Peter Dawson

The New Low-osmolar Contrast Media: A Simple Guide

The rationale of the new, low-osmolality contrast media for intravascular use is briefly discussed and the technical and commercial data are assembled in a form which, it is hoped, will help radiologists to make their own comparisons and conclusions.

The non-ionic dimers: a new class of contrast agents

Non-ionic dimers are a new class of iodinated radiological contrast agents. Some physico-chemical and pharmacological properties of three representative compounds are presented. Non-ionic dimers are iso-osmolar with plasma at all iodine concentrations unless diluted with saline and in terms of chemotoxicity they appear to be closely comparable with the established non-ionic monomeric agents. It is shown that the data lend support to the theory that it is the degree of steric hindrance of the hydrophobic iodine atoms by side-chains which determines this chemotoxicity. Areas in which agents of this type may find applications are indicated.

Contrast media and bronchospasm: A study with iopamido

Conventional radiological contrast media given intravenously are known to produce bronchospasm in the majority of patients, even though this is rarely clinically apparent. We have found that this phenomenon is much less pronounced with the new, low-osmolality contrast medium iopamidol. If the mechanism underlying the clinically silent bronchospasm observed with the conventional media is the same as that underlying the occasional life-threatening or fatal bronchospasm, then this observation is of considerable importance.

Intravenous urography with low-osmolality contrast agents: Theoretical considerations and clinical findings

There are theoretical reasons for expecting some aspects of image quality in intravenous urography to be modified when low-osmolality, rather than conventional, contrast agents are used. The clinical findings in urography using two of these, the non-ionic agents iohexol (Omnipaque 350, Nyegaard) and iopamidol (Niopam 370, Bracco/Merck), are compared with those using sodium iothalamate (Conray 420, May & Baker Ltd), and are discussed against this theoretical background. The best nephrogram obtained with the new agents often occurred later than with the conventional agent, but quantitative differences in its density were explicable on a total iodine dose basis. The pyelographic density obtained with the new agents was significantly greater than with the conventional agent without any evidence, when abdominal compression was used, of the predicted associated poor distension of the collecting system.

Contrast agent nephrotoxicity. An appraisal

Impairment of renal function occasionally occurs following the administration of intravascular contrast agents. As a consequence even intravenous urography in the investigation of renal failure was until 20 years ago generally considered to be contraindicated. In the United Kingdom in recent years this extreme view has been rejected and high dose intravenous urography is frequently performed on patients in renal failure. Contrast agent nephrotoxicity has not been frequently encountered. Experience in the United States, however, has been different, a worryingly frequent impairment of renal function after contrast administration being reported by several authors. The reason for this different experience is unclear but it may be the result of patient selection and preparation or contrast dose regime. The United States studies indicate that the patients most at risk are those with already impaired renal function, diabetics and the elderly. A brief review of the literature is given and a classification of possible mechanisms of nephrotoxicity is proposed. It is argued that on the basis of any or all of these mechanisms the low osmolality contrast agents would be expected to be less nephrotoxic than the conventional agents. This suggests that their use should be considered in those patients defined as high risk.

Vasopressin release in response to intravenously injected contrast media

We have previously reported that intravenously administered contrast media produce a rise in plasma vasopressin (antidiuretic hormone) concentrations. We have now shown that this occurs both when contrast medium is injected into a peripheral vein and when it is centrally injected into the right atrium. The peak vasopressin concentration recorded varies with the osmolality of the contrast medium. The vasopressin response was greater when contrast agent was centrally injected.

Non-ionic contrast agents, red cell aggregation and coagulation

The second generation non-ionic contrast agents were introduced into clinical practice in the UK some 5 years ago. Since that time, and not withstanding their relatively high cost, they have enjoyed ever-increasing clinical application because of the many advantages they offer over previously available agents. These advantages include greater patient comfort (Dawson et al, 1983a; Dawson, 1985), greater patient safety in “high-dose” procedures (Dawson & Pitfield, 1982; Dawson & Hemingway, 1987) and, the evidence would suggest, a lower incidence of major, adverse, life-threatening idiosyncratic reactions (Schrott et al, 1986; McClennan, 1987). There has, of course, been controversy surrounding the cost of the non-ionic contrast agents but, this apart, there has been little to detract from their success story until recently. In the last year, two papers have appeared, one in the European (Raininko & Ylinen, 1987) and one in the American literature (Robertson, 1987), both concerned with the haematological eff...

Dehydration, antidiuretic hormone and the intravenous urogram

The efficacy and safety of attempts to dehydrate patients before intravenous urography in order to improve image quality have been called into question by several authors. Antidiuretic hormone (ADH) assays described here show that dehydration of outpatients as typically practised by radiology departments does not affect plasma ADH levels. Furthermore, the very act of contrast-agent administration causes a rapid rise in plasma ADH concentration to levels higher than would in any case be expected from modest dehydration. Since the likely effect on circulating ADH of attempted dehydration is dwarfed by the physiological response to a contrast agent, it is considered that such efforts are entirely superfluous.

Hexabrix and the sodium problem

In addition to hyperosmolarity related side effects and occasional idiosyncratic reactions, those intravascular contrast media prepared as a sodium salt also present the problem of a sodium load. Reservations have been expressed by some radiologists and by competing manufacturers concerning the sodium content of the new low osmolality contrast medium, Hexabrix. There are two aspects of this problem. Firstly, the sodium concentration is of some importance, particularly in cerebral angiography. Secondly, the total sodium load is an important consideration in certain groups of patients, viz:— (1) those with congestive cardiac failure; (2) those with renal failure; (3) those patients with hepatic disease and ascites; (4) infants.

Isomeric purity and supersaturation of iopamidol

We have found that iopamidol 370 mg I/ml solutions may be greatly supersaturated and iopamidol 300 mg I/ml solutions slightly supersaturated at 4 degrees C and at 25 degrees C. The solubility and degree of supersaturation probably depend on the isomeric purity of the preparation. Some possible implications are discussed.