blood gases

Question 1

Acid:

Answer 1

a substance that can yield a hydrogen ion (H) or hydronium ion when dissolved in water

Question 2

Base:

Answer 2

a substance that can yield hydroxyl ions (OH-)

Question 3

Dissociation constant

Answer 3

(ionization constant K value): describes relative strengths of acids & bases

Question 4

pK:

Answer 4

negative log of ionization constant & pH in which protonated & unprotonated forms are present in equal concentrations

Question 5

Buffer:

Answer 5

combination of weak acid or weak base & its salt; a system that resists changes in pH

Question 6

Acid–base balance

Answer 6

Maintenance of homeostasis of the hydrogen-ion concentration of body fluids
Defined by the degree of acidity or alkalinity of a body fluid
Determined by the pH or negative log of the hydrogen-ion concentration [H+] in moles/L

Question 7

Acidity of a solution

Answer 7

Determined by the concentration of hydrogen ions (cH+)

An acid is a hydrogen donor.

Question 8

Description of Acids & Bases

Answer 8

Carbonic acid (H2CO3) can donate one H+ through dissociation (Acid)
H2CO3 —– H +HCO3
As a base, bicarbonate (HCO3−), is an H+ or proton acceptor (Base)
H + HCO3 —– H2CO3
Equal numbers of H+ and OH− produce water, which is neutral—neither acidic nor alkaline.
H + OH —- H2O

Question 9

Maintenance of H+

Answer 9

Reference values = 7.35 – 7.45

pH is controlled by systems that regulate retention of acid and bases using the lungs & kidney

Question 10

Acidosis:

Answer 10

a pH level below reference range (<7.34)

Question 11

Alkalosis:

Answer 11

a pH level above reference range (>7.44)

Question 12

Buffer Systems

Answer 12

Buffer systems are body’s first line of defense against extreme changes in H concentration.
All buffers consist of a weak acid & its salt or conjugate base.
Bicarbonate-carbonic acid system has low buffering capacity, but is still important buffer for 3 reasons:
1. H2CO3 dissociates into CO2 and H2O, allowing CO2 to be eliminated by lungs and H as water.
2. Changes in CO2 modify ventilation (respiration) rate.
3. HCO3- concentration can be altered by kidneys.

Question 13

Other buffers:

Answer 13

phosphate system
- plays a role in plasma & RBC exhange of Na2+ in urine
Most proteins have a net (-) charge and bind to H+
plasma protein

Question 14

Bicarbonate Buffer System

Answer 14

Most important blood buffer is the carbonic acid–bicarbonate pair
Accounts for the majority of the buffering capacity in the extracellular space
Large amount of carbon dioxide (CO2) is produced within the body as a whole.
Potential for large amounts of acid to build up is greatest

Question 15

Henderson–Hasselbalch Equation

Answer 15

The ratio of bicarbonate to carbonic acid can be determined from this equation and is useful to the clinician who is assessing acid–base disorders.
The ratio of HCO3− to H2CO3 can be deduced.
Important ratio to use when evaluating acid–base disorders
In plasma at 37oC the pKa’ = 6.1
Temperature and solvent affect the constant
When kidneys and lungs are functioning properly a ratio of HCO3- to H2CO3- = 20:1 or pH = 7.40
cHCO3- proton acceptor (base)

Question 16

Oxygen and Carbon Dioxide

Answer 16

CO2 is produced and released into the blood.
The lungs control the cH2CO3 in blood.
Plasma cHCO3− is primarily under the control of the kidneys.

Question 17

Destruction of alveoli

Answer 17

Lung disease( emphysema) causes ↓in O2

Question 18

Pulmonary edema

Answer 18

Pulmonary embolism, pulmonary hypertension or cardiac failure causes ↓ blood to the lungs

Question 19

Airway blockage

Answer 19

Asthma and bronchitis common ailments that prevent air from reaching alveoli
Inadequate blood supply

Question 20

Diffusion of CO2 and O2

Answer 20

O2 diffuses 20 times slower than CO2

O2 concentration ↑ 60% is toxic to lungs

Question 21

Hemoglobin Buffer System

Answer 21

Most O2 in arterial blood is transported to tissue by hemoglobin

Question 22

Role of Hemoglobin

Answer 22

Transports H+, O2, and CO2.
As a buffer, it is the second most important regulator of pH in blood.
Hemoglobin allows for large amounts of CO2 produced by metabolism to be carried in the blood with little or no change in pH.

