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HLT-LPN37-03 · Lesson 2 · 14 clock hours · theory

Fluid, Electrolyte, and Acid-Base Balance

Objective. Students will interpret common fluid, electrolyte, and arterial blood gas imbalances and identify appropriate nursing interventions for each condition.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed clinician in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it to make decisions about any patient or medication. Always follow your instructor, your supervising professional, and current official references and regulations.

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Fluid Compartments and Osmolality

Body fluid is distributed across two main compartments. Intracellular fluid (ICF) accounts for roughly two-thirds of total body water and is found inside cells. Extracellular fluid (ECF) makes up the remaining one-third and includes plasma (intravascular fluid) and interstitial fluid (the fluid between cells). A third, smaller ECF subdivision called transcellular fluid includes cerebrospinal fluid, synovial fluid, and secretions.

Osmolality describes the concentration of solutes dissolved in a fluid, measured in milliosmoles per kilogram (mOsm/kg). Normal serum osmolality is approximately 275–295 mOsm/kg. Water moves by osmosis from areas of lower solute concentration toward areas of higher solute concentration. This principle drives the shifts that cause many fluid and electrolyte disorders. Sodium is the dominant extracellular cation and the primary regulator of ECF osmolality.

Fluid Imbalances

Dehydration (Fluid Volume Deficit): Occurs when fluid output exceeds intake. Causes include vomiting, diarrhea, fever, excessive sweating, inadequate intake, or uncontrolled diabetes. Signs include dry mucous membranes, poor skin turgor, concentrated urine (dark yellow, high specific gravity), tachycardia, hypotension, and elevated serum osmolality and hematocrit. Nursing priority: restore fluid balance with oral fluids when safe, or IV fluids as ordered; monitor urine output (target generally at least 30 mL/hr in adults); document intake and output accurately.

Fluid Volume Excess (Overload): Occurs when the body retains more fluid than it excretes. Common causes are heart failure, renal failure, cirrhosis, and excessive IV fluid administration. Signs include edema (pitting), crackles in the lungs, shortness of breath, weight gain, distended neck veins, and hypertension. Nursing priority: monitor daily weights (same time, same scale, same clothing), restrict fluids and sodium as ordered, elevate the head of the bed, administer diuretics as ordered, and assess lung sounds frequently.

Safety point: Always verify IV fluid orders before hanging a bag. Infusing the wrong fluid or rate can rapidly worsen either deficit or overload.

Common Electrolyte Disorders

Hyponatremia (serum sodium below the normal range, typically < 135 mEq/L): Causes include excess water intake, heart failure, and SIADH. Signs: headache, confusion, nausea, seizures in severe cases. Nursing actions: fluid restriction as ordered, neurological monitoring, seizure precautions.

Hypernatremia (serum sodium above the normal upper limit, typically > 145 mEq/L): Causes include inadequate fluid intake, diabetes insipidus, excessive sodium intake. Signs: thirst, dry mucous membranes, agitation, confusion. Nursing actions: provide free water as ordered, monitor neurological status, gradual correction to avoid cerebral edema.

Hypokalemia (serum potassium below the normal range, typically < 3.5 mEq/L): Causes include diuretic use, vomiting, diarrhea. Signs: muscle weakness, leg cramps, constipation, cardiac dysrhythmias, flattened T-waves on ECG. Safety point: IV potassium must always be diluted and administered slowly via infusion pump; never give IV potassium as a bolus — it can cause fatal cardiac arrest.

Hyperkalemia (serum potassium above the normal upper limit, typically > 5.0 mEq/L): Causes include renal failure, excessive supplementation, acidosis. Signs: muscle weakness, peaked T-waves, bradycardia, cardiac arrest in severe cases. Nursing actions: cardiac monitoring, hold potassium supplements and potassium-sparing medications, dietary restriction, administer treatments as ordered (e.g., sodium polystyrene sulfonate, insulin with dextrose).

Hypocalcemia (serum calcium below the normal range): Causes include hypoparathyroidism, vitamin D deficiency, pancreatitis. Signs: numbness and tingling (especially perioral), muscle cramps, tetany, positive Trousseau's and Chvostek's signs, laryngospasm. Nursing actions: administer calcium supplements as ordered, seizure precautions, fall precautions.

Hypercalcemia (serum calcium above the normal upper limit): Causes include hyperparathyroidism and malignancy. Signs: "bones, stones, groans, and psychic moans" — bone pain, kidney stones, constipation, confusion. Nursing actions: encourage fluids, promote mobility, administer IV fluids and medications as ordered, fall precautions due to muscle weakness.

