Kinetic properties of 99mTc-Q12 in canine myocardium

BACKGROUND

99mTc-Q12 is a new Tc(III) perfusion imaging agent that permits prompt myocardial visualization in humans. We postulated that 99mTc-Q12 myocardial activity is related to actual myocardial blood flow during conditions of myocardial ischemia and pharmacological coronary artery vasodilation and that 99mTc-Q12 shows little or no myocardial redistribution as long as 4 hours after intravenous injection.

METHODS AND RESULTS

In seven anesthetized, open chest dogs, the left circumflex coronary artery was occluded, and dipyridamole (0.32 or 0.56 mg/kg) was infused into the right atrium, followed by 10 mCi of 99mTc-Q12. Myocardial blood flow was measured by radiolabeled microspheres. The animals were euthanized, and a total of 315 myocardial samples were assayed in a well counter for 99mTc activity. One week later, radiolabeled microsphere activity was determined, and myocardial blood flow was calculated. 99mTc activity (y) was related to myocardial blood flow (x) from 0 to 2 mL.g-1 x min-1 by the relation y = 0.64x + 0.35 (r = .88, P = .0001). In 14 additional anesthetized, open chest dogs, an occluder was placed around the left circumflex coronary artery, and an ischemic level of circumflex blood flow was maintained constant over 4 hours as measured by an ultrasonic flowmeter. Dipyridamole (0.56 mg/kg) was infused intravenously beginning 15 minutes after coronary occlusion and then followed by 10 mCi of 99mTc-Q12. Gamma camera images, hemodynamics, microsphere blood flow, and endocardial biopsies (latter in six dogs) were performed at 30, 60, 120, and 240 minutes after 99mTc-Q12 injection. Myocardial blood flow in the distribution of the left anterior descending artery decreased by 29.6% from 30 to 240 minutes (P < .05), whereas left circumflex blood flow increased by 40.4% from 30 to 120 minutes (P < .05) as the dipyridamole hemodynamic effects dissipated. Nevertheless, the ratio of myocardial perfusion defect zone counts to normal myocardial zone counts remained constant over 4 hours, as did the technetium counts from the needle biopsy endocardial samples.

CONCLUSIONS

Over a limited range of myocardial blood flows from 0 to approximately 2 mL.g-1 x min-1, 99mTc-Q12 myocardial activity is related to actual myocardial flow at the time of tracer injection. 99mTc-Q12 shows no evidence of myocardial redistribution.

Oxygen solubility, rheology and hemodynamics of perfluorocarbon emulsion blood substitutes

Perfluorocarbon-based blood substitute emulsions have been under development for more than a quarter century. The first generation emulsions have provided confirmation that the physical principals of high gas solubility and low viscosity can effectively support organ and organism respiration and metabolism. Clinical trials led the US FDA in 1990 to be the first to approve a 20 w/v % perfluorocarbon emulsion for human use as coronary angioplasty adjuvant therapy. Hemodynamic responses to hemodilution with intravascular perfluorocarbon emulsions have varied with species and the mechanisms for adverse reactions are better understood now as second generation emulsions containing up to 100 w/v % perfluorocarbon are under development as blood substitutes, imaging agents, and for other therapeutic applications. This report describes the evolution of perfluorocarbon emulsions as blood substitutes by emphasizing oxygen solubility, rheology and hemodynamic aspects of the emulsions as they have been applied in experimental laboratory animal and human clinical settings.

Lung functions after intravenous or intraperitoneal administration of perfluorooctyl bromide (PFOB) or perfluorotributylamine (FTBA) emulsions

The biomechanical and oxygen transfer functions of lungs of animals administered intravenous or intraperitoneal fluorocarbon emulsions containing either perfluorotributylamine or perfluorooctyl bromide 4 to 7 days earlier were tested in a porcine and a canine model. Lung fluorocarbon content was measured by 19F NMR spectroscopy. The existence of fluorocarbon in the lung tissue produced no measurable effects on lung compliance variables in vivo or ex vivo nor on steady state oxygen transfer from air to blood over a wide range of inspired oxygen partial pressures.

