4.3 Enteral & Parenteral Nutrition, Fluid and Electrolyte Balance & Acid-Base Disorders

4.3 Enteral & Parenteral Nutrition, Fluid and Electrolyte Balance & Acid-Base Disorders

Quick Answer: Clinical management of fluid, electrolyte, and nutritional support requires rigorous scientific principles: verifying nasogastric tube placement via gastric aspirate pH testing (pH ≤ 5.5) or abdominal radiography rather than the discredited air auscultation method; maintaining head-of-bed elevation at 30°–45° to eliminate aspiration risk; and preventing refeeding syndrome in malnourished patients. Acute electrolyte emergencies—such as hyperkalemia—require urgent myocardial membrane stabilization using intravenous calcium gluconate before shifting potassium intracellularly with insulin and dextrose. Arterial blood gas interpretation utilizes systematic analysis of pH (7.35–7.45), PaCO2 (35–45 mmHg), and HCO3 (22–26 mmol/L) to determine the primary disorder and degree of renal or respiratory compensation.

---

Enteral Nutrition and Nasogastric Tube (NGT) Management

Enteral nutrition involves administering nutrients directly into the gastrointestinal tract via a tube when oral intake is compromised but gut mucosal function remains intact (e.g., severe dysphagia following cerebrovascular accident, facial trauma, mechanical ventilation).

Anatomical Measurement: The NEX Method

Prior to introducing a large-bore or small-bore polyurethane NGT, the nurse must measure the required insertion length individually:

• NEX Method: Measure from the tip of the Nose to the Earlobe, and down to the Xiphoid process of the sternum. For adult gastric placement, many evidence-based guidelines recommend adding an additional 5 to 10 cm beyond the xiphoid marking to ensure the distal discharge ports sit fully within the gastric body rather than the lower esophageal sphincter.
• Mark the designated depth on the tube with indelible marker or medical tape.

Insertion Technique and Patient Positioning

Position the conscious patient in a High Fowler's position (90 degrees) with the neck extended initially. Lubricate the distal 10 cm with water-soluble lubricant. Advance the tube gently through the wider naris along the nasal floor toward the ear. When the tube reaches the posterior nasopharynx (often causing gagging), instruct the patient to flex the chin forward toward the chest (which closes the glottis and opens the esophagus) and take small sips of water through a straw while advancing the tube in tandem with each swallow.

                               TUBE PLACEMENT VERIFICATION
                                             |
                   +-------------------------+-------------------------+
                   |                                                   |
         GOLD STANDARD & BEDSIDE                              DANGEROUS & DISCREDITED
- Abdominal X-ray: Definitive confirmation           - Auscultatory "Whoosh" Test: Air
- Gastric Aspirate pH: pH <= 5.5 using                injected into lungs/pleura sounds
  narrow-range pH indicator paper                     identical to epigastric stomach sound!

Verification of Tube Placement

[!CAUTION] Exam Alert: Abandoning the Auscultatory "Whoosh" Test! The historical practice of injecting a bolus of air through the NGT while auscultating the epigastrium for a "gurgle" or "whoosh" is unreliable, unsafe, and completely discredited. Numerous clinical studies and autopsy reports have proven that air injected into a tube inadvertently misplaced into the bronchus, pleural space, or esophagus can produce sounds acoustically indistinguishable from gastric insufflation, leading to fatal pulmonary feedings.

1. First-Line Clinical Bedside Standard (Aspirate pH Testing): Aspirate 1–2 mL of fluid using a 50 mL catheter-tip syringe. Test the aspirate on narrow-range pH indicator paper (graded in 0.5 increments):
   • Gastric Aspirate: Displays an acidic pH of ≤ 5.5 (typically 1.0 to 4.5 in patients not taking gastric acid suppressants). A pH of ≤ 5.5 reliably confirms gastric placement.
   • Respiratory / Pleural Aspirate: Typically displays an alkaline pH ≥ 7.0.
   • Small Intestinal Aspirate: Displays a bile-stained, neutral-to-alkaline pH of 6.0 to 8.0.
   • Caveat: In patients receiving continuous enteral feeds or proton pump inhibitors (PPIs like omeprazole), gastric pH may rise to 6.0; if pH is > 5.5, feeding must be withheld pending radiographic confirmation.
2. Definitive Gold Standard: An abdominal X-ray demonstrating the radiopaque tube course crossing the midline, descending below the left diaphragm, and terminating within the stomach bubble is mandatory prior to initial feed administration in unconscious or high-risk patients.

