Acute profound thrombocytopenia after abciximab therapy during coronary angioplasty

Since the extensive use of abciximab, a potent antiplatelet agent directed against GP IIb/IIIa platelet receptors, to prevent ischemic complications of percutaneous transluminal coronary angioplasty, few cases of thrombocytopenia have been observed. This paper reports a case of acute profound thrombocytopenia (platelet count: 800/mm3) occurring 16 h after abciximab therapy during coronary angioplasty. As thrombocytopenia occurrence is not predictable, platelet count should be evaluated periodically after drug administration. Mechanisms of this adverse effect remain unknown. Platelet transfusion results in a rapid and sustained improvement of platelet count, avoiding the occurrence of major hemorrhagic complications.

A large population of galaxies 9 to 12 billion years back in the history of the Universe

To understand the evolution of galaxies, we need to know as accurately as possible how many galaxies were present in the Universe at different epochs. Galaxies in the young Universe have hitherto mainly been identified using their expected optical colours, but this leaves open the possibility that a significant population remains undetected because their colours are the result of a complex mix of stars, gas, dust or active galactic nuclei. Here we report the results of a flux-limited I-band survey of galaxies at look-back times of 9 to 12 billion years. We find 970 galaxies with spectroscopic redshifts between 1.4 and 5. This population is 1.6 to 6.2 times larger than previous estimates, with the difference increasing towards brighter magnitudes. Strong ultraviolet continua (in the rest frame of the galaxies) indicate vigorous star formation rates of more than 10-100 solar masses per year. As a consequence, the cosmic star formation rate representing the volume-averaged production of stars is higher than previously measured at redshifts of 3 to 4.

Limitation of coronary reserve after successful angioplasty is prevented by oral pretreatment with an alpha1-adrenergic antagonist

Coronary vasoconstriction that occurs after percutaneous transluminal coronary angioplasty (PTCA) is abolished by intracoronary phentolamine. An impairment of coronary vasodilator reserve (CVR) has been observed < or = 7 days after successful PTCA. To ascertain whether pretreatment with the alpha1-adrenergic receptor blocker doxazosin could prevent the limitation of CVR after PTCA, we carried out a randomised, double-blind, controlled study on 26 patients with significant (> 75%) single vessel disease undergoing PTCA. Twelve patients received doxazosin 4 mg daily in addition to their standard treatment, while 14 patients received matching placebo, starting 11 days before PTCA. Myocardial blood flow (MBF) at baseline and after i.v. dipyridamole (0.56 mg/kg) was measured within 5 days after PTCA using positron emission tomography (PET) with oxygen-15-labelled water. Angioplasty was successful in all patients with a residual stenosis < or = 35%. At PET scanning, hemodynamic parameters were comparable in the two groups. In the territory subtended by the dilated artery, CVR was significantly higher in patients treated with doxazosin compared with those receiving placebo (2.78 +/- 0.1.21 vs. 1.95 +/- 0.68; p < 0.01). Conversely, CVR in the remote territories subtended by angiographically normal arteries was similar in the two groups (2.53 +/- 0.92 and 2.48 +/- 0.80, respectively; p = NS). Treatment with oral doxazosin in addition to standard antianginal therapy can prevent the impairment of CVR frequently observed despite successful PTCA.

Coronary vasodilator capacity and epicardial vessel remodeling in physiological and hypertensive hypertrophy

The aim of this study was to compare resting coronary flow velocity, determinants of myocardial oxygen demand, and coronary vasodilator capacity in subjects with physiological, exercise-induced, and hypertensive left ventricular hypertrophy. Sixteen healthy sedentary men, 16 endurance athletes, and 16 hypertensive subjects (mean+/-SEM for left ventricular mass index: 94.9+/-5.5, 184.6+/-8.4, 154.4+/-9.5 g/m(2), respectively) were studied by transesophageal and transthoracic Doppler echocardiography. Coronary flow velocity in left anterior descending artery and cross-sectional area of left main artery were assessed at rest and during dipyridamole-induced vasodilation. Myocardial oxygen demand was estimated through rate-pressure product, left ventricular wall stress, and inotropic function. Coronary flow reserve and minimum coronary resistance were comparable to those of sedentary men in athletes (mean+/-SEM: 3. 23+/-0.16 versus 3.60+/-0.18 and 0.96+/-0.06 versus 1.04+/-0.04 mm Hg. s. cm(-1)), while in hypertensive subjects they were decreased and increased, respectively (mean+/-SEM: 2.31+/-0.08 and 1.21+/-0.10 mm Hg. s. cm(-1); P:<0.05 for both). Resting flow velocity was directly related to rate-pressure product in sedentary men and athletes and also to wall stress in athletes, while these correlations were absent in hypertensives. Dilation of left main artery after dipyridamole was significantly higher in athletes than in sedentary men and hypertensive subjects (mean+/-SEM for area change: 32.9+/-3.7% versus 12.8+/-2.5% and 6.4+/-3.3%; P:<0.05 and 0.01). These data indicate that vasodilator capacity of coronary microcirculation is not impaired in athletes with physiological hypertrophy, in contrast to hypertensive patients. The relationship between resting flow velocity and determinants of oxygen demand is preserved in physiological hypertrophy but missing in hypertensive hypertrophy. Furthermore, the vasodilator capacity of coronary macrocirculation is also enhanced in exercise-trained subjects.

Effect of aspirin and ticlopidine on plasma tissue factor levels in stable and unstable angina pectoris

Patients with unstable angina have an increased activation of the coagulation system. Aspirin and ticlopidine given in combination may potentiate each other by the combination of different action mechanisms and may reduce the risk of coronary occlusion and clinical instability. Plasma tissue factor (TF) levels collected into the stenotic coronary artery may be an index of TF expression within the vasculature. In 160 patients undergoing angioplasty for a 81+/-5% coronary lesion, we measured TF in blood samples collected from a vein and from the coronary ostium. Immediately after and 10 minutes after the dilation procedures the samples were withdrawn also beyond the lesion. Heparin 150 U/kg was given as an anticoagulant. All patients were pretreated with 250 mg/day of aspirin. One hundred twenty patients were randomly assigned to receive 24, 48, or 72 hours of ticlopidine treatment (250 mg/twice daily). TF levels did not increase during angioplasty but there was a significantly higher TF expression in unstable than in stable patients, irrespective of the invasiveness of debulking procedures. When ticlopidine was given for 72 hours, TF levels were similar to normal laboratory values both in stable and unstable patients. This combined antiplatelet pretreatment may be of benefit in unstable angina patients, with a favorable cost/benefit ratio.

alpha-adrenergic coronary vasoconstriction and myocardial ischemia in humans

The use of quantitative coronary angiography, combined with Doppler and PET, has recently been directed at the study of alpha-adrenergic coronary vasomotion in humans. Confirming prior animal experiments, there is no evidence of alpha-adrenergic coronary constrictor tone at rest. Again confirming prior experiments, responses to alpha-adrenoceptor activation are augmented in the presence of coronary endothelial dysfunction and atherosclerosis, involving both alpha(1)- and alpha(2)-adrenoceptors in epicardial conduit arteries and microvessels. Such augmented alpha-adrenergic coronary constriction is observed during exercise and coronary interventions, and it is powerful enough to induce myocardial ischemia and limit myocardial function. Recent studies indicate a genetic determination of alpha(2)-adrenergic coronary constriction.

