|Acute Severe Asthma
|Respiratory Failure |
- Chest wall is composed of sternum, 12 ribs,
intercostal spaces and the thoracic vertebra
- First seven ribs articulate at the sternum.
- Ribs 8,9,10 articulate with manubriosternal
- Ribs 11 and 12 are free floating and do not
- Inferiorly lies the dome shaped diaphragm
- Get atmospheric or medically supplied oxygen
to all of the cells of the body and their mitochondria.
- Mitochondrial oxidative phosphorylation will
cease when PO2 falls below 1 mmHg (0.13 Kpa)
- This will allow the process of oxidative
phosphorylation and krebs cycle with oxygen transport system
- The end product of combining oxygen and
glucose is ATP and CO2 and H20 which need excreted
- Normal respiration rate is 15/minute. Time for
Inspiration to Expiration is 1:2
- The ease at which a lung expands is the
compliance - LOW with LVF, ARDS, Pneumonia
- Airway resistance = pressure gradient from mouth to
alveolus divided by rate of flow - HIGH with Asthma, COPD
- Air is sucked in due to a negative intrathoracic pressure
- Contraction of the diaphragm downwards
becoming flatter (phrenic nerve C3/4/5)
- Contraction of external intercostals pulling
ribs up and outwards
- Occasional use of Accessory muscles -
scalenes, sternocleidomastoids, pectoralis major, trapezius
- Expiration is largely passive using relaxation
of diaphragm and elasticity of lung tissue
- Active expiration can be aided by internal oblique,
external oblique,rectus abdominis, transversus abdominis
- Both actions
- Increase thoracic volume which creates
negative intrathoracic pressure.
- Air moves along pressure gradient inward,
enters through both nostrils
- Air enters the nose where the rich vascular
supply warms it and is humidified and filtered.
- Air then passes via nasopharynx down across the larynx
and into the trachea
- Except when positive pressure ventilated
- Air is pushed against a pressure gradient into
the lungs as respiratory muscles paralysed
- Through an endotracheal tube or with non
Invasive ventilation - a tight fitting face mask/nose
- This creates a positive intrathoracic pressure
which may be continuous or intermittent
- Positive airways pressure can increase risk of lung
trauma and pneumothorax
- Negative pressure ventilation
- Historical interest - The "Iron lung" invented
mainly to manage patients with polio related chest muscle
- Patient lay inside sealed compartment. Driven
often by medical students.
- Stimulated the whole development of ITU
medicine and ventilation
- Lung has a dual blood supply
- Bronchial vessels - 120/80 mmHg
- Pulmonary vessels - 25/10 mmHg
- Pulmonary circulation is low resistance
- Less power needed so RV is thin walled
- Causes localised hypoxic
- Areas of low pO2 have low pulmonary blood flow
- Blood diverted to areas of higher pO2
- The lungs vascular system acts to
- Oxygenate blood
- Excrete CO2
- Filter out microthrombi eturning
from systemic circulation
- Convert Angiotensin I to angiotensin II
- Acts as a blood reservoir - increased with
- Oxygenated blood returning to the heart in the
pulmonary vein also gets some deoxygenated bronchial vein blood.
- The Left ventricle receives deoxygenated
coronary blood via the thebesian veins
- Successful oxygenation requires matching
ventilation with perfusion.
- If areas are perfused but get no oxygen this
is a shunt and leads to deoxygenated blood entering the left atrium.
- No matter how much additional oxygen is given
it will not restore full oxygenation with a shunt.
- This may be seen in pneumonia and consolidation or
atelectasis or cyanotic congenital heart diseases.
- In pulmonary embolism there is oxygenation but not
Ventilation perfusion mismatching can be
estimated by an increase in the Alveolar-arterial pO2 gradient which is
normally < 2 KPa
- The trachea extends from cricoid cartilage to
the bifurcation at the carina at the level of manubriosternal joint
- Trachea in cross section is composed of "C"
shaped cartilage rings deficient posteriorly closely related to
- At the carina at the level of the sternal angle
(T4/5) it bifurcates into right main bronchus and then left main
- The right main bronchus is shorter, wider and
more vertical than the left and so is the destination of aspirated
- There is repeated branching until terminal
bronchioles and alveoli
- Lungs are enveloped by visceral pleural and
then a pleural space and then parietal pleura.
- The pleura are continuous. The pleural space
contains only a small amount of fluid and a virtual space.
