Role of the venous system in baroreceptor-mediated reflexes in man

Although baroreceptor stimulation produced by marked alterations in arterial pressure has been shown to produce reflex changes in venous tone in animals, the effects on venous tone in man of altering arterial pressure within the physiologic range have not been clear. In six subjects, venous tone did not change when mean arterial pressure was raised by 25-40 mm Hg, although heart rate fell reflexly by 40%. Venous tone remained constant in 10 subjects when arterial pressure was lowered. This contrasted to the sustained rise in forearm vascular resistance and the persistent tachycardia that occurred. However, 12 subjects continued to respond to these interventions by transient venoconstriction. To eliminate possible emotional influences on venous tone due to the experimental intervention, venous responses were studied before and during general anesthesia in five of these subjects. In contrast to the response before anesthesia, an equivalent fall in arterial pressure during anesthesia no longer evoked a venoconstrictor response. Venous reactivity and the baroreceptor reflex arc remained intact during anesthesia, since venous tone always rose after a deep inspiration, and tachycardia always accompanied the fall in arterial pressure. It is concluded that changes in arterial pressure in the physiologic range in man do not induce measurable reflex alterations in venous tone, and that the increases sometimes seen with decreases in arterial pressure appear to be due to extraneous psychic factors.

Impairment of autonomically mediated heart rate control in patients with cardiac dysfunction

Patients with cardiac disorders have defective parasympathetic control of heart rate. To evaluate the possibility of similar changes in sympathetic control of heart rate, we compared reflex chronotropic responses to 80 degree upright tilt and nitroglycerin-induced hypotension in 31 cardiac patients and 7 normal individuals before and after partial parasympathetic blockade with atropine. Tilting revealed an attenuation of the normal heart rate increase in patients; the magnitude of this defect was greatest in patients with more severe symptoms (class III) and evidence of left ventricular dysfunction (the heart rate increase averaged 25 plus or minus 3 beats/min in normal subjects, 12 plus or minus 2 beats/min in class I-II patients, and 7 plus or minus 1 beats/min in class III patients). Class III symptoms due to mechanical causes (mitral stenosis), however, were not associated with this defect. A marked reduction in heart rate rise with hypotension was seen only in those class III patients without mitral stenosis (0.4 plus or minus 0.1 beats min-minus 1 mm Hg-minus 1 vs. 3.0 plus or minus 0.5 beats min-minus 1 mm Hg-minus 1 in normal subjects). This abnormality also persisted after atropine administration, thus confirming a defect in the sympathetic as well as the parasympathetic component of baroreceptor-mediated reflex heart rate control in patients with cardiac dysfunction. Infusions of isoproterenol produced equivalent rises in heart rate in patients and normal individuals, excluding a reduction in beta-receptor responsiveness as a cause of impaired sympathetic influence. Norepinephrine depletion, however, is a well-recognized concomitant of cardiac failure. It is possible that the reduction in sympathetically mediated heart rate responses results in part from depletion of the sympathetic neurotransmitter.

Hemodynamic abnormalities in response to mild and intense upright exercise following operative correction of an atrial septal defect or tetralogy of Fallot

Patients are often asymptomatic after operative correction of an atrial septal defect (ASD) or tetralogy of Fallot (TF). However, the maximal functional capacity of their hearts relative to that of normal subjects has not been defined. Twelve asymptomatic patients who had ASDs closed and 10 asymptomatic patients who had total correction of tetralogy of Fallot were evaluated by catheterization studies at rest and during mild and intense treadmill exercise. Except for small right ventricular outflow gradients in the tetralogy of Fallot group (2-21 mm Hg), data recorded at rest were normal or nearly so in each subject. However, at a level of upright exercise sufficiently intense to lower pulmonary arterial oxygen saturation to 30%, cardiac output was below that attained by normal subjects (mean, 8.9 ± 0.3 liters/min/m 2 ) in seven of 12 ASD patients (mean, 6.6 ± 0.6 liters/min/m 2 ) and in eight of 10 tetralogy of Fallot patients (mean, 6.1 ± 0.3 liters/min/m 2 ). Right ventricular (RV) outflow gradients, measured during upright exercise in six TF patients, increased in each; RV systolic pressures reached levels of 75-106 mm Hg in four of the six patients studied. No outflow gradients were observed in the ASD group. Thus, patients with operative closure of an ASD and normal hemodynamic findings at rest may have impairment of their cardiac output response to intense upright exercise in the absence of residual shunts, arrhythmias, or pulmonary arterial hypertension. Although the cardiac output response in patients with corrected tetralogy of Fallot is consistently reduced, it is remarkably good considering the complicated nature of their defect and operative repair; however, the RV outflow gradient and RV systolic pressure may increase markedly with exercise. The longterm significance of these findings remains to be determined.