Alpha-adrenergic blockade improves recovery of myocardial perfusion and function after coronary stenting in patients with acute myocardial infarction

BACKGROUND

AMI reperfusion by thrombolysis does not improve TIMI flow and LV function. The role of infarct-related artery (IRA) stenosis and superimposed changes in coronary vasomotor tone in maintaining LV dysfunction must be elucidated.

METHODS AND RESULTS

Forty patients underwent diagnostic angiography 24 hours after thrombolysis. Seventy-two hours after thrombolysis, the culprit lesion was dilated with coronary stenting. During angioplasty, LV function was monitored by transesophageal echocardiography. Percent regional systolic thickening was quantitatively assessed before PTCA, soon after stenting, 15 minutes after stenting, and after phentolamine 12 microg/kg IC (n=10), the alpha1-blocker urapidil 600 microg/kg IV (n=10), or saline (n=10). Ten patients pretreated with beta-blockers received urapidil 10 mg IC. Coronary stenting significantly improved thickening in IRA-dependent and in non-IRA-dependent myocardium (from 27+/-15% to 38+/-16% and from 40+/-15% to 45+/-15%, respectively). Simultaneously, TIMI frame count decreased from 39+/-11 and 40+/-11 in the IRA and non-IRA, respectively, to 23+/-10 and 25+/-7 (P<0.05). Fifteen minutes after stenting, thickening worsened in both IRA- and non-IRA-dependent myocardium (to 19+/-14% and 28+/-14%, P<0.05), and TIMI frame count returned, in both the IRA and non-IRA, to the values obtained before stenting. Phentolamine and urapidil increased thickening to 36+/-17% and 41+/-14% in IRA and to 48+/-11% and 49+/-17% in non-IRA myocardium respectively, and TIMI frame count decreased to 16+/-6 and to 17+/-5, respectively. Changes were attenuated with beta-blocker pretreatment.

CONCLUSIONS

Our finding that alpha-adrenergic blockade attenuates vasoconstriction and postischemic LV dysfunction supports the hypothesis of an important role of neural mechanisms in this phenomenon.

Postischemic left ventricular dysfunction is abolished by alpha-adrenergic blocking agents

OBJECTIVES

We sought to evaluate the efficacy of alpha-adrenergic blocking agents in counteracting left ventricular (LV) dysfunction occurring after transient ischemia in humans.

BACKGROUND

The mechanisms underlying postischemic LV dysfunction are largely unknown.

METHODS

Percutaneous transluminal coronary angioplasty (PTCA) provides a clinical model of ischemia and reperfusion. In 50 patients undergoing coronary stenting for 77+/-5% stenosis, LV function was monitored by transesophageal echocardiography during and 30-min after PTCA. Fifteen minutes after stenting, 15 patients received 12 microg/kg body weight of the alpha-blocker phentolamine intracoronarily, 15 patients received 600 microg/kg of the alpha1-blocker urapidil intravenously, 10 patients received the combination of phentolamine and 1.2 mg of propranolol intracoronarily, and 10 patients received saline.

RESULTS

Fifteen minutes after successful coronary dilation, significant contractile dysfunction occurred in previously ischemic and nonischemic myocardium. LV dysfunction was accompanied by an increase in coronary resistance and diffuse vasoconstriction. Alpha-blockers counteracted LV dysfunction and coronary resistance and the increase in vasoconstriction. Phentolamine and urapidil increased global LV shortening from 34+/-9% to 45+/-8% and to 49+/-8%, respectively (p < 0.05). After the administration of propranolol combined with phentolamine, LV dysfunction remained unchanged (34+/-6%), as in control subjects.

CONCLUSIONS

LV dysfunction occurs after PTCA, as described in animal models after ischemia. Alpha-blockers abolished LV, macrocirculatory and microcirculatory dysfunction, whereas the alpha-blocker effect was prevented by combining alpha- and beta-blockers. The evidence of diffuse rather than regional dysfunction, together with the opposite effects of alpha- and beta-blockade, supports the hypothesis of neural mechanisms eliciting postischemic LV dysfunction.

The alpha-1 adrenergic blocking agent urapidil counteracts postrotational atherectomy "elastic recoil" where nitrates have failed

Calcium antagonist pretreatment and intracoronary high doses of nitrates (9 mg of isosorbide dinitrate) do not counteract coronary vasoconstriction occurring after rotational atherectomy. In 30 patients undergoing Rotablator atherectomy, intracoronary injection of the alpha 1-sympathetic blocker urapidil abolished or prevented significant vasoconstriction occurring 15 minutes after the procedure despite repeated injections of nitrates.

Ticlopidine and aspirin interactions

The risk-benefit balance when aspirin is compared with aspirin combined with ticlopidine is being investigated in several multicentre trials (MUSIC and WEST II versus TASTE, MUST, and STARS respectively). Cardiologists follow one of two strategies. Some prefer a more aggressive antiplatelet regimen, disregarding the risk of neutropenia (0.7%) because they want to avoid lessening the therapeutic effect of vessel patency obtained with stent implantation. Others give only aspirin (a money saving approach) confident that IVUS inspection (an expensive approach) will allow an adequate evaluation of full stent expansion and lesion coverage, despite a more pronounced activation of the coagulation cascade. Our impression so far is that the combination of ticlopidine and aspirin has a more favourable risk-effect balance.

Ticlopidine and aspirin pretreatment reduces coagulation and platelet activation during coronary dilation procedures

OBJECTIVES

It is unknown whether a therapeutic combination of aspirin (ASA) and ticlopidine might effectively decrease activation of hemostasis.

BACKGROUND

Percutaneous transluminal coronary angioplasty (PTCA), rotational atherectomy and stent implantation are procedures that fracture or ablate endothelium and plaque, a situation that activates hemostasis.

METHODS

In 85 patients undergoing PTCA for a 77.8 +/- 1% stenosis, we measured markers of coagulation and platelet activation (thrombin-antithrombin complexes [TAT], prothrombin fragment 1 + 2 [F1 + 2] serotonin and the presence of circulating activated platelets reacting with monoclonal antibodies against glycoproteins exposed on platelet membranes). Blood samples were drawn from a peripheral vein and from the coronary ostium before the procedures. Both immediately and 10 min after angioplasty, and 10 min afterward, samples were collected from a probing catheter (0.018 in, [0.46 cm]) positioned beyond the stenosis. All patients were being treated with antianginal drugs and ASA, 250 mg/day. Seventy of them had taken ticlopidine, 250 mg, twice daily for < or = 1 day (< or = 24 h) (n = 28) or for > or = 3 days (> or = 72 h) (n = 42). Heparin (150 U/kg) was administered before angioplasty. Thirty patients underwent PTCA; 15 of them were not treated with ticlopidine and 15 were given ticlopidine (> or = 72 h). Thirty-five patients had stent implantation, 20 rotational atherectomy.

