sustained. During week 5, baroreflex activation resulted in sustained reductions in mean arterial pressure, heart rate, and plasma norepinephrine concentration; at the end of week 5, these values were 87±2 mm Hg, 77±4 bpm, and 166±45 pg/mL, respectively. These suppressed values returned to week 4 levels during a 7-day recovery period after baroreflex activation. There were no changes in plasma glucose or insulin concentrations, or plasma renin activity during prolonged baroreflex activation. These findings indicate that baroreflex activation can chronically suppress the sympathoexcitation associated with obesity and abolish the attendant hypertension while having no effect on hyperinsulinemia or hyperglycemia.

Influence of Prolonged Baroreflex Activation on Arterial Pressure in Angiotensin Hypertension

Despite recent evidence indicating sustained activation of the baroreflex during chronic infusion of angiotensin II (Ang II), sinoaortic denervation does not exacerbate the severity of the hypertension. Therefore, to determine whether Ang II hypertension is relatively resistant to the blood pressure-lowering effects of the baroreflex, the carotid baroreflex was electrically activated bilaterally for 7 days in 5 dogs both in the presence and absence of a continuous infusion of Ang II (5 ng/kg per minute) producing high physiological plasma levels of the peptide. Under control conditions, basal values for mean arterial pressure (MAP) and plasma norepinephrine concentration (NE) were 93±1 mm Hg and 99±25 pg/mL, respectively. By day 7 of baroreflex activation, MAP and NE were reduced to 72±4 mm Hg (−21±3 mm Hg) and 56±15 pg/mL, respectively, but PRA was unchanged (control=0.41±0.06 ng ANG I/mL per hour). All values returned to basal levels by the end of a 7-day recovery period. After 7 days of Ang II infusion, MAP increased from 93±3 to 129±3 mm Hg, whereas NE fell from 117±15 to 86±23 pg/mL. During the next 7 days of baroreflex activation/Ang II infusion, further reductions in NE were not statistically significant, and on the final day of baroreflex activation, the reduction in MAP was only 5±1 mm Hg, compared with 21±3 mm Hg in the control normotensive state. These findings indicate that long-term baroreflex-mediated reductions in arterial pressure are markedly diminished, but not totally eliminated, in the presence of hypertension produced by chronic infusion of Ang II.

Chronic Electrical Stimulation of the Carotid Sinus Baroreflex Improves Left Ventricular Function and Promotes Reversal of Ventricular Remodeling in Dogs With Advanced Heart Failure

Background—Autonomic abnormalities exist in heart failure and contribute to disease progression. Activation of the carotid sinus baroreflex (CSB) has been shown to reduce sympathetic outflow and augment parasympathetic vagal tone. This study tested the hypothesis that long-term electric activation of the CSB improves left ventricular (LV) function and attenuates progressive LV remodeling in dogs with advanced chronic heart failure. Methods and Results—Studies were performed in 14 dogs with coronary microembolization-induced heart failure (LV ejection fraction ≈25%). Eight dogs were chronically instrumented for bilateral CSB activation using the Rheos System (CVRx Inc, Minneapolis, Minn) and 6 were not and served as controls. All dogs were followed for 3 months, and none received other background therapy. During follow-up, treatment with CSB increased LV ejection fraction 4.0±2.4% compared with a reduction in control dogs of −2.8±1.0% (P<0.05). Similarly, treatment with CSB decreased LV end-systolic volume −2.5±2.7 mL compared with an increase in control dogs of 6.7±2.9 mL (P<0.05). Compared with control, CSB activation significantly decreased LV end-diastolic pressure and circulating plasma norepinephrine, normalized expression of cardiac &bgr;1-adrenergic receptors, &bgr;-adrenergic receptor kinase, and nitric oxide synthase and reduced interstitial fibrosis and cardiomyocyte hypertrophy. Conclusions—In dogs with advanced heart failure, CSB activation improves global LV function and partially reverses LV remodeling both globally and at cellular and molecular levels.

