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Biomedical subjects

J H Mitchell

Publications and source records attributed to J H Mitchell.

At least 127 records · Page 7Linked to original sources
PubMed1988

Estimation of human myocardial mass with MR imaging.

The accuracy and reproducibility of magnetic resonance (MR) imaging in the determination of left ventricular mass in humans was investigated. Left ventricular wall volume was measured from ten short-axis, end-diastolic MR images that spanned the left ventricle. Mass was estimated on the basis of average left ventricular wall volume and an assumed myocardial density. To establish the accuracy of the technique, the authors imaged ten cadaver hearts and compared true left ventricular weight with the mass estimate based on MR imaging findings. In vivo determination of left ventricular mass was evaluated in 40 subjects, with resultant calculated masses of 156.4-319.3 g. Intra- and interobserver variabilities of the technique were analyzed in ten subjects. Both the intra- (r = .96, standard error of estimate [SEE] = 11.1 g) and interobserver variabilities (r = .91, SEE = 17.8 g) were excellent. Eight subjects were imaged on two separate occasions to evaluate reproducibility of the technique and confidence limits for a given measurement. For these eight, there was good correlation between the two estimates (r = .93, SEE = 21 g). The authors conclude that MR imaging yields highly accurate and reproducible estimates of left ventricular mass in humans in vivo.

Cadaver↗
PubMed1988

Prevalence of nocturnal hypoxaemia amongst men with mild to moderate hypertension.

Thirty men (aged 35-65) with untreated essential hypertension (BP greater than or equal to 140/90), confirmed by 24-h ambulatory monitoring, had overnight recordings of arterial oxygen saturation (SaO2) in their own homes. The overnight saturation records were compared with those from a group of 30 normotensive control subjects matched for age, height and weight. The groups did not exhibit significant differences in any of the following parameters of overnight oxygenation: median SaO2, lowest SaO2 or frequency of 3 or 4 per cent dips in SaO2. We conclude that essential hypertension is not associated with excessive arterial hypoxaemia such as might be due to a sleep apnoea syndrome.

Adult↗
PubMed1987

Cardiovascular responses at the onset of exercise with partial neuromuscular blockade in cat and man.

1. In decerebrated cats the cardiovascular, heart rate and blood pressure responses to static muscle contractions were followed from the onset of stimulation of the cut L7-S1 ventral roots. Heart rate and blood pressure were also followed during maximal voluntary and electrically induced static muscle contractions in man using one leg. In both cat and man contractions were performed under control conditions and tubocurarine-induced neuromuscular blockade. 2. In the cat, heart rate and blood pressure increased 1.7 s after the onset of the contraction. No cardiovascular responses were seen when the muscle contraction was blocked by tubocurarine. 3. In man, both heart rate and blood pressure increased at the onset of voluntary contractions. Partial curarization reduced strength to 39% of control. The heart rate response was unaffected by tubocurarine while the blood pressure response was reduced from 61 to 32 mmHg. 4. Electrical stimulation of the muscles resulted in 75% of voluntary strength in man. The heart rate response was delayed one R-R interval in the electrocardiogram but was as large as during voluntary contractions. During partial curarization the heart rate response was significantly smaller and the blood pressure response was reduced from 11 to 8 mmHg. 5. In conclusion, processes in active muscles elicit an increase in heart rate and blood pressure which depends on the intensity of the muscle contraction developed. However, the immediate cardiovascular responses at the onset of voluntary muscle contractions cannot be accounted for by reflexes generated in the working muscles alone.

Adolescent↗
PubMed1987

Evaluation of the Ohmeda 3700 pulse oximeter.

Arterial oxygen saturation values (Sao2) from 60% to 98% were measured by the Ohmeda 3700 pulse oximeter with the three types of probe available and compared with values of oxygen saturation estimated from direct arterial sampling (arterial oxygen and carbon dioxide tensions and pH) on 65 occasions. The response time of the oximeter was measured after a sudden rise in inspired oxygen concentration. Artefact rejection was assessed by arterial compression proximal to the probe site, and by simultaneous recordings of overnight Sao2 on opposite hands. The ability to recreate patterns of oscillating Sao2 from the data stored in the oximeter was also investigated. With the best probe system the oximeter measured Sao2, relative to arterial values estimated from Pao2, with a mean (SD) difference of -0.4% (1.8%). The response time was comparable with those of previous oximeters. It was not possible to generate artefactual dips in excess of 2% Sao2, and the dual overnight recordings rarely showed even small dips on one tracing alone. The stored data can recreate oscillating Sao2 signals with wavelengths down to about 35 seconds, but not below. The Ohmeda 3700 pulse oximeter appears to be suitable for unattended overnight recordings of Sao2.

Evaluation Studies as Topic↗
PubMed1987

Training effects on regional blood flow response to maximal exercise in foxhounds.

