Here is a basic way to read arterial blood gases.
1. First look at the disease of your patient. That will give you valuable information on whether it is a respiratory or metabolic problem.
2. Look at the pH. Normal is 7.35-7.45. If it is high, then the problem is alkalosis. If it is low, the problem is acidosis.
3. Look at the PaCO2 . Normal is 35-45. If the PaCO2 is high and the pH is low, then the patient probably has respiratory acidosis. Go back to the patient history to check. If the PaCO2 is low and the pH is high, then it might be respiratory alkalosis. Again go back to the patient history.
4. Look at the HCO3. Normal is 22-26. If the HCO3 is high and the pH is high, then the patient probably has metabolic alkalosis. Go back to the patient history. If the HCO3 is low and the pH is low, then the patient probably has metabolic acidosis. Again go back to the patient history.
So why do yo keep going back to the patient history? As the patient begins to compensate, the values of the opposite side will change. For example if the patient has respiratory acidosis over a long period of time, the body will compensate by increasing HCO3 levels. The final step is to determine if the body is compensating for the disorder. Another example of compensation- metabolic acidosis in a diabetic patient. The patient’s respiratory rate will increase and depth of respirations will increase in an attempt to blow off more CO2. So the HCO3 is low due to the diabetic acidosis,then CO2 will lower as the body tries to compensate.
For this class, know how to do the basic analysis. I won’t ask you about compensation- partial or full.
I. Hydrogen ion balance CO2 + H2O <——> H2CO3 <——> H+ + HCO3-
A. Chemical buffer system- proteins, HCO3-, Hgb, immediate
B. Respiratory system- CO2 + H2O —–> H2CO3, rapid
C. Renal system- H2CO3 ——-> H+ + HCO3-, slow
II. Respiratory acidosis- basic cause hypoventilation
1. Inhibition of respiratory center- drugs, oxygen therapy in chronic hypercapnia, cardiac arrest, sleep apnea
2. Disorders of the respiratory muscles and chest wall- neuromuscular diseases, chest cage deformity- kyphoscoliosis, extreme obesity, chest wall injury- fractured ribs.
3. Disorders of gas exchange- COPD, severe pneumonia, asthma, acute pulmonary edema, pneumothorax
4. Acute upper airway obstruction- aspiration of a foreign body or vomitus, laryngospasms
B. Clinical manifestations- dyspnea, disorientation, coma, dysrhythmias, hyperkalemia, hypoxemia, tremor, increased intracranial pressure.
C. Medical management- increase ventilation. Caution in giving O2 to patients with chronic hypercapnia.
III. Respiratory alkalosis- basic cause hyperventilation
1. Cerebral stimulation of respiration- psychogenic, hypermetabolic states such as fever, CNS disorders, head trauma, brain tumor, Salicylate intoxication- early
2. Hypoxia- pneumonia, asthma, pulmonary edema, congestive heart failure, pulmonary fibrosis, high altitude residence
3. Excessive mechanical ventilation
4. Gram negative sepsis, cirrhosis, exercise
B. Clinical manifestations- yawning, A light headedness, circumoral paresthesia, numbness, tingling of fingers and toes, carpopedal spasms, chronic exhaustion, palpitations, anxiety, dry mouth, sleeplessness, cold and clammy hands and feet, inability to concentrate, mental confusion, syncope, hypokalemia
C. Medical management- treat underlying cause, paper bag, sedation
IV. Metabolic acidosis
1. Bicarbonate loss- GI- diarrhea, ileostomy; renal loss- hypoaldosteronism
2. Increased acid load- hyperalimentation
3. Rapid infusion of saline solution
4. Increased acid production- lactic acid, diabetic ketoacidosis, starvation, alcohol intoxication
5. Ingestion of toxic substances- salicylate overdose, methanol, antifreeze
6. Failure of acid secretion- renal failure
B. Clinical manifestations- Kussmal breathing (deep, rapid respirations), peripheral vasodilation, hypotension, cardiac dysrhythmia, lethargy, coma, nausea, vomiting, growth failure, bone disorders, headache
C. Medical management- treat underlying cause, IV lactated ringer’s solution, if severe- NaHCO3.
V. Metabolic alkalosis- increase in plasma bicarbonate
1. GI loss- vomiting, chloride losing diarrhea
2. Renal loss- diuretics, mineralcorticosteroid excess- hyperaldosteronism, Cushing syndrome, excess licorice intake, carbenicillin or penicillin
4. Excess NaHCO3 administration
5. Antacids, milk, milk-alkali syndrome
6. Massive citrated blood transfusion
B. Clinical manifestations- signs of hypokalemia, hypoventilation, dysrhythmias
C. Medical management- mild chloride responsive metabolic alkalosis- parenteral isotonic saline with KCl, treat underlying condition, IV HCl if life threatening
VI. Arterial blood gases