1. ACID-BASE AND ELECTROLYTES
1. How do you analyze arterial blood gas values?
Remember three basic points:
1. pH tells you whether you are dealing with acidosis or alkalosis as the primary event. The
body will compensate as much as it can (secondary event).
2. Look at the carbon dioxide (CO 2) value. If it is high, the patient either has respiratory aci-
dosis (pH: < 7.4) or is compensating for metabolic alkalosis (pH: > 7.4). If CO2 is low, the patient
either has respiratory alkalosis (pH: > 7.4) or is compensating for metabolic acidosis (pH: < 7.4).
3. Look at the bicarbonate value. If it is high, the patient either has metabolic alkalosis (pH:
> 7.4) or is compensating for respiratory acidosis (pH: < 7.4). If bicarbonate is low, the patient
either has metabolic acidosis (pH: < 7.4) or is compensating for respiratory alkalosis (pH: > 7.4).
2. True or false: The body does not compensate beyond a normal pH.
True. For example, a patient with metabolic acidosis will eliminate CO, to help restore a
normal pH. However, if respiratory alkalosis is a compensatory mechanism (and not a rare, sep-
arate primary disturbance), the pH will not correct to > 7.4 . Overcorrection does not occur.
3. List the common causes of acidosis.
Respiratory acidosis: chronic obstructive pulmonary disease, asthma, drugs (e.g., opioids,
benzodiazepines, barbiturates, alcohol, other respiratory depressants), chest wall problems
(paralysis, pain), and sleep apnea.
Metabolic acidosis: ethanol, diabetic ketoacidosis, uremia, lactic acidosis (e.g., sepsis,
shock, bowel ischemia), methanol/ethylene glycol, aspirin/salicylate overdose, diarrhea, and car-
bonic anhydrase inhibitors.
4. List the common causes of alkalosis.
Respiratory alkalosis: anxiety/hyperventilation and aspirin/salicylate overdose.
Metabolic alkalosis: diuretics (except carbonic anhydrase inhibitors), vomiting, volume
contraction, antacid abuse/milk-alkali syndrome, and hyperaldosteronism.
5. What type of acid-base disturbance does aspirin overdose cause?
Respiratory alkalosis and metabolic acidosis (two different primary disturbances). Look for
coexisting tinnitus, hypoglycemia, vomiting, and a history of "swallowing several pills." Alka-
linization of the urine (with bicarbonate) speeds excretion.
6. What happens to the blood gas of patients with chronic lung conditions?
In certain people with chronic lung conditions (especially those with sleep apnea), pH may
be alkaline during the day because they breathe better when awake. In addition, just after an
episode of bronchitis or other respiratory disorder, the metabolic alkalosis that usually compen-
sates for respiratory acidosis is no longer a compensatory mechanism and becomes the primary
disturbance (elevated pH and bicarbonate). As a side note, remember that sleep apnea, like other
chronic lung diseases, can cause right-sided heart failure (cor pulmonale).
7. Should you give bicarbonate to a patient with acidosis?
For purposes of the Step 2 boards, almost never. First try intravenous fluids and correction
of the underlying disorder. If all other measures fail and the pH remains < 7.0, bicarbonate may
be given.