RESULTS

Before and during the procedures, there was greater thrombin generation (expressed by higher TAT and F1 + 2 plasma levels) in patients not taking ticlopidine or taking it for < or = 24 h (p < 0.05). Platelet activation and plasma serotonin levels were also significantly higher in the no ticlopidine or < or = 24-h ticlopidine groups.

CONCLUSIONS

The combined use of ticlopidine, ASA and heparin effectively controls activation of coagulation in patients with stable or unstable angina undergoing coronary dilation.

Effects of amlodipine on coronary hemodynamics and vascular responses to sympathetic stimulation in patients with coronary heart disease

Dihydropyridines (DHPs) exert a powerful coronary vasodilator action, but whether they actually affect the coronary vasomotor effects elicited by an increase in cardiac sympathetic drive is controversial. We assessed the effects of the DHP calcium antagonist amlodipine on coronary hemodynamics and vascular response to sympathetic activation in patients with coronary heart disease. In the control condition, mean arterial pressure (MAP, aortic catheter), heart rate (HR, ECG), rate-pressure product (RPP), coronary sinus blood flow (CBF, thermodilution) and coronary vascular resistance (CVR) (ratio between MAP and CBF) were measured in all our case series (13 patients with angiographically documented severe coronary artery disease) before and during a 2-min cold pressor test (CPT) and a 30-s diving (D) and, in the 8 patients of this case series who were smokers, also before and during smoking a cigarette (S, nicotine content 1.0 mg for 10 min). The same protocol used in control condition was repeated 30 min after intravenous (i.v.) bolus administration of 11 mg amlodipine. CPT, diving, and smoking increased MAP and RPP and caused a marked and significant increase in CVR (+12.1 +/- 4.8, +30.4 +/- 6.8, and +16.8 +/- 7.2%, respectively). Amlodipine reduced MAP, increased CBF, and caused a marked decrease in CBF. The drug did not modify responses to CPT and diving or pressure and HR responses to smoking, whereas the smoking-induced increase in coronary vascular resistance was attenuated after amlodipine administration (+3.2 +/- 2.7%, p < 0.05 vs. control condition). Thus, amlodipine does not attenuate the sympathetic coronary vasoconstrictor effects of CPT and diving.(ABSTRACT TRUNCATED AT 250 WORDS)

Coronary vasoconstriction after percutaneous transluminal coronary angioplasty is attenuated by antiadrenergic agents

BACKGROUND

Vasoconstriction occurs after percutaneous transluminal coronary angioplasty (PTCA) along the dilated vessel. The vasomotor changes, initiated by the mechanical stretch of the stenotic region, are thought to be due to various mechanisms but whether the sympathetic nervous system plays a role in this phenomenon remains unknown.

METHODS AND RESULTS

Quantitative angiography (ARTREK) was performed in 45 patients undergoing an epicardial vessel PTCA for a stenosis of 76 +/- 1% (1) in basal conditions, (2) after PTCA, and (3) 30 minutes after PTCA (vasoconstriction). In 14 control patients, the same measurements were obtained up to 60 minutes after PTCA. Coronary diameters were measured along the PTCA vessel at the narrowest stenosis level and at a level peripheral to stenosis. In 36 patients two diameters were also measured at a proximal segment and at a distal segment along a nonmanipulated vessel. Thirty minutes after PTCA the dilated segment underwent a -31 +/- 2% (mean +/- SEM, ANOVA, P < .05) reduction in diameter when compared with PTCA values, and the segment peripheral to stenosis showed a reduction of -17 +/- 2% (P < .05). In all patients a significant vasoconstriction also was observed along the control vessel (proximal segment, -14 +/- 3%; P < .05 versus basal; and distal segment, -17 +/- 2%). At the time of maximal vasoconstriction (30 minutes after PTCA), the patients (treatment groups) received (1) 18 micrograms/kg IC phentolamine (Phe, n = 7), (2) 14 micrograms/kg IC yohimbine (YO, n = 7), (3) 16 micrograms/kg IC propranolol (Pro) followed by 18 micrograms/kg IC phentolamine (Pro+Phe, n = 7), and (4) 0.2 mg/kg IC bretylium (Bre, n = 10). In 14 patients (control groups) an intracoronary injection of warm saline was given. After drug injections, angiograms were repeated at 5-minute intervals for 20 minutes and ended after a 300-micrograms intracoronary trinitroglycerin injection. At stenosis level, Phe and Bre counteracted vasoconstriction, inducing a dilatation of +19 +/- 3% and +22 +/- 6%, respectively, while Pro+Phe caused a dilatation of +16 +/- 9% above the PTCA values (P < .05 versus PTCA). YO only partially reversed vasoconstriction (from -33 +/- 4% to -12 +/- 4%, P = NS versus PTCA). At peripheral-to-stenosis level, vasoconstriction was abolished by Phe (+26 +/- 7%, P < .05 versus basal), while it was still present after Pro+Phe (-23 +/- 2%) and Bre (-18 +/- 4%). In addition, Phe and Bre dilated the control vessel at the proximal segment (+17 +/- 6% and +8 +/- 4%, respectively, P < .05 versus basal), while YO and Pro+Phe only counteracted vasoconstriction (from -15 +/- 3% to +7.6 +/- 1% and from -16 +/- 3% to +4 +/- 5%, respectively, P = NS versus basal). At the distal segment only Phe produced a vasodilatation of +23 +/- 1%; YO counteracted constriction (from -16 +/- 2% to +9 +/- 6%, P < .05 versus basal), whereas after Pro+Phe and Bre, the vasoconstriction persisted.

CONCLUSIONS

The mechanical stretch and ischemia caused by balloon inflation induced vasoconstriction mediated by alpha-adrenergic receptors (mainly alpha 1), overcoming a beta-mediated dilatation. The use of different antiadrenergic drugs showed that Phe counteracts post-PTCA vasoconstriction, and the simultaneous use of alpha- and beta-receptor blocking agents (Pro+Phe and Bre) reveals the presence of a peripheral, predominant beta-mediated dilatation. The presence of vasoconstriction also along the control vessels not branching from the stretched ramus provides evidence for the existence of neural sympathetic vasoconstrictor reflexes.

Impairment of the arterial baroreflex during symptomatic and silent myocardial ischemia in humans

OBJECTIVES

The aim of this study was to assess whether transient episodes of symptomatic or silent myocardial ischemia after baroreceptor modulation of heart rate.

BACKGROUND

Animal and human studies have shown that myocardial infarction is accompanied by an impairment of the baroreceptor influences on the sinus node. However, whether this also occurs during transient myocardial ischemia has never been documented.

