ACID-BASE IMBALANCES

Nick Alfaro

OBJECTIVESafter the completion of this concept you should be

able to:

• Characterize an acid and a base• Describe the intracellular and extracellular

mechanisms for buffering changes in body pH• Compare the role of the kidneys and

respiratory system in regulation of acid-base balance

• Identify the causes of acid-base imbalances• Analyze an ABG result

ACID-BASE CHEMISTRY

• ACID : a molecule that can release an H+ ion

• BASE : molecule that can accept or combine with H+ ion

• Acid + water = dissociation H+ (+) anion• CARBONIC ACID :the most important acid• BICARBONATE : the most important base

Hydrogen ion

• Expressed in terms of pH– The negative logarithm (p) of H+ ion concentration

in equivalents per liter– Example : pH 7.0

• hydrogen ion concentration of 10 (negative power of 7) equivalents per liter (mEq/L)

• pH is inversely related to H+ ion concentration:– Low pH = high H+ ion concentration– High pH = low H+ ion concentration

Metabolic acid and bicarbonate production

• Acids are byproducts of metabolic processes• 2 GROUPS OF ACIDS

– Volatile acids : carbonic acid (H2CO3)• Leaves the body through the lungs

– Nonvolatile/fixed acids : sulfuric acids, hydrochloric, phosphoric

• Source ; metabolism of dietary proteins• Buffered by body proteins or bicarbonate• Excreted by the kidney

• Lactic acid : incomplete oxidation of glucose

• Ketoacids : incomplete oxidation of fats

• MAJOR SOURCE OF BASE:– Metabolism of amino acids ( aspartate and

glutamate)– Metabolism of anions ( citrate, lactate,

acetate)

Carbon dioxide and bicarbonate

PRODUCTION

CARBON DIOXIDE• TRANSPORTED IN 3 FORMS

– Attached to hemoglobin– Dissolved CO2

• Combines with water in the blood streamcarbonic acid (H2CO3)

– Catalyzed by CARBONIC ANHYDRASE– As bicarbonate

• CARBONIC ANHYDRASE– Present in RBC, renal tubular`cells, other tissues in the body

• to calculate H2CO3:– CO2 X .03 = _____ H2CO3– EXAMPLE : PCO2 45 mmHg X 0.03 = 1.35 mEq/L

Sequence of events 1. CO2 produced by tissue

diffuses to interstitial fluid 2. CO2 diffuses into plasma 3. CO2 diffuses into RBC 4. Some CO2 picked up by Hb 5. Most CO2 reacts with water to

form carbonic acid-carbonic anhydrase

6. Carbonic acid dissociates into H+ and bicarbonate

7. Proton picked up by Hb preventing acidifying blood

8. Bicarbonate diffuses into plasma

9. Bicarbonate carried to lungs

10.Bicarbonate diffuses into RBC

11. H+ released from Hb combines with bicarbonate to form carbonic acid

12. CO2 formed from carbonic acid and unloaded from Hb. Diffuses out of blood into interstitial fluid

13.CO2 diffuses into alveolar space of lung

Regulation of pH

• Intracelular/extracellular

• Lungs kidneys

• Carbon dioxide conserve HCO3 excrete excess H+

Chemical buffer systems• Buffer is a substance that act as a chemical sponge. It

soaks up or release H ions to maintain stable Ph• ACTION TIME OF CHEMICAL BUFFER SYSTEM :

INSTANTANEOUS• CARBONIC ACID-BICARBONATE BUFFER

SYSTEM– Present in ECF– Carbonic acid is formed by the combination of carbon dioxide

and water (CO2+H2O=H2CO3)– 20 parts bicarbonate : 1 part carbonic acid– Maintained by lungs and kidney

• PHOSPHATE BUFFER SYSTEM– Present in cells and ECF– Activate in the kidneys

• PROTEIN BUFFER SYSTEM– Present in plasma and in cells– Albumin and plasma globulins are the major

protein buffers in the vascular compartment– Hgb is one of the proteins involved– Most plentiful buffer system

Respiratory control of pHaction time : minutes to hours

• Dec. in pH (more acid)

• Inc. RR and depth

• Inc. excretion of CO2

• alkalinity

• Inc. pH (more alkaline)

• Dec. RR and depth

• Retention of CO2

• acidity

Renal regulation of pH

• Controls bicarbonate concentration in ECF

• H ions can be exchanged for Na and K ions in the renal tubules

• Excretion or conservation of hydrogen ions can result in imbalances of Na and K

• Action time : hours to days

ABG ANALYSIS

• NORMAL VALUES• Blood pH 7.35-7.45• pO2 80-100 mmHg• pCO2 35-45 mmHg• HCO3 22-26 mEq/L• BE / BD ± 2

DRAWING OF ARTERIAL BLOOD SAMPLE FOR ABGUSING RADIAL ARTERY

Basic knowledge in interpretation

• If pH and pCO2 are primarily affected, respiratory acid-base imbalances will occur

