Kidney, Fluids & Electrolyte: Clinical Application (Dr. Hawthrone Slides)

What are crystalloids?

IV fluids made of water and small dissolved molecules/electrolytes that can distribute from the blood stream into other extracellular compartments.

What are examples of crystalloid IV fluids?

Sodium chloride (saline), Lactated Ringer's, Dextrose solutions

What type of fluid is the most common and what we usually consider as IV fluids in the sense of hydrating someone or providing maintenance fluids?

Crystalloids

What are colloids?

IV fluids containing large molecules, such as albumin, that generally remain in the intravascular space and help retain fluid within the blood vessels

What do we typcially use Colloid IV fluids for?

Used for manipulating fluid movement within the body; not really used for hydrating people. Trying to move water from extracellular to intracellular.

Because colloids are these large molecules, when they go into the intravascular space, what happens to them?

They stay there, because they aren’t going to diffuse across any membranes

What type of solutions are considered to be equal to our blood?

Isotonic solutions such as normal saline (0.9% NaCl) or lactated Ringer's.

What is the tonicity of 0.45% NaCl?

Hypotonic

What is the tonicity of 0.9% NaCl (normal saline)?

Isotonic

What is the tonicity of Lactated Ringer’s (LR)?

Isotonic

What is the tonicity of 3% NaCl?

Hypertonic

Why do we call dextrose 5% “free water” (D5W)?

When we administer it intravascularly, the dextrose piece becomes hydrolyzed, and basically what we’re left with in our vasculature is just water.

What is the electrolyte value of Normal Saline?

Na: 154 mEq/L Cl: 154 mEq/L

What electrolytes are there in Lactated Ringers (LR)?

Na, Cl, K, Ca, lactate

Define serum osmolality.

Concentration of dissolved particles like electrolytes and glucose in the blood for clinical analysis; critical indicator of the body’s water balance and overall hydration status.

Give serum osmolality’s normal value.

280-300 mOsm/L

What is the clinical significance of serum osmolality?

Abnormal serum osmolality values can signal dehydration, overhydration, or underlying metabolic disorders, necessitating further investigation.

How do you estimate total body water?

TBW = body weight (kg) x estimated fraction of body weight that is water.

Example: an 80 kg male at 60% TBW → 80 × 0.60 = 48 L.

How is a patient’s fluid status assessed?

Using the patient’s history, current medications, objective data such as, weight and intake/outtake, physical exam, and lab values to determine whether the patient is hypovolemic, euvolemic, or hypervolemic

Define hypovolemic

The patient has decreased circulating/extracellular fluid volume

Define euvolemic

The patient’s fluid volume is approximately normal

Define hypervolemic

The patient has excess circulating/extracellular fluid volume

What patient history can help assess fluid status?

Ask about vomiting, diarrhea, poor fluid intake, kidney disease, and HF; Vomiting and diarrhea can lead to fluid loss, while kidney disease and heart failure can affect fluid retention/volume overload.

What physical exam findings are used to assess fluid status?

Assess BP, mucous membranes, skin turgor, cardiopulmonary findings, edema, orthostasis, tachycardia, and urine characteristics. These findings help determine whether the patient has too little or too much extracellular fluid.

What is BUN (blood urea nitrogen)?

BUN measures the concentration of nitrogen from urea in the blood. Urea is produced from protein metabolism, primarily by the liver, and is eliminated through the kidneys.

What is the normal BUN range?

~ 8-23 mg/dL

What can an increased BUN indicate?

dehydration/volume depletion, renal dysfunction, GI bleeding, high-dose steroids, or certain medication adverse effects. BUN should be interpreted with SCr and the patient’s clinical picture.

Why can dehydration increase BUN?

With volume depletion, renal blood flow decreases and the kidneys conserve water. This increases urea reabsorption, causing BUN to rise. Therefore, dehydration can cause an elevated BUN even without intrinsic kidney damage.

What is the BUN:SCr ratio used for?

It helps assess causes of acute kidney injury and can provide evidence of volume depletion/dehydration. A BUN:SCr ratio > 20:1 can suggest a prerenal process such as dehydration.

How do you calculate the BUN:SCr ratio and interpret it?

BUN divided by SCr = BUN:SCr ratio; If >than 20 → dehydrated

How do you calculate maintenance fluid requirements for an adult?

30-35 mL/kg/day x body weight (kg) → Then divide the total mL/day by 24 hours to get the maintenance rate in mL/hr. Round the hourly rate to the nearest 5 mL/hr.

Why are Na⁺, K⁺, and Cl^- important in fluid balance?

Their concentrations differ inside vs outside cells, creating concentration and electrical gradients. Na⁺ and Cl^- are mainly extracellular, while K⁺ is mainly intracellular. These gradients help regulate water movement across cell membranes by osmosis.

Why can electrolyte abnormalities affect the heart and nerves?

Na⁺, K⁺, and Ca²⁺ are involved in action potentials. Changes in their concentrations can alter nerve impulses and cardiac electrical activity, potentially causing arrthythmias.

