Primary Teaching - Respiratory Physiology Flashcards
Hb oxygen dissociation curve in healthy individual - PaO2 when sats 50%
3.5 kPa
Hb oxygen dissociation curve in healthy individual - PaO2 when sats 75%
5.3 kPa
Hb oxygen dissociation curve in healthy individual - PaO2 when sats 100%
13.3 kPa
Myoglobin dissociation curve
Hyperbolic curve
Why is myoglobin dissociation curve steeper than Hb dissociation curve
Myoglobin only binds one oxygen molecule
Analogy for environment where Hb has lower affinity for O2
Exercising muscle - eg. heat, higher PaCO2, acidosis, (higher 2,3 DPG + pregnancy don’t fit)
Why is there a delay in increasing oxygen carriage post blood transfusion
Stored blood transfusions depletes levels of 2,3 DPG in circulation and therefore causes delay in increase of oxygen carriage post transfusion
Haldane effect
Deoxygenated Hb has higher affinity for CO2
Bohr effect
Rightward shift of Hb oxygen dissociation curve as PaCO2 / acidosis increases
Oxygen requirements of vital organs per minute
200-250 ml/min
Modes of transport of O2
Bound to Hb (majority at sea level in healthy)
Dissolved in plasma
Oxygen content of blood
CaO2 = (SaO2 x 1.34 x [Hb]) + (PaO2 x 0.023)
(SaO2 x 1.34 x Hb) is oxygen bound to Hb
(PaO2 x 0.023) is oxygen dissolved in plasma
0.023 is oxygen solubility coefficient
1.34 is Hüfner constant, amount of oxygen in millilitres carried by each gram of haemoglobin
Oxygen cascade graph
Saturated water vapour pressure
6.3 kPa
PaO2 in atmospheric air at sea level
21.2 kPa
PaO2 in trachea air where humidification occurs
(Atmospheric pressure - saturated water vapour pressure) x PaO2 in atmosphere
(101 - 6.3) x 0.21 = 19.9
Alveolar gas equation
PAO2 = FiO2 (PATM – PH2O) – PACO2/RQ
(PATM – PH2O) is (Atmospheric pressure - saturated water vapour pressure) i.e previous step to account for humidification
PACO2 is roughly equal to PaCO2
RQ (Respiratory quotient) = 0.8 in most healthy individuals
Nitrogen is not included in equation as Nitrogen pressure is in equilibrium and does not change and therefore does not alter equation
PAO2 = (0.21 x (101-6.3)) - (5.3/0.8)
= 19.9 - 6.6
= 13.3 kPa
Respiratory quotient relevance
0.8
Takes into account CO2 produced and then exchanged
Effected by metabolism so is different in certain food diets e.g pure carbohydrate diets
Alveolar - arterial gradient
Less that 2 kPa
Therefore PaO2 is between 11.3 and 13.3 kPa as PAO2 is 13.3 kPa
2,3 DPG
2,3 Diphosphoglyceric acid
By product of ATP production / glucose metabolism
Methods of CO2 transport in the blood
Conversion to bicarbonate 60%
Carbaminohaemoglobin 30%
Dissolved in plasma 10%
CO2 metabolism equation
CO2 + H2O -> H2CO3 -> HCO3- + H+
H2CO3 is carbonic acid, but almost immediately dissociates to Bicarbonate and Proton ion
Catalysed by carbonic anhydrase enzyme
Goes forward in tissues for transport
Goes backwards in pulmonary capillaries for excretion
Chloride shift
In venous blood, Hb buffers proton ion produced by bicarbonate production.
Bicarbonate diffuses out of RBC into serum via HCO3-/Cl- exchanger in exchange for chloride ion
Therefore venous blood has higher levels of bicarb and lower levels of chloride
Categories of hypoxia
Hypoxaemic hypoxia
Anaemic hypoxia
Histotoxic hypoxia
Circulatory hypoxia
Hypoxaemic hypoxia definition
True hypoxia in atmospheric air / Obstructive lung disease
Circulatory hypoxia definition
Impaired perfusion of tissues due to reduced circulation / stagnation
Circulatory hypoxia examples
Cardiogenic shock / heart failure
Obstructive shock eg tamponade
Distributive shock eg anaphylaxis
Histotoxic / cytotoxic hypoxia definition
Cellular dysfunction for oxygen utility
Histotoxic / cytotoxic hypoxia examples
CO poisoning
Sepsis
Mitochondrial dysfunction
Effect of halothane and sevofluorane on central chemoreceptor response to hypoxaemia
Dampened
Cardiovascular effects of hypoxia
Sympathetic response
Increase in cardiac output
Effect of hypoxia on cerebral vasculature
As PaO2 drops below 8 kPa, cerebral blood flow exponentially increases with vasodilation
Therefore in neuroprotective measures need to maintain normoxia
Approximate spirometry volumes
Total lung capacity 6000 ml
Vital capacity 4500 ml
Tidal volume 500 ml
Inspiratory reserve volume 2500 ml
Functional residual capacity 3000 ml
Expiratory reserve volume 1500 ml
Residual volume 1500 ml
Capacity definition
Is the sum of volumes
Eg. FRC = ERV + RV
Closing volume definition
Volume of the lungs when small airways begin to close
Differs with age and positioning of patient
How to measure closing volume
Fowler’s method
Fowler’s method
Single breath nitrogen washout test
Calculates anatomical dead space and closing volume
Effect of altitude on barometric pressures of PiO2
Higher altitudes have lower barometric pressures of oxygen
Effect of altitude on volatile anaesthetics
Percentage of volatile delivered increased but partial pressure of volatile remains the same and therefore clinical effect remains the same.
Partial pressure of volatile agents determine the depth of anaesthesia, not the percentage
