Acid-base Balance

ACID-BASE BALANCE AND OXYGENATION M.R.CUENO, RN

Blood gases I. Physiological controls A. Chemical buffer systems in ICF and ECF 1. Bicarbonate-carbonic acid a. Ratio of bicarb to carbonic acid is 20:1

b. Responsible for 45% of all H+ buffering 2. Inorganic phosphates 3. Plasma proteins 4. Intracellular buffers - proteins, organic & inorganic phosphates 5. RBC buffers - hemoglobin B. Respiratory - removal of CO2 1. PaCO2 is an acid, serum CO2 is a base 2. Rise in PaCO2 is powerful stimulus for increased respirations - tidal

volume & rate 3. Consistent PaCO2 > 50 mm Hg desensitizes the respiratory center

4. Compensation a. metabolic acidosis - increased respirations b. metabolic alkalosis - decreased respirations c. respiratory compensation has limits C. Renal regulation 1. Bicarbonate is reabsorbed from kidney tubules. Kidney manufactures bicarbonate 2. Hydrogen ion - excreted during acidosis and retained during alkalosis 3. Kidneys can compensate for respiratory imbalances over a period of days II. Tests to measure acid-base A. Anion gap Na+ - (CL- + HCO2) = 12 -15 mEq/L

1. Determines if metabolic acidosis is from excessive acid or loss of bicarbonate B. pH - reflects H+ concentration C. Total serum CO2 - indirect measure of bicarbonate

1. Increased in metabolic acidosis 2. Decreased in metabolic alkalosis 3. Total CO2 is 95% HCO3 and 5% CO2 gas and H2CO3

D. Blood gases 1. Normal values a. pH 7.35-7.45 (usually fatal if < 6.8 or > 7.6) b. PaCO2 35-45 mm Hg c. PaO2 80-100 mm Hg d. HCO3 22-26 mEq/L e. O2 saturation (SaO2) 95-100% - Oxyhemoglobin dissociation

curve f.

Base excess/deficit -2 to +2 mEq/L

g. O2 content - 15-23 vol% - measurement of total O2 in blood,

including that bound to Hgb and free dissolved in plasma) 2. Pediatric procedures a. Transcutaneous pO2 monitoring - 50-100 mm Hg

i. special heated electrode placed on head, chest or thigh ii. warmth causes O2 to diffuse out of capillaries iii. electrode measures O2 at skin surface iv. proportional to capillary O2

b. Fetal scalp vein pH 7.25-7.40 i. Helps determine if cesarean section is needed III. How to analyze ABGs A. Determine if acidosis or alkalosis by looking at pH B. Determine the primary disturbance 1. Look at the HCO3 and CO2 - which one goes in the same direction as the

pH? If the CO2 is in the same direction, it is respiratory; if the HCO3 is in the same direction, it is metabolic. a. CO2 > 40 = acidosis b. CO2 < 40 = alkalosis c. HCO3 > 24 = alkalosis d. HCO3 < 24 = acidosis

e. May have a combined cause C. Compensation 1. Look at the value that is not the same as primary cause. If it is going in the opposite direction (alkalosis or acidosis) as the primary problem, then compensation is occurring. Compensation may be partial or complete. Renal can completely compensate for respiratory. IV. Metabolic acidosis - base deficit A. Causes

1. Excess acid a. Ketoacidosis - diabetes or starvation

b. Lactic acidosis - burns, myocardial infarction, shock c. Salicylate poisoning

d. Uremia e. Methanol or ethylene glycol toxicity 2. Decreased base a. GI losses - diarrhea, fistula, NG drainage b. Ureteroconduits - gain of chloride from urine c. Acetazolamide (Diamox)

B. Clinical picture 1. Headache, confusion, drowsiness 2. Deep rapid respirations (Kussmaul)

3. Nausea & vomiting 4. Peripheral vasodilation 5. Hyperkalemia often present

C. Compensation 1. Hyperventilation to remove CO2 2. Kidneys excrete more H+ and produce more HCO3

D. Nursing considerations 1. Protect confused patients from injury 2. Good pulmonary hygiene 3. Good oral care 4. Monitor potassium V. Metabolic alkalosis A. Causes 1. GI losses - vomiting, gastric suctioning, pyloric stenosis 2. Hypokalemia - potassium-losing diuretics, kidneys excrete H+ and try to

conserve K 3. Hyperaldosteronism, Cushing's syndrome

4. Excessive antacid intake 5. Diuretics 6. Placing patient with COPD on mechanical ventilation, relief of chronic respiratory acidosis B. Clinical picture 1. Depressed respirations

2. Hypocalcemia - decreased ionized Ca - dizziness, tingling of fingers &

toes, circumoral paraesthesia, carpopedal spasm, hypertonic muscles 3. Hypokalemia often present

C. Compensation 1. Hypoventilation - retain CO2 2. Kidneys excrete more HCO3

D. Treatment 1. Treat underlying cause 2. Give NS for kidney to absorb Na and Cl allowing excretion of HCO3

E. Nursing considerations 1. Replace GI losses with isotonic fluids, flush NG tube with NS 2. Teach patient not to use baking soda as antacid 3. Monitor K levels VI. Respiratory acidosis - excess CO2

A. Causes 1. Lung diseases - COPD, ARDS 2. Neuromuscular diseases affecting respirations - Guillian-Barre, multiple

sclerosis 3. Anesthesia 4. Pneumonia B. Clinical picture 1. Acute a. Feeling of fullness in head - cerebral vasodilation

b. Confusion, dizziness, lethargy, restlessness, decreased attention span c. Decreased respirations d. Dysrhythmias 2. Chronic a. Weakness, dull headache b. Barrel chest, use of accessory muscles to breathe C. Compensation 1. Kidneys retain HCO3

D. Treatment 1. Administer oxygen - low concentration if chronic 2. Keep airway clear, postural drainage & vibration, humidified air 3. Force fluids

4. Promote lung expansion - incentive spirometer, blowing up balloons.

E. Nursing considerations 1. Assess respiratory status frequently- effort, accessory muscle use, lung sounds, skin color, mental status 2. Monitor serial ABGs 3. Assist patient into comfortable positions that facilitate lung expansion Semi- or high Fowlers 4. Be prepared to intubate

5. Teach exercises to enhance expiration - pursed lip breathing VII. Respiratory alkalosis - deficit of CO2

A. Causes 1. Hyperventilation a. extreme anxiety b. hypoxemia c. high fever d. early salicylate poisoning

e. bacteremia f. CNS lesions g. pulmonary embolism h. overventilation with mechanical ventilators

i. pregnancy B. Clinical picture 1. Lightheadedness - cerebral vasoconstriction 2. inability to concentrate 3. hypocalcemia - numbness & tingling of extremities, circumoral

paresthesia C. Compensation - kidneys excrete increased HCO3

D. Treatment - treat cause E. Nursing considerations 1. Teach patient to focus on controlled breathing, relaxation techniques 2. Monitor rate and depth on mechanical ventilators