Quantitative pO2 imaging in vivo with perfluorocarbon F-19 NMR: tracking oxygen from the airway through the blood to organ tissues

The physiological redistribution of perfluorocarbon (PFC) compounds to liver, spleen, bone marrow, and lung after intravenous (i.v.) or intraperitoneal (IP) administration of PFC emulsions affords the unique opportunity for non-invasive monitoring of oxygenation status of these organs and tissues utilizing fluorine (F-19) nuclear magnetic resonance (NMR) imaging techniques. PFCs also may be introduced directly into the pulmonary airways by procedures such as liquid ventilation, intratracheal instillation, or aerosol inhalation. Considerations of importance when establishing methodology for accurate quantitation of oxygen partial pressure (pO2) in vivo using F-19 NMR include: 1.) error analysis of the calibration curves which relate pO2 to the measured PFC F-19 relaxation rate, 2.) optimization of the NMR pulse sequence for efficient oxygen sensitive data acquisition and, 3.) fluorine signal independence from emulsion aqueous phase bioconstituents. The porcine model was investigated at 0.14T following i.v. or IP administration of the PFC emulsion containing perfluorotributylamine (FC-43) to demonstrate the capability for tracking oxygen with F-19 NMR from the lung through the blood to selected organ tissues. Quantitative pO2 projection images and isobaric contour graphs were derived for the liver, spleen, and lungs as a function of inspired oxygen. Blood pO2 levels in aorta, pulmonary artery, and hepatic vein were monitored simultaneously with NMR imaging for correlative analysis.

Demonstration of hydroxyl radical and its role in hydrogen peroxide-induced myocardial injury: hydroxyl radical dependent and independent mechanisms

We investigated the mechanism of hydrogen peroxide (H2O2) action on myocardial injury in relation to hydroxyl radical (.OH) formation. Isolated rat hearts were perfused with a concentration of H2O2 (300 microM) known to produce cardiac injury. Perfusion of H2O2 for 15 min caused severe myocardial dysfunction, morphological damage, ATP depletion, and lipid peroxidation. Hydrogen peroxide concentration in the coronary effluent was reduced approximately 40% reflecting a myocardial H2O2 consumption of 12.7 +/- 0.9 mumol/15 min/g wet tissue (n = 12). One of the .OH-generated derivatives, 2,3-dihydroxybenzoic acid (2,3-DHBA), formed from reaction with salicylic acid, was detected in the coronary effluent by high-performance liquid chromatography at 23.16 +/- 4.05 nmol/15 min/g wet tissue. Catalase (200 U/ml, n = 6) added to the perfusate attenuated all parameters of myocardial injury by eliminating H2O2 from the perfusate, and thus .OH was not detected in the effluent. Deferoxamine (5 mM, n = 7) added to the perfusate reduced morphological damage and lipid peroxidation, but not dysfunction or ATP depletion. Deferoxamine significantly reduced .OH production; 2,3-DHBA was 5.22 +/- 3.56 nmol/15 min/g wet tissue. The present study provides evidence that .OH is produced in the H2O2-perfused heart. The adverse H2O2-mediated myocardial outcomes documented in this study appear to arise from both .OH-dependent and .OH-independent mechanisms.

Quantitation of perfluorocarbon blood substitutes in tissues using F-19 magnetic resonance spectroscopy

F-19 Magnetic Resonance Spectroscopy (MRS) has been used to quantitate the biodistribution of perfluorocarbon (PFC) blood substitutes in porcine tissues following intraperitoneal administration of a 20% w/v perfluorotributylamine (FC-43) based emulsion. PFC tissue concentrations were determined for spleen, liver and lung tissues from juvenile pigs ranging in weight from 12 to 15 kg. Typical average values (mmoles FC-43/gram tissue) ranged from 0.23-0.39 for spleen; 0.09-0.13 for liver; and 0.09-0.12 for lung. A description of this spectroscopy based quantitation technique will be presented. F-19 MRS is a specific, rapid, and accurate method for measurement of PFC tissue concentrations.

