Water, Electrolytes & Acid-Base Balance Notes PDF

7 WATER, ELECTROLYTES AND ACID-BASE BALANCE CPY-L5 & L6 Water and sodium Osmolality and tonicity Osmolality: concentration of osmotically active particles, measured by freezing point thermodynamics (or boiling point) (the total number of solute over the volume of fl...

7 WATER, ELECTROLYTES AND ACID-BASE BALANCE CPY-L5 & L6 Water and sodium Osmolality and tonicity Osmolality: concentration of osmotically active particles, measured by freezing point thermodynamics (or boiling point) (the total number of solute over the volume of fluid) o Plasma osmolality: 275 – 295 mOsm/kg (osmolarity: mOsm/L) o Urine osmolality: 50 – 1200 mOsm/kg Calculated osmolality = (2 x [Na] + [urea] + [glucose]) of plasma Osmol gap (OG): measured osmolality - calculated osmolality o Normal OG < 10: osmolality generated by ions e.g. Cl-, K+, Ca2+; serum proteins and lipids o OG > 10: presence of exogenous substances that are osmotically active, e.g. alcohols (polyethylene glycol), sugars (mannitol, sorbitol), lipids, proteins Tonicity: determines the water movement across compartments, caused by effective osmoles (osmotic gradient; dependent on how penetrable solutes are; hypertonic – water leaving cell, hypotonic – water entering cell) o Effective osmoles: Na+, K+, glucose (in diabetics), urea (in extremely high level) o Ineffective osmoles (but osmotically active): glucose (in normal individuals), urea, alcohol o Implications: § Na+ as an effective osmole: ∆Na affects water movement and causes various manifestations in hyper/hyponatremia but heeffective effective osmoles of water no net movement same osmolality osmoles : of water § *Iso-osmolar solutions which are not isotonic: D5 dextrose 5 % moves- net movement (dextrose metabolized in water > - § Urea adds to the osmolality but not tonicity à ­urea may cause a falsely high measured/ calculated osmolality, but should not be regarded as hypertonic and should not be treated with hypotonic IVF : area is a penetrating solute (can cross cell membrane Water metabolism Daily requirement Obligatory losses mL Sources mL Skin 500 Water from oxidative metabolism 400 Lungs 400 Minimum in diet 1100 Gut (faeces) 100 Kidney (urine) 500 Total 1500 Total 1500 Minimum urine output = obligatory osmoles excreted / max urine osmolality = 600 / 1200 = 0.5L Response to water depletion Antidiuretic hormone (ADH) vasopressin = (AVP) Produced in hypothalamus, stored in posterior pituitary Stimulus: osmolality (1% change), non-osmotic stimuli (blood volume (10%), stress, nausea) o In severe hypovolaemia, ADH is secreted to preserve plasma volume at the expense of osmolality and Na regulation Action: (2 mechanisms) apical membrane o Water reabsorption: ­AQP-2 channels at CD & DCT (V2)receptor ↳ o Vasoconstriction (V1a) receptor ↑ PVRABP (in hypovolemia) - : - * feedback loop: minimal ADH when plasma osmolality is < 280 Thirst ­Osmolality à sensed by osmoreceptors à hypothalamic thirst centre (outside of BBB) à activate median preoptic nucleus  8 Disorders of water metabolism Diabetes insipidus Syndrome of inappropriate ADH secretion Cause ①Cranial: (50% idiopathic) X hypothalamus/pituitary gland CNS: encephalitis, stroke 1o hypothalamic (more common) Lung: pneumonia, TB Tumour, vascular, HI, etc Drugs: SSRI, carbamazepine, morphine ②Nephrogenic: (kidney cannot respond; tubular Surgerysmall cell lung cancer injury) Ectopic: SCLC, Ca prostate, Ca thymus Familial Metabolic (hypoK, hyperCa) Li toxicity Manifestation Polyuria of dilute urine (>3L/24h) Concentrated urine Polydipsia Euvolemic, hypotonic hypoNa (dilutional under adequate water intake situation Euvolemic, normal Na & osmo. (dehydrated, hypoNa) ­Na if insufficient water intake) If acute/extreme: cerebral oedema differentiate Investigation Fluid deprivation test* Urine Na, urine osmolality (see below) hypovolemic hyponatrenia burine Nat Treatment Treat underlying cause Treat underlying cause Inappropriate ADH secretion Purine Nat Cranial DI: DDAVP Fluid restriction: 1200ml/day Nephrogenic DI: thiazide (↑ Na+ excretion), Salt: PO, hypertonic IV (rare) administered amiloride (↓K+ loss) - ***create mild orally hypovolemia, encourages salt & water uptake in proximal tubule, improve nephrogenic DI *Fluid deprivation test: HERE To differentiate between primary polydipsia and DI o o 1 polydipsia also presents with euvolaemia and normal Na (hypoNa if with impaired water excretion mechanism) Procedures: o Supervised water deprivation x 8h body weight o Monitor BW, urine volume & osmolality; stop if ¯BW > 5% o To further differentiate between central vs nephrogenic DI, give DDAVP (desmopressin) Uosm 300-750 Primary polydipsia 8h water deprivation Polyuria Normal: Uosm > 750 Uosm > 50% increase Central DI Uosm < 300 DDAVP inj. (no increase) Uosm < 50% increase Nephrogenic DI Water and sodium regulation Water Sodium Sensing parameter Osmolality Intravascular volume XBP By osmoreceptors By baroreceptors, chemoreceptors Vasoconstriction it Endocrine regulator ADH RAAS (sensing & Nat & water) angiotensin It - ADH ANP atrial natriuretic peptide 4 natriuresis sense : 4 BP + volume in heart atrium & diuresis Major compartment ECF + ICF ECF affected Manifestations* Dehydration (more chronic) Hypovolaemia (more acute) Parameter affected [Na]^# ECF volume < /35 mEq/L Hypernatraemia Hyponatraemia Hypervolemia Hypovolemia Clinical Lethargy, coma Mild: nausea, Oedema Poor skin turgor & presentations due to brain lethargy Weight gain Tachycardia # GFRL ? shrinkage Severe: ¯GC, Orthostatic ¯BP (standing :X ↑ blood volume ­JVP ↑ R pressure compensateare C. atrial cellular dehydration vascular rupture seizure, coma SOB : PE ¯capillary refill + blood volume alveolar space blung compliance ↓ Volume + vessel constrictions due to brain > - swelling Implications in Free water, e.g. Water restriction Sodium restriction NS normal Saline treatment water, D5W (since higher water content – dilute Na) ^ECF sodium content (or total sodium content as intracellular contains little sodium c.f. ECF) bears no relationship with [Na] #Measured by indirect ion-selective electrodes or flame photometry *This can be viewed as a clinical spectrum: