Respiratory Flashcards
Fick’s Law of diffusion
Amount of gas that moves across a sheet of tissue is proportional to the area of the sheet and concentration gradient but inversely promotional to its thickness
Anatomical dead space volume
Conducting airways
~150mL
Airway anatomy
23 generations
Divided into conducting zone (0-16, 150mLs) and respiratory zone (17-23, FRC 2.5-3L)
Volume of alveolar region ~2.5-3L
Gas moves by bulk flow down a pressure gradient in the conducting zone
Fas movement in the alveolar region is chiefly by diffusion
Lung volumes
Vt - volume inspired/ expired during normal quiet breathing
-> ~500mL
- 6-8mL/kg
Inspiratory reserve volume - volume of additional air that can be inspired over tidal volume
-> 2500mL
Expiratory reserve volume - volume of additional air that can be expired following normal tidal expiration
-> 1500mL
Residual volume - volume that remains in the lung at maximal expiration
-> 1500mL (20mL/kg)
-> Governed by the balance between the max force generated by expiratory muscles and the elastic forces opposing reduction of lung volume
-> In older subjects or disease, closure of small airways may prevent further expiration increasing RV
Lung Capacities
FRC = RV + ERV
-> Lung volume at end tidal expiration, i.e resting lung volume at equilibrium of lung and chest wall elastic recoil
-> 3000mL (40ml/kg or 30ml/kg supine)
VC = ERV + VT + IRV
-> Volume of maximal inspiration and expiration
-> 4500mL
Inspiratory capacity
-> = VT + IRV
-> 3000mL
TLC
-> Occurs at maximal inspiration - note expiratory muscles also contract strongly at max inspiration
= RV + ERV + VT + IRV
= 6000mL (80ml/kg)
FRC - importance of and factors affecting FRC
Importance
- O2 buffer
- Prevention of alveolar collapse
- Optimal lung compliance
Factors affecting FRC
Overall - body size, sex, age, ethnicity
Reduces - position (falls when supine), raised IAP, anaesthesia, lung disease (restrictive)
Increases - PEEP, emphysema, asthma/ obstructive airway disease
Lung volume measurement
All lung volumes except RV can be measured by spirometry. Any capacities including RV cannot be directly measure by spirometry - TLC and FRC.
FRC can be calculated by
- Gas dilution
-> At end of normal expiration, known conc of He breathed in
-> C1V1 = C2 x (V1 + V2)
-> V2 = FRC
-> Measure only communicating gas/ ventilated lung volume
- Body plethysmography
-> Boyle’s Law, PV = K (constant) at constant temp
-> Large airtight box, end of expiration, mouthpiece shutter closes - inhale against closed mouthpiece and thus gas in lung expands, volume within plethysmograph decreases proportionally
-> P1V1 = P2(V1-∆V); [∆V = change in volume of box (or lung)]
-> P3V2 = P4(V2 + ∆V); [V2 = FRC]
-> Measures titan volume of gas in the lung incl any trapped gas
Alveolar ventilation
Amount of fresh inspired air available for gas exchange
= (VT - anatomical dead space) x RR
V’A = V’E - V’D
Alveolar ventilation (V’A) = (V’CO2/PCO2) x k
Dead space
Volume of inspired air playing no role in gas exchange - conducting airways or non-perfused alveoli such as in PE
Anatomical Dead space
- Volume of upper airways and first 14-16 generations of bronchial tree which form conducting airways
- Approx 150mL, 2ml/kg (roughly 1/3 of Vt)
Alveolar dead space
- Total volume of ventilated alveoli that are unable to take part in gas exchange due to insufficient perfusion
- West zone 1
Physiological dead space
- Total dead space in the lung -> sum of anatomical and alveolar
In normal subject - anatomical and physiological dead space are nearly the same
Anatomical dead space measurement
Fowlers method
- Subject breaths through a valve box, and the sampling tube of a rapid N2 analyser continuously samples gas at the lip
- Following single VC breath of 100% O2, anatomical deadspace fill with 100% O2 and N2 conc in alveoli has been diluted.
- The first part of exhaled gas comes from the anatomical dead space, with zero N2 content
- This is followed by a rapid rise in N2 conc - transition between anatomical dead space gas and mixed alveolar gas
- Follow by a pleateau, represents mixed alveolar gas
- Dead space found by plotting N2 conc against expired volume and drawing a vertical line such that area A = area B. VD is the volume expired up to the vertical line.
