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ABGs445770025146000Normal valuespO2 80-100 (on room air)pH 7.35-7.45pCO2 35-45HCO3- 22-26Osmolar gap <10Anion gap <12Base excess -2 to +2pHpCO2HCO3-Interpretation↓↓ (comp)↓Metabolic acidosis↑↑ (comp)↑Metabolic alkalosis↑↓↓ (comp)Respiratory alkalosis↓↑↑ (comp)Respiratory acidosisThink of CO2 as an acidThink of HCO3- as a baseThe Osmolar gapUsed to detect low molecular weight solutes eg ethanol, methanol, ethylene glycol- elevate the gap= Osmolality – osmolarityOsmolality- from the labOsmolarity = 2(Na+ + K+) + glucose+ ureaThe Anion gapNa+ (+/-K+) – (HCO3- + Cl-)Rarely low- hypoalbuminaemia-causes retention of anions, or increased unmeasured cations e.g Ca2+, Mg2+, lithium toxicityImportant to differentiate causes of metabolic acidosisWhat is the Base excess?Based on pH and HCO3->+2 (ie base excess)= metabolic alkalosis<-2 (ie base deficit)= metabolic acidosisBase deficit with elevated anion gap= addition of acidBase deficit with normal anion gap= loss of bicarbA-a gradient (or multiply oxygen by 4 to get an estimated P02)PAO2= (760-47) x FiO2 – (PaCO2/0.8)The A-A Gradient= PAO2- PaO2 (from ABG)Expected gradient= (age/4) + 4Causes of a Raised A-a gradient:V/Q mismatchPEARDS/ APOPneumoniaDiffusion defectR -> L shuntIncreased O2 extractionMetabolic acidosisCause of a High anion gap acidosis Causes of a Normal anion gap acidosis Cyanide, carbon monoxide- Ureteric diversion Alcoholic/ starvation ketoacidosis - Small bowel fistulaToluene- Extra Cl- (resus, HCl ingestion)Methanol, metformin- DKA (resolving)Uraemia- Carbonic anhydrase inhibitors Diabetic ketoacidosis - AddisonsParaldehyde, propylene glycol, paracetamol- Renal tubular acidosis I, II and IVIsoniazid, iron- Pancreatic fistulaLactic acidosisEthylene glycol/ ethanolSalicylatesMetabolic alkalosis- GI acid lossRenal acid loss- Vomiting, pyloric stenosis- Diuretics- Diarrhoea- Bartter’s/ Gitelmans- NGT- Fistula/ stoma- Overdose of baseEndocrine- Antacids- Cortisol excess (Cushing’s)- Laxatives- Aldosterone excess (Conn’s)- HartmannsRespiratory AcidosisReduced resp driveCNS: haemorrhage/ tumour/ encephalitisDrugs: opiates/ benzosReduced chest wall movementRib fracturesTension pneumothoraxPleural effusionMuscle relaxantsGuillain-Barre/ myasthenia/ muscular dystrophyObstructionCOPDAsthmaPneumoniaRespiratory Alkalosis (can have full compensation)Increased resp driveCNS: CVA/ ICH/ psychogenicMetabolic: thyrotoxicosis/ pregnancy/ feverHypoxaemiaPECCFThe Effects of Exposure to Altitude ................
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