10) Blood gases and pH Flashcards
2 components of bicarb buffer system
Dissolved carbon dioxide gas (CO2)
Bicarbonate ion (HCO3-)
1 atm = —- mmHg
760
gas solubility constant
How much gas dissolves for each 1 mmHg of partial pressure of that gas.
CO2 solubility constant
0.03 mmol/L per mmHg
concentration of CO2
[CO2] = (pCO2)(0.03)
Lung disease can ——- the pressure and hyperventilation can ——- the pressure
increase
decrease
CO2 + H2O →
H2CO3
carbonic acid
H2CO3 →
H + HCO3
enzyme catalyzing change from CO2 to bicarb
carbonic anhydrase
—% of CO2 is present as bicarbonate.
—% is bound to hemoglobin as a carbamino compound.
—% is present as dissolved gas.
75
20
5
buffer
a solution containing a weak acid and its conjugate base that resists changes in pH when a strong acid or base is added
4 blood buffers
Bicarbonate
Hemoglobin
Phosphorus
Proteins
Henderson-Hasselbalch Equation
pH = pKa + log[cHCO3/(𝛼)(pCO2)]
pKa = 6.1
𝛼 = 0.03
bicarb RR
22-26 mmol/L
pCO2 RR
35-45 mmHg
normal bicarbonate:carbonic acid ratio
20:1
Hemoglobin accounts for about —-% of buffering capacity, but is the 2nd most important buffering system.
80
hemoglobin functions
Transports H+, O2 and CO2
Allows for large amounts of CO2 to be carried in the blood with little or no change in pH.
Participates in the chloride shift to maintain neutral charge within the RBC
Accounts for only about 1% of blood buffering capacity
phosphate buffer system
Account for about 14% of blood buffering capacity.
proteins
Proteins are primarily —— buffers.
cellular
diffusion
movement of an uncharged, hydrophobic solute through a lipid bilayer.
external convection system
lungs, the airway, and respiration muscles
internal convection system
circulatory system
Refers to a set of anatomical features involved with respiration and includes the larynx, trachea, bronchi, lungs, and pleura.
respiratory apparatus
6 functions of the respiratory apparatus
- Provide a means of moving outside air to the alveolar air spaces.
- Provide a means to carry O2 and CO2 in the blood.
- Provide a surface for gas exchange.
- Contain an internal convection system and a circulatory system that consists of a four-chamber heart.
- Provide a mechanism for locally regulating ventilation and perfusion.
- Provide a mechanism for centrally regulating ventilation.
Hemoglobin can bind O2 only when the iron is in the…
ferrous (Fe2+) state
Most important amino acid in heme is ——-, which binds Fe2+.
histidine
Shows the relationship between percent hemoglobin saturation with oxygen
The Hemoglobin-Oxygen Dissociation Curve
reasons for S-shaped Hb-O2 curve
involve the four oxygen binding sites on the hemoglobin molecule.
factors that can cause Hb-O2 curve to shift left (higher affinity)
lung environment
- ↑ pH
- ↓ pCO2
- ↓ 2,3-DPG
- ↓ temp
factors that can cause Hb-O2 curve to shift right (lower affinity)
muscle environment
- ↓ pH
- ↑ pCO2
- ↑ 2,3-DPG
- ↑ temp
hemoglobin is intrinsically sensitive to…
High temperatures
High pCO2
Low p H
5 different forms CO2 is transported in
Bicarbonate (primarily)
Carbonate
Carbonic acid
Dissolved carbon dioxide
Carbamino compounds
Cl-HCO3 exchanges
chloride/hamburger shift
difference between serum and urine pH represents…
kidneys’ attempt to excrete nonvolatile acids that are produced by metabolic processes.