Question 23

Diffusion of Oxygen and Carbon Dioxide

Answer 23

Most important fundamental mechanism of O2 and CO2 transport.
Diffusion is the movement of an uncharged, hydrophobic solute through a lipid bilayer.
No expenditure of energy is involved.
The driving force for diffusion is the concentration gradient.

Question 24

Transport Oxygen in the Blood

Answer 24

Hemoglobin can bind O2 only when the iron is in the ferrous (Fe2+) state.
When heme is part of hemoglobin, interactions with about 20 amino acids cradle the heme in the globin so O2 loosely and reversibly binds to Fe2+.
Most important amino acid in this reaction is histidine, which binds Fe2+.

Question 25

Oxyhemoglobin (O2Hb)

Answer 25

O2 reversibly bound to hemoglobin

Question 26

Deoxyhemoglobin (HHb):

Answer 26

hemoglobin not bound to O2 but capable of forming a bond when O2 is availabl

Question 27

Carboxyhemoglobin (COHb):

Answer 27

hemoglobin bound to CO

Question 28

Methemoglobin (MetHb):

Answer 28

hemoglobin unable to bind O2 because iron (Fe) is in an oxidized rather than reduced state

Question 29

Blood Gas Units of Measurement

Answer 29

``` 1 atmosphere (atm) = 760 mmHg *1 torr = 1/760 atm 760 mmHg = 760 torr 1 torr = 1 mmHg 1 mmHg = 0.133 kilopascal (kPa) 1 kPa = 7.5 mmHg 760 mmHg = 101,325 pascals** *A torr is a unit of pressure equal to 1 millimeter rise of mercury in a barometer. **Pascal is the Système International unit of pressure and is equal to n (newton)/m2 or m-1 • kg • s-2 ```

Question 30

Oxygen and Gas Exchange

Answer 30

``` 7 Conditions needed for adequate tissue oxygenation Atmospheric oxygen Adequate ventilation Gas exchange between lungs and arterial blood Loading oxygen onto hemoglobin Adequate hemoglobin Adequate transport (cardiac output) Release of oxygen to the tissues BP @ sea level = 760 mm Hg Atmosphere contains O2 = 20.93% CO2 = 0.03% N = 78.1 % ```

Question 31

Hemoglobin Buffer System

Answer 31

Hemoglobin (Hb) serves several roles in acid–base balance and the respiration process. 75% of CO2 is present as bicarbonate. 5% is present as dissolved gas. 20% is bound to hemoglobin as a carbamino compound. Hemoglobin accounts for about 80% of buffering capacity.

Question 32

Hemoglobin–Oxygen Dissociation

Answer 32

O2 must be released at tissues from its carrier, hemoglobin. Oxygen dissociates from adult (A1) hemoglobin in characteristic fashion (S-shaped curve). Shape of oxygen-dissociation curve & affinity of hemoglobin for O2 are affected by: Hydrogen ion activity pCO2 & CO levels Body temperature 2,3-Diphosphoglycerate (2,3 DPG) A= left shift – retain oxygen B = normal C= right shift – oxygen release

Question 33

What is the p50?

Answer 33

The oxygen tension when the hemoglobin is 50% saturated with oxygen. Its used to measure hemoglobin-oxygen affinity or the ability of the arterial blood to release oxygen to the tissues.

