Sodium & Water Assessment & Therapeutics

Updated August 2016

Instructor

Peter Loewen, B.Sc.(Pharm), ACPR, Pharm.D., FCSHP, RPh | Peter.Loewen@ubc.ca phone 604-506-8011

Pre-Session Objectives

PRIOR TO arriving at the session, participants should be able to describe

  1. The distribution of total body water (TBW)
  2. The influence of changes in TBW on serum Na concentration
  3. The pathophysiology and clinical presentation of diabetes insipidus and SIADH
  4. The clinical parameters for assessing Extracellular Fluid (ECF, “volume”) Status and Intracellular Fluid (ICF) status.

Preparation for the session

  1. Loewen’s Sodium & Water Assessment & Therapeutics 1-pager
  2. McGee S, Abernethy WB, Simel DL. Is this patient hypovolemic? JAMA 1999;281:1022-9. –the classic article on this subject.
  3. Freda BJ, Davidson MB, Hall PM. Evaluation of hyponatremia: A little physiology goes a long way. Cleveland Clin J Med 2004;71:639-50.
  4. Try the self-assessment questions and read the cases.
  5. Install “MedCalc”, “MedMath”, “Mediquations”, or equivalent on your phone and find the fluid&lytes tools in there. Find formulas for “Water deficit” or “Free water deficit”, “Change in serum sodium”, “Fractional excretion of Na”.

Pre-session self-assessment

  • Take a crack at trying to apply the 1-pager approach to some of the cases. This is will start to develop your APPROACH. 

Visuals/Cases for the Session

By the end of the session, and upon further learning and reflection students should be able to

  1. Describe the difference between water and volume, dehydration, and volume depletion
  2. Using physical assessment and laboratory parameters, diagnose the type of water-related defect a patient exhibits (e.g. hypovolemia, hyponatremia, hypernatremia, SIADH)
  3. Demonstrate an APPROACH to evaluating water-related problems in a patient and their potential causes
  4. Design a detailed therapeutic plan for treating the water-related disorder, including:

    • Selecting and writing orders for an appropriate crystalloid solution (if required) or other drug therapy.

    • Quantitative determination of quantities of crystalloid required and infusion rates

    • Writing orders for (or conducting themselves) an appropriate monitoring plan.

Required Skills

  1. assess your patient for postural BP and HR changes
  2. assess your patient's JVP | Jugular venous pressure: a cardinal sign | 

Why Na and H2O Matter in Pharmacotherapy Practice

  • Your patient is on multiple antihypertensives and today complains of dizziness on rising. BP reasonably well controlled. Should you decrease the dose of an antihypertensive?

  • Your patient with parkinson's disease seems to be developing postural hypotension. Does he need midodrine?

  • You're consulted about a patient with severe hyponatremia and recent SSRI initiation. Is the hyponatremia drug-induced?

  • Your CHF patient needs to be started on ACE-I. Her SCr is 155. How worried about a rise in SCr upon starting ACE-I are you?

  • Your CHF patient is stable on ramipril 10mg, B-blocker, and furosemide 40mg. Lately his SCr has been creeping up. Physicians intends to decrease the ramipril dose because everybody knows ACE-I can cause renal failure. Is this a good idea?

  • Your patient with HTN is admitted to hospital with CAP and has a serum Na of 129. Admitting physicians documents that the patient's HCTZ is being held due to "HCTZ-induced hyponatremia" and that it should not be restarted. Is this sensible?

  • Your patient has recurrent angina despite amlodipine+NTG patch. You believe metoprolol will be indicated, but are worried about his standing BP of 110/70 and postural drop. Can his anti-ischemic therapy be augmented?

  • Ever been asked to "please procure some demeclocycline"? Know what tolvaptan is?

  • Your vancomycin-treated patient is starting to show signs of GFR decline. Is it due to vancomycin?

Common Misconceptions and Sticking Points

  1. Explain why serum sodium is a reflection of your patient's ICF.
  2. Explain how giving your patient too much free water causes their serum sodium to go down. EXPLANATION | EXPLANATION (HD)
  3. Explain how causing your patient to lose free water (eg, by administering a loop diuretic) causes their serum sodium to go up. EXPLANATION | EXPLANATION(HD)
  4. Explain why giving your hypovolemic hyponatremic patient normal saline causes their serum sodium to go up.
  5. Explain why it is a mistake to believe that hypernatremia causes ICF contraction, or hyponatremia causes ICF expansion.

Further reading on Na/H2O management