Blood Gas Analysis Flashcards
1kPa =
7.5mmHg
What are the main buffers in the body?
- proteins
- haemoglobin
- carbonic acid/bicarbonate
What are the 2 main organs or excretion?
lungs and kidneys
When do acid-base disturbances occur?
- problem with ventilation
- problem with renal function
- overwhelming acid or base load the body cant handle
What are the normal values for:
- pH
- pO2
- pCO2
- Bicarbonate
in arterial blood gases?
- pH = 7.35-7.45
- pO2 = 12-13 kPa
- pCO2 = 4.5-5.6 kPa
- Bicarbonate = 22-26 mmol/L (standard)
How is the standard bicarbonate calculated?
calculated frim the actual bicarbonate assuming 37°C and paCO2 of 5.3kPa
**Compensates for changes in CO2
What does thw standard bicarbonate show?
reflects the metabolic componenet of acid-base balance
What is step 1 in looking at arterial blood gases?
Look at the pO2
Is the patient hypoxic?
What are the adverse effects of high oxygen levels?
- Increases risk of hypercapnic respiratory failure in acute exacerbations of COPD
- Increased mortality survivors of cardiac arrest
- increased mortality intensive care patients
- increased mortality in acute severe asthma
What can high oxygen levels generate? and what are the consequences of this?
- Generate free radicals
- Lung toxicity
- collapse of alveoli due atelectasis
- irritating mucois membranes
What are the two main oxygen targets in hypoxia?
- Normal = 94-98%
- Type 2 resp. failure = 88-92%
What are the theraputic uses of high inspired concentration of oxygen?
- pneumothorax
- high oxygen conc. decreases the volume of N2 due to concentration gradient
- carbon monoxide poisoning
- pulse oximetry cannot differentiate between CO and O2 bound to Hb
Draw a diagram indicating how oxygen partial pressure changes in different parts of the body
What is the normal alveolar-arterial (A-a) gradient
less than 3kPa
expect arterial pO2 to be approx. 2/3 FiO2
What is the P/F ratio?
PaO2/FiO2
kPa divided by the fraction of inspired oxygen (0.2)
Give the P/F ratio in:
- healthy person
- acute lung injury
- Acute respiratory distress syndrome
- healthy = >50
- acute lung injury = < 40
- ARDS = <26.7
What is step 2 in looking at arterial blood gases?
Assess the pH
- <7.35 = acidaemia
- >7.45 = alkalaemia
- 7.35-7.45
- Normak
- mixed acid-base abnormality
What is step 3 in looking at arterial blood gases
Determine the primary problem
What is supected if pH and pCO2 are changing in the opposite direction?
respiratory problem
What is suspected in pCO2 and pH are changing in the same direction?
metabolic problem
What is step 4 when looking at arterial blood gases?
Is compensation occuring?
What is compensation?
altering of function of the respiratory or renal system in an attempt to correct an acid-base imbalance
What happens to pH and HCO3- if compensation is possibly occuring?
they move in the same direction (proportional)
If both values move in opposite durections more than 1 pathology must be present
How do the kidneys compensate for chronic respiratory acidosis?
retaining bicarbonate
takes a few days to reach maximum
Causes of hyperventilation
- acute severe asthma
- pulmonary embolism
- pulmonary oedema
- anxiety attack
What can cause an abnormal level of central respiratory drive?
- hypoxia
- stimulation lung mechanoreceptors/chemoreceptors
- direct stimulation of respiratory centre
- psychogenic