Enteral Feeding Protocols and Aspiration Prophylaxis

• Aspiration Prevention: The patient's head of bed (HOB) must be maintained elevated at 30° to 45° at all times during continuous feeding, and during plus at least 30 to 60 minutes after intermittent bolus feeds. If the patient must be placed flat for hygiene or linen changes, the enteral infusion must be paused.
• Gastric Residual Volume (GRV): For continuous feeds, aspirate residual stomach contents every 4 to 6 hours. While modern critical care guidelines discourage routine cessation for isolated mild elevations, a GRV exceeding 250 to 500 mL accompanied by abdominal distension, nausea, or vomiting requires pausing the feed, evaluating gut motility, and considering prokinetic agents (e.g., metoclopramide).
• Tube Flushing: Flush the tube with 30 mL of sterile or potable water before and after feeds or medication administration, and every 4 hours during continuous infusions to preserve lumen patency.

Refeeding Syndrome Pathophysiology

In chronically malnourished patients (severe anorexia, prolonged starvation, chronic alcoholism, Kwashiorkor/Marasmus), introducing concentrated carbohydrate enteral or parenteral feeding triggers a dangerous metabolic shift:

Prolonged Starvation (Fat/Protein Catabolism) ---> Sudden Carbohydrate Influx
                                                              |
                                                    Massive Insulin Surge
                                                              |
                                  +---------------------------+---------------------------+
                                  |                                                       |
                 Rapid Cellular Uptake of PO4, K+, Mg2+                   Renal Sodium/Water Retention
                                  |                                                       |
                  Severe Hypophosphatemia, Hypokalemia                     Volume Overload, Congestive
                  Cardiac Arrhythmias, Respiratory Failure                   Heart Failure, Pulmonary Edema

• Clinical Manifestations: Profound hypophosphatemia causes diaphragmatic contractility failure, acute respiratory failure, hemolytic anemia, and ventricular arrhythmias.
• Prevention: "Start low and go slow." Initiate feeds at 25% of caloric requirements, check baseline electrolyte levels, and proactively replete phosphate, potassium, and magnesium alongside thiamine supplementation before advancing caloric intake.

---

Total Parenteral Nutrition (TPN) Principles

Total Parenteral Nutrition bypasses the gastrointestinal tract entirely, delivering amino acids, hypertonic dextrose, lipids, electrolytes, trace elements, and vitamins directly into the vascular tree. Indicated when the GI tract is non-functional (e.g., massive bowel resection, high-output enterocutaneous fistulae, mechanical intestinal obstruction, severe intractable pancreatitis).

Vascular Access and Infusion Mechanics

TPN solutions exhibit extreme hypertonicity (hyperosmolar, >1,000 mOsm/L due to 20%–50% dextrose concentrations). Infusion into peripheral veins produces rapid phlebitis, endothelial damage, and venous thrombosis. Therefore, TPN requires a Central Venous Catheter (CVC) or Peripherally Inserted Central Catheter (PICC) with the distal catheter tip located in the high-flow superior vena cava. (Peripheral Parenteral Nutrition [PPN] is limited to low-osmolar solutions with dextrose < 10%).

Nursing Management and Complications

1. Strict Surgical Asepsis: TPN solutions provide an ideal bacterial and fungal culture medium. Central line dressing changes require sterile gloves and mask; IV tubing and inline filters must be changed every 24 hours. The TPN lumen must be dedicated strictly to nutrition—never use the TPN line for blood draws, blood transfusions, or piggyback medications.
2. Blood Glucose Surveillance: Check blood glucose levels every 4 to 6 hours. High dextrose infusions stimulate pancreatic insulin secretion; sliding-scale regular insulin is administered as needed.
3. Preventing Rebound Hypoglycemia: Never abruptly terminate a TPN infusion. The high circulating insulin level will cause rapid, profound rebound hypoglycemia. The infusion must be tapered down over several hours. If a TPN bag runs out unexpectedly and a replacement bag is delayed from the pharmacy, immediately hang an infusion of 10% Dextrose in Water (D10W) at the same hourly rate to maintain euglycemia.

