Arterial Blood Gases Flashcards
How would you convert H+ to pH?
80 - [H+] = 2 decimal places after 7
Describe the normal physiology of the acid base balance (including the buffers and excretion methods of the body)
human body continually producing acid
- volatile: CO2
- non-volatile: breakdown of proteins in body
pH must be maintained (through buffers):
- proteins
- haemoglobin
- carbonic acid/bicarbonate
excretion:
- lungs
- kidney
What causes acid-base disturbances?
- problems with ventilation
- problems with renal function
- overwhelming acid/base load that the body can’t handle
What are the normal values for pH, PO2, PCO2 and bicarbonate (std)?
- pH: 7.35-7.45
- pO2: 12-13 kPa
- pCO2: 4.5-5.6 kPa
- bicarbonate: 22-26 mmol/l
What is standard bicarbonate?
calculated from actual bicarbonate but assuming 37 degrees and a pCO2 of 5.3 kPa (normal)
What are the steps of assessing arterial blood gas results?
- step 1: assess oxygenation (look at pO2) to see if hypoxic/too high
- step 2: assess pH (acidaemia/alkalaemia)
- step 3: determine primary problem
- step 4: determine if compensation is occuring
What can happen if the pO2 is too high?
- can result in retinopathy causing visual impairment
- increased risk of hypercapnic respiratory failure in acute exacerbations of COPD
- increased mortality in survivors of cardiac arrest, intensive care patients and in those with acute severe asthma
Describe the physiological effects of high oxygen levels
- generates free radicals
- lung toxicity:
- collapse of alveoli due to atelectasis
- irritation of mucous membranes
- ocular toxicity
- myocardial damage
- neurological damage
Describe how you would treat a respiratory emergency and the targets for pO2 levels
- oxygen to treat hypoxia but won’t solve dyspnoea
- give high concentration of oxygen then titrate to target level once stable:
- normal: 94-96%
- type 2 resp faolure: 88-92%
Describe how high concentration oxygen treats pneumothorax and CO poisoning
- pneumothorax: increases pressure causing nitrogen to leave pleural cavity to faster reduction of pneumothorax
- CO poisoning: CO bonded with haemoglobin, increases dissolved O2 in bloodstream and pushes CO off haemoglobin
What is the normal alveolar-arterial gradient? (A-a)
- less than 3kPa
- arterial pO2 should be 2/3 FiO2 (fraction of inspired O2)
What do different pO2/FiO2 numbers mean?
- > 50 = healthy
- <40 = acute lung injury
- <26.7 = ARDS (acute respiratory distress syndrome)
What are the 2 reasons for pH levels to be within the normal range?
- normal
- mixed acid base abnormality (2 opposing pH abnormalities)
How can you determine whether the patient has a respiratory/metabolic problem?
- respiratory problem: if pH and pCO2 levels are changing in opposite directions
- metabolic problem: if pCO2 and pH are changing in same direction
How can you determine if compensation is occuring?
- if pCO2 and HCO3- are changing in the same direction
- possible that compensation if occuring
- if moving in opposite directions - more than 1 pathology
