Fluid management and sodium disorders

1. Fluid management and sodium disorders

2. Definitions and normal values: • Volume depletion: Results from a deficit in total body Na content that exceed Na intake. Water losses alone can also cause volume depletion, but the quantity required to do so is larger. • Dehydration: Free water losses alone, or with Na but the water lose exceed the Na lose, that result in high [Na].

3. Normal range for plasma [Na]: 135-145 mEq/L. • Hyponatremia: plasma [Na] < 135 mEq/L. • Hypernatremia: plasma [Na] > 145 mEq/L. • Measured serum Osmolality = 280- 300 mOsm/L • Individual who consume a typical western diet generate approximately 600- 800 mOsm/d as a solute load.

4. Maximum urine diluting capacity in healthy kidneys is 50 mOsm/L. • Maximum urine concentrating capacity in healthy kidneys is 1200 mOsm/L. • Maximum renal excretory capacity of water in healthy people is 12 L/d. • Minimum renal urine output in healthy people is 0.5 L/d.

5. General principles: • Distribution of total body water: • Total body water (TBW). Wt (Kg) X 0.6 (y.m) 0.5 ( y.f) =TBW in liters. e .g for (y.m) 70 * 0.6 = 42 L • Next, consider the different fluid compartments within the body. 2/3, 1/3 rule.

6. Fluid compartments

7. 70 Kg male: TBW= 42 L Intracellular volume = 0.66 * 42 = 28 L Extracellular volume = 0.34 * 42 = 14 L Interstitial volume = 0.66 * 14 = 9 L Intravascular volume = 0.34 * 14 = 5 L

8. Fluid compartments

9. Free water is distributed evenly throughout the TBW “compartment” 1-1000cc D5W: 2/3 to intracellular space = 660cc 1/3 to extracellular space = 330cc • 2/3 to interstitial space = 220cc • 1/3 to intravascular space = 110cc 1000/110 = 11% only effective

10. 2-1000 cc 0.45% saline: 500 cc free water throughout TBW 55 cc intravascular space 500 cc to ECV 2/3 to interstitial space = 330 cc 1/3 to intravascular space = 170 cc 225 cc, 22% into the intravascular space 3-1000 cc 0.9% saline: Essentially all confined to extracellular compartment 2/3 to interstitial space = 660 cc 1/3 to intravascular space = 340 cc Approximately 33 % i

11. Osmolality: Is the solute or particle concentration of a fluid. solutes that are restricted to the ECF (Na and its accompanying anions) or the ICF (K salts and organic phosphate esters) determine the effective osmolality or tonicity of each of those compartments. • The osmolality of the ICF and ECF compartments is always equal because water diffuses rapidly across cell membranes in order to achieve osmotic equilibrium, because of that, changes in water content of one compartment affect both the ECF and ICF, as opposed to changes in Na

12. content, which affects primarily the ECF to which it is restricted. • TBW osmolality = ECF osmolality of the TBW = plasma osmolality = 2 * [Na] + ([BUN]/2.8) + ([glucose]/ 18) Since urea crosses plasma membranes, it is an ineffective osmole. The contribution of glucose to osmolality in euglycemic individuals is small (~ 5 mOsm/L) So… Effective TBW osm = 2 * [Na]

13. Difference between calculated and measured osmolality should agree within 10 . • Hypernatremia and hyponatremia are primarily disorders of water balance or water distribution across the fluid compartments.

14. 85-90% of total body Na is extracellular, so alteration in Na content are manifest clinically as ECF volume depletion (hypotension, tachycardia) or ECF volume overload ( peripheral and/or pulmonary). • Na concentration is distinct from Na content, since one can have reduced serum [Na] but increased total body Na as occurs in heart failure.

15. Regulation of water balance occurs primarily through antidiuretic hormone (ADH) and the thirst control centers of the hypothalamus. • A persistent abnormality in [Na] would thus require an alteration in the action of ADH or the thirst response.

16. In summary: ICF Osm. = ECF Osm. Interstitial Osm= Serum Osm. Hypothalamus is the serum osmostat, it stimulates thirst and appropriate ADH secretion Primary Defense for increase Osmolality Thirst Primary Defense for decrease Osmolality Renal excretion of water via stop the secretion of ADH. Any defect in these defense mechanisms lead to hyponatremia or hypernatremia.

17. Indications of intravenous fluid: • NPO. • Significant volume deficit. • On going losses. • Specific goal to fluid therapy, (e.g. hydration prior to contrast dye). • Contra indications of intravenous fluid: • Congestive Heart Failure. • Severe Renal Disease (End Stage).

18. Potential complications of intravenous fluid: • Fluid overload. • Dangerous electrolyte derangements. • Line infections. • Consider the water and the electrolyte needs of the patient separately when prescribing IVF therapy.

19. Minimum water requirements for daily fluid balance can be approximated from the sum of the urine output, stool water loss, and insensible losses. 1-The minimum urine output: necessary to excrete the daily solute load = solute consumed each day/maximum urine concentration capacity. i.e.: [600-800 mOsm/d]/[1200 mOsm/L] = 0.5 L/d. 2-The water lost in stool is 200 mL/d.