3 mechanisms that facilitate renal excretion of acid, and retention of bicarb
- Na-H exchange
- production of ammonia, excretion of ammonium
- reclamation of bicarb
chloride shift in venous blood
HCO3 out of rbcs
Cl into rbcs
chloride shift in arterial blood
HCO3 into rbcs
Cl out of rbcs
An acid-base disorder is primary if…
the imbalance results from a change in pCO2 or HCO3- (one disturbance only)
Result of more than one pathologic process occurring in an individual
mixed acid-base disorders
pO2 RR
85-108 mmHg
tCO2 RR
22-28 mEq/L
Siggaard-Andersen Acid-Base Chart
metabolic acidosis general causes
- Impaired renal excretion of fixed acids
- Overproduction or administration of fixed acids
- Primary bicarbonate loss via kidneys or gastrointestinal tract (base out the bottom)
- Secondary bicarbonate loss attributable to elevated serum chloride levels
metabolic acidosis + normal AG
- Renal tubular acidosis
- Increased serum chloride, which results in suppressed bicarbonate ion resorption
- Diarrhea
- DKA recovery
- TPN
- Carbonic anhydrase inhibitors
- Ureterosigmoidostomy
metabolic acidosis + high AG
- Renal failure
- Diabetic ketoacidosis
- Lactic acidosis
- Acetylsalicylic acid, methanol, formic acid, isopropyl alcohol, and ethylene glycol (MUDPILES)
metabolic alkalosis general causes
Addition of base to the body
Decrease in the amount of base leaving the body
Loss of acid-rich fluids
——– causes are categorized by Cl responsiveness or resistance
metabolic alkalosis
causes of metabolic alkalosis
Via gastrointestinal tract
Vomiting and nasogastric suction
Chloride-losing diarrhea states
Renal failure
Hypercalcemia (milk-alkali syndrome)
Hormonal (hypoaldosteronism, adrenal hyperplasia, Cushing’s, pituitary adenomas)
Hypokalemia
Retention of bicarbonate ion
Administration of alkali (antacids)
Overuse of diuretics
respiratory acidosis causes
Diffusion
Alveolar destruction (emphysema)
Cancer (thickened alveolar membrane)
Congestive heart failure (fluid buildup)
CNS
Impaired respiratory drive
Trauma
Tumor
Vascular disorders
Epilepsy
Hypoxia
Drug ingestion (barbiturates, narcotics)
Degenerative disorders
Sleep apnea
Impaired respiratory mechanics
Polio
Respiratory muscle (dystrophy)
Trauma to the ribs
Airway obstruction
Tumors
Food/vomitus
COPD and asthma
causes of weakened respiratory muscle function
Myasthenia gravis
Periodic paralysis
Intraperitoneal aminoglycosides
Guillain–Barré syndrome
Botulism
Poliomyelitis
Amyotrophic lateral sclerosis (A L S)
Myxedema
chloride-responsive metabolic alkalosis
- diuretics
- vomiting
- NG tube suction
- villous adenoma
- carbenicillin
- contraction alkalosis
chloride-resistance metabolic alkalosis
- hyperaldosteronism
- cushings
- exogenous steroids
- licorice (glycyrrhizin)
- bartter syndrome
- milk-alkali syndrome
respiratory alkalosis causes
Hypoxemia
Pneumonia, emboli
Congestive heart failure
High-altitude exposure
Severe anemia
Stimulation of the Medullary Respiratory Center
Hyperventilation syndrome (anxiety)
Hepatic encephalopathy
Salicylate intoxication
Pregnancy (increased progesterone)
Neurologic disorders (CVA)
GN sepsis
Excessive mechanical ventilation
Recovery from metabolic acidosis
3 general methods of compensation
buffers
renal
respiratory
One acid/base component is abnormal, while one is normal
uncompensated
The second component is now starting to offset the other by going in the same direction to help pH
partial compensation
The pH is now normal, because the acid/base components have responded
fully compensated
compensation for met. alk.
Lungs hypoventilate to hold CO2. Kidneys reclaim less HCO3.
compensation for met. acid.
