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Biomedical subjects

J Keegan

Publications and source records attributed to J Keegan.

At least 55 records · Page 3Linked to original sources
PubMed1993

Magnetic resonance imaging of coronary arteries: technique and preliminary results.

BACKGROUND: Coronary artery imaging is an important investigation for the management of coronary artery disease. The only reliable technique presently available, x ray contrast angiography, is invasive and is associated with a small morbidity and mortality. Alternative non-invasive imaging would be useful, but the small calibre and tortuosity of the coronary vessels, and cardiac and respiratory motion create formidable imaging problems. OBJECTIVE: The development of rapid magnetic resonance imaging of the coronary arteries. PATIENTS: 21 healthy controls and five patients with coronary artery disease established by x ray contrast angiography, of whom two had undergone bypass grafting. METHODS: Magnetic resonance imaging was performed with gradient echoes and a segmented k-space technique, such that a complete image was acquired in 16 cardiac cycles during a breathhold. The signal from fat was suppressed and images were acquired in late diastole to reduce artefact from cardiac motion. An imaging strategy was developed for the proximal arteries, including longitudinal imaging from oblique planes defined according to the origins and the continuation of the arteries in the atrioventricular grooves or interventricular sulcus. RESULTS: Of the 26 subjects studied, 22 were imaged successfully. Identification of the artery was possible for the left main stem, left anterior descending, right coronary, and left circumflex arteries respectively in 95%, 91%, 95%, and 76%. The arterial diameter at the origin could be measured in 77%, 77%, 81%, and 63%. The mean (SD) arterial diameter in each case (4.8 (0.8), 3.7 (0.5), 3.9 (0.9), and 2.9 (0.6) mm) was not significantly different from reference values. The mean length of artery visualised was 10.4 (5.2), 46.7 (22.8), 53.7 (27.9), and 26.3 (17.5) mm. In 12 healthy men the total coronary area was 30.9 (9.2) mm2 and the ratio compared with body surface area was 16.4 (4.4) mm2m2 (both p = NS compared with reference values). In seven patients in whom x ray contrast coronary angiography was available, the proximal arterial diameter was 3.9 (1.1) mm measured by magnetic resonance and 3.7 (1.0) mm by x ray contrast angiography (p = NS). The mean difference between the measurements was 0.2 (0.5) mm, and the coefficient of variation was 13.7%. All five occluded coronary arteries were identified, as were all three vein grafts. In two patients insertion of the graft into the native arteries was identified. CONCLUSIONS: Magnetic resonance coronary angiography is feasible. Good results were obtained by a breath-hold, fat suppression technique, gated to late diastole. Arterial occlusions and vein grafts were readily identified. Further studies are required to establish its value in the detection of coronary stenosis and to develop the measurement of coronary flow velocity which could be used to quantify the severity of the stenosis.

Coronary Angiography↗
PubMed1992

Chromatographic analysis of cis- and trans-mevinphos in poisoned wildlife.

A method is described for the analysis of the cis- and trans-isomers of mevinphos in baits and avian tissues. The procedure involves extraction of the mevinphos isomers with acetone-dichloromethane (1:1) followed by clean-up on alumina. Bait samples were initially screened by thin-layer chromatography and identity of extracts confirmed by infrared spectrophotometry and gas chromatography-mass spectrometry.

Animals↗
PubMed1992

Effects of coronary artery bypass surgery and angioplasty on the total ischemic burden: a study of exercise testing and ambulatory ST segment monitoring.

To assess the effects of standard therapeutic interventions on the total ischemic burden, 86 patients with stable angina underwent 48 hours of ambulatory ST segment monitoring and treadmill exercise testing before and at a mean of 10 weeks after coronary artery bypass surgery (CABG) (group 1, N = 46) or percutaneous transluminal coronary angioplasty (PTCA) (group 2, N = 40). There were 72 male and 14 female patients with a mean age of 56.4 years. All patients had documented coronary artery disease (24, single-vessel; 28, two-vessel; 34, three-vessel disease). Both groups were characteristically similar apart from more severe coronary artery disease (p less than 0.001) and more previous myocardial infarctions (p less than 0.05) in group 1. Groups with CABG and PTCA had significant prolongation of exercise time after intervention (group 1: 7.6 to 9.8 minutes, p less than 0.0001; group 2: 8.1 to 10.0 minutes, p less than 0.001), and both interventions led to a significant reduction in ischemic responses (group 1: 33 to 4, p less than 0.001; group 2: 20 to 13, p less than 0.05) to exercise. During a total of 7643 hours of ST segment monitoring, 253 episodes of ischemia were recorded in 3768 hours before and 44 ischemic episodes in 3875 hours after intervention (group 1, 113 episodes in 24 patients and 21 episodes in 10 patients; group 2, 140 episodes in 13 patients and 23 episodes in six patients). Both interventions reduced the mean frequency of ischemia per 24 hours (group 1: 1.24 to 0.22 episodes per 24 hours; p less than 0.01; group 2: 1.9 to 0.3 episodes per 24 hours; p less than 0.05). Almost 28% (N = 24) of resting electrocardiographic findings were altered as a result of intervention.(ABSTRACT TRUNCATED AT 250 WORDS)

