Acid Base

Question 1

Arterial Blood Gases

Answer 1

• arterial pH, carbon dioxide tension (pCO2), oxygen tension (pO2), bicarbonate concentration ([HCO3-]), and percent oxyhemoglobin saturation (SaO2).
• The pH, pCO2, and the pO2 are measured directly from the sample.
• The bicarbonate value is calculated from the pH and pCO2 data using the HendersonHasselbalch equation
• In addition, some laboratories also calculate the percent oxyhemoglobin saturation from the measured pO2 value rather than measuring the saturation directly. This is done using the values obtained from an O2 dissociation curve.
• At UAMC, oxyhemoglobin saturation is measured directly and values for hemoglobin, methemoglobin and carboxyhemoglobin are also reported routinely.

Question 2

Basic Metabolic Panel

Answer 2

• Electrolytes (Na+ , K+ , Cl- , CO2**) are usually obtained as part of a “basic metabolic panel” (BMP), which includes creatinine, BUN, glucose and calcium in addition to the basic electrolytes.
• Electrolytes are also included in a “comprehensive metabolic panel” (CMP) or may be ordered individually.

Question 3

Acid

Answer 3

•a molecule that releases hydrogen ions in solution. Acid: A substance that when added to a solution brings about an increase in [H+]

Question 4

Base

Answer 4

•a molecule that can accept hydrogen ions. Base: A substance that when added to a solution brings about a decrease in [H+]

Question 5

Buffer

Answer 5

•a substance that can reversibly bind hydrogen ions. pH—negative log (base 10) of the hydrogen ion concentration. Acidemia — An arterial pH below the normal range

Question 6

Alkalemia

Answer 6

•An arterial pH above the normal range

Question 7

Acidosis

Answer 7

•A process that tends to lower the extracellular fluid pH (hydrogen ion concentration increases). This can be caused by a fall in the serum [HCO3-] and/or an elevation in pCO2

Question 8

Alkalosis

Answer 8

•A process that tends to raise the extracellular fluid pH (hydrogen ion concentration decreases). This can be caused by an elevation in the serum [HCO3-] and/or a fall in pCO2

Question 9

Metabolic Acidosis

Answer 9

•A disorder that causes reductions in the serum [HCO3-] and pH

Question 10

Metabolic Alkalosis

Answer 10

•A disorder that causes elevations in the serum [HCO3-] and pH

Question 11

Respiratory Acidosis

Answer 11

•A disorder that causes an elevation in arterial pCO2 and a reduction in pH

Question 12

Respiratory Alkalosis

Answer 12

•A disorder that causes a reduction in arterial pCO2 and an increase in pH

Question 13

Simple Acid Base Disorder

Answer 13

•The presence of one of the above four disorders with the appropriate respiratory or renal compensation for that disorder

Question 14

Mixed Acid Base Disorder

Answer 14

•The simultaneous presence of more than one acid-base disorder

Question 15

Normal pH

Answer 15

7.38-7.42

Question 16

Normal pCO₂

Answer 16

38-42 mmHg

Question 17

Normal [HCO₃^-]

Answer 17

22-26 mEq/L

Question 18

Volatile Acid

Answer 18

•Volatile acids are defined as those acids which can be converted into a gaseous form and can thus be eliminated by the lungs. The primary volatile acid of the body is carbon dioxide which is produced in substantial amounts by the processes of cellular respiration

Question 19

Nonvolatile

Answer 19

•A nonvolatile acid (also known as a fixed acid or metabolic acid) is an acid produced in the body from sources other than carbon dioxide, and is not excreted by the lungs. They are produced from e.g. an incomplete metabolism of carbohydrates, fats, and proteins.

Question 20

Henderson-Hasselbach Equation

Answer 20

pH = 6.1 + log HCO3/0.03 x pCO₂

Note: pH is defined by the ratio of HC03- to pCO2 and not by the absolute value of either alone

Question 21

Renal Compensatory Response

Answer 21

•the renal compensatory response to respiratory disturbances occurs slowly (taking place over days—usually described as occurring over ~72 hours). Thus, respiratory disorders are divided into “acute” and “chronic” respiratory acidosis or alkalosis. The “acute” response is due to immediate buffering primarily by intracellular proteins. The “chronic” response is due to renal compensation.

