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P Dawson

Publications and source records attributed to P Dawson.

At least 91 records · Page 5Linked to original sources
PubMed1996

Review article: the quantification of renal function with enhanced computed tomography.

The changes with time in renal parenchymal and vascular attenuation following injection of an intravascular contrast medium can be exploited to study renal physiology in several ways. Shortly after injection, both contrast medium clearance and fractional vascular volume per unit volume of kidney can be measured on a regional basis by obtaining sequential CT data from one anatomical level and the application of a graphical analysis to the baseline subtracted CT numbers for tissue and vascular regions of interest: c(t) and b(t), respectively. By plotting the ratio c(t)/b(t) against integral of b(t)dt/b(t), both fractional vascular volume and contrast medium clearance can be estimated graphically. At later time points, a semilogarithmic plot against time of the declining attenuation of vascular or parenchymal regions of interest gives information about overall, as opposed to regional, renal function: the gradient estimates the glomerular filtration rate per unit volume of extracellular fluid. Absolute glomerular filtration rates can also be calculated with blood sampling. Although further experimental evaluation is needed, these techniques have a strong theoretical basis, and experimental work performed so far is encouraging. Possible applications of these techniques are discussed.

Contrast Media↗
PubMed1995

Current usage of contrast agents, anticoagulant and antiplatelet drugs in angiography and angioplasty in the UK.

Currently, there is no consensus in the UK on what constitutes best angiographic practice. To provide a basis for discussion a questionnaire was sent to all Radiology and Cardiology departments in the UK. Information was requested on practice during diagnostic angiography and angioplasty regarding the type of contrast agent used, whether and how flush solution or contrast agents themselves were heparinised, and whether bolus doses of heparin were administered. The use of other supplementary drugs including corticosteroids was also explored. Two hundred of 353 (57%) of questionnaires were returned. Over 80% who replied used non-ionic contrast agents for all angiographic procedures. The majority of the smaller group, using ionic contrast agents for uncomplicated procedures, resorted to non-ionic contrast agents in a range of circumstances in both diagnostic angiography and angioplasty. Heparinized flush solutions were used by over 75% for both types of angiographic procedures, but employing a wide range of doses. Bolus doses of heparin were administered by over 80% performing angioplasty, again in a wide range of doses, with only (a few) cardiologists monitoring the anticoagulant effect by measuring the activated whole blood clotting time in the angiographic suite. Over 70% used aspirin or dipyridamole as supplementary agents, at the time of the angioplasty and, subsequently, continued these medications for a variable period. Corticosteroid prophylaxis for high risk patients, very variably defined, was felt necessary by 58%. A wide range of regimes of both dose and timing was noted.

Adrenal Cortex Hormones↗
PubMed1995

Effect of arterial cannulation and contrast agents on blood coagulation.

RATIONALE AND OBJECTIVES: Nonionic contrast media have been considered by some to have thrombogenic properties. We prospectively assessed the effect of femoral artery catheterization and both nonionic and ionic contrast media on the coagulation parameters--fragment 1 + 2 (F1 + 2) and fibrinopeptide A (FpA)--during clinical angiography. METHODS: Seventeen patients undergoing aortography were included. Blood samples were obtained before and after arterial puncture and before and up to 30 min after contrast administration. RESULTS: An increase in FpA was observed after arterial puncture (range = 8.4 +/- 1.9 to 13.6 +/- 2.3 ng/ml, p < .004; data are written as mean +/- standard error of the mean). There was an observed increase in F1 + 2 after arterial puncture that was not statistically significant (2.0 +/- 0.4 to 2.3 +/- 0.4 nmol/l). No further increase was observed in either FpA or F1 + 2 levels after nonionic or ionic contrast media administration. CONCLUSION: The increased activity of the coagulation system during angiography is related to the arterial puncture, and nonionic and ionic contrast media have no thrombogenic potential in vivo.

Adolescent↗
PubMed1994

Gadolinium chelate MR contrast agents.

