The relation of heart rate to cardiovascular dynamics. Pacing by atrial electrodes

Heat-related changes in the velocity and kinetic energy of flowing blood influence the human heart's output during hyperthermia

Passive whole-body hyperthermia increases limb blood flow and cardiac output (Q ̇ $\dot Q$ ), but the interplay between peripheral and central thermo-haemodynamic mechanisms remains unclear. Here we tested the hypothesis that local hyperthermia-induced alterations in peripheral blood flow and blood kinetic energy modulate flow to the heart andQ ̇ $\dot Q$ . Body temperatures, regional (leg, arm, head) and systemic haemodynamics, and left ventricular (LV) volumes and functions were assessed in eight healthy males during: (1) 3 h control (normothermic condition); (2) 3 h of single-leg heating; (3) 3 h of two-leg heating; and (4) 2.5 h of whole-body heating. Leg, forearm, and extracranial blood flow increased in close association with local rises in temperature while brain perfusion remained unchanged. Increases in blood velocity with small to no changes in the conduit artery diameter underpinned the augmented limb and extracranial perfusion. In all heating conditions,Q ̇ $\dot Q$ increased in association with proportional elevations in systemic vascular conductance, related to enhanced blood flow, blood velocity, vascular conductance and kinetic energy in the limbs and head (all R2 ≥ 0.803; P < 0.001), but not in the brain. LV systolic (end-systolic elastance and twist) and diastolic functional profiles (untwisting rate), pulmonary ventilation and systemic aerobic metabolism were only altered in whole-body heating. These findings substantiate the idea that local hyperthermia-induced selective alterations in peripheral blood flow modulate the magnitude of flow to the heart andQ ̇ $\dot Q$ through changes in blood velocity and kinetic energy. Localised heat-activated events in the peripheral circulation therefore affect the human heart's output. KEY POINTS: Local and whole-body hyperthermia increases limb and systemic perfusion, but the underlying peripheral and central heat-sensitive mechanisms are not fully established. Here we investigated the regional (leg, arm and head) and systemic haemodynamics (cardiac output:Q ̇ $\dot Q$ ) during passive single-leg, two-leg and whole-body hyperthermia to determine the contribution of peripheral and central thermosensitive factors in the control of human circulation. Single-leg, two-leg, and whole-body hyperthermia induced graded increases in leg blood flow andQ ̇ $\dot Q$ . Brain blood flow, however, remained unchanged in all conditions. Ventilation, extracranial blood flow and cardiac systolic and diastolic functions only increased during whole-body hyperthermia. The augmentedQ ̇ $\dot Q$ with hyperthermia was tightly related to increased limb and head blood velocity, flow and kinetic energy. The findings indicate that local thermosensitive mechanisms modulate regional blood velocity, flow and kinetic energy, thereby controlling the magnitude of flow to the heart and thus the coupling of peripheral and central circulation during hyperthermia.

The heart, a secondary organ in the control of blood circulation

Circulation of the blood is a fundamental physiological function traditionally ascribed to the pressure-generating function of the heart. However, over the past century the 'cardiocentric' view has been challenged by August Krogh, Ernst Starling, Arthur Guyton and others, based on haemodynamic data obtained from isolated heart preparations and organ perfusion. Their research brought forth experimental evidence and phenomenological observations supporting the concept that cardiac output occurs primarily in response to the metabolic demands of the tissues. The basic tenets of Guyton's venous return model are presented and juxtaposed with their critiques. Developmental biology of the cardiovascular system shows that the blood circulates before the heart has achieved functional integrity and that its movement is intricately connected with the metabolic demands of the tissues. Long discovered, but as yet overlooked, negative interstitial pressure may play a role in assisting the flow returning to the heart. Based on these phenomena, an alternative circulation model has been proposed in which the heart functions like a hydraulic ram and maintains a dynamic equilibrium between the arterial (centrifugal) and venous (centripetal) forces which define the blood's circular movement. In this focused review we introduce some of the salient arguments in support of the proposed circulation model. Finally, we present evidence that exercising muscle blood flow is subject to local metabolic control which upholds optimal perfusion in the face of a substantive rise in muscle vascular conductance, thus lending further support to the permissive role of the heart in the overall control of blood circulation.

Physiological Function during Exercise and Environmental Stress in Humans-An Integrative View of Body Systems and Homeostasis

Claude Bernard’s milieu intérieur (internal environment) and the associated concept of homeostasis are fundamental to the understanding of the physiological responses to exercise and environmental stress. Maintenance of cellular homeostasis is thought to happen during exercise through the precise matching of cellular energetic demand and supply, and the production and clearance of metabolic by-products. The mind-boggling number of molecular and cellular pathways and the host of tissues and organ systems involved in the processes sustaining locomotion, however, necessitate an integrative examination of the body’s physiological systems. This integrative approach can be used to identify whether function and cellular homeostasis are maintained or compromised during exercise. In this review, we discuss the responses of the human brain, the lungs, the heart, and the skeletal muscles to the varying physiological demands of exercise and environmental stress. Multiple alterations in physiological function and differential homeostatic adjustments occur when people undertake strenuous exercise with and without thermal stress. These adjustments can include: hyperthermia; hyperventilation; cardiovascular strain with restrictions in brain, muscle, skin and visceral organs blood flow; greater reliance on muscle glycogen and cellular metabolism; alterations in neural activity; and, in some conditions, compromised muscle metabolism and aerobic capacity. Oxygen supply to the human brain is also blunted during intense exercise, but global cerebral metabolism and central neural drive are preserved or enhanced. In contrast to the strain seen during severe exercise and environmental stress, a steady state is maintained when humans exercise at intensities and in environmental conditions that require a small fraction of the functional capacity. The impact of exercise and environmental stress upon whole-body functions and homeostasis therefore depends on the functional needs and differs across organ systems.

