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How do you analyze arterial blood gas values?
Remember three basic points: -
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). -
Look at the carbon dioxide (CO2) value. If it is high, the patient either has respiratory
acidosis (pH: less than 7.4) or is compensating for metabolic alkalosis (pH: greater than
7.4). If CO2 is low, the patient either has respiratory alkalosis (pH: greater than 7.4) or is
compensating for metabolic acidosis (pH: less than 7.4). -
Look at the bicarbonate value. If it is high, the patient either has metabolic alkalosis (pH:
greater than 7.4) or is compensating for respiratory acidosis (pH: less than 7.4). If
bicarbonate is low, the patient either has metabolic acidosis (pH: less than 7.4) or is
compensating for respiratory alkalosis (pH: greater than 7.4). -
True or false: The body does not compensate beyond a normal pH.
True. For example, a patient with metabolic acidosis will eliminate CO2 to help restore a normal pH.
However, if respiratory alkalosis is a compensatory mechanism (and not a rare, separate primary
disturbance), then the pH will not correct to greater than 7.4. Overcorrection does not occur. -
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
carbonic anhydrase inhibitors. -
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. -
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.”
Alkalinization of the urine (with bicarbonate) speeds excretion. -
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
compensates 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). -
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 less than 7.0,
bicarbonate may be given. -
The blood gas of a patient with asthma has changed from alkalotic to normal,
and the patient seems to be sleeping. Is the patient ready to go home?
For Step 2 purposes, this scenario means that the patient is probably crashing. Remember that
pH is initially high in patients with asthma because they are eliminating CO2. If the patient
becomes tired and does not breathe appropriately, CO2 will begin to rise and pH will begin to
normalize. Eventually the patient becomes acidotic and requires emergency intubation if
appropriate measures are not taken. If this scenario is mentioned on boards, the appropriate
response is to prepare for possible elective intubation and to continue aggressive medical
treatment with beta2 agonists, steroids, and oxygen. Fatigue secondary to work of breathing
is an indication for intubation. Asthmatic patients are supposed to be slightly alkalotic during
an asthma attack. If they are not, you should wonder why. -
List the signs and symptoms of hyponatremia.
n Lethargy
n Seizures
n Mental status changes or confusion
n Cramps
n Anorexia
n Coma -
How is hyponatremia treated?
For hypovolemic hyponatremia, the treatment is normal saline. Euvolemic and hypervolemic
hyponatremia are treated with water/fluid restriction; diuretics may be needed for hypervolemic
hyponatremia. -
What medication is used to treat SIADH if water restriction fails?
Demeclocycline, which induces nephrogenic diabetes insipidus. -
What happens if hyponatremia is corrected too quickly?
You may cause brainstem damage (central pontine myelinolysis). Hypertonic saline is used
only when a patient has seizures from severe hyponatremia—and even then, only briefly
and cautiously. Normal saline is a better choice 99% of the time for board purposes. In chronic
severe symptomatic hyponatremia, the rate of correction should not exceed 0.5 to 1 mEq/L/hour. -
What causes spurious (false) hyponatremia?
n Hyperglycemia (once glucose is greater than 200 mg/dL, sodium decreases by 1.6 mEq/L
for each rise of 100 mg/dL in glucose)
n Hyperproteinemia
n Hyperlipidemia
In these instances, the lab value is low, but the total body sodium is normal. Do not give the
patient extra salt or saline. -
What causes hyponatremia in postoperative patients?
The most common cause is the combination of pain and narcotics (causing SIADH) with
overaggressive administration of intravenous fluids. A rare cause that you may see on the
USMLE is adrenal insufficiency; in this instance, potassium is high and the blood pressure is low. -
What is the classic cause of hyponatremia in pregnant patients about to deliver?
Oxytocin, which has an antidiuretic hormone-like effect. -
What are the signs and symptoms of hypernatremia?
Basically the same as the signs and symptoms of hyponatremia:
n Mental status changes or confusion
n Seizures
n Hyperreflexia
n Coma -
What causes hypernatremia?
The most common cause is dehydration (free water loss) due to inadequate fluid intake relative
to bodily needs. Watch for diuretics, diabetes insipidus, diarrhea, and renal disease as well as
iatrogenic causes (administration of too much hypertonic intravenous fluid). Sickle cell
disease, which may lead to renal damage and isosthenuria (inability to concentrate urine), is a
rare cause of hypernatremia, as are hypokalemia and hypercalcemia, which also impair the
kidney’s concentrating ability. -
How is hypernatremia treated?
Treatment involves water replacement, but the patient often is severely dehydrated; therefore,
normal saline is used most frequently. Once the patient is hemodynamically stable, he or she
often is switched to ½ normal saline. Five percent dextrose in water (D5W) should not be used
for hypernatremia. -
What are the signs and symptoms of hypokalemia?
Hypokalemia causes muscular weakness, which can lead to paralysis and ventilatory failure.
When smooth muscles also are affected, patients may develop ileus and/or hypotension. Best
known and most tested, however, is the effect of hypokalemia on the heart. EKG findings
include loss of the T wave or T-wave flattening, the presence of U waves, premature ventricular
and atrial complexes, and ventricular and atrial tachyarrhythmias.