Recapitulation of Acid-Base Concepts
Blood gas analysis (PaO ; Pa2O ) and b2ood pH are the best overall tests of lung
Arterial blood gas values cannot be determined precisely by clinical assessment – even
by experience clinicians (examples: cyanosis doesn’t appear until PaO around 50 2 60
mm Hg and % saturation of 80%; auscultation for air exchange is poor indicator of lung
If the PaCO is2normal, is the patient necessarily exhibiting normal lung function? Not
necessarily since the individual could be hyperventilating to decrease the PaCO to 2
Oximetry measures % saturation between hemoglobin and oxygen, NOT PaO 2
Henderson-Hasselbach equation and its importance: Analysis of acid-base status of
blood for the bicarbonate buffer system. If the ratio of bicarbonate to CO is 20, 2he pH
will be 7.4 (normal)
Compensations brought about by the lungs or kidneys help maintain this ratio.
The 4 possible alterations in A-B balance are respiratory acidosis, respiratory alkalosis,
metabolic acidosis, metabolic alkalosis. Respiratory defects are brought about by
alterations in ventilation whereas metabolic defects are due to a gain or loss of either
hydrogen or bicarbonate ions.
Primary change is a decrease in HCO -
Cause can be acids in blood due to diabetes mellitus or tissue hypoxia (increase in lactic
Shift initially is downward
Respiratory compensation is via increased ventilation (due to H stimulation of
chemoreceptors) which then decreases pCO and then i2creases the ratio and the pH.
Shift is to the right along the lower buffer curve.
Metabolic Alkalosis Increased ratio due to increase in HCO 3-
Cause can be ingestion of bases or antacids or loss of acids due to vomiting
Shift initially is upward
Respiratory compensation is via decreased ventilation which increases pCO and 2
movement is along the upper buffer line to the left. The compensation is minor.
Metabolic Causes of Acidosis
Drug ingestion (methanol, ethanol, ethylene glycol, ammonium chloride)
Lactic acidosis (shock, acute respiratory distress syndrome (ARDS), carbon monoxide)
Ketoacidosis (diabetes, starvation, alcoholism)
Metabolic Causes of Alkalosis
Vomiting (nasogastric suctioning)
Steps in analyzing arterial blood gas samples
Step 1: Is there acidosis of alkalosis?
Step 2: Is the primary disorder of respiratory or metabolic origin? In other words, does
the pH move in appropriate direction with pCO ? If 2o, this is respiratory disorder. If pH
does not move in the appropriate direction, then it is a metabolic disorder with
Step 3: Is there evidence of compensation – respiratory or metabolic? If so, is it acute or
Acute: For each increase in 10 mm Hg of PaCO , the pH2will drop .08 units if no
time for renal compensation. Chronic: For each i