Question 22

Respiratory Compensatory Response

Answer 22

•In contrast, the respiratory compensatory response to metabolic acidosis occurs quickly (usually beginning within minutes). The respiratory compensatory response to metabolic alkalosis is often variable and may take hours (see below for more explanation). Nonetheless, both metabolic acidosis and alkalosis are not separated into acute and chronic phases

Question 23

Metabolic Acidosis

Answer 23

• decrease in pH
• decrease in [HCO₃^-]
• respiratory compensation - Winter’s Formula!

pCO₂ = (1.5 x HCO₃₎ + 8 +/- 2

Question 24

Winter’s Formula

Answer 24

pCO2 = (1.5 x HCO3) + 8 +/- 2

Question 25

Metabolic Alkalosis

Answer 25

* increased pH * increased [HCO₃] * respiratory compensation pCO₂ = variable increase or pCO₂ = (0.7 x HCO₃) + 20

Question 26

Respiratory Compensation for Metabolic Alkalosis

Answer 26

pCO2 = (0.7 x HCO3​) + 20

Question 27

Respiratory Acidosis

Answer 27

* decrease pH * increase pCO₂ * renal comlensation - acute: HCO₃ increases 1 mEq for every 10 mmHg increase in pCO₂ - chronic: HCO₃ increases 3.5 mEq/L for every 10 mmHg increase in pCO₂

Question 28

Compensation for Acute Respiratory Acidosis

Answer 28

HCO₃ increases 1 mEq/L for every 10 mmHg increase in pCO₂

Question 29

Compensation for Chronic Respiratory Acidosis

Answer 29

HCO₃ increases 3.5 mEq/L for every 10 mmHg increase in pCO₂

Question 30

Respiratory Alkalosis

Answer 30

* increased pH * decreased pCO₂ \*renal compensation - acute: HCO₃ decreases 2 mEq/L for every 10 mmHg decrease in pCO₂ - chronic: HCO₃ decreases 5 mEq/L for every 10 mmHg decrease in pCO₂

Question 31

Compensation for Acute Respiratory Alkalosis

Answer 31

HCO₃ decreases 2 mEq/L for every 10 mmHg decrease in pCO₂

Question 32

Compensation for Chronic Respiratory Alkalosis

Answer 32

HCO₃ decreases 5 mEq/L for every 10 mmHg decrease in pCO₂

Question 33

Metabolic Acidosis

Answer 33

* A metabolic acidosis is an abnormal primary process leading to a fall in the plasma bicarbonate. The decrease in the plasma HCO3- can be caused by two mechanisms: a gain of fixed (nonvolatile) acid or loss of base. * The fall in plasma bicarbonate with gain of acid is due to titration of HCO3- by H+ . This initial pathophysiology causes a reduction in the serum [HCO3-] producing a decrease in pH. * The most common causes are due to excess generation of metabolic acids (e.g., lactic acid and ketoacids), exogenous loss of HCO3- (e.g., loss in stool or urine), reduced kidney excretion of acid (e.g., renal failure).

Question 34

Causes of Metabolic Acidosis - Elevated Anion Gap

Answer 34

* Diabetic Ketoacidosis * Alcoholic Ketoacidosis * Lactic Acidosis * Renal Failure * Toxins (ethylene glycol, methanol, ASA)

Question 35

Causes of Metabolic Acidosis - Normal Anion Gap

Answer 35

* Normal tubular acidosis * Diarrhea * Carbonic Anhydrase Inhibitors \*Admiistration of HCL or NH₃Cl •Acidosis associated with a normal anion gap has a limited number of causes. Basically, the causes are 1) loss of HCO3- through the gastrointestinal tract and 2) renal loss of HCO3- / failure to acidify the urine. Theoretically, normal anion gap acidosis could also occur with administration of HCl or NH4Cl (addition of acid with Cl as the anion), but this is a rare occurrence.