Dehydration reduces stroke volume and cardiac output during exercise because of impaired cardiac filling and venous return, not left ventricular function

Dehydration accrued during intense prolonged whole-body exercise in the heat compromises peripheral blood flow and cardiac output ( Q ˙ ). A markedly reduced stroke volume (SV) is a key feature of the dehydration-induced cardiovascular strain, but whether the lower output of the heart is mediated by peripheral or cardiac factors remains unknown. Therefore, we repeatedly quantified left ventricular (LV) volumes, LV mechanics (LV twist, a marker of systolic muscle function, and LV untwisting rate, an independent marker of LV muscle relaxation), left intra-ventricular pressure gradients, blood volume and peripheral blood flow during 2 hr of cycling in the heat with and without dehydration (DEH: 4.0 ± 0.2% body mass loss and EUH: euhydration control, respectively) in eight participants (three females and five males). While brachial and carotid blood flow, blood volume, SV, LV end-diastolic volume (LVEDV), cardiac filling time, systemic vascular conductance and Q ˙ were reduced in DEH compared to EUH after 2 hr, LV twist and untwisting rate tended to be higher (p = .09 and .06, respectively) and intra-ventricular pressure gradients were not different between the two conditions (p = .22). Furthermore, LVEDV in DEH correlated strongly with blood volume (r = .995, p < .01), head and forearms beat volume (r = .98, p < .05), and diastolic LV filling time (r = .98, p < .05). These findings suggest that the decline in SV underpinning the blunted Q ˙ with exercise-induced dehydration is caused by compromised LV filling and venous return, but not intrinsic systolic or diastolic LV function.

Noncontact measurement of emotional and physiological changes in heart rate from a webcam

Heart rate, measured in beats per minute, can be used as an index of an individual's physiological state. Each time the heart beats, blood is expelled and travels through the body. This blood flow can be detected in the face using a standard webcam that is able to pick up subtle changes in color that cannot be seen by the naked eye. Due to the light absorption spectrum of blood, we are able to detect differences in the amount of light absorbed by the blood traveling just below the skin (i.e., photoplethysmography). By modulating emotional and physiological stress-that is, viewing arousing images and sitting versus standing, respectively-to elicit changes in heart rate, we explored the feasibility of using a webcam as a psychophysiological measurement of autonomic activity. We found a high level of agreement between established physiological measures, electrocardiogram, and blood pulse oximetry, and heart rate estimates obtained from the webcam. We thus suggest webcams can be used as a noninvasive and readily available method for measuring psychophysiological changes, easily integrated into existing stimulus presentation software and hardware setups.

The impact of cardiac resynchronization therapy on obstructive sleep apnea in heart failure patients: a pilot study

BACKGROUND

Cardiac resynchronization therapy (CRT) has been shown to improve cardiac function and reduce Cheyne-Stokes respiration but has not been evaluated in patients with obstructive sleep apnea (OSA). In this pilot study, we investigated the impact of both CRT and CRT plus increased rate pacing in heart failure (ie, congestive heart failure [CHF]) patients with OSA. We hypothesized that through increased cardiac output CRT/pacing would reduce obstructive events and daytime symptoms of sleepiness.

METHODS

Full polysomnograms were performed on CHF patients who were scheduled for CRT, and those patients with an apnea-hypopnea index (AHI) of > 5 events per hour were approached about study enrollment. Patients had a pre-CRT implant baseline echocardiogram and an echocardiogram a mean (+/- SEM) duration of 6.6 +/- 1.4 months post-CRT implant; polysomnography; and responded to the Minnesota Living with Heart Failure questionnaire, the Epworth sleepiness scale, and the Functional Outcomes of Sleep Questionnaire. An additional third polysomnography was performed combining CRT with a pacing rate of 15 beats/min above the baseline sleeping heart rate within 1 week of the second polysomnography. Assessments for the change in cardiac output during the polysomnography were performed using circulation time to pulse oximeter as a surrogate.

RESULTS

Twenty-four patients were screened, and 13 patients (mean age, 68.6 years; body mass index, 28.7 kg/m(2)) had evidence of OSA. The mean AHI decreased from 40.9 +/- 6.4 to 29.5 +/- 5.9 events per hour with CRT (p = 0.04). The mean baseline ejection fraction was 22 +/- 1.7% and increased post-CRT to 33.6 +/- 2.0% (p < 0.05). The reduction in AHI with CRT closely correlated with a decrease in circulation time (r = 0.89; p < 0.001) with CRT. Increased rate pacing made no additional impact on the AHI or circulation time. CRT had a limited impact on sleep architecture or daytime symptom scores.

