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!