Intro to blood gas analysis Flashcards
pH normals and changes when
Acidity or alkalinity of the blood
Scale of 1–14
Increase H+ Decrease pH [ACIDOTIC]
Decrease H+ Increase pH [ALKALOTIC]
NORMAL BODY pH = 7.35–7.45
Normal cellular metabolism, enzymatic and cellular reactions require normal pH
What is acidosis and waht are the cs
Decreased pH
Decreased excitability of cells
Clinical signs:
Alteration in cardiac contraction
Decreased vascular response to catecholamines
Can lead to loss of consciousness
What is alkalosis and what are the cs
Increased pH
Increased excitability of cells
Clinical signs:
Impaired neurological function
Impaired muscle function
Tingling sensations, nervousness, muscle twitching
What are buffers in you body
Chemical–immediate response
Respiratory–seconds to minutes
Renal–hours to days
What are electrolyte shifts
ACIDOSIS–compensatory response:
- H+ moves into the cell while K+ moves out of the cell
- Result is HYPERKALEMIA
ALKALOSIS–compensatory response:
-H+ moves out of the cell while K+ moves into the cell
- Result is HYPOKALEMIA
What is a resp buffer
Normal byproduct of cell metabolism = CO2
CO2 travels via the blood to the lungs where it is excreted
Excessive CO2 combines with H2O → H2CO3
Blood pH changes according to how much CARBONIC ACID is present
Respiratory rate changes via stimulation of chemoreceptors in the carotid and aortic bodies
Chemoreceptors respond to changes in blood pH, PaO2 and PaCO2
What is a renal buffer
To maintain blood pH–kidneys retain or excrete bicarbonate (HCO3-) to compensate
When pH decreases (acidosis)–kidneys retain base (HCO3-)
When pH increases (alkalosis)–kidneys excrete base (HCO3-)
Gas transport and gas composition
Oxygen
- Transported attached to hemoglobin
-Taken up in the lungs and delivered to the tissues
- Inhale 21% oxygen in the atmosphere (FIO2=21%)
- Exhale ≅15% (ETO2 = 15%)
Carbon dioxide
-Transported as bicarbonate ion in plasma
-Byproduct of metabolism with H+ ions
- Exhaled (ETCO2) = 5-6%
-Negligible amounts are inhaled from the atmosphere
FI= Fraction inspired (inhalation)
ET= end tidal (exhalation)
What does our air consist of
Obey daltons law of partial pressures
The sum of all pressures of a gas = atmospheric pressure
We breathe air which consists of
20-95% O2
0.038% CO2
78.09% N2
0.247% H2O
0.93% Argon
Other trace gasses
Conversion of % to partial pressure
Atmospheric pressure at sea level = 760 mmHg
If all gasses in the atmosphere add up to 760 mmHg (100%) what is the partial pressure of gases in the atmosphere?
O2 = (20.95/100) x 760 = 159.22 mmHg
CO2= (0.038/100) x 760 = 0.28 mmHg
What is exhaled (ETCO2)?
(5-6/100) x 760 = 38–45.6 mmHg
When do you take arterial blood for a blood gas analysis
Required if we want to assess lung function
Take an arterial blood sample from any systemic artery
Arterial blood has left the lungs and not passed through tissues
Can examine any compensatory changes made by lungs/brain
A paul oximeter will also provide arterial oxygenation status
When do you take venous blood for a blood gas analysis
For indices of metabolism, look at blood running from the whole body (mixed venous blood), NOT the paw (peripheral vein):
- Ideally, sample is taken from the pulmonary artery
-Clinically we use a jugular sample
-Good for lactate, pH, PvO2, PvCO2, and co oximetry
A peripheral venous sample can be used for
- Glucose, electrolytes, hemoglobin
-Gasses and lactate only if good circulation
Do not assess a venous sample to look at resp compensation by the lungs
- Need to look at how lungs (arterial sample) and brain (arterial blood flow) are interacting to control body pH
Blood sampling errors
Use pre heparinized commercial syringes to avoid damaging analyzer
Blood sample taken from wrong site
Peripheral venous sites do not reflect whole body in hypovolemic states
Venous sites do not accurately assess lung function. Need arterial sample
Air bubble
Always remove air bubbles from blood sample
pO2 and pCO2 will reflect atmospheric partial pressures from the air bubble
Accidental sampling from artery and vein together (bizarre results)
Erroneous low PaO2 and high PaCO2
Continuous metabolism in sample (white cells, avian cells); analyze immediately
Oxygen terminology
Function of inspired (FI) O2
- Must know FIO2 to assess oxygenation status of animal
Partial pressure O2
- pO2 (tissue); PaO2 (artery); PvO2 (vein)
- O2 dissolved in tissue or plasma
- ‘Driving force’ for O2 to attach to (saturate) a hemoglobin molecule with 4 molecules of O2
Saturation of hb with O2 (sO2)
-Relationship of sO2 with pO2 of blood
Oxygen Content (O2Ct)
- Total number of mLs O2 per litre of blood (arterial: 200mL)
- Sum of dissolved O2 in plasma and O2 carried by hemoglobin
How does a normal lung function
Mammals: PaO2 is less than atmospheric pO2 (150 mmHg)
Normal PaO2 in mammals = 90-100 mmHg
Mammals can store O2 during breath-hold
PaO2 is dependent on lung function
Some V/Q mismatch
Arterial blood leaving the lung is mix of well oxygenated and poorly oxygenated blood
General Rule:
PaO2= 5 x FIO2(%)
What controls resp
In the blood–concentrations of the following stimulate the RESPIRATORY RATE:
CO2
pH
O2
Central chemoreceptors located on the ventral surface of the medulla respond to changes in pH of the CSF
Peripheral chemoreceptors located in the CAROTID and AORTIC arches
What does typical BG analyzer measure
Electrodes measure:
pH
Partial Pressure CO2 (pCO2)
Partial Pressure O2 (pO2)
Does not measure Nitrogen, Nitrous oxide
Calculates bicarbonate, base excess
Co-oximeter measures:
Hemoglobin content
Saturation of hemoglobin molecule with O2
Re-calculates pO2 and pCO2 for body temperature
Electrolytes; Glucose; Lactate; Anion Gap
BG analysis: rules of thumb
FIRST ➔ Look at the pH
SECOND ➔ Look at the PaCO2
THIRD ➔ Look at the HCO3-
Once the primary disturbance +/- compensation has been determined, assessment of oxygenation is next.
This is only reliable if what type of sample has been obtained??
Arterial
Then look at:
PO2
sO2
O2ct
What is a normal and abnormal PaCO2
Tells us about the RESPIRATORY component
Opposite of the pH
Normal PaCO2 is 35–45 mmHg
PaCO2 < 35 = ALKALOSIS
PaCO2 > 45 = ACIDOSIS
What is a normal and abnormal HCO3-
Tells us what the KIDNEYS are doing (METABOLIC component)
Follows the pH
Normal HCO3-= 22–26 mEq/L
HCO3- < 22 = ACIDOSIS
HCO3- > 26 = ALKALOSIS
Understanding O2 content and O2 delivery
O2 delivery = O2 content× Cardiac
Output O2 content = (1.34 × SaO2×[Hb]) + (0.003×PaO2)
O2 Hb is carrying Dissolved O2
Cardiac output = Heart Rate (HR) x Stroke Volume (SV)