Experimental acute myocardial infarction. Characterization and treatment of the malignant premature ventricular contraction

The majority of deaths from acute myocardial infarction (AMI) occur prior to the arrival of medical aid and appear to be due to ventricular fibrillation (VF). Since the premature ventricular contraction and bradycardia are believed to predispose to VF, it has been suggested that administration of atropine or lidocaine during the prehospital phase of AMI may effectively reduce mortality in AMI. To test the efficacy of these drugs in treating arrhythmias during the acute phase of AMI we studied 72 closed-chest conscious dogs in which AMI was produced by inflating a balloon cuff previously implanted around the left anterior descending coronary artery. Ventricular arrhythmias developed in 52 dogs either during occlusion or within 2 min of release of occlusion. Arrhythmias were treated by (1) atropine, (2) atrial pacing, (3) lidocaine, or (4) atropine plus lidocaine. Of the 18 dogs that developed VF, all had ectopic ventricular beats that followed a preceding beat (R-R PVC interval) by ≤0.43 sec; no dog with a ventricular arrhythmia that exhibited only R-R PVC intervals >0.43 sec developed VF. On this basis arrhythmias were defined as "benign" (R-R PVC >0.43 sec) or "malignant" (R-R PVC ≤0.43 sec). Using this classification, we found that benign arrhythmias were successfully suppressed by atropine (10 of 13 arrhythmias), pacing (nine of 11), and lidocaine (three of five); however, atropine suppressed only two of 18, pacing two of 14, and lidocaine none of six malignant arrhythmias. We conclude that (1) not all ventricular arrhythmias are potentially lethal and (2) while atropine, pacing, and lidocaine successfully suppress most benign arrhythmias, they appear considerably less effective in suppressing those faster arrhythmias that frequently lead to the precipitation of VF.

Effects of chronic heart failure on the capacity of glucagon to enhance contractility and adenyl cyclase activity of human papillary muscles

Glucagon has a positive inotropic effect in normal hearts but is ineffective in animals with chronic cardiac failure. To assess directly the influence of glucagon on the human myocardium, we measured contractility and activation of adenyl cyclase, the enzyme thought to mediate the inotropic action of glucagon, in left ventricular papillary muscles obtained from 12 patients at mitral valve replacement. On the basis of preoperative ventricular end-diastolic pressures and cardiac output (independent of papillary muscle data) patients were classified in three groups: normal, cardiac failure, and condition indeterminate. Concentration-response curves showed that glucagon caused a mean rise of 11% in peak papillary muscle tension and a rise of 12% in peak rate of tension development in the normal patients; myocardial adenyl cyclase activity from each normal patient rose after glucagon (average, 84%). In the papillary muscles of the patients with cardiac failure, glucagon did not augment either tension or adenyl cyclase activity. In contrast, contractility and adenyl cyclase activity increased after norepinephrine in both normal patients and those with cardiac failure. The indeterminate group had two patients whose papillary muscles responded to glucagon and two whose papillary muscles did not respond. Thus, direct study of human papillary muscles shows that chronic cardiac failure is uniformly associated with complete loss of the normal enhancement of contractility and associated activation of adenyl cyclase after glucagon. This perhaps explains the inefficacy of this drug in treating patients with chronic cardiac failure.