- Surface tension keeps the layers together and
normal intrapleural pressure is between - 2 and - 5 cm H2O.
- Parietal pleura is innervated by the intercostal and
phrenic nerves so pleuritic pain can go to the shoulder and to the
- The right lung is composed of 3 lobes Upper,
Middle, Lower and has both transverse and oblique fissures.
- The Left lung is composed of 2 lobes Upper (+ lingula)
and lower separated by the oblique fissure
- Vessels, nerves and lymphatics all enter the lung
medially at the lung hila.
- Lung segments
- Lungs can be divided into separate surgically
resectable bronchopulmonary segments
- Each with their own segmental bronchus and artery and
- Vascular supply
- Airway branches below the terminal bronchioles
receive their arterial supply from the bronchial arteries
- These originate as an aortic branch
- R/L Main Bronchi
- Lobar bronchi
- Segmental Bronchi
- Small bronchi - successive branches
- Terminal bronchioles
- Respiratory bronchioles
- Alveolar ducts and sacs
- Surface area available for gas exchange is 90
- 300 million alveoli of less than 1/5th of a mm
in diameter in two lungs
- The gases must traverse
- alveolar epithelium
- basement membrane of alveolus
- basement membrane vascular endothelial cell
- This works out at about 0.4 um.
- Oxygen passes from high oxygen tension alveoli
to low oxygen tension venous blood.
- CO2 passes from high tension venous side to low
tension alveolus side
- Both gases pass either way from alveolar air
across alveolar epithelium basement membrane and capillary endothelium
- Oxygen binds erythrocyte Haemoglobin and a
small volume of oxygen dissolves in blood but most is carried bound to
- Haemoglobin bound oxygen carriage is determined
by O2 dissociation curve.
- Type 1 pneumocytes
- Line the alveoli forming a thin layer allowing
- They are devoid of most organelles.
- Type 2 Pneumocytes
- Secrete surfactant and some also become Type 1
- They have larger nuclei and microvilli and cytoplasm
containing storage vesicles for surfactant release.
- Surfactant is composed of phospholipids
including sphingomyelin and lecithin.
- The molecules have a hydrophobic and
- Surfactant reduces alveolar collapse and
improves alveolar stability.
- Lines the alveoli and lowers surface tension
- Allows easier inflation and deflation of the
- Loss of surfactant causes stiff lungs - neonates
- The curve is sigmoidal and not a straight line.
At low oxygen tension binding is poor and Hb releases oxygen to surrounding hypoxic tissues - enhances oxygen delivery where it is needed.
- Hb is more "sticky" for oxygen when in high oxygen tension area and is good at binding oxygen when there is plenty such as in the lungs and giving it away when there is none. These properties are affected by certain environmental factors and changes also in Hb.
- The steep middle section means that when there is a small
drop in local PaO2 in the midzone there is a large release of oxygen
- Room air contains 21% oxygen. Small increases in
arterial pO2 can cause significant improvements in oxygen carriage and
- Nasal cannula can deliver 24-40% depends on flow
rate. Simple face masks variable FiO2 depending on flow and respiratory
- Venturi mask - uses venturi effect to mix air
and oxygen to a more accurate concentration. Recommended in COPD
- Non rebreather - used to give very high FiO2 where COPD is
not an issue
- Structure and function
- Four subunits of Haem + globin each of which
binds an O2 molecule at differing strengths giving rise to the
- A process called Cooperative binding.
- Anaemia makes the curve move vertically down
and reduces oxygen delivery.