The effect of training on the regional blood flow response to maximal exercise was investigated in the foxhound. Training consisted of 8-12 wk of treadmill running at 80% of maximal heart rate 1 h/day for 5 days/wk and resulted in a 31% increase in maximal O2 consumption, a 28% increase in maximal cardiac output, and a 23% decrease in systemic vascular resistance during maximal exercise. Blood flow to the heart, diaphragm, brain, skin, and 9 of 10 muscles investigated was similar during maximal exercise pre- and posttraining; however, blood flow to the gastrocnemius muscle was greater posttraining than it was pretraining. Blood flow to the stomach, small intestine, and pancreas decreased during maximal exercise pre- and posttraining; however, blood flow to the large intestine, spleen, liver, adrenal glands, and kidneys decreased during maximal exercise only posttraining. In addition, a larger decrease in blood flow to the stomach during maximal exercise was found posttraining compared with pretraining. These results demonstrate that blood flow to skeletal muscle, the kidneys, and the splanchnic region of the foxhound during maximal exercise can be significantly altered by dynamic exercise training.

Animals↗
PubMed1987

Regional distribution of blood flow of dogs during graded dynamic exercise.

The regional blood flow response to progressive treadmill exercise was measured with radioactive microspheres in 25 untrained mongrel dogs. Incremental increases in work intensity resulted in corresponding increases in blood flows to the gracilis, gastrocnemius, semimembranosus, and semitendinosus muscles of the hindlimb and to the heart. During maximal exercise, blood flow was greatest in the semimembranosus muscle and lowest in the semitendinosus muscle (342 and 134 ml-1.100 g tissue-1.min-1, respectively). Exercise produced a decrease in blood flow to the temporalis muscle, which was classified as nonlocomotive in function. Blood flows to the stomach, pancreas, and large intestine decreased at the lowest exercise work load and remained diminished throughout the continuum to maximal exercise. Blood flows to the small intestine and spleen were maintained during submaximal exercise but were reduced by 50% at maximal O2 consumption (VO2max). No changes in blood flows to the kidneys, adrenal glands, liver, and brain were found. These results demonstrate that 1) renal blood flow is maintained at resting levels during exercise in untrained dogs; 2) blood flow changes in the various organs of the splanchnic region of dogs during exercise are heterogeneous; and 3) blood flows to the working skeletal muscles of dogs progressively increase with increasing work loads up to VO2max.

Animals↗
PubMed1987

Cardiovascular and risk factor evaluation of healthy American adults. A statement for physicians by an Ad Hoc Committee appointed by the Steering Committee, American Heart Association.

Cardiovascular disease is the major cause of death in American adults. The chief form of cardiovascular disease is coronary heart disease (CHD). Prevention of CHD depends on the identification of risk factors in asymptomatic individuals. The American Heart Association recommends that all adults be examined periodically for the presence of silent cardiovascular disease and coronary risk factors. The major risk factors for CHD are smoking, high blood pressure, and high blood cholesterol. Additional factors associated with CHD are high blood triglycerides, reduced levels of high-density lipoproteins, diabetes mellitus, obesity, sedentary lifestyle, and certain behavioral characteristics. Available data suggest that the predominance of CHD among Americans can be attributed to these risk factors, and increasing evidence indicates that appropriate modification of these factors will markedly reduce coronary risk. The purpose of this report is to identify the risk factors, indicate their relation to coronary disease, and recommend an approach to their detection in adults during periodic health examinations.

Adult↗
PubMed1986

Regional blood flow responses to stimulation of the subthalamic locomotor region.

Recent studies have suggested that descending central command from an area in the diencephalon (subthalamic locomotor region - STLR) is involved in the control of ventilation, arterial pressure and heart rate during exercise. The purpose of this study was to determine if electrical activation of the STLR in anesthetized cats elicits changes in regional blood flow and vascular resistances similar to those evoked by exercise. Therefore, organ blood flows (radioactive microsphere technique), arterial pressure (AP), heart rate (HR), and respiratory output (quantified from phrenic nerve activity) were recorded during resting conditions and during STLR stimulation. Stimulation of the subthalamic locomotor region produced increases in AP, HR and respiratory output similar to those reported previously. These changes were accompanied by increased blood flow to the heart, diaphragm and limb skeletal muscles. A concomitant decrease occurred in blood flow to the kidneys. In addition, the vascular resistances of the intestines, gallbladder and stomach increased. These vascular and respiratory responses are similar to those occurring during static exercise in conscious cats.

Animals↗
PubMed1986

Effects of alpha-adrenergic blockade on cardiovascular responses to static exercise in cats.