Effects of chronic right ventricular volume and pressure loading on left ventricular performance

The effects of chronic right ventricular (RV) distension on left ventricular (LV) function were assessed in dogs 3 weeks after pressure and volume loading of the RV had been produced by the emplacement of a constricting band around the pulmonary artery and the creation of tricuspid insufficiency. This resulted in ascites, RV hypertrophy and dilatation, an increase in RV end-diastolic pressure (EDP), and a reduction of RV and LV norepinephrine concentrations. Peak LV pressure, wall stress, and dp/dt were measured during isovolumic beats at LVEDP's of 1 to 20 mm Hg, and comparisons at matched EDP's were made among the normal, sham-operated, and RV stressed dogs. Under these circumstances, LV function appeared to be depressed appreciably. The effects of RV volume loading on the pressure-volume curve of the LV in the potassium-arrested heart were assessed in a separate group of dogs. The pressure-volume curve of the LV was shifted so that any given LV volume was associated with a higher LV pressure. Thus, when peak pressure and wall stress were related to LV end-diastolic volume rather than to LVEDP, the relationship did not appear to differ from normal; peak dp/dt and peak V CE , however, remained depressed. This depression may represent a decrease in LV contractility consequent to the chronic RV stress or may reflect alterations in LV geometry. Thus, when the RV is subjected to a chronic flow and pressure load, assessment of LV function may be unreliable when indices of contractility are employed that require measurements of LVEDP.

Alterations in the circulatory response to exercise following a meal and their relationship to postprandial angina pectoris

In order to study the mechanisms responsible for the more rapid precipitation of angina in the postprandial state, we evaluated the circulatory response to upright bicycle exercise in 12 patients with angina before and after a meal. Eleven of 12 subjects developed angina sooner after eating (average 1.3 min, P < 0.001). Comparison of circulatory responses revealed that a given amount of postprandial exercise resulted in faster heart rate (12 beats/min, P < 0.001) and greater blood pressure (6 mm Hg, P < 0.05). The product of blood pressure and heart rate (an index of myocardial oxygen demand) at onset of angina during postprandial exercise was the same as corresponding preprandial values. Our results suggest that the accelerated development of angina during exercise after meals is primarily due to a more rapid rise in heart rate and blood pressure, factors tending to augment myocardial oxygen requirements, rather than the result of a deleterious effect of digestion and absorption on myocardial oxygen delivery.

Importance of the design of an exercise protocol in the evaluation of patients with angina pectoris

The severity of angina and the effects of therapeutic interventions in patients with coronary artery disease have been assessed by determining changes in both exercise performance and the triple product (TP) of heart rate, systolic pressure, and ejection time occurring at angina. However, the validity of conclusions based on such changes is uncertain since the effects of different exercise protocols on these variables have not been determined. Twelve patients with angina were studied during upright bicycle exercise; repeated bouts of exercise using a standard protocol of 20-w increments every three minutes produced no consistent changes in TP at angina. When exercise began 20 to 60 w above the work load of the standard protocol that produced angina, exercise capacity was reduced (average 1'40'' vs. 4'40'', P < 0.001), and triple product at angina exceeded control anginal values (average 4,840 vs. 4,150, P < 0.001). In the control studies nitroglycerin (TNG) and carotid sinus nerve stimulation (CSNS) enabled patients to exercise to a higher level, although the triple product at angina was unaltered. However, at the higher work load TNG and CSNS exerted only minimal effects on exercise capacity, indicating that if the work load is excessive, a reduction in myocardial oxygen consumption produced by a therapeutic intervention may be comparatively minor so that a potentially salutary effect would be masked. We conclude that work loads causing angina in less than three minutes cannot reliably be used for studying the effects of therapy. However, if progressive work loads are chosen which cause angina in the control studies in three to six minutes, exercise capacity and triple product at angina provide important information about the efficacy and mechanism of action of a therapeutic intervention.