METHODS

In 12 patients undergoing coronary angiography, systolic blood pressure (intraarterial catheter) was reduced by an intravenous bolus of nitroglycerin during a spontaneous episode of transient chest pain and myocardial ischemia (ST segment depression on the electrocardiogram) and 30 min after recovery. The slope of the linear regression between the decrease in systolic blood pressure and the RR interval shortening was taken as the measure of baroreflex sensitivity.

RESULTS

During ischemia, baroreflex sensitivity was 1.3 +/- 0.3 ms/mm Hg (mean +/- SEM), whereas after recovery it was markedly and significantly greater (2.6 +/- 0.5 ms/mm Hg, p < 0.01). Similar results were obtained in eight other patients who experienced a silent ischemic episode either spontaneously or during coronary angioplasty. The reduction in baroreflex sensitivity was similarly pronounced during inferior (10 patients) and anterior (10 patients) ischemia, and its magnitude showed little or no relation to the ischemia-dependent changes in blood pressure and heart rate.

CONCLUSIONS

Transient myocardial ischemia is associated with marked baroreflex impairment. The impairment occurs even during symptomless ischemic episodes and is therefore not related to pain or to other nonspecific influences on the baroreflex.

ACE inhibition attenuates sympathetic coronary vasoconstriction in patients with coronary artery disease

BACKGROUND

In humans, angiotensin converting enzyme (ACE) inhibition attenuates the vasoconstriction induced by sympathetic stimulation in a number of peripheral districts. Whether this is also the case in the coronary circulation is unknown, however.

METHODS AND RESULTS

In nine normotensive patients with angiographically assessed coronary atherosclerosis, we measured the changes in mean arterial pressure (intra-arterial catheter), heart rate, rate-pressure product (RPP), coronary sinus blood flow (CBF, thermodilution method), and coronary vascular resistance (CVR, ratio between mean arterial pressure and CBF) induced by the cold pressor test (CPT, 2 minutes) and diving (30 seconds), i.e., two stimuli eliciting a sympathetic coronary vasoconstriction. The measurements were performed in the control condition and 30 minutes after captopril 25 mg p.o. In the control condition, CPT caused an increase in mean arterial pressure and heart rate. Despite the increase in RPP (+20.7 +/- 3.2%, p less than 0.01), CBF did not change and CVR increased (+12.2 +/- 4.0%, p less than 0.05). diving caused an increase in mean arterial pressure and a reduction in heart rate. RPP increased (+14.3 +/- 3.5%, p less than 0.01), but despite this increase, there was a reduction in CBF and a marked increase in CVR (+37.3 +/- 7.4%, p less than 0.01). Captopril did not modify the blood pressure and heart rate responses to both stimuli except for a slight accentuation of the bradycardia to diving. Despite the unchanged or only slightly reduced RPP response, the increase in CVR was markedly and significantly attenuated (p less than 0.01).

CONCLUSIONS

ACE inhibition attenuates sympathetic coronary vasoconstriction in patients with coronary artery disease. This is probably due to removal of the facilitating influence of angiotensin II on sympathetic modulation of coronary vasomotor tone.

Coronary response to diving in subjects with mild and severe left coronary artery disease

The cold pressor test (CPT) is commonly used to determine the vasospastic origin of angina and to investigate the factors modulating coronary vasomotor tone. However, coronary vasoconstriction associated with this manoeuvre is often limited, particularly in patients with mild coronary atherosclerosis. To identify stimuli that can more powerfully constrict the coronary arteries we compared the effects on coronary blood flow (thermodilution) and vascular resistance (mean aortic pressure divided by coronary blood flow) of CPT (2 min) and diving (D, 45 s) in subjects with angiographically documented mild (n = 11) or severe (n = 11) left anterior descending coronary artery stenosis. In subjects with severe coronary artery stenosis the rate-pressure product increased to a similar extent with CPT and D. The latter stimulus, however, caused a more marked fall in coronary blood flow and a much more pronounced increase in coronary resistance as compared to CPT (+44 +/- 3.1% vs +19 +/- 1.6%, P less than 0.01). In the face of a similar increase in rate-pressure product, D caused a significant increase in coronary vascular resistance also in patients with mild coronary artery stenosis (less than or equal to 50%) in which CPT failed to induce any coronary vasoconstriction (+16 +/- 1.8% vs +0.3 +/- 1.3%, P less than 0.01). Thus, diving is a much more powerful coronary vasoconstrictor stimulus than CPT. It can thus replace CPT when an increase in coronary resistance is needed for diagnostic purposes or for investigating abnormalities in coronary vascular regulation.

Use of intravenous digital subtraction ventriculography in the evaluation of left ventricle volumes and ejection fraction

Although intravenous digital subtraction ventriculography (IDSV) is increasingly used to estimate end-diastolic left ventricular volume (EDV), end-systolic left ventricular volume (ESV) and left ventricular ejection fraction (EF), its ability to reproduce the precise estimates provided by left ventricle cineangiography (LVCA) and its role in clinical cardiology have not been unequivocally established. In 32 patients subjected to cardiac catheterization for a variety of cardiac disorders and a normal or reduced left ventricular function the EDV, ESV and EF provided by a 30 degrees right anterior oblique LVCA were compared with those provided by a 30 degrees right anterior oblique IDSV. The mean EDV, ESV and EF obtained by IDSV in the 32 patients were superimposable on those obtained by LVCA. The individual EDV, ESV and EF values provided by the two methods were all related in a close linear fashion. For EF the correlation coefficient was 0.98 and the 90% confidence interval of the mean difference between the two series of values was +/- 6.1%, i.e. +/- 10% error compared to the mean EF provided by LVCA. Thus IDSV is a reliable and not too invasive method for estimating left ventricle volumes and ejection fraction. It might provide serial estimations with a better assessment of the evolution of a patient's disease and the effect of treatment.

Haemodynamic effects of ACE inhibitors

This paper reviews the haemodynamic effects of angiotensin-converting enzyme (ACE) inhibitors in hypertension, focusing on their ability to cause a fall in systemic vascular resistance, with no change in cardiac output and no reduction and even an increase in blood flow to vital organs such as the brain, the kidney and the heart. The haemodynamic effects of ACE inhibitors are qualitatively similar in congestive heart failure, except that, in the presence of impaired cardiac function, the fall in resistance is accompanied by a pronounced increase in cardiac output and tissue perfusion. In both conditions ACE inhibition opposes sympathetic influences and enhances vagal influences and, in hypertension, this intervention is followed by a regression of left ventricular hypertrophy providing a multifold background for a cardioprotective action. The new ACE inhibitor quinapril appears to share the haemodynamic effects of other ACE inhibitors with an improvement of cardiovascular function in congestive heart failure.