- This method measure the volume of the conducting airways down to the midpoint of the transition from dead space to alveolar gas
~2ml/kg
Physiological dead space measurement
Bohr’s method
- All expired CO2 comes from the alveolar gas and none from the dead space
- Blood gas to be taken at the sam time as capnography is measured
-> VT. FECO2 = VA. FACO2
-> VT. FECO2 = (VT - VD). FACO2
Bohr’s equation -> VD/VT = (PACO2 - PECO2)/ PACO2
PACO2 ~ PaCO2 in healthy
Enghoff modification -> VD/VT = (PaCO2 - PECO2)/ PaCO2
- Measure volume of lung that does not eliminate CO2 - physiological dead space
Alveolar Gas Equation
PAO2 = [(PB - PH2O)xFiO2] - [PaCO2/0.8)
= PIO2 - (PACO2/R) + F
PAO2 = alveolar partial pressure O2
PB = barometric pressure
PH2O = saturate vapour pressure of water at 37oC
PaCO2 = arterial partial pressure CO2
R = respiratory quotient = 0.8
Rate of diffusion
Proportional to SA, concentration gradient and solubility; inversely proportional to thickness and √MW
Diffusion limitation
Amount of gas transferred from alveolus to capillary blood, per unit time, is limited by the diffusion properties of the blood-gas barrier, and not by the amount of blood flow.
Partial pressures in blood and alveolus DO NOT reach equilibrium.
E.g CO, and O2 in abnormal conditions, e.g when diffusion properties of lung are impaired
Perfusion limitation
Amount of gas transferred from alveolus to capillary blood, per unit time, is limited by the amount of blood flow, and not by the diffusion properties of the blood-gas barrier.
Partial pressures in blood and alveolus reach equilibrium.
E.g N2O, and O2 in normal conditions
DLCO
DL = V’CO/PACO
- CO used to measure as it is diffusion limited
Single breath method
- Single VC breath of 0.3% CO held for 10s and then exhaled
- Inspired and expired PCO are measured via infrared gas analyser
- difference is the amount of CO now bound to Hb
- Alveolar CO is not constant in breath hold but allowances made
- He is also added to measure lung volume by dilution
- Adjustments made for Hb as this effects DLCO
Normal DLCO = 25ml/min.mmHg - can increase 2-3x with exercise
Decreased in
- thickened alveolar capillary membrane (ILD)
- reduced SA (emphysema, PE, lobectomy)
Increased in
- exercise (recruitment and distension)
- alveolar haemorrhage (Hb present with lung binds CO)
- asthma (potentially due to increased apical flow)
- obesity (potentially due to incr CO)
Reaction rates with Hb
O2 reaction with Hb is fast (0.2s), but can become limiting
Uptake of O2 (or CO) occurs in 2 stages
1. Diffusion of O2 through the blood-gas barrier (incl plasma and red cell interior)
2. Reaction of O2 with Hb
Two elements resisting diffusion of O2
1. Resistance of the blood gas barrier which is equal to the inverse of DL, i.e 1/DM where M is membrane
2. Rate of reaction with Hb (diffusion capacity of the blood)
⍬ describes the rate of reaction of O2 with Hb in mL/min. ⍬ multiplied by volume of capillary blood Vc gives the effective diffusing capacity of O2 with Hb, the inverse gives the resistance
Total diffusion resistance 1/DL = (1/DM) + (1/⍬.Vc)
DLCO is affected by volume of alveoli, distribution of diffusion properties, and capillary blood
Reaction rates with Hb
O2 reaction with Hb is fast (0.2s), but can become limiting
Uptake of O2 (or CO) occurs in 2 stages
1. Diffusion of O2 through the blood-gas barrier (incl plasma and red cell interior)
2. Reaction of O2 with Hb
Two elements resisting diffusion of O2
1. Resistance of the blood gas barrier which is equal to the inverse of DL, i.e 1/DM where M is membrane
2. Rate of reaction with Hb (diffusion capacity of the blood)
⍬ describes the rate of reaction of O2 with Hb in mL/min. ⍬ multiplied by volume of capillary blood Vc gives the effective diffusing capacity of O2 with Hb, the inverse gives the resistance
Total diffusion resistance 1/DL = (1/DM) + (1/⍬.Vc)
DLCO is affected by volume of alveoli, distribution of diffusion properties, and capillary blood
PulmVR decreases with exercise?
As arterial or venous pressure increases, pulmonary vascular resistance decreases via two mechanisms
- Recruitment and distension of capillaries
Starling’s equation
Net fluid out = K[(Pc-Pi) - σ(πc-πi)]
K = filtration coefficient (constant)
Pc = hydrostatic pressure in capillaries
Pi = hydrostatic pressure in ISF
σ = reflection coefficient
πc = colloid osmotic pressure of proteins in blood
πi = colloid osmotic pressure of proteins in ISF
Net pressure is outward
4 causes of hypoxaemia
- Hypoventilation
- Diffusion limitation
- Shunt
- V/Q inequality
Shunt + shunt equation
Venous blood entering the arterial system that doesn’t pass through the ventilated areas of the lung
- Passes from R to L heart without participating in gas exchange
- Deoxygenated venous blood passes directly into arterial system reducing PaO2
Shunt equation
- Used to calculate the proportion of the CO that is shunted from the venous to the arterial system (actually calculated venous admixture)
- Qs/Qt = (CcO2 - CaO2) / (CcO2 - CvO2)
Qs/Qt = shunt fraction (shunt flow divided by total cardiac output)
CcO2 = pulmonary end-capillary O2 content, same as alveolar O2 content
CaO2 = arterial O2 content
CvO2 = mixed venous O2 content
2 causes of hypercapnia
Hypoventilation
V/Q inequality