Clinical and circulatory effects of isosorbide dinitrate. Comparison with nitroglycerin

In order to resolve current controversies on isosorbide dinitrate (ISDN), we employed a particularly sensitive testing protocol to evaluate effects of sublingual ISDN and nitroglycerin on the exercise capacity of patients with angina. Ten minutes after ISDN 21 of 23 patients exercised longer (average 2.7 minutes, P < 0.001) than after placebo. Benefit was evident in only a minority of patients tested one hour and in none tested two hours after either ISDN or nitroglycerin. A given amount of exercise resulted in lower mean blood pressure (average 13 mm Hg, P < 0.001), higher heart rate (average 10 beats/min, P < 0.001), and shorter ejection time (average 0.04 second, P < 0.001) after ISDN. Similar changes were seen after nitroglycerin. The product of blood pressure, heart rate, and ejection time, an index of myocardial O 2 consumption, was unchanged at angina after ISDN or nitroglycerin despite the increased exercise capacity, suggesting that clinical improvement after these drugs may be due to circulatory changes causing decreased myocardial O 2 demand. We conclude that sublingual ISDN closely resembles nitroglycerin in its alteration of circulatory responses to exercise and in the duration of the resultant improvement in exercise capacity.

Comparison of the peak inotropic effects of a catecholamine and a digitalis glycoside in the intact canine heart

The relative peak effects of isoproterenol and ouabain on myocardial contractility and cardiac output were compared by infusing increasing amounts of these two drugs into seven open-chest anesthetized dogs until toxicity developed. Just prior to the development of toxicity isoproterenol increased contractile force an average of 149% and the peak rate of force development (df/dt) an average of 278% of control values, compared to an increase of only 49% and 35%, respectively, with the administration of ouabain. Cardiac output and stroke volume were also significantly greater with the catecholamine than the glycoside. The combination of isoproterenol and ouabain produced essentially the same contractile force and stroke volume achieved by isoproterenol alone. Suppression of ouabain-induced arrhythmias by ventricular pacing allowed additional glycoside to be infused until ventricular fibrillation terminated the study. With pacing and ouabain, contractile force increased 131% above control, a level similar to that achieved by isoproterenol; peak df/dt increased to 200% above control, a value significantly lower than that obtained with isoproterenol. However, stroke volume decreased despite a substantial increase in left ventricular end-diastolic pressure. It is concluded that maximal doses of isoproterenol produce significantly greater increases in myocardial contractility and cardiac output compared to ouabain, even when the toxicity produced by the latter is suppressed by electrical stimulation.

Blood fibrinolytic activity in man. Diurnal variation and the response to varying intensities of exercise

This investigation was undertaken in normal subjects to define the relationship between the intensity of exercise and magnitude of fibrinolytic response and to examine the effect of diurnal variations on the exercise response. Fibrinolytic activity was measured on fibrin plates and expressed as mm 2 . Diurnal variations occurred with lowest activity at 8:00 AM (mean, 66 mm 2 ), and peak activity between 5:00 and 8:00 PM (mean, 266 mm 2 , P <0.001). Five minutes of maximal treadmill exercise caused a marked increase in mean activity from 90 to 658 mm 2 ( P <0.001). Five minutes of 70% maximal exercise produced no significant increase, but 30 minutes increased activity to 626 mm 2 ( P <0.005). In contrast, 30 minutes of 40% maximal exercise produced a small elevation from 80 to 173 mm 2 ( P <0.005). Maximal and 40% maximal exercise evoked greater responses at 4:00 PM than 8:00 AM . Exercise produces increases in fibrinolytic activity which are related to the relative intensity of exercise, its duration, and the time of day it is performed. Short bursts of intense exercise cause marked increases, but more prolonged bouts of moderate exercise are required to produce similar increases. The increases with prolonged mild exercise are small and comparable to those observed during resting diurnal variations.