Calcium antagonists and neural control of circulation in essential hypertension

Data from animals and from man suggest that calcium antagonists interfere with alpha-adrenergic receptors and that this mechanism may be responsible for some of the vasodilation induced by these drugs. However, alpha-adrenergic receptors play a primary role in baroreceptor regulation of the cardiovascular system and blood pressure homeostasis, which might therefore be adversely affected by calcium antagonist treatment. We addressed this question in 14 essential hypertensives studied before treatment, 1 h after 20 mg oral nitrendipine and 5-7 days after daily administration of 20 mg oral nitrendipine. Blood pressure was measured by an intra-arterial catheter, heart rate by an electrocardiogram, cardiac output by thermodilution and forearm blood flow by venous occlusion plethysmography. Total peripheral and forearm vascular resistances were calculated by dividing mean blood pressure by blood flow values. Plasma norepinephrine was also measured (high performance liquid chromatography) in blood taken from the right atrium. Compared with the pretreatment values, acute nitrendipine administration caused a fall in resting blood pressure, an increase in the resting heart rate and cardiac output, and a fall in resting peripheral and forearm vascular resistance. The resting hypotension and vasodilation were also evident during the prolonged nitrendipine administration, which was, however, accompanied by much less resting cardiac stimulation than that observed in the acute condition. Baroreceptor control of the heart rate (vasoactive drug method) was similar before and after acute and prolonged nitrendipine treatment. This was also the case for carotid baroreceptor control of blood pressure (neck chamber technique) and for control of forearm vascular resistance as exerted by receptors in the cardiopulmonary region (lower-body negative-pressure and passive leg-raising techniques).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of altering the interval between the stimulus and the reflex response in the analysis of the baroreceptor control of the sinus and atrioventricular nodes in man

Baroreceptor control of the sinus node may be determined by raising or lowering blood pressure with intravenous bolus injections of phenylephrine or glyceryl trinitrate and calculating the slope of the linear regression between the drug induced changes in systolic blood pressure and RR interval using shift 1 coupling--namely, coupling of each systolic blood pressure value with the interval of the following cardiac cycle. To assess whether shift 1 coupling provides the best linear fit and the highest regression slope nine subjects received phenylephrine and glyceryl trinitrate injections both during spontaneous sinus rhythm and during atrial pacing to evaluate baroreflex control of the sinus and of the atrioventricular node respectively. In regression analysis of the data, for each drug injection nine different shifts (from 0 to 8) were used to couple systolic blood pressure with RR or StQ intervals. When the mean results from all subjects were compared the use of shift 1 was equal or superior to the use of any other shift for both the RR and the StQ interval responses evoked by either phenylephrine or glyceryl trinitrate. In many instances, however, the shift that provided the highest correlation and regression coefficient was different from shift 1, and the use of these best individual shifts provided results considerably different from those obtained with the standard shift 1. It is concluded that in the regression analysis of baroreflex cardiac responses to vasoactive drugs the regular use of shift 1 does not invariably provide the best estimation of baroreflex sensitivity. This is better achieved by calculating the best shift in individual responses.

Control of circulation by arterial baroreceptors and cardiopulmonary receptors in hypertension

This article reviews observations made on reflex control of circulation in essential hypertension and in a few models of experimental hypertension. It is emphasized that a wide consensus exists on the fact that baroreceptor control of heart rate is impaired in essential hypertension and that this occurs not only in severe but also in mild and borderline hypertensive conditions. On the other hand, baroreceptor control of blood pressure is much less affected by chronic elevations in arterial blood pressure due to central or peripheral factors that differentially affect cardiac and vascular responses to the vagus and the sympathetic drive, respectively. Cardiopulmonary receptor control of peripheral circulation and renin release is also little affected by hypertension, and the suggestion has been made that its sensitivity may even be enhanced in early hypertensive stages and then undergoes a subsequent reduction as cardiac hypertrophy develops. Finally, the possible consequences of preservation of reflex control of circulation in hypertension are discussed. It is speculated that this phenomenon may be in part responsible for the similar percentage of blood pressure oscillations that can be observed in normotensive and borderline, moderate, and severe hypertensive subjects.

Evaluation of an index of peripheral vascular resistance in human subjects

Based on evidence in animal studies, an index of resistance in man has been evaluated. In 10 human subjects, ranging in age from 31 to 60 years, direct brachial arterial pressure was measured, and a computer program using linear regression calculated the index of resistance from the slope of the logarithm of the pressure versus time during the second half of diastole. Resistance was independently calculated from 20-s averages of pressure and cardiac output obtained by thermodilution. Resistance was varied by infusion of phenylephrine and nitroprusside. The values of the index of resistance were compared with independently calculated resistance by linear regression and correlation. The index of resistance was variably correlated with independently calculated resistance. However, when the comparison was limited to points with a model correlation coefficient greater than or equal to 0.98 (i.e. a good fit of a straight line to the logarithm of pressure versus time), the correlation of the index of resistance with calculated resistance approached or exceeded 0.9 in eight of 10 subjects. In the two subjects showing poor correlation of the index with calculated resistance, estimated compliance from the same model was much lower than in the other subjects. In the same two subjects pressure dependence of estimated compliance was much higher than in the other subjects, suggesting the presence of significant atherosclerosis. We concluded that the diastolic decay of pressure may be used to calculate a useful index of resistance, provided that a single exponential decay fits the observed diastolic waveform well, and arterial compliance is not significantly reduced by atherosclerosis.

Twenty-four-hour blood pressure profile and blood pressure variability in untreated hypertension and during antihypertensive treatment by once-a-day nadolol

First, blood pressure and heart rate variability was studied in 89 normotensive and hypertensive ambulatory subjects with the use of an intra-arterial monitoring device. Short- and long-term variabilities were analyzed by computer. Absolute variabilities (standard deviations) were greatest in patients with hypertension, but relative variabilities (variation coefficients, i.e., standard deviations as percent of means) were slightly lower in the subjects with more severe hypertension. Second, the effect of nadolol on 24-hour blood pressure and heart rate values and on their variability was assessed in seven ambulatory patients with essential hypertension by means of the same intra-arterial device and computer analysis. Two recording sessions were performed (1) without treatment and (2) after 10 days' administration of nadolol once a day (dose range, 80 to 320 mg). Nadolol, given once a day, was shown to reduce blood pressure uniformly throughout the 24-hour period without loss of activity in the hours farthest from administration. The lack of alteration in relative blood pressure variability suggests that nadolol lowers blood pressure without interfering with the mechanisms involved in cardiovascular homeostasis. Reduction in heart rate variability after nadolol suggests less chance of tachycardia episodes in patients with angina and/or arrhythmias receiving nadolol.