Compliance to nicotine gum recommendations in a multicomponent group smoking cessation program: an exploratory study

This is an exploratory study of compliance to nicotine chewing gum in a multicomponent group smoking cessation program where usage was optional. Two-thirds (24 out of 36) of the participants across three group sessions complied with the recommendation to use nicotine chewing gum. All subjects received standard instructions. Demographic characteristics (age, sex, site of employment) and smoking variables (number of years smoked, pretreatment number of cigarettes smoked per day, and nicotine dependence) failed to predict compliance, but complaint participants reported significantly greater confidence at their ability to not smoke in various challenging situations as measured by the Smoking Self Efficacy Questionnaire. Those who did comply mostly failed to chew the gum in the way instructed. Reduced craving was cited as the most positive feature of gum use.

A comparison of models describing reports of disability associated with chronic pain

We examined pain-related disability from two perspectives. A disability model, suggesting that disability is a direct consequence of pain, is compared with a symptom perception model emphasizing individual differences in perception and report of physical symptoms. Disability estimates (reported activity interference and employment status) were obtained from a sample of 179 patients with chronic pain. Using multiple regression analyses, we showed that distress, symptom reporting, and pain intensity accounted for comparable levels of variance in reports of activity interference. When we controlled for distress, the frequency of reporting physical symptoms made the largest contribution to prediction of this kind of disability (14%). Neither symptom report or pain intensity was useful in predicting the more objective disability criterion of employment status. Results were interpreted as probable evidence against a disability model of pain-related disability and suggest the relative importance of individual cognitive differences in symptom responsivity.

A critical review of questionnaires for assessing pain-related disability

Standardized questionnaires offer a practical and cost-effective means for measuring moderate disability. Fourteen questionnaires that can be used for assessing pain-related disability are reviewed in this article. Their comparative attributes are summarized. Most of the questionnaires obtain information about specific activities of daily living. They vary in terms of structure, content, and intended applications. Psychometric theory provides the optimal method for evaluating these questionnaires, highlighting the importance of reliability and validity. Being mindful of psychometric qualities will help the evaluator to select an appropriate questionnaire. The present array of existing instruments points toward a need for comparison studies that may eventually result in more uniform methods for evaluating pain-related disability.

Construct validity of practical questionnaires for assessing disability of low-back pain

Four practical questionnaires used for assessing disability associated with chronic low-back pain were investigated. Comparison of findings among 93 patients disclosed a moderate level of agreement (r = 0.44-0.63) between the scales, providing limited evidence that a single construct is being measured by each. The questionnaires were more strongly related to each other than to psychosocial scales, which suggests that findings are distinguishable from distress. However, higher distress was significantly associated with greater reports of disability. This was most discernible in the form of more enduring characteristics that may reflect neuroticism. These findings allow modest support for the construct validity of the four questionnaires and suggest a need for caution with their use because persistently negative affect may be a biasing factor.

Nursing assessment of positive adjustment for individuals with multiple sclerosis

The chronic, progressive nature of multiple sclerosis (MS) often interferes with effective psychosocial adjustment. When presented with physical limitations and/or cognitive impairment, the client with MS may experience problems such as depression, family dissolution, and loss of important roles. However, some persons are able to adjust positively and enjoy life. This article explains a model for assessing psychosocial adjustment of those with MS and describes its practical application for the rehabilitation nurse.