CONCLUSIONS

CRT improved cardiac function and reduced the AHI. Reduced circulatory delay likely stabilized ventilatory control systems and may represent a new therapeutic target in OSA.

Atrial overdrive pacing compared to CPAP in patients with obstructive sleep apnoea syndrome

AIMS

Obstructive sleep apnoea (OSA) is associated with oxygen desaturation, blood pressure increase, and neurohumoral activation, resulting in possible detrimental effects on the cardiovascular system. Continuous positive airway pressure (CPAP) is the therapy of choice for OSA. In a recent study, nocturnal atrial overdrive pacing (pacing) reduced the severity of sleep apnoea in pacemaker patients. We compared the effects of CPAP with those of pacing in patients with OSA but without pacemaker indication or clinical signs of heart failure.

METHODS AND RESULTS

Ten patients with OSA on CPAP therapy were studied for three nights by polysomnography. During the nights that followed a night without any treatment (baseline), the patients were treated with CPAP or pacing in a random order. Pacing was performed with a temporary pacing lead. The pacing frequency was 15 b.p.m. higher than the baseline heart rate. The apnoea-hypopnoea index was 41.0 h(-1) (12.0-66.6) at baseline and was significantly lower during CPAP [2.2 h(-1) (0.3-12.4)] compared with pacing [39.1 h(-1) (8.2-78.5)]. Furthermore, duration and quality of sleep were significantly improved during CPAP when compared with pacing.

CONCLUSION

Nocturnal atrial overdrive pacing is no alternative therapeutic strategy to CPAP for the treatment of OSA in patients without clinical signs of heart failure and without conventional indication for anti-bradycardia pacing.

Atrial Overdrive Pacing in Patients with Sleep Apnea with Implanted Pacemaker

RATIONALE

Atrial overdrive pacing markedly improved sleep-disordered breathing in a recent study.

OBJECTIVES

Using a single-blind, randomized, crossover design, we aimed to reproduce these findings and investigate the possible underlying mechanisms.

METHODS

Twenty ambulatory patients with an implanted pacemaker or cardioverter defibrillator were studied by polysomnography on 3 consecutive nights in a randomized, single-blind, crossover study in which devices were programmed for nonpacing or for overdrive pacing at 7 or 15 beats/minute faster than the mean nocturnal heart rate. Ventilation and biomarkers (urinary norepinephrine excretion, amino-terminal portion of the precursor of brain natriuretic peptide, or NT-proBNP, were also evaluated.

MEASUREMENTS AND MAIN RESULTS

Neither the primary endpoint apnea-hypopnea index, nor the apnea index, oxygen desaturation, ventilation, or biomarkers were affected by the nocturnal atrial overdrive pacing. A small, clinically insignificant, rate-dependent reduction in the hypopnea index was evoked by pacing (nonpacing, 13.4 +/- 1.4; pacing 7, 12.9 +/- 1.4; pacing 15, 10.9 +/- 1.0; p < 0.01, analysis of variance).

CONCLUSIONS

The lack of effect on the apnea-hypopnea index means that atrial overdrive pacing is inappropriate for treating sleep-disordered breathing.

Rate-dependent hemodynamic responses during incremental atrial pacing in chronic constrictive pericarditis before and after surgery

Chronic constrictive pericarditis is a frequent cause of diastolic dysfunction, and results in impaired ventricular filling. Unlike in normal subjects, ventricular filling in constrictive pericarditis occurs almost entirely in the initial one third of diastole, and cardiac output is dependent predominantly on heart rate. Tachycardia impairs ventricular filling in normal subjects, but its effects in patients with constrictive pericarditis have not been studied. The effect of increasing heart rate alone with atrial pacing on the central and peripheral hemodynamics of patients with untreated chronic constrictive pericarditis before and after pericardiectomy was evaluated. Increased heart rate with atrial pacing increased cardiac output, whereas stroke volume remained unchanged up to heart rates of 140 beats/min. Further increases in heart rate resulted in reductions of cardiac output and stroke volume. There were no significant changes in ventricular filling pressures. Infusion of 300 ml of saline solution at peak pacing rates did not improve cardiac output. After successful surgical pericardiectomy, the hemodynamic effects of atrial pacing returned to normal. It is concluded that moderate tachycardia improves the hemodynamic profile of patients with constrictive pericarditis.

Quantitation of chronotropic response: comparison of methods for rate-modulating permanent pacemakers

OBJECTIVES

The aim of this study was to develop a technique for quantitating chronotropic response.

BACKGROUND

Although the importance of chronotropic response for optimizing cardiac output during exercise is widely recognized, methods for quantitating the rate-modulating behavior of permanent pacemakers have not been developed. For a method of quantitating chronotropic response to be clinically useful, the rate-modulating characteristics of a pacing system should be defined at the onset of exertion, over a variety of exercise work loads and during recovery.

METHODS

Three methods for quantitation of rate modulation were assessed in 10 patients during treadmill exercise testing using the chronotropic assessment exercise protocol with expired gas exchange analysis. To compare the observed chronotropic response with a standard, the "expected" heart rate throughout exercise was calculated by using the concept of heart rate reserve as described by Wilkoff. The pacing rate observed during exercise was analyzed with 1) standard linear regression analysis, 2) comparison of observed and expected pacing rates at the midpoint and end of each quartile of exercise, and 3) integration of the area under the rate-response curve with comparison with the area under the expected curve.

