Acid-Base Balance

Acid-Base Balance

1. Acids

   • form constantly as byproducts of metabolism 
   • Volatile acids
     • Respiratory acids eliminated by lungs
   • Nonvolatile acids
     • Metabolic acids eliminated by kidney or metabolized by liver 

2. pH

   • -Indicates acid/alkaline state of fluids including body fluids pH=-log[H+] small change in pH --> large change in [H+]
   • Balance
     • Ration of 1:20, more important than absolute values  -Change in pH: diet, organ failure, metabolic abnormalities
     • Carbonic acid (H2CO3)
     • Bicarbonate (HCO3-)
   • Mechanisms for normal pH
     • -Failure of mechanisms --> acidosis or alkalosis 
     • Blood buffer system
       • -First line of defense against AB imblanace -Works instantly to buffer strong acids and bases -Bicarbonate system most important in plasma -Other buffer systems: protein, phosphate, Hgb 
       • H2CO3
       • NaHCO3
     • Respiratory system
       • -Lungs excrete CO2
     • Kidneys
       • -Kidneys excrete H+ ions (helps blood become less acidic) -Takes hours to days -Synthesize ammonia to excrete more H+ -Retain/reabsorb HCO3-

3. Acidosis

   • pH < 7.35
   • Neuro
     • Lethargy, CNS depression
   • CV
     • severe negative inotropic effect (decreased cardiac contractility), dysrhythmias
   • Electrolytes
     • (rise in extracellular cations) -Hyperkalemia  -Hypercalcemia
   • O2 delivery
     • -Shifts oxyhemoglobin dissociation curve to the right (decreased affinity of hemoglobin to oxygen) -Physiologic: Allows more oxygen to be released to tissues 
     • Oxyhemoglobin Dissociation Curve
       • -Relationship between pressure of O2 in the blood and O2 saturation -PO2=60, O2 sat is at 90% -

4. Alkalosis

   • Neuro
     • Overstimulation of CNS
   • CV
     • -Increased inotropy, coronary vasospasm, dysrhythmias
   • Electrolytes
     • -Hypokalemia -Hypercalcemia -Hypocloremia
   • O2 delivery
     • -Shifts OHD curve to the left (increased infinity to O2, difficult to get oxygen to tissues 

5. Diagnosis

   • Diagnosis of arterial blood
   • PaCO2
     • -Partial pressure of CO2 in blood -Ventilation indicator -Carbonic acid
   • HCO3
     • Metabolic indicator
   • pH
   • Base deficit/excess
     • -Calculation based on level of bicarbonate. -
   • PaO2
     • -Not 

6. Respiratory Acidosis

   • -Low pH -Retains Co2 (high pCO2) -Pathophys: hypoventilation --> retention of CO2 -S/S

7. Respiratory Alkalosis

   • High pH, low paCO2 -Patho: hyperventilation, leads to blowing off too much CO2 -paresthesias (overstimulation of CNS) -Decrease amount of ventilation (rebreath own CO2, decrease respiratory rate and depth)

8. Metabolic Acidosis

   • -Low pH, low HCO3 -Patho:    >Excess acid: ketoacidosis, lactic acidosis, renal failure (can't excrete H+ ions    >Loss of bicarb
   • Anion gap
     • -Ketoacidosis: diabetic, alcoholic, starvation -Lactic acidosis
   • Normal anion gap
     • -Renal tubular metabolic acidosis -Acidosis from GI losses

9. Metabolic Alkalosis

   • High pH, high HCO3 -Excess base (diuretics) or loss of acid (vomiting) rebreathe own CO2

10. Compensatory Mechanisms

    • High PaCO2
      • -Kidneys retain HCO3 --> "compensated" respiratory acidosis
    • High HCO3
      • Lungs hypoventilate, retain HCO3 --> a "compensated" metabolic alkalosis

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