Clinical Blood Gases Flashcards
Why do we measure blood gases?
To assess very sick patients
To diagnose respiratory failure
To diagnose metabolic problems
How do we quantify oxygen carriage?
Haemoglobin saturation
-Because it’s very easy to do!
-Assuming Hb is normal, it’s an accurate reflection of oxygen content
Arterial blood gases
-More complicated and invasive
-PaO2 reflects haemoglobin saturation but is a measure of the partial pressure of O2 in the blood
How is haemoglobin saturation measured?
Oxygenated haemoglobin is RED
Deoxygenated haemoglobin in BLUE
Using absorption spectroscopy, it is possible to estimate the degree of saturation of haemoglobin
SpO2, pulse oximetry
How do we measure arterial blood gases?
Single arterial puncture technique -Radial artery -Femoral artery -Brachial artery Measurement from in-dwelling arterial catheter or A-line -Only really an option in HDU/ITU
What does a blood gas measure?
PaO2 PaCO2 Hydrogen ion/pH Bicarbonate Some analysers may also measure electrolytes and Hb Other forms of haemoglobin: Carboxyhaemoglobin
What is the difference between PA and Pa?
PA= partial pressure in alveolus Pa= partial pressure in arterial circulation
What are normal blood gas values?
H+ 36-44 nmol/l PO2 12-15 kPa PCO2 4.4-6.1 HCO3 21-27.5 BE +2 to -2 mmol/l
What is kPa of oxygen?
Partial pressure of oxygen in the air is 21 kPa
The total pressure in the atmosphere is 100 kPa
21% of the air is oxygen, therefore 21% of the total pressure is the partial pressure of oxygen
This depends on environment
What is ‘normal’ PaO2?
‘Normal’ PaO2 = 12-15 kPa
Breathing air
At normal atmospheric pressure
This is because the amount of oxygen available in the alveolus (PAO2) is around 14-15 kPa
What is ‘normal’ pCO2?
“Normal PaCO2” = 4.4 – 6.1 kPa
pCO2 and CO2 content vary with ventilation
More ventilation = low pCO2 and content
Less ventilation = high pCO2 and content
Hypoventilation causes build up of alveolar CO2 and therefore less is removed from blood
Increase in blood CO2 leads to acidosis
What is carbon monoxide poisoning?
Carbon monoxide (CO) is produced from incomplete combustion of hydrocarbons (faulty gas boilers etc)
Carbon monoxide binds to haemoglobin in the place of oxygen to form carboxyhaemoglobin
(also interfers with mitochondrial respiration)
Death by asphyxia
Treatment is high concentration oxygen (to displace the CO from the haemoglobin)
How can respiratory failure be defined?
Low oxygen level in the blood -Hypoxaemia Respiratory failure -PaO2 < 8.0 kPa Caused by either V/Q mismatch or hypoventilation (or both)
How does pneumonia lead to type 1 respiratory failure?
Low O2 due to blood not being oxygenated on passage through pneumonic lung
Patient breathes faster so can get rid of excess CO2 but can’t increase O2
What are other causes of Type 1 Respiratory Failure?
Lung diseases effecting the parenchyma
Interstitial lung disease
Bronchiectasis
Obstructive airways disease e.g. asthma and COPD (but these can also cause type 2 RF)
Pulmonary embolism
Treat with oxygen whilst treating underlying cause
Describe Type 2 Respiratory Failure?
Low PaO2 High PaCO2 Caused by hypoventilation May be acute or chronic If acute will have respiratory acidosis
How does Type 2 Respiratory Failure lead to acidosis?
Low oxygen level due to hypoventilation of (diseased) lungs
High CO2 due to increased levels in alveolar space and less removed from blood
Acute rise in blood CO2 leads to respiratory acidosis
What are the causes of Type 2 Respiratory Failure?
Hypoventilation due to any cause Opiate toxicity Neuromuscular disease COPD Acute severe asthma Important to differentiate acute from chronic type 2 resp failure
What is acidosis?
CO2 + H2O –H2CO3– H+ + HCO3-
H+ is increased by:
An increase in pCO2 (respiratory acidosis)
An increase in acid production or decrease in excretion (metabolic acidosis)
What is respiratory acidosis?
H+ is increased by:
An increase in pCO2 (respiratory acidosis)
An increase in acid production or decrease in excretion (metabolic acidosis)
What is acute vs chronic type 2 respiratory failure?
Acute hypoventilation e.g. due to opiate toxicity leads to hypoxia, hypercapnia and acidosis
Chronic hypoventilation e.g. neuromuscular disease or severe COPD leads to hypoxia and hypercapnia but may not have acidosis due to compensation
Describe metabolic compensation in chronic Type 2 respiratory failure
Increased bicarbonate retention by the kidney compensates for acidosis
How can a sudden increase in PO2 in oxygen therapy worsen hypoventilation?
CO2 normally stimulates ventilation
Some patients with chronic type 2 respiratory failure are dependant on hypoxia to stimulate breathing
Chronic high CO2 is compensated so no longer stimulates breathing
What is respiratory alkalosis?
Not usually associated with respiratory failure
Caused by hyperventilation
Have low PCO2 and low H+
How does metabolic problems lead to acidosis?
Excess acid production by the body e.g. lactic acidosis or diabetic ketoacidosis
Kussmal breathing is a classical clinical sign of acidosis as a compensatory mechanism to increase CO2 removal from the blood
Full compensation is difficult: need to treat the underlying cause of increased acid load e.g. treatment of DKA
How do we interpret bicarbonate?
HCO3- is increased by: An increase in pCO2 HCO3- is decreased by: An increase in acid production or decrease in excretion CO2 + H2O -- H2CO3 -- H+ + HCO3- Blood gas machine
What is the difference between actual and standard bicarbonate?
Actual bicarbonate:
Calculated with actual H+ and pCO2 values
Standard bicarbonate:
Calculated with actual H+ and a pCO2 of 5.3kPa (normal pCO2)
Standard bicarbonate is therefore only influenced by metabolic effects
What is base excess?
The amount of base needed to be removed from a litre of blood at a normal pCO2 in order to bring the H+ back to normal
It is calculated with a normal CO2, so it only looks at the metabolic component
Normal value is zero (-2 to 2 mmol/l)
A big negative value indicates a metabolic acidosis
A positive value seen in compensated respiratory acidosis
Why is acidaemia purely respiratory?
because the standard bicarbonate is normal (and because the BE is normal
How do we determine the cause of acidosis?
Is it respiratory?
-Yes, if the pCO2 is high
Is it metabolic?
-Yes, if the standard bicarbonate is low, or the base excess has a more negative value than -2mmol/l, e.g -3mmol/l
