CLINICAL CHEMISTRY II

Blood Gases & Acid-Base Balance

INTRODUCTION

DefinitionsACID• a substance that can yield a hydrogen ion (H+) or hydronium

ion (H3O+) when dissolved in water

BASE• a substance that can yield a hydroxyl ion (OH-) when dissolved

in water

INTRODUCTION

DefinitionsBUFFER• the combination of a weak acid or weak base and its salt; a

system that resists changes in pH

pH• the negative (or inverse) log of the hydrogen ion concentration;

-log[H+] or log 1/[H+]

INTRODUCTION

DefinitionsACIDOSIS• a pH below the reference range

ALKALOSIS• a pH above the reference range

INTRODUCTION

DefinitionsPARTIAL PRESSURE• the pressure exerted by an individual gas in the atmosphere;

equal to the barometric pressure times the percentage for the gas

pO2

• the partial pressure of oxygen

pCO2

• the partial pressure of carbon dioxide

PHYSIOLOGIC THEORY

Maintenance of H+

The arterial pH is controlled by systems that regulate the production and retention of acids and bases, including• buffers• the respiratory center and lungs• the kidneys

PHYSIOLOGIC THEORY

Buffer SystemsThe bicarbonate-carbonic acid system; HCO3

- and H2CO3

• when acid is added to the bicarbonate-carbonic acid system, HCO3

- will combine with the H+ from the acid to form H2CO3

• when a base is added, the H2CO3 will combine with the OH- group to form H2O and HCO3

-

PHYSIOLOGIC THEORY

Buffer SystemsThe bicarbonate-carbonic acid system is important for three reasons

1. H2CO3 dissociates into CO2 and H2O, allowing H+ to be eliminated as CO2 by the lungs

2. changes in pCO2 modify the ventilation rate

3. HCO3- concentration can be altered by the kidneys

PHYSIOLOGIC THEORY

Buffer SystemsOther buffer systems• the phosphate buffer system; HPO4

-- and H2PO4-

• the plasma proteins

PHYSIOLOGIC THEORY

Respiratory SystemPlasma• the end product of most aerobic metabolic processes is CO2

• in the plasma, small amounts of CO2 remain as dCO2 or combine with proteins to form carbamino compounds; most of the CO2 combines with H2O to form H2CO3, which quickly dissociates into H+ and HCO3

-

PHYSIOLOGIC THEORY

Respiratory SystemLungs• inspired O2 diffuses from the alveoli into the blood and is

bound to hemoglobin, forming oxyhemo-globin; o the H+ that was carried on the (reduced) hemoglobin in the venous

blood is released to recombine with HCO3- to form H2CO3, which

dissociates into H2O and CO2

• the CO2 diffuses into the alveoli and is eliminated through ventilation

PHYSIOLOGIC THEORY

Renal System

Kidneys• the kidney’s main role in maintaining acid-base homeostasis is

to reclaim HCO3- from the glomerular filtrate and add it to the

blood

PHYSIOLOGIC THEORY

the Henderson-Hasselbalch EquationEquation that mathematically expresses the dissociation characteristics of weak acids and bases:

• pH = pK’ + log [cA-]/[cHA+]

or

• pH = 6.1 + log [bicarbonate]/[carbonic acid]

PHYSIOLOGIC THEORY

Dalton’s lawin a gas mixture, the total barometric pressure equals the sum of the individual components

• for example, in atmospheric air… pAtm = pO2 + pCO2 + pN2 + pH2O

SPECIMEN

SourceVenous blood • if pulmonary function or O2 transport is not being assessed

Arterial blood• radial, brachial, femoral

“Arterial lines”

SPECIMEN

Handling• dry heparin• ice water slurry• immediate transport to lab

METHODS OF ASSAY

pH electrode systemMeasuring electrode• a glass membrane sensitive to H+ is placed around an internal

Ag-AgCl electrode

Reference electrode• calomel (Hg-HgCl) or Ag-AgCl

Voltmeter (potentiometry)• millivoltmeter

METHODS OF ASSAY

pCO2 (Severinghaus) electrode system

Modified pH electrode (potentiometry)• an outer semipermeable membrane allows CO2 to diffuse into a

bicarbonate buffer;

• the CO2 that diffuses across the membrane reacts with the buffer, forming carbonic acid, which then dissociates into bicarbonate plus H+;

• the change in activity of the H+ is measured by the pH electrode and related to pCO2

METHODS OF ASSAY

pO2 (Clark) electrode system

• anode• gas-permeable membrane• cathode• ampmeter (amperometry)

CLINICAL CORRELATIONS

Reference Range (Arterial)• pH: 7.35-7.45 pH units• pCO2: 35-45 mm Hg• pO2: 80-110 mm Hg• HCO3-: 22-26 mmol/L• Total CO2: 23-27 mmol/L• SO2: > 95%