Activities of Daily Living↗
PubMed1992

Predictors of prognosis in severe chronic heart failure.

A total of 127 patients with chronic heart failure referred to our exercise laboratory were studied retrospectively to identify parameters predictive of prognosis. Patients were followed for a mean of 14.6 months. The group as a whole had severe ventricular dysfunction with a median ejection fraction of 17% and a median peak rate of oxygen consumption of 13.7 ml/kg/min. During the follow-up period 23 patients (18%) died and 18 (14%) underwent cardiac transplantation. The effect of the following variables on outcome (death or transplantation) were examined: age, cause of heart failure, cardiothoracic ratio on chest radiography, left ventricular end-systolic dimension on echocardiography, left ventricular ejection fraction on radionuclide ventriculography, mean dose of diuretic, plasma sodium and urea concentrations, and peak oxygen consumption during exercise. Although all variables except cause of heart failure affected outcome on univariate analysis, multivariate analysis identified three variables that were statistically significant and independent predictors of outcome. In order of importance these were plasma sodium level, left ventricular ejection fraction and peak oxygen consumption. Even in this group of patients with severe heart failure, these variables were predictive of outcome.

Exercise Test↗
PubMed1992

Reduced pulmonary microvascular permeability in severe chronic left heart failure.

Pulmonary edema is a serious complication of heart failure, but often patients with chronic heart failure resist pulmonary edema despite elevated pulmonary venous pressures. This protection might be a result of decreased pulmonary microvascular permeability. Double-isotope scintigraphy with 113mindium-labeled transferrin and 99mtechnetium-labeled erythrocytes allows noninvasive estimation of pulmonary microvascular permeability; an index of transferrin accumulation is calculated that reflects microvascular permeability. Fourteen patients with severe chronic left ventricular dysfunction were compared with a control group of 15 patients with mild coronary artery disease. In the control group the transferrin accumulation index was 0.35 (range -0.3 to 1.0) x 10(-3)/min, and in patients with heart failure the index was 0.0 (range -1.0 to 0.7) x 10(-3)/min, which was significantly lower (p less than 0.01). The reduction in the transferrin accumulation index correlated weakly with the duration of heart failure (R = -0.5, p less than 0.02). These data indicate reduced protein efflux consistent with a decrease in pulmonary microvascular permeability in patients with severe chronic heart failure. Similar changes have been observed in severe mitral stenosis and may reflect a generalized adaptation to chronic pulmonary venous hypertension.

Aged↗
PubMed1991

The influence of right heart catheterisation on pulmonary arterial pressure in chronic heart failure: relationship to neuroendocrinal changes.

Continuous ambulatory measurement of pulmonary arterial pressure was used to investigate changes following right heart catheterisation in patients with chronic heart failure. Ten males, mean age 56 years, with chronic heart failure, underwent 24 hour pressure recording using a micromanometer tipped catheter with in vivo calibration and frequency modulated recording. Eight patients were taking diuretics and 3 vasodilators. Blood was drawn for catecholamines, plasma renin activity and atrial natriuretic peptide 1 hour before catheterisation (-1 h), at the time of catheterisation (0 h) and 1, 2, 3, 4 and 6 hours later and aldosterone, cortisol and growth hormone at -1, 0 and 6 hours. Analysis of variance was used to determine changes in pulmonary arterial pressure, heart rate and hormones from the time of catheterisation in lying, sitting and standing postures. There was no significant change in pulmonary arterial pressure or heart rate over the 12 hours following or 24 hours after catheterisation in any posture. In the majority of patients plasma noradrenaline, plasma renin activity, atrial natriuretic peptide, aldosterone and cortisol were elevated. There was no significant change in hormone levels during the 6 hours following catheterisation. These findings suggest that the effect of invasive haemodynamic monitoring and chronic medical therapy on central haemodynamics is minor, and that a delay between insertion of catheters and measurement of pressure is unnecessary.