Lungs hyperventilate to lose CO2. Kidneys reclaim more HCO3.
compensation for resp. alk
Kidneys reclaim less HCO3.
Lungs hypoventilate to hold CO2.
compensation for resp. acid.
Kidneys reclaim more HCO3.
Lungs hyperventilate to lose CO2.
venous tCO2 RR
22-29 mmol/L
recommended site for ABG draw
radial artery
pH and blood gas sample
whole blood
anticoag for pH & blood gas
balanced heparin
sources of ABG error
Metabolism (glycolysis continues)
Diffusion of gases through the plastic container
Elevated potassium values (hemolysis)
preanalytical handling of ABG samples
Every effort must be made to remove air bubbles immediately and without agitation.
Sample should be mixed immediately to dissolve the heparin and prevent clotting.
Samples should be mixed thoroughly before injection into the blood-gas analyzer.
sample requirements for ABG
- Arterial or venous specimen tightly stoppered
- Heparinized syringe
- On ice (slow metabolism) — good for 2 hours, or 30 mins not on ice
- No air bubbles
- Perform ASAP
indirect pH/ABG measures
HCO3-
H-H equation
——- of RBCs used in order to spill and measure hemoglobin
Ultrasonic lysis
Measurement of potential (voltage) between two electrodes in a solution.
potentiometry
explain reference and indicator electrode
Reference electrode: Electrode with a constant voltage
Indicator electrode (Na, K, Cl): The measuring electrode
Measured cell potential is related to the molar concentration by the —— equation
nernst
4 important attributes of a reference electrode
- Potential is reversible and obeys the Nernst equation
- Electrode exhibits a potential that is constant with time
- Electrode returns to its original potential after being subjected to small currents
- Electrode exhibits little hysteresis or lag with temperature cycling
ion selective electrode
membrane-based electrochemical transducer capable of responding to a specific ion
ISE advantages over other methods
Fast, sensitive, direct measurement, no reagents needed, selective, easy to maintain, precise, and cost-effective (can be reused)
pH electrode membrane
layers of hydrated and unhydrated glass
usually silver/silver chloride serves as a reference electrode
pH
pCO2 electrode membrane
gas-permeable membrane (Teflon or silicone)
measuring the quantity of electricity needed to convert the analyte quantitatively to a different oxidation states
coulometry
the quantity of electricity or charge that is transported in one second by a constant current of one ampere
coulomb
Measurement of the current flow produced by an oxidation-reduction reaction
amperometry
sweat chloride methodologies
Coulometry
Amperometry
gas-permeable membrane, usually polypropylene, that allows dissolved oxygen to pass through, but nothing else
Clark pO2 gas-sensing electrode
information about the analyte is derived from the measurement of current as a function of an applied potential obtained under conditions that promote polarization of an indicator, or working, electrode
voltammetry
Measurement of lead in whole blood samples methodology
Anodic stripping voltammetry
a measure of the ability of a solution to carry an electric current.
conductometry
The electrical resistance in ohms measured between opposite faces of a 1.00-centimeter cube of an aqueous solution at a specific temperature.
resistivity
pH values of calibration materials
6.8
7.4
frequency of gas calibration
One-point calibrations done every 30 min and 4 hrs
why is calibration important?
the calibration curve, or data, is used to generate all other results for quality control and patient samples
frequency of pH/ABG QC
all three levels of controls assayed within a 24h period (one each 8hr interval)
most ABG errors occur in the —– stage
preanalytical
temperature correction is useful for…
patients who have been exposed to low temps for a prolonged period
room air contamination effects on results
↑ pH
↓ pCO2
↑ pO2
old sample effects on results
↓ pH
↑ pCO2
↓ pO2
the mEq/L base needed to change whole blood pH by 0.01 unit
Hemoglobin-dependent base excess
Siggaard-Andersen Alignment Nomogram