CLINICAL CORRELATIONS

Reference Range (Venous)• pH: 7.32-7.42 pH units• pCO2: 40-50 mm Hg• pO2: 30-50 mm Hg

CLINICAL CORRELATIONS

Panic Values• pH: < 7.2 or > 7.6

• pO2: < 40 mm Hg

• pCO2: < 20 mm Hg or > 70 mm Hg

CLINICAL CORRELATIONS

Metabolic AcidosisEtiology• excessive formation of organic acids

o diabetic ketoacidosis; starvation

• decreased excretion of acids

o renal tubular acidosis

• excessive loss of bicarbonate

o diarrhea; drainage from a biliary, pancreatic or intestinal fistula

CLINICAL CORRELATIONS

Metabolic AcidosisEtiology (continued)• direct administration of an acid-producing substance

o ammonium chloride; calcium chloride

CLINICAL CORRELATIONS

Metabolic AcidosisCompensation• respiratory: hyperventilation; an increase in alveolar

ventilation

o an increase in the rate or depth of breathing ...”blowing off” CO2

CLINICAL CORRELATIONS

Respiratory AcidosisEtiology• hypoventilation; a decrease in alveolar ventilation:

o emphysema; bronchopneumonia; asphyxiation (strangulation or aspiration)

o congestive heart failure, with decreased cardiac output

o effects of drugs--barbiturates, morphine, or alcohol

CLINICAL CORRELATIONS

Respiratory AcidosisCompensation• metabolic (renal): the kidneys increase the excretion of H+ and

increase the reabsorption of HCO3-

CLINICAL CORRELATIONS

Metabolic AlkalosisEtiology

A gain in HCO3-

• excess administration of sodium bicarbonate; ingestion of bicarbonate-producing salts such as sodium lactate, citrate, or acetate

Excessive loss of acid

• vomiting; nasogastric suctioning• prolonged use of diuretics that augment renal excretion

of H+

CLINICAL CORRELATIONS

Metabolic AlkalosisCompensation• respiratory: hypoventilation, increasing the retention of CO2

CLINICAL CORRELATIONS

Respiratory AlkalosisEtiology• hyperventilation; an increased rate of alveolar ventilation;

causing excessive elimination of CO2 by the lungs

o stimulation of the respiratory center by drugs, such as salicylates

o an increase in environmental temperature; feverohysteriaopulmonary emboli; pulmonary fibrosis

CLINICAL CORRELATIONS

Respiratory AlkalosisCompensation• metabolic (renal): the kidneys compensate by excreting HCO3

- and retaining H+

NOTES

Assessment of Oxygen StatuspO2

• the partial pressure of oxygen

Oxygen saturation• the ratio of O2 that is bound to hemoglobin, compared with the

total amount the hemoglobin could bind

NOTES

Assessment of Oxygen StatusPulse oximetry• differentiates between the absorption of light due to

oxyhemoglobin and deoxyhemoglobin in the capillary bed and calculates hemoglobin saturation

• through the tissue of the toe, finger, or ear

Fractional oxyhemoglobin (FO2Hb)

• the ratio of the concentration of oxyhemoglobin to the concentration of total hemoglobin

NOTES

Co-OximetryPrinciple• spectrophotometric, based on the fact that each type of

hemoglobin has a characteristic absorbance curve

Application• measurement of oxyhemoglobin, O2Hb; deoxyhemoglobin,

HHb; carboxyhemoglobin, COHb; and methemoglobin, MetHb

NOTES

Hemoglobin-Oxygen DissociationOxygen dissociates from hemoglobin in a characteristic fashion• if this dissociation is graphed with the pO2 on the x-axis and

percent SO2 on the y-axis, the resulting curve is sigmoid, or slightly S-shaped

NOTES

OxygenationAdequate tissue oxygenation requires:• available atmospheric oxygen• adequate ventilation• gas exchange between the lungs and arterial blood

• loading of O2 onto hemoglobin

• adequate hemoglobin• adequate transport (cardiac output)

• release of O2 to the tissues

NOTES

OxygenationFactors that influence tissue oxygenation:• destruction of the alveoli (e.g. emphysema)• pulmonary edema• airway blockage (e.g. asthma, bronchitis)• inadequate blood supply (e.g. pulmonary embolism or

congestive heart failure)

NOTES

OxygenationFactors that influence tissue oxygenation• the concentration and type(s) of hemoglobin• the presence of nonoxygen substances, such as carbon

monoxide (CO)• the pH• the temperature of the blood • the levels of pO2 • the level of 2,3-DPG

NOTES

Correction to Patient Temperature• because pH is temperature-dependent, blood gas

instruments are maintained at 37 + 0.05o C

• accordingly, the blood pH must be corrected to the patient’s body temperature, because significant deviations occur in patients with high fever or low body temperature