Blood Gases


                                Acid-Base Balance & Blood Gas Interpretation

Acid – Base Balance

   -  Acid-base balance is critical in maintaining homeostasis in the body.
   -  Concentration of hydrogen ions in the body expressed as pH. Inverse relationship:
        More H+ ions = lower pH (acidic)

        Fewer H+ ions = higher pH (alkaline)

        Notice on scale - The relationship  of 1 part carbonic acid (H2CO3) to 20 parts 
        bicarbonate (HCO3) maintains hyrogen ion concentration (pH) within normal limits.

        Accumulation of acid in blood or loss of base = acidosis

        Accumulation of base in blood or loss of acid = alkalosis 

ABG: Arterial Blood Gas
   -  Arterial blood is used because venous blood is not suitable for the assessment 
      of oxygen tension & pH.
   -  Nurse draws blood from an artery without exposure to air in a heparinized syringe.
   -  When analyzing ABG’s, the most important component to look at first is the pH
   -  Let’s look at normal values that need to be memorized:
         pH	7.35-7.45
         PaCO2  35-45 mmHg
         HCO3	  22-26 mEq/L
         PaO2	  80-100 mmHg
         SaO2  95-100%
         BE (base excess) +2  (the amt of H+ ions that would be required to return the 
         pH of the blood back to normal

*The main 3 you will need to know are pH, CO2, & HCO3.

Examples using line chart:

pH = 7.29 (A)                           pH = 7.28 (A)
PaCO2 = 50 (A)                       PaCO2 = 40 (Normal)
HCO3 = 24 (Normal)               HCO3 = 20 (A)
Respiratory Acidosis            Metabolic Acidosis

pH = 7.5  (B)                            pH = 7.51  (B)
PaCO2 = 33  (B)                      PaCO2 = 39 (Normal)
HCO3 = 25 (Normal)               HCO3 = 28 (B)
Respiratory Alkalosis            Metabolic Alkalosis

  Always look at pH 1st
       -Below normal – acidosis
       -Above normal – alkalosis

  PaCO2 ↑or↓ - Respiratory – lungs (resp system controls CO2 concentration in 
  ECF by changes in rate & depth of respiration)
        -Hypoventilation, depressed breathing, drug overdose, airway obstruction 
        (build-up of CO2↑) = Resp Acidosis
	-Hyperventilation (blowing off lot of CO2↓), anxiety, fever = Resp Alkalosis

  HCO3 ↑or↓ - Metabolic – kidneys cause inc or dec quantity of HCO3 in ECF, 
  they combine bicarb or hydrogen with other substances & excrete them in urine.
	-Shock, diabetic ketoacidosis, diarrhea, renal failure = Metabolic acidosis 
         (pH ↓, HCO3 ↓)
	-Vomiting, loss of gastric juices = (pH ↑HCO3 ↑)  = Metabolic alkalosis

*Remember pneumonic ROME – Respiratory Opposite, Metabolic Equal.

     -Memorize the normal values (pH, CO2, HCO3)

     -Make a line chart (ABA on left, BAB on right)

     -Look at pH first

     -Which is abnormal? 
   	   CO2 (lungs/respiratory) 
	   or HCO3 (kidneys/metabolic)?

     -Follow these steps & you should get all your ABG problems right!


Mometrix eLibrary