RESULTS

With use of a normalized scale relating change in heart rate to change in metabolic work load, with values of heart rate and metabolic work load at rest set to 0 and those at maximal exertion set to a value of 1, the mean y intercept for the study group was 0.10 +/- 0.20 (range -0.14 to +0.45), with a mean slope of 0.81 +/- 0.25 (range 0.31 to 1.19). The correlation coefficient relating change in heart rate to change in exercise work load was a mean of 0.90 +/- 0.09 (range 0.63 to 0.98). Integration of the area under the rate-response curve observed during exercise yielded a mean area that was 101 +/- 36% of that expected. When the range of exercise work loads was divided into quartiles, the area under the observed rate-response curve was 151 +/- 114% of that expected during the first quartile of exercise, 113 +/- 70% during the second, 96 +/- 38% during the third and 92 +/- 20% during the fourth. The mean area under the curve during recovery was 93 +/- 29% of that expected. Although calculation of the observed heart rate as a percent of that expected at the midpoint and end of each quartile of exercise used fewer observations, it provided similar results.

CONCLUSIONS

Quantitation of the rate-response curve with comparison with the expected heart rate curve provides accurate methods for quantitation of chronotropic response. Adoption of this method would facilitate comparisons of artificial sensors and provide a framework to address issues of optimal rate modulation.

Effect of increase in heart rate on interatrial shunt in atrial septal defect

The effect of increases in heart rate by atrial pacing was investigated in 20 children [mean 9 +/- 4 (SD) years] with atrial septal defect. Systemic blood flow increased by 13 +/- 10 and 27 +/- 22% with a 25 and 50% increase in heart rate, respectively. Pulmonary blood flow, however, remained unaltered. Thus, rapid pacing decreased the pulmonary to systemic blood flow ratio significantly (-15 +/- 10 and -22 +/- 13%, respectively). This hemodynamic alteration was attributed to the difference in diastolic distensibility (compliance) between the left and right ventricles, and the ratio of right ventricular to left ventricular compliance (defined by dV/dP) near the end-diastolic pressure was estimated to be 6.5 +/- 4.2. It is suspected that the increase in heart rate may contribute to the lowering of pulmonary to systemic flow ratio during exercise in children with atrial septal defect.

Effect of cholinergic blockade on heart rate, blood pressure and plasma catecholamine responses to mental stress in normal subjects

The effects of cholinergic blockade on haemodynamic reactivity to standardized mental stress has been studied in nine normotensive males during infusion of atropine (bolus dose 10 micrograms x kg-1 followed by a constant-rate infusion of 0.02 microgram x kg-1 x min-1) or placebo given in a randomized order on two different days. Partial cholinergic blockade increased resting heart rate by 25-30 beats per minute. The magnitude of the heart rate response to stress (reactivity) however was unaffected by the atropine infusion. Also, in four subjects who received a higher dose of atropine (approximately 1.8-1.9 mg), heart rate responses to stress were the same as during placebo infusion. Cholinergic blockade was associated with a small but prolonged increase in diastolic blood pressure. These findings suggest that parasympathetic withdrawal does not contribute to the tachycardia caused by mental arithmetic, and that the pattern of neurogenic activation may differ from that elicited during a classic defence-alarm reaction and by somatomotor activation.

Stroke volume and the three phase cardiac output rate relationship with ventricular pacing

Knowledge of how stroke volume (SV), and hence cardiac output (CO), changes with ventricular pacing rate (R) constitutes a key aspect of sensor driven, variable rate pacemakers. It has been established that the relationship between CO and pacing rate exhibits three phases for rest and constant exercise. At low rates (phase 1), CO increases with increasing R; with additional rate increase (phase 2), CO either remains constant or increases slightly; and above some critical rate, CO decreases (phase 3). However, the nature of the relationship between SV and pacing rate has not been as clearly described. Therefore, the objectives of this study were (1) to describe and document the relationship between SV and R, and (2) to demonstrate the consequence of this relationship in terms of the three phase CO versus R relationship. In six anesthetized dogs, right ventricular SV was determined from pulmonary artery blood flow measured using an electromagnetic flow meter, and the right ventricle was paced over a range of rates. In general, SV decreased with increasing R, although the exact nature of the relationship varied from animal to animal. The results demonstrate that it is the manner in which SV decreases with increasing R that determines the three phase relationship between CO and R. The relationships described in this study have important implications for choosing pacing rates for patients receiving sensor driven, variable rate pacemakers.