Modification of the baroreceptor control of atrio-ventricular conduction induced by digitalis in man

Several studies in animals and in man have suggested that the inhibitory influence of baroreceptors on heart rate and peripheral circulation is enhanced by digitalis. Because the atrio-ventricular node represents a key site for the clinical action of digitalis we studied how baroreceptor control of atrio-ventricular conduction is modified by digitalis at therapeutical doses. In eight subjects heart rate was kept constant by atrial pacing to assess neural influences on atrio-ventricular conduction rate without the modifications caused by simultaneous changes in cardiac cycle length. Arterial baroreceptors were stimulated by increasing or reducing blood pressure (intra-arterial recording), via an iv bolus of phenylephrine or nitroglycerine. The baroreflex sensitivity was assessed in ms . mmHg-1 as the slope of the linear regressions relating the rise or fall in systolic blood pressure to the lengthening or shortening in St- (atrial stimulus artifact) Q interval (ECG recording). The study was performed before and 45 min after iv administration of digitalis (0.8 mg of Lanatoside C). Baroreflex sensitivity during baroreceptor stimulation was 2.9 +/- 1.1 ms . mmHg-1 (mean +/- SE) before digitalis, whereas after digitalis a significantly and markedly greater value of 5.6 +/- 1.5 ms . mmHg-1 was found. Baroreflex sensitivity during baroreceptor deactivation was 0.9 +/- 0.1 ms . mmHg-1 before digitalis, and was not significantly affected by the drug. Thus in man the baroreceptor control of atrio-ventricular conduction is strikingly potentiated by digitalis although this potentiation is only evident in the upper portion of the stimulus-response curve of the reflex.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of blood-pressure measurement by the doctor on patient's blood pressure and heart rate

Changes in blood pressure in 10 or 15 min periods during which a doctor repeatedly measured blood pressure by the cuff method were monitored by a continuous intra-arterial recorder. In almost all the 48 normotensive and hypertensive subjects tested the doctor's arrival at the bedside induced immediate rises in systolic and diastolic blood pressures peaking within 1 to 4 min (mean 26.7 +/- 2.3 mm Hg and 14.9 +/- 1.6 mm Hg above pre-visit values). There were large differences between individuals in the peak response (range, 4--75 mm Hg systolic and 1--36 mm Hg diastolic) unrelated to age, sex, baseline blood pressure, or blood-pressure variability. There was concomitant tachycardia (average peak response 15.9 +/- 1.5 beats/min, range 4--45 beats/min) which was only slightly correlated with the blood-pressure rise. After the peak response blood pressure declined and at the end of the visit was only slightly above the pre-visit level. A second visit by the same doctor did not change the average size of the early pressor response or the slope of its subsequent decline.

Plasma catecholamines do not invariably reflect sympathetically induced changes in blood pressure in man

Plasma concentrations of noradrenaline and adrenaline were measured radioenzymatically in nine subjects during 4 min pressor and depressor responses (intra-arterial measurements) induced by increasing and reducing sympathetic vasoconstrictor tone via carotid baroreceptor deactivation and stimulation (neck chamber technique). During the pressor response (15 +/- 3 mmHg, mean +/- SE) plasma noradrenaline and adrenaline showed various changes in the different subjects and on average were not significantly increased above control. During the depressor response (-9 +/- 2 mmHg) plasma noradrenaline and adrenaline also showed various changes in the subjects and were on average not significantly reduced below control. In contrast the same subjects all showed an increase in noradrenaline and adrenaline (average 76 and 117%) at the fourth minute of a tilting manoeuvre with a return to pretilting values no more than 4 min after resumption of the supine position. These results suggest that the moderate and/or restricted alterations in sympathetic tone produced by manipulating a single baroreflex, though capable of affecting blood pressure, are not reflected by alterations in plasma catecholamines. To modify these humoral indices significantly, the more drastic or more diffuse alterations in sympathetic activity that may be produced by manipulating low as well as high pressure reflexogenic areas are needed.

Twenty-four-hour hemodynamic profile during treatment of essential hypertension by once-a-day nadolol

The effect of nadolol (N) on 24-hour blood pressure (BP) and heart rate (HR) values and on their variability was examined in ambulant patients with essential hypertension, using the Oxford method to obtain continuous intraarterial recording and a computer to have a beat-to-beat analysis of the data. The recording was carried out without treatment and after 10 days' administration of N once daily by mouth (dose range: 80-320 mg). After N, 24-hour BP and HR were reduced by 17 +/- 3% and 27 +/- 4% respectively as compared to before N, the effect being similar for both systolic and diastolic BP. The hypertension and bradycardia were significantly more marked during the day than during the night, neither showing any attenuation in the hours furthest from the administration of the drug. During N, there was a reduction in the 24-hour variation coefficient for HR but the reduction was limited to the longer term component of this phenomenon, the moment-to-moment variations remaining unaffected. The long- and short-term variation coefficients for BP were not modified under N. These findings suggest that N once a day can reduce BP for 24 hours in ambulant hypertensive patients. The lack of alteration in variability of BP and moment-to-moment HR suggests that the hypotension is achieved without interfering with the mechanisms involved in cardiovascular homeostasis.

Mechanisms of antihypertensive action of beta-adrenergic blocking drugs: evidence against potentiation of baroreflexes

A possibility that can be advanced to explain the antihypertensive effect of beta-blocking drugs is that they act through the baroreflex control of the cardiovascular system. In 38 essential hypertensive patients we measured 1) The lengthening and shortening in R-R interval caused by stimulation or deactivation of arterial baroreceptors (vasoactive drug technique); 2) The fall and rise in blood pressure caused by stimulation and deactivation of carotid baroreceptors (neck chamber); 3) The rise in forearm vascular resistance caused by deactivation of cardiopulmonary receptors (lower body suction). The study was made before and after 6-10 days' administration of nadolol (80-360 mg once a day) or acebutolol (200-600 mg t.i.d.). Nadolol and acebutolol similarly reduced blood pressure and heart rate. Either drug increased heart rate responses to arterial baroreceptor manipulation but the increase fell short of statistical significance. Blood pressure and vasomotor responses to carotid baroreceptor and cardiopulmonary receptor manipulation were also not significantly modified by beta blockade. The baroreceptor control of heart rate and blood pressure showed a modification, however, insofar as a resetting towards the lower blood pressure values occurred. These findings demonstrate that arterial baroreceptor and cardiopulmonary receptor control of circulation is not potentiated by beta-blocking drugs, and that therefore this mechanism cannot account for their antihypertensive effect. The resetting of the baroreflex that occurs during beta blockade may, however, contribute to maintain the hypotension obtained.

Modification of arterial baroreflexes by captopril in essential hypertension

Captopril lowers blood pressure without increasing heart rate and plasma norepinephrine, which suggests that this drug may potentiate arterial baroreflexes. In eight subjects with untreated essential hypertension, blood pressure was monitored intraarterially and the effects of baroreceptor stimulation or deactivation were assessed by measuring (1) the slopes of the relations between increase or reduction in systolic pressure (intravenous phenylephrine or nitroglycerin) and the resulting lengthening or shortening in R-R interval, and (2) the increase or decrease in mean arterial pressure induced by increasing and decreasing carotid transmural pressure (neck chamber). The measurements were made before and after a hypotensive oral dose of captopril (50 mg). Before captopril, the slopes of the R-R interval changes with increase and reduction in systolic pressure were 8 and 4 ms/mm Hg, respectively. The slopes of the mean arterial pressure changes with increase and reduction in carotid transmural pressure were 0.51 and 0.40 mm Hg, respectively. After captopril, the responses to baroreceptor stimulation were unaltered but those to baroreceptor deactivation were augmented. The pressor and heart rate responses to hand-grip and cold exposure were unchanged by captopril. Administration of captopril is accompanied by a baroreflex potentiation which involves the lower portion of the stimulus-response curve of the reflex. This phenomenon (which may originate at the afferent baroreceptor fibers or centrally) may avoid a reduction in the tonic baroreflex influence during captopril-induced hypotension, thus contributing to the hemodynamic effects of the drug.