Question 36

Renal Causes of Hyperchloremic Acidosis

Answer 36

•The renal causes of hyperchloremic acidosis can be divided into 4 major categories: 1. Primary defects in ammoniagenesis 2. Hypoaldosteronism 3. Disorders of the proximal tubule 4. Disorders of the distal tubule

Question 37

"MUDPILES" - Metabolic Acidosis with Elevated Anion Gap

Answer 37

**M**ethanol **U**remia **D**KA **P**ropofol / Propylene Glycol **I**ron / Isoniazid **L**actic Acidosis **E**thylene Glycol **S**alicylates

Question 38

"MUDPALES" - Metabolic Acidosis with Elevated Anion Gap

Answer 38

**M**ethanol **U**remia **D**KA **P**ropofol / Propylene Glycol **A**lcoholic Ketoacidosis **L**actic Acidosis **E**thylene Glycol **S**alicylates

Question 39

Metabolic Alkalosis

Answer 39

* A metabolic alkalosis is a primary disorder that causes the plasma HCO3- to rise, usually initiated by gastric loss of H+ (vomiting or nasogastric suction) or diuretic therapy. * The pathogenesis of metabolic alkalosis requires both the generation and maintenance of this process. * Regardless of the process generating the increase in HCO3- , the process must include a maintenance stage in which renal excretion of HCO3- is reduced. * GI H+ Loss – Vomiting – Nasogastric Suction • Renal H+ Loss – Diuretics * Loop * Thiazide – Primary Mineralocorticoid Excess – Bartter’s, Gitelman’s, & Liddle’s Syndromes • Alkali Administration

Question 40

Causes of Metabolic Alkalosis

Answer 40

•The most common causes of metabolic alkalosis (vomiting, nasogastric suction, loop or thiazide diuretic use) are due to Cl depletion/deficiency and have low urinary Cl- . These types of metabolic alkalosis rapidly respond to Cl replacement (usually as infusion of “normal saline” in hospitalized patients [normal saline = 0.9% NaCl solution]).

Question 41

Respiratory Acidosis

Answer 41

•The underlying pathophysiology (most often alveolar hypoventilation causing hypercapnic respiratory failure) increases the pCO2 because volatile acid (CO2) is not removed by lung ventilation

Question 42

Respiratory Alkalosis

Answer 42

•The initial process is a decrease in the pCO2 due to an increase in ventilation (hyperventilation). Several (of many) causes are hypoxemia, anxiety, pregnancy, liver failure, mechanical ventilation, progesterone, salicylate poisoning, and pain.

Question 43

Anion Gap

Answer 43

Total Cations = Total Anions Anion gap = measured cations – measured anions

Question 44

Normal Anion Gap

Answer 44

8-12 mEq/L

Question 45

Formula to Calculate Anion Gap

Answer 45

AG = Na- (Cl + HCO₃) correct for albumin AG_corrected = AG + 2.5(4 - measured albumin) •If there is a significantly elevated anion gap, there is a primary metabolic acidosis.

Question 47

Net Renal Acid Excretion

Answer 47

* Net Renal Acid Excretion = Titratable acidity + NH4 + - urinary HCO3 - * HCO3 - Reabsorption * Titratable Acid * Ammoniagenesis and NH4 + excretion

Question 48

HCO₃ Reabsorption

Answer 48

* ~4300 mEq/day filtered * 80% reabsorbed proximal tubule

Question 49

HCO₃ Reabsorption Proximal Tubule

Answer 49

Question 50

HCO₃ Reabsorption Collecting Duct

Answer 50

•principal cells respond to - ADH --\> more H₂O reabsorption (aquaporins) - aldosterone --\> reabsorbing NaCl ---\> more H₂O reabsorption (water flows where Na goes!) •intercalated cells secrete or reabsorb H+

Question 51

Titratable Acid

Answer 51

Question 52

Ammoniagenesis and NH₄ + excretion

Answer 52

•Ammoniagenesis: in proximal tubule metabolism of glutamine produces NH₄+ and HCO₃-