Regional myocardial blood flow and cardiac mechanics in dog hearts with CO2 laser-induced intramyocardial revascularization

Laser-induced intramyocardial revascularization (LIR) has been used to promote direct communications between blood within the ventricular cavity and that of the existing myocardial vasculature in an attempt to increase perfusion in patients with ischemic heart disease. This study was conducted to measure the effects of LIR channels on regional myocardial flood flow (microspheres), cardiac mechanics (sonomicrometers), and myocardial tissue pressures in 18 dogs. Under baseline hemodynamic conditions (mean HR = 165.2 +/- 11.4 bpm, LVP = 123.6 +/- 22.9/4.0 +/- 1.8 mm Hg, AoP = 112.8 +/- 27.1/77.0 +/- 22.5 mm Hg), myocardial blood flow in laser-treated tissue (mean = 1.11 +/- .10 cc/min/gm before laser; .71 +/- .19 cc/min/gm after laser) was reduced as compared to blood flow in control tissue (mean = 1.12 +/- .15 cc/min/gm before laser; 1.25 +/- .22 cc/min/gm after laser). Regional myocardial systolic shortening (11.32% +/- 3.82% before laser; 7.49% +/- 2.86% after laser) was decreased by 33%. During simultaneous reversible ligation of the LAD and LCCA for 2 min, when intramyocardial channels represented the only tissue access for the injected microspheres, blood flow in laser-treated tissue was not increased above that of the control non-lasered tissue. However, regional blood flow was greater in laser-treated ischemic tissue (mean = .61 +/- .12 cc/min/gm) than in untreated ischemic areas (mean = .04 +/- .03 cc/min/gm) when left ventricular pressure (LVP) was acutely elevated (mean SLVP = 207.0 +/- 16.1 mm Hg). Using these measurements, a model is proposed to predict regional systolic pressure gradients between the left ventricular cavity and coronary intramyocardial vasculature required to permit restoration of blood flow to ischemic myocardium. We conclude that improved perfusion via laser-induced intramyocardial channels does not occur in otherwise normal myocardium exposed to acute coronary ligation and only small improvements in perfusion are noted when LVP is significantly elevated. Consideration of further clinical application of this approach is seriously cautioned awaiting additional experimental studies.

Amlodipine, a long-acting calcium antagonist drug reduces ischemia-induced ventricular conduction delay in pig hearts

The effects of amlodipine, a novel, long-lasting calcium channel blocking agent, on ischemia-induced myocardial conduction delay was studied in anesthetized pigs paced at a constant heart rate. Acute coronary occlusion (3 minutes) significantly lengthened time to onset, time to peak and duration of bipolar electrograms recorded from both subendocardial and subepicardial left ventricular sites. After intravenous injection of amlodipine (0.3 mg/kg, n = 6), subsequent periods of ischemia greatly reduced (p less than 0.01) all indexes of subepicardial conduction delay. In the subendocardium, amlodipine decreased only time to onset (-25 +/- 4%, p less than 0.01) within the ischemic zone. Significant delays in all indexes were present during repeated ischemic periods in the placebo-treated control group (n = 5). Amlodipine also increased regional myocardial blood flow within the nonischemic myocardium by 25 +/- 10% and decreased mean aortic pressure by 7 +/- 2% without altering flow in the ischemic region. Left atrial pressure remained unchanged. Indexes of ischemia-induced conduction delay were more rapidly restored after reperfusion in amlodipine-pretreated than in control animals. In conclusion, amlodipine produced a beneficial blood flow-independent effect on ischemia-induced injury potentials. The effect may help to reduce the likelihood of development of lethal ventricular arrhythmias in the early stage of myocardial ischemia in the clinical setting.

Hemodynamic and electrophysiologic effects of amlodipine, a new calcium channel blocker

The effects of amlodipine (300 micrograms/kg administered intravenously), a new, long-acting, dihydropyridine class, calcium channel blocking drug, were studied in atrially paced (120 beats/min), autonomically blocked dogs. Hemodynamic and electrophysiologic parameters were measured before and up to 3 hours after amlodipine. Coronary blood flow was significantly increased 10 and 30 minutes after drug administration, whereas cardiac output and mean arterial pressure were unaffected. Coronary vascular resistance was decreased but total systemic vascular resistance did not change. Atrioventricular (AV) nodal conduction was slightly prolonged, as reflected by increases in atrial-His bundle and AV conduction times and PR interval, 30 minutes after administration. All parameters returned toward their control values within 3 hours after drug administration. Comparison of coronary vascular resistance and AV conduction changes with those previously reported for other calcium channel blocking drugs where autonomic blockade existed suggests that at equivalent levels of coronary vasodilation, amlodipine's effects more closely resemble the effects of diltiazem or verapamil than other dihydropyridines.

