Diagnostic imaging matters


CONTRAST-INDUCED NEPHROPATHY: potential role of viscosity


Reviewer: Dr Donal Reddan, Consultant Nephrologist, Galway University Hospitals, Ireland


Introduction Renal impairment is a known (and usually transient) side effect of iodinated contrast media.1


of contrast-induced nephropathy (CIN) is not yet fully understood, but a variety of factors are believed to be involved (see Box 1), including the osmolality and direct cellular toxicity of the contrast agent.2


but not confirmed


in studies in humans. This article looks primarily at the potential role of viscosity, but also covers some of the direct cellular impact of contrast media.


Viscosity basics Viscosity refers to the resistance that a fluid exerts against movement. The viscosity of a fluid can be calculated using Poiseuille’s equation (Box 2), which measures the pressure of fluid flowing through a cylindrical pipe (such as an injection needle or catheter).7


Poiseuille’s equation has several implications for administration of contrast media: • With increasing viscosity, the flow slows and the injection pressure increases


• Increased injection pressure results in a faster delivery rate • Longer needles or catheters result in the need for: • Higher pressures at a constant delivery rate or • Slower delivery rate at a constant pressure


• Changing the radius of the needle or catheter has a dramatic effect on flow rate and pressure (Figure 1)


• With high-flow catheters and needles, only the distal part (i.e. the part of the needle that enters the patient, or the part of a catheter that is placed into a narrow-branch artery) needs to have a compatible size, while the remaining length can be slightly wider, thus significantly lowering injection pressure and increasing delivery rate


For most examinations requiring catheter-based contrast media outside the left ventricle and aorta, the maximum


2F


0.66 mm diameter catheter


4F


1.33 mm diameter catheter


10% needle/catheter 10% wider 20% needle/catheter 20% wider The pathophysiology


• Osmolality-related effects • Viscosity: • Increased tubular hydrostatic pressure • Decreased single-nephron glomerular filtration rate


The roles of other


properties of the contrast medium, such as viscosity have been explored in animal models,3–6


• Direct cellular toxicity • Cellular effects (tubular reabsorption, vacuolisation) • May be due to increased viscosity, resulting in increased transit time (i.e. prolonged contact with tubular cells)


Box 1: Properties of contrast media that may be involved in the pathogenesis of contrast-induced nephropathy2


v = π ∙ r4


∙ p 8 ∙ ρ ∙ l


p = 8 ∙ v ∙ ρ ∙ l π ∙ r4


Where: v = velocity (flow rate or injection rate) in ml/s p = injection pressure π = circumference of the catheter or needle divided by 2r r = radius (inner lumen) of catheter or needle ρ = viscosity in mPa.s l = length of catheter/needle


Box 2: Poiseuille’s equation (simplified)7


Increases flow


1,500%


50%


100%


Decreases pressure


94%


33%


50%


Figure 1: Impact of changes in needle/catheter radius on delivery rate and injection pressure


3


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