6 Blood Gases Flashcards
How much acid per day is produced by our body doing its normal processes versus the level maintained in the body (mmol/L)?
Normal cellular functions produces acids: Carbonic acid, lactic acid, β-hydroxybutyrate, sulfuric acid, phosphoric acid.
o 50 – 100 mmol/L of acid produced/day
o Maintained at 36 – 44 nmol/L
What are the main ways our bodies eliminate acids?
Eliminate acids by:
- Buffering
- Through organs
a) In lungs acid is lost as CO2
b) Kidneys (carried there by blood plasma)
What are buffers and what does it consist of?
Buffers resist pH changes
Combination of weak acid and a salt of its conjugate base
What are two major buffering systems in the body?
Major buffering systems:
1. Bicarbonate-Carbonic acid Buffering System
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-
2. Phosphate Buffer System
Primary buffer in urine
What is the Henderson - Hasselbalch Equation?
pH=pKa+log([A-]/[HA] ), where:
Ka is dissociation constant (pH at which there is an equal concentration of protonated and unprotonated species)
A- proton acceptor or base
HA- proton donor or weak acid
What is the chemical reaction formula for the most important buffering system in extracellular fluids? What are the weak acids and salt of the conjugate base?
• Bicarbonate-Carbonic Acid Buffering - Most important buffering system in extracellular fluids
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-
• Carbonic acid = H2CO3 = weak acid
• Bicarbonate = HCO3- = salt of the conjugate base
In the bicarbonate-carbonic acid buffering system where does the Co2 (end-product) go?
CO2 is the end-product of most metabolic processes:
- Most diffuses into RBCs –> Causes diffusion of HCO3- into plasma
- Small amount of CO2 is dissolved in plasma
What happens in the Bicarbonate-Carbonic Acid Buffer system as CO2 enters red blood cells?
- CO2 diffuses into RBC and combines with H2O to form H2CO3 which dissociated into H+ and HCO3-
↑ in HCO3- (bicarbonate) causes it to diffuse into plasma
To maintain electroneutrality, Cl- diffuses into the cell
Known as the Chloride Shift - CO2 is carried by hemoglobin as carbaminohemoglobin (HbCO2) and eliminated by lungs through alveolar gas exchange
What is the process that happens in the lungs with the Bicarbonate-Carbonic Acid Buffering System?
In lungs (process is reversed):
- O2 is inspired, diffuses from alveoli into RBC, becomes bound to hemoglobin (oxyhemoglobin = O2Hb)
- H+ that was on hemoglobin is released to combine with HCO3- which dissociates into CO2 + H2O
- CO2 diffuses into alveoli and is exhaled
When the balance is off with blood gases what happens if removal of CO2 is slower than production and when it is faster than production?
Regulation through hypo- or hyperventilation
1. If removal is slower than production:
o Accumulation of CO2 in blood
o Causes an ↑ in H+
2. If removal is faster than production (hyperventilation):
o Causes a ↓ in H+
o ↓ in blood pH
What is the body’s first defense against acid-base issues?
Changes in H+ concentrations from non-respiratory disturbances will cause changes in ventilation so blood pH is stable
–> Buffer system + lungs = first defense against acid-base issues.
What do the kidneys do to help maintain the body’s acid/base balance?
Excrete acids, but also need to re-absorb bicarbonate (HCO3-) by exchanging sodium in the filtrate for H+ in the tubular cells
Note: Don’t want to lose HCO3- in urine as it would impact bicarbonate-carbonic acid buffer system (↑ H+)
Compare the concentration of hydrogen ions in an acidotic state and an alkalotic state?
↑ H+ = acidotic state
↓ H+ = alkalotic state
It is important to assess blood gases with electrolytes because of this relationship
What does it suggest when the anion gap is above its reference range?
An anion gap above its reference range:
Suggests metabolic acidosis
What can a venous sample be used for in regards to acid/base balance analysis in the medical lab?
Can be used to monitor metabolic acid-base imbalances where oxygenation is not usually a problem
What type of tube should an arterial sample be taken with and what precautions used?
Arterial • Pre-heparinized glass or plastic syringe • Without tourniquet anaerobically • Placed on ice • Immediate analysis
What type of tube and precautions are done with a blood gas capillary sample?
Capillary
• Heparinized capillary
• Poke in finger or heel
• Analyzed immediately
What can happen if a patient’s in distress?
Patient distress and hyperventilation:
Can cause HCO3- ↓ pCO2 ↓ pH↑ pO2 ↑
What type of anticoagulant should be used for blood gas lab samples and why is the tube put on ice?
Anticoagulant:
o Lithium heparin in the dry state
o Liquid will dilute sample = falsely ↓ results
Ice:
o Stabilize values
o Cell metabolism and glycolysis will continue to use O2
Why is it important to ensure that the blood gas tube sample is sealed?
Seal sample:
Exposure to room air / bubbles causes pH ↑ and pO2 ↑ and pCO2 ↓
Note: O2 is higher in air and CO2 is lower. Gases move by diffusion from higher concentration to lower
Within what time frame should a blood gas sample be analyzed and can happen if not done in time?
Time:
- Run immediately (within 30 minutes) or remove from cells and freeze
- After 30 minutes pO2 ↓ pCO2 ↑ pH ↓
By what method are pH, pCO2 and pO2 measured and what are the inferences that can occur in the measurement?
pH –> Potentionmetry, build up of proteins on membrane surface can interfere.
pCO2 –> same as pH
pO2 –> Amperometric Electrode (or Clark electrode), build up of proteins on membrane surface can interfere; Contamination of sample with room air
What are the iSTAT capabilities for measuring blood gases and how does it determine them?
G3+ cartridge provides results for: pH, pCO2, pO2, TCO2, HCO3-, BE, sO2
G4+ adds lactate
pH and pCO2 are measured by ion‑selective electrodes (potentiometry) –> Concentrations calculated using Nernst equation
pO2 is measured amperometrically
HCO3- is calculated from the pH and pCO2 using the Henderson-Hasselbalch equation
Other analytes are calculated as well
