Blood Gases Flashcards
Gas
Substance that can exist above it’s critical temperature
Acid
substance that can yield H+ when dissolved in H20 (low pH)
Base
substance that can yield H+ when dissolved in H20 (low pH)
Buffer
substance that helps resist a change in pH
Boyle’s Law
volume inverse to pressure
P1V1 = P2V2
Charles Law
volume proportional with temperature
V1/T1 = V2/T2
Gay-Lussac’s Law:
Temperature proportional with pressure
P1/T1 = P2/T2
Dalton’s Law
sum of partial pressures of gases equals total pressure
Total gas pressure = % gas + Barometric Pressure
Barometric Pressure:
total pressures of all gases at 1atm or at 760mm Hg at sea level
Ideal Gas Law
PV = nRT
P = absolute pressure of gas V = volume of the gas n = amount of substance of gas measured in moles R = gas constant T = absolute temperature of gas
how PCO2 can be converted to total CO2?
Total CO2 = HCO3- + H2CO3 + PCO2
(bicarbonate) + (carbonic acid) + (partial CO2)
how PCO2 can be converted to total CO2?
Total CO2 = HCO3- + H2CO3 + PCO2
(bicarbonate) + (carbonic acid) + (partial CO2)
Describe the different means of CO2 transportation in plasma and red cells.
- About 5 % of carbon dioxide is transported unchanged, simply dissolved in the plasma since it is more soluble in blood than oxygen
- About 10 % of carbon dioxide is transported bound to hemoglobin and plasma proteins
- The majority of carbon dioxide is transported as bicarbonate
a. Carbon dioxide enters blood cells to be carried back to lungs (CO2)
b. It combines with water to form carbonic acid (H2CO3) this is catalyzed by carbonic anhydrase
c. Carbonic acid dissociates into Bicarbonate (HCO3-) and Hydrogen Ions (H+) - Bicarbonate ions diffuse out of the red blood cell into the plasma whilst chloride ions (Cl-) diffuse in to take their place. This is known as the chloride shift.
role of carbonic anhydrase.
Within the RBC, carbonic anhydrase is an enzyme that catalyzes carbon dioxide (CO2) combining with water (H2O) to from carbonic acid (H2CO3). This is then dissociated into bicarbonate and Hydrogen Ions. Carbonic Anydrase is an important enzyme in maintaining the acid/base balance. Must buffer CO2 carried in blood to prevent toxicity.
normal ratio of bicarbonate to carbonic acid
20:1 ratio HCO3- : H2CO3
20 bicarbonates for every 1 carbonic acid
substance exchanged for CL- in the chloride shift.
HCO3
Bicarbonate exiting the RBC into plasma is exchanged for CL- entering the RBC from plasma to maintain acid/base balance
Describe CO2 elimination in the lungs.
- Inspiration of oxygen (O2)
- Oxygen (O2) diffuses from alveoli into blood and bound by hemoglobin (O2Hb)
- H+ that was carried on hemoglobin is released to recombine with Bicarbonate to form carbonic acid (H2CO3)
- Carbonic Acid disassociates to from H2O and CO2
- CO2 diffuses through alveoli and exits through ventilation
pH
a measure of the acidity or basicity of an aqueous solution.
pH less than 7 are said to be acidic
pH greater than 7 are basic or alkaline.
Henderson Hasselbach Equation
pH = 6.1 + log(HCO3- / PCO2 * 0.03)
Bicarb pK = 6.1
Dissociation Constant = 0.03
HCO3 = Concentration of base
PCO2 = concentration of acid
primary blood buffer
Bicarbonate (HCO3)
Bohr effect
States that hemoglobin’s oxygen binding affinity is Inversely related to both acidity and concentration of CO2
↑ pH or ↓ CO2 Hemoglobin pick up more oxygen
↓ pH or ↑ CO2 hemoglobin proteins release Oxygen
effect of pH on the respiratory system.
↑ pH less H+ slower breathing to compensate Respiratory Alkalosis
↓ pH more H+ faster breathing to compensate Respiratory Acidosis
Describe the respiratory component of acid-base balance.
