ABG
• Determines the blood pH and the partial
pressures of O2 and CO2
• Measures the respiratory and metabolic
effects of the disease on the acid-base
balance
• Cannot be determined by clinical assessment
alone
↑CO2
↑RR = ↓CO2
Respiratory Effects
CO2 + H2O H2CO3 HCO3
-...
Arterial Blood Gas (ABG)
Arterial Blood Gas (ABG) - Answer Arterial Blood Gas (ABG) is defined as:
"The measurement of the acidity and the levels of oxygen and carbon dioxide in the
blood from an artery."
ABG are tests which are used to check how well a patient's respiratory efforts are able
to move oxygen into the blood and remove carbon dioxide from the blood..
Arterial Blood Gas (ABG), Components - Answer ABG are broken down into 5 main
components, which are then further grouped together into their areas of use.
List of Components
1) pH
* The hydrogen concentration in the Arterial Blood
* ≈ 7.35 - 7.45
2) PaCO2
* Partial Pressure (PP) of CO2 within the Arterial Blood
* ≈ 35 - 45 mmHg
3) PaO2
* Partial Pressure of O2 within the Arterial Blood
* ≈ 80 - 100 mmHg
4) HCO3
* Concentration of Bicarbonate within the Arterial Blood
* ≈ 24 - 30 mEq/l
5) SaO2
* Saturation of Hemoglobin with Oxygen within the Arterial Blood
** Sometimes referred to as: % O2-Hgb
* ≈ 93 - 100%
- ABG components are either measured directly from the sample or are calculated
based off of measurements or other component values.
1) Measured
* pH
* PaCO2
* PaO2
2) Calculated
* HCO3
* SaO2
,Arterial Blood Gas (ABG), Components, Groupings - Answer The components of ABGs
can be generally grouped into 3 sections
1) Acid-Base Calculations
- Based on the components:
* pH, PaCO2, HCO3
2) Ventilation Efficiency
- Based on the component:
* PaCO2
3) Oxygenation Efficiency
- Based on the components:
* PaCO2, PaO2, SaO2
Arterial Blood Gas (ABG), ABG vs. VBG - Answer There is always debates about which
set of values to use:
* Venous Blood Gas (VBG)
- or -
* Arterial Blood Gas (ABG)
- VBG are not able to provide information about the oxygen status of the patient.
* But are viewed as being an acceptable substitute
** For A/B status analysis
Common VBG Values:
- Venous pH ≈ Arterial pH - 0.03
- PvCO2 ≈ PaCO2 - 4
Arterial Blood Gas (ABG), Effect of Temperature - Answer ABG analysis is temperature
control, but the temperature of the blood at which is was collected and the temperature
of the lab equipment can impact the gathered results
- Typically lab machines will report 2 ABG results for patients, one with the temperature
of the blood when it was taken, and one with temperatures at the normal set point of the
equipment.
* ≈ 37°C
Effects of Temperature:
1) Increased Temperature
,- Causes more Gas to dissolve into the blood, increasing readings
2) Decreased Temperature
- Causes more Gas to evaporate out of the blood, decreasing readings
The Journey of Oxygen - Answer The fundamental principle of human physiology is the
delivering of oxygen to cells and removing of CO2 from cells to enable Aerobic
Respiration.
- All organ systems and individual biochemical processes exist to support this purpose
- As oxygen is the critical element in this equation, its movement into the body, through
the body, and use in the body are thoroughly evaluated.
Journey of Oxygen can be summarized as:
1) Ventilation
- Oxygen enters the Alveoli
- Dependent on the Communication between the Lungs and Brain
- Summarized by the PaCO2 Equation
2) Diffusion
- Oxygen moves from within the Alveoli into the Blood stream
- Dependent of the Tissue of the Lungs
- Summarized by the Equations of Oxygenation
* Alveolar Gas Equation
* A-a Gradient
3) Transport & Delivery
- Oxygen is:
* Bond to Hgb
* Taken to the tissues
** Typically as O2-Hgb complex
* Oxygen is released from Hgb into the tissues
Ventilation - Answer The process of ventilation can be generically described as:
" The exchange of air between the lungs and the atmosphere so that oxygen can be
exchanged for carbon dioxide in the alveoli."
- This process is summarized by the PaCO2 equation
- The PaCO2 equation puts into physiological perspective one of the most common of
all clinical observations:
* A patient's respiratory rate and breathing effort.
PaCO2 Equation
PaCO2 = (VCO2 x 0.863)/(VA)
Where:
VCO2 = Rate of CO2 production from respiration
* ml of CO2/min
0.863 = Constant
* This constant is necessary to equate dissimilar units for VCO2 and VA
VA = RR(Vt-Vd)
*aka Alveolar Ventilation Equation
- Where:
*RR = Respiratory Rate
** Breathes/min
*Vt = Tidal Volume
** L
*Vd = Dead Space Volume
** L
Ventilation, Respiratory Input Centers - Answer These receptors provide information to
the Medullary Respiratory Center in the Brainstem, which controls RR and Vt.
- These centers are divided into 2 divisions
1) Central Input
- 4 types
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