Adult↗
PubMed1991

Therapeutic implications of dynamic coronary stenosis in patients with single vessel coronary artery disease.

Ten patients with proven single-vessel coronary artery disease and a positive exercise test for ischaemia were investigated to establish the importance and therapeutic implications of dynamic coronary stenosis in such patients. All patients interrupted their anti-anginal therapy and under took serial exercise testing in an attempt to identify variability in the ischaemic threshold. Ergonovine testing was performed in nine patients and all underwent 48 h of ambulatory ST segment monitoring while treatment was discontinued. Patients then entered a randomized double-blind study of atenolol and nifedipine; treadmill exercise testing and 48 h of ambulatory ST segment monitoring were performed at the end of each treatment phase. Six (60%) patients showed evidence of variability in coronary vasomotor tone four of whom developed significant ST segment changes during administration of ergonovine; a further two had greater than 30% variability in time to onset of ischaemia during serial treadmill exercise testing. Atenolol significantly increased the time to ischaemia on exercise testing, both in the group as a whole and in the subgroup with evidence of altered vasomotor tone when compared with no therapy, and led to a non-significant reduction in the frequency and duration of ischaemia during the patients' daily lives. Nifedipine, conversely, did not significantly increase time to ischaemia on exercise testing or reduce the frequency or duration of ambulatory ischaemia in either the whole group or the subgroup. With evidence of altered vasomotor tone when compared to no therapy however it was beneficial in terms of reduction in chest pain and requirement for glyceryl trinitrate during daily life.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1991

Pulmonary microvascular permeability in patients with severe mitral stenosis.

Patients with rheumatic mitral stenosis often have no pulmonary oedema despite considerably increased pulmonary venous pressure. Pulmonary microvascular permeability was measured non-invasively by a previously validated method of double isotope scintigraphy with indium-113m and technetium-99m. This permits calculation of an index reflecting transferrin efflux and thus, indirectly, the microvascular permeability. Fifteen patients with severe mitral stenosis (defined as valve area less than 1.0 cm2) were compared with a control group of 11 patients with mild coronary artery disease. The permeability index was significantly lower in patients with mitral stenosis than in the control group. Furthermore, the extent of reduction of the permeability index correlated with the severity of mitral stenosis as reflected by the Gorlin valve area. This finding may account for the relative resistance of these patients to pulmonary oedema despite chronic pulmonary venous hypertension.

Adult↗
PubMed1991

Airborne infection. Theoretical limits of protection achievable by building ventilation.

Of 67 office workers 27 (40%) had documented tuberculin skin test conversions after an estimated 4-wk exposure to a coworker with cavitary tuberculosis. Worker complaints for more than 2 yr before the tuberculosis exposure prompted investigations of air quality in the building before and after the tuberculosis exposure. Carbon dioxide concentrations in many parts of the building were found to be above recommended levels, indicating suboptimal ventilation with outdoor air. We applied a mathematical model of airborne transmission to the data to assess the role of building ventilation and other transmission factors. We estimated that ventilation with outside air averaged about 15 feet 3/min (cfm) per occupant, the low end of acceptable ventilation, corresponding to CO2 levels of about 1,000 ppm. The model predicted that at 25 cfm per person 18 workers would have been infected (a 33% reduction) and at 35 cfm, a level considered optimal for comfort, that 13 workers would have been infected (an additional 19% reduction). Further increases in outdoor air ventilation would be impractical and would have resulted in progressively smaller increments in protection. According to the model, the index case added approximately 13 infectious doses (quanta) per hour (qph) to the office air during the exposure period, 10 times the average infectiousness reported in a large series of tuberculosis cases. Further modeling predicted that as infectiousness rises, ventilation would offer progressively less protection. We conclude that outdoor air ventilation that is inadequate for comfort may contribute to airborne infection but that the protection afforded to building occupants by ventilation above comfort levels may be inherently limited, especially when the level of exposure to infection is high.