The role of skin and muscle resistance vessels in reflexes mediated by the baroreceptor system

The role of skin and muscle vascular beds in baroreceptor-mediated alterations of peripheral vascular resistance was evaluated in six normal subjects in whom the skin circulation in one forearm was temporarily suppressed by epinephrine iontophoresis. Baroreceptor activity was enhanced by application of negative pressure to the neck (neck suction) and inhibited by application of lower body negative pressure. Forearm blood flow was measured simultaneously in both arms with strain gauge plethysmographs. Since blood flow in the treated arm consisted entirely of muscle flow, skin flow was calculated from the difference between total forearm flow in the intact arm and muscle flow in the treated arm. Vascular resistances were calculated as the ratio of mean arterial pressure to the blood flow of each vascular bed. During neck suction, mean arterial pressure decreased from an average of 89 to 75 mm of Hg (P < 0.005), heart rate decreased from an average of 60 to 55 beats/min (P < 0.005), and total skin and muscle flows remained essentially unchanged. Cutaneous vascular resistance decreased from an average of 75 to 49 mm of Hg/ml per 100 g per min (P < 0.05), muscle vascular resistance from 68 to 51 (P < 0.005), and total forearm vascular resistance from 36 to 24 (P < 0.025). During lower body negative pressure, heart rate increased from an average of 59 to 69 beats/min (P < 0.005), mean arterial pressure did not change significantly, and significant decreases occurred in forearm blood flow from 5.4 to 2.7 ml/100 g per min, in skin blood flow from 3.1 to 1.4, and in muscle blood flow from 2.3 to 1.3. Cutaneous vascular resistance increased from an average of 47 to 110 mm of Hg/ml per 100 g per min (P < 0.05), muscle vascular resistance from 43 to 72 (P < 0.005), and total forearm vascular resistance from 20 to 38 (P < 0.001). These results demonstrate that both the skin and muscle resistance vessels participate in reflex changes initiated by alterations in baroreceptor activity.

Circulatory effects of electrical stimulation of the carotid sinus nerves in man

The effects of carotid sinus nerve (CSN) stimulation were studied at rest and during a mild level of supine bicycle exercise in seven patients in whom CSN stimulators had been implanted for the treatment of angina pectoris. At rest, CSN stimulation produced a fall in mean arterial pressure (MAP) averaging 23% and an 8% decrease in cardiac output (CO). Total peripheral resistance (TPR) fell by 14% and forearm vascular resistance (FVR) by 16%. During exercise, MAP fell 16%, but no significant change occurred in CO. Thus, the fall in MAP could be attributed to a reflexly induced decrease in peripheral vascular resistance. Only small decreases occurred in heart rate. No changes in venous tone, central venous pressure, or the maximum transverse end-diastolic diameter of the heart were produced by stimulation either at rest or during exercise. Thus, at rest, CSN stimulation reduces MAP by reflexly decreasing both vascular resistance and CO; during exercise, the diminution in CO no longer occurs. Finally, the venous system does not appear to participate in reflexes activated by CSN stimulation.

Hemodynamic effects of diuresis at rest and during intense upright exercise in patients with impaired cardiac function

Although diuretic therapy appears to improve the exercise capacity of patients with moderately impaired cardiac function, the hemodynamic basis for this improvement is not clear. It is also unknown to what extent the moderate diuresis that often occurs during the first few days of hospitalization contributes to the normal or nearly normal hemodynamic measurements obtained in certain patients with cardiac impairment who are thought clinically to have signs and symptoms of pulmonary congestion. Accordingly, the circulatory response to moderate diuresis resulting in a loss of weight averaging 3.4 kg was investigated in 15 patients with heart disease. At rest in the supine position mean pulmonary arterial wedge pressure fell after diuresis from an average of 24 to 13 mm Hg. Reductions also occurred in mean pulmonary arterial pressure (42 to 26 mm Hg), mean right atrial pressure (9 to 4 mm Hg), and right ventricular end-diastolic pressure (11 to 6 mm Hg). Cardiac output decreased by an average of 20%, mean systemic arterial pressure by 12%, right ventricular stroke work by 44%, and left ventricular stroke work by 25%. Diuresis also caused similar reductions in these values in the sitting position at rest and during mild and intense levels of treadmill exercise. Despite the reductions in cardiac output, all but one of the patients studied achieved substantial clinical improvement from the diuresis. Such improvement probably resulted from the fact that the beneficial effects of lower pulmonary vascular pressures outweighed the deleterious effect of a reduction in cardiac output. Thus, moderate changes in body weight brought about by either fluid retention or fluid loss may result in substantial alterations in circulatory dynamics. These changes, if unrecognized, can lead to considerable confusion when attempts are made to correlate the hemodynamic findings with the degree of cardiac decompensation as judged clinically.