Question 34

RIGHT SHIFT:

Answer 34

In general, as pH ↓ or as PCO2, temp and 2,3 DPG increase, the O2 curve shifts to the right, indicating that hemoglobin has a lower affinity for oxygen and the p50 or the midpoint of the curve (see ----- line) will be increased. Hemoglobin readily gives up the oxygen its carrying so that oxygen is able to diffuse into the tissues. The deoxyhemoglobin is then free to act as a buffer by picking up H+. High rates of cellular metabolism causes increased levels of PCO2 which results in the formation of more carbonic acid that dissociates into bicarbonate (HCO3-) and H+. Right shift happens when tissues become acidic following exercise, with decreased perfusion or at high altitudes, and in anemic states. Anemia results in a decrease of oxygen carrying capacity and is characterized by a low hemoglobin. As long as the 2,3-DPG remains high, the patient doesn’t suffer effects of hypoxia

Question 35

LEFT SHIFT:

Answer 35

In cases of hypothermia, hyper ventiliation, transfusion, abnormal hemoglobin (fetal, hemoglobinopathies) or alkalosis, O2 curve shifts to the left, indicating that the hemoglobin has a higher affinity for oxygen and the P50 will be decreased. Higher oxygen-hemoglobin retention means that tissues aren’t getting the oxygen it needs resulting in poor tissue perfusion, and results in organ ischemia. Shows the relationship between percent hemoglobin saturation with oxygen Can be used to determine the percent hemoglobin saturation for a given PO2 and O2 content The curve is sigmoid or S-shaped. The reasons for this involve the four oxygen binding sites on the hemoglobin molecule. Presence of disease can result in significant changes in the dissociation curve.

Question 36

Factors Affecting the Hemoglobin–Oxygen Dissociation Curve: Temperature

Answer 36

High temperature decreases the O2 affinity of hemoglobin | O2 is released

Question 37

Factors Affecting the Hemoglobin–Oxygen Dissociation Curve: pH

Answer 37

Presence of excess H+ in the blood—and hence a lowered pH—will cause a decrease in O2 affinity for hemoglobin. Effect of pH on hemoglobin–oxygen affinity is known as the Bohr effect.

Question 38

Factors Affecting the Hemoglobin–Oxygen Dissociation Curve: PCO2

Answer 38

``` Hypercapnia Increase in blood CO2 concentration Results in a shift to the right of the dissociation curve An increase in PCO2 causes: Hemoglobin to release O2 Hemoglobin to unload CO2 ```

Question 39

Factors Affecting the Hemoglobin–Oxygen Dissociation Curve: 2,3-diphosphoglycerate

Answer 39

Affinity of Hb for O2 is highly sensitive to the presence of the glycolytic metabolite 2,3-DPG. 2,3-DPG affinity of oxygenated hemoglobin is only about 1% as great as that of deoxygenated hemoglobin. Binding of 2,3-DPG to Hb destabilized the interaction of Hb with O2.

Question 40

Metabolically active tissues are characterized as:

Answer 40

``` Having a high demand for O2 Being warm Producing large amounts of CO2 Being acidic Hemoglobin is intrinsically sensitive to: High temperatures High PCO2 Low pH ```