Left ventricular inotropic effect of atrial pacing after coronary artery bypass grafting

The effect of atrial pacing on left ventricular (LV) performance was studied in 19 patients, 24 hours after coronary artery bypass grafting (CABG). LV volumes were calculated from simultaneous radionuclide-thermodilution measurements at rest (heart rate 82 +/- 12 beats/min), 10 minutes after the start of atrial pacing (100 beats/min), and with atrial pacing plus volume loading to return preload toward baseline. Atrial pacing reduced preload as reflected by LV end-diastolic volume index (69 +/- 14 vs 60 +/- 14 ml/m2, mean +/- standard deviation) (p less than 0.0001), but returned to baseline with volume loading. Afterload, as reflected by arterial end-systolic pressure, did not change with atrial pacing (63 +/- 9 at baseline vs 64 +/- 8 mm Hg with pacing, difference not significant). Afterload increased with volume loading (68 +/- 10 mm Hg, p less than 0.025 vs baseline and pacing). LV stroke volume decreased with atrial pacing due to reduced preload, but returned to baseline with volume loading. Cardiac index increased with atrial pacing and increased further with volume loading. Compared with baseline, LV end-systolic volume index was reduced during atrial pacing both before and after volume loading, despite unchanged or augmented afterload. The combination of atrial pacing and volume loading resulted in augmentation of LV stroke work, despite no increase in preload compared with baseline. Thus, after CABG, increased (paced) heart rate augments inotropic state, as indicated by reduced LV end-systolic volume under conditions of unchanged or increased afterload, and elevated LV stroke work without an increase in preload or a decrease in afterload.

Cardiac output versus pacing rate at rest and with exercise in dogs with AV block

To achieve maximum benefit from exercise (rate)-responsive pacing in subjects with sinus node dysfunction and AV block, it is necessary to determine the pacing rate (HR) which produces maximum cardiac output (CO) under specified exercise conditions. However, the CO-HR relationship for exercise has not been systematically investigated. To permit determination of the optimum HR, CO was measured at rest and with exercise for different pacing rates. Seven dogs with complete AV block and permanently implanted ventricular pacemakers were exercised on a treadmill for 5 min at each of four pacing rates (55, 76, 101, 116/min) and at two constant exercise levels (225 and 560 kg.m/min). CO was determined by impedance cardiography during the resting state preceding exercise and during a brief period (10-20 s) immediately after exercise, and was expressed as a percent of the CO determined at rest with HR = 55/min. A three-phase pattern of CO versus HR appears to exist for exercise as for rest. For exercise, starting from a low HR, CO increases markedly; a "plateau" is reached during which moderate increase in CO is achieved by increasing HR. At very rapid pacing rates, CO may actually decrease with further increase in HR. The results of this study suggest that a subject-specific optimum HR exists for each constant exercise level. Moreover, the methodology employed in the study is applicable to the identification of optimum HR for any exercise (rate)-responsive pacemaker.

Radionuclide left ventricular function curve during atrial pacing in normal subjects and in patients with coronary artery disease

We used radionuclide angiography during right atrial pacing to assess left ventricular function in 7 normal subjects and 20 patients with coronary artery disease. A left ventricular function curve relating stroke volume to end-diastolic volume was plotted for each patient. The normal pacing ventricular function curve was a straight line passing through the origin of axes. The pacing ventricular function curve was abnormal in 18 of the 20 patients with coronary artery disease, and three different shaped curves were obtained, reflecting decreased contractile force for the same end-diastolic volume during ischemia. Cardiac output and blood pressure do not change during atrial pacing, thus the Frank-Starling relationship is evaluated by this method during almost experimentally controlled conditions. Relating stroke volume to end-diastolic volume, and not end-diastolic pressure, distinguishes between overall left ventricular systolic function and left ventricular compliance.

Heart rate influence on the systolic gradient across the stenotic aortic valve: theoretical evaluation and implications

Peak systolic gradient across the aortic value, measured invasively or noninvasively is used in many laboratories for estimation of the severity of the stenosis (= actual valve area). This study shows that when cardiac output is normal the gradient across the valve is influenced by heart rate (HR) apart from valve area. A rate-corrected peak systolic gradient (PSGc) is defined by the formula (formula; see text) and its importance for clinical decision making is demonstrated. The significance of bradycardia in the pathogenesis of syncope in aortic stenosis is discussed.

Relation between QT interval and heart rate. applications and limitations of Bazett's formula

We studied the relationship between QT interval and RR interval when the heart rate was changed by atrial pacing, atropine, isoproterenol, and exercise in nine healthy adult males. The following results were obtained: (1) QT shortening with an increase in heart rate was minimal in atrial pacing and an equation QT = k4 square root RR was obtained from the QT-RR relationship during atrial pacing (r = 0.64, p less than 0.001); (2) shortening of QT with shortening of RR was least in atrial pacing; (3) during recovery from exercise it was largest and almost coincident with Bazett's formula; (4) Bazett's formula, therefore, is clearly applicable in the evaluation of ECGs recorded at rest or after exercise; (5) Bazett's formula can not be applied in the evaluation of an effect of some drug or maneuver on the QT when such a drug or maneuver can affect both the QT and the RR directly. The RR-dependent QT change can be estimated from the equation QT = k4 square root RR and the difference between a measured QT and the estimated RR-corrected QT would be a genuine effect of the drug or maneuver on the QT at that heart rate. We conclude that atropine, isoproterenol and exercise actually shorten the QT interval.

Left ventricular function during atrial pacing: a radionuclide angiographic study

Multigated radionuclide angiography was performed at rest and during atrial pacing in 10 normal subjects. Left ventricular volumes in diastole and systole were measured by total counts in the region of interest of the left ventricle. Stroke volume, cardiac output, ejection fraction, and rates of ejection for each heart rate were calculated. As heart rate increased, there was a gradual decrease in end-diastolic, end-systolic, and stroke volumes without a significant change in cardiac output and ejection fraction. Ejection fraction rate increased and the uncorrected ejection rate decreased with the increase in heart rate. The normal response to tachycardia induced by atrial pacing was a decrease in ventricular volumes without a change in ejection fraction or cardiac output.