Protection of regional mechanics and mitochondrial oxidative phosphorylation by amlodipine in transiently ischemic myocardium

The effects of amlodipine (0.3 mg/kg administered intravenously, n = 5) and placebo (n = 5) on recovery of myocardial function and respiration of isolated mitochondria were examined in "stunned" myocardium of normotensive pentobarbital-anesthetized dogs. Measures of myocardial wall motion, using sonomicrometers, and tissue blood flow, by radioactive microspheres, were obtained at baseline, 15 minutes after administration of amlodipine or placebo, after 10 minutes of left circumflex artery ligation and at 60 minutes of reperfusion. Mean aortic pressure decreased from 115 +/- 6 to 101 +/- 5 mm Hg after administration of amlodipine and remained decreased throughout the experiment. Heart rate was not significantly affected at any time. Both groups showed similar degrees of ischemia: elevation of ST segment to 4.7 +/- 1.4 vs 6.2 +/- 1.9 mV; reduction of ischemic zone shortening fraction to 0.6 +/- 1.9 vs -3.4 +/- 2.7%; and reduction of epicardial and endocardial blood flows (epicardial = 40 +/- 13 vs 43 +/- 13 ml/100 g/min; endocardial = 7 +/- 4 vs 13 +/- 6 ml/100 g/min [values for amlodipine vs placebo]). Mitochondrial state 3 rate of respiration and respiratory control index indicative of rate of adenosine triphosphate synthesis and membrane integrity in myocardial samples taken after 10 minutes of ischemia were significantly reduced in the placebo but not in the amlodipine group. Myocardial function showed significantly greater improvement in amlodipine vs placebo at 60 minutes of reperfusion as indicated by shortening fraction (17.7 +/- -1.4 vs 5.8 +/- -3.5%, amlodipine vs placebo), which may have been related to increased myocardial blood flow and decreased blood pressure during reperfusion. Thus, amlodipine pretreatment prevented mitochondrial dysfunction during ischemia and accelerated recovery of both myocardial mechanical function and blood flow when compared with placebo.

The Functional Assessment Screening Questionnaire: application for evaluating pain-related disability

The Functional Assessment Screening Questionnaire (FASQ) is a 15-item checklist which was developed for primary care populations and may serve as a questionnaire method for evaluating disability which is associated with chronic pain. One hundred fifty-eight patients completed the FASQ as part of an initial multidisciplinary evaluation of chronic pain. Reliability was reaffirmed through split-half and alternate-form methods. Responses were examined to explore relationships to aspects of disability. Although job functions were not directly assessed, the scores of employed vs unemployed respondents differed significantly. Findings were related to Minnesota Multiphasic Personality Inventory scale scores but appeared to reflect a separate phenomenon. Scale 1 (Hs) was the most useful scale for predicting level of reported impairment. Nonparametric methods showed levels of functioning varying significantly by site of pain complaint. Patients with back pain reported the most difficulty; those with head pain and genital pain reported the least difficulty. An internal structure appropriate to chronic pain populations was discerned, with two general factors measuring either physical-motoric or cognitive-social aspects of disability. Aside from constraints associated with the use of self-report methods, the FASQ may be helpful as part of efforts to portray pain-related disability.