• If pH and HCO3 are primarily affected, metabolic acid-base will be experienced

• The kidneys and lungs attempt to compensate one another in maintaining acid-base balance

• In acid-base imbalances, the normal bicarbonate-carbonic acid ratio of 20:1 is lost. The body attempts to compensate in an effort to maintain the normal 20:1 ratio

• In compensation, the kidneys attempt to compensate for changes in blood CO2 by making a corresponding adjustment in blood bicarbonate. Normally, almost all the bicarbonate formed by the kidneys are retained

• The lungs attempt to compensate for abnormal changes in blood bicarbonate by making corresponding adjustment in blood CO2

• Another compensatory mechanism for acid-base imbalances is shifting of hydrogen ions from the ECF to the ICF or vice versa

Shifting of hydrogen ions

• Inc. H ions (metabolic acidosis)

• H ions shift into ICF

• K moves out from ICF

• Inc. K (hyperkalemia)

• Dec. H ions ( metabolic alkalosis)

• H ions shift out from ICF

• K moves into ICF

• Dec. K (hypokalemia)

METABOLIC ACIDOSIS

• IMPAIRED ELIMINATION OF METABOLIC ACID– Kidney failure

• EXCESS PRODUCTION OF METABOLIC ACIDS– Diabetic ketoacidosis– Fasting and starvation– Poisoning (salicylate, methanol, ethylene glycol)

Metabolic acidosis

• EXCESS BICARBONATE LOSS– Diarrhea– Intestinal suction– Hyperaldosteronism (CUSHING SYNDROME)

• INCREASED CHLORIDE LEVEL– Excess reabsorption of chloride by the kidney– Parenteral hyperalimentation

METABOLIC ALKALOSIS

• EXCESS GAIN OF BICARNONATE– Administration of sodium bicarbonate– Blood transfusion (citrate-containing)

• EXCESS LOSS OF HYDROGEN ION– Vomiting– Gastric suction– Diuretic therapy (potassium deficit)– Hyperaldosteronism– Loss of body fluid

RESPIRATORY ACIDOSIS• DEPRESSION OF RESPIRATORY CENTER

– Drug overdose– Head injury

• LUNG DISEASE– COPD– Pneumonia– Pulmonary edema– Respiratory distress syndrome

• AIRWAY OBSTRUCTION/DISORDER OF CHEST WALL AND RESPIRATORY MUSCLE– Chest injuries– Extreme obesity– Respiratory muscle paralysis

• BREATHING AIR WITH HIGH CO2 CONTENT

RESPIRATORY ALKALOSIS

• EXCESSIVE VENTILATION– Anxiety and psychogenic hyperventilation– Hypoxia and reflex stimulation of ventilation– Stimulation to respiratory center

• Elevated blood ammonia level• Salicylate toxicity• Encephalitis• fever

• MECHANICAL VENTILATION

interpretations• pH• < 7.35 • acidosis• > 7.45• alkalosis• pCO2 (RESPIRATORY SIDE)• < 35• alkalosis• >45• acidosis• HCO3 (METABOLIC SIDE)• < 22 • acidosis• >26• alkalosis

4. Determine the extent of compensation

• ABSENT: the value that does not match the acid-base status of the pH is normal

• PARTIAL: both the value that doesn’t match the acid-base status of the pH and the pH itself are abnormal

• COMPLETE: the value that doesn’t match the acid-base status is abnormal but the pH is normal

Interpret the following• 1. pH 7.60, PaCO2 25, HCO3 24• Respiratory alkalosis, uncompensated• 2. pH 7.49, PaCO2 48, HCO3 38• Metabolic alkalosis partially compensated• 3. pH 7.36, PaCO2 66, HCO3 35• Respiratory acidosis completely compensated

• 4. pH 7.45, PaCO2 47, HCO3 34• Metabolic alkalosis completely compensated

• 5. pH 7.25 PaCO2 60, HCO3 26• Respiratory acidosis uncompensated

• 6. pH 7.45 PaCO2 47 HCO3 34 • 7. pH 7.30 PaCO2 30 HCO3 19• 8. pH 7.32 PaCO2 34 HCO3 17• 9. pH 7.52 PaCO2 49 HCO3 40• 10. pH 7.33 PaCO2 55 HCO3 29

• 6. pH 7.45 PaCO2 47 HCO3 34 • Alkalosis metabolic completely compensated• 7. pH 7.30 PaCO2 30 HCO3 19• Acidosis metabolic partially compensated• 8. pH 7.32 PaCO2 34 HCO3 17• Acidosis metabolic partially compensated• 9. pH 7.52 PaCO2 49 HCO3 40• Alkalosis metabolic partially compensated• 10. pH 7.33 PaCO2 55 HCO3 29• Acidosis respiratory partially compensated