- Polycythaemia makes the curve move vertically up
improving O2 delivery "altitude training"
- Anaemia becomes very significant in terms of
oxygen delivery when Hb < 7g/dl
- Makes Hb hold onto to O2 tighter "Collection" (Leftward
- Low PCO2, Alkalosis, Low temperature, Reduced
- Carbon monoxide binds to HbO2 sites avidly
forming COHb and actually increases O2 affinity worsening tissue
- Fetal Hb - has to remove oxygen from maternal Hb
- Markers of metabolism makes Hb release O2 more easily
"Delivery" (Right shift)
- Raised PCO2, Acidosis, Raised temperature,
Increased 2,3 DPG
- Hb Kansas and some other abnormal Hb
- Central chemoreceptors in ventral medulla
- Responds to increasing pCO2 indirectly by
detecting increased CSF [H+]
- Stimulates inspiratory centre - increased rate
and depth of respiration
- Affected by sedation, drugs, sleep, alcohol
- Peripheral chemoreceptors in Carotid body (IX cranial
nerve) and Aortic arch (X cranial nerve)
- Detect low pO2 primarily but also raised pCO2
- Stimulates inspiratory centre - increased rate and
depth of respiration
- Senation of Dyspnoea comes from
- Afferent receptors in respiratory muscles
- Juxtacapillary J receptors in lung sense
- Chemoreceptors sensing hypoxia and hypercarbia
- Normally pCO2 is the main gas that determines
resp rate and depth
- Under hypoxia (PO2 < 8Kpa) hypoxia along
with hypercarbia and acidosis drives the response
- In those with COPD hypoxia can become the main
regulator normally "hypoxic drive"
Causes of hypoxia
- Lung gets 4-6 L/min of air and 5L/min blood.
Ventilation/perfusion ratio = 0.8 (4/5)
- Lung apices V > Q so ratio > 0.8. Lung
bases V < Q so ration < 0.8
- Pulmonary embolism Q falls . V >> Q and so
there is a VQ mismatch as aerated lung is under perfused "dead space"
- Asthma/Pneumonia V falls. V < Q and there is a VQ
mismatch as perfused lung is not aerated "shunt"
Physics of Respiration
- Dalton's law - for 2 individual unreactive gases
the total pressure exerted is the pressure of the first plus the second
- Normal atmospheric pressure at sea level is 760
mmHg = 101 kPa. Air contains 21% oxygen and 78% nitrogen and 1% inert
- Partial pressure of oxygen at sea level -
Fraction in inspired air x atmospheric pressure = 0.21 x 760 = 159 mmHg
= 21.2 Kpa
- Partial pressure of oxygen On summit of Everest - Fraction
in inspired air x atmospheric pressure = 0.21 x 252 = 52 mmHg = 6.9 Kpa
so high FiO2 must be given
- Spirometry involves blowing out as fast as possible into a special piece of equipment called a Spirometer. When the maximal blow is performed together with a maximal suck this test is sometimes referred to as a 'Flow Volume Loop'.
- The test will be performed with you seated and your nose may be sealed with a nose clip. You will be asked to breathe normally into a mouthpiece and instructed to take a deep breath in and then to blow out as fast as possible and try and keep blowing until your lungs are empty. This will be performed a minimum of three times, but you will be given adequate rest between each blow.
- This test measures the volumes and speed of the air you can blow out from your lungs and will give the doctor an indication of how clear the airways are in your lungs. For example, the airways may be narrower in conditions such as COPD or Asthma.
- Spirometry may then be repeated after you have been given an inhaler or nebuliser. This will be to see if there is any improvement in your airways as a result of taking this medication.
Forced expiratory volume in 1 second (FEV1)
- The FEV1 (forced expiratory volume 1) is the volume of air forcefully exhaled in 1 second
Forced Vital Capacity (FVC)
- The FVC is the volume of air that can be maximally forcefully exhaled - and therefore contains the FEV1 within it.
- If the FEV1/FVC ratio is <80%, it indicates that an obstructive defect is present.
Summary of lung volumes
- TV (Tidal volume) is the volume that flows in or out of the lungs with each breath during quiet breathing. (Normally about 7 cm3/kg)
- IRV (Inspiratory reserve volume) is the maximum amount of air that can be inspired in excess of the tidal volume. (Normally approximately 3.3 dm3
in men and 1.9 dm3 in women)
- ERV (Expiratory reserve volume) is the maximum amount of air that can be expired in excess of the tidal volume. (Normally approximately 1.0 dm3
in men and 0.7 dm3 in women).
- RV (Residual volume) is the volume left in the lungs after maximum expiration. (Normally approximately 1.2 dm3 in men and 1.1 dm3 in women).
advanced applied science: ? IC (Inspiratory capacity) is IRV + TVFRC (Functional residual capacity) is ERV + RV ie volume remaining in the lungs at the end of a normal expiration.
- TLC (Total lung capacity) is IRV + TV + ERV + RV or SVC + RV. It is typically about 3 - 5 dm3. TLC increases when elasticity of the lungs is lost, e.g. in COPD, due to emphysema.
- VC or SVC (Vital capacity) is IRV + TV + ERV and is the maximum breath volume.