Effects of calcium channel antagonists on carotid sinus baroreflex control of arterial pressure and heart rate in anesthetized dogs

Our study was designed to determine whether the calcium channel antagonists verapamil, diltiazem, and nifedipine and the nitrate vasodilator sodium nitroprusside modulate carotid sinus (CS) baroreflex control of mean arterial pressure (MAP) and heart rate (HR). Pentobarbital-anesthetized, vagotomized dogs were surgically prepared for reversible vascular isolation of the CS regions. Open-loop performance of the CS baroreflex was determined under control conditions and after intravenous infusion of each agent for 20 minutes at four rates (nitroprusside: 0.3-10 micrograms/kg/min; verapamil and diltiazem: 1-30 micrograms/kg/min; nifedipine: 0.1-3 micrograms/kg/min). With the CS baroreflex loop closed, each vasodilator decreased MAP from control (nitroprusside: 127 +/- 3 to 69 +/- 5 mm Hg; verapamil: 137 +/- 7 to 86 +/- 5 mm Hg; diltiazem: 137 +/- 9 to 100 +/- 5 mm Hg; nifedipine: 140 +/- 6 to 109 +/- 7 mm Hg). Each compound also caused a dose-dependent downward shift in the open-loop CSP-MAP relations. The higher doses of each vasodilator also depressed the total range of control of MAP (i.e., maximum MAP minus minimum MAP) by the baroreflex and significantly attenuated the peak open-loop MAP/CSP gains (nitroprusside: 1.21 +/- 0.19 to 0.56 +/- 0.12; verapamil: 1.36 +/- 0.16 to 0.64 +/- 0.10; diltiazem: 1.52 +/- 0.34 to 0.89 +/- 0.11; nifedipine: 1.35 +/- 0.20 to 0.83 +/- 0.14) but did not alter the CSP at which the peak gain was manifest. Only verapamil and diltiazem significantly shifted downward the CSP-HR relations, whereas none of the drugs affected the total range of baroreflex control of HR (i.e., maximum HR minus minimum HR) or the peak open-loop HR/CSP gains. Our results suggest that 1) it is unlikely that calcium channel antagonists act directly on the baroreceptors or the neural components of the baroreflex loop (i.e., afferent, central and efferent nerves) because they impair CS baroreflex control of MAP but not HR and 2) the impairment of MAP control is predominantly due to a nonspecific blunting of adrenergic vasoconstriction.

Sympathetic innervation of the cardiovascular system in the giraffe

The extreme blood pressure gradients in the giraffe, produced by gravity and behaviour, present a special challenge to blood flow and vascular capacity regulation, e.g. via sympathetic nerves. We report the distribution of nerves in vascular tissue from giraffe extremities and neck based on immunofluorescence against specific antisera to dopamine-beta-hydroxylase, neuropeptide Y, neurofilament, and synapsin I. Sympathetic innervation of gravitation-dependent arteries in the limbs was restricted to the adventitia-media border, while in carotid arteries fibers penetrated deep into the media. Surprisingly, limb veins appeared to be devoid of sympathetic innervation, while jugular veins had a sparse innervation. The morphological response, medial hypertrophy, that parallels the hydrostatic pressure gradients in the circulation combines with the thick skin and tight underlying fascia, the 'g-suit', and an exquisitely distributed sympathetic innervation pattern to provide an effective array of mechanisms for cardiovascular regulation in the adult giraffe.

Microcirculatory effects of somatostatin in acute pancreatitis

Somatostatin, a 28-amino-acid inhibitory polypeptide has been advocated for the treatment of upper gastrointestinal bleeding and acute pancreatitis. This study examines the effect of somatostatin in acute hemorrhagic pancreatitis in piglets (n = 12), weighing 8-12 kg. Six animals served as controls, and received only fluid resuscitation (0.9%, NaCl, 20 ml/kg/h). Six animals received somatostatin treatment consisting of a 15 micrograms/kg bolus i.v. given simultaneously with the induction of pancreatitis, and treatment continued with an intravenous infusion (15 micrograms/kg/h) for 5 h. Cardiac output, heart rate, blood pressure, arterial pO2, hematocrit and serum amylase were recorded before and each hour during the experiment. Regional blood flow in the gastrointestinal area was measured using the microsphere method. The microspheres labelled with three different isotopes were administered before the experiment and at 2 and 5 h, respectively. There was a significant decrease in the cardiac output (p less than 0.05) and an increase in systemic blood pressure in the somatostatin-treated group (p less than 0.025). Pancreatic blood flow decreased by 43% following somatostatin infusion. The decreases at 2 and 5 h were highly significant (p less than 0.005). Blood flow to the mucosal but not muscular region of the stomach was decreased by somatostatin. This study suggests that somatostatin might be harmful in acute pancreatitis due to its adverse effects on pancreatic blood flow and cardiac output. However, somatostatin may be effective in reducing gastrointestinal bleeding. If the drug is used clinically, careful monitoring of the cardiac output is necessary.

