Airway Flashcards
What is the Fi02 of atmospheric air?
21%
What is the oxygen cascade and values?
Describes the incremental successive drops in p02 from atmosphere to arterial circulation
Air = 21.0 kPa Trachea = 19.8 kPa Alveolar = 14.0 kPa Arterial = 13.3 kPa
How is oxygen transported in the body?
Via binding to haemoglobin 99%
OR
1% is in solution in the blood
What is Henrys law?
Oxygen at 37 degrees?
Gas content of solution = solubility x partial pressure of gas
Oxygens solubility at 37 degrees = 0.03…
Gas content = 0.03 x Pa02
what is haemoglobin composed of?
How many oxygens can bind?
A haem and globin chain.
Globin = 2 alpha and beta units and a 2,3-DPG
Haem = Fe2+ and protoporphyrin ring
4 oxygens
What other molecules may bind to haemoglobin?
Carbon monoxide - binds to globin chain
H+ - Bind to globin chain
2,3-DPG - byproduct of red cell metabolism. Form covalent bonds with B-subunits.
What are the main sites of haematopoiesis?
- In first few weeks gestation = Yolk sac
- up to 7 months gestation = liver and spleen. (Also in adults but this is pathological e.g. myelofibrosis)
- From first few weeks of life = bone marrow
What is the Bohr effect, and what causes it?
Bohr effect is the shift of the oxygen dissociation curve to the right
Cause by:
Increased temperature, H+, 2,3-DPG
Hypercarbia
What physiological effect does the Bohr effect have?
Why useful?
Shifts curve to the right
This means Hb has lower affinity for oxygen, so offloads oxygen more easily
Useful if tissues are acutely/chronically under perfused, we can supply more oxygen to them.
How does the oxygen dissociate curve in the foetus compare to that of an adult?
In a foetus it is shifted to the left. This is due to the replacement of a B subunit with a y subunit.
This means it has greater affinity for oxygen, so can more readily bind to maternal oxygen
How much. oxygen is bound to haemoglobin when fully saturated?
1.34ml per gram of Hb, when fully saturated
Therefore oxygen carrying capacity is 1.34 x Hb (at full sats)
How would we figure out total oxygen content in blood?
Need to figure out the 99% carried by Hb, and 1% carried in solution.
(0.03 x Pa02) + (1.34 x Hb x % sats)
FiCO2 of atmospheric air?
0.035%
Outline the figures of the carbon dioxide cascade?
Why no difference between alveolar and arterial?
Why might this difference increase?
Air = 0.03 kPa Alveolar = 5.30 kPa Arterial = 5.30 kPa Venous = 6.10 kPa Exhaled air = 4.00 kPa
Because CO2 has high water solubility so rapidly diffuses across respiratory membrane
This difference may increase under pathological conditions which increase V/Q mismatch, or when there is increased C02 production.
How is CO2 transported within the body?
85-90% is transported via bicarbonate ions
5-10% is transported via carbamino compounds, formed when CO2 binds with plasma proteins
*the most significant of these is haemoglobin, binding to globin chain
5% dissolved in solution
How does CO2 transport differ in venous blood vs arterial?
In arterial blood there are less carbamino compounds, and more bicarbonate carriage.
This variation is due to pH affecting binding properties.
How does the amount of CO2 in solution compare to the amount of oxygen in solution?
CO2 = 5%
Oxygen is 1%
how does CO2 come to be carried in bicarbonate ions?
What catalyses this?
CO2 + H20 = H2CO3 = H+ + HCO3-
The interchangeable reaction is catalysed by carbonic anhydrase
What happens to the H+ generated by CO2, being converted to HCO3-?
what effect does this have on transport of oxygen via haemoglobin?
The H+ is mopped up by buffer systems.
Namely haemoglobin molecule via imidazole groups
The Hb taking up CO2 and H+ leads to a reduced oxygen affinity (as in the BOHR EFFECT)
What happens to the bicarbonate ion generated in the red cell when it carries CO2?
How does this affect osmotic balance?
Bicarbonate diffuses out of the red cell into the plasma (unlike the H+ it can diffuse through the membrane)
The bicarbonate is exchanged for a chloride ion = CHLORIDE SHIFT
Bicarbonate ion and chloride generated following CO2 carriage = increased intracellular osmotic pressure causing cell to swell:
Which is why haematocrit of venous blood is 3% higher than arterial
Why can the amount of CO2 not be expressed as saturation percentage like oxygen?
Because it is so soluble in liquid, it never reaches full saturation
Bohr effect vs Haldane effect?
Bohr effect = affinity for oxygen, affected by H+, temperature and 2,3DPG as well as hypercarbia
Haldane affect is affinity for CO2.
- As PaO2 increases, the affinity for CO2 decreases
- This would be see as a downward shift on the CO2 curve
What equation describes the relationship of PaO2 with PaCO2?
What is the use of this equation?
Alveolar gas equation
PAO2 = PiO2 - PCO2/R
(PA02 = partial alveolar pressure)
R is respiratory exchange ratio, usually 0.8
This equation gives PA02 which can be used to calculate the A/a gradient.
What is the danger of oxygen therapy In chronic retainers of CO2?
- Loss of the hypoxic drive - those who retain CO2 rely I hypoxia to stimulate respiration. Get rid of this hypoxia = apnoea
- Abolishment of hypoxia: This can reverse the normal compensatory hypoxic pulmonary vasoconstriction
What is hypoxic pulmonary vasoconstriction (HPV)?
During time of hypoxia, smaller pulmonary arteries constrict
This diverse blood flow to the better ventilated regions of the lung
Name 3 problems with oxygen therapy?
- Absorption atelectasis = normally nitrogen is absorbed slowly, so ‘splints’ open the airway. With pure oxygen it is absorbed rapidly and can collapse
- Pulmonary toxicity - oxygen can irritate mucosa
- Risk of combustion.
What is pulse oximetry and what does it measure?
Works via spectrophotometry. Measures the differing light absorbed by saturated and unsaturated Hb and calculates sats from this ratio.
Measures oxygen saturation (SaO2) and pulse
Disadvantages of pulse oximetry?
Delay in real time sats ad what comes up on display of about 20 seconds.
Poor peripheral perfusion means poor signal quality e.g. in shock
Poor accuracy below 70%
Abnormal pigmentation e.g. jaundice can affect it
Arrhythmias can causes interference with signal too
What is methaemoglobin?
2 causes
What will patients look like?
Mx?
Hb where it has Fe3+ (ferric) instead of Fe2+ (ferrous)
Can be causes due to congenital deficiencies in enzymes OR acquired via exposure to chemical agents e.g. prilocaine
Patient will look blue, as darker colour of methaemoglobin
Mx = methylene blue
How may ventilation be assessed?
Ventilation measures ability to blow off CO2
Assessed using capnography
Three types of surgical airways?
Needle cricothyroidotomy (Jet insufflation) Cricothyroidotomy Tracheostomy
Indications for a surgical airway?
Failed non-surgical intubation e.g. due to oedema
Traumatic # larynx
Where are surgical airways sited?
Cricothyroidotomy inserted into cricothyroid membrane
Tracheostomy between 2-5th tracheal ring.
How is jet insufflation performed and what is main precaution to consider?
Inserted as needle cricothyroidotomy
Connected to oxygen and short bursts of oxygen given
Patient is well oxygenated but poorly ventilated, so can lead to progressive hypercarbia
Only use for 45 minutes maximum