Pressor and depressor responses after cholinergic blockade in humans

To determine whether cholinergic mechanisms contribute to blood pressure responses during arterial baroreflex inhibition and stimulation, we assessed the effects of atropine on pressor and depressor responses during and after release of the Valsalva maneuver, upon administration of intravenous phenylephrine and nitroglycerin, and during neck suction in 12 healthy people. Atropinization augmented the depressor response during the Valsalva maneuver (269%), the pressor response after release of the maneuver (544%), the pressor response to phenylephrine (109%), and the depressor response to nitroglycerin (76%), whereas the depressor response to externally applied neck suction was attenuated or abolished in all subjects. Cardiac output as indicated by impedance cardiography was unchanged during carotid baroreceptor stimulation. The results are best explained by inhibition by atropine of compensatory cholinergic heart rate, vasodilator, and negative inotropic responses.

Left ventricular volumes and function during atrial pacing in coronary artery disease: a radionuclide angiographic study

This study set out to determine the pathophysiologic changes in the left ventricle during atrial pacing in 22 patients with coronary artery disease. Graduated right atrial pacing to a rate of 160 beats/min, or the induction of angina pectoris or significant ST depression was undertaken. Ventricular volumes were measured at rest and at rates of 100, 120, 140 and 160 beats/min using radionuclide angiography. The volumes at a pacing rate of 100 beats/min were used as a reference standard (100%). In the 22 patients with coronary artery disease, left ventricular end-diastolic volume decreased from 118 +/- 3% at rest to 80 +/- 5% at a rate of 160 beats/min; stroke volume from 121 +/- 3% to 54 +/- 5%; and ejection fraction (EF) from 49 +/- 3% to 37 +/- 5%. End-systolic volume decreased from 118 +/- 4% at rest, reached its minimal value of 94 +/- 5% at a rate of 120 beats/min and then increased slightly to 106 +/- 9% at 160 beats/min. Cardiac output and blood pressure did not change significantly. Compared to the control group of 10 normal subjects, the patients had a significantly smaller decrease in end-diastolic volume and end-systolic volume than in normal control subjects. EF in the normal subjects did not change. Blood pressure, cardiac output and stroke volume were similar in both groups. Atrial pacing tachycardia induced reversible ventricular dysfunction with a decrease in EF. Stroke volume was maintained because of relative ventricular dilatation.

Diagnosis of coronary stenosis by two-dimensional echographic study of dysfunction of ventricular segments during and immediately after pacing

The adequacy of two-dimensional echocardiography during right atrial pacing for the detection and characterization of coronary artery stenosis was examined in 10 closed chest dogs. Pacing at successively higher rates up to 210 beats/min was carried out in the control state and again during a 70% left anterior descending coronary artery stenosis-induced with intracoronary plugs. Left ventricular short-axis echographic cross sections were obtained at several levels of the left ventricle. After computer-aided standardized subdivision, contractile function of the global section and its subsegments was characterized by computed systolic fractional area change percent and wall thickening percent. Ventricular segments supplied from the site of the 70% coronary stenosis were delineated in a low papillary level cross section by a myocardial contrast echographic technique, and these segments demonstrated significant dysfunction during pacing at 150 to 210 beats/min. Echographic observation of the involved segments immediately after pacing revealed a maximal depression of function 5 seconds after pacing, equivalent to dysfunction at peak pacing, with function returning to control levels within about 2 minutes. Both maximal pacing and early postpacing studies facilitated satisfactory discrimination of ischemic from normally perfused myocardial segments. These experiments show that right atrial pacing study with quantitative two-dimensional echocardiography may serve to detect and assess a coronary stenosis associated with minor or no cardiac dysfunction in the rest state.

Atrial synchronized ventricular pacing: contribution of the chronotropic response to improved exercise performance

In contrast to asynchronous ventricular pacing (VOO, VVI), atrial synchronized ventricular pacing (VAT, VDD, DDD) maintains the normal sequence of cardiac chamber activation and permits a chronotropic response to exercise, thereby improving exercise performance. To assess the separate contributions of these two factors to improved work capacity, 14 patients with implanted programmable VAT pacemakers were exercised according to the Bruce protocol, in three different pacing modes, selected in a random order and on a double blind basis: (a) VAT; (b) chest wall stimulation triggered ventricular (V-CWS-T) pacing, during which the pacemaker was programmed to VAT mode but driven externally using chest wall stimulation at rates fractionally above the patients' atrial rate, thereby providing a chronotropic response to exercise without atrioventricular synchronization; and (c) VOO mode at 70 beats per minute. There was a significant improvement in exercise performance in all patients during both VAT and V-CWS-T pacing as compared to VOO mode; the average increase in work capacity being similar: VAT: 44 +/- 31, (range, 12 to 140) percent and V-CWS-T; 40 +/- 24 (range, 5 to 85) percent. It is concluded that in patients with adaptive pacing systems, the chronotropic response is the major determinant of any improvement in exercise performance.

