WEM1 Lab diagnostics MK, JSC 2015/2016 Sodium deficit Postoperatively a 70-kg patient has a serum sodium value of 120 mEq/L (120 mmol/L) to increase this value to 140 mEq/L would require the administration of how much sodium in mEq? what is the amount of fluid (0.9% NaCl, ml/hour)? what is the amount of fluid (3% NaCl, ml/hour)? rate of replacement 12 mEq/L in the first 24 hours (osmotic melinolysis!!) rate of replacement 6 to 8 mEq/L in the first 24 hours Example Free water clearance (FWC) Correcting hyponatraemia in hypovolemic hyponatraemia the goal is to replace Na, K, and water deficit in dilutional (euvolaemic, hypoervolaemic) hyponatraemia the goal is to induce negative free water intake electrolyte-free water intake must be less than urinary electrolyte-free water excretion (assuming insensible loss and fecal water loss approximate ingested water and metabolic water!) Electrolyte-free water clearance (EFWC) Example urinary volume (flow)? urinary [Na + ] concentration? Urinary [K + ] concentration? EFWC, MEFWC? Example 2 Volume 600/300=2 L Na 80/2=40 mmol/L K 20/2=10 mmol/l MEFWC=1.3L Example If a patient was placed on 1 L free water restriction, a negative balance would be -0.3 L Example 2 Patient 55 yo, presented with hyponatremia secondary to the SIADH. On initial presentation, plasma [Na + ] was 120 mmol/l, urinary [Na+K] was 100 mmol/l, urinary volume (flow) was 1.5 L/day Example 2 As the urinary volume was 1.5L/day, the patient was placed on 1 L free water restriction. However, this degree of free water restriction resulted in a decrement in plasma Na. Why was free water restriction ineffective in this patient? Example MEFWC=0.43L Example 2 Difficult to adhere! Not a therapeutic option! A urine osmolality value of less than 100 mOsm/kg indicates complete and appropriate suppression of antidiuretic hormone secretion Example 3 Patient 62 yo, who presented with hyponatraemia secondary to SIADH. On initial presentation, Plasma Na 120 mmol/l Urinary somolality was 800 mOsm/kg Urinary Na excretion was 100 mmol/day Urinary K excretion was 40 mmol/day Daily solute excretion was 600 mOsm/day Volume 600/800=0.75 l Urina Na=100/0.75=133 mmol/l Urine K=40/0.75=53 mmol/l MEFWC=-0.25L!! Example 3 Negative value! ADH leads to reabsorption of electrolyte-free water rather than excretion! Any amount of electrolyte-free water intake will lead to hyponatraemia! In such cases increasing urinary electrolyte- free water excretion in excess of electrolyte- free water intake Example 3 Patient was treated with vasopresin-2 receptor antagonist (VRA, vaptans, demeclocycline, lithium) Urinary osmolality decreased to 100 mOsm/kg Volume 600/100=6 liters Urine Na =100/6=17 mmol/l Urine K 40/6=7 mmol/l MEFWC=0.62 (increased by 0.8!)