How do the kidneys contribute to acid-bace balance?

They regulate H⁺ and HCO₃^-. They can excrete H⁺ and conserve/generate HCO₃^- to help maintain normal blood pH

What role do electrolytes play in metabolism?

They can serve as cofactors or help support enzymatic reactions; for example, Mg²⁺ and K⁺ participate in metabolic enzyme activity.

What is the normal serum sodium range, and how are sodium disorders classified?

Normal Na⁺ = 135-145 mEq/L Hyponatremia and Hypernatremia; Sodium disorders must also be assessed with patient’s volume status

What volume states can occur with hypernatremia?

Hypovolemic, euvolemic, and hypervolemic.

What volume states can occur with hyponatremia?

Hypovolemic, euvolemic, and hypervolemic, and can also be isotonic or hypertonic hyponatremia

What are common signs and symptoms of hypernatremia and hyponatremia?

Both can cause neurologic symptoms such as agitation, reduced deep tendon reflexes, disorientation, headaches, lethargy, muscle cramps, N/V, and seizures. (Severe sodium abnormalites affect the brain because changes in tonicity causes water to move into or out of brain cells.)

What are common causes of Hypernatremia?

GI losses, diuretics, renal failure, diabetes insipidus, iatrogenic (sodium-containing fluids)

What are common causes of hyponatremia?

GI losses, diuretics, renal failure, CHF (congestive heart failure), and syndrome of inappropriate antidiuretic hormone secretion (SIADH).

How do you estimate serum osmolality?

(2 x Na⁺) + (glucose / 18) + (BUN / 2.8) = estimated serum osmolality in mOsm/kg

Normal = 285-295 mOsm/kg

What is the first step when evaluating hyponatremia?

If serum Na⁺ is <135 mEq/L, first assess serum osmolality/tonicity to determine whether the hyponatremia is isotonic, hypotonic, or hypertonic.

What is hypertonic hyponatremia?

Low Na⁺ with high serum osmolality (>300) , usually due to another osmole such as glucose, mannitol, or glycine pulls water into the vascular space and dilutes the serum Na⁺.

What is isotonic hyponatremia?

Low Na⁺ with normal serum osmolality and is usually pseudohyponatremia, such as with triglycerides >1,000 mg/dL or hyperpoteinemia.

What is hypotonic hyponatremia?

Low Na⁺ with low serum osmolality (<280). This is the type where you next assess the patient’s volume status.

After identifying hypotonic hyponatremia, what is the next step?

Assess the patient's volume status to determine if they are hypovolemic, euvolemic, or hypervolemic.

What is hypovolemic hypotonic hypponatremia?

The patient has lost both sodium and water, but proportionally more sodium than water. Therefore, total body Na⁺↓ ↓ and total body water ↓.

What is hypervolemic hypotonic hyponatremia?

The patient has too much sodium and too much water overall, but the increase in water is greater. Therefore, total body Na⁺ ↑ but total body water increasing, causing diluted serum Na⁺ . Think heart failure or cirrhosis.

What is the key difference among the 3 volume states of Hypernatremia?

Hypovolemic = loses Na⁺ and water, but lose MORE water.

Euvolemic = lose water without much Na⁺

Hypervolemic = gain Na⁺ and water, but gain MORE Na⁺

What is the easiest way to think about sodium disorders?

Serum sodium is about the ratio of sodium to water, not simply how much sodium is in the body. Hypernatremia = too much Na⁺ relative to water. Hyponatremia = too little Na⁺ relative to water.

How does diabetes insipidus cause hypernatremia?

The kidneys cannot appropriately retain water because of a problem with ADH production/release (centra DI) or the kidney’s response to ADH (nephrogenic DI) → large amounts of dilute urine → free- water loss → serum Na⁺ and osmolality increase.

What electrolyte abnormality is especially important in refeeding syndrome?

Hypomagnesemia (low Mg2+). The P2 specifically emphasized magnesium with refeeding syndrome.

Why can a hemolyzed blood sample cause a falsely elevated potassium level?

RBCs contain a large amount of intracellular K+. When RBCs rupture during hemolysis, K+ is released into the serum sample → falsely elevated serum K+. Think: hemolyzed sample → RBCs burst → K+ comes out.

If a potassium result is unexpectedly high and the blood sample is hemolyzed, what should you suspect?

Pseudohyperkalemia (a falsely elevated K+) caused by K+ leaking out of ruptured RBCs.

What is the major intracellular electrolyte?

K+ is the major intracellular electrolyte. Think: K+ lives primarily INSIDE cells.

What is the major extracellular electrolyte?

Na+ is the major extracellular electrolyte. Think: Na+ lives primarily OUTSIDE cells.

How should you approach a patient with an abnormal sodium level on an exam?

1. Identify hyponatremia vs hypernatremia → 2. Assess serum tonicity/osmolality → 3. Assess volume status (hypovolemic, euvolemic, or hypervolemic) → 4. Use the combination to determine the likely cause.