Question 41

Carbon Dioxide Transport

Answer 41

``` Carbon dioxide is transported in the blood in five different forms: Bicarbonate Carbonate Carbonic acid Dissolved carbon dioxide Carbamino compounds ``` ``` Depends on: Carbonic anhydrase Cl−-HCO3− exchanges Chloride or Hamburger shift Hemoglobin Approximately 11% of CO2 remains in blood plasma and travels to the lungs. Most (89%) enters the erythrocytes. ```

Question 42

Phosphate Buffer System

Answer 42

Important in the RBC and other cells Enables the kidneys to excrete H+ 95% of phosphate is present as NaH2PO4, which neutralizes strong acids Accounts for only about 1% of blood buffering capacity

Question 43

Proteins Buffer System

Answer 43

Proteins are primarily a cellular buffers. Some amino acids are acids, some are bases. They have ionizable side chains to pick up or release H+. Proteins account for about 14% of blood buffering capacity.

Question 44

Kidney Role in Acid-Base Balance

Answer 44

Average pH of plasma and of the glomerular filtrate is ∼7.4. Average urinary pH is ∼6.0. Values represent kidney’s attempt to excrete nonvolatile acids that are produced by metabolic processes. Three mechanisms that facilitate renal excretion of acid and conservation of HCO3−: Na+–H+ exchange Production of ammonia (NH3) and excretion of ammonium (NH4+) Reclamation of HCO3−

Question 45

Regulation of Acid–Base Balance: Lungs

Answer 45

Ventilation affects pH of blood (Respiratory). O2 is inspired & diffuses from alveoli into blood and binds to hemoglobin →oxyhemoglobin H+ carried on the reduced hemoglobin recombines with HCO3- → H2O + CO2 CO2 diffuses into alveoli from blood & is eliminated via ventilation. When lungs cannot remove CO2 it accumulates in the blood causing ↑ H+. If CO2 is removed faster than rate of production (hyperventilation) it causes ↓ H+

Question 46

Regulation of Acid–Base Balance: Kidneys

Answer 46

regulate acid–base balance by excreting acids or bases (Metabolic) HCO3- is reclaimed from glomerular filtrate to prevent excessive acid gain in blood from loss of HCO3- in urine. Main site for HCO3- is the proximal tubules → same HCO3- levels in plasma Na2+ is exchanged for H+ in the cell. H+ combines with HCO3- in the filtrate → H2CO3- → H2O + CO2 CO2 diffuses back into the tubule and reacts with H2O to reform H2CO3- → HCO3- which is reabsorbed into the blood with Na2+ Reabsorption/reclamation – process to reenter bloodstream Secretion / excretion – by tubules to removal of substances from filtrate Body produces a net excess of H+, urine pH = 4.5 Renal tubules generate NH3 from glutamine and other aa and will ↑ in response to acid output. Factors that affect HCO3- reabsorption: Blood / plasma level > 26-30 mmol/L Excess in lactate or acetate Loss of Cl- (sweating, vomiting, nasogatric suction) ↓ HCO3- seen in: Diuretics Chronic nephritis infection

Question 47

Acidemia:

Answer 47

excess acid or H concentration (pH < reference range

Question 48

Alkalemia:

Answer 48

excess base (pH > reference range)

Question 49

Nonrespiratory (metabolic) acidosis:

Answer 49

decrease in bicarbonate resulting in decreased pH Ammonium or Ca2+ chloride Starvation, diabetic ketoacidosis Compensates by hyperventilation

Question 50

Respiratory acidosis:

Answer 50

decrease in alveolar ventilation, causing decreased elimination of CO2 by lungs Hypoventilation, congestive heart failure COPD, bronchophneumonia

Question 51

Nonrespiratory (metabolic) alkalosis:

Answer 51

gain in HCO3-, causing increase in pH

Question 52

Respiratory alkalosis:

Answer 52

Increase in alveolar ventilation, causing excessive elimination of CO2 by lungs Hypoxemia, salicylates, hysteria (hyperventilation), fever, pulmonary emoboli, pulmonary fibrosis Compensates by breathing in a paper

Question 53

How to Determine Acid-Base Status

Answer 53

``` Three steps: Look at the blood pH. pH >7.45 indicates base pH <7.35 indicates acid Look at the respiratory component, PCO2. PCO2 >48 indicates respiratory acidemia PCO2<35 indicates respiratory alkalemia Look at the metabolic components (HCO3–) and base excess. Metabolic alkalemia Metabolic acidemia ```

Question 54

Metabolic Acidosis

Answer 54

Represents a base deficient disorder that results from either an accumulation of fixed acids or a loss of extra-cellular buffers Blood pH decreases as a result of a decrease of cHCO3−

Question 55

Expected pH and blood-gas values

Answer 55

Acidic pH Decreased HCO3– Normal PCO2 Increased anion gap (AG)

Question 56

Causes Metabolic Acidosis

Answer 56

Impaired renal excretion of fixed acids Over-production or administration of fixed acids Primary bicarbonate loss via kidneys or gastrointestinal tract Secondary bicarbonate loss attributable to elevated serum chloride levels

Question 57

Increased anion gap

Answer 57

Renal failure Diabetic ketoacidosis Lactic acidosis Acetylsalicylic acid, methanol, formic acid, isopropyl alcohol, and ethylene glycol

Question 58

Normal anion gap

Answer 58

Gastrointestinal loss of HCO3– Renal tubular acidosis Increased serum chloride, which results in suppressed bicarbonate ion resorption.

