blood gas Flashcards
(25 cards)
What can be measured from blood gas?
Oxygen (O2) •Carbon dioxide (CO2) •pH •Oxygen Saturation (SaO2) •Bicarbonate (HCO3-) •Base excess (BE)
What are the indications for measuring blood gases?
Identification of respiratory, metabolic, and mixed acid-base disorders
- Measurement of the partial pressures of respiratory gases involved in oxygenation and ventilation, including assessment of severity of impairment
- Assessment of the response to therapeutic interventions such as mechanical ventilation in a patient with respiratory failure
Give some advantages of capillary blood gas:
- Relatively pain free
- Simple procedure
- Physiologists can perform
- No doctors required for procedure
- If performed well can reflect PO2from an ABG
- Only small sample needed (90-150ul)
Give some disadvantages of capillary blood gas:
- Poor arterialisation – misleadingly low PO2
- Not the gold standard
- Sample may be exposed to air
- ## now not recommended for LTOT assessment
What are the contraindications for blood gas testing?
- Patient does not consent to procedure
- Negative modified Allen’s test (ABG)
- Abnormal puncture site (e.g. inflammation, infection etc)
- Aortic valvular incompetence (ABG)**
What needs to be done before taking blood sample?
History taking, asking contraindications, consent and adequate skin prep
What are the advantages and disadvantages of ABG?
CONS:
•Painful
•Difficult to perform
•If you want to take repeated samples e.g.
LTOT assessment – hard to take
repeated samples
•Risk of arterial occlusion (clot) / damage
Pros:
•Gold standard
•Accurate in acute settings
•Minimal exposure to air
What are the advantages and disadvantages of ABG?
CONS:
•Painful
•Difficult to perform
•If you want to take repeated samples e.g.
LTOT assessment – hard to take
repeated samples
•Risk of arterial occlusion (clot) / damage Pros:
•Gold standard
•Accurate in acute settings
•Minimal exposure to air
What is Allen’s test?
The Allen test is a first-line standard test used to assess the arterial blood supply of the hand
What are the three different electrodes?
Sanz electrode, Severinghaus electrode, Clark electrode
What do the 3 electrodes directly measure?
- pH-Sanz electrode
- PCO2-Severinghaus electrode
- PO2-Clark electrode
How often should Blood gas machines be calibrated?
•Calibration – normally automatic on modern analysers:
One point calibration – every 1-2 hours (ARTP 2020)
Two point calibration – every 4-6 hours (ARTP 2020)
What is one point Calibration?
One point calibration:
•Simplest type
•Corrects sensor for offset errors
•Only one measurement point needed
What is two point calibration?
Two point calibration:
•More precise
•Sensor offset is adjusted at 2 different values
•Leads to accurate measurement across the range
•Two point calibration additionally allows slope of calibration curve to be adjusted
What are the normal values for PaO2, PaCO2, pH, HCO3, BE, SaO2?
Pa02 – 10-13.5kPa
PaCO2- 4.8-6.0kPA
PH- 7.35-7.45
HCO3- 23-27mmol/L
BE –-3- +3
SaO2- 97-99%
Type 1 respiratory failure is defined as…
Low Pa02 along with normal or low PaCO2
Type 2 respiratory failure is defined as
Low PaO2 along with an Increased PaCO2
What other factors does the equation help us to understand?
Patient with emphysema with low SPO2
Affect of abnormal hb
Altitude ?
The equation for oxygen carrying capacity is…
CaO2 (ml/dl) = (SaO2 x Hb x 1.34 ) + (0.003 x PaO2)
(bound to Hb) YOUT (Plasma)
Where:
SaO2 – expressed as a fraction (e.g. 0.99 not 99%)
1.34 – capacity of Hb to carry O2 (g/dl) (Hufner’s constant)
0.003 – derived from oxygen solubility constituents (37oC) ( referred to as aO2)
PaO2 – in mmHg
What is the normal Ca02?
16-22ml/dl
How do you know if it is resp or metabolic?
Resp- the PaCO2 will be abnormal
Metabolic- the HCO3 will be abnormal
1- When is it alkalosis?
2- When is it acidosis?
1- when the pH is greater than 7.45
2-when the pH is smaller than 7.35
The mode of action for a sanz electrode
oUses AgCl and Calomel reference electrodes
oPotential difference between blood sample and buffer solution is measured
The mode of action for servinghaus electrode
oCO2 forms H+
oDifference in H+ ions between electrolyte & buffer solution is measured
