ACID BASE

Acid-base disorders

Acid-base disorders are divided into two broad categories:

Those that affect respiration and cause changes in CO2 concentration are called respiratory acidosis (low pH) and respiratory alkalosis (high pH).

Respiratory acid-base disorders are commonly due to Lung diseases or conditions that affect normal breathing.

Acid-base disorders

Disorders that affect HCO3-

concentration are called metabolic acidosis (low pH) and metabolic alkalosis (high pH).

Metabolic acid-base disorders may be due to, kidney diseases electrolyte disturbances, severe vomiting or diarrhea, ingestion of certain drugs and toxins, and diseases that affect normal metabolism.

Acid-base disorders

Acute acidosis Increased rate and depth of

breathing, confusion, and headaches, and it can lead to seizures, coma, and in some cases death.

Alkalosis Symptoms of alkalosis are often due

to associated potassium (K+) loss and may include irritability, weakness, and cramping.

Acidosis Due to increased acid production within the body Consumption of substances that are metabolized

to acids Decreased acid excretion, or increased excretion

of base. Alkalosis Due to electrolyte disturbances caused by, for

example prolonged vomiting or severe dehydration

Administration or consumption of base, and hyperventilation

Respiratory acidosis

Reduced CO2 elimination Decreased breathing rate

(respiratory drive) due to drugs or central nervous system disorders

Respiratory muscle/nerve disease (myasthenia gravis.

Airway obstruction (food or foreign object)

Respiratory alkalosis

Increased CO2 elimination Hyperventilation due to anxiety, pain,

shock Drugs (aspirin) Pneumonia, pulmonary (lung) congestion,

or embolism Exercise, fever Central nervous system tumor, trauma,

infection Liver failure

Metabolic acidosis

Decreased HCO3-, due to loss or to

increased acid Alcoholic ketoacidosis Diabetic ketoacidosis Kidney failure Lactic acidosis Toxins – overdose of salicylates (aspirin),

methanol, ethylene glycol Gastrointestinal bicarbonate loss, such as

from prolonged diarrhea

Metabolic alkalosis

Increased HCO3-, due to loss of acid

or gain of bicarbonate Diuretics Prolonged vomiting Severe dehydration Administration of bicarbonate,

ingestion of alkali

Acid-Base Disorder pH HCO3

- PCO2

Body Compensation

Metabolic acidosis Less than 7.4 Low Low

Increased breathing rate (hyperventilation) to increase CO2 elimination

Metabolic alkalosis

Greater than 7.4 High High

Slowed breathing (hypoventilation) to decrease CO2 elimination

Respiratory acidosis Less than 7.4 High High

Kidney increases retention of HCO3

- and excretion of acid

Respiratory alkalosis

Greater than 7.4 Low Low

Kidney decreases retention of HCO3

- and excretion of acid

Metabolic Compensation

Acute Hypercapnia:HCO3 increases 1

mmol/L for each 10 mmHg increase in PaCO2 >40

Chronic Hypercapnia:HCO3 incr. 3.5 mmol/L

for each 10 mmHg increase in PaCO2 >40

Acute Hypocapnia:HCO3 decreases 2

mmol/L for every 10 mmHg decrease in PaCO2 <40

Chronic Hypocapnia:HCO3 decreases 5

mmol/L for every 10 mmHg decrease in PaCO2 <40

CO2 + H2O H2CO3 H + HCO3

Respiratory Compensationfor

Metabolic Acidosis: Occurs rapidly Hyperventilation

“Kussmaul Respirations”

Deep > rapid (high tidal volume)

Is not Respiratory Alkalosis

Metabolic Alkalosis: Calculation not as

accurate Hypoventilation Not Respiratory

Acidosis Restricted by

hypoxemia PCO2 seldom > 50-55

pCO2=1.5 x HCO3 + 8 +/- 2Winter’s formula

pCO2=0.9 x HCO3 + 15

Case 1

A 26 year old man with unknown past medical history is brought in to the ER by ambulance, after friends found him unresponsive in his apartment. He had last been seen at a party four hours prior.

ABG: pH 7.25 Chem 7: Na+ 137 PCO2 60 K+ 4.5 HCO3

- 26 Cl- 100 PO2 55 HCO3

- 25

Case 3

A 68 year old woman with metastatic colon cancer presents to the ER with 1 hour of chest pain and shortness of breath. She has no known previous cardiac or pulmonary problems.

ABG: pH 7.49 Chem 7: Na+ 133 PCO2 28 K+ 3.9 HCO3

- 21 Cl- 102 PO2 52 HCO3

- 22

Case 4

A 6 year old girl with severe gastroenteritis is admitted to the hospital for fluid rehydration, and is noted to have a high [HCO3

-] on hospital day #2. An ABG is ordered:

ABG: pH 7.47 Chem 7: Na+ 130 PCO2 46 K+ 3.2 HCO3

- 32 Cl- 86 PO2 96 HCO3

- 33

Urine pH: 5.8

Case 5A 75 year old man with morbid obesity is sent to the ER by his skilled nursing facility after he developed a fever of 103° and rigors 2 hours ago. In the ER he is lucid and states that he feels “terrible”, but offers no localizing symptoms. His ER vitals include a heart rate of 115, and a blood pressure of 84/46.

ABG: pH 7.12 Chem 7: Na+ 138 PCO2 50 K+ 4.2 HCO3

- 13 Cl- 99 PO2 52 HCO3

- 15

Urine pH: 5.0

Case 6

A 25 year old man with type I diabetes presents to the ER with 24 hours of severe nausea, vomiting, and abdominal pain.

ABG: pH 7.15 Chem 7: Na+ 138 PCO2 30 K+ 5.6 HCO3

- 10 Cl- 88 PO2 88 HCO3

- 11Cr 1.1

Urine pH: 5.0

Case 7A 62 year old woman with severe COPD comes to the ER complaining of increased cough and shortness of breath for the past 12 hours. There are no baseline ABGs to compare to, however, her HCO3

- measured during a routine clinic visit 3 months ago was 34 mEq/L.

ABG: pH 7.21 Chem 7: Na+ 135 PCO2 85 K+ 4.0 HCO3

- 33 Cl- 90 PO2 47 HCO3

- 34

Urine pH 5.5

Case 8

A 36 year old man with a history of alcoholism is brought to the ER after being found on the floor of his apartment unresponsive, soiled with vomit, and with an empty pill bottle nearby.

ABG: pH 7.03 Chem 7: Na+ 134 PCO2 75 K+ 5.2 HCO3

- 19 Cl- 90 PO2 48 HCO3

- 20

Urine pH 5.0