Question 59

MUDPILES is a mnemonic for:

Answer 59

``` Methanol Uremia Diabetic ketoacidosis Paraldehyde Iron Lactic acid Ethanol Salicylate ```

Question 60

Metabolic Acidosis- Compensation- Lung

Answer 60

``` Increased hydrogen ion concentration stimulates the respiratory center. Kussmaul breathing (deep, gasping) lowers PCO2, thus lessening the acidemia. ```

Question 61

Metabolic Acidosis- Compensation: Kidney

Answer 61

Retain bicarbonate

Question 62

Metabolic Alkalosis

Answer 62

Characterized as a primary excess of HCO3−. Can result from: Addition of base to the body Decrease in the amount of base leaving the body Loss of acid-rich fluids Commonly arises from the loss of Cl from body fluids.

Question 63

Metabolic Alkalosis Expected pH and blood-gas values

Answer 63

``` Alkaline pH Normal PCO2 Increased plasma bicarbonate levels Decreased blood ionized calcium Caused by acute alkalosis Decreased serum potassium levels Decreased serum chloride levels ```

Question 64

Metabolic Alkalosis Symptoms

Answer 64

Tetany and increased neuromuscular irritability | Confusion, leading to stupor and coma, may also be seen in very severe metabolic alkalosis

Question 65

Metabolic Alkalosis-Compensation: Respiratory

Answer 65

Increased PCO2 by reducing rate and depth of respiration

Question 66

Metabolic Alkalosis-Compensation: Kidneys

Answer 66

Increased excretion of alkali | Decreased Na+–H+ exchange

Question 67

Metabolic Alkalosis-Compensation: Buffers

Answer 67

Excess base reacts with carbonic acid

Question 68

Respiratory Acidosis

Answer 68

Characterized by an increase of PCO2 above normal limits. Defect in the excretion of CO2 Person tends to retain CO2 (hypercapnia). Expected pH and blood-gas values Increased PCO2 Decreased blood pH

Question 69

Respiratory Acidosis-Compensation: Buffers

Answer 69

Increase the amount of bicarbonate buffer

Question 70

Respiratory Acidosis-Compensation: Renal

Answer 70

Retention of bicarbonate ions and excretion of hydrogen ions

Question 71

Respiratory Acidosis-Compensation: Shift in electrolytes

Answer 71

Sodium and hydrogen ions move into the cells while potassium ions move from the cells into the blood.

Question 72

Respiratory Alkalosis

Answer 72

Expected pH and blood-gas values Decreased PCO2 Alkaline pH Always the result of excessive pulmonary excretion of CO2 attributable to hyperventilation

Question 73

Respiratory Acidosis

Answer 73

Causes Diffusion effect, Alveolar destruction-Emphysema,Cancer, Congestive heart failure, Impaired respiratory drive Central nervous system disorders, Trauma, Tumor, Vascular disorders, Epilepsy, Hypoxia, drug ingestion Impaired respiratory mechanics, Polio, Respiratory muscle (dystrophy), Trauma to the ribs Airway obstruction, Tumors, Food vomitus, COPD and asthma Abnormal ventilation/perfusion ratio, Circulatory impairment

Question 74

Respiratory Alkalosis - Compensation: Tissue buffers

Answer 74

Carbon dioxide is lost and bicarbonate ion is used up. Hydrogen ions are made available for bicarbonate ion to be consumed. Hemoglobin buffer systems contribute about 30% of the buffering capacity.

Question 75

Respiratory Alkalosis - Compensation: Renal mechanisms

Answer 75

Retain acid and ammonium

Question 76

Blood Gas Analysis: Sample Collection

Answer 76

Arterial heparinized whole blood (0.05 heparin/mL) Syringes or capillary tubes NO air bubbles or needles False ↑ pH, pO2 and ↓pCO2 Sample delivered on ice Optimum = test within ½ hour of draw; max = 60 minutes

Question 77

Blood Gas Analysis: Pre analytical Errors due to:

Answer 77

Sample ID Variation in anticoagulant Improper mixing Exposure to warm temperature

Question 78

Blood Gas Analysis: What can go wrong?

Answer 78