Static exercise performed by conscious cats elicits increases in heart rate (HR), left ventricular systolic pressure (LVSP), and the maximal rate of left ventricular pressure development [LV(dP/dt)max]. The increased HR is mediated primarily by withdrawal of parasympathetic tone, whereas a beta-adrenergic mechanism is responsible for the LV(dP/dt)max increase. In the present study the cardiovascular responses to static exercise in awake cats was recorded before and after alpha-adrenergic blockade. Pressure transducers were implanted into the left ventricle of cats who had been trained operantly to perform static exercise. Significant increases in LVSP, LV(dP/dt)max and HR occurred in all cats during static exercise before blockade. In contrast, alpha-adrenergic blockade (phentolamine, 2.5 mg/kg iv) abolished the exercise-induced increase in LVSP but did not prevent increases in HR and LV(dP/dt)max. The cats performed fewer exercise bouts per day during alpha-blockade than when unblocked. We conclude that an alpha-adrenergic mechanism mediates the increase in LVSP in response to static exercise in conscious cats.

Adrenergic alpha-Agonists↗
PubMed1986

The effect of pericardiectomy on maximal oxygen consumption and maximal cardiac output in untrained dogs.

To test the hypothesis that the pericardium limits maximal oxygen consumption by limiting stroke volume and cardiac output, we studied 10 untrained dogs during submaximal and maximal exercise before and after pericardiectomy. Seven additional dogs were studied before and after a sham operation. All dogs were instrumented chronically with aortic and pulmonary artery catheters. Dogs were tested by running on a motor-driven treadmill, 4-6 times before and after pericardiectomy or sham operation. We measured cardiac output (dye dilution), heart rate, and arteriovenous oxygen difference. Oxygen consumption and stroke volume were calculated from these variables. After pericardiectomy, there were significant (P less than 0.01) increases in maximal oxygen consumption, maximal cardiac output, and maximal stroke volume. Maximal oxygen consumption decreased significantly in the sham group. There was no change in maximal heart rate following pericardiectomy, or in maximal cardiac output, heart rate, or stroke volume following sham operation. Both groups of dogs experienced similar significant decreases in hematocrit, arterial and venous oxygen contents, and arteriovenous oxygen difference. Neither pericardiectomy nor sham operation had any effect on oxygen consumption during submaximal exercise. However, the sham group had significant increases in cardiac output and heart rate during submaximal exercise, and the pericardiectomy group demonstrated a trend toward an increased cardiac output during submaximal exercise. These results support the hypothesis that the pericardium limits maximal oxygen consumption by limiting stroke volume and cardiac output during maximal exercise in untrained dogs. Further, these findings suggest that maximal oxygen consumption is limited by the oxygen transport capacity of the cardiovascular system, and not by the oxidative capacity of skeletal muscle in the untrained dog.

Animals↗
PubMed1985

Ventricular performance in human hearts aged 61 to 73 years.

The effects of increasing and decreasing cardiac preload by 15% on the left ventricular (LV) performance of 11 carefully screened normal subjects aged 61 to 73 years were examined. Comparisons were made with 11 subjects aged 21 to 28 years. Two-dimensional echocardiograms were obtained before and at the termination of 5 degrees of head-down tilt for 90 minutes and at the termination of graded lower body negative pressure to -40 mm Hg. Heart rates and blood pressures were unchanged after physiologic interventions. Changes in LV end-diastolic and stroke volumes were similar but of a smaller magnitude in the older subjects compared with changes in younger subjects. When LV end-diastolic volumes obtained at each extreme of preload variation were compared, the range of mean change was less in the older (23 ml, 26%) than in the younger subjects (31 ml, 41%). Control LV end-diastolic and end-systolic volumes were greater in the older subjects. This study shows that despite larger control LV volumes, alterations in preload produce changes in the LV end-diastolic and stroke volumes of these older subjects that conform to the normal LV function curve, but that these responses are diminished compared with changes in younger subjects, suggesting an age-related change in diastolic stiffness.

Age Factors↗
PubMed1985

The effect of cardiac autonomic blockade on ventricular response to changes in preload.

Echocardiographic (E) assessment of left ventricular response to increases (I) and decreases (D) in preload of 9 volunteer male subjects (mean age 25 +/- 2.0 years) was made during both control (C) and cardiac blocked (B) conditions. Cardiac autonomic blockade was produced by intravenous administration of atropine (0.04 mg/kg) and propranolol (0.2 mg/kg), while I was produced by 5 degrees head-down tilt (T) for 90 min and D by lower body negative pressure to -40 Torr. Increases in resting heart rate of 44% and diastolic blood pressure of 13% occurred after B, (P less than 0.05). During C, the alterations in preload produced mean changes in end-diastolic volume (EDV) ranging from 135 +/- 10 cm3 for I to 96 +/- 9.1 cm3 for D. Changes in stroke volume during condition C were significantly related to changes in EDV during preload alterations and conformed to a normal LV function curve and were described by SV = 0.6 EDV + 4.5 cm3 (r = 0.85; P less than 0.001). Similarly during B, SV = 0.43 EDV + 5.8 cm3 (r = 0.75; P less than 0.001). Comparison of the LV function curves produced during C and B showed that both slope of regression and r were significantly different (P less than 0.01). These data indicate that cardiac autonomic blockade produces a downward shift in the LV function curve indicating a depression in myocardial contractility. In addition, the data indicate that changes observed in LV function during wide variations in preload are independent of autonomic nervous system influences and are independent of the intrinsic heart rate.

