1.6 - Acid Base Disorders Flashcards
What are acid-base disorders?
- Disturbances in the processes that maintain a constant body hydrogen ion concentration (H+) or pH throughout all body fluids
- The arterial pH points to the primary disorder in acid-base distrubances
How is acid-base balance maintained in the body?
Acid base balance is maintained by:
- Normal pulmonary excretion of carbon dioxide
- Metabolic utilization of organic acids
- Renal excretion of nonvolatile acids.
Maintenance of pH is essential for normal cellular function
What are the 3 mechanisms that maintain body pH?
1.Chemical buffering
- It is mediated by HCO3 in the ECF and by protein and phosphate buffers in the ICF.
- The normal HCO3 is 24 + 2 mEq/L.
2. Alveolar ventilation
- This minimizes variations in the pH by altering the pCO2.
3. Renal H+ handling
- This allows the kidney to adapt to changes in the acid-base balance by HCO3 reabsorption.
What are the 2 mechanisms the body uses to compensates for changes in acid-base balance?
- For respiratory abnormalities - the body will compensate metabolically by making changes in the serum bicarbonate through the kidney (renal regulation) by secreting hydrogen in exchange for sodium. The time of full compensation may vary from 3-5 day to begin and 7 days until full compensation is seen.
- For metabolic disorders - compensation is through the pulmonary regulation of partial pressure of carbon dioxide (pCO2). Occurs more rapidly than metabolic compensation
What are the normal values seen in an ABG?
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Normal pH = 7.35-7.45
- Acidemia = pH <7.35
- Alkalemia = pH >7.45
- Normal HCO3 = 22-26 mEq/L
- Normal pCO2 = 35-45
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Normal AG = 8-10 meq/L
- AG = NA+ - ([Cl-] + [HCO3-])
Normal values for peripheral venous blood gases differ from those of arterial blood due to the uptake and buffering of metabolically produced CO2 in the capillary circulation and the addition of organic acids that are produced by the tissue bed that is drained by the venous circulation. In addition, careful attention should be paid to the use of the tourniquet. If a tourniquet is used it should be loosened about one minute before the sample is drawn to avoid changes that may occur with ischemia
What are the 6 steps to ABG interpretation?
Step 1 – Check the arterial blood gas. See above for pH values that determine academia and alkalemia
Step 2 – Establish the primary disturbance by determining whether the change in HCO3- or pCO2 can account for the abnormal pH.
- In acidemia a decreased HCO3 suggests metabolic acidosis and an elevated CO2 suggests respiratory acidosis.
- In alkalemia an elevated HCO3 suggests metabolic alkalosis and a decreased pCO2 suggests respiratory alkalosis
- A combined disorder is present when the pH is normal but you see BOTH an abnormal pCO2 and HCO3 that subsequently cause an alteration in the pH.
Step 3 – Determine whether compensation is appropriate. The compensatory mechanism is our body’s way of trying to fix the abnormal pH. A respiratory process that shifts the pH in one direction will be compensated by a metabolic process that shifts the pH in the opposite direction and vice versa.
- The effect of compensation is to attenuate but not completely correct the primary change in pH
- An inappropriate compensatory response suggests the presence of a combined disorder
Step 4 – Determine the anion gap
- The anion gap is a way of demonstrating the accumulation of unmeasured anions. Certain forms of acidosis are characterized by these unmeasured anions.
Step 5 – Assess the delta gap – To maintain a stable total anion content, every increase in an unmeasured anion should be met with a decrease in HCO3. Comparing the change in the AG with the change in the HCO3 is a simple way of making sure that each change in the AG is accounted for. If the delta anion gap= delta HCO3, this is a simple anion gap metabolic acidosis. If the delta anion gap > delta HCO3, the HCO3 did not decrease as much as expected. This would be considered a metabolic acidosis and an anion gap metabolic acidosis.
