Biomedical subjects
A R Michell
Publications and source records attributed to A R Michell.
Fallibility of faecal consistency as a criterion of success in the evaluation of oral fluid therapy for calf diarrhoea.
It is often said that the success of oral rehydration in humans depends on the adequacy of the improvement in the composition and volume of extracellular fluid, not reduction of faecal output. Indeed, the latter may increase initially. Such increases do not prevent the treatment from being effective but they may, falsely, undermine its acceptability to patients or those caring for them. This paper provides data to show that standard oral rehydration solutions used to treat experimentally induced calf diarrhoea procure identical improvements in plasma volume during the first 48 h, whether faeces improve or not, and those calves whose faecal consistency improved actually showed greater deterioration of extracellular fluid volume. While it is important for this to be appreciated by clinicians and explained to owners, it is absolutely imperative that those responsible for the approval of new therapeutic products for registration understand and accept that faecal consistency offers no reliable insight into the effectiveness of oral rehydration therapy for calf diarrhoea. It was, however, interesting that there was some relationship with correction of acidosis--perhaps because some of the contributing factors arise from colonic dysfunction.
Evaluation of iohexol as a marker for the clinical measurement of glomerular filtration rate in dogs.
The contrast medium iohexol can be used as a marker for the measurement of glomerular filtration rate. It has the advantages of sample stability and the availability of a simple, automated (though expensive) analyser. The clearance of iohexol was compared with the clearance of Tc-DTPA in 24 dogs with known or suspected renal impairment. The results were comparable but fractionally lower with iohexol. The correction factors developed for human beings were found to be satisfactory in the dogs, thus validating the software routines used in the automated analyser with the canine samples.
Epidemiological study of blood pressure in domestic dogs.
Previous experience has shown that a non-invasive (indirect) technique using an oscillometric monitor in conjunction with a tail cuff makes routine clinical blood pressure measurement practicable in dogs. The relationship between indirect and direct readings has been evaluated in both anaesthetised and conscious dogs (Bodey and others 1994, 1996). In this study, more than 2000 pressure measurements were taken from 1903 dogs. It was found that systolic is the most variable pressure parameter and that it depends on age, breed, sex, temperament, disease state, exercise regime and, to a minor extent, diet. Diet was not a significant determinant of diastolic and mean arterial pressure. Age and breed were the major predictors for all parameters. Heart rate was primarily affected by the temperament of the animal, though other factors also play a part in prediction. The distribution of systolic, diastolic, mean arterial pressure and heart rate across the dog population approximates to a log normal distribution. On the basis of these results it is possible to describe normal ranges for canine blood pressure; definition of hypertension, though, demands attention to age and breed normal values. The existence of statistically defined hypertension in an individual or breed does not imply adverse effects justifying therapy. Among the secondary causes of hypertension, such as diabetes, obesity and hyperadrenocorticism, hepatic disease was a new addition also undocumented in humans. The hypothesis that dogs, though classic model animals for hypertension, are resistant to its development found support from the modest increase in mean pressure values observed among dogs with renal disease, notably those with substantial reduction of glomerular filtration rate. The existence of breeds such as deerhounds with average pressures in the borderline range for hypertension in humans (and many individuals, therefore, well above) suggests that dogs may also be resistant to some of the adverse effects of high blood pressure.
Clinical science, veterinary science and comparative medicine: problems of continuing neglect.
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A simple method for measuring glomerular filtration rate in dogs.
A technique for measuring glomerular filtration rate (by six-sample plasma clearance) and extracellular fluid volume is described. The results obtained for glomerular filtration rat per extracellular fluid volume with a three blood sample technique are also presented. Concurrently with the blood sample techniques measuring plasma clearance, glomerular filtration rate per extracellular fluid volume was also measured using an external radiation detector and no blood samples. The validity of expressing glomerular filtration rate per extracellular fluid volume (rather than per body weight or surface area) and the clinical utility of the results obtained with the external radiation detector are discussed.
Responses to reduced water intake, including dehydration natriuresis, in sheep excreting sodium predominantly in urine or in faeces.
Sheep which were predominantly urinary excretors (U) or faecal excretors (F) of sodium were exposed to a 75% reduction of water intake for 72 h. The experiment was performed on moderate, low or high sodium intakes (0.4, 0.05 or 1.2 mmol kg-1 day-1) to test the hypothesis that dehydration natriuresis was not a cause of sodium depletion but a defence against hypernatraemia. Dehydration caused elevation of plasma sodium concentration, osmolality, antidiuretic hormone (ADH) and oxytocin but, as in other experiments, a fall in haematocrit. The two higher levels of sodium intake were associated with dehydration natriuresis but also a smaller increase in faecal sodium excretion in both U and F sheep. On low sodium intake, however, neither urinary nor faecal sodium excretion increased in either group of sheep although the rise in plasma sodium concentration caused by dehydration was similar. Thus, when there is a risk of sodium depletion, due to low sodium intake, dehydration natriuresis does not occur, consistent with the hypothesis. Active sodium transport inhibitor (ASTI) and atrial natriuretic peptide (ANP) fell rather than rose during dehydration. Since aldosterone is suppressed by the higher levels of sodium intake, none of these hormones is likely to mediate dehydration natriuresis in sheep. F sheep showed more effective renal and faecal water conservation when dehydrated. During water restriction, the urinary potassium excretion of U sheep was significantly reduced, unlike that of F sheep; moreover, the latter maintained an identical plasma potassium concentration between baseline and restriction period, whereas in U sheep it was 0.3 mmol l-1 higher during water restriction. Increased drinking rather than reduced urine output was the basis of rehydration when ad lib. water intake was restored.