Gravitational haemodynamics and oedema prevention in the giraffe

Because it is so tall, the giraffe, Giraffa camelopardalis, provides an important animal model for investigating adaptive mechanisms to orthostatic (gravitational) pressure changes. Previous physiological studies of the giraffe have concentrated on arterial blood pressures in the heart and neck. Briefly, these investigations revealed that arterial pressure near the giraffe heart is about twice that in humans, to provide more normal blood pressure and perfusion to the brain. Another important question is that of how giraffes avoid pooling of blood and tissue fluid (oedema) in dependent tissues of their extremities. As monitored by radiotelemetry, the blood and tissue fluid pressures that govern transcapillary exchange vary greatly with exercise. These pressures, combined with a tight skin layer, move fluid upward against gravity. Other mechanisms that prevent oedema include precapillary vasoconstriction and low permeability of capillaries to plasma proteins.

Defining the mechanical border zone: a study in the pig heart

Page H88: Kikuo Sakai, Kozo Watanabe, and Ronald W. Millard. “Defining the mechanical border zone: a study in the pig heart.” Page H94: the acknowledgments were omitted and are as follows.

We are grateful to Dr. Arnold Schwartz, Dept. of Pharmacology and Cell Biophysics, University of Cincinnati College of Medicine, and Dr. Mootomi Nakamura, Angiocardiology and Cardiovascular Clinic, Kyushu University, Fukuoka, Japan, for their support and encouragement. The expert technical assistance of Victoria Rapien and John Erickson was invaluable.

This study was supported in part by Grant P01-HL-22619 from the National Heart, Lung, and Blood Institute.

Preliminary findings of this study were first reported at the Annual Scientific Session of the American Heart Association held at Anaheim, CA, November 1983. An abstract of this work has appeared ( Circulation 68, Suppl. III: 194, 1983).

Coronary blood flow and myocardial segment dimensions during simulated dives in seals

Three harbor seals Phoca vitulina richardsi and five spotted seals Phoca vitulina largha were used in studies of acute episodes of local myocardial ischemia in open-chest, anesthetized animals and of coronary blood flow and regional function as indicated by left ventricular segment dimensions during experimentally simulated dives of conscious, instrumented animals. We observed that seal myocardium, in which there are few coronary anastomoses, responded to brief local occlusion with prompt local dysfunction and systolic bulging; coronary flow in the nondiving seal oscillated irregularly and declined with spontaneous apnea and related falling heart rate; flow continued to oscillate but was much reduced during dives, frequently ceasing entirely for periods as long as 45 s; ventricular segment dimension shortening was reduced intermittently during dives; and elevated heart rate induced during dives by cardiac pacing or by administration of atropine diminished or eliminated the reductions in coronary blood flow. Responses of seal heart reflect the reduction in cardiac metabolic demand during diving and the seal's myocardial adaptation for enhanced anaerobic glycolysis. The seal heart can maintain mechanical function during dives with minimal coronary perfusion, despite the progressive and ultimately profound hypoxia, hypercapnia, and acidosis. Reduced cardiac metabolism, copious glycolytic reserves, and metabolite washout by intermittent brief bursts of coronary blood flow are apparently sufficient to support continued cardiac function, even though the seal heart has little tolerance for acute localized ischemia.