Blood Gas Analysis Flashcards
State the scale of kPa to mmHg. Which of kPa or mmHg should you use ?
1kPa = 7.5mmHg
kPa
Identify the main volatile and non-volatile acids in the body.
Volatile: carbonic acid (H2CO3)
Non-volatile: lactic acid, phosphoric acid, sulfuric acid
How are non-volatile acids produced ?
Through the breakdown of proteins
What are the main buffers in the body ? What is the function of buffers ?
♦ Proteins
♦ Haemoglobin
♦ Carbonic acid / bicarbonate
Keep pH within tight range.
Why is it important to maintain pH within a tight range ?
Because enzymes work sub-optimally above or below optimal pH.
What are the main ways of excreting acid ?
Lungs (by exhaling CO2)
Kidneys (by excreting acids in urine)
What would happen to the buffer system if an acid overload occur ?
Since buffers have limited capacity, an overload would result in more acidic pH. However, the body can get rid of volatile acids (through lungs) and non-volatile acids (through kidneys), to prevent this happening.
Where are blood gases usually taken from ?
From radial artery, but it can be any palpable artery.
Briefly explain the process of taking blood gases.
Local anaesthetic used first because procedure painful.
Then, puncture performed at the distal part of the limb.
Sample then kept cool until analysed.
What are the main components of the blood gases analysis results.
pH
pCO2
PO2
Bicarbonate
Define standard bicarbonate.
Concentration of bicarbonate in the plasma from blood which is equilibrated with a normal PaCO2 (40 mmHg, i.e. 5.3kPa) and a normal pO2 (over 100 mmHg) at a normal temperature (37°C).
How do we calculate standard bicarbonate ?
Obtained by solving the Henderson-Hasselbalch equation to get a bicarbonate value when the pH is known and PaCO2 is 40mmHg.
State the equation for the bicarbonate buffer system.
H2O + CO2 ↔ H2CO3 ↔ H+ + HCO3-
What is the impact of respiratory failure on blood gases ?
Increased CO2 (cannot blow it off) and thus increased bicarbonate (since equation shifts to the bicarbonate)
What happens if, the calculation of standard bicarbonate by the machine in hospital is for a patient with respiratory failure (i.e. not normal pCO2 5.3 kPa) ?
High pCO2 will result in high bicarbonate because of equilibrium. Machine would standardise it (i.e. would say what bicarbonate would be if pCO2 was 5.3 kPa).
Which of the components of the bicarbonate buffer system reflects the metabolic component of acid base balance ?
Bicarbonate
Which of the components of the bicarbonate buffer system reflects the respiratory component of acid base balance ?
CO2
Once the patient (e.g. with accident induced pneumothorax) is in and inspection has been performed, what are ways to gather more clues about their condition ?
- History
- Examination
- What are they breathing
- Urea and Electrolytes
- Haemoglobin
- Glucose (i.e. diabetic ?)
- Arterial blood gases
- CXR
Then, look at arterial blood gas results
What are the main chronological steps in analysing arterial blood gas results ?
Step 1: Assess oxygenation (i.e. look at PO2)
Step 2: Assess pH
Step 3: Determine the primary problem
Step 4: Is compensation occurring?
Why is it necessary to assess oxygenation first and foremost in a blood gas analysis ?
Because hypoxia would kill the patient before anything else
What are the main ways to assess oxygenation ?
- Looking at pO2 (more precise than other two)
- Sometimes, arterial blood gas results are not even necessary, obvious central cyanosis
- Pulse oximetry can also be used
What are potential issues with oxygenation ?
Hypoxia
Oxygenation too high