Adult↗
PubMed1991

Analysis of tissues of ducks poisoned by alpha-chloralose.

A method is described for the analysis of alpha-chloralose in avian tissues. The procedure involves extraction of chloralose with acetone, drying of extracts with anyhydrous magnesium sulphate, followed by derivatisation with dichlorodimethylsilane in the presence of triethylamine. The dimethylsilyl derivatives were cleaned-up using mini-columns containing Florisil and the eluates subsequently analysed by gas chromatography. Extracts were initially screened by thin-layer chromatography and the identity of derivatised extracts confirmed by gas chromatography/mass spectrometry.

Animals↗
PubMed1990

Characteristics of silent and painful ischaemia during ambulatory monitoring in patients with coronary arterial disease.

We compared the characteristics of silent and painful ischaemia during ambulatory ST segment monitoring in 288 patients with documented coronary arterial disease and stable angina. During 12,436 hours of monitoring, 890 ischaemic episodes were recorded, of which 652 (73%) were silent. Silent and painful ischaemic episodes were similar in terms of heart rate at onset of ischaemia, increase in heart rate prior to ischaemia, duration of ischaemia, and percentage of episodes not preceded by an increase in heart rate. Change in the mean maximal ST segment was greater during painful ischaemic episodes (P less than 0.01). Silent ischaemia is characteristically painful ischaemia without the pain.

Adult↗
PubMed1990

Pulmonary artery pressure changes during exercise and daily activities in chronic heart failure.

Long-term continuous pulmonary artery pressure monitoring was used to investigate pressure changes during different types of exercise and normal daily activities in patients with chronic heart failure. Nine men (mean age 55 years) with treated chronic heart failure underwent continuous pulmonary artery pressure measurement with use of a micromanometer-tipped catheter with in vivo calibration and frequency-modulated recording. The mean (+/- SD) maximal systolic pulmonary artery pressure (in mm Hg) was 59.4 +/- 26.1 on treadmill exercise, 54.9 +/- 30.6 on bicycle exercise, 52.5 +/- 26.1 walking up and down stairs and 43.5 +/- 23.9 walking on a flat surface. The mean maximal diastolic pressure (in mm Hg) was 27.8 +/- 14.6 on treadmill exercise, 25.5 +/- 14.9 on bicycle exercise, 24.9 +/- 14.8 walking up and down stairs and 20.4 +/- 12.5 walking on a flat surface. The increase in pulmonary artery pressure did not correlate with the severity of the limiting symptoms except during walking on a flat surface. All patients had marked postural changes in pressure, with the systolic pressure difference from lying to standing ranging from 8 to 25 mm Hg and the diastolic pressure difference ranging from 3 to 13 mm Hg. Eating meals caused an increase in pressure in three patients, but less than that when lying flat. There was an increase in pressure during urination in four patients equal to that when walking on a flat surface. None of these activities was associated with symptoms. Neither symptoms nor pulmonary artery pressure during maximal exercise is the same as during daily activities. This may restrict the value of maximal exercise tests in assessing patients with chronic heart failure.

Activities of Daily Living↗
PubMed1990

Circadian variation of heart rate is affected by environment: a study of continuous electrocardiographic monitoring in members of a symphony orchestra.

Twenty four hour ambulatory ST segment monitoring was performed on 48 members (43 players and five members of the management/technical team) of the British Broadcasting Corporation (BBC) symphony orchestra without a history of cardiac disease. This period included final rehearsals and live performances (for audience and radio) of music by Richard Strauss and Mozart at the Royal Festival Hall (n = 36) and Rachmaninov and Tchaikovsky at the Barbican Arts Centre (n = 21). During the period of monitoring one person (2%) had transient ST segment changes. Mean heart rates were significantly higher during the live performances than during the rehearsals. Mean heart rates during the live performance of Rachmaninov and Tchaikovsky were significantly higher than during Strauss and Mozart in those (n = 6) who were monitored on both occasions. Mean heart rates in the management and technical team were higher than those of the players. The recognised circadian pattern of heart rate, with a peak in the morning waking hours, was altered similarly during both concert days, with a primary peak occurring in the evening hours and a lesser peak in the morning for both musicians and management/technical staff. This study showed that environmental factors are of primary importance in defining the circadian pattern of heart rate. This has important implications when identifying peak periods of cardiovascular stress and tailoring drug treatment for patients with angina pectoris.