Role of the capacitance and resistance vessels in vasovagal syncope

Withdrawal of sympathetic tone to the veins resulting in peripheral pooling of blood has been suggested as an important factor contributing to the decrease in cardiac output and hence arterial pressure that occurs during vasovagal syncope. However, no measurements of venous tone during syncope have been reported. In the course of other studies on the circulatory effects of negative pressure below the iliac crests, and 80° head-up tilt, vasovagal reactions occurred in 10 subjects. Heart rate, central venous pressure, arterial pressure, forearm blood flow, forearm vascular resistance, and forearm or hand venous tone were measured. The typical vasovagal reaction could be divided into two phases. A gradual fall in arterial pressure signified the onset of phase I, during which forearm vascular resistance did not change significantly. The duration of phase I was highly variable. The onset of phase II was denoted by an abrupt fall in arterial pressure and heart rate and a decrease of 62% in forearm vascular resistance, from 36 to 14 mm Hg/ml/100 g/min. However, venoconstriction rather than venodilatation occurred in the forearm or hand veins. Since central venous pressure did not change prior to or during the onset of the reaction, it is unlikely that venodilatation occurred in other vascular beds. It is concluded that two of the major mechanisms responsible for the hypotension of vasovagal syncope initiated by orthostasis or lower body negative pressure are bradycardia and dilatation of the resistance vessels. In contrast, it appears that the venous bed, by constricting, tends to maintain filling pressure and thereby cardiac output, and thus works in an opposite direction.

Characterization of the circulatory response to maximal upright exercise in normal subjects and patients with heart disease

The circulatory responses to mild and maximal upright exercise were studied in six normal subjects and 21 patients with various types of cardiac disease. It was found that the usual hemodynamic indices employed to evaluate cardiac performance during exertion were unreliable in separating patients from normal persons. In contrast, the cardiac index achieved at a pulmonary arterial (PA) O 2 saturation of 30% was found to be highly reliable and sensitive for distinguishing the normal from the abnormal response to exercise. At a level of exercise that lowers PA O 2 saturation to 30%, all normal subjects achieved a cardiac index greater than 7.0 L/minm 2 while no patient exceeded 4.8 L/min/m. 2 In addition, the maximal capacity to extract O 2 was greater in patients than in normal subjects; thus, during maximal exercise it was not unusual for patients to achieve PA O 2 saturations of 15% or less, while the lowest value achieved in normal subjects was 23%.

Control of heart rate by the autonomic nervous system. Studies in man on the interrelation between baroreceptor mechanisms and exercise

The control of heart rate by the autonomic nervous system was investigated in conscious human subjects by observing the effects of β-adrenergic blockade with propranolol, of parasympathetic blockade with atropine, and of combined sympathetic and parasympathetic blockade. The increase in heart rate with mild exercise in supine men was mediated predominantly by a decrease in parasympathetic activity; at higher levels of work, however, sympathetic stimulation also contributed to cardiac acceleration. When the response to 80° head-up tilt was compared with the response to exercise in the same subject supine, it appeared that the attainment of an equivalent heart rate was associated with a significantly greater degree of sympathetic activity during tilting than during exercise. Although heart rate was always higher at any given pressure during exercise than it had been at rest, the changes in heart rate that followed alterations in arterial pressure were found to be of similar magnitudes at rest and during exercise; it was therefore concluded that the sensitivity of the baroreceptor system was not altered during exercise. Investigation of the efferent pathways concerned in mediating the baroreceptor-induced changes in heart rate suggested that the relative roles of the sympathetic and parasympathetic systems were nearly equal in the resting state. During exercise, on the other hand, changes in sympathetic activity appeared to be the predominant mechanism by which speeding and slowing of the heart was achieved. It thus appears that baroreceptor-induced alterations in heart rate may be mediated by increased or decreased activity of either efferent system; the ultimate balance, however, is critically dependent on the preexisting level of background autonomic activity.