Effects of isometric exercise on the carotid baroreflex in hypertensive subjects

Previous studies indicate that arterial baroreceptor modulation of heart rate is drastically reduced during static or dynamic exercise. We have investigated whether this reduction also occurs with regard to blood pressure modulation by the baroreflex. The study was performed in 19 subjects with uncomplicated untreated essential hypertension in whom blood pressure was measured intraarterially, and R-R interval was obtained by an electrocardiogram. The carotid baroreceptors were stimulated by neck suction of 30 seconds' duration, and equal stimuli were applied at rest and during hand-grip exercise performed at 40% of the subjects' maximal strength. Baroreceptor stimulation at rest increased R-R interval and reduced blood pressure. During hand-grip, the R-R interval responses to the baroreceptor stimulus were diminished by 61%. In contrast, the blood pressure responses were not significantly altered. Similar results were obtained when two subgroups of subjects with a lesser or greater degree of hypertension were separately considered. Thus, the carotid baroreceptor influence on blood pressure is largely preserved during exercise in contrast to the carotid baroreceptor influence on the sinus node, which is markedly impaired.

Blood pressure response to labetalol in twice and three times daily administration during a 24-hour period

1 The anti-hypertensive effect of labetalol given twice or three times daily was evaluated in ambulant subjects with essential hypertension by recording blood pressure directly for 24 h before and after 15 d of labetalol administration (daily dose 600-1800 mg). 2 Labetalol reduced 24 h systolic and diastolic blood pressures by about 20%. The reduction was evident throughout the whole 24 h period, although it was less marked during sleep. The hypotensive effect was similar when the drug was given twice or three times daily. 3 The 24 h heart rate was reduced during labetalol treatment. However, this effect was less marked than the hypotensive effect and was not present in all subjects. 4 There was a reduction in the standard deviations of blood pressure and heart rate values. However, in neither case was the coefficient of variation altered, indicating that labetalol did not have any significant effect on the shape of the 24 h blood pressure measurements.

Carotid sinus reflex control of renin release in hypertensive subjects with high renin secretion

1. Carotid baroreceptor manipulation (neck-chamber technique) and passive head-up tilting were used in ten patients with renovascular hypertension and in five subjects with essential hypertension under diuretic treatment to study reflex control of renin secretion at high basal-renin production rates. 2. Reflex effects of carotid baroreceptor manipulation on renin secretion were only minor. During baroreceptor deactivation there was a moderate increase in mean arterial pressure, but an inconsistent change in the renal venous--arterial difference in plasma renin activity (PRA). 3. During baroreceptor stimulation there was a modest fall in mean arterial pressure and a marked rise in the renal venous--arterial difference in PRA. This was opposite to the fall which might have been predicted as a result of the sympathetic depressor influence of the baroreceptor stimulus. Conversely, tilting increased the venous--arterial PRA difference by about 200%. 4. It is concluded that when renin production rate is high carotid baroreceptors exert little control over renin release, just as when renin production is low. Reflex control of renin, however, is very active in subjects with a high renin production, probably due to receptors in the cardiopulmonary region.

Evaluation of a slow-release clonidine preparation by direct continuous blood pressure recording in essential hypertensive patients

The antihypertensive effect of a slow-release (SR) clonidine preparation was evaluated in hypertensive patients not confined to bed using a 24-h intraarterial blood pressure monitoring (Oxford) method. The monitoring was carried out during placebo administration and after 7-10 days of daily administration of SR clonidine in a single oral dose (250 or 500 microgram). An analysis of all the blood pressure values obtained was performed by computer. The results were as follows: (a) SR clonidine significantly reduced the 24-h systolic and diastolic arterial pressure by 16 and 10 mm Hg, respectively (mean +/- SE). (b) The reduction was well evident throughout the 24-h period, the average decrease in mean arterial pressure observed during the first, second, and third 8-h period after the administration of the drug being 11, 14, and 14 mm Hg, respectively. (c) The blood pressure reduction was similar during the daytime and the nighttime, despite the lower base-line value that occurred in the latter condition. (d) There was no consistent change in blood pressure variability, as measured in each patient by the standard deviation of the blood pressure values, with SR clonidine. SR clonidine caused a reduction in heart rate that, as that in blood pressure, was well evident throughout the 24 h. It is concluded that SR clonidine is an effective antihypertensive agent and that a single administration of this drug can produce a clear-cut reduction in the blood pressure values of hypertensive patients for a 24-h period.

Digitalis and baroreceptor reflexes in man

Data in animals indicate that large amounts of digitalis potentiate arterial baroreflexes and that this factor may be important for the cardiovascular effects of the drug. To determine if arterial baroreflex potentiation also exists after administration of therapeutic doses of digitalis in man, we studied how stimulation and deactivation of arterial baroreceptors by phenylephrine and nitroglycerin injection affect heart rate and how stimulation and deactivation of carotid baroreceptors by neck suction and pressure affects blood pressure and heart rate. The study was performed in 29 normotensive or hypertensive subjects before and after injection of Lanatoside C (0.8 mg i.v.). Baroreceptor stimulation reduced heart rate and blood pressure, while baroreceptor deactivation increased both of these variables. The bradycardic and hypotensive effect of baroreceptor stimulation increased significantly after digitalis both in normotensive and hypertensive subjects. However, the tachycardic and hypertensive responses to baroreceptor deactivation were not affected by digitalis. Thus, therapeutic doses of digitalis in man enhance baroreceptor reflexes, and both the heart rate and the blood pressure reflex effects are involved. However, the enhancement occurs to a marked degree only with baroreceptor stimulation and is not evident with baroreceptor deactivation.