Adult↗
PubMed1990

Changes in pulmonary microvascular permeability accompanying re-expansion oedema: evidence from dual isotope scintigraphy.

The pathophysiological mechanism of pulmonary oedema following rapid re-expansion of a collapsed lung is poorly understood. It has been suggested that the period of collapse or subsequent reinflation produces an increase in pulmonary microvascular permeability. To investigate this, the pulmonary accumulation of the plasma protein transferrin was measured by radiolabelling it in vivo with indium-113m. Plasma protein accumulation was calculated after correcting the accumulation of transferrin for changes in intrathoracic blood distribution by simultaneously monitoring technetium-99m labelled red blood cells. Functional images of plasma protein accumulation were constructed for the lung fields on a pixel by pixel basis. Investigations were performed on 14 subjects after drainage of a pleural effusion (n = 9) or evacuation of a pneumothorax (n = 5), and on 11 control subjects. Plasma protein accumulation was greater over the regions of lung re-expansion (-0.1-9.6, mean 2.9 x 10(-3)/min) than over the corresponding region of the contralateral lung (-1.2-0.8, mean 0.01 x 10(-3)/min; p less than 0.001). Patients who had undergone re-expansion procedures also had significantly greater plasma protein accumulation than normal controls. Nine of the 14 patients in the re-expansion group had clearly identifiable areas of increased plasma protein accumulation that corresponded to the part of the lung that had been re-expanded; no regional abnormalities were recorded in the control group. These results suggest that the reinflated lung displays abnormal microvascular permeability.

Adult↗
PubMed1989

Ischemia in the ambulatory setting--the total ischemic burden: relation to exercise testing and investigative and therapeutic implications.

To establish the relation between treadmill exercise testing and ambulatory St segment monitoring in the detection of ischemia in patients with coronary artery disease, and to assess whether standard medical therapy affects any such relation, 277 patients with stable angina and angiographically documented coronary artery disease were studied with treadmill exercise testing and 48 h ambulatory ST segment monitoring. One hundred forty-six patients (52%) were studied while receiving no routine antianginal therapy, and 131 (48%) while receiving standard medical therapy. In 187 patients (67%) the exercise test was positive for ischemia. During 11,964 h of ambulatory monitoring, 881 episodes of ischemia (645 [73%] silent) were recorded, of which 809 (92%) occurred in patients with a positive exercise test. The mean heart rate at the onset of ischemic episodes during ambulatory monitoring was significantly less than that at the onset of 1 mm ST segment depression during exercise testing (94.5 versus 105.9 beats/min, p less than 0.0001). However, the frequency of ambulatory ischemic episodes was strongly related to a positive exercise test (p less than 0.001), and this relation was similar for both silent and painful ischemia (p less than 0.0001 for both) and in patients who were and were not receiving therapy (p less than 0.0001 for both). The total duration of ischemia was similarly related to a positive exercise test (p less than 0.0001). Only one patient with a negative exercise test had frequent (greater than 5/day) episodes of ischemia on ambulatory monitoring and had documented coronary artery spasm. Thus, exercise testing identifies the majority of patients likely to have significant ischemia during their daily activities.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1989

Circadian patterns of myocardial ischemia.

In this overview we studied evidence for the occurrence of silent myocardial ischemia, its pathophysiologic mechanisms, and the circadian distribution of ischemic episodes. The correlation between many hemodynamic parameters and their potential as drug targets was assessed. ECG evidence of myocardial ischemia (ST segment), a parameter often used in exercise tests, also occurs during ambulatory monitoring. Use of the isotope rubidium-82 and positron emission tomography helped confirm the significance of these changes in the ST segment, both painful and silent. Ischemia was detected by a reduction in the uptake of the isotope in a region of the heart reflecting a decrease in myocardial blood flow. Also it was shown that both painful and silent ischemia are caused by similar pathophysiologic mechanisms; such mechanisms include alterations in coronary blood flow and increases in myocardial oxygen demand. Episodes of ST segment depression show a circadian rhythm similar to that seen for heart rate and catecholamine release. The basic pattern shows a low incidence of ischemia at night, followed by a primary peak on rising, a decrease at midday, and then a secondary peak in the evening, with a final decrease again at night. Similar circadian rhythms have been described for acute cardiovascular events, and this may be important for treatment.

Angina Pectoris↗