PCO2 decreases
less H+
pH increases (alkalosis)
PCO2 increases
more H+
pH decrease (acidic)
Acidosis vs. Alkalosis
Acidosis pH < 7.35 (Lethargy, Coma)
Alkalosis pH > 7.45 (Irritability, Convulsions)
Causes of Respiratory Alkalosis ( ↑pH ↓PCO2)
and Compensation…
Sepsis
Exercise
Pain, fever,
Anxiety
Compensation: Renal excretion (HCO3-) balances Slow breathing (retains H+)
Causes of Respiratory Acidosis (↓pH ↑PCO2)
and Compensation
Over Sedation
Lung Disease
Respiratory Failure
Compensation:
renal Retention of (HCO3-)
rapid breathing (excretes H+)
(acidic, H+ must be balanced)
Identify the metabolic component of acid-base balance
Metabolic component is the renal component HCO3-
Regulated by re-absorption and excretion of Bicarbonate
Define Base Excess
The amount of bases in the blood
Increased Base Excess Alkalosis
Decreased Base Excess Acidosis
Causes of Metabolic Alkalosis (↑pH ↑HCO3)
NAHCO3 administration
Vomiting
Diuretics
Potassium (K+) depletion
Causes of Metabolic Acidosis (↓pH ↓HCO3-)
Normal Gap:
Elevated Cl-
Renal Failure
Diarrhea
Increased Gap Overdose Sepsis Ketoacidosis Diabetic
Compensation vs. Correction
Compensation: body is adjusting to bring pH closer to normal
Correction: pH is in normal range; fully compensated
oxygen, carbon dioxide, and pH electrodes
pO2 Clark electrode sensitive to O2
pH glass electrode sensitive to H+
pCO2 modified pH electrode (severinghaus)
tonometry in blood gas analysis
Reference method that produces known concentrations for pCO2 and pO2 by bubbling gasses into liquid
Used for QC: timely and cumbersome
Calibration for Blood Gases
One Point: pH 7.38 CO2 5% O2 12% or 20%
Two Point: pH 6.83 CO2 10% O2 0%
Protein Fouling of electrode:
build up of protein on electrode due to improper cleaning/maintenance
Sluggish or erratic results
effect of temperature on accurate blood gas analysis
PO2 falsely increased
Diffuses through plastic and shifts left on curve
K+ increases
State the reference (normal) ranges for pH, PCO2, HCO3-, Base Excess.
pH: 7.35 - 7.45
PCO2: 35 - 45
HCO3: 22 - 26
Base Excess: -2 to +2
anticoagulant used for blood gas samples
Lyphilized lithium heparin (dry)
better than liquid lithium heparin since it can dilute the sample and alter equilibration with room air.
quality control of blood gases.
Tonometry: reference method produces known concentrations by bulling gases into liquid
Surrogate liquid (use known values of pH, PCO2, PO2)
Non-Surrogate (automated internal checks to check electronic quality)
Physiologic effects of decreased PO2.
Tachycardia (increased heart rate) Hypertension (increased blood pressure) Vasoconstriction (increased blood pressure) Confusion / loss of judgment Dysrhythmia (irregular heartbeat) Increased respiration
Causes of Increased PO2
O2 administration
Hyperventilation (fast breaths)
Causes of Decreased PO2
Hypoventilation (slow breaths)
Pneumonia or asthma
Diffusion defect pulmonary edema
Altitude sickness
Right Shift
↓ pH (acidosis/more H+)
↑ PCO2
↑ 2,3-DPG
↑ Temp (fever)
* Never reach PO2, low affinity for O2*
Left Shift
↑ pH (alkalosis/fewer H+)
↓ PCO2
↓ 2,3-DPG
↓ Temp (hyperthermia)
* Reach PO2 max sooner, higher affinity for O2*
Oxygen Saturation
measure of how much oxygen the blood is carrying as a percentage of the maximum it could carry.
Define and illustrate the hemoglobin dissociation curve
The hemoglobin dissociation curve is a graph that shows the percent saturation of hemoglobin at various partial pressures of oxygen. It displays the non-linear tendency for oxygen to bind to hemoglobin.
Increasing levels of PO2 after 90 plateaus, it does not yield more saturation
pH, PCO2, HCO3-, pH, and O2 Sat of Venous Blood
SAT 02% 35% – 45% PO2 <50 (venous only pH and pO2 usable) HCO3 24 – 25 PCO2 41 – 51 pH 7.32 – 7.42
pH, PCO2, HCO3-, pH, and O2 Sat of Arterial Blood
SAT 02% >95% pH 7.35 – 7.45 PCO2 35 – 45 HCO3 22 – 26 PO2 80 – 110 Base Excess -2 to +2
Bicarbonate (HCO3-):
Carries majority of CO2 as primary blood buffer (metabolic component retain/excrete)
Carbonic Acid (H2CO3):
weak acid (carbon dioxide dissolved in water)
PCO2
partial CO2 dissolved in plasma (respiratory component blow off H+)
Base Excess:
amount of acid or base to bring pH to 7.4
pH:
Ideal is 7.4 (Acidosis 7.35)