Renal Denervation Does Not Abolish Sustained Baroreflex-Mediated Reductions in Arterial Pressure

Recent studies indicate that suppression of renal sympathetic nerve activity and attendant increments in renal excretory function are sustained baroreflex-mediated responses in hypertensive animals. Given the central role of the kidneys in long-term regulation of arterial pressure, we hypothesized that the chronic blood pressure–lowering effects of the baroreflex are critically dependent on intact renal innervation. This hypothesis was tested in 6 dogs by bilaterally activating the carotid baroreflex electrically for 7 days before and after bilateral renal denervation. Before renal denervation, control values for mean arterial pressure and plasma norepinephrine concentration were 95±2 mm Hg and 96±12 pg/mL, respectively. During day 1 of baroreflex activation, mean arterial pressure decreased 13±1 mm Hg, and there was modest sodium retention. Daily sodium balance was subsequently restored, but reductions in mean arterial pressure were sustained throughout the 7 days of baroreflex activation. Activation of the baroreflex was associated with sustained decreases in plasma norepinephrine concentration (≈50%) and plasma renin activity (30% to 40%). All of the values returned to control levels during a 7-day recovery period. Two weeks after renal denervation, control values for mean arterial pressure, plasma norepinephrine concentration, plasma renin activity, and sodium excretion were comparable to those measured when the renal nerves were intact. Moreover, after renal denervation, all of the responses to activation of the baroreflex were similar to those observed before renal denervation. These findings demonstrate that the presence of the renal nerves is not an obligate requirement for achieving long-term reductions in arterial pressure during prolonged activation of the baroreflex.

Systemic and Renal-Specific Sympathoinhibition in Obesity Hypertension

Chronic pressure-mediated baroreflex activation suppresses renal sympathetic nerve activity. Recent observations indicate that chronic electric activation of the carotid baroreflex produces sustained reductions in global sympathetic activity and arterial pressure. Thus, we investigated the effects of global and renal specific suppression of sympathetic activity in dogs with sympathetically mediated, obesity-induced hypertension by comparing the cardiovascular, renal, and neurohormonal responses to chronic baroreflex activation and bilateral surgical renal denervation. After control measurements, the diet was supplemented with beef fat, whereas sodium intake was held constant. After 4 weeks on the high-fat diet, when body weight had increased ≈50%, fat intake was reduced to a level that maintained this body weight. This weight increase was associated with an increase in mean arterial pressure from 100±2 to 117±3 mm Hg and heart rate from 86±3 to 130±4 bpm. The hypertension was associated with a marked increase in cumulative sodium balance despite an approximately 35% increase in glomerular filtration rate. The importance of increased tubular reabsorption to sodium retention was further reflected by ≈35% decrease in fractional sodium excretion. Subsequently, both chronic baroreflex activation (7 days) and renal denervation decreased plasma renin activity and abolished the hypertension. However, baroreflex activation also suppressed systemic sympathetic activity and tachycardia and reduced glomerular hyperfiltration while increasing fractional sodium excretion. In contrast, glomerular filtration rate increased further after renal denervation. Thus, by improving autonomic control of cardiac function and diminishing glomerular hyperfiltration, suppression of global sympathetic activity by baroreflex activation may have beneficial effects in obesity beyond simply attenuating hypertension.

Sustained suppression of sympathetic activity and arterial pressure during chronic activation of the carotid baroreflex

Following sinoaortic denervation, which eliminates arterial baroreceptor input into the brain, there are slowly developing adaptations that abolish initial sympathetic activation and hypertension. In comparison, electrical stimulation of the carotid sinus for 1 wk produces sustained reductions in sympathetic activity and arterial pressure. However, whether compensations occur subsequently to diminish these responses is unclear. Therefore, we determined whether there are important central and/or peripheral adaptations that diminish the sympathoinhibitory and blood pressure-lowering effects of more sustained carotid sinus stimulation. To this end, we measured whole body plasma norepinephrine spillover and alpha(1)-adrenergic vascular reactivity in six dogs over a 3-wk period of baroreflex activation. During the first week of baroreflex activation, there was an approximately 45% decrease in plasma norepinephrine spillover, along with reductions in mean arterial pressure and heart rate of approximately 20 mmHg and 15 beats/min, respectively; additionally, plasma renin activity did not increase. Most importantly, these responses during week 1 were largely sustained throughout the 3 wk of baroreflex activation. Acute pressor responses to alpha-adrenergic stimulation during ganglionic blockade were similar throughout the study, indicating no compensatory increases in adrenergic vascular reactivity. These findings indicate that the sympathoinhibition and lowering of blood pressure and heart rate induced by chronic activation of the carotid baroreflex are not diminished by adaptations in the brain and peripheral circulation. Furthermore, by providing evidence that baroreflexes have long-term effects on sympathetic activity and arterial pressure, they present a perspective that is opposite from studies of sinoaortic denervation.