Acid-Base Balance: A Four-Step Approach

The lungs and kidneys work together to maintain blood pH within approximately 7.35–7.45. Arterial blood gas (ABG) analysis is the primary tool for evaluating acid-base status.

Step 1 — Assess pH: Below 7.35 = acidosis; above 7.45 = alkalosis.

Step 2 — Assess PaCO2 (normal ~35–45 mmHg): CO2 is a respiratory acid. If PaCO2 is elevated, the condition is respiratory acidosis; if reduced, it is respiratory alkalosis. Ask: does the PaCO2 match the pH direction?

Step 3 — Assess HCO3 (normal ~22–26 mEq/L): Bicarbonate is a metabolic base. If HCO3 is low, the condition is metabolic acidosis; if high, it is metabolic alkalosis. Ask: does the HCO3 match the pH direction?

Step 4 — Assess for compensation: The system not responsible for the primary disorder will try to correct the pH. Compensation is partial if pH is still outside normal range; full if pH has returned to normal.

Common respiratory acidosis causes: Hypoventilation, COPD exacerbation, opioid overdose, pneumonia. Nursing priority: improve ventilation — position, encourage deep breathing, administer oxygen or support mechanical ventilation as ordered; for opioid overdose, prepare to administer reversal agent per order.

Respiratory alkalosis: Hyperventilation (anxiety, pain, fever). Nursing priority: treat underlying cause; coach slow breathing.

Metabolic acidosis: Diabetic ketoacidosis, renal failure, severe diarrhea. Nursing priority: treat underlying cause; monitor airway and breathing; administer bicarbonate only as specifically ordered.

Metabolic alkalosis: Prolonged vomiting, excessive antacid use, NG suctioning. Nursing priority: replace fluid and electrolytes as ordered, treat cause.

Safety note: Always compare ABG values as a set — interpreting any single value in isolation can lead to incorrect treatment and patient harm.

Key terms8

OsmolalityThe concentration of solutes in a fluid, expressed in milliosmoles per kilogram (mOsm/kg); normal serum osmolality is approximately 275–295 mOsm/kg.
HypokalemiaA serum potassium level below the lower limit of normal (typically < 3.5 mEq/L), associated with muscle weakness and cardiac dysrhythmias.
HyperkalemiaA serum potassium level above the upper limit of normal (typically > 5.0 mEq/L), which can cause life-threatening cardiac dysrhythmias.
Respiratory AcidosisAn acid-base imbalance characterized by low pH and elevated PaCO2, caused by inadequate ventilation and CO2 retention.
Metabolic AlkalosisAn acid-base imbalance characterized by elevated pH and elevated HCO3, often caused by prolonged vomiting or excessive use of antacids.
Trousseau's SignA clinical indicator of hypocalcemia in which carpal spasm is elicited by inflating a blood pressure cuff on the upper arm above systolic pressure for a few minutes.
Fluid Volume DeficitA state in which the body loses more fluid than it takes in, leading to signs such as tachycardia, hypotension, dry mucous membranes, and decreased urine output.
CompensationThe physiologic response by the lungs or kidneys to correct a primary acid-base disturbance and return the pH toward the normal range.

