Brief Radiology Resident Notes on Iodinated contrast media

 Introduction:   An attempt is made to discuss  the overview with existing  practice recommendations including by ACR and should not be construed as either complete  or perfect.  Submitted by Dr MGK Murthy

Definition : medium that increases contrast of  various anatomical structures  to differentiate between them

 Mechanism:  Increased absorption of X rays by them

Types: Positive(iodinated) and Negative (air or Co2)

Iodinated could be Oily/ water insoluble(not used) or Water soluble (WS)

WS could be monomeric , and dimeric(both having Ionic and non ionic  varieties)

Ionic ( high osmolar for practical purposes)

·        Monomeric salts of triiodinated  benzoic acid

·        Dissociates  while becoming  soluble

·        For every 3 iodine molecules present, 2 charged particles  (one  cation and one anion)produced (also called 3:2  compounds) which can interfere with brain and heart electrical activity to produce neuro and cardiotoxicity, apart from  increasing the osmolality(1400 to 1800 mosm/kg of water)

·        Anion is responsible for radio-opacity

·        If we make  them iso-osmolar to plasma/CSF (300),iodine  will be only 6% and not useful

·        Examples include, diatrizoate,iothalamate,and metrizoate

NON IONIC (Low osmolar)

·        Hypertonicity decreased(400 to 750 mosm/kg of water) as triodinated substitutes in the ring compound do not dissociate while becoming  soluble (side chains altered to make this possible )

·        For every 3 iodine molecules present, 1 neutral molecule(non charged) produced,  making them as 3:1 compounds

·        Osmotic dilution by body fluids is much less  leads to sharpness of image lasting longer  apart from increased opacity due to accumulation

·        Examples include  Metrizamide,iopamidol/iohexol

Reactions

·        Most spoken about (hence not being discussed), exact cause not known

·        Usually due to physiochemotoxic(due to altered dynamics) or idiosyncrasies (higher iodine concentration may lead to higher risk), they are anaphylactoid in nature.

 Prevention

·        ACR  advises in some given high risk situations, steroids with prednisolone 50 mg orally 13,7,and 1 hour before procedure  along with 50mg of Diphenhydramine (benadryl) iv/im/oral with the last dose of prednisolne

SERUM CREATININE 

·        ACR recommends testing  of serum creatinine  only  if there is clinical possibility of  renal dysfunction, or in high risk groups   (hx of renal disease including in the family, nephrotoxic drugs, diabetes , collagen diseases ,paraproteinaemias and all inpatients etc )

·         Between the pathologists serum creatinine level and nephrologists  GFR levels of  renal dysfunction(normal adult level is 120 to 130 ml/mt/1.73 sqm), lies the  Radiologists  domain and no consensus  exists

Various surveys have put levels of acceptance of (radiological) normal as 1.5 to 2.0 mg%, corresponding to appx 133 mmol/L, or Expected GFR of> 60 ml/mt/1.73 sqm, excluding  diabetics where   <1.68 mg% is  preferred.

·        Definition of nephrotoxicity is controversial as std European teaching of creatinine raising >25%(within72 hrs of contrast) is dented by studies claiming  normal variations of creatinine  could range from  0 to 40%

·        Pts on Metformin do not have  increased risk of contrast induced nephropathy(CIN). However if pt develops CIN, Metformin could produce  lactic acidosis

·        Preventive medication   like n- acetyl cysteine (600mg twice day  before and on day of test)(favoured by cardiogists) has not been accepted as radiological standard  of practice, however  may be used.

·        The most important thing seems to be adequate hydration{100ml oral or iv(0.9%N saline)per hour  starting 6 to 12 hours before , continuing  4 to 12 hours after the procedure}

·         Renal  insuffiency is defined(American  kidney society ) as GFR<60 ml/mt/1.73  sq m or  creatinine above 1.5 mg% (133 mmol/l),with loss of half or more of adult level of normal kidney function

·        Renal failure  is  GFR of <15 ml/mt/1.73 sq m ,which is when dialysis  etc is initiated