``` Too much heparin Air in sample No ice on sample Poorly mixed or clotted sample Venous vs. arterial blood ```

Question 79

Blood gas analyzers measure pH, PCO2, & PO2 with electrodes: Oxidation

Answer 79

Loss of electrons by a particle

Question 80

Blood gas analyzers measure pH, PCO2, & PO2 with electrodes: Reduction

Answer 80

Gain of electrons by a particle

Question 81

Blood gas analyzers measure pH, PCO2, & PO2 with electrodes: Amperometric (PO2)

Answer 81

Amount of current flow indicates oxygen present

Question 82

Blood gas analyzers measure pH, PCO2, & PO2 with electrodes: Potentiometric (PCO2, pH)

Answer 82

Change in voltage indicates analyte activity

Question 83

Blood gas analyzers measure pH, PCO2, & PO2 with electrodes: Cathode

Answer 83

1) negative electrode 2) site to which cations tend to travel 3) site at which reduction occursBlood gas analyzers measure pH, PCO2, & PO2 with electrodes

Question 84

Blood gas analyzers measure pH, PCO2, & PO2 with electrodes: Electrochemical cell

Answer 84

formed when two opposite electrodes are immersed in a liquid that will conduct curre

Question 85

Blood gas analyzers measure pH, PCO2, & PO2 with electrodes: Anode

Answer 85

1) positive electrode 2) site to which anions tend to travel 3) site at which oxidation occurs

Question 86

Blood Gas Analyzers: Calibration

Answer 86

pH & blood gas measurements are extremely sensitive to temperature. Electrode sample chamber must be maintained at constant temperature (37oC + 0.1oC). pH electrode is calibrated with 2 buffer solutions, traceable to standards prepared by NIST. Two gas mixtures are used to calibrate for pCO2 & pO2. Most instruments are self-calibrating & are programmed to indicate a calibration error if electronic signal from electrode is inconsistent with programmed expectedm

Question 87

Blood Gas Analyzers: Quality Assurance: Sample Considerations

Answer 87

``` Only experienced personnel should draw blood gas specimens 1 to 3 mL self-filling, plastic, disposable syringe with anticoagulant No vacutainer tubes Dry heparin preferred to liquid Arterial (best) or venous samples okay Sample 20-30 minutes in cool, RT room Sodium / lithium heparin recommended Warming of skin affects arterial pO2 levels Sample collection from arterial line Fluid contamination Sources of Error [Heparin] Slow-filling syringes Hemolysis Air and bubbles in syringe Transport time Ice slushymust be labeled appropriately Mix thoroughly ```

Question 88

Spectrophotometric Determination of Oxygen Saturation

Answer 88

Actual percent oxyhemoglobin (O2Hb) can be determined using cooximeter designed to measure various hemoglobin species. Each species has a characteristic absorbance curve. Number of wavelengths incorporated into instrument determines number of species that can be measured, from 4 to hundreds. 4 most common hemoglobin species: HHb, O2Hb, COHb, MetHb Potential sources of error: faulty instrument calibration & spectral-interfering substances

Question 89

WHAT IS THE RXN PRINICPLE?

Answer 89

Saturated O2 measured as oxyhemoglobin versus total hemoglobin is measured by spectrophotometer taking multiple absorbance readings at various wavelengths. O2 and total hemoglobin is measured.