---

Fluid Balance, Intake & Output, and Insensible Losses

Maintaining fluid homeostasis requires balancing total fluid intake against measurable and unmeasurable outputs.

Normal Fluid Balance Components

• Intake: Oral fluids, fluid in food, enteral feedings, IV fluids, IV flushes/medications, and blood products.
• Sensible Output: Measurable liquid losses: urine (normal adult minimum 0.5 mL/kg/hour or approximately 30 mL/hour), gastrointestinal drainage (vomitus, nasogastric aspirate, liquid diarrhea, ostomy output), and wound drainage.
• Insensible Losses: Unmeasurable evaporation from the skin (perspiration) and respiratory tract (exhaled water vapor), averaging 600 to 900 mL/day in a temperate climate.

[!TIP] Tropical Climate Consideration: Insensible Fluid Adjustments In the warm, humid climate of Ghana, cutaneous perspiration increases substantially. Furthermore, febrile states augment insensible losses by approximately 10% to 15% for every 1°C elevation in body temperature above 37°C, demanding proactive fluid replenishment to avoid hypovolemic dehydration.

Hypovolemia vs. Hypervolemia Assessment

---

Key Electrolyte Imbalances and Emergency Protocols

Electrolytes maintain neuromuscular membrane resting potentials, cellular volume, and biochemical enzymatic pathways.

                       CRITICAL ELECTROLYTE DERANGEMENTS
                                       |
       +-------------------------------+-------------------------------+
       |                               |                               |
   POTASSIUM (K+)                 SODIUM (Na+)                   CALCIUM (Ca2+)
  3.5 - 5.0 mmol/L               135 - 145 mmol/L               2.15 - 2.55 mmol/L
- Hyperkalemia: Peaked T,      - Hyponatremia: Cerebral        - Hypocalcemia: Tetany,
  QRS wide, arrest               edema, confusion, seizure       Chvostek's & Trousseau's
- Hypokalemia: U waves,        - Hypernatremia: Cellular       - Hypercalcemia: Stones,
  cramps, paralytic ileus        shrinkage, thirst, coma         bones, groans, moans

1. Potassium Derangements (Normal: 3.5 – 5.0 mmol/L)

• Hypokalemia (< 3.5 mmol/L):
  • Causes: Loop diuretics (furosemide), profuse diarrhea, prolonged vomiting/NGT suction, hyperaldosteronism.
  • Signs & ECG: Skeletal muscle weakness, paresthesias, hyporeflexia, paralytic ileus; ECG displays flattened T waves, ST-segment depression, and prominent U waves.
  • Management: Oral or cautious intravenous replacement. Maximum peripheral IV infusion rate is 10 mmol/hour (20 mmol/hr via central line with cardiac monitor). Never give IV push!
• Hyperkalemia (> 5.0 mmol/L):
  • Causes: Chronic kidney disease, acute renal failure, tissue crush injuries, potassium-sparing diuretics, severe acidosis.
  • Signs & ECG: Neuromuscular irritability, ascending flaccid paralysis; ECG displays tall peaked T waves, flattened P waves, prolonged PR interval, widening QRS complex, degenerating into sine-wave patterns and ventricular fibrillation / asystole.
  • Emergency Treatment Protocol (Step-by-Step):
    1. Myocardial Membrane Stabilization: Administer IV Calcium Gluconate (10% solution, 10 mL over 2–5 minutes). Antagonizes potassium-induced membrane excitability within 1–3 minutes without changing serum potassium levels.
    2. Intracellular Potassium Shifting: Administer 10 units of Regular Insulin IV bolus accompanied immediately by 50 mL of 50% Dextrose (D50W). Insulin drives potassium into cells via the Na+/K+ ATPase pump within 15–30 minutes (dextrose prevents hypoglycemia). Additionally, administer nebulized Salbutamol (albuterol 10–20 mg) to promote intracellular shift.
    3. Potassium Elimination: Administer loop diuretics (IV furosemide), oral or rectal cation-exchange resins (Calcium Polystyrene Sulfonate / Resonium), or initiate emergent hemodialysis.