Adult↗
PubMed1985

Partial neuromuscular blockade and cardiovascular responses to static exercise in man.

In human subjects sustained static contractions of the quadriceps femoris in one leg were performed with the same absolute and the same relative intensity before and after partial neuromuscular blockade with either decamethonium or tubocurarine which reduced strength to about 50% of the control value. During the contractions performed with the same absolute force, the magnitude of the cardiovascular responses (heart rate and blood pressure) was greater during neuromuscular blockade than during control contractions. During the contractions involving the same relative force the magnitude of the cardiovascular responses was almost the same with and without neuromuscular blockade. These findings were independent of the drug used. The metabolic part of the exercise pressor reflex was assessed by the application of an arterial cuff 1/2 min before cessation of exercise and for the following 3 min of rest. Although heart rate and blood pressure decreased after cessation of exercise, application of the tourniquet resulted in higher post-exercise values and this effect was seen both with and without neuromuscular blockade. Muscle biopsies from the subjects' m. vastus lateralis were analysed for fast- and slow-twitch fibre composition showing 27-66% slow-twitch fibres. No correlation was found between cardiovascular responses to static exercise, with or without neuromuscular blockade, and fibre type predominance. The results suggest that the involvement of fast- or slow-twitch muscle fibres does not play a dominant role in the cardiovascular responses to static exercise in man. Both central command and reflex neural mechanisms are of importance, and it appears that these two control mechanisms are redundant and that neural occlusion may be operative. However, when partial neuromuscular blockade induces a disproportion between an increase in central command and a constant or decreasing muscle tension and metabolism, the larger signal arising from central command determines the magnitude of the cardiovascular responses.

Adult↗
PubMed1985

Effects of barodenervation on cardiovascular responses to static muscular contraction.

The purpose of this study was to measure blood flow to various tissues during static muscular contraction in anesthetized cats and to evaluate if the baroreflex modulates the cardiovascular responses to muscular contraction. Contraction of the hindlimb muscles induced by ventral root stimulation caused increases in arterial pressure (delta 37.8 +/- 5.5 mmHg) and heart rate (delta 13.9 +/- 3.1 beats/min). Increases in blood flow to the heart, working skeletal muscles, and selected areas of the central nervous system occurred during muscular contraction. Blood flow to visceral organs did not change during muscular contraction. Baroreceptor-denervated cats showed a greater rise in arterial pressure (delta 55.5 +/- 5.5 mmHg) during muscular contraction than did the baroreceptor-intact cats. However, blood flow responses were similar in both groups. Thus the baroreceptor reflex modulates the pressor response without changing the alteration in blood flow during induced muscular contraction in anesthetized cats.

Anesthesia, General↗
PubMed1985

Exercise, dobutamine, and combined atropine, norepinephrine, and epinephrine compared.

We compared the cardiovascular effects evoked in conscious dogs by 1) submaximal exercise; 2) infusion of dobutamine (40 micrograms X kg-1 X min-1); and 3) infusion of a combination of atropine (0.15 mg/kg), norepinephrine (0.19 micrograms X kg-1 X min-1), and epinephrine (0.05 micrograms X kg-1 X min-1). Myocardial O2 demand, as estimated by the double product (heart rate X systolic blood pressure), was similar during all three interventions. Cardiac output and heart rate increased significantly (P less than 0.05) during each of the three interventions. Arteriovenous O2 difference and total body O2 consumption, however, increased only during submaximal exercise. Although myocardial blood flow increased similarly during each of the three interventions, blood flow to skeletal muscle and the tongue increased only during exercise. Exercise and the combined infusion of atropine, norepinephrine, and epinephrine produced similar increases in blood flow to the diaphragm and similar decreases in blood flow to the stomach. These changes in blood flow were associated with appropriate changes in vascular resistance. Additionally, blood flow to the brain, kidney, adrenal glands, liver, and intestine did not change during any of the three interventions. Thus, in dogs, submaximal exercise, infusion of dobutamine, and infusion of a combination of atropine, norepinephrine, and epinephrine to evoke a given level of estimated myocardial O2 consumption produce similar increases in cardiac output, heart rate, and myocardial blood flow. In contrast, the changes in total body O2 consumption, arteriovenous O2 difference, regional blood flow, and regional vascular resistance that occur during each of these three interventions are different.

Animals↗