20. 3-Insensible water losses: *From the skin and respiratory tract amount to roughly 400 to 500 mL/d. *The volume of water produced from endogenous metabolism ~250 to 350 mL/d should be subtracted.  insensible water losses: 150 to 250 mL/d. *Insensible water losses depend on respiratory rate, room temperature, humidity, and body temperature. Water losses increase by 100 to 150 mL/d for each degree of body temperature over 37C. *Fluid losses from sweating can vary (100-2000 mL/hr).

21. since there is no advantage to minimizing urine output; The usual maintenance IVF rate, assumes normal fluid loss (Urine, Stool, Insensible) and no significant renal or cardiac disease and NPO. 1-Normal adult requires approximately 35cc/kg/d. e.g.: A 70 Kg man need 70 * 35 = 2450 cc/d

22. If the patient is taking some PO, the IVF rate must be decreased accordingly. • Fluid from drain losses must be factored in as well. • The electrolytes that are usually administered during maintenance fluid therapy are Na and K salts. • calcium, magnesium, phosphorus, vitamins, and protein replacement are necessary after one week of parenteral therapy without PO intake.

23. Electrolyte Requirements: * A typical 2-g Na diet provides 86 mEq Na. ** If renal function is normal. *** To minimize protein catabolism and prevent starvation ketoacidosis.

24. Sodium: • Na: 1-3 mEq/kg/day. e. g: 70 kg male requires 70-210 mEq NaCl, 2600 cc fluid per day. 0.45% saline contins 77 mEq NaCl per liter. 2.6 x 77 = 200 mEq Thus, 0.45% saline is usually used as MIVF assuming no other volume or electrolyte issues.

25. potassium: • Potassium: 1 mEq/kg/day • K can be added to IV fluids. Remember this increases osm load. 20 mEq/L is a common IVF additive. • This will supply basal needs in most patients who are NPO. • If significantly hypokalemic, order separate K supplementation. • Oral potassium supplementation is always preferred when feasible.

26. Commonly used parenteral solutions: *also contains 4 mEq/L K, 1.5 mEq/L Ca, and 28 mEq/L lactate.

27. Hyponatremia: • To maintain a normal plasma [Na], the ingestion of solute-free water must eventually lead to the loss of the same volume of the electrolyte-free water. • Tree steps are required for the kidney to excrete a water load: 1- glomerular filtration. 2-active reabsorption of Na and Cl without water at diluting segments (distal convoluted tubule and thick ascending limb of the loop of Henle)

28. 3- maintenance of a dilute urine due to impermeability of the collecting duct to water in the absence of ADH. Abnormalities of any of these steps can result in impaired free water excretion and hyponat- remia.

29. Most causes of hyponatremia are associated with a low plasma osmolality. • The ECF volume, reflecting total body Na content, may be decreased (hypovolemic), normal (euvolemic), or increased (hypervolemic) in hyponatremia. • Etiology: 1-Hypo-osmolar hyponatremia: a. Hypovolemic hyponatremia. b. Hypervolemic hyponatremia. c. Euvolemic hyponatremia.

30. 2- hyponatremia with normal osmolality (pseudo- hyponatremia). 3- hyperosmolar hyponatremia. * post-transurethral resection of the prostate (post-TURP) syndrome.

32. 1-Hypo-osmolar hyponatremia: a-Hypovolemic hypo-osmolar hyponatremia: • A decreased effective circulating volume stimulates the hypothalamic thirst centers and vasopressin release, which both cause hyponatremia from free water accumulation. • Thiazide use is a classic and very common cause of hyponatremia by tow mechanisms: a. impaired water excretion by inducing a decreased effective circulating volume. b. secondary hyperaldosteronism.

33. Cerebral salt wasting: a. Hypovolemic patient. b. Related to neurosurgery and CNS trauma especially SAH. c. Excessive renal Na excretion. d. Hyponatremia dose not always correct with volume resuscitation. e. Labs: same labs of patient with SIADH. b-Hypervolemic hypo-osmolar hyponatremia: • The increase in total body Na is exceeded by the rise in total body water.

34. The degree of hyponatremia often correlates with the severity of the underlying condition and is therefore an important prognostic factor. • As a result of a decrease in effective circulating volume, leading to increased thirst and vasopressin levels. • In oliguric renal failure, if free water intake exceeds the kidney’s limited ability to excrete an equivalent volume of free water, hyponatremia will occurs.

35. Prepared by : Dr. Abeer Shurbaji

36. Remember…….. • Your fluid orders are incomplete until you have ordered monitoring labs (elyctrolytes, Cr, etc), I/O, daily weights. • IVF orders are often more art than science. • There may be several reasonable rates to order be sure that you have thought your orders out. • The basic fluid, i.e. isotonic, hypotonic, hypertonic, should be clear.

37. Thank you