Future of the State Veterinary Service.
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Veterinary manpower.
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A comparison of direct and indirect (oscillometric) measurements of arterial blood pressure in anaesthetised dogs, using tail and limb cuffs.
Oscillometric measurements of arterial blood pressure were compared with direct measurements in anaesthetised dogs being monitored during routine surgery. Readings were obtained with tail cuffs and limb cuffs and the reproducibility (precision) of the readings from various cuff sites was also compared in conscious dogs. Tail cuffs gave the best precision in conscious dogs and the closest correlation with direct measurements in anaesthetised dogs, especially for systolic pressure. The proximal hindlimb site gave results with a slightly lower precision.
Salt, hypertension and renal disease: comparative medicine, models and real diseases.
Dogs are well established as experimental animals for the study of both renal disease and hypertension, but most work is based on surgical or pharmacological models and relatively little on spontaneous diseases. This review argues for the latter as an underexploited aspect of comparative medicine. The most important feature of canine hypertension may not be the ease with which models can be produced but the fact that dogs are actually rather resistant to hypertension, and perhaps to its effects, even when they have chronic renal failure. The importance of natural models of chronic renal failure is strengthened by the evidence that self-sustaining progression is a consequence of extreme nephron loss, that is, a late event, rather than the dominant feature of the course of the disease. The role of salt in hypertension is discussed and emphasis given to the importance of understanding the physiological basis of nutritional requirement and recognizing that it is unlikely to exceed 0.6 mmol/kg/day for most healthy adult mammals except during pregnancy or lactation. Such a perspective is essential to the evaluation of experiments, whether in animals or humans, in order to avoid arbitrary definitions of 'high' or 'low' sodium intake, and the serious misinterpretations of data which result. An age-related rise in arterial pressure may well be a warning of excess salt intake, rather than a normal occurrence. Problems of defining hypertension in the face of variability of arterial pressure are also discussed.
What is the importance of salt appetite?
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Qualifications in Europe.
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Non-invasive measurement of arterial blood pressure in dogs: a potential indicator for the identification of stress.
The reproducibility of indirect measurements of arterial pressure in dogs was assessed using the Dinamap 1846 SX oscillometric monitor and tail cuffs. Measurements on different days correlated excellently in 20 working dogs, with indistinguishable group means for systolic, diastolic and mean arterial pressure, whereas heart rates differed, though not significantly. The similarity of the means did not simply result from 'cancelling out' of individual variations; the rank correlation was also highly significant (P < 0.01 diastolic, P < 0.002 systolic). Remeasurement in the veterinary clinic, rather than 'neutral territory' showed significant increases in pressure but not heart rate. Anticipation of work raised systolic pressure whereas heart rate and diastolic pressure were unchanged. Exposure of a dog to a known stressor for that individual (sound) raised systolic and diastolic pressure as well as heart rate; diastolic pressure was the fastest to recover, heart rate the slowest. The data are compared with other observations on non-invasive blood pressure measurement in both dogs and humans.
Clinical teaching: a case for treatment.
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Sodium preference in sheep excreting sodium predominantly in urine or faeces.
On moderate sodium intakes (0.5 mmol/kg/day approx.) sheep are readily divided into those that are predominantly urinary or faecal excretors of sodium (U or F sheep). Water turnover and obligatory sodium loss are lower in F sheep. In view of these differences, sodium preference was examined in U and F sheep offered water and sodium bicarbonate solutions (40 or 300 mmol/l). The F sheep took significantly more of the bicarbonate solutions and, therefore, had sodium intakes that were 51% higher on the lower concentration, 94% higher on the higher concentration. Thus, F sheep not only conserve sodium more readily when it is scarce, they select it more readily when it is available.
Plasma and extracellular volume in calves: comparison between isotopic and 'cold' techniques.
While isotopic techniques have largely superseded traditional markers for the determination of the volume of fluid compartments in vivo, they are not always convenient, especially with diarrhoeic animals. A direct comparison was therefore made in week-old calves between Evans blue and radio-iodinated serum albumin as measures of plasma volume and thiocyanate or 24sodium as measures of extracellular fluid space. The correlation coefficients were excellent (1.00, 0.96; P < 0.001) and the calves had plasma and extracellular fluid volumes of 72 +/- 2 and 438 +/- 2 ml kg-1, respectively. The latter value is, though high, typical of young animals and comparable with other data in calves.