AV pacing and LV performance

Five patients with impaired left ventricular function (LV) and implanted AV sequential pacemakers underwent serial radionuclide angiograms. The goal was a non-invasive evaluation of the rapid changes in left ventricular performance elicited by rate, pacing mode and AV interval manipulation. End diastolic volume, end systolic volume, stroke volume and cardiac output were increased by AV sequential pacing in comparison with ventricular pacing at 70 beats per minute. No significant change in ejection fraction and blood pressure were noted with changing AV sequential pacing rates at usual pacing rates. Our data suggest that a short AV interval (150 ms) improved LV performance more than a long AV interval (250 ms). A non-invasive technique to optimize left ventricular performance on an acute basis by varying heart rate, AV interval and pacing mode with the implanted AV sequential pacemaker is feasible and may be useful in selective clinical situations.

The interrelationship of factors controlling cardiac output

Our understanding of the mechanics of circulation may be broadened by testing old and new concepts on a new hydraulic model made possible by a unique pump which resulted from the evolution of open heart surgical equipment. Findings in human physiology which have corollaries in the model can be analyzed more easily in the model, with the resulting conclusions transferable with reasonable validity. Factors thought to control cardiac output were tested. The results indicate that there are two separate and distinct sets of control factors, with only one set being operative at a time. It is the set that is potentially limiting cardiac output the most that is the determinant at any time. Past paradoxes in our perception of human circulatory physiology are accommodated by the resulting concept.

Relative roles of heart rate and ventricular stroke volume for the regulation of cardiac output during controlled hypotension with sodium nitroprusside in man

The effects of N-allyl clonidine (St 567, alinidine), (0.5 mg/kg i.v.) a substance with specific bradycardic action at the sinus node, were studied on a total of thirteen patients in neuroleptanaesthesia and during controlled hypotension with sodium nitroprusside (SNP). Invariably, the fall in blood pressure was associated with an increase in heart rate (20.0 +/- 4.3+; P less than 0.01), presumably due to an activation of the arterial baroreceptor reflex. Alinidine decreased heart rate to the original level but no fall in cardiac output occurred a ventricular stroke volume and the calculated left ventricular stroke work were increased compensatorily (35.9 +/- 7.2% and 35.9 +/- 6.7%, P less than 0.01, respectively). In patients who received alinidine before the onset of controlled hypotension (n = 5) SNP failed to elicit an increase in heart rate. It is concluded that in patients under neuroleptanaesthesia tachycardiac does not play an important role for the maintenance of an adequate cardiac output during controlled hypotension with SNP.

Assessment of left ventricular function by radionuclide angiography during induced supraventricular tachycardia

Electrocardiographically synchronized radionuclide angiography was performed before, during and after induced paroxysmal supraventricular tachycardia in 13 patients. Data were acquired with a computer-interfaced Anger camera in a left anterior oblique projection. No data were acquired during tachycardia until tachycardia had been sustained for 1 minute. Patients ranged in age from 20 to 64 years (mean +/- standard deviation 42 +/- 14.5). Three patients had organic heart disease and 10 did not. Baseline and tachycardia heart rates (beats/min) were 59 to 99 (73 +/- 11) versus 141 to 228 (157 +/- 22). Baseline and tachycardia left ventricular measurements (mean +/- standard error) were as follows: ejection fraction 64 +/- 2 versus 62 +/- 4 percent (not significant), ejection rate 3.0 +/- 0.1 versus 4.3 +/- 0.4 mean ventricular counts/s (p less than 0.001), normalized end-diastolic counts 72.7 +/- 7.8 versus 48.7 +/- 6.7 X 10(3) counts (p less than 0.001), normalized stroke counts 37.1 +/- 3.4 versus 23.3 +/- 2.7 X 10(3) counts (p less than 0.001) and normalized count cardiac output 2,717.5 +/- 273.0 versus 3,620.2 +/- 403.7 X 10(3) counts/min (p less than 0.005). Although ejection fraction for the whole group did not change significantly, it decreased during tachycardia by 5 percentage points or more in five patients. These were the three patients with heart disease and the two normal patients with the fastest heart rate during tachycardia (228 and 214 beats/min, respectively). In summary, paroxysmal supraventricular tachycardia was characterized by a marked decrease in left ventricular end-diastolic and stroke volumes but increased ejection rate and cardiac output without significant change in ejection fraction. Heart disease or rapid heart rate during tachycardia appeared to have a deleterious effect on ejection fraction.

The effect on myocardial contractility of a new beta-adrenergic receptor blocking drug, penbutolol

1 The inotropic effect of penbutolol, a new beta-adrenergic receptor blocking drug, has been compared with that of propranolol in two similar groups of six subjects each. 2 Inotropic changes were assessed from changes in myocardial contractility index [LV dp-dt/11 T] as well as the slope of the regression line relating LV dp/dt to LVED during incremental pacing. 3 Penbutolol was found to have a significant negative inotropic effect of an order similar to that of propranolol in the respective doses studied.