Several Gadolinium chelates are now available or will shortly become available. They differ in being, variously, ionic or non-ionic and in being of higher or lower osmolality. Although they differ in detail of chemical structure, they are all based on the same general principle and all possess the same extracellular distribution pharmacokinetics (Gd BOPTA excepted), typical for their molecular size. In terms of efficacy there is no convincing evidence that any one is clinically superior to the others. As regards reducing osmolality, it is not self evident that, in the recommended and currently widely used dosage regimens, high osmolality presents any real clinical problems. However, if 'high dosage' regimens or dynamic bolus imaging come to be more widely used, then the newer agents may be perceived to have clinical advantages.

Contrast Media↗
PubMed1994

The hazards of prescribing from serum levels.

Clinical audit at a mental handicap hospital suggested that doses of antiepileptic drugs were being altered in response to suspiciously high or low serum level results. In order to assess the reliability of such results, sequential serum levels for individual patients were examined for consistency. Correlation between results which should have been the same was found to be so low as to call into question the value of drug serum level sampling in this hospital. Investigation of the source of the variability continues. In the meantime, clinical staff are reminded that it is unsafe to make any decision on the basis of a single drug serum level.

Anticonvulsants↗
PubMed1994

Pharmacokinetics of bupivacaine enantiomers during interpleural infusion.

Interpleural infusions (24 hr) were given to 14 adults (29-81 yr) having surgery via right subcostal incisions. Patient-controlled analgesia with morphine was also available for 72 hours following surgery. An infusion of 0.25% bupivacaine with adrenaline 1/400,000 was commenced at 0.1 ml/kg/hr at the conclusion of surgery after an initial 20 ml bolus. The 24 hr cumulative morphine dose increased by 62% (P < 0.01) on the second postoperative day after the bupivacaine infusion was ceased, but the pain score did not change. There was no clinical evidence of bupivacaine toxicity although the maximum bupivacaine concentration ranged from 0.74-6.52 mg/l, mean 2.90 (SEM 0.44) mg/l and vascular uptake was rapid in two patients. The concentration of S-bupivacaine was consistently greater than R-bupivacaine (P < 0.05) and total body clearance was less. The unbound bupivacaine fraction also changed from 3.6% (SEM 0.6) at the start of the infusion to 2.0% (SEM 0.3) at 24 hr (P < 0.005). The total body clearance of each enantiomer was reduced during the 24 hours (P < 0.001). No relationship between pharmacokinetic parameters and weight, age or sex was found (P > 0.25 for each). Variation between patients was reduced when parameters were estimated for the free (unbound) bupivacaine (P < 0.001).

Adult↗
PubMed1994

What is the nephrogram?

One of the oldest questions in radiology, "what is the nephrogram?", has been approached in a novel manner using dynamic bolus contrast enhanced computed tomography to monitor the arrival and distribution of contrast agent in the normal kidney. A modification of a mathematical technique first developed for applications in positron emission tomography has been used to analyse the data. It is concluded that the blood pool contribution to the nephrogram is greater in the first minutes after rapid bolus contrast injection than generally supposed and that the "immediate" nephrogram is a predominantly blood pool contrast phenomenon.

Aorta, Abdominal↗
PubMed1994

Laser angioplasty: enough to make your blood boil!

Some of the effects on blood and contrast agents of a laser angioplasty device of the "hot tip" variety have been studied. This device liberates iodine from contrast agents both in the form of soluble free iodide and in the form of solid elemental iodine debris. Some of the solid iodine does not adhere to the probe but remains free and, in vivo, would be available to embolize distal vessels. The liberation of iodine occurs more readily with dimeric agents than with monomeric contrast agents regardless of whether they are ionic or non-ionic. Blood and blood/contrast agent mixtures exhibit local boiling in the vicinity of the hot tip laser probe. Blood so treated exhibits markedly accelerated clotting as indicated by measurements of whole blood clotting times. Untreated blood to which aliquots of blood treated as described above are added also exhibits accelerated clotting. The laser tip, operating in blood/iodine mixtures, becomes coated with a friable black/brown deposit which analysis reveals to be a mixture of carbon and iodine. Again, some of this particulate debris fails to adhere to the probe and is available as free embolic material. The possible practical clinical implications of these observations are discussed.

Angioplasty, Laser↗