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Biomedical subjects

R I Stewart

Publications and source records attributed to R I Stewart.

24 records · Page 2Linked to original sources
PubMed1985

Exercise in patients with chronic obstructive pulmonary disease.

Patients with chronic obstructive pulmonary disease (COPD) may incur exercise limitation by any one or combination of disturbances in breathing mechanics, oxygen transport, respiratory muscle metabolism or respiratory regulation and sensation. In spite of the increased ventilation demand/capacity ratio in these patients, the relationship between breathing mechanics, respiratory muscle fatigue, the adequacy of alveolar ventilation and the development of exertional dyspnoea is neither clearly defined nor predictable from data obtained with the patient at rest. The issue of oxygen transport during exercise has been complicated by confusion between arterial hypoxia and inadequate volume of oxygen transported to the tissues, which frequently may differ qualitatively and quantitatively. The cardiac output response to exercise in patients with COPD is therefore critical in determining oxygen transport. This response is also impossible to predict from resting lung mechanics, pulmonary arterial blood pressure, arterial oxygen tension or clinical disease profile. Without exercise testing, which includes measurement of all the variables mentioned, it is impossible to define clearly the cause of exercise-induced symptoms in patients with COPD. Exercise training with and without supplemental oxygen has been shown to improve exercise tolerance in these patients, but the precise mechanism of this improvement remains obscure.

Hemodynamics↗
PubMed1985

Venous crossover PO2. A reference point for assessing the physiological significance of alterations in oxyhaemoglobin equilibrium.

A method is described for the quantitative analysis of the physiological significance of either an increase or decrease in P50. The venous crossover PO2 (PvxO2) refers to that PO2 of the mixed venous blood on the standard oxyhaemoglobin equilibrium (OHE) curve at which the extraction of oxygen from the blood is the same for both the standard and the in vivo arterial and venous OHE conditions. A measured PvO2 which is greater than the PvxO2 indicates that the in vivo OHE conditions conferred a benefit over the standard conditions inasmuch as the same volume of oxygen was delivered to the tissues at a higher venous PO2. The method of determining the venous crossover PO2 from the measured arteriovenous difference in haemoglobin saturation of oxygen is described using a mathematical model of the oxyhaemoglobin equilibrium curve. The presence of carboxyhaemoglobin is accounted for in this analysis. Clinical examples in which the method is applied are presented.

Humans↗
PubMed1983

The reliability of the carbon dioxide-rebreathing, indirect Fick method of cardiac output determination in patients with pulmonary disease.

1. The CO2-rebreathing indirect Fick method of cardiac-output measurement was compared with the standard invasive direct Fick method for oxygen during rest and treadmill exercise in 15 patients with pulmonary disease characterized by the common feature of expiratory-airflow limitation. 2. The correlation coefficient for all 37 determinations was 0.94 (P less than 0.01); in patients who had a Pa-ETCO2 (arterial end-tidal PCO2 gradient) of greater than 5 mmHg, the coefficient was 0.69 (P less than 0.05), and in the remainder, with a normal Pa-ETCO2, 0.98 (P less than 0.01). 3. The difference in correlation between these two latter groups is statistically significant. 4. It is concluded that the indirect method is valid in most patients with pulmonary disease, but that the correlation with the direct Fick method is significantly worse when the Pa-ETCO2 exceeds 5 mmHg.

Carbon Dioxide↗
PubMed1981

[A non-invasive approach to pulmonary hemodynamics].

Currently available investigative methods employed in the evaluation of the pulmonary circulation involve cardiac catheterization and, in respect of exercise determinations, only supine data have been recorded. Consequent upon a 3-year study of pulmonary haemodynamics and gas exchange at rest and during exercise on the treadmill in patients with chronic hypoxaemic lung disease, correlations have been established which accurately characterize the venous admixture (QVA/Qt, the quantitative expression of hypoxaemia), cardiac output and pulmonary artery pressure (PA), permitting the prediction of these variables in the erect posture. The employment of fibre-optic oximetry obviates the need for arterial cannulation. The relevant regression formulae are as follows: QVA/Qt = 8,85 x ln (Hb (100 - SaO2) x heart rate/VO2) - 3,66 (r = 0,97; SEE = 2,43) PAm (resting) = 0,4516 (QVA/Qt) + 8,2 (r = 0,90; SEE = 2,7) PAm (exercise) = 1,453 (QVA/Qt) + 3,8 (r = 0,96; SEE = 4,0) Cardiac output may be derived directly from VO2 and the known components of QVA/Qt, and total pulmonary vascular resistance in turn from Qt and PAm. Direct comparison of the predicted values derived by this means with actual determinations yielded no systematic differences. The clinical application of these principles ensures reliable estimates of haemodynamic variables, without resort to invasive techniques, during free movement and graded exercise testing.

Adolescent↗
PubMed1981

Supravalvular aortic stenosis in the adult. A case presentation with unique associated features.

A 42-year-old symptomatic woman with possible adult Williams's syndrome (mental retardation, 'elfin facies' and supravalvular aortic stenosis (SVAS)) is documented. This patient displayed many unique features in addition to the severe SVAS (peak systolic gradient 96 mmHg): there was an associated persistent left superior vena cava draining into the coronary sinus, mitral valve prolapse (Barlow's syndrome) and complete right bundle-branch block on ECG. Peripheral pulmonary artery stenosis was absent. M-mode echocardiograhy in the adult with SVAS is described for the first time in the literature, as is the use of the 60 degrees cross-sectional scan. Both these non-invasive procedures proved of value in the diagnosis. The degree of subendocardial ischaemia, as determined by the 'endocardial viability ratio', was calculated and the possible mechanisms producing angina pectoris with 'coronary artery hypertension' in this condition are discussed. The literature on associated mitral valve abnormalities in SVAS is reviewed. A successful surgical result was obtained; the various forms of operation are outlined.

Adult↗