Blood pressure variability in man: its relation to high blood pressure, age and baroreflex sensitivity

1. Intra-arterial blood pressure and heart rate were recorded for 24 h in ambulant hospitalized patients of variable age who had normal blood pressure or essential hypertension. Mean 24 h values, standard deviations and variation coefficient were obtained as the averages of values separately analysed for 48 consecutive half-hour periods. 2. In older subjects standard deviation and variation coefficient for mean arterial pressure were greater than in younger subjects with similar pressure values, whereas standard deviation and variation coefficient for mean arterial pressure were greater than in younger subjects with similar pressure values, whereas standard deviation aations and variation coefficient were obtained as the averages of values separately analysed for 48 consecurive half-hour periods. 2. In older subjects standard deviation and variation coefficient for mean arterial pressure were greater than in younger subjects with similar pressure values, whereas standard deviation and variation coefficient for heart rate were smaller. 3. In hypertensive subjects standard deviation for mean arterial pressure was greater than in normotensive subjects of similar ages, but this was not the case for variation coefficient, which was slightly smaller in the former than in the latter group. Normotensive and hypertensive subjects showed no difference in standard deviation and variation coefficient for heart rate. 4. In both normotensive and hypertensive subjects standard deviation and even more so variation coefficient were slightly or not related to arterial baroreflex sensitivity as measured by various methods (phenylephrine, neck suction etc.). 5. It is concluded that blood pressure variability increases and heart rate variability decreases with age, but that changes in variability are not so obvious in hypertension. Also, differences in variability among subjects are only marginally explained by differences in baroreflex function.

Effects of prazosin on autonomic control of circulation in essential hypertension

Prazosin, an antihypertensive agent that reduces blood pressure (BP) mainly through a blockade of alpha-adrenergic receptors, may, in theory, affect sympathetic control of circulation to an extent that impairs circulatory hemeostasis. This possibility was studied in subjects with essential hypertension by examining the cardiovascular effects of several stimuli that induce a powerful and diversified activation of the sympathetic noradrenergic activity (dynamic and isometric exercise, cold exposure) and of stimuli that exert a powerful inhibitory influence upon the sympathetic nervous system (carotid baroreceptor reflex). Before and after 15 days of continuous administration of prazosin (2-5 mg), 3 times a day, measurements were made of BP (intraarterial catheter), heart rate, cardiac output (thermodilution), and peripheral resistance. Prazosin reduced mean arterial pressure (10%) and peripheral resistance (9%) at rest, and it did not affect heart rate and cardiac output. Neurally mediated changes in arterial pressure, cardiac output, and peripheral resistance during dynamic or isometric exercise and cold exposure were unaffected by the drug; also unaffected were the cardiovascular responses to increase and decrease in carotid baroreceptor activity obtained by varying carotid transmural pressure through a variable neck pressure chamber device. Thus, the hypotensive and vasodilating effect of prazosin in essential hypertension is not accompanied by an impaired response to neural excitation influences upon the cardiovascular system. Also, the inhibitory influences originating from the carotid baroreflex are unchanged. These data indicate that circulatory homeostasis is largely preserved during administration of prazosin at clinically effective doses.

Plasma catecholamines and blood pressure responses to the carotid baroreceptor reflex in essential hypertension

1. Slight decreases and increases in carotid baroreceptor activity were induced in subjects with essential hypertension by slight alterations in carotid transmural pressure (variable pressure neck-chamber technique) in order to obtain limited increases and reductions in sympathetic adrenergic activity. 2. When sympathetic activity was reflexly increased there was a rise in arterial pressure but no significant increase in plasma catecholamines. Likewise when sympathetic activity was reflexly reduced there was a fall in arterial pressure but no significant reduction in plasma catecholamines. 3. Plasma noradrenaline and adrenaline significantly and markedly increased in the same subjects when sympathetic activity was increased by activation of both arterial and low pressure baroreceptor reflexes with tilting. 4. It is suggested that measurements of catecholamines in systemic plasma may reveal marked degrees of sympathetic activation but may not be a sensitive index of more moderate changes in sympathetic tone.

Clonidine and carotid baroreflex in essential hypertension

Clonidine is believed to reduce blood pressure by a neural action and animal experiments suggest that this consists in potentiation of baroreflexes. In 16 patients with essential hypertension we studied the effects of alterations in carotid sinus baroreceptor activity (neck chamber technique) on arterial blood pressure (catheter measurements) and heart rate, before and after intravenous administration of 150 microgram and 300 microgram of clonidine. The magnitude of the reflex responses was assessed by the slope of the linear regressions relating applied increase and decrease in tissue pressure at the carotid sinus (and therefore applied decrease and increase in carotid sinus transmural pressure) and resulting changes in mean arterial pressure and R-R interval. Clonidine caused a marked reduction in mean arterial pressure (-26 +/- 3 mm Hg) and a slight but significant reduction in heart rate (-5 +/- 1 b/min). There was no evidence for a potentiation of the baroreceptor influence on blood pressure, although a slight potentiation of the baroreceptor influence on heart rate was observed in few instances. We conclude that in man clonidine can exert a pronounced hypotensive effect without potentiating baroreceptor influence on blood pressure. Therefore this mechanism does not play a prominent role in the clinical antihypertensive action of the drug.

Baroreceptor control of atrioventricular conduction in man

Although human baroreflexes are known to exert a powerful physiological control on heart rate, little information exists on the physiological control they exert on the atrioventricular conduction system. In 11 normotensive subjects with normal atrioventricular conduction, we altered baroreceptor activity by injection of pressor and depressor drugs (phenylephrine and trinitroglycerin) and recorded mean arterial pressure (MAP, catheter measurements), R-R interval, and pre-His and post-His intervals (A-H and H-V, His bundle recording). With the subjects in sinus rhythm, increasing MAP by 21+/- 1 mm Hg caused a marked lengthening (250 +/- 28 msec), and decreasing MAP by 17 +/- 2 mm Hg a marked shortening (142 +/- 16 msec) of the R-R interval. There was little change in the A-H interval and no change at all in the H-V interval. However, when the R-R interval was kept constant in these subjects by atrial pacing, a similar increase and decrease in MAP caused, respectively, a marked lengthening (49 +/- 6 msec) and shortening (19 +/- 3 msec) of the A-H interval, although the H-V interval remained unaffected. Thus physiological ranges of baroreceptor activation have a marked influence on the atrioventricular node but apparently not on the ventricular portion of the atrioventricular conduction system. This influence is unmasked when pacing prevents the baroreceptor influence on the sinoatrial node.

Haemodynamic effects of isometric handgrip exercise in patients convalescent from myocardial infarction

1. Cardiac performance in response to 30% maximal isometric handgrip exercise was studied in fourteen patients convalescing uneventfully from a first myocardial infarction. In each patient, heart rate, mean arterial blood pressure and cardiac index were measured, and total peripheral resistance was calculated. The covariance of changes in the mean arterial blood pressure and cardiac index in these patients was matched against tolerance limits calculated from published data for normal subjects. 2. All patients had normal haemodynamic values at rest, and showed the usual rise of heart rate and mean arterial pressure during handgrip exercise. However, in six patients (group 1) the rise in mean arterial pressure was, as in normal subjects, accounted for mainly by a rise in cardiac index, with no consistent change in peripheral resistance. In eight patients (group 2), the mean arterial pressure rose to the same level as in group 1, but with a consistent increase in peripheral resistance and a smaller rise in the cardiac index. 3. It is suggested that in a substantial proportion of patients who are making a seemingly uncomplicated recovery from myocardial infarction, it may be possible to unmask an impairment of left ventricular function by means of isometric handgrip exercise.