Worked examples4

1. Interpret the following ABG: pH 7.30, PaCO2 55 mmHg, HCO3 24 mEq/L. Classify the disorder and identify one nursing priority.
  1. Step 1 — pH: 7.30 is below 7.35, so the patient is in acidosis.
  2. Step 2 — PaCO2: 55 mmHg is above the upper limit of normal (45 mmHg). Elevated CO2 drives acidosis, and this matches the low pH, so the primary problem is respiratory.
  3. Step 3 — HCO3: 24 mEq/L is within the normal range (22–26 mEq/L), indicating the kidneys have not yet compensated.
  4. Step 4 — Compensation: HCO3 is normal, so there is no metabolic compensation; this is an uncompensated respiratory acidosis.
Answer: Uncompensated respiratory acidosis. Nursing priority: promote ventilation — reposition the patient to optimize lung expansion, encourage deep breathing and coughing, administer supplemental oxygen as ordered, and notify the provider.
2. Interpret the following ABG: pH 7.50, PaCO2 30 mmHg, HCO3 23 mEq/L. Classify the disorder and identify one nursing priority.
  1. Step 1 — pH: 7.50 is above 7.45, so the patient is in alkalosis.
  2. Step 2 — PaCO2: 30 mmHg is below the lower limit of normal (35 mmHg). Low CO2 (blown off) drives alkalosis and matches the high pH, so the primary problem is respiratory.
  3. Step 3 — HCO3: 23 mEq/L is within the normal range, indicating no metabolic compensation.
  4. Step 4 — Compensation: HCO3 is normal; this is an uncompensated respiratory alkalosis.
Answer: Uncompensated respiratory alkalosis. Nursing priority: identify and treat the cause of hyperventilation (anxiety, pain, fever); coach the patient to slow their breathing rate.
3. Interpret the following ABG: pH 7.28, PaCO2 38 mmHg, HCO3 16 mEq/L. Classify the disorder and identify one nursing priority.
  1. Step 1 — pH: 7.28 is below 7.35, indicating acidosis.
  2. Step 2 — PaCO2: 38 mmHg is within the normal range (35–45 mmHg), so the respiratory system is not the primary cause.
  3. Step 3 — HCO3: 16 mEq/L is below the lower limit of normal (22 mEq/L). Low bicarbonate drives acidosis and matches the low pH, so the primary problem is metabolic.
  4. Step 4 — Compensation: PaCO2 is normal, meaning the lungs have not yet significantly compensated; this is an uncompensated metabolic acidosis.
Answer: Uncompensated metabolic acidosis. Nursing priority: identify and treat the underlying cause (e.g., diabetic ketoacidosis, renal failure, severe diarrhea); monitor airway and respiratory status closely, as the lungs will attempt to compensate by increasing the respiratory rate.
4. A patient has a serum potassium of 2.9 mEq/L. The provider orders potassium chloride 40 mEq in 100 mL normal saline IV. What is the nurse's primary safety consideration before and during administration?
  1. Step 1 — Verify the order: confirm dilution, concentration, and infusion rate are within safe parameters per institutional policy; IV potassium should never be given as a bolus.
  2. Step 2 — Use an infusion pump: set the pump to deliver the solution at the ordered rate to prevent accidental rapid infusion.
  3. Step 3 — Monitor during infusion: assess the IV site for phlebitis or infiltration (potassium is irritating to veins); place the patient on continuous cardiac monitoring to detect dysrhythmias.
  4. Step 4 — Reassess: recheck serum potassium level after infusion as ordered and document response.
Answer: The primary safety consideration is that IV potassium must always be administered diluted, via an infusion pump, at the ordered rate — never as an IV bolus. Rapid infusion of potassium can cause fatal cardiac dysrhythmias. Continuous cardiac monitoring is required during administration.
In class: Illustrated lecture on fluid compartments, osmolality, and the pathophysiology of dehydration, fluid overload, and common electrolyte disorders (hypo/hypernatremia, hypo/hyperkalemia, hypo/hypercalcemia) using diagrams and clinical photos. · Lab-value interpretation drill: students work in pairs to analyze a set of ten printed lab reports, flag abnormal values, state clinical significance, and list two nursing actions for each abnormality. · Acid-base worksheet: students use a systematic four-step approach (pH, PaCO2, HCO3, compensation) to classify six ABG results as respiratory acidosis, respiratory alkalosis, metabolic acidosis, or metabolic alkalosis and identify one nursing priority for each.
Materials: Lewis's Medical-Surgical Nursing, printed lab-value reference ranges, ABG interpretation worksheet, colored pens for flagging, calculator
Check: Students correctly classify at least 5 of 6 ABG results and identify appropriate nursing interventions on a short quiz; pass = 80%.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Physiological Integrity: Physiological Adaptation
1. A nurse is caring for a patient with heart failure who has gained 3 pounds overnight. Lung sounds reveal bilateral crackles. Which intervention is the nurse's highest priority?
Physiological Integrity: Reduction of Risk Potential
2. A patient's ABG results are: pH 7.32, PaCO2 58 mmHg, HCO3 25 mEq/L. Which interpretation is correct?
Safe and Effective Care Environment: Safety and Infection Control
3. A nurse is preparing to administer an IV potassium replacement to a patient with hypokalemia. Which action is most important for patient safety?
Physiological Integrity: Physiological Adaptation
4. Which clinical finding is most consistent with hypocalcemia?
Physiological Integrity: Reduction of Risk Potential
5. A patient with prolonged vomiting has the following ABG: pH 7.49, PaCO2 44 mmHg, HCO3 32 mEq/L. Which classification is correct?
Physiological Integrity: Physiological Adaptation
6. A nurse is assessing a patient with hypernatremia. Which sign or symptom is most expected?
Physiological Integrity: Reduction of Risk Potential
7. The nurse is reviewing laboratory results for a patient on loop diuretics. Which finding requires immediate notification of the provider?
Physiological Integrity: Basic Care and Comfort
8. Which nursing intervention is most appropriate for a patient with fluid volume deficit?
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