Prothrombin Complex Concentrate Versus Standard Therapies for INR Reversal in Trauma Patients Receiving Warfarin

Background: Prothrombin complex concentrate (PCC) is recommended as a therapy to be considered for the reversal of warfarins effects. Few published data are available on the use of PCC for this indication in traumatically injured patients. Objective: To determine whether the addition of PCC to standard approaches to warfarin reversal more rapidly corrects the international normalized ratio (INR) in injured patients. Methods: A retrospective analysis was performed in trauma patients who were on warfarin preinjury from January 2007 to September 2009 at North Memorial Medical Center. Data were collected from medical records and the trauma registry. Patients were separated based on whether or not they received PCC. The groups were compared on the basis of demographics, units of fresh frozen plasma (FFP), vitamin K use, units of PCC, number of patients achieving an INR of 1.5 or less, time to an INR of 1.5 or less, mortality, intensive care unit (ICU) and hospital length of stay, and the incidence of thromboembolic events during hospitalization. Results: Thirty-one patients were included in the analysis; 13 patients who received a total mean (SD) dose of 2281 (1053) units (25.6 [12.2] units/kg) of PCC (Profilnine SD) were compared to 18 patients who did not receive PCC. There was no significant difference between groups in FFP units received or the number of patients who received vitamin K. Most patients in both groups achieved an INR of 1.5 or less (92% PCC vs 89% no PCC). However, the mean time to achieve an INR of 1.5 or less was 16:59 (20:53) hours in the PCC group versus 30:03 (23:10) hours in the no PCC group (p = 0.048). There were 3 deaths in the PCC group and no deaths in the no PCC group (p = 0.06). ICU and hospital length of stay and number of thromboembolic events did not differ significantly between the 2 groups. Conclusions: PCC, when added to FFP and vitamin K, resulted in a more rapid time to reversal of the INR.

Primary repair of ultra-long-gap esophageal atresia: Results without a lengthening procedure

Ultra-long-gap esophageal atresia, defined as a gap length of 3.5 cm or greater, has proved difficult to repair. When primary repair has been attempted, even with bougienage, circular myotomy, or intraabdominal esophageal mobilization to lessen anastomotic tension, leaks, anastomotic disruptions, and recurrent tracheoesophageal fistulas are frequent. Consequently, interposition grafts are commonly used. For long-term function the intact native esophagus should be preferable to an interposition graft or the consequences of circular myotomy. Therefore, even when an ultra-long gap is present, we have carried out a primary repair using our single-layer technique without myotomies. Since 1979, 8 of 58 infants (14%) with esophageal atresia had gaps ranging from 3.5 to 6 cm. All had a primary repair with follow-up from 1 to 11 years. Despite severe anastomotic tension in all cases, there were no anastomotic leaks, disruptions, recurrent tracheoesophageal fistulas, or deaths. The tension, however, may have led to major gastroesophageal reflux in 5 of 8 patients (62.5%), all treated by a Nissen fundoplication, and a stricture in 4 of 8 infants (50%). Three strictures responded to dilation and one was resected. Now, all children are eating a normal diet for age. In conclusion, this technique has allowed primary repair of ultra-long-gap esophageal atresia. Although the severe tension may contribute to strictures needing dilation and gastroesophageal reflux requiring fundoplication, primary repair resulted in a clinically functional native esophagus.