Hemodynamic effects of rapid atrial stimulation in adult and young dogs

We evaluated the use of rapid atrial stimulation (RAS) to produce 2:1 atrioventricular (AV) block as a method for treating supraventricular tachycardias. We studied the cardiovascular hemodynamic changes before and after block in eight adult dogs and nine young dogs (3-4 1/2 weeks old). In each dog an electrocardiogram; cardiac index; aortic, pulmonary, and left ventricular (LV) systolic and end-diastolic pressures; and LV (dp/dt) p were recorded simultaneously and pulmonary ans systemic resistances were calculated. Measurements were obtained during (1) the control state, (2) RAS with maximal 1:1 AV conduction, (3) RAS with 2:1 AV block, and (4) RAS with 1:1 AV conduction at 50% of the rate which resulted in 2:1 AV block. Comparison of the hemodynamic effect of RAS with maximal 1:1 AV conduction and hemodynamics in the control state showed that there was a significant decrease in cardiac index and aortic mean pressure of 39% and 15%, respectively. When hemodynamic changes during RAS with 2:1 AV block were compared with those during RAS with maximal 1:1 AV conduction, cardiac index, LV systolic pressure, and aortic mean pressure increased by 52%, 17%, and 22%, respectively. LV (dp/dt) p increased by 20% and was significantly higher during RAS with 2:1 AV block than at higher ventricular rates obtained with RAS and maximal 1:1 AV conduction. We found that in this acute study the hemodynamic findings for the young dogs were similar to those for adult dogs. The data suggest that (1) significant hemodynamic improvement is obtained by RAS causing 2:1 AV block when compared to higher ventricular rates associated with 1:1 AV conduction; (2) in addition to increasing the diastolic filling period, reducing the very high ventricular rates improves the cardiac contractile state, and (3) RAS and 2:1 AV block may provide a useful technique for the management of some patients with intractable supraventricular tachycardia.

Influence of heart rate increase on uncorrected pre-ejection period/left ventricular ejection time (PEP/LVET) ratio in normal individuals

In 26 normal volunteers, increase in heart rate from mean 73-94 +/- 1-97 to 103-61 +/- 2-72/min, by either intravenous atropine administration or rapid right atrial pacing, produced definite changes in the uncorrected systolic time intervals. As expected, total electromechanical systole (QS2) and left ventricular ejection time (LVET) were shortened, while the pre-ejection period (PEP) was unaffected. There was a consistent and significant increase of the PEP/LVET ratio (P less than 0-001). It is postulated that when this ratio is taken to express left ventricular contractility, it should probably be corrected for heart rate. Appropriate regression equations for such a correction were calculated (PEP/LVET=0-249 + 0-0168 HR).

[The effect of sudden changes in the stimulation rate of the heart on peak systolic pressure (author's transl)]

The changes in systolic pressure were studied in 48 patients during artificially (by electrical stimulation) induced sudden increases and consecutive decreases in heart rate. Various patterns could be separated respective to phase A (sudden switch from low (80-120 impulses/min) to higher heart rate (140-180 impulses/min)) and to phase B (sudden switch-back from high to lower (control) beating rate): AI) The systolic peak-pressure of the first contraction after the shortened stimulation interval is very low and often stays ineffective (below the aortic opening pressure). The second contraction develops already a higher pressure than the first one, during the consecutive beats the systolic pressure increases gradually until a new steady state is reached, which is usually lower than the systolic pressure during the foregoing lower beating rate. Sometimes however it can be equal or even higher. Accordingly after an elevation of heart rate arterial mean pressure can drop, stay constant or increase. AII) The systolic peak-pressure of the first contraction after the shortened stimulation interval stays relatively high and drops continuously with the succeeding contractions until a new steady state is reached according to the higher heart rate, it is however always lower than the one at a lower stimulation rate. Therefore arterial mean pressure is always decreased. AIII) Finally we observed changes in arterial systolic pressure in some patients that could not be grouped according to one or to the other pattern described above.- BI) The systolic peak of the first contraction after the switch-back from the high to the lower (control) rate is much higher than that of the last at the higher rate. The peak pressures of the consecutive contractions are then dropping continuously to the new steady-state, that can be higher, equal or lower than that at the higher stimulation rate. BII) ....

Atrial and ventricular pacing after open heart surgery

The effect of cardiac pacing, through a wide range of pacing rates, has been studied in 13 patients in stable sinus rhythm within 24 hours of operation. Using first atrial and then ventricular pacing, the cardiac rate was raised to 60% above control value. The effects of atrial and ventricular pacing were compared by observing the differences in the various parameters measured at corresponding paced heart rates. With atrial pacing there was no significant difference in the cardiac output, mean aortic pressure, mean left atrial pressure, or left ventricular work. During ventricular pacing there were significant falls in cardiac output (p<0·05), mean aortic pressure (p<0·01), and left ventricular work (p<0·01). There was a significant rise in mean left atrial pressure (p<0·01).

In this study atrial pacing had no significant effect on the parameters measured, through a wide range of cardiac rates. An inotropic effect resulting from increase of rate was not seen. Reported increases in cardiac output following pacing must therefore have resulted from correction of a dysrhythmia. Ventricular pacing incurs a definite haemodynamic penalty if used in patients with an intact atrioventricular conducting pathway. If atrial pacing is not available, this penalty must be balanced against the possible haemodynamic advantage of suppressing a dysrhythmia.