2. Sodium Derangements (Normal: 135 – 145 mmol/L)

• Hyponatremia (< 135 mmol/L):
  • Pathophysiology: Extracellular hypo-osmolality causes water to shift into brain cells, producing cerebral edema. Manifests as headache, lethargy, confusion, muscle cramps, seizures, and herniation.
  • Management: Mild cases managed with fluid restriction. Severe symptomatic hyponatremia with seizures warrants slow, controlled infusion of hypertonic 3% Saline. Correction must not exceed 8–10 mmol/L per 24 hours to prevent Osmotic Demyelination Syndrome (central pontine myelinolysis).
• Hypernatremia (> 145 mmol/L):
  • Pathophysiology: Water exits brain cells into hyperosmolar blood, causing cellular shrinkage. Manifests as intense thirst, dry sticky mucous membranes, restlessness, delirium, and subarachnoid hemorrhage from retracted bridging veins. Correct slowly with hypotonic fluids (0.45% NaCl or 5% Dextrose) to prevent cerebral edema.

3. Calcium Derangements (Normal: 2.15 – 2.55 mmol/L / 8.5 – 10.5 mg/dL)

• Hypocalcemia (< 2.15 mmol/L):
  • Causes: Hypoparathyroidism (often post-thyroidectomy), severe acute pancreatitis, vitamin D deficiency, massive citrated blood transfusions.
  • Clinical Assessment Signs:
    • Chvostek's Sign: Tapping the facial nerve anterior to the earlobe elicits unilateral, involuntary twitching of the ipsilateral facial and lip muscles.
    • Trousseau's Sign: Inflating a sphygmomanometer cuff on the upper arm to 20 mmHg above systolic BP for 3 minutes precipitates painful carpopedal spasm (wrist flexion, metacarpophalangeal flexion, adducted thumb).
    • Neuromuscular: Laryngeal stridor, muscle tetany, prolonged QT interval on ECG. Managed with emergent IV calcium gluconate.
• Hypercalcemia (> 2.55 mmol/L):
  • Mnemonic: "Stones (nephrolithiasis), Bones (bone pain/osteopenia), Abdominal Groans (constipation, pancreatitis, peptic ulcers), and Psychic Moans (confusion, depression, lethargy)." Treated with vigorous normal saline hydration, IV bisphosphonates, and furosemide.

---

Arterial Blood Gas (ABG) Analysis & Acid-Base Disorders

Acid-base balance reflects systemic hydrogen ion ($H^+$) concentration, tightly regulated by chemical buffers, respiratory excretion of $CO_2$, and renal tubular excretion/reabsorption of $HCO_3^-$.

Normal ABG Reference Ranges

• pH: 7.35 – 7.45 (Acidosis < 7.35; Alkalosis > 7.45)
• $PaCO_2$ (Respiratory Component): 35 – 45 mmHg (Acidic gas; >45 indicates respiratory acidosis; <35 indicates respiratory alkalosis)
• $HCO_3^-$ (Metabolic Component): 22 – 26 mmol/L (Basic buffer; <22 indicates metabolic acidosis; >26 indicates metabolic alkalosis)
• $PaO_2$: 80 – 100 mmHg
• Base Excess (BE): -2 to +2 mmol/L

Systematic 5-Step ABG Interpretation Method

1. Step 1 (pH): Is the pH acidic (<7.35), normal (7.35–7.45), or alkalotic (>7.45)?
2. Step 2 ($PaCO_2$): Is the $PaCO_2$ elevated (>45), normal (35–45), or reduced (<35)?
3. Step 3 ($HCO_3^-$): Is the bicarbonate reduced (<22), normal (22–26), or elevated (>26)?
4. Step 4 (Primary Driver): Match the parameter that shifts in the same direction as the pH derangement:
   • If pH is acidic and $PaCO_2$ is high $\rightarrow$ Respiratory Acidosis.
   • If pH is acidic and $HCO_3^-$ is low $\rightarrow$ Metabolic Acidosis.
   • If pH is alkalotic and $PaCO_2$ is low $\rightarrow$ Respiratory Alkalosis.
   • If pH is alkalotic and $HCO_3^-$ is high $\rightarrow$ Metabolic Alkalosis.
5. Step 5 (Determine Compensation):
   • Uncompensated: pH is abnormal; the opposite system parameter remains within normal reference ranges.
   • Partially Compensated: pH is abnormal; the opposite system parameter has moved outside its normal range in an attempt to normalize pH.
   • Fully Compensated: pH has returned to the normal range (7.35–7.45), but both $PaCO_2$ and $HCO_3^-$ remain abnormal.

Acid-Base Diagnostic Summary Table